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 Farm Dams
Planning,

Construction

and Maintenance

Barry Lewis

SOFTbank E-Book Center Tehran, Phone: 66403879,66493070 For Educational Use.

 © Barry Lewis 2002

All rights reserved. Except under the conditions described in the Australian Copyright Act 1968 and
subsequent amendments, no part of this publication may be reproduced, stored in a retrieval system or
transmitted in any form or by any means, electronic, mechanical, photocopying, recording, duplicating
or otherwise, without the prior permission of the copyright owner. Contact Landlinks Press for all
permission requests.

National Library of Australia Cataloguing-in-Publication entry

Lewis, B. (Barry), 1942– .

Farm dams: planning, construction and maintenance.

ISBN 0 643 06576 8 (paperback).

ISBN 0 643 06992 5 (eBook).

1. Dams – Design and construction. I. Title.

627.8

Available from
Landlinks Press
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Collingwood VIC 3066
Australia

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Freecall: 1800 645 051 (Australia only)
Fax: +61 3 9662 7555
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Web site: www.landlinks.csiro.au

Cover photos by Barry Lewis

Set in 10/12 Sabon

Cover design by James Kelly
Typeset by J&M Typesetting
Printed in Australia by Ligare

Disclaimer
This book is intended for use as a guide to owners and operators of farm dams. It
suggests prudent approaches to normal surveillance and maintenance practice
with a view to enhancing the long-term safety and survival of farm dams. It is not
intended as a source of detailed information to cover all possible eventualities. In
the event of any suspected imminent or potential failure condition, expert advice
should be sought immediately.

SOFTbank E-Book Center Tehran, Phone: 66403879,66493070 For Educational Use.

 Table of Contents
Preface viii
Acknowledgements ix
Introduction x
Section 1 Planning 1
1.1 Assessing water needs 1
1.1.1 Planning water supplies 1
1.1.2 Water quality 2
1.2 Assessment of catchment yield 3
1.2.1 Factors controlling catchment yield 3
1.2.2 Methods of estimating catchment yield 4
1.2.3 How trees affect yield 7
1.2.4 Artificial catchments 8
1.3 Dam site selection 8
1.3.1 Choosing a dam site 9
1.4 Types of farm storages 12
1.4.1 Gully dams 13
1.4.2 Hillside dams 14
1.4.3 Ring tanks 15
1.4.4 Turkey’s nest tanks 16
1.4.5 Excavated tanks 17
1.4.6 Weirs 19
1.4.7 Off-waterway storages 19
1.5 Dam storage size 19
1.5.1 Evaporation losses 20
1.5.2 Ways of controlling evaporation 21
1.5.3 Seepage losses 21
1.5.4 Average water consumption 21
1.6 Using a dam in drought 24
1.7 Fire fighting 27
1.7.1 Maximum daily consumption 27
1.7.2 Peak rate of demand 27
1.7.3 Monthly and annual consumption 28
1.8 Farm dams and trees 28
1.9 Dam cost justification 28
Section 2 Investigation 31
2.1 Soil testing 31
2.1.1 Foundation 31
2.1.2 Borrow pit for embankment material 32
2.1.3 Spillway site 32
2.2 Site selection criteria 33
2.2.1 Seepage losses 33
2.2.2 Stability of dam sides 34
2.2.3 Sedimentation in dams 34
2.3 Foundation materials 34
2.4 Embankment materials 35
2.5 Site investigation of materials 37

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 iv • Farm Dams

2.5.1 Soil texture tests 37

2.5.2 Unified soil classification 38
2.6 Analysis of soil 39
2.6.1 Core trench 39
2.6.2 Embankment soil 40
2.7 Location of soil 40
2.8 Unsuitable material 41
Section 3 Design 43
3.1 Items that need to be considered 44
3.1.1 Embankment types 44
3.1.2 Core trench 46
3.1.3 Embankment batter slope 47
3.1.4 Crest width 48
3.1.5 Freeboard 49
3.1.6 Alternative ways of batter protection 50
3.1.7 Topsoil cover 51
3.1.8 Fencing 51
3.2 Flood flow estimation 52
3.2.1 Peak flow estimation 53
3.3 Outlet structures 54
3.3.1 Earth spillways 54
3.3.2 Design spillway capacity 55
3.3.3 Selecting spillway dimensions 57
3.3.4 Chute spillways 58
3.4 Pipelines through embankments 59
3.4.1 Trickle pipes 60
3.4.2 Drop inlet structures 61
3.4.3 Cut-off collars 62
3.5 Earth and water computations 63
3.5.1 Embankment material 63
3.5.2 Floor slope 64
3.5.3 Area beneath embankment 64
3.5.4 Excavated tanks 66
3.5.5 Water storage capacity computations 66
3.5.6 Storage excavation ratio 68
3.6 Estimate of costs 68
3.6.1 Economics 68
3.6.2 Dam quality pays over time 70
Section 4 Documentation 71
4.1 Collation of plans and specification 71
4.2 Collecting basic design data 72
4.2.1 Catchment map 72
4.2.2 Location (topographical) map 72
4.2.3 Profiles and cross-sections 73
4.2.4 Soils 73
4.3 Assembly of data 74
4.3.1 Analysis of data 74
4.3.2 Design 74

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 Contents • v

4.4 Construction documents and drawings 74

4.4.1 Specifications 75
4.4.2 Checklist 75
4.5 Final review and approval 75
4.5.1 Records 75
Section 5 Construction 77
5.1 Approval for dam building 77
5.1.1 Details that may need to be submitted 77
5.1.2 Referral dams 78
5.2 Selecting your dam builders 79
5.2.1 Selecting an engineer 79
5.2.2 Details that an engineer can provide 80
5.3 How to build a dam 81
5.4 Steps in constructing a dam 81
5.4.1 Setting out 81
5.4.2 Diversion of water 83
5.4.3 Clearing and grubbing 83
5.4.4 Stripping topsoil 83
5.4.5 Core trench 83
5.4.6 Borrow pit material 84
5.4.7 Selection and placing of material 84
5.4.8 Spillway and outlet structures 84
5.4.9 Batters and topsoil 84
5.5 Compaction 85
5.5.1 Compaction when constructing a dam 86
5.5.2 Recommendations for compaction 86
5.6 Soil moisture 87
5.6.1 Adjusting soil moisture 87
5.7 Allowance for settlement 88
5.8 Equipment 88
5.8.1 Rollers 90
5.8.2 Features of roller compaction 91
5.8.3 Other machinery 92
5.9 Installation of outlet pipe 92
5.9.1 Testing of the pipe 92
5.9.2 Foundations on rock 92
5.10 Checking for compliance with standards 93
5.10.1 Checking the contractor’s work 93
5.10.2 Inspection during construction 93
5.10.3 Good work takes time 93
5.10.4 Extra ‘bonuses’ 94
5.10.5 Changing your mind 95
5.10.6 Progress payments 95
5.11 Final inspection and measurements 95
Section 6 Maintenance 97
6.1 Safety surveillance 97
6.1.1 Equipment for inspection 98
6.1.2 Observations to be recorded 98

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 vi • Farm Dams

6.2 Inspection procedures 99

6.2.1 General techniques 100
6.2.2 Specific techniques 101
6.2.3 Evaluation of observations 102
6.2.4 Frequency of inspection 102
6.3 Causes of dam failures 103
6.3.1 Dispersive clays 104
6.3.2 Seepage and leakage 105
6.3.3 Cracking and movement cracks 105
6.3.4 Erosion 107
6.3.5 Deformation and movement 111
6.3.6 Defects in associated structures 113
6.3.7 Vegetation 115
6.3.8 Total catchment protection 115
6.3.9 Weed control 116
6.4 Dam leakage 116
6.4.1 Rebuilding 116
6.4.2 Use of soil additives 117
6.4.3 Artificial liners 117
6.4.4 Using a clay liner 118
Section 7 Water 121
7.1 Water quantity 121
7.2 Water quality 122
7.2.1 Domestic use 124
7.2.2 Stock water 125
7.2.3 Irrigation water 125
7.3 Water treatment for human consumption 125
7.4 Algae in farm water supplies 126
7.4.1 Problems 128
7.4.2 Identification 128
7.5 Salt in dam water 129
7.5.1 Minimising salinity in dams 129
7.5.2 Interception of sub-surface saline water 130
7.5.3 Maintenance 130
7.5.4 Downstream effects 130
7.5.5 Minimising the effects of evaporation 130
7.5.6 Stock and salty water 131
Section 8 Ecology 133
8.1 Wildlife and plants in dams 133
8.1.1 Water regime 134
8.1.2 Basic topography 134
8.1.3 Vegetation 134
8.1.4 Grazing by stock 135
8.1.5 Waterways and swamps 135
8.1.6 The role of hunting 136
8.1.7 Waterfowl management 136
8.2 Water plants in dams 138
8.2.1 Aquatic plants 138

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 Contents • vii

8.3 Using herbicides near water 141

8.3.1 Hazards 141
8.4 Vegetation on and around dams 142
8.5 Yabbies 142
8.5.1 Physical removal 142
8.5.2 Biological removal 143
8.5.3 Chemical control 143
Section 9 Commercial 145
9.1 Fish farming 146
9.1.1 Established freshwater species 146
9.1.2 Dam conditions that control productivity 147
9.1.3 Feeding 148
9.1.4 Aquatic vegetation 148
9.1.5 Fish loss by escaping 148
9.1.6 Muddy water 148
9.1.7 Algae 149
9.1.8 Other species 149
9.2 Yabby farming 149
9.2.1 Types of dams for yabby production 150
9.2.2 Water properties 150
9.2.3 Stocking and feeding 151
9.2.4 Harvesting 151
9.3 Native fauna and total ecosystem management 151
9.3.1 Native fauna 151
9.3.2 Managing predation by native fauna 152
9.4 Licensing process 153
9.4.1 Licences required 153
9.4.2 Other steps that may need to be taken 154
Section 10 Legal 155
10.1 Legal and policy aspects in Australia 155
10.2 Liability 158
10.3 Responsibility of dam owners 159
10.3.1 Liabilities of dam owners 160
10.3.2 Permission to build a dam 160
10.4 Dam failure 161
10.4.1 Potential hazard and risk of farm dams 162
10.4.2 Minimising the risk of dam failure 162
10.4.3 In case of dam failure 162
10.4.4 Abandonment of farm dams 164
10.5 Designer and earthmoving contractor(s) 164
10.6 Property insurance 165

Appendix 1 A glossary of terminology 167

Appendix 2 Engineering specification for an earth-fill farm dam 172
Appendix 3 Metric and imperial conversion tables 178
References and suggested further reading 181

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 Preface
Farmers are well aware of the need to boost productivity. In the face of greater
competition for domestic and overseas markets, the farmer who wants to succeed
has to take a business person’s approach to increasing efficiency, reducing costs
and improving output. In this environment, water becomes an economic factor
and its provision a matter for careful deliberation.
This book is designed as a guide for dam owners, engineering students,
Government agencies, developers, and earthmoving contractors who are
responsible for designing, building and using the majority of farm water storages
constructed in Australia. It is also designed for engineers who have not specialised
in small earth dam design for agricultural hydrology, but who may be called upon,
from time to time, to design small water storage schemes. It is not intended to
replace standard procedures currently used by those specialised engineers who are
engaged in farm water design, although many of the design methods described
herein are based on their procedures. It does, however, attempt to provide such
engineers with a comprehensive array of design data and a concise reference to
basic design techniques that are not otherwise readily available.
To cover all aspects of water conservation and use in detail is not possible in a
book of this size. However, it will give the landowner an insight to those aspects
of planning which must precede the establishment of a feasible and economic
water supply project.
The information for this book has come from a number of sources. One is a
series of farm dam pamphlets (which I wrote for the Rural Water Authorities in
Victoria over a number of years), another is data from papers I have presented at
conferences.

Barry Lewis
February, 2002

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 Acknowledgements
There are many people who have supported and provided information, ideas and
procedures in the collation of this book on farm dams. Perhaps a major dilemma
for me is the incorporation of sources of information, which are better explained
in the anonymous quote:

How about the many ideas and procedures that one picks up from
discussion with colleagues? After the passage of time, one can no longer
remember who originated what idea. After the passage of even more time,
it seems to me that all of the really good ideas originated with me, a
proposition that I know is indefensible.

It becomes clearer with time that nothing is new, but people forget the original
sources and whether those sources were really original or were the result of slight
modifications of other people’s ideas. It is like the wheel. Nobody knows who
conceived the idea, yet it is used universally. So, credit must be given to those who
have gone before me in this area.
My family has been supportive in assisting in many ways. My wife, Marion,
has typed and corrected a number of drafts. Richard, my son, has shown me some
of the intricacies of the computer, whilst Janette, Helen and Kenneth have done
their bit in other ways.
In the practical areas of planning, design, construction, maintenance and legal
issues of farm dams and other related areas I would like to thank all of those who
have been instrumental in the completion of this book. Some of these people (in
no particular order) are: K.D. Nelson, Dr M. Papworth, Dr S. Beavis, K. Brown,
J. McMullan, D. Plant, S. Perera, Dr I. Rutherfurd, T. Reid, K. Murley,
L. Churchward, T. Hamilton, Dr M. Hunter, N. Hunter, A. Loguisto and
A. Tseberg.

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 Introduction
People have always gathered water during wet seasons so as to have enough for
themselves, their animals and their crops in dry spells. The earliest known dams
were in China in the sixth century BC. The ruins of ancient dams also exist in the
Tigris and the Nile River Valleys. Some Roman dams built in Italy, Spain and
North Africa are still being used today.
Today, dams are built to allow storage of water to give a controlled supply for
domestic or industrial consumption, for irrigation, to generate hydro-electric
power, or to prevent flooding. Large dams are built of earth, rock, concrete or a
combination of these materials (for example, earth and rock fill). They are built
as: gravity dams, where the stability is due entirely to the great weight of material;
arch dams, where abutments at either side support the structure; or, arch gravity
dams, which are a combination of the two.
Those who plan, design, construct, maintain, use and administer significant
infrastructure developments which have a real potential to harm people and
property if something goes wrong, are potentially subject to significant legal
liabilities. These liabilities need to be taken into account when decisions are made
in relation to such developments.
It is well known that Australia is a dry continent characterised by variable
rainfall. It is less well known that, in response to widespread harvesting of water
on the small and large scale, Australia has the highest water storage per capita in
the world (Lewis and Perera, 1997; Lewis, 2001b). Farm dam development has
occurred in response to agricultural expansion, and to the need for a reliable
source of water for stock, domestic and irrigation use, particularly during periods
of drought. However, there is a growing belief in the community that small farm
dams are impacting on water resources in many major catchments by reducing
stream flow and flow duration. Potential and actual impacts of such reductions on
the conflicting needs of the environment, agriculture and industry are cause for
concern. These concerns are set against a background of changes including
increased areas of intensive land uses such as viticulture and horticulture, and the
development of farmland into rural residential subdivisions in commuter belts
surrounding major cities and rural centres. These changes are associated with
increased farm dam development. In a significant proportion of cases, particularly
where intensive land use changes have occurred, farm dams are constructed that
exceed the available water resource. Where this occurs, downstream impacts on
stream flow will be the cause of conflict between users (including the environment).
In addition, landowners may experience financial losses by constructing dams of a
size inappropriate for the catchment.
In most Australian States, provisions of the relevant Water Act(s) require that
environmental consideration be taken into account in relation to new and renewed
licences. These requirements apply to both regulated and unregulated waterway
systems. Since unregulated waterways may not have large storage capacities from
which environmental releases can be made, flow needs to be specified as a
proportion of daily stream flow, or as a minimum daily flow. Many factors such
as in-stream habitat requirements need to be considered when allocating an
entitlement to environmental flows. In the case of regulated waterways,
environmental flows can be released as part of the normal operating procedure.

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 Section 1
Planning
1.1 Assessing water needs
Water is the most plentiful and vital liquid on earth. No life, as we know it, could
exist without water as it provides the essential medium in which all things grow
and multiply.
In recent years the steady growth in the numbers of agricultural businesses,
including viticulture, aquaculture, irrigating crops and grazing animals, has
increased the demand for water supplies. No longer can the small, muddy farm
dam or the small bore with windmill provide adequate supplies of clean water for
these uses.
Landholders are lucky if their property can be serviced from a local water trust
or authority. In most cases, the only option is to utilise on-farm water resources
and this requires careful planning prior to costly construction. In a majority of
cases farmers use roofs of buildings to collect rainwater for domestic and farm
purposes. Rainwater is the main source of high quality water for human
consumption, providing that guttering and tanks are kept in good order. In some
cases bore water is used, but it may require treatment to remove or neutralise the
minerals it contains. Where the terrain permits, rainfall and run-off gravitating to
various categories of dams (for example, off-stream and on-stream catchment
dams) provide the farm water supplies.

1.1.1 Planning water supplies

When deciding the most appropriate source of water for a farm, the following
steps should be taken:
• determine the purpose for which the water is to be used (for example,
household, garden, irrigation, stock, aquaculture, viticulture or a combination
of all of the above);
• determine the quantity of water required for the different purposes, time of
year and seasonal use and conditions (for example, summer, winter, flood or
drought);
• ascertain whether the source of water supply is regulated (controlled from a
reservoir that releases flows subject to availability) or unregulated (for
example, bore water, rainfall or combination of both) and can be maintained
when supplies are needed.
These factors are a guide to planning and they differ with almost every
property. The most likely base can be established by calculating the average daily

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 2 • Farm Dams

or average annual requirements for each proposed use of the dam. It is only after
these facts are established that harvesting, storing and distributing the water
resource can be planned (Lewis, 1993).
1.1.2 Water quality
In project planning the quality of water can determine success or failure of a farm
dam for several reasons. First, the quality of water available from a farm water
system will determine the use to which that water can be put, and hence govern
the overall feasibility of the system. Many natural waters have impurities that can
make them directly harmful to crops. Therefore, knowledge of the quality of the
water supply is essential.
Second, water quality is a factor in determining storage capacity. For example, if
saline water is intended for irrigation use, additional quantities of non-saline water
must be applied from time to time to avoid damage to the irrigated crops or soils.
Third, the presence of certain mineral constituents (such as calcium, potassium,
magnesium and/or sodium cations) in stored water may cause tunnel erosion in
some soil types. Therefore, the quality of water to be stored in a farm dam must
be considered in the design and construction of the impounding embankment.
Finally, overland flows mobilise and transport nutrients, fertilisers and silt
which can contaminate the dam water supply. Precautions need to be taken to
minimise the risk of this occurring, as algal blooms and weed infestation could
result (see Section 7.4.1).
When planning for farm water supply, the presence of minerals, sediments,
nutrients, agricultural chemicals, biota or bacteriological contamination needs to
be considered, according to whether the water is to be used for:

i Water for consumption for humans and animals

• Micro-organisms and other harmful organisms which may produce illness or
disease.
• Soluble minerals and other substances which give the water an unpleasant taste
or make a person unwell.
• Silts, suspended impurities or algae which may cause discolouration, odour or
taste problems and in some cases are toxic to both humans and animals.

ii Water for household, dairy or general farm use

• Suspended minerals and compounds which cause problems with clothes
washing and the build-up of scale in equipment such as pipes, hot water
systems and cooking utensils.
• Impurities which can cause water discolouration and associated staining and
odour problems.
• Acids, dissolved minerals and gases which corrode pipes and equipment.

iii Water for irrigation

• Dissolved minerals which can alter water and soil chemical and physical
properties. For example, an altered pH may inhibit plant growth.

iv Water to be stored behind farm dams

• Dissolved and suspended particles can affect the soil structure of a dam, which
can lead to tunnel erosion.

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 Planning • 3

The testing of water samples to determine the presence and likely effect of these
impurities requires laboratory facilities (see Section 7.2).

1.2 Assessment of catchment yield

The initial question to be asked when considering the development of any storage
dam on a property is how much water can be harvested in a catchment from
overland run-off.
1.2.1 Factors controlling catchment yield
Estimating catchment yield by the methods described needs to be done within the
context of a full understanding of the run-off process, so that the limitations of the
method can be appreciated. Catchment yield is the volume of water that flows
from a catchment past a given point (such as a stream gauging station) and is
generally calculated on an annual basis (Beavis and Lewis, 1999). It comprises
surface run-off and base flow (discharge from shallow and deep groundwater).
Catchment yield will vary according to a number of hydrological and physical
factors, which control how much water is delivered to, retained by and
transported from a catchment.

i Hydrological factors
Run-off may be regarded as the residue of rainfall after losses due to interception
by vegetation, surface storage, infiltration, surface detention and waterway
detention. There is always a time lag between the beginning of rainfall and the
generation of run-off. During this time lag, rainfall is intercepted by vegetation,
infiltrates the soil, and surface depressions start to be filled. Run-off occurs when
the infiltration capacity is exceeded, or the precipitation rate exceeds the rate of
infiltration. Thus the depth of water builds up on the surface until the head is
sufficient to result in run-off. As the flow moves into defined waterways, there is
a similar build-up in the head with volume of water, and this is termed waterway
detention. The water in surface storage is eventually diverted into infiltration or
evaporation pathways (Beavis and Howden, 1996; Beavis and Lewis, 1999).
Rainfall duration, intensity, and distribution influence both the rate and
volume of run-off. Infiltration capacity normally decreases with time, so that a
short storm may not produce run-off, in contrast to a storm of the same intensity
but of long duration. However, an intense storm exceeds the infiltration capacity
by a greater margin than a gentle rain. Therefore, the total volume of run-off is
greater for an intense storm even though total precipitation is the same for the two
events. In addition, an intense storm may actually decrease the infiltration rate by
its destructive action on the structure of the soil surface. Generally, the maximum
rate and volume of run-off occur when the entire catchment contributes. However,
an intense storm on one portion of the catchment may result in a greater run-off
than a moderate storm over the entire catchment.

ii Physical factors
Physical factors affecting run-off include topography, soil type and antecedent soil
moisture conditions, catchment size, shape and orientation, and management
practice. Topographic features such as slope and the extent and number of
depressed areas influence the volume and rate of run-off. Catchments having

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 4 • Farm Dams

extensive flat or depressed areas without surface outlets have lower run-off than
areas with steep, well-defined drainage patterns. A catchment with a northerly
aspect will dry out faster than one with a southerly aspect. Soil texture, fabric, clay
mineralogy and antecedent moisture conditions have major impacts on the
infiltration rate and capacity, and thus influence run-off. Sandy soils with an open
fabric have high infiltration rates and generate less run-off than clayey soils with
a closed fabric (where intergranular spaces are smaller and there is less
connectivity between pores). Both run-off volumes and rates increase with
catchment area. However, run-off rate and volume per unit area decrease as the
area increases. Vegetation cover and management practices influence infiltration
rate by intercepting precipitation and modifying soil structure respectively.
Vegetation also retards overland flow and increases surface detention to reduce
peak run-off rates. Structural works such as dams, weirs, pipe culverts and levees
all influence run-off rates by either directing surface water into preferential flow
lines or storing water.
1.2.2 Methods of estimating catchment yield
The most readily available source of water is the surface water in rivers and lakes.
This water is usually stored in dams. In certain parts of Australia, fortunate
farmers have ‘run of the river’ schemes, that is, they do not need storages because
the flows in the rivers are so reliable that they can meet all requirements. This is
the situation in areas of consistently high rainfall, an uncommon circumstance in
Australia. It also applies to farms that draw their water from a river downstream
from a large public storage.
In the absence of a reliable flow in the river, a storage must be constructed to
meet all the water requirements for the storage period as well as evaporation and
seepage losses. The storage period is that interval during which there is no run-off
into the dam. Table 1.1 gives a ‘rule-of-thumb’ estimate of the storage period
required in areas with different average rainfall.

Table 1.1 Storage period

Average annual rainfall (mm) Storage period (months)

>650 12
450–650 18
250–449 24
<250 30–36
Source: Modified from Soil Conservation Authority, 1983.

Run-off from a catchment must be adequate to meet requirements. In practice,

the catchment should be neither too small nor too large. If it is too small, there are
two dangers. First, it will not meet the requirements, and second, the lack of silt,
which all streams carry, will result in excessive seepage from the dam. All earth
dams seep to a greater or lesser degree. However, after a couple of years, the silt
from the stream impregnates the floor and this forms a relatively watertight seal.
If the catchment is too large, the dam will be subject to very heavy floods that,
in turn, necessitate a concrete spillway. This is an expensive item and frequently
makes a project uneconomical.

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 Planning • 5

Two methods are currently used in Australia for estimating the yield of a
catchment. One is the United States Department of Agriculture (USDA, 1969)
method of estimating daily run-off. This is based on daily rainfall records for the
district. The longer the period of record, the better the results. The other method
is based on the assumption that catchment yield is a percentage of the average
annual rainfall. Variability of rainfall in Australia limits the accuracy of
forecasting, and hence, the reliability of this method. It therefore follows that,
despite the most careful calculations, it is difficult to guarantee that a dam will
always meet requirements. However, a method of estimating the potential
catchment yield must be adopted so that a farm water supply scheme can be
planned on a reasonably sound basis. The run-off yield method was first published
by the Water Research Foundation of Australia (Burton, 1965) and has proved to
be of continued practical value in water resource management in most States.
Table 1.2 provides an estimate of yields from small natural catchments. The
reliability column relates to the number of years in a ten-year period in which the
given percentage yield will be equalled or exceeded. For irrigation and stock, a
reliability of eight years out of ten is acceptable, and for domestic schemes the aim
is nine years out of ten.
The selection of percentage yield within the given range depends on local
experience. For example, the lower limits in the yield column usually include
forests, areas of cultivation and improved pastures.

Table 1.2 Yield from natural catchments

Yield as percentage of average annual rainfall (Y)
Average Total Reliability Shallow Sandy Elastic Clay pans
Annual Annual (years out sand or clay clay or inelastic
Rainfall Evaporation of ten) loam soils (%) (%) clays
(R) (mm) (mm) (%) (%)

>1100 <1000 8 10–15 12–20 15–25 15–30

900–1100 <1000 8 10–15 10–15 12.5–20 15–25
500–899 1000–1300 8 5–8 7.5–12 7.5–15 10–15
500–899 1300–1800 8 5–8 5–12 5–12 10–15
400–499 1300–1800 8 3–4 3–7.5 4–6 7.5–12.5
250–399 <1800 8 1.5–3 1.5–5 1–3 2–5
<250 >1800 8 1–2 1.5–3 1–2 2–5
Source: Modified from Burton, 1965 and Nelson, 1983.
Notes: i For 9 out of 10 years reliability multiply percentages by 2/3.
ii Numbers should be halved if catchment sown to improved perennial pastures.

The run-off, in megalitres, from the catchment is calculated from Table 1.2 and
according to the following formula:
V=K×A×R×Y
where V = run-off yield volume (ML)
K = conversion to megalitres = 0.01
A = catchment area (hectares),
R = average annual rainfall (millimetres), and
Y = yield as percentage (using Table 1.2, and expressed as a decimal,
for example, 12.5% = 0.125; 7.5% = 0.075).

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 6 • Farm Dams

Example:
A small catchment of 50 hectares is forested and the soil is a sandy clay. It receives
an average annual rainfall of 1000 mm and has an evaporation of 1000 mm. What
would the estimated run-off yield be for a domestic and stock scheme?

K = 0.01
A = 50 ha
R = 1000 mm
Y = 10%, or 0.10 (reliability 8)
V = run-off yield volume (ML)
Run-off yield V = 0.01 × 50 × 1000 × 0.10
= 50 ML

For normal stock dams, and for the irrigation of annual pasture, an 80 per cent
reliability is generally used. An 80 per cent reliability yield means that the
catchment yield or run-off will be exceeded 80 per cent of the time, for example,
in four out of five years. If the water is for domestic use, or is to be an adequate
drought storage, a 90 per cent reliable yield should be used. Alternatively,
additional water should be stored to provide water during the critical storage
period, that is, the period of no run-off. However, in the absence of reliable data,
the ‘rule-of-thumb’ method can be applied to estimate the reliable yield, which is
defined as the percentage of the total volume of water that falls on the catchment
during the year. While it is acknowledged that this method is basic and arbitrary,
it is also apparent that it will continue to be used in small rural catchments. This
estimate is of critical importance when landholders request a storage of optimum
capacity. In this method, the percentage yield is calculated using the factors of
average rainfall and soil type, derived from Table 1.2. In those parts of southern
Victoria which are subject to prevailing westerly influences in the winter months,

Figure 1.1 Water yield

Yield % Rainfall
8% 7% 6% 5% 4% 3% 2% 1400 mm
1200 mm
9%

10% 1000 mm

12%
800 mm

15%
600 mm
20%
25% 400 mm
30%
35%
200 mm

100 80 60 40 20 0 20 40 60 80 100
Yield (ML) Catchment area (ha)
Source: Beavis and Lewis, 1999.

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 Planning • 7

the annual pattern of run-off, while not so sharply defined as in the north, involves
a winter season. This is when replenishment of farm storages can normally be
expected, followed by a comparatively dry summer and autumn.
It should be noted that estimations of yield from these tables are largely
tentative. They may be over-conservative in some districts and may be modified if
good evidence of their unreliability exists. Any increase in percentage run-off
should, however, be accepted with caution, bearing in mind that the values in
Table 1.2 are for 80 per cent reliability conditions and are not mean values. Their
usefulness is that they represent a ‘first-cut’ methodology in assessing water
resources in the field.
The estimation of yield volume (V) megalitres in the example can also be shown
by using Figure 1.1 once the catchment area (A) hectares, annual rainfall (R) mm and
yield as a percentage (%) in Table 1.2 has been selected. Simply place a mark on the
catchment area (ha) axis, trace up to rainfall (R), then move across to yield % that
has been obtained from Figure 1.1. Directly below is the storage volume or yield (V).
The run-off yield method can be used to take into account existing and future
land use even if the land will be periodically rotated after cropping. The end
results are not affected, particularly when irrigation areas are taken out and then
used for stock and domestic purposes. The basic criteria used are fixed and based
on catchment area (hectares), application rate (megalitres per hectare) and annual
rainfall (millimetres). The only variable is the yield (percentage). This will vary
only if the land has been subdivided for closer settlement and more intensive
farming practices. Figure 1.1 can be used to determine the run-off yield for small
catchments, given the storage capacity of an existing dam when it is full without
overtopping.
The storage design procedure should be based on the provisions of sufficient
storage capacity to meet water demands over a selected ‘critical storage period’.
Usually, in both case study areas (northern and southern Victoria) it is the period
from October to March, inclusive. This is the period without appreciable inflow
when the storage must supply all water demands and losses without
replenishment. For design of minor storages, ‘rule-of-thumb’ values of critical
storage period are useful. For the design of larger storages, the critical storage
period must be determined from an analysis of past run-off estimates. In both
cases it is necessary to calculate a quantity, and time pattern, of water use for the
critical period in order to determine the storage volume needed to meet all
demands and losses over this period.
1.2.3 How trees affect yield
The catchment for every farm dam is unique. In a small catchment, or where
rainfall is low, widespread tree cover will reduce yield. In a large catchment, or
where rainfall is high, water yield is usually high and the amount of tree cover is
less relevant. In most cases, a tree cover of approximately 5–10 per cent
comprising shelterbelts, small woodlots or scattered trees is unlikely to have any
noticeable effect on water yield. If water yield from a farm dam catchment covered
in good pasture were compared with yield from the same catchment with 5–10 per
cent of mature tree cover added, the difference in water yield would be hard to
measure. At the same time, the trees provide many other benefits to water quality,
pasture and crop production (Greening Australia, 1990).

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 8 • Farm Dams

1.2.4 Artificial catchments

Relatively low percentage yields have made engineers attempt to produce larger
yields by treating catchments. In the United States these techniques are called
‘water harvesting’, in Australia, the preferred term is ‘artificial catchments’.
There are past records of humans modifying the landscape to augment surface
run-off for agricultural purposes. Ancient cultures in the Middle East and Europe
had an extensive system of channels and aqueducts to collect run-off and transport
the water to irrigation areas.
A practical technique of water harvesting has been developed in Western
Australia. The catchment area is graded, compacted into a series of parallel roads,
and then drained into a large channel leading to the storage. However, there are
obvious issues of soil erosion using this technique. Technical analysis of
dimensions and slopes is given in Hollick (1975).
In some regions, traditional construction materials such as sheet metal and
concrete have been used to make artificial catchments. Gibraltar, for example, is
totally independent of outside drinking water supplies and has 40 hectares of
metal roofing, concrete paving and treated rock surface on the eastern slopes of
the Peninsula. The run-off is collected by gutters and passed to dams.
Other materials include plastic and metal films that are either anchored or
bonded to the soil, or cheap bitumen paving. Unfortunately, oxidation of the
bitumen frequently means that the run-off is discoloured, but work on this
material still continues. Chemical treatments to reduce the permeability of soil in
a catchment are another means of increasing yield. However, they do not last for
long and usually increase the erodability of the soil.
Construction costs vary from site to site for different treatments:
• a roaded catchment could cost as little as $500/hectare where there is clay close
to the surface, or as much as $3000/hectare if there is a lot of sand and gravel;
• a bitumen treatment could range from $500/hectare to $25 000/hectare and
last for five years to ten years respectively.
In common with other construction works, the unit cost will decrease as the
area of treated surface increases.

1.3 Dam site selection

Surveys have indicated that a large proportion of farm dams fail because of poor
planning, unsatisfactory siting, faulty construction or lack of maintenance. Such
costly failures can usually be avoided. The choice of a suitable dam site should
begin with preliminary studies of possible sites. Where more than one site is
available, each should be studied separately with a view of selecting the one that
proves most practical and economical (DoA Vic, 1978; SRW, 1995).
From an economic viewpoint, a dam should be located where the largest
storage volume can be obtained with the least amount of earthworks. This
condition will occur, generally, at a site where the valley is narrow, side slopes are
relatively steep, and the slope of the valley floor will permit a large deep basin.
Such sites tend to minimise the area of shallow water. Except where the dam is to
be used for wildlife, large areas of shallow water should be avoided due to the
potential for excessive evaporation losses. Value of the land flooded by the storage
should also be considered.

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 Planning • 9

Dams to be used for watering livestock should be spaced so that livestock will
not have to travel more than 200 m to reach them in rough, broken country, nor
more than 1000 m in smooth, relatively level areas; that is, the maximum spacing
between dams is 1 km. Forcing livestock to travel long distances for water is
detrimental to both the livestock and the grazing area. Overgrazing near water,
and unused feed far from water, are characteristics of inadequate water
distribution.
Where water must be conveyed for use elsewhere, such as for irrigation, fire
protection or stock and domestic use, dams should be located as close to the point
of use as practical. The economics of gravity flow compared with pumping must
also be considered.
Pollution of farm dam water should be avoided by selecting a site where
drainage from houses, piggeries, dairies, sewerage lines and similar areas will not
reach the dam. Where this cannot be done practically, the drainage from such
areas should be diverted from the dam.
The dam should not be located where sudden release of water due to failure of
the dam would result in loss of life, injury to persons or livestock, damage to
residences, industrial buildings, railroads or highways, or interruption of the use
or service of public utilities. Where the only suitable site presents one or more of
these hazards, a more detailed investigation should be made.
Powerlines present a hazard to people constructing, using or desilting farm
dams. Sites under such lines should be avoided. Permission from the electric
supply company is recommended before construction is commenced beneath
powerlines.
1.3.1 Choosing a dam site
When choosing a dam site (Lewis, 1995b), the following points need to be
considered:
i Storage yield from the catchment
Yield is the volume of water harvested from the dam catchment area. It depends
on rainfall, plant cover, slope, soil type, area and other factors (see Section 1.2.1).
Three questions need to be asked when selecting a dam site:
1 what is the catchment area above the dam?
2 how much water will the catchment yield?
3 would the catchment yield be substantially reduced if another dam were to be
built in the same catchment?
If most of the catchment is outside the property and is eroded, a silt trap may
have to be built. This is usually a small dam above the main storage (Figure 1.6).
Its function is to slow down the run-off so that silt is deposited before it is carried
into the main dam. Silt traps have to be cleaned out periodically.
ii Increased catchment harvest
Often dams cannot fill because the catchment area produces insufficient run-off.
The catchment, or source of water of a dam, should generate enough water each
year to fill it.
If the catchment is large enough, graded drains can be constructed to divert
run-off from adjacent areas. Hard surface areas such as roads and roofs can also
be used to increase the yield (see Section 1.2.4).

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 10 • Farm Dams

iii Choosing dam type for site

The larger the quantity of water stored for each cubic metre of soil to be moved,
the larger is the cost-saving on construction. Wherever possible, a site with a good
storage excavation ratio (S:E) should be selected. The average stock dam site
would have a storage to excavation ratio of 2 or 3 to 1, that is, 1 m3 of soil moved
for each 2 to 3 m3 of water stored (see Section 1.4).

iv Soil assessment and testing

Although clay soils are imperative to prevent leaking dams, it should be noted that
all clays do not hold water. Physical and chemical properties can make some clay
soils prone to seepage or tunnelling, both of which result in bank failure.
To test the suitability of the soil, samples should be taken from the borrow pit
(see Section 2.1.2) and along the bank centre-line using a soil auger, which
penetrates at least just below the proposed excavation depth. These tests show
whether or not the soil contains clay. Further tests are needed to find if this clay
is of a water-holding type.
If testing indicates that the soil is suitable in terms of clay content and type, the
following points should be checked:
• are there any sand or gravel layers or seams? A dam should not be sited where
there are seams, fissured rock or soaks. A stream or soak is an indication that
the water-holding layer has not been reached. Where this occurs, it will be
necessary to dig down to the impermeable layer and repack with a suitable clay
to stop water leaking out along the original soak line.
• will the clay content of different soil samples be sufficient to seal the bank
against leaks? Soils from different locations should not be mixed to form a
composite. Test results from such a sample can be meaningless because even
though the mixture may hold water, one of the soils in the mixture may leak.
The dam will then leak where this soil is exposed in the excavation.
• is there sufficient material with correct clay content readily available within the
excavation area? Unless extra care and time are spent, there will almost
certainly be layers of unsuitable material in a borrow pit which will leak when
placed in an embankment. It is better to keep the soils separately zoned, as
explained in Section 3.1.1.
• is the layer of permeable topsoil overlying the clay soil too deep? If there is
more than 1 m of topsoil to be removed, costs should be estimated carefully.
Removal of topsoil may become a large proportion of the total costs.
• if the dam is to be filled with groundwater, special care must be taken in testing
soils. Some groundwater contains chemicals which react with clays and cause
leakage even though the soil is watertight to rainwater.
If you have any doubts, seek advice from an experienced soil/farm dams engineer.

v Planning outlet structures

Many dams fail because of an inadequate or incorrectly located spillway or
insufficient freeboard. If a spillway is too small to cope with storm flood-flows,
water will flow over the top of the bank which may then breach. A badly designed
or constructed spillway can cause erosion of the spillway and lead to complete
failure of the dam. Therefore, the spillway should be large enough to handle and
dispose of flood-waters safely, without damaging the dam bank or causing erosion
of the spillway.

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 Planning • 11

When planning to divert water a landholder should make sure it does not leave
the property in a different drainage line from its natural alignment. If it is
advantageous to do this then it would be wise to obtain permission from down-
slope neighbours and, if road works will be affected, from the municipality. The
permission for this diversion should be in writing from all parties concerned to
prevent future arguments.
In some cases, flows from the catchment are so great that there is no
economical way of constructing an in-stream dam with an adequate spillway. In
these cases consideration should be given to an off-stream storage. Where it is
anticipated that water may flow over the spillway for a prolonged period, for
example one week, problems may occur due to deterioration of the vegetation
cover on the spillway. This problem can sometimes be overcome with the
installation of a trickle pipe to carry the prolonged flow.

vi Legal considerations
Before constructing any dam across a gully or depression, it should be checked
first that it is not a legally recognised waterway, and that it does not have an
existing easement. This may be found on the land title or in the records of the local
municipality. If the gully or depression does have either or both of these
conditions, then permission must be obtained from each responsible authority,
agency, instrumentality or department. Permission should also be obtained from
any neighbours who may be affected.
In some States of Australia, the construction of a dam on a river, stream,
watercourse or waterway, is governed by common law and/or legislation. In
addition, a separate licence may be required to take and use the water that is to
be stored in the dam.
For details on owning a dam as an asset or a liability see Section 10.

vii Environmental issues

It is preferable to design and build in-stream dams in a manner that inherently
provides for fish passage. However, where existing dams are to be modified, or
where it is not practical to modify the design of new structures to provide for fish
movement, it will be more appropriate to install some type of dedicated fish way
(Lewis and O’Brien, 2001). This decision will be based on a number of site-specific
factors such as the fish species present, site topography, flow characteristics and
the potentially increased costs. Over the years, a large number of in-stream
structures have been built without any provision for fish movement. Some of these
are large dams and weirs that require a fully engineered device such as a fish lift
or a vertical-slot fish way to facilitate fish movement. However, the vast majority
of barriers are smaller structures for which other options, such as rock fish ways,
are considered more appropriate. There is also a need for those planning to install
an in-stream structure to conform with the current legislative requirements and
practices (Lewis et al., 1999; O’Brien et al., 1999).
Conditions regarding installations of in-stream structures have been
formulated over many years in Victoria to minimise the impact on aquatic biota
and maintain their natural environment. This Section does not discuss the
legislative responsibilities associated with providing fish passage, but does
demonstrate graphically an in-stream structure that assists fish passage (see
photograph on following page).

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 12 • Farm Dams

Typical in-stream barrier with rock-ramp fish way, Merri River, Victoria

Source: T. O’Brien.

viii Improving biodiversity and aesthetic values

Finishing touches to a dam include:
• fencing the entire pond area to keep livestock out, and installing drinking
troughs away from the dam. This minimises pollution entering the storage.
• planting trees and shrubs to provide windbreaks which prevent wave action
and therefore soil erosion, while also providing shelter for wildlife (do not
plant trees on the banks as it will create seepage through the banks).
• stocking the dam with suitable fish which can provide food and recreation.

1.4 Types of farm storages

Farm dams are small earth structures designed to impound water for stock
watering, domestic supply, aquaculture, irrigation or as a component of an
integrated erosion control network on agricultural or pastoral land.
Dams can be divided into two main types:
1 drainage line or unregulated waterway dams include those dams located in
drainage lines or waterways which are filled by run-off from the catchment
upstream;
2 off-waterway dams include structures which are filled by a diversion channel
or by pumping from a waterway or groundwater.
A stock, domestic or irrigation dam can be of either type. The purpose of the
dam influences the shape that is required and this in turn influences the kind of
site that is best. For instance, a stock dam must be deep enough to allow for
evaporation and still have enough water left over for the stock. Evaporation losses
from a shallow storage are important and can easily amount to 30 per cent over
a hot summer (see Section 1.5.1). In a deep storage these losses may be only 15

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 Planning • 13

per cent. Therefore, the ideal site for a stock dam is where there is adequate depth
of a suitable soil. The same applies for fire fighting and domestic dams.
Where security of supply is important, two years supply of water should be
stored to guard against a drought year. In most parts of Australia loss to
evaporation over the two years can be minimised by a greater depth of storage.
The ideal site for this kind of dam is one with a good depth of suitable soil as close
as possible to where the water is to be used.
The ratio of the volume of water which can be held to the volume of the earth
excavated (m3) to form a farm dam, is one means used to compare different
storages and storage sites. This ratio is called the storage : excavation ratio (S:E).
High ratio values imply a low capital cost of storage. Many landowners seem to
have the wrong idea when looking for large storages. A common belief is that a
short, high embankment across a deep valley will store many megalitres. This may
be true, but a high bank is costly to build. Usually a long bank, which is low in
comparison to its length, will store more water for less capital outlay. The reason
for this is that when the height of bank is doubled, the cost of the dam increases
four times. However, when the length is doubled, the cost of the dam is doubled.
This last point may be disputed since evaporation losses will be very high with this
type of storage. The objection is real, but the fact is that deep, narrow valleys with
a flat floor are unusual. Furthermore, a narrow valley will not hold as much water
as can be stored over a wider area. Obviously, a one metre depth of water over a
one hectare surface area, in a narrow valley cannot possibly be equal to a one
metre depth over five hectares on a flat site.
There are many types of farm storages constructed, but the terminology in
describing them is not standardised. A selection with associated design and
construction details are listed in the following sections.
1.4.1 Gully dams
A gully dam consists of an earth embankment built across a waterway, valley,
depression or drainage line (Figure 1.2). The embankment generally incorporates
an earth spillway at one or both ends to pass surplus water. The earth spillway is
sometimes supplemented by a pipe spillway (trickle pipe), which helps protect the
earth spillway from the effects of long-term flows.
Outlet pipes (also called compensation or low level pipes) are commonly
provided through the embankment to service stock, domestic irrigation or
environmental flows.
Conditions when most used
These dams are normally built from material located in the storage area upstream
of the dam site. If possible, excavation should be above the level of the outlet from
the reservoir to maximise yield. The ideal site for a gully dam is where the sides of
a valley are close together and then widen out above the point where there is flat
ground.
If the slope of the valley floor is sufficiently flat a relatively low dam will
impound water over the natural surface, for a considerable distance upstream. An
S:E ratio of up to 10:1 can be achieved at a favourable site.
Gully storages must incorporate some means of passing flood flows. They do,
however, provide the most economical form of storage and are particularly
suitable for irrigation development as well as for stock and domestic purposes.

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 14 • Farm Dams

Figure 1.2 Gully dam

Freeboard
1 m minimum
Full supply level

5 m minimum
SECTION 'AA' Core trench

Full supply level

Borrow
A pit
Bank A

Contour lines
Spillway

PLAN
Source: SR&WSC, 1970.

1.4.2 Hillside dams

An earth dam located on a hillside or slope, and not in a defined depression or
waterway, is called a hillside dam and is usually three-sided or curved (Figure 1.3).
Conditions when most used
To achieve the maximum S:E ratio the embankment should cross the natural
contours at right angles. On flatter terrain the hillside storage can have an S:E
ratio of up to 5:1 and is suitable for irrigation use in areas of low evaporation. On
steeper slopes the S:E ratio can become less than 1:1 and long downhill batters are
necessary to ensure embankment stability. Material for the bank is usually taken
from within the storage area. As with gully storages, provision of a spillway clear
of the actual embankment is needed to pass surplus flows from the natural
catchment.
The inflows to hillside storages can often be increased by the construction of
catch drains, or under favourable circumstances, by diversion of surplus stream
flows in addition to natural run-off.

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 Planning • 15

Figure 1.3 Hillside dam

Full supply level
Freeboard
1 m minimum

SECTION 'AA' Core trench

Contour Bank
lines

Full supply level

Borrow pit
A A

Spillway

PLAN
Source: SR&WSC, 1970.

1.4.3 Ring tanks

A ring tank consists of a storage confined entirely within a continuous
embankment which is built from material obtained within the storage basin
(Figure 1.4). Consequently, water is held partly above and partly below the
natural surface. This is achieved by radial bulldozing or a scraper working in
annular ring borrow pits. Compaction equipment is also necessary. Ring tanks are
filled by pumping from groundwater or streams during periods of surplus flow.
The size is therefore determined by the pumping plant available and the periods
of surplus flow.
Conditions when most used
Generally the surface area is large and, in relation to depth and high evaporation
rates, losses can be a problem if surplus stream flows are infrequent or irregular.

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 16 • Farm Dams

The S:E ratio increases with diameter, ranging in practice from approximately
1.5:1 for a small tank to 4.5:1 for a large tank.
Ring tanks are used primarily for irrigation in flat terrain. They have the
advantage that inflow and outflow can be controlled without expensive spillway
provision. In pumping from streams, use can be made of catchments which might
otherwise be unavailable.
Figure 1.4 Ring tank
Full supply level
Freeboard
Bank 1 m minimum

Core trench
Core trench SECTION 'AA'

Bank

A A
Natural
surface

Borrow pit

PLAN

Source: SR&WSC, 1970.

1.4.4 Turkey’s nest tanks

The ring tank is often incorrectly called the ‘turkey’s nest tank’. A true turkey’s
nest tank is constructed from material borrowed from outside the storage area
(Figure 1.5). All water is therefore held above ground level and is made available
by gravity through an outlet pipe in the lowest point of the embankment.

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 Planning • 17

Conditions when most used

Although storage water can be discharged through a low level outlet pipe, pumping
is still required to fill the storage. These storages are usually much smaller than ring
tanks and have a lower S:E ratio. The principal use of turkey’s nest dams is as a
balancing reservoir between a pump supply, windmills, and stock troughs.
Figure 1.5 Turkey’s nest tank
Full supply level Freeboard
Bank 1 m minimum

Core trench
Borrow pit
SECTION 'AA'

Bank

A A
Floor

B o r r o w p it

PLAN

Source: SR&WSC, 1970.

1.4.5 Excavated tanks

Excavated tanks are restricted to flat sites and comprise excavations below the
natural surface in locations where suitable sites for gully and hillside storages do
not exist (Figure 1.6). The excavated material is wasted, and is usually placed
adjacent to the excavation in neat spoil banks. Although excavated tanks can be
any shape, they are usually in the form of an inverted pyramid.

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 18 • Farm Dams

Conditions when most used

An excavated tank that is filled on an annual basis only should be as deep as
possible (3 m minimum) to reduce the surface area in relation to depth in order to
reduce the effects of high evaporation. Catch drains can be constructed to collect
surface run-off which, on sloping country, can raise the ponded water above
surface level.
Figure 1.6 Excavated tank

Freeboard
Silt tank Full supply level
1 m minimum

Core trench Core trench

SECTION 'AA'

Spillway

Bank

Berm

Inlet pipe

A A
Borrow pit

Silt tank

PLAN
Source: SR&WSC, 1970.

When the storage is gravity filled by surface run-off which contains appreciable
quantities of silt, it is desirable to divert this run-off into a small excavated silt
tank about one-tenth the size of the main storage. This is to reduce the velocity of
the incoming water so that the silt is deposited in the silt tank rather than in the
main storage. A suitable pipe connects the two tanks.
A storage collecting water from its own catchment requires a spillway, or by-
wash, to pass excess run-off. The catch drains are generally constructed to
incorporate the spillway, which must be located so that the overflow does not

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 Planning • 19

damage the tank walls. A silt tank and spillway are not required for off-stream
storages that are filled by pumping or a controlled artificial channel.
Excavation tanks are generally limited to stock and domestic uses, and the
irrigation of high value crops, because of low S:E ratios and the high unit cost of
water storage. The excavated storage may require fencing to prevent fouling and
pollution by stock. Troughs supplied by windmill or pump are then provided for
stock watering.
1.4.6 Weirs
A weir is a structure which is not used to store water but to raise the upstream
water level to allow diversion into off-waterway storages or create a pump sump.
It can be constructed of earth, concrete or timber.
Conditions when most used
Small weirs are quite frequently used as farm storages, usually where most of the
water supply required is available from low flow in the waterway concerned. In
the past, these have all been hard weirs, built to resist the erosion effects of high
velocities and high-energy losses over the weir. Because of the expensive
construction materials, these weirs are located at sites with a short crest length,
where a rock bar or other feature causes a natural constriction of the stream.
A valid criticism of weirs on waterways with a low flow supply is that they may
be used to obtain an excessive share of the flow. To avoid this, all weirs require a
bed outlet, so that once the storage is used, remaining low flows can be passed to
downstream users. Operation of this is difficult to police, resulting in private weirs
generally not being favoured in major streams. However, if operated correctly, a
weir can provide useful storage to supply irrigation or stock water during periods
of insufficient low flow.
1.4.7 Off-waterway storages
Off-waterway storages are used to store water that is diverted or pumped from
groundwater, an adjacent waterway or catchment. These off-waterway storages
are usually in the form of a gully dam, hillside dam, ring tank or excavated tank.
They usually have either very little direct catchment, or none at all.
Conditions when most used
An off-waterway storage has the advantage of having fewer foundation problems
and, if the waterway has a large catchment, that it does not need an expensive
spillway.

1.5 Dam storage size

In order to determine the required capacity of a dam, it is necessary to allow for
environmental requirements, and losses such as evaporation and seepage.
Accurate calculations of the losses can be complex. However, as an
approximation, it is advisable to allow for losses in the order of 40 per cent of
total storage volume, depending on locality and weather conditions. This means
that only 60 per cent of water stored is available for consumption, and this is
commonly referred to as the useable volume (Figure 1.7).

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 20 • Farm Dams

Figure 1.7 Water losses and needs

Freeboard
Water level at full volume
1 m minimum

Evaporation loss
Seepage loss Embankment
Useable volume

Environmental reserve

During low rainfall periods, replenishment rates can be too low to replace
consumed water. Therefore, storages need to be large enough to accommodate
these periods. Dams should be able to cope with drought periods at least equal to
18 months, including two summer seasons (see Section 1.2.2).
When a pump or windmill must be used to supply dam water, it is advisable
to:
• make sure the storage is large enough to meet anticipated daily needs; and
• locate the dam in a position that allows supply to the whole farm over two
days by gravity feed, even if at reduced water pressure.
1.5.1 Evaporation losses
Evaporation can significantly reduce production. More than 40 per cent of water
can be lost by evaporation from stored water in a 12-month period in certain
locations. This loss will reduce the amount of water available for irrigation, with
possible significant production losses.
Since evaporation is often the biggest consumer of water from a dam, it must
be taken into account when choosing dam size. Evaporation will vary according
to climatic zone, time of year, dam size, dam shape and the specific location of the
dam.
A first approximation of annual loss to evaporation can be calculated from the
following relationship:
LE = 0.67 E × AF
where LE = evaporation loss (L)
E = local annual evaporation (mm)
AF = surface area of the dam at full supply level (m2)

To take an example: for a dam with a surface area of 5000 m2 (0.5 hectare)
and an annual evaporation of 1275 mm, the volume of water lost from the storage
through evaporation is estimated as:

LE = 0.67 × 1275 × 5000 = 4 270 000 L or 4.27 ML

Evaporation, however, varies considerably through the year. During the
summer it is usually about twice that of either the spring or autumn months.
Hence the three summer months account for approximately half of the yearly
total. Estimation of evaporation for periods of less than a full year needs to take
this seasonal variation into account.

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 Planning • 21

The Bureau of Meteorology in conjunction with CRC Catchment Hydrology,

on 5 July 2001 released two books, titled Climatic Atlas of Australia—
Evapotranspiration and Climatic Atlas of Australia—Rainfall. This information
could be used to assess both evapotranspiration losses and rainfall.
1.5.2 Ways of controlling evaporation
Various means of controlling evaporation have been suggested, including chemical
films, floating or suspended material, windbreaks and emergent water plants.
However, more information is needed in order to measure the relative efficiency
of these methods.
A recent study by RMIT University, Victoria, into aspects of evaporation from
off-waterway storage, recommended the following ways of decreasing evaporation:
i floating rings with reflective caps significantly reduced evaporative losses, and
further development of this option is proceeding. The rings are formed from
strips of buoyant plastic, covered with a cap of bubble plastic and painted with
white reflective paint.
ii initial tests indicated that windbreaks have potential for reducing evaporation,
but further development of this idea is needed.
iii two simple methods for reducing losses are to have deeper storages, and to split
large storages into cells.
Note: Floating water plants do not achieve significant savings in evaporative losses.
1.5.3 Seepage losses
Pervious sub-surface soils are not always detected in the initial investigation
because landowners cannot afford, or do not understand, the necessity for
comprehensive soil testing of the base of a storage.
The borrow pit is usually located beneath the proposed storage. Material from
this source that is used in the embankment is not necessarily impervious and may
result in seepage.
When the borrow pit is within a storage there are underlying strata that may
not be watertight. Excavating these pervious seams will result in seepage and
leakage through the base. One problem that often occurs with high storage ratio
dams, in wide flat bottomed valleys, is that they may contain a considerable depth
of sand, gravel or silt.
For a simple field method of testing for seepage in the borrow pit and other
areas, see Section 2, and for remedies Section 6.
1.5.4 Average water consumption
The information provided in the compiled figures below (Tables 1.3 to 1.7,
Figures 1.8 and 1.9), is based on average conditions using Department of Natural
Resources and Environment (Victoria) data and Agnotes (see also Table 7.2 and
SRW, 1995—Farm Dam Series of pamphlets). In most cases the data can be
applied to other areas, by adjusting for local climate and other variables.
Peak consumption rates tend to govern the size of dam storages, but it is
difficult to estimate peak demands for stock. However, it can be assumed that
average daily consumption may occur over 20 per cent of the day (approximately
five hours). In domestic consumption the period is less than 5 per cent of a day
(approximately one hour). Sizes based on these quantities are normally adequate
for most installations.

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 22 • Farm Dams

Table 1.3 Annual rainfall and roof run-off relationships

Rainfall (mm) Roof area (m2)
150 200 250 300

Volume (litres)

100 15 000 20 000 25 000 30 000

400 60 000 80 000 100 000 120 000
600 90 000 120 000 150 000 180 000
800 120 000 160 000 200 000 240 000
1 000 150 000 200 000 250 000 300 000
Notes: To achieve 100 per cent run-off, the following need to be considered:
• spouts and downpipes have sufficient capacity to deal with peak flows, otherwise losses occur;
• small evaporation losses do not occur from the roof.
(Information based on various roof areas and a range of climatic conditions.)

Table 1.4 Method of calculating dam or tank size (litres)

Month Storage at Inflow (L) Outflow (L) Difference Storage at
start of month between inflow end of month
(L) and outflow (L)
(L)

October 45 000 15 000 14 000 +1 000 46 000

November 45 000 12 000 14 000 -2 000 43 000
December 43 000 5 000 16 000 -11 000 32 000
January 32 000 5 000 16 000 -11 000 21 000
February 21 000 3 000 14 000 -11 000 10 000
March 10 000 10 000 13 000 -3 000 7 000
April 7 000 12 000 9 000 +3 000 10 000
Notes: i In first month 1000 litres is lost as overflow.
ii It is necessary to work figures over two summer periods to establish number and size of storage(s).
iii On small farms (for example, 10 hectare) in areas with reasonable rainfall, large concrete tanks (45 000 litres
capacity) filled from roof run-off are often adequate, on their own, to provide both stock and domestic water.

Table 1.5 provides an estimate of how much drinking water stock will need in
a week.

Table 1.5 Daily domestic water usage

Type of use Average daily requirement (Litres)

Household 180 per person

Garden 3 000 per 0.1 hectare
Note: Based on summer months only.

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 Planning • 23

Table 1.6 Water application rates for different uses

Culture Application rate – average annual quantity of
water required (megalitres/hectare)
Annual Crops 3.0
Berries 3.0
Cereals 3.0
Citrus 3.0
Flowers 3.0
Golf Course 3.0 (south of Divide)
6.0 (north of Divide)
Nursery 3.0
Lucerne 6.0
Market Garden 3.0
Orchard 3.0
Native Pasture 3.0
Tobacco 3.0
Vines 3.0
Permanent Pasture 6.0
Annual Pasture 3.0

Figure 1.8 Assessing weekly needs for different livestock

Assessing water needs for different livestock

35
Lambs (dry pasture)
30 Sheep (dry pasture)
Calves
Kilolitres per week

25
Beef Cattle
20

15

10

0
0 100 200 300 400 500
Number of animals

Key: 1 kilolitre = 1000 litres (or 220 gallons).

Table 1.7 Daily animal water usage

Type of stock Average daily requirements (litres/day)

Sheep 7
Cows (milking) 70
Horses 50
Pigs 25
Poultry 32 per 100
Note: Does not take into account whether or not stock are on dry pasture (see also Table 7.2).

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 24 • Farm Dams

Table 1.7 provides estimates only, which are affected by:

• pregnant and lactating animals. These animals require larger volumes of water
than non-brood animals.
• the age of the stock. Older animals need greater amounts of water than young
or newborn animals.
• type of pasture and/or feed. Hand-feeding stock with dry feed increases their
water requirements significantly.
• weather conditions (hot/cold/windy etc.).
• presence of shade (trees, shelterbelts, etc.). Shade and protection from the
elements can help reduce stock water requirements.
• distance to water/availability.
• quality of water. Saline water will increase water consumption.

1.6 Using a dam in drought

Every State of mainland Australia is subject to periodic droughts, which makes it
necessary to cart water for stock and domestic purposes. A well-planned water

Figure 1.9 How long dam water lasts

4000 Water requirement per week – m3 4000 Water requirement per week – m3

100 60 40 20 10 100 60 40 20 10

3000 3000
Water area – m2

Water area – m2

2000 2000

1000 Depth = 1.0 m 1000 Depth = 2.0 m

0 0
10 20 30 40 50 60 70 80 90 100 10 20 30 40 50 60 70 80 90 100
Weeks to last Weeks to last

4000 Water requirement per week – m3 4000 Water requirement per week – m3

100 60 40 2010 100 60 40 20

3000 3000
Water area – m2

Water area – m2

2000 2000 10

1000 Depth = 1.5 m 1000

Depth = 3.0 m

0 0
10 20 30 40 50 60 70 80 90 100 10 20 30 40 50 60 70 80 90 100
Weeks to last Weeks to last

Source: SRW, 1995.

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 Planning • 25

supply scheme can alleviate such hardships. Below are some facts and some simple
tips and techniques to help plan stock water use and water conservation during a
long dry season.
Drought is a word that strikes fear into the hearts of dryland farmers across
Australia. Even in so-called ‘good’ years, you may be asking yourself, ‘Is there
going to be enough drinking water for my stock?’ and ‘How can I manage my
water in case of drought?’
To plan for drought effectively the following questions need to be addressed:
i How much water does my stock need?
ii How long will my water supplies last?
Once these two questions have been answered, it will be possible to make
informed choices about water and stock management. The first question can be
answered by referring to Section 1.5.4, whilst some rules-of-thumb approaches to
help answer the second question are shown in Figure 1.9.
The graphs in Figure 1.9 provide rough estimates of how long dam water lasts
for dams with a maximum depth of 1.0 m, 1.5 m, 2.0 m and 3.0 m, taking into
account evaporation losses. Note that if a dam is extremely silty it will reduce the
volume of water by as much as 20 per cent.
To use the graphs:
• measure the surface area of the water in the dam at the beginning of January;
• choose the graph that shows the maximum depth for the dam being assessed;
• measure across the graph horizontally to the amount of your weekly water
requirements, then draw a line to the bottom axis.
This will show the number of weeks you will have water in the dam, if no
further rain falls during that time.

Example
Maximum depth January 1: 2m
Surface area January 1: 1200 m3
Weekly water requirement: 40 m3

In this case the available water would last about 25 weeks, or until the end of
June.
Key:
kL = kilolitre
L = litre
m3 = cubic metre
m2 = square metre
mm = millimetre
m = metre
km = kilometre

If the graph shows that water supplies will be depleted before the next expected
rainfall, a number of choices are available:

A Reduce/conserve your water usage

As a first step, this is probably the most cost-effective and easiest option to
achieve. Below are some hints for making your water go further and to help
manage your water resources during dry times:

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 26 • Farm Dams

• fence off springs, soaks and dams, and pipe water to covered troughs. This
helps prevent losses by evaporation and bogging by stock as they go in search
of water.
• pump water from shallow dams to one central dam to reduce evaporation
losses.
• build drains from road catchments and other hard surfaces to dams, to catch
any run-off from summer thunderstorms.
• de-sludge, deepen and/or carry out maintenance on empty dams while you
have the opportunity, and to prepare for rain when it comes again.
• plant or build shelterbelts to reduce stock water losses through perspiration.
• use pumps and pipes that best suit your needs, to reduce wastage and pumping
costs.
• if you use streams or bores for stock drinking water, check them for salinity
levels. If they are saline, clean your troughs and uncovered tanks each week as
they will corrode quickly.

B Cart water in
This is not only an expensive option, but can also be soul-destroying and time-
consuming. When looking at carting water in, consider the costs.
• Costs of carting water vary depending on whether you hire a contractor or cart
it using your own truck.
• Contract rates vary from $45 for 10 000 L (in areas where milk tankers are
used for back-loading to dairy farms) to $60 for 4500 L in areas where there
are fewer contractors or supplies are harder to obtain.
When deciding whether to cart your own water, you will need to consider
operating and labour costs (see example below).

Estimating costs of carting water

Example:
If water has to be carted 9 km for 500 sheep, the daily costs could be calculated
as follows:

Daily water requirement (from Tables 1.5 and 1.7, Section 1.5.4):
500 sheep × 7 L/day = 3500 L/day (770 gallons/day)
Truck operating costs: 18 km at $0.80 per km = $14.60
Labour costs: 2 hours @ $15.00 per hour = $30.00
Total cost = $44.60

This represents approximately 9 cents per sheep per day or $3 per sheep per
month.
If you decide to cart water, there are some things you can do to optimise the
use of that precious resource:
• store the water in a tank rather than a dam, if possible, to reduce losses by
seepage and evaporation.
• round-shaped, above-ground swimming pools are the cheapest form of
temporary storage for carted water;
• consider bulk cartage of water by a contractor—it could be much cheaper in
the long run.

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 Planning • 27

C Sell stock
This is a difficult decision but one worth planning well in advance, before stock
start suffering and the decision is taken out of your hands. If you are unsure where
to start, contact the nearest Agriculture Department for advice.

1.7 Fire fighting

Rural water and country fire authorities suggest an approximate supply of 15 000 L
(3300 gallons) for fire fighting at a rural house. This represents about one-third of
the capacity of many rainwater tanks on rural properties. This quantity is
considered to be additional to other water uses such as domestic supply. This is
important because the highest fire danger periods often occur during drought
when water is scarce.
A supply of 15 000 L is considered a minimum quantity, particularly if
buildings are located in bushland with a high fire risk. At least 50 000 L may be
needed to protect a house in an isolated location.
Rural water authorities suggest that if household tanks are used, two outlets
should be provided—an upper outlet for domestic use and a lower one for fire
fighting. If the tank has a capacity of 45 000 L, the outlet for domestic use would
need to be about one-third the height of the tank from its base. Unfortunately,
rainwater tanks are not manufactured with this feature and in practice, additional
water for domestic use often has to be carted in during drought periods.
If a dam is used to provide water for fire fighting, it may need to have a 50-
year reliability, that is 2 per cent chance of such an event occurring in a 50-year
period (the figure often used for protection of houses from flood damage). This is
because of the strong temptation to use all of the water during drought periods,
particularly if water for livestock or domestic use is scarce. This was found to be
the case during the 1982–83 drought, followed by bushfires in Victoria, when
dams had to be filled by water freighted in by rail.
1.7.1 Maximum daily consumption
Information from fire fighting authorities suggests that volumes of water required
for bushfire control should range between 12–40 L/m2 for buildings, and 7–25
L/m2 for grass areas. This would typically mean about 14 400–48 000 L for a
homestead and shed and 70 000–250 000 L for 1.0 ha of grass. The volume of
water needed to control forest fires would be much greater. These quantities
should be considered as the minimum amounts used within a 24-hour period.
1.7.2 Peak rate of demand
A minimum flow rate of 155 to 180 L/min at a pressure of 240 to 340 kilopascals
is suggested in most references, depending on the fire risk. In steep forested areas
it may be desirable to reticulate water supply from a single large dam at greater
pressures and flow rates to several properties. The need for a reticulated town
water supply was demonstrated during the 1983 fires in Victoria and the 1994
fires in New South Wales.
Fire fighting literature states that up to 340 L/min for a hose is needed for
fighting forest fires. It is important to note that many small domestic pumps and
garden hoses connected to a gravity tank may have adequate pressures but
insufficient flow rates for fire control.
Country Fire Authority advice should be sought on the above points.

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 28 • Farm Dams

1.7.3 Monthly and annual consumption

One method of reducing fire risk is to burn off vegetation surrounding a
homestead. Control of this burn-off is dependent on terrain, and may require a
large quantity of water (40 000 to 250 000 L) for each hectare of vegetation burnt.
It is recommended that at least 15 000 L be provided each year for protecting
a house in accessible locations and at least 50 000 L in isolated locations. If a dam
is to be used to provide water for fire control in bushland, at least 500 000 L is
suggested.

1.8 Farm dams and trees

A farm dam is a long-term investment and proper design and construction will pay
future dividends. Strategies to lower farm costs and improve water yield and
quality include the planting of appropriate trees, shrubs and grasses in the right
places around farm dams to provide protection from wind and sunshine.
Dams are primarily intended to provide water for stock, irrigation or domestic
use and often provide greater flexibility in pasture and stock management.
Important secondary uses can include soil erosion control, bushfire protection,
wildlife habitat, recreation and aesthetics. Whatever the purpose of the shelterbelt
zone, the inclusion of trees, shrubs, grasses and other groundcovers in land and
water management plans can provide the following benefits:
• a filter for cleaning water
• shade and shelter to reduce evaporation
• shade and shelter for stock
• emergency fodder and timber
• habitat for wildlife
• soil erosion control
• salinity control, and
• improved farm appearance (Greening Australia, 1990).

1.9 Dam cost justification

The money spent on storing water in dams varies according to how the water is
to be used (see Section 3.6). A suitably sized stock dam may make the difference
between carrying 1000 head of sheep, or none at all. Similarly, a fire fighting dam
may help to prevent thousands of dollars of damage. In these cases an S:E ratio
even as low as 1:1 is justified (DoA SA, 1967). For irrigation however, more
consideration is warranted. Again, the capital outlay for a certain amount of water
varies with the crop that is to be grown. A vegetable or fruit grower can afford
more expensive water than the person who will use it to grow pasture. In every
case the landowner should consider whether the money would be better spent on
pasture improvement or fencing. In general, irrigation should not be undertaken
until a whole farm plan has been developed.
For permanent pasture irrigation from a catchment dam, a rule-of-thumb
method for assessing storage cost is that the maximum economic outlay will be
approximately $2500 per 5 ML. Therefore, at normal earthmoving costs this
means an S:E ratio of 6:1. In the case of horticulture it pays to store water at an
S:E ratio as low as 2:1, while stud breeders should consider an S:E ratio of 4:1

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 Planning • 29

approximately. Below this ratio the costs should be worked out very carefully, as
the dam is only part of the outlay. Distributing the water can entail heavy
additional expense. Irrigation for drought insurance alone is unlikely to pay on
any ratio.
When underground water is available, the cost of building a dam can be
compared with the costs of sinking a bore. If there is good quality water
underground in reasonable quantity, it takes a very favourable irrigation dam to
outclass a bore. It is the high evaporation loss that makes dam water expensive.
The unreliability of run-off to fill a dam may also favour a bore. In the case of
dams filled by pumping, filling the dam adds further to the cost of stored water.
It is difficult for the average landowner to predict whether or not a water
conservation and irrigation project will pay. This is because some landowners lack
detailed facts and figures on the likely increase in production to be expected from
a change in irrigation method (DoA SA, 1970).
However, a good method for comparing irrigation with other investments is as
follows:
i calculate the capital value and net returns of the property in its present state.
ii work out the net returns as a percentage return on capital.
iii determine the extra capital involved in setting up the irrigation project, and
add this to the present capital.
iv estimate the increased returns due to irrigation, not forgetting to deduct
pumping costs, depreciation, extra labour, fertiliser costs and loss of
production from land flooded by the dam. Add this figure to the present
returns from the property.
v calculate a new percentage return, using the new capital and return figures.
Steps iii, iv, and v are then repeated for the other proposed investment or
improvement, and this new percentage return on capital is worked out. Whichever
project gives the higher percentage return is the better one from an economic point
of view.

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 Section 2
Investigation
The suitability of a dam site depends not only on the embankment but also on the
ability of the soils in the storage area to hold water. The soil profile should contain
a layer of material that is sufficiently impervious and thick enough to prevent
seepage losses. Excellent materials for this purpose are clays and silty clays,
although sandy clays are usually satisfactory.
Coarse textured sands and sand-gravel mixtures are highly pervious and
therefore generally unsuitable. In some areas, such as flood plains, it is often
possible to impound a limited depth of water in areas where no impervious layer
exists in the soil profile but where a high water table exists at, or near, the ground
surface. Some limestone areas are especially hazardous for use as dam sites. They
may form tunnels, cracks or channels in the limestone below the ground surface,
which are not visible from the surface. These lines of seepage may drain a dam in
a very short period of time. In addition, soils in these areas are often granular. The
granules do not break down readily in water and the soils remain highly
permeable.

2.1 Soil testing

Without extensive investigations and laboratory tests, it is difficult to recognise all
of the factors that might make a site undesirable. One of the best guides to a site’s
suitability is the degree of success experienced by neighbours with similar dams.
The absence of a layer of relatively impervious material over a storage area does
not necessarily mean that the site must be abandoned. It usually means that the
area will have to be treated by one of the several methods described in Section 6
under the heading ‘Dam leakage’. However, any of these methods may prove to be
expensive.
2.1.1 Foundation
The foundation includes the valley floor and its side slopes or abutments. The
requirements of a foundation for an earth-fill dam are that it provides support for
the overlaying embankment under all conditions of saturation and loading, and
that it provides sufficient resistance to seepage to minimise loss of water. Poor
foundations can lead to failure of a dam due to cracking, tunnelling, sliding,
settlement, or uplift.
The soil conditions under an embankment should be investigated to ensure
that the site is suitable and that a safe structure can be designed. The complexity

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 32 • Farm Dams

of the foundation survey will depend upon the site conditions and on the height
of the dam wall. Test holes should be taken at intervals along the centre-line of the
dam (Figures 2.1 and 2.2), using either a hand auger or backhoe to cut a trench
to determine the nature of the soil profile. The depth and spacing of the holes
should be sufficient to determine the suitability of the foundation. The location of
these holes will depend on the occurrence of significant changes in the soil profile.
The holes should be deep enough to identify the underlying materials that may
affect the design or safety of the structure. A record, or log, of each hole or test
pit should be made. This should show the location, depth and classes of materials
encountered. Each test hole location should be marked on the ground for future
reference, in case more detailed surveys are required.

Figure 2.1 Depth of test holes along centre-line

Top of bank
Spillway Embankment

1m e
a
1m 2
_ e
3
b c d
TH1 2
_a TH6
3

TH2
2
_ 2
_
3 b 3 d

2
_
TH3 3 c TH5

TH4

Source: WAWA, 1991.

2.1.2 Borrow pit for embankment material

Suitable excavated material needs to be found in sufficient quantities for
construction of the embankment. The further the excavation is from the
embankment site, the greater the placement costs. This is a good reason for having
the borrow pit inside the storage area. Test holes should be made in the chosen
borrow pit areas to establish whether sufficient volumes of suitable material are
available.
Selected materials for construction of a dam wall must have both the strength
for the embankment to remain stable, and a sufficiently low permeability (when
compacted) to prevent excessive seepage of water through the dam wall.
2.1.3 Spillway site
During a storm event, it becomes necessary to bypass run-off around the
embankment of a farm dam through an earth spillway. For safety reasons, the
spillway should be located on a natural surface and not on the fill material. Thus,
auger holes should be placed along the centre-line of the proposed spillway to
determine the type of material that will be encountered, its erodability, and its
suitability.

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 Investigation • 33

2.2 Site selection criteria

A problem sometimes associated with selected sites is that there may be substantial
depths of pervious materials (for example, 3 m). This could require a deep cut
excavation for the core trench placement beneath the dam. The presence of
pervious materials could also result in more expensive de-watering problems
during excavations.
2.2.1 Seepage losses
Seepage losses are affected more by the prevailing groundwater conditions than by
permeable soils. For example, if an excavation is cut into sandy soil, which lies
well above the water table, water will tend to seep out of the excavation. On the
other hand if the water table is higher than the floor of the excavation, then
obviously water will seep in. However, because most storages are built well above
the water table, sands and gravels are likely to be sources of seepage.
If the site is doubtful, there is a simple test often used by geo-technical people
to evaluate a site. During the site exploration, several soil-sampling test holes
(Figure 2.1) will be dug in the borrow pit areas under the embankment. It is
important to consider the proximity of auger holes, as they can have an effect on
results. These holes are usually done by hand auger (100 mm diameter and at least
3 m deep). Several holes should be selected for the seepage tests. The following
procedure should be used in initially testing sub-surface soils for suitability.
• After excavating auger holes, carefully fill each hole to two-thirds its depth
with water so as to saturate the sides of the sub-surface soils. It is important
not to disturb the wall of the hole during filling. If it is necessary to support the
sides of the test hole, this can be done by filling gravel around a slotted pipe.
• When water fills the auger hole to two-thirds its total depth, the depth to water
level below the ground surface should be maintained in each hole. The quantity
of water required, and the time intervals when water is added to maintain this
depth, will need to be recorded.
• It is important that the trials be carried out over a number of hours and that
the effect of seasonal variation on moisture content are considered in the end
results.
A tried and proven method of assessing the results of field testing for seepage
is given below. This procedure is also used in the selection of effluent line sites for
sewerage systems.
The recommendations listed in Table 2.1 are suggested guides for site selection,
based on seepage loss where the auger hole has a diameter of 100 mm, a depth of
3 m and is filled 2 m deep of water (that is, the water level is 1 m below the ground
surface).

Table 2.1 Guide to site selection based on seepage loss

Seepage loss rate (added water – L/hr) Recommendation

<3 L/hr (50 ml/min) Site should be satisfactory.

3–30 L/hr (50–500 ml/min) Site should be regarded as doubtful.
This indicates the need for further tests.
>30 L/hr (500 ml/min) The site is too permeable for a dam.
Source: Information from Nelson, 1985.
Note: L/hr = litres/hour and ml/min = millilitres/minute.

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 34 • Farm Dams

If seepage loss rates greater than 3 L/hr (50 ml/min) are confirmed, then other
factors must be considered before proceeding with the project. These would
include the purpose of the water supply scheme, the availability of water for the
dam and the likely cost of treating the seepage area.
A comparative evaluation of falling water levels over an area can then provide
an indication of permeability and may indicate relative clay contents. Infiltration
rings are a more sophisticated way of assessing infiltration capacity for irrigation
design purposes.
Like sands and gravels, some jointed formations of rock are permeable and
encourage seepage. Dams have failed due to sinkholes developing in the floor of
the storage. This usually occurs when limestone underlies the soil. Water from the
storage dissolves the rock to form vertical holes, which in turn lead to
underground cavities and springs.
2.2.2 Stability of dam sides
As the level of stored water rises, so does the water table in the sides of the dam.
Soils and rocks, which are quite stable when dry, may become weak when
saturated. This could cause a landslip, which in turn will reduce the capacity of
the storage. Frequently this problem is aggravated by cutting the borrow pits too
close to the reservoir sides.
2.2.3 Sedimentation in dams
Sedimentation is a problem that occurs in catchments with active soil erosion. In
these cases, advice should be sought from a soil scientist. Sometimes it is possible
to remedy the problem at its source; if not, it may be necessary to resort to filter
strips and silt traps. Filter strips are dense stands of stiff, long-stemmed plants
intermingled with grass. These strips reduce the velocity of the water and so cause
silt to be deposited.
In the case of choosing a gully site where sedimentation may be a problem, the
following topographical features should also be borne in mind.
• The storage should be located in a wide valley just upstream of a narrow gorge.
This will initially provide maximum storage for minimum earthworks. Over
time, silting will reduce the storage capacity of the dam and it will require
maintenance (see Section 6.3.8).
• It should be located on the flat slope of a stream rather than on the steep slope.
This provides a larger capacity of stored water for any given wall height of a
dam.

2.3 Foundation materials

It is possible to construct an earth-fill embankment on a suitable foundation if this
has been thoroughly investigated and the design and construction procedures are
adapted to site conditions. Some foundation conditions require construction
measures that are relatively expensive which, in the case of small farm dams,
cannot be justified. Sites with such foundation conditions ordinarily should be
abandoned.
The best foundation comprises, or is underlaid by, a thick layer of relatively
impervious, consolidated material, which occurs at a shallow depth. Such
foundations cause no stability problems. Where a suitable layer occurs at the

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 Investigation • 35

surface, no special measures are required. It is sufficient to remove the topsoil, and
scarify or disc plough the area to provide a bond with the material in the dam wall.
A compacted clay cut-off trench can be constructed to extend from the surface of
the ground into the impervious layer. This prevents possible failure by tunnelling
and excessive seepage.
A detailed investigation should be made where the foundation consists of either
pervious sand or a sand–gravel mixture, and the impervious clay layer is beyond
the reach of equipment. While such a foundation might be satisfactory in terms of
stability, corrective measures will be required to prevent excessive seepage and
possible failure.
In the case of a foundation consisting of, or underlain by, a highly plastic clay
or unconsolidated material, very careful investigation and design is required in
order to obtain stability.
Water stored on bedrock foundations rarely gives cause for concern unless the
rock contains seams, fissures or crevices through which water may escape at an
excessive rate. Where rock is encountered in the foundation, careful investigation
of the type and physical properties of the rock is required.
Foundations must be capable of supporting the weight of the dam and must be
sufficiently watertight to prevent seepage under the dam. Springs, soaks or
landslips indicate unstable soil conditions and should be avoided.
Therefore, the three main kinds of foundation material are:
Clay Clay foundations are usually satisfactory, provided they are of the
same material as that placed in the earth bank. However, if they are
soft and saturated it may become necessary to remove them or place
additional stabilising fills. Highly expansive clays, which shrink and
swell during cycles of wetting and drying, may be unsuitable because
of risks associated with tunnelling and high seepage rates.
Rock Most rock can support the weight of the dam. Care must be taken to
ensure that seepage does not occur between the rock foundation and
the earth-fill dam so that weathering of the rock does not lead to
weakening of the foundation, or that permeable zones are not
created by joints and faults. Care should also be taken where
expansive rock is being excavated, since elastic recovery (or
expansion) of rock material occurs in response to reduced pressures,
as the overburden is removed.
Sands and The problem with this type of foundation is high seepage losses.
gravels While it is possible to build dams with these materials, the cost is
frequently prohibitive. Such sites are best avoided and an alternative
location found.

2.4 Embankment materials

Soils placed in dams must fulfil two conditions; they must be sufficiently
impervious to keep the seepage at a safe rate, and they must have sufficient
strength to ensure stable side slopes.
There are three kinds of gully dams: homogenous, zoned, and diaphragm. The
homogenous dam is built from one type of soil and is the most common kind in
Australia. A zoned bank consists of a centre clay core with pervious material on
either side. It is considered the most stable form of farm dam. The diaphragm dam

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 36 • Farm Dams

is built when there is only a limited amount of clay available at the site. The bulk
of the bank is constructed from relatively pervious material with a thin layer (that
is, a diaphragm) of clay on the upstream slope. This layer varies from 0.6 to 1.0
m thick, depending on the height of the dam.
Good, impervious material contains about 25 per cent clay with the balance
made up of silt, sand and some gravel. Too much clay results in the embankment
being weak and prone to expansion and contraction with changes of moisture
content. Insufficient clay can cause excessive seepage through the bank.
The usual method for exploring the material at a potential dam site is hand
auger boring. This is the cheapest method, although it is very hard work for the
operator and provides a disturbed sample. It is therefore advisable to sink a test
pit or trench so that the soil can be examined in its natural state.
Dam sites are tested on a fixed pattern. Small dams (up to 3 m high) have a
minimum of six test holes, four in the centre-line (including one on the spillway)
and at least two in the borrow pit area (Figure 2.2). For larger farm dams the
number of test holes is increased, with holes at 20 m intervals in borrow pits

Figure 2.2 Test hole plan for a small dam

30 m
Borrow pit

TH9

30 m

TH8

TH7
Direction of flow

Embankment

Centre-line

TH1 TH2 TH3 TH4 TH5 TH6

Spillway

Source: WAWA, 1991.

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 Investigation • 37

where sites are steep or complicated. This spacing can be increased to a 70 × 100 m
grid when the site is flat or uncomplicated. The test holes on the centre-line of
larger dams are spaced at about 30 m intervals. The test holes in the borrow pits
are sunk to about 3 m or to rock, while those in the dam centre-line are put down
to at least two-thirds of the dam height or to rock.
When the exploration has been completed, all test holes and pits should be
carefully filled to prevent human and stock injury and compacted to prevent
seepage/leaks.

2.5 Site investigation of materials

All the earth fill for the embankment should be excavated from within the water
storage area when possible, and if required, from any cut spillway areas (SCA,
1979). At the investigatory stage possible borrow areas should be identified—
initially by eye—to ascertain soil type from vegetation, visible soil, position on
slope and so on. Most favourable areas can then be investigated.
Initial investigations involve the use of a backhoe to dig borrow pits and an
auger to assess the condition of topsoil, subsoil and foundation in the
embankment area. Auger holes dug on a grid to depths of 3 m throughout a
potential source area will allow a general assessment of soil types to be made. A
series of trial pits using a backhoe, can then be dug in more promising areas to
allow a visual assessment of the soil profile according to soil classification
techniques. Samples can be taken for texture and laboratory analysis.
2.5.1 Soil texture tests
Soil texture tests are carried out to determine soil types. The relative proportions
of sand, silt and clay are used to determine the textural class of a soil. The
internationally accepted and most used tool for initially identifying soils for dam
building is the United States Department of Agriculture (USDA) texture diagram
shown in Figure 2.3. Also included, in Table 2.2, are the texture classes as defined
by a percentage for sand and clay.

Table 2.2 Texture classes

Textural class % Sand % Clay %

Sand >85% –
Loamy Sand 70–85% –
Sandy Loam 50–70% <20%
Sandy Clay Loam 45–80% 20–35%
Clay Loam <45% 40%
Sandy Clay 45–65% >35%
Clay <45% >40%
Source: Fietz, 1969; Stephens, 1991.

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 38 • Farm Dams

Figure 2.3 Textural classification chart

100 0

80 20

Sil
t, i
np
60 Clay
40

erc
nt

en
rce

t
Silty
pe

Sandy clay
, in

40 clay
ay

60
Cl

Clay loam Silty

clay loam
Sandy clay loam
20 Loam 80
Silt loam
Sandy loam
Loamy Silt
0 Sand sand 100
100 80 60 40 20 0
Sand, in percent

Source: SCS USDA, 1967.

2.5.2 Unified soil classification

Materials are classified according to the ‘Unified Soil Classification’ system. This
system of classification uses grouped symbols to indicate the physical properties of
the material. The system is based on the size of the particles, the relative amounts
and the characteristics of the very fine particles.
Table 2.3 provides the list of grouped symbols used under the ‘Unified Soil
Classification’, with their common names, organised in order from largest to finest
particles, that is gravels, sands, silts, clays and organic material.
To define the textural classification of soils precisely requires laboratory
techniques. However, with experience and specific local knowledge, hand testing
to determine texture can prove important in the initial stages of identifying
appropriate earth fill materials. Clay soil sites (that is, soils with a higher
percentage of clay) can be defined in the field. These soils are used for the core and
upstream batter of the embankment. Silts are often similar in both appearance and
feel to wet clays when dry, but can usually be differentiated when wet as the clay
will exhibit sticky, plastic-like characteristics while silt has a silky, smooth feeling
with a tendency to disperse.
Hand-testing techniques involve taking a small sample of a soil—usually in the
hand not required for making notes—dampening it (avoiding soaking it), and
rolling it into a ball to examine its cohesive constituents. A good clay can be
manipulated into a thin strip without breaking up, rolled into a ball and dropped
onto a flat surface from waist height without cracking unduly, and cut to exhibit
a shiny, smooth surface.

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 Investigation • 39

Table 2.3 Unified Soil Classification

Texture class Soil Group Soil characteristics

Gravel GW Well-graded gravels; gravel-sand, little or no fines

GP Poorly graded gravels; gravel-sand mixture, little or no fines
GM Silty gravels; poorly graded gravel-sand-silt mixture
GC Clayey gravels; poorly graded gravel-sand-clay mixture
Sands SW Well-graded sands; gravelly sands, little or no fines
SP Poorly graded sands; gravelly sands, little or no fines
SM Silty sands; poorly graded sand-silt mixture
SC Clayey sands; poorly graded sand-clay mixture
Silts ML Inorganic silts and fine sands; rock flour, silt or clayey fine sands with
slight plasticity
CL Inorganic clays low medium plasticity; gravelly and sandy clays; silty clays;
lean clays
MH Inorganic silt; micaceous or diatomaceous fine sandy or silty soils; elastic
silts
Clays CH Inorganic clays of high plasticity; fat clay
OH Organic clays of medium to high plasticity
Pt Peat and other highly organic soils
Source: SCS USDA, 1967.

2.6 Analysis of soil

Analysis of soil samples should be carried out to assess components and
compaction characteristics, and to check for other factors that may make
apparently good soil unsuitable. Correlation of these results with previous work
will allow estimates to be made of available earth fill, overburden to be removed
and unsuitable areas to be avoided.
The importance of a correct analytical approach to determine the various soil
types for a zoned embankment cannot be over-emphasised. Although using a soil
laboratory is expensive, the results can more than repay the cost involved and,
more often than not, will ensure the exclusion of doubtful material in the
construction process (Stephens, 1991).
2.6.1 Core trench
In the construction of a core trench, soil is required that will limit the passage of
water through or underneath an embankment, but not to such an extent that
undesirable differential pressures could build up across and within the
embankment (see Section 3.1.2). The impermeability of the soil used will vary
between sites, but some standardisation of watertightness can be achieved through
varying the degree of compaction involved, with a more pervious material
requiring more compaction. Generally, soils containing a significant percentage of
clay are ideal for the core but clays with a tendency to crack should be avoided. If
the latter are used they should be carefully compacted or placed in parts of the

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 40 • Farm Dams

dam which are unlikely to dry out, such as in the core trench or covered by topsoil
with grass.
2.6.2 Embankment soil
Semi-pervious materials, such as sandy clays and clay loams which contain a
proportion of fines as clay, are suitable for inclusion in the upstream batter. These
will allow limited passage of water and will resist slumping when wet in a properly
constructed embankment. Where poorer soils are used, special attention to
compaction techniques will have to be given to minimise the number of air voids
in the soil and to maximise its stability when wet.
Pervious materials such as coarser grained sand and gravels can be used in the
downstream batter slope and sections of the embankment which require a large
mass of material and good drainage. A dry downstream face will prevent slippage
and reduce the risk of failure.

2.7 Location of soil

Within a valley a variety of soils may be available. Valleys where less leaching has
occurred can provide soils with a higher proportion of clay, while the more heavily
leached areas can provide higher amounts of sands, gravels and/or silts. The
stream-bed can be a source for silts, sands and gravels, with the latter being useful
for drains and concrete work. The need to find such materials close to the dam
site, preferably within the reservoir area and in large enough quantities to justify
their removal is of great economic importance. Complete removal of impervious
materials should be avoided, as exposure of more permeable underlaying layers
could lead to seepage problems in later years, especially when under pressure of
several or more metres of water. Investigation of proposed borrow areas is a
necessary feature of any dam survey. This is done by using a hand auger, soil pits,
groundwater bore data and utilising existing features such as animal burrows or
exposed road cuttings to gain a knowledge of the area.
Clays Any material with more than 55 per cent clay can be considered a clay
(see Figure 2.3). A sandy-clay is a soil with between 35–55 per cent clay
and up to 65 per cent sand and a sandy-clay-loam is between 20–35 per
cent clay with up to 80 per cent sand and loam.
Materials with a high clay percentage should always be kept for the core
trench and central zoned embankment and must be well compacted.
Basically, the poorer the clay percentage (to an arbitrary minimum as
low as 3–5 per cent), the more compaction and care in construction is
required.
The upstream batter does not require highly impermeable clays as these
could lead to undesirable uplift pressures developing beneath this
section of embankment. Sandy clay soils are most suited for inclusion in
this upstream batter as they compact well, have much reduced seepage
characteristics but do not allow the build-up of high soil–water
pressures. Clays are not required in the downstream batter as it is
essential that this portion is free draining (see Figure 3.2).

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 Investigation • 41

Silts These soils should be avoided in any section of the embankment. The
lack of cohesion, poor structure, fine material and difficulty in
compaction are their main drawbacks. A small proportion of silt is
permissible in a silty-clay, but care must be taken in its use and
application.
Sands A soil with a high proportion of sand should not be used in dam
construction. A sandy soil can be used in the downstream batter but
should not be used elsewhere unless there is no alternative. If a sandy
soil is used in other parts of the dam, attention must be paid to
compaction. Furthermore, the best soil should be kept for the core and
consideration given to ensuring embankment watertightness.

2.8 Unsuitable material

There are some materials that should never be used in dam construction, including
the following:
• organic material;
• decomposing material;
• calcitic soils such as clays derived from limestone which, although generally
stable, are usually very porous;
• fine silts;
• cracking clays that fracture when dry and may not seal up if rapidly wetted in
time to prevent piping; and
• sodic soils, which are fine clays with a high proportion of sodium. They are
difficult to identify in the field, so any fine clay should be analysed.

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 Section 3
Design
Australians spend millions of dollars each year on the construction of small dams
for rural communities. This money is wasted because a large proportion of these
dams fail. The solution to this problem rests in more thorough investigation and
improved standards of design and construction. Unfortunately such proposals are
often regarded with apprehension because many people consider that higher
standards mean increased costs (Lewis, 1995a).
The combined cost of field investigation and design usually represents only 5 to
10 per cent of the total capital cost of the dam. Furthermore, if the dam is
constructed to a well-designed plan many cost-saving features can be incorporated.
So you may well ask ‘Can I afford the extravagance of a cheap dam?’
Figure 3.1 Cross-section and elevation view

Crest
Freeboard
Full supply level
1 Desirable to spread
1 2 top soil over bank
Compensation pipe 3 Batter slope
Strainer
Compacted earthfill Gate valve
Creek bed

Strip topsoil from entire foundation area Core trench constructed under
Cut-off collars entire bank length into solid foundation

TYPICAL EMBANKMENT CROSS-SECTION

Desirable to allow minimum additional 5–10%

Natural surface of bank height to allow for settlement
1
2 1
Cut batter may 2 Str
ip t Creek bed
h

be steeper Spillway dimensions ops

nc

oil
than 1:2 if determined by design tr e
in sound rock r e
Co

Compensation pipe in trench

EMBANKMENT ELEVATION
(Cross-section of creek)

Source: SRW, 1995.

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 44 • Farm Dams

3.1 Items that need to be considered

An earth-filled dam of any size must be designed so that it is structurally sound
and stable during its operational life. They are simple structures that rely mainly
on their mass to resist sliding and overturning. Modern construction techniques
and developments in soil mechanics have greatly increased the safety and
durability of these structures.
The following design considerations must be met if safety is to be maintained:
• the batter slopes must be stable and resistant to movement under different
operating conditions, including rapid drawdown.
• the batter slope on the upstream side must be able to handle across-water wave
action.
• the earth embankment must be safe from overtopping by both flood inflow and
wave action. This is the reason for providing a 1 m freeboard from full supply
level to the top of the crest.
• seepage through the embankment and beneath the foundation needs to be
controlled to prevent piping or tunnelling along a line of weakness through, or
under the dam.
• the embankment needs to be protected from a major failure due to earthquake,
if the site is known to be earthquake-prone.
3.1.1 Embankment types
i Homogenous
The earliest farm dam embankments were constructed on the principle of a
homogenous wall of earth, whether impervious or not, across waterways or
streams. When built properly such embankments can still be cheap and reliable.
However, they are generally inferior to the zoned embankments described in the
next section.
This method of construction is simple, straightforward and suitable for those
sites where there is sufficient suitable material. Protection from seepage and
slipping is provided by flattening the downstream batter, that is from a 2:1 to 3:1
gradient (horizontal to vertical), and providing a thick covering of topsoil to carry
any seepage to the toe of the bank.
Seepage problems can occur when the dam is constructed on impervious
foundations and comprises:
• upstream and relatively impervious sections, and the highly impermeable
material of the central core, (which effectively seal the dam against seepage);
and
• a downstream segment of poor, coarse material which allows freer drainage of
the embankment and which, by its weight, not only holds the completed wall
to its foundation but also prevents slippage and movement.
Figure 3.2 illustrates this situation and shows the incorporation of a processed
drain material (sand/gravel or similar).
Embankment materials vary widely from place to place, particularly in respect
to gradation and permeability. If the difference in permeability between the
impervious core and the downstream batter is great, no internal drainage is
required. If the variation of the permeability between the inner core and the batter
is not sufficient, then the embankment will become saturated after prolonged
storage at full supply. Consequently, the downstream slope will show seepage to a

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 Design • 45

Figure 3.2 Zoned dams with treatment of seepage flow lines for internal drainage

Storage water surface level Freeboard

at spillway crest (typical) Phreatic water surface (typical)
1m
2:1
Natural drain
H 3:1 A
"H" less than 4.5 m Natural surface
Core trench

Trench drain 2:1

"H" less than 7.5 m 3:1
A or B A B

Drainage material
with drain pipe

Toe drain 2:1

3:1
"H" less than 7.5 m
A

Drainage material

Blanket drain 2:1

3:1
"H" less than 15 m
A or B A A or B

Drainage material

Chimney drain
"H" less than 15 m
3:1 2:1

A or B A A or B

Drainage material
LEGEND
A -– impervious
B – semi-impervious to pervious
Drainage material – rock or gravel

Source: modified from CDWR, 1986.

Legend:
A—impervious zone, which under the Unified Soil Classification would be used for soil types, GC, SC, CL and CH.
B—semi-impervious layer, which under the Unified Soil Classification would be used for soil types, GW, GP, gravelly SW,
and gravelly SP.
Note: Batter gradelines on upstream and downstream sides would need to be 3:1 (horizontal to vertical) and 2:1 (horizontal
to vertical) respectively. This is to accommodate for rapid drawdown of the water level. Gradelines are totally dependent on
soil types and need to be carefully tested and checked before adoption. Each of the cross-sections (top to bottom) in Figure
3.2 is based on a bank height of less than 4.5 m, 7.5 m, 7.5 m, 15 m, and 15 m respectively.

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 46 • Farm Dams

height of approximately one-third the depth of the high water level. Such
saturation reduces the stability of the dam and creates maintenance problems.
This pressure line is also shown in Figure 3.2 and is called the ‘phreatic’ water
surface line. Major maintenance problems can occur if this is not taken into
account. These problems include undesirable growth of vegetation on the batter
and minor slippage from trafficking by animals and vehicles.

ii Zoned
At those sites where there are varying soil materials with widely differing
construction properties, and where high dams are being considered, a zoned
embankment should be considered. Zoned embankments consist of an impervious
core/blanket held in place and protected by a more pervious ‘shell’. The core may
be centrally placed, sloping or placed on the upstream slope in the form of a
blanket (Figure 3.2).
A zoned embankment is a better alternative for larger dams; this allows for the
use of construction machinery. Compared with homogenous sections, costs are
likely to be higher, mainly because the earthworks materials are divided into three
categories, that is
• pervious for the downstream section;
• impervious for the inner core section; and
• semi-impervious for the upstream section.
All of these materials need to be excavated from different borrow pits (preferably
from within the storage), thus increasing excavation and movement costs.
The use of pervious (sandy) soil materials in the shell can greatly reduce the
volume of the embankment as steeper batters can be used. Upstream blankets are
commonly used in conjunction with lining of the storage area of the dam.
For high embankments (greater than 10 m) the use of toe drains and blanket
drains allows the downstream slope to be maintained at a reasonable slope of
approximately 3:1 (horizontal : vertical). Without these provisions, much flatter
slopes would be required to maintain slope stability, particularly for clayey soils.

iii Artificial liners

Artificial impermeable materials, like heavy-duty plastic liners, have been used as
an alternative to clay cores and zoning. However, these materials have been found
to be particularly attractive to yabbies and burrowing vermin; and have not
resisted settlement of the wall after construction. In Section 6, details are given on
other alternatives that can be used to minimise leaks in dams. These materials are
currently being tried in new dams in areas prone to problems.
3.1.2 Core trench
Under the middle of the dam a core trench must be provided to minimise seepage
beneath the dam wall. The core trench should have side slopes no steeper than 1:1
(horizontal to vertical) for a depth up to 3 m and no steeper than 1.5:1 (horizontal
to vertical) for greater depths. The bottom width of the trench should be 1.5 times
the height of the dam, or a minimum width equal to the width of a bulldozer or
scraper. The depth of the core would generally extend to bedrock, or to an
impervious stratum of soil sufficient to prevent excessive seepage beneath the dam.
The core trench material should be placed in layers with a maximum thickness
of 100 mm. It is important that every layer is well compacted. The whole dam

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 Design • 47

length should be completed at the one time. If this is not possible, then each
section must be well keyed and bonded to the next. To avoid seepage and
structural problems, the core trench and embankment shell should be designed as
one homogenous unit. Any water collected in the core trench should be removed
before backfill operations are started.
The value of installing the core correctly cannot be over-emphasised. Failure to
properly carry out these comparatively inexpensive techniques could lead to
inherent problems that remedial works will rarely solve. If the core and cut-off
trenches have not been placed on good foundation material, or layered correctly
with enough moisture to allow compaction, the structural integrity will be
compromised. In severe cases the dam will probably fail.
Where the foundation consists of pervious materials at or near the surface, with
rock or impervious materials at a greater depth, seepage through the pervious layer
should be reduced to prevent piping and excessive losses. Usually a cut-off joining
the impervious stratum in the foundation with the base of the dam is needed.
The trench should be filled with successive, thin layers of relatively impervious
material, with each layer being thoroughly compacted at near optimum moisture
conditions, before the next layer is placed.
3.1.3 Embankment batter slope
For homogenous dams, the following batter slopes are recommended for
embankments built of soils classified according to the Unified Soil Classification
system (see Section 2.5.2).

Table 3.1 Batter slope with soil classification and height

Case type A B
Homogenous or modified Modified homogenous

Purpose Detention or storage Storage

Subject to rapid drawdown No Yes
Soil classification GW GC CL CH GW GC CL CH
GP GM ML MH GP GM ML MH
SW SC SW SC
SP SM SP SM
Dam Height (m) & Slope
0–3 U/S P 2.5 : 1 2.5 : 1 3.5 : 1 P 3 : 1 3.5 : 1 4:1
D/S 2:1 2:1 2.5 : 1 2 : 1 2.5 : 1 2.5 : 1
3–7 U/S P 2.5 : 1 3:1 3:1 P 3.5 : 1 4:1 4:1
D/S 2.5 : 1 2.5 : 1 3:1 2.5 : 1 3:1 3:1
7–10 U/S P 3:1 3:1 3.5 : 1 P 3.5 : 1 4:1 4:1
D/S 3:1 3:1 3:1 3 : 1 3.5 : 1 3.5 : 1
Notes: U/S = upstream slope & D/S = downstream slope.
Rapid drawdown rates of 1000 mm or more per day may follow prolonged storage at high water level.
P (pervious) denotes soils which are not suitable. OL and OH soils are not recommended for major portions of homogenous
earth-fill dams. Pt (Peat) soils are unsuitable.
See Table 2.3 for soil groups.

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 48 • Farm Dams

The batter slopes for most embankments on strong foundations can be 3:1
(horizontal to vertical) upstream and 2:1 (horizontal to vertical) downstream.
However, flatter batter slopes should be considered for dams structured on inorganic
clay or highly plastic and very fine inorganic silt. Organic soils are not useable as an
embankment material. They tend to be placed on the outside batters to establish
grassy vegetation. They act as a blanket by reducing internal dam moisture losses.
The batter slopes of a dam depend primarily on the stability of the material in
the embankment. The greater the stability of the fill material, the steeper the slopes
may be. The more unstable materials require flatter slopes. Table 3.1 contains
recommended maximum slopes for the upstream and downstream faces of dams
constructed of various materials.

3.1.4 Crest width

The crest width should be increased as the height of the dam increases. The
generally accepted empirical formula for crest width is:-
Crest width (m) = H0.5 + 1
where H is the height of the crest of the embankment above the bed of the gully
in metres.
The minimum crest width should be about 2.5 m irrespective of H, so that
machinery can work on the crest.
Table 3.2 contains recommended top widths for embankments of various
heights based on above formula.

Table 3.2 Height to crest width

Dam height (m) Crest width (m)

3 2.75
4 3.00
5 3.25
6 3.50
7 3.65
8 3.85
9 4.00
Source: SCA, 1983.

Where the top of the embankment is to be used as a road access track then the
top width should provide for a shoulder on each side of the roadway. The crest
width in such cases should not be less than 4 m. An exception to this is a turkey’s
nest dam where usually the embankment is not high, and filling of the storage (by
pumping) is under control. Since there are no hazards, the crest width may be
reduced to as little as 1.5 m, if necessary (and if plant is available to work or trim
such a narrow crest).

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 Design • 49

3.1.5 Freeboard
Care must be taken to allow for wave action, especially on dams that have 0.7 km
or more of exposed water surface. For less than 0.7 km see Figure 3.4. Erosion by
wave action has certainly occurred on some of the large ‘shallow storages’ and on
large ring tanks. It is accentuated on very friable soils and/or fast slaking clays. It
is important to establish good grass protection on the upstream batter (as difficult
as this might be with changing water levels) to minimise wave erosion. Rock-
beaching would be the conventional engineering solution to wave erosion. However,
this is too expensive for most farm dams. A cheaper and more risky alternative—
trusting the landowner to do everything possible to establish the necessary grass
cover, and to replace eroded material when necessary—is recommended.
Freeboard is the added height of the dam provided, as a safety factor, to
prevent waves or run-off from storms greater than the design allows from
overtopping the embankment. It comprises the vertical distance between the
elevation of the water surface in the dam, when it is full, and the elevation of the
top of the dam after all settlement has taken place. A large number of dams have
failed due to overtopping and consequently greater attention must be paid to this
feature. Freeboard should not be less than 1.0 m and should include provision for:
a depth of flood surcharge to pass water through the spillway;
b wave action, which can be calculated from Hawksley’s formula:
H = 0.0138√F
where H = wave height in metres,
F = fetch distance (the longest exposed water surface in metres).
See Figures 3.3 and 3.4;
c an additional allowance of 0.3 m for unevenness in the crest level.
Using these provisions:
Freeboard (m) = depth of flood surcharge + wave action + 0.3.

Figure 3.3 Wave action

0.40

0.35
Height of wave (metres)

0.30
0.25

0.20
0.15
0.10

0.05

0.00
0 100 200 300 400 500 600 700
Length of fetch (metres)

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 50 • Farm Dams

Figure 3.4 Wave action based on fetch distance across the storage

Fetch distance
Full supply level

Embankment Spillway

3.1.6 Alternative ways of batter protection

A farm dam should not be considered complete until proper protection from
erosive wave action, livestock and other sources of damage has been provided.
Dams that lack such protection may be short-lived, and the cost of maintenance
is usually high. In most areas, the exposed surfaces of the dam, spillway, borrow
areas and other disturbed surfaces can be protected against erosion by establishing
a good cover of sod-forming grass. Occasionally, there is a need for better
protection against wave action than will be provided by grass cover. Methods used
to provide this protection include earth berms, log booms, and rock riprap.
i Berms
A berm, 2.5 to 3 m in width and located at normal dam level, will often provide
adequate protection from wave action. The face of the dam above the berm should
be protected by vegetation.
ii Floating booms
A boom may consist of a single or double line of pine logs chained together and
securely anchored to each end of the dam. The boom should be tied end-to-end as
close together as practical. There should be enough slack in the line to allow the
boom to adjust itself to fluctuating levels in the dam. Double rows of logs should
be framed together to act as a unit. The boom should be placed in order to float
about 1.5 m upstream from the face of the dam for best results. In the case of a
curved dam, anchor posts may be required at the ends in order to prevent the
boom from riding on the slope. Booms afford a high degree of protection and are
relatively inexpensive, especially in areas where timber is readily available.
iii Rock-beaching
Where the water level in the dam can be expected to fluctuate widely, or where a
high degree of protection is required, the use of rock-beaching is a most effective
method of control (Figure 3.5). Rock-beaching should extend from the top of the

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 Design • 51

Figure 3.5 Typical rock-beaching

Crest

Freeboard
Full supply level
Rock-beaching layer
(large rocks keyed in place)
Depth of storage
fluctuation Embankment

Filter layer or bedding

(composed of sand and gravel)

Source: Modified SCS USDA, 1969.

dam, down the upstream face to a level at least 1 metre below the lowest expected
level of the water in the dam. Rock-beaching may be placed by machine or by
hand. Machine placing requires more stone but less labour. The layer of stone
should be durable and large enough not to be displaced by waves. Where rock-
beaching is not continuous with the upstream toe, a berm should be provided on
the upstream face to support the layer of rock-beaching. In some circumstances
graded gravel filters may be required under the rock-beaching layer.

iv Other methods
Other methods include increasing the crest width of the dam, flattening the
upstream slope of the embankment, and applying a layer of coarse sand and gravel
on a 10:1 (horizontal : vertical) slope. These methods are applicable to arid areas
where vegetation is not reliable and rock and timber is not readily available.
Old tyres tied together and secured to the dam face have also been used to
maintain vegetation cover (Figure 3.6; see also Section 3.1.8 on fencing).
3.1.7 Topsoil cover
Prior to the placement of topsoil onto the compacted embankment, the surface
should be roughened to assist in combining the different soil types. Topsoil must
be placed over the entire embankment to a depth of at least 100 to 150 mm and
grassed with a good holding grass. The purpose of the topsoil cover is to:
• reduce surface erosion on either side of the batter slope;
• minimise surface cracking in the embankment;
• lessen the tendency of the surface material in contact with storage water being
dispersed; and
• lessen wide fluctuations in embankment moisture content.
3.1.8 Fencing
The complete fencing of embankment type dams is recommended where livestock
are grazed or fed in adjacent areas. Fencing provides the protection needed to
develop and maintain vegetation cover. When combined with a water facility

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 52 • Farm Dams

Figure 3.6 Floating tyre systems

Tyres fixed to batter
slope at full supply
Tyres on upstream batter slope level

Bank crest

Separation depends on
water level fluctuations

Floating tyres
with tubes
fixed to anchor
on batter slope

Wind direction

Source: Floating Tyre System, Pty Ltd.

below the dam, fencing allows good quality drinking water and eliminates the
danger of pollution by livestock. Fencing also improves wildlife habitat.

3.2 Flood flow estimation

All dams formed by embankments across natural drainage ways require the
protection of a carefully designed spillway or a combination of spillways. The
function of outlet structures is to pass storm run-off around or under the
embankment to prevent overtopping. In addition, the structures must convey
water from the dam safely to a stable outlet below, without damaging the
downstream slope of the embankment. Spillways can consist of more than one
component: trickle pipes, drop inlet structures, chute or earth spillways (see
Section 3.3).
Spillway design dimensions are linked to the size and the character of the
catchment. A catchment with rocky or steep surfaces (and therefore high run-off)
will have higher peak floods than a catchment of the same area, within the same
climatological zone and with flatter, well-vegetated slopes. Similarly, a long
narrow catchment will have a greater time of concentration of floodwater after a
rainstorm than a broad catchment with the same area, climate, gradient and
vegetation.
The peak flood is the maximum flood to be expected from a catchment
following a rainfall of estimated intensity and duration for a selected return
period. In many parts of the world information is not available or smaller streams
are not gauged to allow estimation of such floods for spillway design purposes. It
is economically prudent to study the hydrology, climate, and topography of a
catchment supplying run-off to a large dam in order to determine reasonably
accurate estimates of peak floods. However, for smaller dams, unless this

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 Design • 53

information is already available, the engineer can rarely justify the cost of such an
exercise and must resort to other means to estimate the maximum possible flood.
Where no other data is available, a very approximate peak flood estimate can
be made by taking the highest daily rainfall figure for the catchment and making
the assumptions that all dams in the same catchment are 100 per cent full, the
ground is saturated, and 100 per cent run-off will occur.
An important element in designing spillways of a dam is to establish run-off
within a specified return period (recurrence interval). Selection of a return period
depends on the economic balance between cost of periodic repair or replacement
and the cost of providing additional capacity to reduce the cost of repair or
replacement. In some instances, the possibility of causing damage to other works
by a failure may dictate the choice of the return period.
To protect a dam, flood flows entering the storage must be able to pass around
the embankment by a spillway or, as it is sometimes called, a by-wash. Most
spillways on farm dams are cut into the earth because concrete is too expensive.
They are generally designed to pass a 2 per cent probable flood. This refers to a
flood so large that there is only a 2 in 100 chance of it occurring in any year.
The generally accepted flood frequency return periods used are:
Minor dams (depending on consequences of overtopping) 10–50 years
Detention dams 50–100 years
Large dams >100 years
3.2.1 Peak flow estimation
The wide range of variation of Australian climatic conditions makes it difficult to
provide a simple method for estimating flood flows that can be applied generally for

Table 3.3 Typical design discharge data for 50-year ARI

“CY” Design run-off coefficient contours (%)
0.05 0.10 0.15 0.20 0.25 0.30 0.35
Catchment area Rainfall “QY” Spillway design rainfall intensity
(ha) intensity (m3/s)
(mm/hr)
50 65 0.5 1.1 1.6 2.2 2.7 3.4 3.8
100 60 1.0 2.0 3.0 4.0 5.0 6.0 7.0
150 55 1.4 2.8 4.1 5.5 6.9 8.3 9.6
200 50 1.7 3.3 5.0 6.7 8.3 10.0 11.7
250 48 2.0 4.0 6.0 8.0 10.0 12.0 14.0
300 46 2.3 4.6 6.9 9.2 11.5 13.8 16.1
350 44 2.6 5.1 7.7 10.3 12.8 15.4 18.0
400 42 2.8 5.6 8.4 11.2 14.0 16.8 19.6
450 41 3.1 6.2 9.2 12.3 15.4 18.5 21.5
500 40 3.3 6.7 10.0 13.3 16.7 20.0 23.4
750 38 4.8 9.5 14.3 19.0 23.8 28.5 33.3
1 000 36 6.0 12.0 18.0 24.0 30.0 36.0 42.0
1 250 34 7.1 14.2 21.3 28.4 35.4 42.5 49.6
1 500 33 8.3 16.5 24.8 33.0 41.3 49.5 57.8
Source: Lewis, 2001.
Note: For details of “Cy” Design run-off coefficient contours, see Australian Rainfall and Run-off (ARR)—A Guide to Flood
Estimation, Volume No. 2.

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 54 • Farm Dams

small to medium-sized rural catchments. The methods more often used are based on
Australian Rainfall and Run-off, A Guide to Flood Estimation (IEA, 1987). For an
understanding of the methods discussed and selection of appropriate design values,
it is important that the probabilistic nature of the methods is recognised.
Engineers can, in most cases, calculate peak discharge for flood flow conditions
based on available observed flood flow data in the region of interest. For most of
Australia, sufficient data is available for the derivation of methods that reasonably
reflect reality. The methods in Australian Rainfall and Run-off (ARR) take into
consideration rainfall intensity, catchment characteristics and size, average slope
of the waterway and its length from source to the dam site. These same methods
of estimating peak discharge are used by local government and drainage and rural
water authorities for the design of most structures on waterways.
Estimation of required spillway size for farm dams is given in Section 3.3.3.
This is based on design discharge for small to medium rural catchments for an
Average Recurrence Interval (ARI) of 50 years, based on ARR (ibid.). This has
been calculated in Table 3.3 (Lewis, 2001a). The information in the tables should
only be used as a guide for assessment. An engineer experienced in hydrology and
farm dam design should be consulted before any works are commenced.
The formula as used in design of Table 3.3 is:
QY = 0.278 CY. It .A
c.Y
where QY = peak flow rate (m3/s) of average recurrence interval (ARI)
of Y years
CY = run-off coefficient (dimensionless) for ARI of Y years
It = average rainfall intensity (mm/h) for design duration of tc
c.Y
hours and ARI of Y years
A = area of catchment (km2). If area is in hectares instead of
km2, the conversion factor is 0.00278 (or 1/360).

3.3 Outlet structures

There are various types of components, storm run-off, compensation flows and
environmental flows that are incorporated into an embankment to allow for the
passing of water flows under a licence condition. Some of the common ones are
explained in the rest of this section. The tables and information provided should
only be used as a guide in the selection of sizes of pipes and spillway dimensions.
It should be remembered that these numbers are not necessarily applicable to all
cases. They are general values rather than site-specific. It is recommended that an
engineer be employed to advise on these types of issues when the dam is being
designed.
3.3.1 Earth spillways
An earth spillway is an earth or vegetated channel, designed to discharge the peak
flow calculated for the catchment. Where catchments are small and long duration
flows are not a problem, it may be feasible to handle the run-off safely with only
a vegetated spillway.
Earth spillways, as discussed here, apply to both vegetated and non-vegetated
spillways, the latter being used where climatic or soil conditions make it impossible
to grow or maintain a suitable grass cover. Earth spillways are usually excavated,

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 Design • 55

Figure 3.7 Spillway cross-section

Natural surface

Freeboard minimum Top of bank

depth of 1 metre

H = Surcharge depth Batter slopes 2:1

of flood discharge (horizontal:vertical)
in spillway (m)
B = Spillway width (m)

but may exist as a natural spillway on a well-vegetated saddle or drainage line. In

either case, the spillway must discharge the design peak flow at a non-erosive
velocity to a safe point of release. Exiting earth spillways, whether vegetated or non-
vegetated, should not be built on fill material.
Earth spillway structures have certain limitations. They should be used only
where the soils and topography permit safe discharge of the peak flow at a point well
away from the dam at a velocity that will not cause appreciable erosion. Temporary
flood storage provided in the dam has been used to reduce the design flow or
frequency of use in the spillway, if a trickle pipe is incorporated in the design.
3.3.2 Design spillway capacity
Emergency earth spillways should have the capacity to discharge the peak flow
from the catchment resulting from a storm. The procedure for determining peak
flood flow is presented in Section 3.2 of this book.
The definition of an excavated earth spillway consists of the four elements in
Figures 3.8. These are:
• approach (entry) channel
• control section
• exit channel, and
• spill section.
Each element has a special function. The flow enters the spillway through the
approach channel. The flow is controlled in the level portion and exit section and

Figure 3.8 Earth spillway

Batter slope 2:1
(horizontal:vertical)

Spillway

Outlet width
Inlet
am return
width of d
p
To

Return slope grade (%)

Embankment

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 56 • Farm Dams

passes through critical depth at the downstream edge of the level portion. The
flow is then discharged through the spill section.
The direction of slope of the spill section must be such that overflow will not
flow against any part of the embankment. Wing banks may be used to direct the
outflow to a safe point of release. The floor of the spillway should be excavated
into original earth for the full design width. Where this is not practical, the end of
the dam embankment and any earth fill constructed to confine the flow, should be
protected by vegetation or riprap. It is desirable that the entrance to the approach
channel be widened so it is at least 50 per cent greater than the designed bottom
width at the control section (Figure 3.7). The approach channel should be
reasonably short and should be planned with smooth, easy curves for alignment.
It should have a slope toward the dam of not less than 2 per cent except in rock,
to ensure drainage and low inlet losses.
The control section should be located near the intersection of the extended
centre-line of the dam with the centre-line of the spillway. A level section, at least
6 m in length, should be provided immediately upstream from the control section.
The exit spillway must have a slope that is adequate to discharge the peak flow
within the channel. A slope which is adequate for drainage is generally sufficient

Table 3.4 Minimum inlet width of spillway

Spillway Depth of flow (m)
discharge(m3/s)

0.30 0.40 0.50 0.60 0.70 0.80

Inlet width (m)

1 4 4 4 4 4 4
2 7 5 4 4 4 4
3 11 7 5 4 4 4
4 15 10 7 4 4 4
5 19 12 8 6 5 4
6 23 15 10 7 6 4
7 27 17 12 9 7 5
8 31 20 14 10 8 6
9 35 22 16 12 9 7
10 39 25 18 13 10 8
12 47 30 21 16 12 10
14 35 25 19 14 11
16 40 28 21 17 13
18 45 32 24 19 15
20 50 36 27 21 17
22 39 30 23 19
24 43 32 25 20
26 47 35 28 22
28 50 38 30 24
Source: Modified WAWA, 1991.
Notes: i These widths are for well grassed spillways. Poorly grassed should be wider.
ii For practical purposes the minimum width has been taken as 4 m and the maximum 50 m.
iii The inlet width in the shaded area should be two-thirds of the outlet width obtained from Table 3.5.

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 Design • 57

(for example, 0.2 per cent). The exit spillway should be straight or gently curved
to spread the flow to a uniform depth over the lip of the spill section.
The spill section is the natural slope over which the peak flood-flow passes.
The width of the spill section is selected so that the permissible velocities for the
soil type, or the planned grass cover, are not exceeded.
3.3.3 Selecting spillway dimensions
With the required discharge capacity (Q) known (see Section 3.2) and the natural
slope of the spill section determined from plotting the spillway centre-line profile,
the bottom width of the control section, the width of the spill section and the
depth of the flow can be selected. Tables 3.4 and 3.5 give the appropriate control
section width for discharge capacity at various depths of flow. A spillway requires
a level section with a minimum length of 8 m and a cross-sectional area calculated
from the formula:
Q = 1.546 BH1.5
where Q = either a 1 in 50 or 1 in 100-year peak discharge (m3/s)
B = spillway width (m)
H = surcharge depth of flood discharge in spillway (m).

Table 3.5 Minimum outlet width of spillway

Spillway Return slope (m)
discharge (m3/s)

3% 4% 5% 6% 7% 8% 9% 10% 15% 20%

Outlet width (m)

1 6 6 6 6 6 6 6 6 6 6
2 6 6 6 6 6 6 6 6 7 9
3 6 6 6 7 8 8 9 10 14 18
4 6 6 8 9 10 11 13 14 19 23
5 6 8 10 11 13 14 16 17 23 29
6 8 10 12 14 16 17 19 20 28 35
7 9 12 14 16 18 20 22 24 33 41
8 11 13 16 19 21 23 25 27 37 47
9 12 15 18 21 24 26 29 31 42
10 13 17 20 23 26 29 32 34 47
12 16 20 24 28 31 35 38 41
14 19 24 28 33 37 41 45 48
16 22 27 33 38 42 46
18 25 31 37 42 47
20 27 34 41 47
22 30 38 45
24 33 41 49
26 35 45
28 39 48
Source: Modified WAWA, 1991.
Notes: i These widths are for well grassed spillways. Poorly grassed should be wider.
ii Lined spillways may be more economic in the range above 47 m.

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 58 • Farm Dams

Spillway side slopes should be no steeper than 2:1 (Figure 3.7) unless the
spillway is excavated into rock, in which case the side slopes may be vertical.
Usually, the selected bottom width of the channel should not exceed 35 times the
design depth of flow. Where this ratio of bottom width to depth is exceeded, the
channel is likely to be damaged by meandering flow and accumulated debris.
Whenever the required bottom width of the spillway is excessive, consideration
should be given to the use of a spillway at each end of the dam. These two
spillways need not be of equal width so long as their total capacity meets
requirements. In cases where the required discharge capacity exceeds the ranges
shown (Tables 3.4 and 3.5) or topographic conditions will not permit the
construction of the exit channel bottom with a slope that falls within the required
ranges, the need for a piped or concrete lined spillway is indicated.
Wherever there is a good vegetative cover in the spillway area and the
topography (such as a natural saddle) is suitable, first consideration should be
given to the use of a natural spillway.
3.3.4 Chute spillways
Chute spillways on farm dams are not common because of their high cost, but
they may be used where grassed spillways are not possible. They are commonly
built of concrete, but may be of sheet steel, bituminous concrete, or other erosion
resistant material such as plastic sheeting. A natural rock surface, or a surface
allowed to erode down to hard rock may also be regarded as a chute spillway.

Figure 3.9 Chute spillway

Inlet at
full supply Crest
level

Cut-off wall Downstream batter slope

2:1 maximum

Vertical side wall

Stilling blocks
before exit from
spillway chute

Cut-off wall Rock-beaching

at outlet

The materials used in chute spillways are harder, and more expensive than
earth. Therefore:

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 Design • 59

i high velocities can be tolerated because of better erosion resistance;

ii greater depth of flow and higher velocities are used to reduce the overall size;
and
iii use of efficient hydraulic shapes to further reduce the size and cost becomes
important.
Thus, instead of spreading the flow as widely as possible for grassed earth
spillways, a narrow and deep chute is usually used. The inlet should occur over a
relatively short distance in the direction of flow to achieve a high weir discharge
coefficient. Additional surcharge, and therefore freeboard on the embankment,
will normally be economical to reduce the width of the inlet. The plan shape of
the spillway will normally be wider at the inlet than further down the slope to take
advantage of the accelerating flow velocities down the slope and thereby reducing
the amount of concrete required.
Finally, because of the smooth, high-velocity flow down the chute, the water
arrives at the bottom with high kinetic energy per metre width. This energy has to
be dissipated in a structure specifically designed for the spillway if serious
downstream erosion is to be avoided. However, this is not usually necessary for
grassed spillways because most of the energy is dissipated on the way down the
slope (SCA, 1979). See Figure 3.9 above.

3.4 Pipelines through embankments

Pipelines (see figure 3.10 below) are often placed through or under embankments
for any of the following purposes:
Figure 3.10 Inlet and outlet pipeline layout
2.0 m

Strainer

Embankment Natural surface 500 mm allowance for silt

Cut-off collar

Outlet pipe
1000 mm

Flow

500 mm

1000 mm Concrete anchor block

INLET DETAIL

3.0 m 1.0 m
Gate valve
Embankment

Cut-off collar
Outlet pipe
Stone beaching
Flow

Concrete apron
Concrete anchor block (500 mm square)
2.5 m
Cut-off wall
OUTLET DETAIL

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 60 • Farm Dams

• to provide a gravity supply from the storage;

• as a suction pipe for a pumped supply (this always maintains a positive prime
to the pump);
• to maintain flow downstream from the storage; and
• to bypass a significant flood-flow through a piped spillway.
Pipelines may be constructed using reinforced concrete pipes (rubber-ring
joints), cement-lined cast iron pipes, galvanised or black steel pipes, high density
polyethylene or PVC pipes.
Special care needs to be taken with joining of rubber-ring jointed pipes. An
anchor block at the outlet valve is essential to avoid movement of the pipes. If
concrete pipes are used, the class of the pipe suitable for the depth of fill above
must be used. PVC is liable to brittle fracture if impacted by construction
equipment. All pipes should be pressure-tested to check welds, joints and seams
on placing. In addition, a further test may be required after covering the pipe is
completed, taking into account:
• the proposed full supply level;
• the superimposed load on the pipe;
• permit conditions if applicable; and
• siltation of the entry.
The pipe should be located underneath the embankment in a trench excavated
in natural material. If required, the embankment core trench should be deepened
where it crosses the outlet pipe to a depth of at least 0.6 m below the pipe. If outlet
levels permit, the pipe should be located so that the tops of the core collars are at
or below the natural surface to minimise the risk of damage by construction
equipment. The barrel of the pipe should be evenly and firmly bedded in the
trench and should not be laid directly on rock or stony ground.
Cut-off collars are required to prevent seepage flows along the pipe. The pipe
should always be laid in a trench, and the cut-off collars should extend down and
sideways (1 m2), well into natural ground or fully compacted material. An
absolute minimum of two and preferably three cut-off collars should be provided.
The pipe should be positioned so that it is trenched into natural ground, and
so that the inlet strainer extends into an excavated area, in order to avoid blockage
by silt. If this is not possible, the use of a bent pipe with a vertical strainer may be
necessary.
For small pipes (150 mm), heavy galvanised steel is usually used. For larger
pipes, high-density polyethylene with concrete surround will probably be the most
economical (in terms of installation) and will have the best life. Reinforced
concrete with rubber ring joints can be used with care and adequate thrust and
anchor blocks. Class–12 PVC (with concrete surround) can be used, but is not
favoured. The chance of brittle fracture during construction is quite high. The type
and class of pipe used must be selected on the basis of its suitability for the height
of fill and bedding conditions.
Pipe smaller than 150 mm in diameter should not be used because of the
danger of becoming clogged. The crest elevation of the emergency spillway should
be located a distance above the invert or crest elevation of the trickle pipe inlet at
least equal to the value of the minimum head required to provide full pipe flow.
3.4.1 Trickle pipes
A small pipe spillway is provided in many farm dams to protect the vegetative
cover in the emergency spillway from prolonged saturation by continuous flow,

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 Design • 61

spring flow, or flows that may continue for several days following a storm. This
type of spillway is designed to discharge such a small percentage of the peak flow
that it has no measurable effect on the emergency spillway design (see Section 6.3.6,
Figure 6.10).
The design capacity of a trickle pipe should be adequate to discharge long-
duration, continuous, or frequent flows without flow through the earth spillway.
3.4.2 Drop inlet structures
Generally a drop inlet structure consists of a vertical pipe (concrete or steel)
connected to a horizontal pipe (fitted with cut-off collars) which passes through
the bank and takes the flow away clear of the toe. These structures can be most
useful on a storage with a small catchment area where:
• all the flood flows might be flood-routed in surcharge and all the discharge can
go through a drop inlet;
• on a steep gully where a normal spillway is not practicable; or
• where the gully has an appreciable base flow.
Two precautions are taken to prevent unstable (full pipe) flow conditions from
occurring:
i There must be a significant gradient on the outlet pipe towards the
downstream end so that any flow entering the pipe through its inlet orifice can
flow away at less-than-full pipe flow. The entrance should be sharp, not
smoothly transitioned, to retain the orifice effect.
ii The top of the outlet pipe must be ventilated immediately downstream of the
drop pipe so that any air which is evacuated by mixing with the water in the
outlet pipe is immediately replaced at atmospheric pressure. This vent pipe has
to extend above maximum surcharge level of the storage.
It is also an advantage to have the drop pipe diameter about three or more
times the diameter of the outlet pipe, so that weir flow is likely to be the discharge
control under most surcharge conditions. This ventilated system will have less
capacity than one of the same size allowed to flow full, but is a much more

Figure 3.11 Drop inlet structures

Upstream batter

Crest
Trash rack
Downstream batter

Compacted embankment
Drop
inlet
Outlet pipe

Breathing pipe Seepage

toe drain

Cut-off collars Rock-beaching at

stilling pool on
return to waterway
Source: Modified SCS USDA, 1969.

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 62 • Farm Dams

satisfactory and hydraulically stable system in practice. If greater capacity is

required, a larger sized pipe should be used.
Calculations of discharge and head through drop inlet spillways have to be
done as a three-stage process checking:
1 the head-discharge over the inlet weir;
2 the head-discharge at the orifice entry of the outlet pipe; and
3 the flow conditions in the outlet pipe, to ensure part-full flow.
The weir is essentially a pipe spillway with a significant flow, that is the flow
is taken into account when determining the emergency spillway.

3.4.3 Cut-off collars

The object of cut-off collars is to prevent seepage along the barrel of the pipe (see
Table 3.6).

Table 3.6 Separation distance of cut-off collars

Length of pipe (m) <20 20 25 30 40 60

Number of collars 3 4 5 6 8 12

Note: An absolute minimum of 3 cut-offs located upstream should be used. Cut-off collars may be constructed of concrete,
mild steel plate or PVC.

Figure 3.12 Pipeline through embankment with cut-off collars

Crest Low-level outlet pipe

Foundation level
Inlet
Outlet valve A
Natural surface

6m
Compacted soil
= = =
around pipe or
Outlet rock-beaching Cut-off collars concrete encased

A1 Cut-off trench

150 mm 150 mm

Natural surface
200 mm
Compacted soil around pipe
or concrete encased Pipe size (mm)
150 mm
300 mm
Cut-off collar 1 m square
400 mm

300 mm 300 mm

ENLARGEMENT DETAIL OF A–A1

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 Design • 63

3.5 Earth and water computations

3.5.1 Embankment material
The estimate of the volume of material required should include the embankment,
allowance for settlement (see Section 5.7), backfill core trench (see Section 5.4.5),
stripping topsoil (see Section 5.4.4), backfill gullies and any other banks that the
contractor is required to construct, for example, diversion banks.
Volume estimates for dams are made on the basis of cubic metres (m3) of earth
fill in place, that is, compacted volume and not excavated volume or loose
(transported) volume. The symbols used for the calculation of all earthwork
computations in this Section are shown in Figure 3.13.
Formula for calculating the volume of earthwork:
This formula can be stated as:
V = 0.5 P × ( A l + A 2 ) (1)
where V = volume in cubic metres (m3)
Al & A2 = cross-sectional area at different points along an
embankment (m2)
P = perpendicular distance between end areas A l & A 2 (m).
Figure 3.13 Computational symbols

P P P P P

H1 H2 H3 H4

A1 A2 A3 A4

Cross-sectional view A-A1 of embankment

T
L
b
a H
Hw

A1
Longitudinal view – embankment and storage

Cw

Hw H
Top of embankment
Spillway Full supply level
B

Embankment symbols used in formula

Source: SCA, 1983.

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 64 • Farm Dams

For more than two cross-sections then:

V = volume (m3 ) = P × (0.5 A1 + A2 + A3 + …An-1 + 0.5An) (2)
where n = number of cross-sections A1, A2 ,A3 …An involved.
Table 3.7 can be used to give the cubic metres per metre length for Al , A2, A3
and A4 for the different crest width, fill height and batter slopes of an embankment.

Example:
An estimate of the volume (m3) of material for an embankment in a gully is
required. The following information has been provided in Figure 3.13.
Cw = Embankment length = 25 m
T = Crest width = 3 m
H = Fill heights (m): H1 = 4.00, H2 = 4.5, H3 = 4.25 and H4 = 3.75
Batter slopes: a = 3: 1 (U/S), b = 2:1 (D/S)
As the 4 cross-sectional areas are evenly spaced P = 5 m; otherwise P the
perpendicular distance between end areas (Al , A2, A3 & A4) would need to be
calculated separately for each segment.
By using information provided, that is, crest width (T), fill height (H), batter
slope (‘a’ U/S & ‘b’ D/S), and Table 3.7—fill required for earth dams, the volume
(cubic metres per metre—m3/m) can be obtained between each cross-sectional area
for use in formula (2):
Areas at each section A1, A2, A3 and A4 from Table 3.7 are:
A1 = 52.0 (m3/m),
A2 = 64.1(m3/m),
A3 = 57.9 (m3/m),
A4= 46.4 (m3/m)
Substitute this data into formula (2) gives:
V = P × (0.5 A1 + A2 + A3 + 0.5A4)
= 5 × (0.5 × 52.0 + 64.1 + 57.9 + 0.5 × 46.4)
= 5 × 171.2
= 856 m3
3.5.2 Floor slope
All methods of calculating volumes assume a horizontal valley floor. This is
generally not the case. For floor slopes up to 5 per cent and banks of up to 6 m
high the difference in volume is not significant (less than 4 per cent).
3.5.3 Area beneath embankment
It is sometimes necessary to know the volume of stripping required from beneath
the embankment. This volume can be calculated by the use of the formula:
Volume = depth of stripping x area beneath banks
V = S × P CT + (a + b) ΣH
{P } (3)

where V = volume (m3)

S = depth of stripped topsoil (m)
P = distance between sections (m)

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 Design • 65

C = length of embankment (m)

T = crest width (m)
a + b = upstream + downstream slopes
ΣH = sum of heights (m) at all sections
i.e. 3H :1V = a = 3, 2H :1V = b = 2

Table 3.7 Fill required for earth dams

Crest width (T)
2.5 (m) 3 (m) 3.5 (m)

Batter slopes (a = Upstream : b = Downstream, i.e. Horizontal : Vertical)

(a) U/S 31 ⁄ 2:1 3:1 3:1 3:1 31 ⁄ 2:1 3:1 3:1 3:1 31 ⁄ 2:1 3:1 3:1 3:1

(b) D/S 3:1 3:1 21 ⁄ 2:1 2:1 3:1 3:1 21 ⁄ 2:1 2:1 3:1 3:1 21 ⁄ 2:1 2:1

Fill Volume of fill required

height cubic metres per metre (m3 per m)
(m)
0.25 0.83 0.82 0.80 0.79 0.95 0.94 0.92 0.90 1.08 1.06 1.05 1.03
0.50 2.1 2.0 1.9 1.9 2.3 2.3 2.2 2.1 2.6 2.5 2.4 2.4
0.75 3.7 3.6 3.4 3.2 4.1 3.9 3.8 3.7 4.5 4.3 4.2 4.1
1.00 5.8 5.5 5.3 5.0 6.3 6.0 5.8 5.5 6.8 6.5 6.3 6.0
1.25 8.2 7.8 7.4 7.0 8.8 8.4 8.0 7.6 9.5 9.1 8.7 8.3
1.50 11.1 10.5 9.9 9.4 11.8 11.3 10.7 10.1 12.6 12.0 11.4 10.9
1.75 14.3 13.6 12.8 11.9 15.2 14.4 13.7 13.0 16.1 15.3 14.5 13.7
2.00 18.0 17.0 16.0 15.0 19.0 18.0 17.0 16.0 20.0 19.0 18.0 17.0
2.25 22.1 20.8 19.5 18.2 23.2 21.9 20.7 19.5 24.3 23.1 21.8 20.5
2.50 26.6 25.0 23.4 21.9 27.8 26.3 24.7 23.1 29.1 27.5 25.9 24.4
2.75 31.5 29.6 27.7 25.8 32.8 30.9 29.0 27.1 34.2 32.3 30.4 28.5
3.00 36.8 34.5 32.2 30.0 38.3 36.0 33.8 31.5 39.8 37.5 35.3 33.0
3.25 42.5 39.8 37.2 34.2 44.1 41.4 38.8 36.2 45.7 43.1 40.4 37.7
3.50 48.6 45.5 42.4 39.4 50.3 47.3 44.2 41.1 52.1 49.0 45.9 42.9
3.75 55.1 51.6 48.0 44.4 57.0 53.4 49.9 46.4 58.8 55.3 51.8 48.3
4.00 62.0 58.0 54.0 50.0 64.0 60.0 56.0 52.0 66.0 62.0 58.0 54.0
4.25 69.3 64.8 60.3 55.8 71.5 66.9 62.4 57.9 73.6 69.1 64.5 60.0
4.50 77.1 72.0 66.9 61.9 79.3 74.3 69.2 64.1 81.6 76.5 71.4 66.4
4.75 85.2 79.6 73.9 68.3 87.6 81.9 76.3 70.7 90.0 84.3 78.7 73.0
5.00 93.8 87.5 81.3 75.0 96.3 90.0 83.8 77.5 98.8 92.5 86.3 80.0
5.25 105.3 98.4 91.5 84.6 108.0 101.1 94.2 87.3
5.50 114.8 107.3 99.7 92.1 117.6 110.0 102.4 94.5
5.75 124.7 116.4 108.2 100.0 127.6 119.3 111.0 102.7
6.00 135.0 126.0 117.0 108.0 138.0 129.0 120.0 111.0
6.25 145.7 135.9 148.8 139.1
6.50 156.8 146.3 160.1 145.9
6.75 168.3 156.9 171.7 160.3
7.00 180.3 168.0 183.8 171.5
Source: SCA, 1983.

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 66 • Farm Dams

Example:
Use the data from the example in Section 3.5.1 to find the volume (m3) of material
to be stripped.
S = 0.10 m
P=5m
C = 25 m
T=3m
ΣH = 4.00 + 4.50 + 4.25 + 3.75 = 16.5 m
3H :1V is 3, 2H :1V is 2, so a + b = 5
Substitute data into formula (3) gives:
V = S × P CT + (a + b) ΣH
{ P }
V = 0.1 × 5 (25 × 3) + 5 × 16.5
{5 }
3
= 48.7 m
3.5.4 Excavated tanks
Volumes of excavated tanks should be calculated by the prismoidal formula which
is:-
d (Al + 4Am + A2)
V= (4)
6
V = volume (m3)
d = depth between Al and A2 (m)
Al = top natural surface area (m2)
Am = mid area (m2)
A2 = bottom excavated area (m2)

3.5.5 Water storage capacity computations

Storage volume for small gully dams can be approximated by the use of either
formula (5) or (6):-
(Hw × L) (2B + Cw)
V= (5)
6 × 1000
where the symbols are shown in Figure 3.13 and
V = capacity in megalitres (ML)
Hw = depth of water at Full Supply Level at embankment (m)
L = distance from embankment to tailwater at Full Supply Level (m)
B = length of embankment at bed level (m)
Cw = length of embankment at Full Supply Level (m)

Example:
Use the data from the example in Section 3.5.1 to find the volume (m3) of water
stored.
Hw = 3.5 m (includes 1.00 m freeboard)
L = 50 m
B = 15 m
Cw = 25 m

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 Design • 67

Substitute values into formula (5):

(Hw × L) (2B + Cw)
V=
6 × 1000

(3.5 × 50) (2 × 15 + 25)

=
6 × 1000
= 1.6 ML

K × A × Hw
V= (6)
1000

where V = capacity in megalitres (ML)

A = surface area of water stored at full supply level (m2)
Hw = depth of water at embankment (m)
K = constant with default value 0.4. The value of K increases as the
side slopes of land become steeper, reaching K = 1 for vertical
sides.
Example:
K = 0.4
A = 50 × 25 = 1250 m
Hw = 3.5 m
Substitute data into formula 6:
0.4 × 1250 × 3.5
V=
1000
= 1.75 ML
When an accurate assessment of the storage volume is required (e.g. irrigation
dam), a contour plan should be produced by grid survey or tacheometry.
Storage volume can then be calculated by the use of a planimeter and end area
formula.
All the above measurements can be made on site with a level or theodolite. This
can be achieved either in the form of a cross-section survey at the centre-line of the
proposed dam or by a contour survey. The latter approach is more accurate and
time consuming, but also more useful where comparison of similar sites is being
undertaken.
Using the formula, approximate figures for the capacity at various heights of
dam can be derived. The capacity estimated in this way is accurate to within 20
per cent, but it must be revised by a more detailed survey when the site has been
approved for possible construction. An approximate formula considers the water
volume to be an inverted triangular pyramid with surface area of water = (LT ⁄ 2) and
depth = (D ⁄ 3). With experience, one is able to judge fairly accurately how an
individual valley will compare with such an idealised picture and therefore to
adjust the resulting conclusions.
If all the excavation is from below the full supply level contour the volume held
in storage is the volume calculated from formula above plus excavated volume
minus half bank volume. This is approximately the same as storage volume plus
volume calculated above plus half excavated volume.

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 68 • Farm Dams

3.5.6 Storage excavation ratio

The storage excavation ratio is a measure of the unit cost of the stored water. It is
the ratio between the total storage and total earth moved. See Section 1.4 for
different types of farm storages.

3.6 Estimate of costs

The term ‘estimate of costs’ means an estimate of the contract cost of construction
of physical works and the purchase of items of equipment. Cost estimates are an
essential part of the process of decision making in farm projects. They are used by:
• the designer in deciding which option is the best design solution;
• the owner/client in deciding whether the proposed dam is likely to be
profitable; and
• the contractor, who prepares estimates on the basis of how much to charge for
the work. A contractor has to estimate very accurately to stay in business.
In some circumstances cost estimates may also be used by finance organisations
to decide on the approval of money for the scheme. They have also been used as
a convenient method of calculating charges for planning work, particularly
subsidised charges made by Government services.
The accuracy of the estimate is often determined by the proposed use. Very
rough approximations may be quite suitable for comparing different solutions to
a design problem. In fact if very accurate estimates are needed to choose between
options then they are so close in cost that the choice may be made on other factors.
Similarly, very close estimates may not be warranted for decision making on
profitability, especially when the part of the project being estimated is not a major
part of the total cost, as is often the case with farm dams. Generally, estimates to
arrange finance for a project should be fairly accurate and will usually be finalised
only after agreement is reached on details of the design. The most accurate
estimates are those prepared for contract purposes by the contractor to determine
a quotation figure and by the designer for comparison with contract prices. The
contractor has to be accurate—too low and a loss is incurred, too high and the
contractor does not get the project. Both scenarios relate to the viability of the
business. The design estimate needs to be accurate, and not disclosed to
contractors, so that quotation prices may be compared on an objective basis.
Quotations well below a design estimate should be examined very closely to sort
out, in advance, problems that are likely to arise during construction.
3.6.1 Economics
The value of land occupied by the work may be important and may influence the
design. It will usually be necessary to include land value in farm dam estimates
only when comparisons are being made between different design options.
A farmer’s decision to build a small and deep tank when a large shallow tank
would have much cheaper earthworks will usually be determined by the value of
the additional land which is occupied or disrupted by the lower cost storage.
Farmers may seek to save money by building dams themselves, either with their
own machinery or with hired plant, particularly if the farmer has time available
which would otherwise not be used to earn more than the savings on a contract
construction price. This has the advantage that the dam builder has a direct

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 Design • 69

interest in achieving a good result. However, these potential savings should not be
deducted from the cost estimate.
The estimate of costs should be based on rates which would reasonably be
expected to cover construction and materials. Should the client be able to obtain
a quotation at a cost lower than the design estimate, there is no reason why it
should not be accepted, provided the contractor has suitable equipment and agrees
to carry out the work in accordance with the plans and specifications.
For many purposes the future annual cost of a dam is important. This cost
determines whether the project is profitable (compared with the annual value of
increased production). It is also needed to combine with other annual costs to
determine the best project design. For example, a dam built at low capital cost,
but which had high annual cost for repairs and maintenance, might well cost more
than one built to a higher standard and initial cost with minimal maintenance cost.
The objective is an optimal combination of annual costs of capital and recurring
items. Comparison of all options should be considered before proceeding with a
choice of project.
Contractors generally work on an hourly rate of hire for each piece of
construction plant. The hire charge is negotiated between the landowner and
contractor before work commences.
Other forms of contract can be used:

i Schedule of rates contract

When using this method of contract the actual measurement of each item becomes
very important.

Table 3.8 Schedule of rates contract

Item Description Unit Quantity Rate Amount

1 Site Investigation Sum …………. …………. ………….

2 Design Engineers’ Fees 5–10% …………. …………. ………….
3 Movement Charge Sum …………. …………. ………….
4 Site Clearing – $/m2 …………. …………. ………….
5 Core Trench Excavation – $/m3 …………. …………. ………….
6 Backfilling Core – $/m3 …………. …………. ………….
7 Outlet Structures-Supply – $/m …………. …………. ………….
8 Outlet Structures-Install – $/m …………. …………. ………….
9 Embankment-Construct – $/m3 …………. …………. ………….
10 Levee Banks – $/m3 …………. …………. ………….
11 Spillway-Install – $/m3 …………. …………. ………….
12 Replace Topsoil – $/m3 …………. …………. ………….
13 Gravel on Access Track – $/m3 …………. …………. ………….
14 Seed and Plants – $/m2 …………. …………. ………….
15 Fence Out areas Sum …………. …………. ………….
16 Gauge Boards Sum …………. …………. ………….
17 Other(s) Sum …………. …………. ………….
Sum Total ……….
GST@ × ……% ……….
Contingencies @ × ……% ………..
Total $………

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 70 • Farm Dams

ii Lump sum contract

The contractor estimates the cost of construction making allowances for any
unknown factors that may affect the cost. This form of contract, while common
in structural engineering jobs, is not used to any great extent for earthmoving
works because of the greater number of unknown factors, including water and the
inconsistency in earth materials (SCA, 1983).
A mixture of the above forms of contract may sometimes be used, for example,
an hourly rate for the core and a lump sum for the embankment.
3.6.2 Dam quality pays over time
Dam quality does not cost—it saves. There are points to consider when planning
to build a dam in relation to cost. In general, you get what you pay for, and good
quality work pays for itself in the long run (Lewis, 1995).
In any kind of business transaction it is common sense to expect good value for
money. The same argument applies to employing a contractor to build a dam on
your farm. The lowest quote may turn out to be expensive in the end and the
person whose quote is much cheaper than another’s may only do half the job.
Whoever you get to design and build your dam, make sure you have considered
the points provided in Section 5.2. Building a dam is costly, and it is well worth
your time and money to make sure you get a top quality product.

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 Section 4
Documentation
An engineering plan for the dam should be developed using information collated
during the design process.
This plan should include:
• all pertinent elevations;
• dimensions;
• extent of the cut-off trench;
• other areas requiring backfill;
• location;
• dimensions of the trickle pipe and other planned outlet structures; and
• all other factors crucial to the construction of the dam.
It should also take into consideration a schedule of materials documenting the
quantity and type of all the necessary construction materials. To ensure that the
quality of construction meets requirements, both the landowner and contractor
should receive copies of the plans and specifications to assist them in understanding
the demands.
The Australian National Council of Large Dams (ANCOLD) and most
Australian State water authorities have developed uniform engineering standards
and construction specifications for dams.

4.1 Collation of plans and specification

Engineering plans are prepared methodically through a process of collection,
recording and analysis of the facts and data to obtain a satisfactory outcome to a
project. The final product is generally a written report, with diagrams and tables,
outlining the class, scope and quality of work to be accomplished.
All of the material presented in this section is to be used as a guide in analysing
site conditions and preparing engineering plans. It is therefore critical that
designers collect sufficient data to ensure the elimination of inherent problems
after they have been identified. However, it is equally important to observe an
orderly approach in the collation of site data, the consideration of alternate
possibilities, and the use of accepted procedures for the development of designs
and plans.
Certain basic procedures should be taken in examining the project and in
preparing relevant engineering plans, regardless of the size and complexity of the
work. Indeed, the same process of thought and action applies even when the job
involves a small farm dam.

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 72 • Farm Dams

The basic steps are shown below.

1 Identify the project and its scope.
2 Investigate site conditions.
3 Collect all relevant design data.
4 Examine and collate data.
5 Design plan based on the above conditions.
6 Prepare plans and specifications for construction.
7 Review the plan and seek approval from responsible authorities.

4.2 Collecting basic design data

Subsequent items represent data that may be applicable in the development of an
engineering plan. Not all of these items may apply in every situation, and
furthermore, additional investigations may be required for more complex projects.
The designer should carefully analyse the available data in order to ascertain what
additional information is required.
4.2.1 Catchment map
All catchment information relevant to the analysis and design of the proposed
works should be assembled and recorded on a prepared form or map. The amount
of detail needed will depend on the complexity of the structure. Catchment
information includes the area of the contributing catchment, physical
characteristics of the catchment, and the location of the proposed works within
the catchment. Such information may include some or all of the following:
1 average slope of various lengths of the main waterway;
2 average slope of the land in various parts of the catchment (generally this can
be obtained from a 1:25 000 topographical map of the area);
3 categorisation of land use, that is, irrigation, grassland and woodland;
4 the main soil types or groups within the catchment;
5 the location of the proposed dam wall and submerged land, i.e. storage;
6 subdivisional information and title description; and
7 proprietary interest on large catchments.
In order to prepare a catchment map, any combination of the following
methods may be used:
• mark information on an aerial photograph;
• relay data from aerial photograph onto a work sheet;
• proportionally enlarge data from aerial photograph onto cross-sections; and
• draw information on a 1:25 000 topographical map.
4.2.2 Location (topographical) map
A location map is used for planning details and for outlining the project. It is
frequently referred to as a plan view of the proposed works. It is normally drawn
to a scale larger than the catchment map, but may be combined with a catchment
map on some of the smaller projects, especially on small drainage work. A
location map should show as many of the following items as required in the
design, construction, and future maintenance of the project.
• Position of survey centre-line, or other survey lines, connected to land fixtures.
• Location and plan view of all features (including horizontal dimensions) of
proposed works with reference to survey lines.

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 Documentation • 73

• Location and elevation of benchmarks at Australian Height Datum (AHD).

• Location of existing waterways (with names) or other features when these
affect design.
• Location of soil testing holes to a Geographic Information System (GIS).
• Surface or sub-surface water elevations at the time of investigation.
• Location of existing fences, property lines, buildings, roads, culverts, bridges,
springs, wells, borrow pits.
• Contour lines to Australian Height Datum (AHD).
4.2.3 Profiles and cross-sections
Defining the detailed shape and elevation of the existing terrain of the site is often
necessary in the preparation of engineering plans. Generally, field data provide
baseline information to compile plan views, profiles, longitudinal and cross-
sections. The detail and accuracy of the survey should be in line with the
complexity of the site and the structure design.
4.2.4 Soils
The engineering characteristics and land capabilities of the soil are important in
determining the suitability of a dam site. Records of soil holes, cut trenches or
other investigations are used to determine:
• the ability of the foundation materials to support the structure adequately;
• the ability of a storage site to retain water;
• suitability of the materials for embankment;
• permeability of soils for effective drainage systems; and
• depth to rock, groundwater, or other conditions that may affect the structure.
The depth, method, and scope of the soil and foundation investigations will
vary according to the size of the structure and the hazard of the site. Generally,
adequate investigation can be made with the use of a hand auger. When inspection
of undisturbed soils is required, the use of heavier equipment, such as backhoes
and bulldozers, has proven to be efficient and economical. All sites at which
investigations are made should be numbered and plotted on the engineering plan,
and the findings recorded.
Most storage dams control, store, or provide discharge capacity for a certain
volume or flow of water. The expected safety and efficiency of the dam is related
to the accurate determination of the design run-off volume, or peak discharge of
the contributing drainage area or other source of supply.
Certain site data and capacity information are required to evaluate the
hydraulic requirements of the dam. Items that often require consideration are:
• the alignment and slope of waterway;
• grades and cross-sections;
• the critical elevations;
• upstream and downstream capacities and controls;
• the stream-bed conditions, that is bed load and deposition patterns that affect
velocity and erodability;
• rate of release from controlled supply;
• the design flow from hydrologic data; and
• earthquake and structural fault lines.

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 74 • Farm Dams

4.3 Assembly of data

After collecting all relevant site data, it is important that all the facts and
information be arranged and recorded in an orderly manner. Survey notes should
be used in the assessment and in plotting plans in accordance with standard
procedures. Time can be used more efficiently by preparing the engineering plans
to scale and in detail, as the field data is obtained. This will generally suffice for
final design and preparation of plans for the project.
For many projects the use of approved, standard work sheets can (and should)
be used to record all of the necessary data. This information can then be used later
in the project, if problems arise.
4.3.1 Analysis of data
It is important to consider the relationship of the proposed solution to the overall
use of the land, when reviewing the collected data.
Detailed review of the data is necessary to determine:
• the type or series of measures required;
• the limitations in location and size of facility imposed by the site; and
• the design procedures and criteria that apply to the proposed structures or
practices.
Throughout the process of analysis, consideration needs to be given to
alternative methods and construction materials. A small, additional outlay in
installation cost may prove a saving in future maintenance and operation.
4.3.2 Design
Design procedures include:
1 rechecking or amending the estimated catchment flow regime;
2 hydraulic calculations to provide information on control, capacity, and safety;
3 structure design (see Section 3);
4 location, dimensions and elevations of important parts of the dam wall and its
accessories;
5 estimate of material quantities (see Section 3.5);
6 estimate of construction costs (see Section 3.6); and
7 specifications for materials and construction (see Appendix 2).
It is important that design calculations be documented in an orderly manner
and checked for accuracy.

4.4 Construction documents and drawings

The preparation of the construction plans or drawings provides a detailed record
of the design and structural requirements of the dam. This allows a contractor
who is unfamiliar with the project to lay out and ensure that the work is
constructed as designed. Construction drawings for complex dams usually involve
drafting the topographic, cross-section and profile data collected in the
investigation stage. Additional layout and detail drawings, as required, may be
prepared as the design calculations are completed.
For many on-farm measures, the preparation of the construction drawings may
be simplified by using approved standard pre-design layouts. The standard
drawings require careful review to see that:

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 Documentation • 75

• the plan will fit the site so that the structure will function properly; and
• all required dimensions, elevations and modifications are complete.
A common error in the preparation of the construction drawings is the
omission of required details, sections, and dimensions. All plans should be
carefully reviewed for completeness and accuracy.
4.4.1 Specifications
In addition to the detailed construction drawings, the construction plan often
requires written specifications to clarify how the work will be done, the quality of
workmanship, and methods of testing (see Appendix 2). Another important
component of the specifications is the required quality of the manufactured
materials that will be used in the work.
For small projects, the material and construction specifications may be
documented in the form of notes on the drawings. For larger projects, the
preparation of a separate specification document, or the use of Standards
Association of Australia (SAA) or ANCOLD or State guidelines are more
practical. In all cases, readability of drawings is vital to the success of the project.
4.4.2 Checklist
The amount of detail required in the construction plans will vary with the type
and size of the project. However, all projects regardless of size should be
adequately planned. The following list may be useful in checking the adequacy of
the drawings and specifications.
1 Can the farm be located from the plans?
2 Is the project site clearly shown?
3 Can the survey lines be relocated and the job pegged for construction as
designed?
4 Are all dimensions and construction details clearly shown?
5 Are material and construction specifications complete for all parts of the work?
6 Are material quantities shown?
7 Has the title block been completely filled, including the date and who designed,
drafted, amended and approved the work?
8 Has the cost estimate been prepared?
9 Have licence/permit documents been lodged with the responsible authorities?

4.5 Final review and approval

Before the construction plans are delivered to the contractor, the design and
construction plans should be reviewed by an independent designer.
When required, plans should be submitted to the appropriate responsible
authorities for approval. The construction plans should also be properly signed
and dated by all parties involved in their preparation and approval.
4.5.1 Records
A complete copy of survey notes, useful basic data, soils logs, design calculations
and other relevant data, including a copy of the plans and specifications, should
be assembled and filed in an orderly manner.
All dams or practices, regardless of type or kind of material, will require
maintenance. Changes or additions made during construction should be recorded

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 76 • Farm Dams

in coloured pen on the office copy of the plans. These ‘as constructed’ plans are
often useful when making maintenance recommendations to the contractor. They
are also useful for structural design improvement and for the evaluation of
hydraulic performance. Moreover, complete ‘as constructed’ records may be
valuable in case of a legal dispute (see Section 10).

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 Section 5
Construction
Despite the apparent super-efficiency of modern equipment, a high proportion of
dams built today do not seem to have the long life of those farm dams constructed
in the old days with a horse and scoop, followed by a driven flock of sheep to give
compaction. Although bulldozers, scrapers and tractors are heavy machines, they
are designed to create as little pressure as possible and are usually used when the
soil is at its driest—a time when soil moisture is well below the optimum for good
compaction. The modern answer for the compaction process is a wheeled tractor
or sheepsfoot roller, particularly where troublesome soils are encountered, and a
crawler tractor to carry out the excavation.

5.1 Approval for dam building

Under the provisions of different State Legislation in Australia, people wishing to
build dams, including weirs, must have the approval of the responsible authorities
before commencing any works (see Section 10).
It is important, first, to contact the responsible authorities to ascertain whether
the proposed dam would need to conform with design, planning and licensing
requirements. In some States of Australia (see Section 10, Table 10.1), certain
procedures must be followed when applying for the issue of a licence to construct.
On satisfactory completion of the works a licence containing guidelines and
controls is issued. This is necessary to safeguard water supplies, the stream
environment and existing water users’ requirements.
Upon receipt of the information needed, the responsible authorities will make
an evaluation, taking into account the water available, environmental issues,
planning and any possible detrimental impacts on existing diverters. Submission
of this information will not preclude additional information being requested when
it is considered necessary.

5.1.1 Details that may need to be submitted

In order to assess an application and obtain authorisation to commence
construction of a dam, an engineer/contractor/landowner may need to submit
details to the responsible authorities.
Dams constructed in Victoria on waterways defined under the Water Act
(1989) need to meet the following licence conditions:

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 78 • Farm Dams

• approvals in regard to the location of a storage dam from other relevant

authorities such as the Environment Protection Authority (EPA), Catchment
Management Authority (CMA) or Rural Water Authority (RWA);
• a planning permit from the municipality in which the proposed private dam is
to be operated;
• an occupancy licence, if any part of the proposed works is to be located on
Crown Land, or adherence to conditions may be set if the dam is to be within
a proclaimed water supply catchment; and
• approval for any instream works from the river management authority if such
an authority exists.
Other requirements may relate to the storage of water:
• farm dams may only be filled during the months May to October inclusive
providing sufficient flows are available. From November to April inclusive, all
incoming flows must be passed downstream through the low level outlet pipe
to which a gate valve has been connected;
• a staff gauge in the storage to measure the water depth and ‘V’ notch weir
upstream of storage is installed to allow water inflow fluctuations to be observed;
• maintenance is to be carried out to ensure continuing compliance with
specified dimensions and levels and the safety of the structure; and
• appropriate written dam-sharing agreements, and approval with reference to
potential flood risks on neighbours’ properties, should be obtained before
commencing any works on a dam.
Upon satisfactory completion of the storage construction, and payment of the
relevant charges, the Water Authority will then authorise an annual licence. If the
water is to be used for irrigation, then 1 ha of crop or pasture for each 3 ML of
storage capacity may be used, depending on locality and the type of crop.
In New South Wales, the new Farm Dams Policy (1999) allows the use of 10
per cent of the mean annual run-off as a harvestable right. When water
requirements exceed this amount, it is necessary to apply for a farm dam licence
(see also Section 10). The process of determining the harvestable right for a
property, and applying for a licence, is managed by the New South Wales
Department of Land and Water Conservation (DLWC). Supporting
documentation is available from the DLWC website.
5.1.2 Referral dams
Many States in Australia have laws that require permits or licences to construct
dams for water storage for any intended use. The engineer/contractor should
know the requirements in their State and comply with them in planning, design
and layout of dams. Referral dams, sometimes called prescribed dams in New
South Wales, also come under the provision of most State legislation, and are
defined as being:
• 5 m or more high with a capacity of at least 50 Megalitres, or
• 10 m or more high with a capacity of at least 20 Megalitres. These dams will
require additional information to be supplied. Further information on these
requirements can be obtained by contacting one of the responsible authorities
in that State.
The following requirements tend to apply to most States of Australia for dams
in excess of wall height and/or storage capacity stated above, and, in certain
circumstances, to dams of lesser size:

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 Construction • 79

• they must be designed by a professional engineer who is experienced in the field

of small dam engineering;
• certified engineering designs, plans, computations and specifications must be
submitted to the responsible authorities for consideration before any works will
be allowed to commence; and
• the construction of the works must be supervised by the designer or a similarly
qualified professional engineer, who will certify that such works have been
completed to design specifications and provide as-constructed details.

5.2 Selecting your dam builders

Common causes for dam failure are listed in Section 6. Your contractor should
encourage you to hire an experienced farm dams engineer to plan your dam before
the contractor starts moving any earth. The contractor will do a much better job
for you if they have been provided with a plan. Do not jump to the conclusion that
the contractor must be getting a commission from the engineer. A plan is vital to
the success of the whole project, because it ensures that the money spent on the
rest of the project is not wasted. Contractors who say they don’t need a design plan
are your worst enemies! Beware of the person who says, ‘I can do it all and save
you money’. This kind of ‘bargain’ can end up being very costly.
5.2.1 Selecting an engineer
Before building any large dam, talk to a consultant engineer experienced in farm dam
work. Furthermore, consult the engineer during the design and construction phases,
in the event of problems or uncertainties with the dam contractor. Many farm dams
fail because of poor planning, poor investigation and design, unsatisfactory siting,
faulty construction practices or lack of maintenance after construction. The larger
the dam, the more critical these factors become. Many of the failures result in total
loss of the dam. In those cases where the damage can be corrected, repair costs can
be more expensive than the original cost of building the dam.
The engineer must consider a range of issues when designing a dam. The issues
crucial to the dam’s safety include:
• a good understanding of local geology—and whether soils in the vicinity of the
dam are suitable for storing large volumes of water;
• knowledge of the properties of the foundation material beneath the dam—
whether it will support the load without excessive deformation, and control
seepage to within acceptable levels;
• understanding the materials from which the dam will be built—whether they
have adequate strength, durability and low permeability, and from where they
can be most economically obtained;
• a knowledge and understanding of spillway design requirements;
• an appreciation of local meteorological and hydrological conditions, in
particular for determining spillway capacity. Too often the importance of an
adequate spillway is not reflected in the initial planning. In many cases,
determination of spillway capacity is simply based on normal winter flows of
the nearby creek or stream. However, ‘normal’ flows can be exceeded many
times during or after heavy rain, and the safety of the dam embankment
depends entirely on the ability of the spillway to operate under peak storm flow
conditions;

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 80 • Farm Dams

• a sound knowledge of relevant dam design principles and methods, to ensure

safety, economy and durability. Experience in good practice covers such areas
as foundation cut-offs, internal and external drainage, outlet pipes, trickle
pipes and erosion protection; and
• a broad knowledge of other factors that may be relevant in particular
situations, including knowledge of legal obligations under relevant State
legislation and awareness of potential problems such as siltation, wave action,
other forms of erosion, and pollution.
5.2.2 Details that an engineer can provide
The designer/engineer will need to supply the responsible authorities with a brief
report outlining the design of the storage to obtain authorisation to commence
construction of a dam. The report must contain the details shown below.
• Location of the proposed dam using the following parameters—Australian
Map Grid co-ordinates, Australian Height Datum, Land Title description and
distance from major landmarks or roads. It is desirable that other improvement
sites are located within the proper section and subdivision as accurately as
possible from existing map or field data.
• A brief description of the catchment area, its soil type, vegetation and
topography. This may also contain legal land descriptions and the dam shown
on the catchment or location map.
• Description of foundations and foundation material of the dam site, including
the downstream area as far as the point of return of the spillway channel to the
waterway.
• The nature and properties of the proposed embankment material (laboratory
testing of material to be carried out by a registered company experienced in
this field).
• An indication of the embankment design methods and cases, and the factors of
safety adopted (including earthquake prone areas).
• Calculations of the design storm inflow (with a 1 in 100 probability of
occurrence in any year) or such longer period as may have been specified.
• Calculation of the earth or culvert spillway size to pass the design outflow,
including the downstream channel as far as the point of return to the waterway
(earth spillways are usually 1 m deep with a freeboard allowance of 500 mm
under design flood conditions) and an assessment of possible erosion effects
under design conditions.
• Plans of the proposed embankment showing full details and dimensions,
including spillway, capacity of storage, compensation pipe and trickle pipe,
berms, beaching, crossing culverts.
• A brief outline of housing, utilities or other development downstream of the
storage.
• Details of the proposed method of construction and equipment to be used, the
degree of supervision to be provided, and the names of the supervising engineers.
• When the dam has been constructed, the engineer will be required to certify
that such works have been completed to design and to provide as-constructed
details.
• On request the engineer may provide a maintenance and safety surveillance
program for ongoing use.

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 Construction • 81

5.3 How to build a dam

Most of the element of risk can also be eliminated from the building of a dam by
careful attention to several basic requirements. The builder should ensure that:
• the site selected is the most economical, that is a natural depression is usually
best because it considerably reduces the amount of excavation;
• the storage has low seepage losses and the soils present are capable of holding
water—clay is usually the best (sometimes a sealer can be used with sandy
clays, but this increases costs significantly);
• data relating to dam capacity, that is, the limitations of the drainage area are
available;
• the foundations are watertight and capable of supporting the dam;
• there is sufficient dam-building material; and
• 75 per cent of the drainage area is permanent pasture or woodland to prevent
excessive silting.
The cost and effort expended on making sure that these conditions are met
will, in the long run, save the builder both time and money.

5.4 Steps in constructing a dam

Building a good dam embankment is rather like making a layer cake; it is a
methodical process requiring exactly the right ingredients and careful attention to
building each layer. However, unlike a cake, the aim of building a dam wall is to
force air out from between soil pores, making the soil denser and less permeable.
The following steps should be considered before constructing an embankment, as
listed in Appendix 2.
5.4.1 Setting out
Setting out or pegging out transmits the information on the farm dam plans to the
ground. This information provide lines, grades, and elevations for construction of
the works in accordance with the plans. Consideration should be given to the
contractor’s requests to ensure optimum efficiency of the set out. The quality and
appearance of the completed work will reflect the care and thoroughness exercised
in the set out procedure (SCA, 1983).
The areas to be cleared usually will consist of the embankment site, the
spillway site, the borrow pit, and the area over which water is to be stored. Each
of these areas should be clearly marked with pegs. In the case of the storage area,
the proposed waterline should be located accurately with a level. Clearing pegs
should be at least 4 metres outside this waterline to give an indication of the area
to be cleared around the edge of the storage.
The embankment is located by setting pegs along its centre-line at intervals of
10 metres or less. Usually this will have been done during the course of the initial
planning survey. Fill and slope pegs are then set both upstream and downstream
from the centre-line pegs marking the points of intersection of the side slopes with
the ground surface.
The earth spillway is located by pegging the centre-line and then setting cut and
slope pegs along the lines of intersection of the spillway side slopes with the
natural ground surface. The procedure for setting these pegs is the same as for

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 82 • Farm Dams

pegging the embankment, except that they are cut pegs rather than fill pegs. They
should be offset so they will remain in place for referral during construction.
Where suitable fill material must be obtained from a borrow pit, it is essential
that this area be clearly demarcated. Cut pegs should be set to control excavation
within the limits of suitable material and to drain the borrow pit.
A spillway or trickle pipe should be located by pegs offset from the centre-line
of the pipe and placed at intervals not exceeding 10 m. The spillway should be
located where it will rest on a firm foundation. Cuts from the tops of the stakes
to the grade elevation of the tube should be plainly marked on the pegs. The
locations of the low level pipe and gate valve, cut-off collars, outlet structure, and
other appurtenances should be identified by clearly marked, additional pegs.
Setting out of a dam site can vary from the pegging of the Full Supply Level
(storage area) at each end of the embankment to placing pegs all along the toes of
the embankment and spillway and the placing of offsets. For small dams offset
centre-line pegs (steel posts), full supply pegs (coloured) and one upstream toe peg
are normally sufficient.
Centre-line pegs should be placed well away from the construction area. Two
pegs placed 6 m apart at one end of the bank enable the centre-line to be easily
established during construction. Batter peg distances can then be calculated simply
from a longitudinal centre-line survey. It is important that all pegs and the bench
mark (see Section 4.2.2) are shown on the plan to minimise confusion.
The distance of toe pegs from the dam centre-line is calculated from a
longitudinal centre-line survey as follows:
Distance of toe peg = (Height at centre-line x batter slope) +
(half of the crest width).
3
= (6 × 3) +
2
= 19.5 m
This formula is only applicable to non-sloping sites. Adjustments must be made
to correct for the slope, when measuring a site for a high dam or where steep
slopes occur (for an example with 5 per cent slope [1:20, vertical : horizontal], see
Figure 5.1).
19.5
In this example the correction is distance = × 3 = 3 m (approx)
20

Figure 5.1 Location of toe pegs on slopes

3m

Crest width
Upstream batter slope
Downstream batter slope
3:1 (horizontal:vertical)
3:1 (horizontal:vertical)
Peg 6m

Borrow pit area

Peg
1
20
m = metres 3 x 6 + 1.5 = 19.5 m 3 x 6 + 1.5 = 19.5 m ~3 m

16.5 m 22.5 m
~3 m
19.5/20 x 3 (batter) = approx. 3 m
Source: SCA, 1983.

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 Construction • 83

5.4.2 Diversion of water

Provision must sometimes be made to divert water flowing through the site. If a
pipe is being placed through the embankment, its early installation may be
required to divert small flows during construction.
In wet drainage lines it is commonly necessary to construct a small coffer-dam
(protective wall) to keep the site dry while the core trench is constructed. In this
case, flows are either pumped or piped across the site.
5.4.3 Clearing and grubbing
Where the site is timbered, clearing and grubbing should be carried out to at least
6 m from the downstream and upstream toes of the embankment. Trees flooded
by the dam should be cut off at stump height and then removed from the site. All
holes made by grubbing under the embankment should be filled with sound
material which is well compacted.

Figure 5.2 Compactor with blade stripping topsoil

Source: Caterpillar of Australia.

5.4.4 Stripping topsoil

All plants and silty soil should be stripped from the embankment site and ‘borrow’
area (the site from which additional soil is to be taken). The topsoil should be kept
separate from subsoil, and stockpiled for use during the later stages of
construction, such as covering the embankment and spillway (Figure 5.2).
5.4.5 Core trench
The core trench (cut-off trench) must extend at least 0.6 m into impervious
material or 0.3 m into rock and extend for the full length of the bank. All water,
mud, loose soil and rock must be removed before backfilling commences. The
backfill should be specially selected and placed in layers of no more than 100 mm
thick. If a central core is used, the core trench must be continuous with the core.
The core trench should be a minimum of 2 m wide, extend on both banks to
above top water level, and reach down to impervious material below. This last
factor may not be practical but it is essential in deep soils that crack during
seasonal changes, as it will create a ‘cut-off’ zone to stop the cracking in the
embankment.

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 84 • Farm Dams

5.4.6 Borrow pit material

Where the borrow material or soil has adequate clay content, an impermeable
embankment can be constructed without having to ‘zone’ the embankment (see
Sections 2.1.1, 3.1.1 and Figure 3.2). If the borrow material is highly variable, it
is advisable to place the material with higher clay content (free from stones and
decomposed rock) in a zone along the axis. Soil with lower clay content is then
placed on either side, upstream and downstream, of this zone. It should be noted
that silty and sandy materials are not suitable for construction of a watertight
embankment.
The waterway (river, stream or creek) bed soils should not be used either as a
borrow pit within a water storage or to increase the volume of the storage. To do
so can create a low point that prevents environmental flows passing through a low
level pipe within the embankment (see Section 3). This can also lead to seepage
problems if the borrow pit is in close proximity to the dam wall.
5.4.7 Selection and placing of material
If the material is uniform, no selection needs to be made. However, in a great
many cases the soil is variable and a selection of materials is found to be necessary.
Where good quality, impervious material is in short supply, it should be kept
separate and placed as a central core. In general, where the soil materials are
variable, they should be placed as shown in Figure 3.2. It is sometimes necessary
to remove very poor quality material. This can be placed in the downstream side
of the bank by flattening the downstream batter.
Soil should always be placed longitudinally in an embankment to avoid lines
of weakness across the bank. Bulldozer-constructed banks require material to be
spread as well as pushed up.
The thickness of soil layers should not exceed the capacity of the compacting
plant being used, because each successive layer must be bonded onto the previous
layer. Where material has dried out it must be scarified and watered before the
next layer is emplaced.
5.4.8 Spillway and outlet structures
A spillway should be designed and installed so that it is of sufficient capacity for
a 1-in-50-year or 1-in-100-year storm flow, and at a level which maintains
adequate freeboard. A spillway return should be constructed clear of the
embankment toe and with the least possible disturbance to the natural ground in
order to minimise erosion. To protect the spillway from erosion during continuous
flows, a trickle pipe with inlet pit should be installed through the embankment.
A low level pipe (compensation pipe) of an appropriate size, determined by
catchment area and rainfall characteristics, should be installed at the base of the
embankment with cut-off collars at intervals along the pipe as indicated in Section
3.4.3. It is preferable to place the pipe in natural ground rather than in fill
material. All material around the pipe must be carefully compacted, preferably
with a mechanical rammer.
5.4.9 Batters and topsoil
After the embankment and spillway have been completed, all loose uncompacted
material must be removed before topsoiling. Topsoil is spread onto all the batters

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 Construction • 85

and crest of the embankment and all the cut surfaces of the spillway. It may be
necessary to cover the cut batters of the borrow pit if erosion is likely.
The thickness of topsoil necessary for erosion protection, insulation and
promotion of vegetation cover varies with slope, environment and soil type. A
common fault is an excessive thickness of topsoil on a steep batter that slides off
with the first heavy rainfall event. A thickness of 50 mm is generally sufficient to
establish a vegetative cover while a 200–300 mm thickness is required to provide
an effective insulation of the embankment against excessive moisture changes
(that is shrinkage and cracks).
The upstream batter is compacted more than the downstream batter to give
greater stability where the soil is wetter. Sandy clays are usually more stable under
moist conditions than soils with a higher clay content.
A ‘freeboard’ of about 1 m is required to protect the embankment from
overtopping. To minimise drying and cracking of the impervious embankment, it
is recommended that most of the freeboard zone is constructed from silty or sandy
topsoil. Increasing the minimum crest level required for freeboard by 5–10 per
cent allows for settlement of the embankment crest. If a wide crest is required for
vehicle access, make sure that wheel tracks do not form as water will pool in them
and increase the risk of erosion.
Topsoil stripped from the borrow area and embankment foundation should
only be used in the outer metres of the embankment, and never in lower regions.
The higher proportion of silt and organic material protects against rapid moisture
loss from the embankment, stopping it from cracking and providing an
environment for rapid growth of ground cover. Seeding the embankment surface
to protect against erosion is also recommended. All disturbed areas must be
protected against erosion by planting and maintaining a suitable holding grass for
the geographic area involved. The use of fertiliser and watering may be necessary
particularly during the establishment stages.

5.5 Compaction
Compaction is the most important factor in achieving a stable, durable and solid
earth embankment, which is resistant to the constant seepage of water through the
soil. Notably, many dams fail because of poor compaction.
Compaction occurs when pressures are applied to the soil so that the individual
soil grains are pushed together as air is expelled. Compaction in the field is
directed at reducing the percentage voids to less than 5 per cent. When soils are
saturated, the application of sustained pressure will also expel water from the pore
spaces separating the soil grains, resulting in a measurable reduction in soil
volume. This latter process is called consolidation.
When soil is disturbed, transported and used in construction without being
compacted by machinery, it is loose and friable with a high proportion of air
spaces between the soil grains. The soil mass will have high permeability and low
strength. Over time, this material will settle and become denser. However,
settlement will be uneven and deformation in a number of different directions may
occur. This will result in extensive cracking of the surface and eventual failure of
the structure. To protect against this, artificial compaction is used in the
construction of roads, earth dams and embankments (including levees and graded
banks) so that soil physical and engineering properties are modified. After

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 86 • Farm Dams

compaction, the soil mass should be characterised by high soil strength and low
permeability (the rate at which water will infiltrate and flow through the soil). In
addition, vulnerability to settlement in response to repeated loading should be
significantly reduced.
Compaction produces a uniform product and requires not only the appropriate
equipment and specifications, but also an understanding of the inherent properties
of the soil to be used. Therefore, before compaction is undertaken, the
permeability, shear strength and density of the undisturbed soil should be known.
In addition, because over-compaction can cause failure of the structure, soils need
to be tested to determine the maximum possible compaction.
5.5.1 Compaction when constructing a dam
Good compaction relies on a range of factors including soil characteristics, soil
moisture levels, the type of machinery, weather conditions and construction
methods. The soil to be compacted must be moist but not too wet, and must be
layered along the full length of the embankment at depths appropriate to the
equipment used. Farm machinery and hand methods are usually only sufficient to
compact layers less than 50–75 mm deep. Heavier equipment such as rollers can
work with layers up to 200 mm thick, and should be used where large quantities
and wide areas of soil are involved. In addition to compactive effort (roller
pressure and number of passes), the moisture content in the soil is vitally
important.
Each soil layer should be bonded to the previous layer by light scarifying along
the axis. Rolling should be continued until the roller feet do not completely
penetrate the soil. Large stones and lumps of partly decomposed rock, which
cannot be broken down by rolling, should not be incorporated.
This information has been written for those who are constructing their own
dam. However, it is recommended that expert advice is sought before taking any
action.
5.5.2 Recommendations for compaction
Compaction must form a part of the construction of all embankments.
• If a bulldozer is the only plant used for compaction, the correct moisture
content is critical. This is determined by a qualified engineer or geo-technical
expert.
• The moisture content for bulldozer compaction is higher than that required for
compaction by a roller.
• Standard Optimum Moisture Content (SOMC) is roughly equivalent to
moisture content at the plastic limit in clayey soils.
• When using a scraper, it is generally impossible to work with material that is
too wet. In these conditions, the equipment will bog when the moisture content
is greater than 4 per cent over optimum.
• Embankments of 3 m in height or exceeding 3m3 in volume should be rolled
with a ‘sheepsfoot’ or ‘pad’ roller.
• Flat-wheeled rollers are not recommended for the construction of small dams.
• If a dry crust forms during construction it must be broken up, watered and
mixed or removed from the embankment.
• If the soil is dispersive, soil moisture content during construction must be at
least OMC and a sheepsfoot roller should be used.

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 Construction • 87

5.6 Soil moisture

The dam embankment should be constructed from soil that is sufficiently moist to
be pliable without crumbling, but not so wet as to excessively stick to, or flow
away from machinery. A simple test is to roll a small ball of the soil into a ‘rod’
between the hand and a smooth, hard surface. If you can roll it into the thickness
of a pencil (about 7 mm) then the moisture content is near optimum. Soil that is
too dry will crumble. Soil that is too wet will stick to your palm. A soil of the
correct moisture is critical for construction purposes. It will layer and compact
more easily, reducing the tendency of the embankment to ‘settle’ after the dam has
been built, and will hold water in the dam for longer periods.
If the soil is too dry it will contain a high percentage of air spaces, even after
compaction. Dry soils will take up moisture easily to the point of saturation,
losing the necessary characteristics of strength and impermeability. If the soil is too
wet, it becomes soft and unable to be compacted effectively.
5.6.1 Adjusting soil moisture
In some situations, such as where dam walls are higher than 5 m or when the soil
has very high or low clay content, it may be necessary to measure accurately, and
adjust the soil moisture content during construction. Soil should be tested in a
laboratory to determine its clay content. The required moisture content for dam
building varies from about 10 per cent for soils with low clay content to 34 per
cent for the heaviest clays.
If soil is spread in thin layers when conditions are hot and dry, the moisture
content could be too low because of losses through evaporation. This can be
overcome by minimising the length of time the soil is exposed, either by waiting
for cloudy conditions, or by adding water to the soil during construction. The
addition of water can be undertaken by:
• mixing water into the soil by cultivation with a disc plough or rotary hoe;
• irrigating as the soil is spread out on the embankment; or
• deep ripping and irrigating the soil before excavation, also known as ‘borrow
pit irrigation’.
Borrow pit irrigation is more economical than adding water directly to the
construction surface. It results in more even distribution of water, and saves time
by avoiding the necessity to water the construction surface between each layer.
Borrow pit irrigation is undertaken by initially ripping and ploughing the
‘borrow’ area before irrigating it thoroughly, and leaving the water to soak in over
one or more days before excavation. If you are unable to add water artificially,
you may have to wait until sufficient rain falls. However, the amount of rain
needed varies widely. For this reason, you will need to dig one or more holes at
least 2 m deep, before excavation starts, to help assess the soil moisture content.
If you think rain may interfere with construction it may be feasible to install drains
to direct water away from the site or to pump water from a hollow in the
excavation area.
Since additional time and money are involved in manipulating soil moisture
content under dry climatic conditions, it is recommended that you avoid dam
construction during very hot summer periods.

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 88 • Farm Dams

5.7 Allowance for settlement

Settlement in small dams is due to consolidation or saturation. Consolidation is
the process of squeezing out the pore water by the weight of the embankment
itself. Consolidation settlement can be settlement of the embankment or the
foundations. In general, most consolidation settlement takes place during
construction, particularly with coarse grained (silty and sandy) materials.
Consolidated settlement will continue for an appreciable time after construction
when materials, such as highly plastic clays, are used. This is particularly evident
when the materials are wet during construction. Under these conditions,
consolidation settlement of up to 5 per cent can be expected (SCA, 1983).
Compacted soils may exhibit a sudden settlement when saturation occurs in
response to seepage flow. This may lead to settlement of the soil below the seepage
line. The joint between the settlement soil and the overlying unsettled soil is a
common location of tunnel failure in dispersive soils. There is no simple method
of predicting the amount of saturation settlement. However, it is greatest in clay
soils and least in sandy soils.
Settlement includes consolidation of both the fill materials and the foundation
materials due to the weight of the dam and the increased moisture caused by the
storage of water.
Figure 5.3 Settlement allowance

Constructed level Spillway

Final crest level

Allowance for settlement

Settlement by consolidation depends on the properties of the materials in the

embankment and foundation and on the method and speed of construction. The
design height of earth dams should be increased by an amount equal to the
estimated settlement. According to different construction techniques, this increase
should not be less than:
Rolled fill (Sheepsfoot roller) 5%
Scraper placed 8%
Dozer placed 10%

5.8 Equipment
The type of equipment you should use for building your dam will depend on:
• the size of the proposed dam;
• soil type;
• the amount of soil and distance it is to be moved; and
• operating conditions.

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 Construction • 89

i Bulldozer
Topsoil should first be removed from the borrow pit and embankment areas and
stockpiled in the immediate area. Trees should not be included in the topsoil
stockpile area and room needs to be made for the bulldozer to push the topsoil
back over the embankment (that is, clear of any fences and trees). The topsoil
stockpile should be graded so that any rainwater will not form ponds and make it
too wet to push the soil material back onto the built wall.
Where there is a small amount of topsoil over uniform clay, it is better to
construct the dam as shown in Figure 5.5. This technique allows the material to
be pushed into place quickly because of the flatter grade. Suspect material should
be dug out so that there is a layer of good clay connected to a sound foundation.
Optimal compaction is achieved by running over each layer with a bulldozer.
Figure 5.4 The bulldozer—a track type of tractor

Source: Caterpillar of Australia.

The embankment should be built in progressive layers if a bulldozer is used.

The excavation area usually needs to be ripped prior to pushing, with cross-
ripping of the excavation area being necessary in hard conditions to gain greater
depth and to obtain better broken material for pushing. As material closest to the
proposed wall is pushed up to create the wall, dirt will spill around the sides of
the blade and cause windrows. Successive amounts of material are then pushed
between the windrows until there is a blade-full of soil available. This is then
pushed up the wall and dropped when the topsoil is reached. Each ripped area is
fully excavated.
The ends of the wall and corners are built first so that it is not necessary to
push material a long way to complete the section. This also leaves the centre open
to allow rainwater to drain away. It is important that all the poorer material is not
utilised in one section of the main wall.

Figure 5.5 In-situ layers of different soil types used to form an embankment

Embankment

Topsoil Topsoil
Borrow pit
5m minimum
Core trench
separation

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 90 • Farm Dams

Steep gully edges, which would be under the wall, should be excavated to a 3:1
batter (horizontal:vertical). This will allow better compaction and prevent sheer
cracks forming if the wall material settles. A core trench the width of the tractor
blade is then dug down to clay, and the trench is ripped and refilled with good
clay.
Figure 5.5 is an example of how a bulldozer could be used to move individual
layers of different soil types to form an embankment.

ii Scraper
Heavy earthmoving machines such as elevating or push-loading scrapers are not
generally necessary for dam building unless you are in a hurry, short-haul
distances are involved, or you can obtain the machinery at economical rates. A
wheeled tractor or crawler-drawn dam scoop are sufficient for building some farm
dams. However, a vibrating roller is still considered the preferred machine.

Figure 5.6 Wheel tractor scraper

Source: Caterpillar of Australia.

5.8.1 Rollers
Rollers are designed for the purpose of soil compaction. They may have pronged feet
or smooth drums, they may be vibrating or static, and be self-propelled or towed by
a bulldozer. The type and size of the roller required depends on factors including soil
type, size of the project and required thickness of the soil layers. The maximum
recommended thickness of loose soil for compaction by a roller is 200 mm.

Figure 5.7 Self-propelled roller with blade to level off soil layers

Source: Caterpillar of Australia.

To achieve adequate compaction, four to eight passes of the roller are required
over the same area. The cost of using a roller is usually less than 30 per cent of the
total dam construction cost. Sheepsfoot rollers can compact layers up to 200 mm
deep (before compaction) and satisfactory densities can be achieved with six to
twelve passes at a roller speed of 3 to 6 km per hour. It is important to keep the
rollers clean to avoid soil collecting between the feet.

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 Construction • 91

Sheepsfoot rollers are more effective in compacting dry clay than other rollers
because the soil is blending and water is being distributed throughout the
constructed surface. Vibrating rollers are more suited to compacting soils with
lower cohesion characteristics and where very high soil densities are required.
Smooth wheeled rollers are more efficient at reducing air spaces, and continue
the compaction of lower layers of the embankment to a greater extent than
sheepsfoot rollers. Under conditions of comparable layer depths and speeds, a
smooth wheeled roller would probably require fewer passes than a sheepsfoot
roller to obtain similar soil densities. However, sheepsfoot rollers are often more
appropriate for dam construction as their lighter weight and versatility allow them
to be pulled by farm machinery over a range of surfaces (Stephens, 1991).
During construction (assuming the material is at optimum moisture content),
the compaction for banks up to 3 m may be done by bulldozer or dozer-scraper,
provided the layers do not exceed 100 mm. Where embankments are between 3
and 9 m, there should be at least four passes of a sheepsfoot roller over a 150 mm
layer. A guide to the minimum number of passes is provided in Table 5.1.

Table 5.1 Minimum number of sheepsfoot roller passes

Dam water height (m) Soil classification
GC, GM, SC, SM,CL ML,CH, MH
3–6 4 6
7–9 6 8

Notes: • USDA Soil Classification (see Section 2.5.2 and Figure 3.2);
• dispersible material should be compacted with two additional passes;
• homogenous banks of dispersible material, such as ML, MH, and CH should not be greater than 6 m in height.

5.8.2 Features of roller compaction

The most important features of roller compaction are shown below.
• Subsequent passes of the same roller have less compactive effect at the same
moisture content. This is due to the increasing resistance to compaction with
increased density.
• The optimum moisture content for the first roller pass may be slightly higher
than for the second pass, and thereafter for each successive pass. Consequently,
some drying can be advantageous.
• The speed of the roller has little effect on the compaction achieved.
• Different rollers are better suited to different materials. Vibrating rollers are
best for sandy soils at low moisture contents; sheepsfoot rollers for loam and
dry soils at low moisture contents; and pneumatic type rollers for clays at high
moisture contents.
• No roller will compact the soil adequately if the layer is greater than 400 mm
thick in the loose state.
• The use of the heaviest roller that does not cause subgrade failure is always
recommended.
• A sheepsfoot roller requires more passes.
• Compared with a sheepsfoot roller, a higher moisture content is required for
compaction when using a bulldozer, because of the low loading intensity.
• Flat wheeled rollers tend to cause layering.

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 92 • Farm Dams

5.8.3 Other machinery

It is not generally recommended to use bulldozers to achieve soil compaction.
Bulldozers are designed to excavate soils and, unlike rollers, exert a low pressure
and compactive effort on the ground. However, bulldozers can achieve sufficient
compaction by thoroughly track-rolling the soil in layers less than about 50 mm
thick, depending on factors such as soil type, moisture content and weather
conditions. Four to eight passes are needed over the same area, limiting the
amount of soil that can be efficiently compacted by bulldozer alone. Nevertheless,
very small dams made from impermeable soil up to 2 m high can be constructed
successfully by using only a bulldozer.
Machines such as rubber-tyred scrapers or tractors exert greater pressure on
the ground than bulldozers but can be less effective than rollers. Moreover, soil
that is sufficiently moist for compaction by a rubber-tyred machine can cause the
tyres to slip. The distance between the tyres also means that extra passes of the
machine are needed after the soil has been unloaded. These machines should
compact the soil in layers no thicker than 100 mm. Tractor-drawn scoops tend to
be slow and require a tractor of at least 40 kw capacity. If using this type of
machinery, remember to calculate the length of time it will take to excavate and
compact the soil, and allow for this when scheduling the project.

5.9 Installation of outlet pipe

Seepage and piping along outlet pipes (see Section 3.4) are a major cause of failure
of farm dams. As pipes become larger in diameter it becomes more difficult to
compact material properly, down the sides of, or under, the pipe. Therefore,
backfill should comprise material of a similar moisture content to the in-situ
material to avoid shrinkage cracks at the excavated interface. Mixing with
expanding clay is also good practice. Large pipes (450 mm diameter and above)
require layer thickness no greater than 100 mm (loose) to ensure adequate
compaction by pneumatic or mechanical hand-held rammers. Compaction and
filling should continue until there is sufficient cover to avoid disturbance and/or
crushing of the pipe.
5.9.1 Testing of the pipe
After the pipe is in place, and before any backfilling, the pipe should be tested for
leakage, applying pressure through the use of compressed air. Testing should be
carried out at a pressure of 150 per cent of the maximum working pressure. No
leakage is permitted over a two-hour test period.
Where pipes that are prone to damage are used, such as rubber ring-jointed
PVC pipes, a further test is recommended after 1 m of soil cover has been
emplaced. Failure at this time can be corrected at a much lower cost than after
completion of the embankment.
5.9.2 Foundations on rock
Where rock or ‘hard ground’ is exposed in the embankment foundation, all loose,
shattered or flaky rock and soil should be removed. Any ‘overhanging’ rock
sections should be broken down to a 1:1 (horizontal:vertical) slope by barring or
wedging. Before any fill is placed in the core trench or in the embankment all

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 Construction • 93

exposed rock should be cleaned and any holes left by loose rock should be filled
with good quality CL material.

5.10 Checking for compliance with standards

5.10.1 Checking the contractor’s work
It takes time to set out a project properly at the start. Ideally, the designing
engineer undertakes the dam layout, but if the contractor does it, you must expect
to pay for it. The contractor should certainly check the work by taking levels
during and after the project and this, too, will add to the cost. However, for you,
it is money well spent (McMullan, 1995).
While the owner, engineer and the contractor have responsibilities for
compliance with standards, inspection is also required if high standards are to be
maintained.

5.10.2 Inspection during construction

There are many items in the construction of a farm dam that need to be checked
during construction. Responsibility must be assumed by the owner, since the
designer of the dam may not always be able to spend sufficient time on the project.
The landowner should be encouraged to watch the construction so that he/she can
inform the contractor/engineer if corrective action is necessary. The following are
items that might be checked by the landowner:
1 all clearing and grubbing operations should be completed according to
specifications before any work on the embankment is started.
2 before embankment construction begins, the foundation should be properly
prepared. The completed cut-off trench should be inspected to ensure that it is
excavated to impervious material and is free of water before it is backfilled.
3 the completed installation of the spillway, pipeline cut-off collars and other
outlet structures should be inspected before embankment construction is
started. Materials used, location, alignment, grades and dimensions should be
checked for compliance with the plans.
4 the earth excavation and the selecting, placing, spreading and compacting of
the material in the embankment should be inspected frequently to ensure that
specifications are met.
5 the landowner may wish to photograph key stages in the project for record
purposes.

5.10.3 Good work takes time

There are no short cuts available if you want a top quality project completed.
First, the dam site must be ripped. If the project is done well, you won’t finish
up with any hard patches on the soil surface, and it is important to note that
quality of finish is evident in the hard-to-get-at places. Good contractors tidy up
the crest and batters, and especially the awkward corners (SRW, 1995).
The vital test of a top-quality project is the compaction of soil. It doesn’t matter
how good the equipment looks, or how big the tractor is, time must be spent
achieving good compaction.

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 94 • Farm Dams

Defective dam-building includes the following practices:

i The use of inexperienced contractors and/or inadequate supervision

Nothing can take the place of a reputable contractor, experienced machine
operators and appropriate equipment. For large dams or those that have specific
problems, working from plans, specifications and material test requirements
prepared by an experienced engineer is also vital. Even the best contractor may be
tempted to take the occasional short cut in the absence of supervision.

ii Failure to strip topsoil from foundations, or having topsoil

incorporated in the bank
A large number of dams are still being constructed where the topsoil is
incorporated into or left under the clay material that forms the bank. Such dams
are more likely to leak, or ultimately fail, because topsoil is more permeable than
clay and any organic matter in topsoil will rot, allowing seepage to occur.

iii The use of inappropriate or defective materials

Farm dams are built from different soil types ranging from sand to black clays.
Each soil type has its own characteristics and problems that must be considered
when designing a dam. The two soils most susceptible to problems are dispersive
clays and pervious soils. A dispersive clay is one that erodes spontaneously in the
presence of water. This is evident where local dams and creeks have a constant
‘muddy’ look. Swelling and cracking clays also present potential problems. Using
these soils can result in dam leakage, which in turn can lead to piping or tunnel
erosion. If unchecked, this will lead to rapid dam failure.

iv Inadequate compaction
This can result from using soils that are too wet or too dry during dam
construction, or using tracked plant rather than rollers. Even one layer of
inadequately compacted material in a bank can result in seepage and eventual dam
failure.

v The incorporation of outlet pipes or other structures in the dam wall

Unless backfilling and compaction are carried out with extreme care, and cut-off
collars provided, outlet pipes can provide an easy path for leakage. As far as
possible, outlet pipes should be placed in the original ground rather than the
embankment. Care is needed if such items are placed in the embankments or
foundations of dispersive clays, as dam failure is highly likely unless the job is
overseen by an expert.
5.10.4 Extra ‘bonuses’
The conscientious contractor will think of ‘extras’ such as leaving a load of soil
where stock troughs and channel crossings are to go. It helps if this soil is dumped
at the right spot while the equipment is in the paddock. A good contractor will
also think about where to put the topsoil and clay. Clay should be buried
underneath topsoil, not the other way around! Some contractors will make it
easier to spread seed on batters, and protect the batters against erosion, by
‘roughening’ the topsoil.

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 Construction • 95

5.10.5 Changing your mind

Problems arise if you get ‘bright ideas’ halfway through a project. If you pushed
the contractor hard on the original price, don’t expect to get extra work done
without an increase in the total cost. For example, if you have a plan of the
proposed dam, it will show the contractor what to do. You create difficulties if
you decide, when the job is half-done, that you want a vehicular crossing or trickle
pipe constructed. Avoid changing your mind after you have agreed to the plan.
5.10.6 Progress payments
The contractor does not want a big overdraft any more than you do. When you
are dealing with thousands of dollars, it is reasonable for the contractor to receive
progress payments, similar to house-builders. It is your responsibility to negotiate
agreed progress payments with your contractor (see Sections 1.9 and 3.6).

5.11 Final inspection and measurements

The final inspection by the design engineer should include sufficient profile and
cross-section readings to ensure that the height, crest width, batter slopes and
other dimensions shown on the plans have been met. Elevations of the top of the
trickle pipe or spillway in relation to the control section of the spillway, cross-
sections and profile of the spillway should be surveyed to ensure that construction
has been undertaken in accordance with the plan dimensions and left in the
specified condition. The final inspection should be made immediately after
completion of the work and before the contractor moves his equipment from the
site. All observations and measurements made during the final inspection of the
dam construction should be recorded. Some State authorities responsible for
licensing dams require a formal completion report and ‘as constructed plans’ on
completion of works.

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 Section 6
Maintenance
There is always the possibility that the dam you build might experience partial or
total failure causing extensive damage downstream. This could result in loss of
life, injury to people or livestock, damage to residences, industrial buildings or
railways, and interruption to the service of public utilities (for example,
electricity). In addition, loss of income resulting from lack of water could make
the economic consequences of a dam failure substantial.
With newer farm dams, surveys have shown that many fail due to lack of
maintenance in the first few years after construction. Although owners enjoy the
benefits of having a farm dam, they also have to take total responsibility for the
dam’s safety and must, therefore, undertake adequate monitoring and
maintenance.
This section sets out to explain some of the structural risks and hazards of the
farm dam. It also describes how to carry out routine monitoring and maintenance
of a dam’s condition. It is intended to cover all farm storages. The guidelines, if
followed, could avoid costly repairs and extend the useful life of dams as well as
decrease risks to the public (DC&NR, 1992).

6.1 Safety surveillance

Safety surveillance of a dam is a program of regular visual inspection using simple
equipment and techniques. It is the most economically efficient means of ensuring
the long-term safety and survival of the dam. Its primary purpose is to monitor the
condition and performance of the dam and its surroundings, and to ensure
maintenance before the development of potential hazards.
The procedure for dam safety surveillance is unique to each dam but essentially
it consists of:
• regular, close examination of the entire surface of the dam and its immediate
surroundings;
• taking appropriate measurements; and
• keeping concise, accurate records of observations.
This inspection would normally occupy less than an hour. Frequency of
inspection is discussed in Section 6.2.4.
For continuity and consistency of approach, the same person should normally
carry out each inspection. An exception would be when an experienced engineer
is brought in to inspect or advise on a particular problem.

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 98 • Farm Dams

6.1.1 Equipment for inspection

The following items are useful when conducting an inspection:
• dam inspection checklist to identify the items to be examined;
• notebook or diary and pencil—to write down observations at the time they are
made, thus reducing mistakes and avoiding reliance on the memory;
• camera—to provide photographs of observed field conditions. Colour
photographs taken from the same vantage points are valuable in comparing
past and present conditions;
• shovel—useful for clearing drain outfalls and removing debris;
• stakes and tape—used to mark areas requiring future attention and to stake the
limits of existing conditions such as wet areas, cracks and slumps for future
comparisons;
• probe—a 10 mm diameter by 1 m long blunt-end, metal rod with right angle
‘T’ handle at one end. The probe can provide information on conditions below
the surface such as depth and softness of a saturated area. Soft areas indicate
poor compaction or saturated material. (An effective and inexpensive probe
can be made by removing the head from a golf club); and
• hammer—to test soundness of concrete structures.
6.1.2 Observations to be recorded
All measurements and descriptive details that provide an accurate picture of the
dam’s current condition must be recorded. This information falls into three
categories as shown below.
1 Location—any problematic area or condition must be accurately described to
provide adequate evaluation. The location along the length of the dam should
also be noted. For this purpose, a fixed reference peg at one end of the dam can
be useful. The height above the toe, or distance down from the dam’s crest,
should also be measured and recorded. The same applies to conditions
associated with the outlet or spillway.
2 Extent of problem area—record the length, width and either the depth or
height, as appropriate, of any area where a suspected problem is found.
3 Descriptive detail—a brief, yet detailed, description of a condition or
observation must be given, including:
– volume of seepage from point and area sources;
– colour or volume of sediment in water;
– length, width, displacement, and depth of cracks;
– soil moisture condition in the area (dry, moist, wet or saturated);
– adequacy of protective cover (topsoil);
– surface drainage;
– steepness of batter slopes;
– presence of bulges or depressions on the slopes;
– where deterioration has occurred, describe whether it is rapid or slow; and
– changes which have occurred since the previous inspection.
Note: A sketch plan of the dam assists the recording of observations. Ideally it should be approximately to scale and the
locations of observations should be indicated on it.

This is not a complete list, but serves as a guide. If a condition has changed
since the last inspection, records should be kept in the diary as follows:
• describe the change;

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 Maintenance • 99

• take photographs of the site, including a close up of the changed condition, if

possible; and
• note the date and a description of the scene shown in the photograph.
Remember, the primary purpose of the inspection is to pick up changes that
have occurred since the previous inspection. If a situation looks as if it is
worsening, or causing concern, the owner should not hesitate in seeking
professional help. Table 6.1 is a typical checklist for noting defects and keeping
long term records.

Table 6.1 Dam inspection checklist

Areas of dam Items to address Observations

a. Upstream slope • Protection

b. Crest • Uniformity
c. Downstream slope • Displacements, bulges, depressions
• Cracking
• Erosion
• Rabbit, wombat or yabby activity
• Obscuring growth (trees)
• Wetness
• Changes in condition
• Stock damage
Seepage • Location
• Extent of area
• Characteristics of area (i.e. soft, boggy, firm)
• Quantity and colour
• Transported or deposited material
• Spring activity
• Boils
• Piping and tunnel erosion
• Changes in vegetation
Outlet • Outlet pipe condition
• Operability
• Leakage
• Downstream erosion
• Gate valve leakage
Spillway • Condition of downstream slope protection
• Spillway obstructions
• Erosion or back cutting in spillway
Source: DC&NR, 1992.

6.2 Inspection procedures

Some preparatory work before an inspection provides direction and purpose. This
may include a review of the previous diary entries to note any areas which will
require special attention. If the inspection aims to re-evaluate suspected defective

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 100 • Farm Dams

conditions discovered during the last inspection, any available construction

drawings should be examined first for a possible explanation of the situation.
The best results can be achieved by developing consistent records of
observations. Following a specific sequence of observations during the inspection
is a useful approach, such as:
• upstream slope
• crest
• downstream slope
• any seepage areas
• outlet, and
• spillway.
This will lessen the chance of an important condition being overlooked. It is
also wise to report inspection results in the same sequence to ensure consistent
records.
6.2.1 General techniques
The inspection is conducted by walking along and over the dam as many times as
necessary to see every square metre, a person can usually obtain a detailed view
for a distance of 3 to 10 m in each direction (Figure 6.1), from any given location.
The distance will vary according to the smoothness of the surface or the type of
material on the surface (for example, grass, concrete, rock, brush).
Figure 6.1 Sight distance

To cover extensive surfaces properly, several passes are required (Figure 6.2).
Adequate coverage can also be achieved using parallel or zigzag paths (Figure 6.3).
Figure 6.2 Successive passes

Figure 6.3 Coverage path

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 Maintenance • 101

On the downstream slope a zigzag path is recommended to ensure that any

cracking is detected. At several points on the slope, you should stop and look
around through 360° to check alignments and ensure that some important feature
of the slope has not been overlooked.
6.2.2 Specific techniques
Monitoring changes that will occur over a period of time can include a number of
methods, including:

Sighting
A sighting technique, similar to that used when selecting straight pieces of timber,
can be used in identifying misalignment as well as high or low areas along a
surface. This technique is illustrated in Figure 6.4.
Figure 6.4 Sighting technique

Line viewing

Person sighting Through eyes Through eyes

1 2 3

The same method can be used to sight along the crest of a dam. Centre the eyes
along the line being viewed. Sighting along the line, move from side to side a little
to view the line from several angles (Figure 6.5). Looking through a pair of
binoculars will make any variations more obvious.
Figure 6.5 Sighting along crest
Crest of dam

Crest of dam

Crest of dam

Crest of dam

1 2 3 4
Straight Bowed

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 102 • Farm Dams

Probing
The blunt end of the probe (see Section 6.1.1) is pressed into the earth on the
batter slopes, crest or at places being inspected. Conditions below the surface,
such as depth and softness of a saturated area can then be noted. By observing the
moisture brought up on the probe’s surface and the resistance to penetration, it is
possible to decide whether an area is saturated or simply moist.

Pegging-stakes
The best way to find out if there is a leak is to check how fast water is
disappearing from a storage by marking the waterline with a peg at regular
intervals, for example weekly. If the storage is used for stock or irrigation, the
waterline should be pegged before and after use.
Measuring in this way is much better than simply guessing. A suspected leak,
when measured, may turn out to be only evaporation loss. Evaporation can easily
be 5 mm per day, and as much as 10 mm per day in dry and windy conditions.

Uneven surfaces and displacements

Slides are as difficult to spot as cracks. Their appearance is subtle, since there may
be less than 1 m of depression or bulging in relation to the normal slope for a
distance of perhaps 10 m. This may be complicated further in situations where the
dam was not uniformly graded by the bulldozer operator during construction.
Therefore, familiarity with how the slope looked at the end of construction helps
identify any slides. One method of monitoring batter surface movement is to place
a straight line of stakes down the slope with a string tape attached to the top of
each stake. The point at which a slide takes place will cause the uphill stakes to be
uprooted, whilst those just downhill of the movement will show a slackening of
the string tape.
6.2.3 Evaluation of observations
The record of observations taken at periodic inspections is used to develop a
mental picture of the dam’s performance over time. Accurate measurements pay
off here because small changes, which could go undetected if simply looked at, will
show a pattern or trend.
Immediately following the inspection, the observations should be compared
with previous records to see if there is any condition, measurement, or trend that
may indicate a developing problem. The owner can then begin to address any
potential problem before it becomes a threat to the dam. When a significant
change is detected, any design drawings for the dam should be examined carefully
to see if an obvious reason for the change is apparent. If there is any uncertainty
regarding a change, a professional engineer experienced in the field of farm dam
engineering should be engaged immediately to determine if any action, such as
increased monitoring or detailed investigation of the condition, is required. These
actions will help ensure the safety, safe operation, and long, useful life of the dam.
6.2.4 Frequency of inspection
The required frequency of inspection required for an effective program of
monitoring is dependent on a variety of factors including:
• size or capacity of the dam;
• condition of the dam (risk category); and

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 Maintenance • 103

• potential for damage resulting from failure of the dam (hazard category).
The larger a dam, the more frequently inspections are required. Notifiable
dams that have a significant hazard category, for example, need to be inspected
more frequently than once a year. Dams in this class are defined as being >5 m in
height with a 50 ML storage capacity, or 10 m in height with a 20 ML storage
capacity and their failure would result in significant economic damage or loss of
life (see Section 10.4.1).
The inspection frequency for a particular dam is the responsibility of the
owner, although large dams, or those suspected of being in a high risk or hazard
category, require professional advice. Detailed, comprehensive inspections can be
alternated with more frequent, rapid, visual inspections aimed at detecting
unusual changes that have occurred in the interim period.
Following a regular routine like this should enable the dam owner to become
aware of faults before partial or total failure occurs. Times when additional close
inspections are recommended are:
• before a predicted major rainstorm (check spillway and outlet pipe);
• during and after severe rainstorms (check spillway and outlet pipe);
• during and after a severe windstorm (check upstream batter slope for damage
from wave action); and
• after any earthquake or tremor, whether directly felt on the owner’s property
or reported by local news media.
Inspections should be made before and during construction, and also during
and immediately after the first filling of the storage.

6.3 Causes of dam failures

There is little that a dam owner can do to prevent dam failure in response to
earthquakes, extreme storm activity and failure of upstream dams. However,
normal margins of safety should be capable of accommodating earthquakes of a
magnitude that is appropriate for the region, based on geological information.
Advance emergency planning to minimise risk and uncertainty is the only measure
available to the dam owner (Lewis, 1995a; DC&NR, 1992).
Ingles (1984) observed that failures fell into the following categories, as
follows: overtopping 20%, piping 26%, slope failure 20%, foundation failures
17%, all other 17%. Overtopping occurs when the actual flow over a spillway
exceeds the flow from which it has been designed (see Section 3). It may therefore
be regarded as a ‘natural hazard’, resulting from extreme low probability weather
conditions. The other main types of failure listed may be regarded as related to
human error. The high percentage corresponds broadly with overseas
observations. Of these human-error-related failures, two-thirds (piping and slope
stability) are more related to improper construction and operation controls. One-
third (foundation failures) are more related to errors of judgement in design and
geological assessment (see Section 2).
Effective monitoring and maintenance programs can protect dams against the
main causes of failure. A systematic program of safety monitoring should ensure
detection, in good time, of processes that require major repairs or cause dam
failure. Typical problems, which can be remedied if detected early enough, include:
• seepage/leakage
• cracking

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 104 • Farm Dams

• erosion, rilling and piping

• deformation and settlement
• concrete structure defects
• spillway blockage/erosion, and
• animal damage.
Figure 6.6 Problem areas on a dam site

Spillway blockage Spillway erosion

Settlement
Seepage and leakage
Rilling

Sedimentation Wave action

Blocked pipe
Stock traffic damage

Rabbit damage

6.3.1 Dispersive clays

Dispersive clays are common in many parts of Australia. They occur in soils whose
clay minerals separate into single grains when placed in contact with water.
Typically, water is cloudy or turbid where dispersive clays occur. Dispersive clays
are associated with high soil erodability and their distribution often coincides with
the occurrence of erosion gullying, rilling and piping. Dispersive clays in a dam
embankment can result in the leaching out of material from the embankment with
consequent erosion and failure. The failure may be gradual but often occurs very
rapidly, with little or no warning. A significant proportion of such failures has
taken place during the first filling.
As far as possible the use of dispersive clays in dam construction should be
avoided, but when this is not possible, the addition of a small proportion of lime
or gypsum, well mixed in with the embankment material, can help to stabilise it.
In existing dams, application of these chemicals to the surface layer of the
upstream face of the embankment may be beneficial. As a guide, application rates
of 3–5 per cent well mixed with the upper 200 mm would be used, but
professional advice should be obtained.
Compaction of dispersive clays must be carried out to very high standards in
accordance with specifications prepared by an experienced engineer. The moisture
content during compaction must be carefully controlled to be on or marginally above,
the optimum level. This can only be determined by laboratory testing of the material.

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 Maintenance • 105

6.3.2 Seepage and leakage

Water escaping from the dam can occur locally or over a wide area. It might be
visible on the embankment, the downstream toe of batter slope or the abutments,
and be either clear or muddy. The rate of flow might be small or large, steady or
increasing. Unless the flow is clear and the rate only small and not increasing,
most forms of leakage represent a warning of potentially serious problems and
indicate the need for early remedial work. The known or suspected presence of
dispersive clays in the embankment or foundations would be an indication for
even greater urgency. It is important that the embankment is well maintained and
grass is kept relatively short so that seepage is readily identified if it occurs. Unless
the cause of seepage and leakage is readily apparent and the repairs immediately
effective, expert professional advice should be sought.

6.3.3 Cracking and movement cracks

During dry periods there will always be minor cracking as the embankment dries
out. This may be prevented by good topsoil and grass cover. However, some soil
types are more prone to cracking than others. Where these types of soil are
common, cracking is often a serious problem.
Transverse cracks running across an embankment can promote seepage.
Longitudinal cracks running along the embankment can fill with water during a
storm and saturate lower layers. This may cause part of the embankment to slump.
Ideally, large cracks should be filled as soon as possible with compacted clay,
preferably mixed with expansible clay (with trade names of Bentonite or Volclay).
In practice, this can be difficult and it may be necessary to trench out the cracks
before filling them so the clay can be compacted.
It is well known that when highly plastic clays are subjected to alternating wet
and dry conditions, they will alternately expand and contract and develop thin
shrinkage cracks on the surfaces of dams. These cracks rarely exceed 600 mm in
depth, and consequently with properly designed freeboard allowances are unlikely
to be a serious problem. However, some failures have occurred in cheaply built
dams that are less than 3 m high.
Tensile cracking is considered to be the most dangerous form of cracking in
terms of dam failure. Tensile cracks are caused by differential settlement strains
induced within the bank by earth movements which are, in turn, products of non-
uniform settlement or consolidation.
The extent of this earth movement or deformation depends mainly on the
relative compressibility and geometry of the earth dam, the foundations and the
abutments. The influencing factors can distort the dam in different directions and
so create a variety of cracking patterns that may be continuous or local. These
cracks can be up to 150 mm wide. The two most common forms of tensile
cracking are longitudinal and transverse (Figure 6.7).
Longitudinal cracks develop roughly parallel to the centre-line of the dam and
are always in a vertical plane, which distinguishes them from shrinkage cracks,
which develop in a plane at right angles to the soil surface (compare Figure 6.7).
Earth movements creating longitudinal cracks result from:
• differential settlement in zoned dams
• foundation settlement
• foundation heave.

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 106 • Farm Dams

Figure 6.7 Different types of problems in an embankment

Shrinkage cracks

Longitudinal cracks

Transverse crack

Pervious material

Clay

Longitudinal crack in a zoned dam

Settlement caused by crack in the bed

Dry material

Seepage line Saturated material

Saturated settlement crack

Piping

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 Maintenance • 107

Longitudinal cracks in the crest of the embankment

Source: B. Lewis.

Longitudinal cracks are not usually dangerous but have proved troublesome
when combined with other weaknesses.
By contrast, transverse cracks need careful monitoring and can be particularly
serious because they tend to run straight through the dam. The main causes of this
type of cracking are:
• differential settlement due to steep and/or incompressible abutments;
• settlement of foundation; and
• saturated settlement of embankment.
Where saturated settlement of the embankment occurs, this will be due to
construction of the dam with material that is too dry. Consequently when the
storage fills, the dam material becomes saturated and then slumps. However, the
material above the seepage line remains dry and firm, resulting in an arched-type
failure.

6.3.4 Erosion
Erosion is also a problem with many causes and forms; the presence of dispersive
clays will usually increase its severity. The following are among the most common
forms of erosion associated with farm dams:

i Rilling of the bank (small erosion gutters down the bank)

This usually happens where there is no topsoil on the bank, and run-off becomes
channelled. To rectify the problem, pack rills with grass sods or cover the bank
with topsoil and sow down. Maintain a good grass cover on the embankment, and
exclude stock.

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 108 • Farm Dams

ii Damage from wave action

Wind across the water creates wave action damage (see Section 3.1.5). Protecting
the bank with rocks, hay mulch and netting, or grasses such as kikuyu can help.
For larger dams, more substantial rock protection is warranted and the use of
wind breaks should be considered.

iii Spillway erosion

By far the cheapest form of spillway is an earth type spillway (see Section 3.3.1),
comprising either soil clay material or grassed, well-compacted earth. In the
majority of cases, the nature of the site is such that an adequately sized spillway
can be cut into the side of a hill without too much trouble. Where the site, type of
soil, or extremely high flow prevents a normal ‘earth spillway’ from being
constructed, concrete-lined spillways (either on an abutment or over the centre of
the dam, see Section 3.3.4) can be provided.
A spillway return should be designed to spread the flow out to as great a width
as possible and to keep water flow velocity to a minimum. The spillway return
slope is that section of the spillway that returns the dam overflow to the creek.
This section is also subject to erosion. Consequently, it should also be grassed and,
preferably, should cross over natural, undisturbed ground. The gradient should be
as flat as possible. The spillway return should be located to direct flow away from
the batter toe of the dam to avoid erosion during floods.
Heavy flows over spillways can lead to erosion. Where this occurs, minor
erosion should be filled with grass sods, covered with hay mulch and pinned down
with plastic netting. The spillway should remain as level as possible across its
entire width. The installation of a trickle flow pipe (see Section 3.4.1) with a pit
will protect the vegetation cover on the spillway from prolonged saturation
following a storm. This is not an alternative to the outlet pipe. If erosion persists,
more substantial protection is probably required.

iv Sinkholes
Sinkholes are holes or depressions at the surface resulting from internal erosion
that has caused underground cavities into which the surface material eventually
subsides. Sinkholes are often a sign of severe and widespread hidden damage
caused by processes such as piping. It is necessary to determine the nature and
trace the extent of such damage, and to backfill all eroded areas with well
compacted (non-dispersive) clay before dealing with the visible surface holes or
depressions in a similar manner. Unless the defect is found to be superficial and
associated with surface run-off rather than leakage from the dam, professional
advice should be sought.

v Wind erosion
Erosion due to wind action can occur when the dam wall material has a high sand
or silt content, and vegetation cover is poor. It is often associated with the passage
of stock. Re-establishment of good grass cover is the best remedy.
A fence to exclude stock should be constructed around the perimeter of the
embankment, storage area, spillway and spillway downstream slope as soon as
possible after the dam is completed. Apart from damaging the grass cover and
creating ‘cattle pads’ which can lead to serious erosion, the water in the storage
can become turbid and polluted by continued stock access. The best alternative is

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 Maintenance • 109

to provide stock water at a trough remote from the dam, or install a fenced
walkway to a restricted area of the dam.

vi Rabbit and wombat damage

Animal burrows can be a source of seepage. They need to be dug out and repacked
with clay. If the burrows are extensive, the storage should be emptied before they
are dug out, or professional advice sought. Effective rabbit control needs to be
maintained.

vii Erosion at outlet valve

Erosion can be caused by water discharging through low level pipes beneath a
dam. Provision should be made at the outlet pipe point of discharge to reduce the
velocity of exiting water. It is recommended that rock with minimum diameter of
300 mm should be placed on a layer of crushed rock to minimise erosion, or an
effective outlet structure constructed to dissipate the energy of the discharging
flow.

viii Piping
When a failure is the result of an internal pipe or tunnel forming through an earth
dam, it is usually referred to by contractors as ‘piping’ and this may be a result of
a number of causes. The three most common causes are given below and are
illustrated in Figure 6.7.

1 Conventional piping
Water seeps through all earth dams, whether large or small. It is not the
occurrence of seepage that is problematic, but the rate of seepage. If the rate is
high, water will have the capacity to mobilise and transport soil particles. Viscous
drag forces within the dam will oppose this movement of water and so reduce the
eroding forces operating through the soil. If the drag forces exceed the eroding
force no piping will occur, but if they do not, then there is a serious likelihood that
piping failure will occur. Any line of weakness within the earth dam can accelerate
piping failure. This process may develop because no dam can be built of truly
homogenous (identical) material.
Conventional piping occurs in cohesionless soil (such as a silt). This form of
piping starts at the downstream side of the dam and then slowly proceeds to the
water face upstream. Collapse of the pipe also accelerates surface erosion.

2 Tunnelling
Tunnelling occurs in dispersive soils, which are soils that can be broken down or
separated into single grain components when placed in contact with water. The
process is initiated when water enters the dry soil through a crack, and then
disperses the soil exposed on the sides of the crack. The dispersed particles are
carried back, in suspension, to the dam storage. This process continues as more
water comes into contact with fresh dry soil and it will gradually take the form of
a tunnel (or pipe) which will eventually develop through the dam.
Since tunnelling starts at the water side of the dam it is more difficult to detect
than conventional piping, which starts on the downstream face. This makes
tunnelling far more insidious and dangerous than conventional piping.

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 110 • Farm Dams

3 Leaking pipe
Another form of piping results from poor laying techniques used on outlet pipes
as described in Section 5.9. This includes poor compaction around, and
particularly beneath the pipe, which is a difficult zone to reach and is frequently
neglected. The disturbance of a laid pipe by the movement of earthmoving plant
over it can also initiate problems. For example, a dozer may hit a cut-off collar
(see Section 3.4.3) and this in turn may create a ‘roofing’ condition (Figure 6.8).
Figure 6.8 Pipe bedding problems

Pipe through embankment

Poorly compacted

A frequent cause of outlet pipe failure is leakage in a poorly jointed pipe. This
is usually accelerated where a downstream valve has been included in the dam
design (Figure 6.9). The use of an upstream valve would at least decrease the
pressure on the joint and ensure that this reduced leakage flow would be confined
within the pipe.
Figure 6.9 Pipe failure above and below gate valve

Homogenous cross-section dam

Full supply level Phreatic line Crack in pipe which causes

water to leak out

Downstream gate valve

Inlet
Outlet

Valve at downstream end

Phreatic line
Full supply level Cavity formed due to seepage
Upstream gate valve leaking into crack in outlet pipe

Inlet
Outlet

Valve at upstream end

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 Maintenance • 111

6.3.5 Deformation and movement

i Settlement
Settlement occurs in response to poor compaction (see Section 5.7). Monitoring
for settlement includes checking the distance between full water level and the top
of the bank (the freeboard). Hard and fast rules cannot be laid down, but as an
indication, it should not normally be less than about one-fifth the height of the
embankment, or an absolute minimum of 0.5 m, whichever is the greater. If it is
less than this, the embankment should be raised or the spillway lowered.

ii Slides
Slides can be described as the downward movement of soil on the slopes of dams.
The slide usually has three distinct features. First, tension cracks develop on the
crest, then a substantial portion of the dam slips downwards, and finally a
pronounced heave occurs near the toe. Warnings of likely slides are first given by
these surface cracks on the crest or on the upper sections of the sloping sides (see
Figure 6.7).
Material that is likely to slide can occur on upstream and/or downstream
batter slope sides of an embankment. Slope failure and slides occur where dams
are built:
• on soft or clayey foundations with low strength;
• where batter slopes are too steep; or
• where the bank is insufficiently compacted.
This type of failure is most dangerous because it usually occurs when the dam
is full and therefore can cause a catastrophic flood wave downstream of the dam.
Additional problems can arise if a slide obstructs the outlet pipe, because it would
be impossible to release stored water. The danger, if detected early enough, can
sometimes be controlled by the reduction of water level in the storage, or by the
placement of stabilising rock-fill at the toe of the potential slide.
Slump extends to the centre-line of the embankment

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 112 • Farm Dams

Slump on the downstream batter slope

Slides are major structural defects, normally requiring major remedial works
such as flattening of batter slopes, improved drainage or the addition of rock-fill
as a stabilising weight at the toe of a slope. Selection of an economical and
effective remedy normally requires expert professional advice. In the short term, it
may be necessary to drain or pump out the stored water.
Slides can be divided broadly into three categories.
1 Construction slides which develop during the construction phase and may
occur on either the upstream or the downstream slopes. Almost without
exception, construction slides occur where dams are being built on soft or
brittle clayey foundations. This type of slide failure is seldom responsible for
serious damage because it takes place before the storage is filled, but it can be
embarrassing to both the designer and contractor. Two forms of construction
failure can occur. In the first case there is a rapid downward movement of soil
over a very short period (from 5 to 15 minutes). The vertical drop of the slide
could easily be as much as 3 m on a 6 m dam. In the second case, there is a
slower, uniform movement spread over a much longer period, usually up to
about two weeks, followed by a very slow creep. These two kinds of
construction slides are due to differences in the soil structure of the foundation.
2 Downstream slides which may develop after construction. The two kinds of
downstream slides are the deep slide and the shallow surface slide.
• Seepage through or under the dam creates internal pore water pressure
causing a slide which moves deep into the clay foundation and often
develops into the upstream slope of the dam. A frequent complication arises
when it completely obstructs the outlet pipe making it impossible to release
water as an immediate response.
• The shallow surface slide is often more due to saturation of the downstream
slope after heavy rains. It may vary in thickness from 50 mm to 1.2 m and
is most frequently found in poorly compacted farm dams. It is not generally
dangerous but can be expensive to repair.

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 Maintenance • 113

3 Upstream slides which usually occur after a rapid draw-down of water in the
storage. Like the deep downstream slide, it has a slide surface that cuts well
into the foundation. Rapid draw-down refers to any water level drop in excess
of 1000 mm per day (Table 3.1). Dam engineers make special allowances in
their design for storages where rapid draw-down is anticipated.
Also hillside dams can be impacted upon by uphill slides, even though they are
not located on the structure itself. In these cases, the balance between equilibrium
and landslide conditions may be particularly sensitive on the slope, causing the
least disturbance to mobilise vast volumes of soil downhill. The excavation for a
hillside storage, together with the presence of groundwater, sometimes creates
conditions resulting in an uphill slide into the storage.
6.3.6 Defects in associated structures
i Spillway blockage
The construction of an earth dam is not just a matter of pushing up a wall across
a gully. Many dams built this way fail because they do not have an adequate
spillway capacity to prevent floodwater overtopping the dam, or erosion on the
spillway return cuts back into the downstream sloping batter. As with most
problems, prevention is a cheaper and simpler strategy, so the following points are
recommended when designing your dam.
• All too frequently the importance of an adequate spillway is not fully
appreciated. When considering the size of a spillway, many people try to relate
it to the normal everyday winter flows that occur in the creek or stream. What
they do not realise is that these flows can be exceeded many times over and that
the safety of the embankment, on which they spend so much money, depends
entirely on the ability of the spillway to operate under high storm flow
conditions.
• The costs of repairing a failed storage and paying compensation for
downstream damages will be far greater than the cost and effort involved in
providing a few extra metres of spillway width.
• If spillways are blocked in order to increase the storage capacity, the bank may
overtop. This is a situation fraught with danger and must be rectified. Debris,
bushes, trees, shrubs, fences and tall grass should be regularly cleaned from all
parts of the spillway, including the approach area.

ii Outlet pipe blockage

The cleaning of an outlet pipe is a problem if the gate valve has been closed over
the winter since accumulated trash, fish and siltation can clog the inlet on the
upstream side of the embankment. This type of blockage can place a dam owner
in conflict with the public regulatory authorities since approval for the
construction of a dam on a waterway sometimes requires the gate valve (low level
pipe) to be operated to ensure flows through the drier months of summer. These
conditions cannot be met if the outlet pipe is blocked. Regular flushing is therefore
recommended to minimise the risk of blockages.
In the event of a blockage there are different techniques and types of equipment
available to dislodge an obstruction, including cleaning rods and flexible sewer
pipe cleaners. However, most equipment can only be used when the storage is
empty or close to it.

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 114 • Farm Dams

iii Trickle pipe

A trickle pipe is a high-level outlet pipe that will discharge before the earth
spillway comes into operation (Figure 6.10).

Figure 6.10 Trickle pipe for a farm dam

Trash rack
300 mm min. clearance from
embankment to top of pit

Full supply level

Internal diameter of pit
to be three times the
diameter of the trickle
pipe (minimum Trickle pipe.
diameter 600 mm) Diameter as specified
Unreinforced concrete base
100 mm deep
TRICKLE PIPE INLET PIT
NOT TO SCALE

Embankment crest

Freeboard 1 m Spillway floor level

100 mm minimum full supply level Cut-off walls at 6.0 m centres
1 metre square min.
Outlet pipe grade to suit
See detail for trickle downstream batter slope
pipe pit dimensions
Outlet trickle pipe
to be a minimum
of 1 metre
downstream of
embankment toe

Trickle pipe laid through Embankment material to be

embankment at minimum well compacted in layers Outlet trickle pipe to be beached
Compensation pipe grade 2 in 100 around pipe and cut-off walls with rock of nominal diameter
300 mm min. placed on a layer
of graded crushed rock

CROSS-SECTION THROUGH EMBANKMENT

NOT TO SCALE

To ensure the best performance of a spillway, small flows on a continuous basis

should never be allowed to pass for long periods (several days). These small flows
are more likely to cause erosion than much larger flows of short duration. Dams
supplied by springs are particularly subject to erosion of this type. To minimise
this problem, a trickle pipe can be installed to carry the low flows through the dam
without crossing the earth spillway. The size of the trickle pipe will depend on the
volume of these low flows.

iv Trickle outlet construction

Where erosion of the spillway by perennial or ephemeral low flows is an issue, a
trickle outlet can be simply constructed to minimise the risks. The information
provided, however, should be used as a guideline only, since widely varying
circumstances can apply on individual properties.
A simple and cheap drop inlet pit can be made from a standard 400 mm × 400
mm concrete sewage distribution pit. These commonly have two opposite outlets
to fit 110 mm pipes (the outlet size may need to be adjusted). For larger farm dams
the trickle pipe could be a 150 mm diameter pipe with a 1 m diameter drop inlet
pit. Alternatively a 200 Litre (44 gallon) drum can be used as a mould to make the
drop inlet pit.

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 Maintenance • 115

The trickle pipe is laid under the dam crest and run down the downstream
slope of the dam in a trench. Cut-off collars are used to prevent water flowing
along the trench outside the pipe.
The larger the trickle pipe, the larger the flow that can be passed and the greater
the cost of installation. For pipes up to 225 mm in diameter, PVC sewerage piping
is generally used because it is cheaper than steel, although concrete pipes are used
in some cases. The cover over the pipes should be not less than 350 mm deep. In
selecting a pipe class, it is advisable to contact the manufacturer to determine the
most suitable pipe and depth of cover to support anticipated vehicle traffic loads.
The outlet pipe on the downstream side of the storage should direct water onto
a rock mattress which is composed of graded layers of crushed rock to dissipate
energy and prevent scouring in the creek.

6.3.7 Vegetation
Trees
Planting trees and shrubs can provide windbreaks that prevent wave action and
associated soil erosion, and provide shelter for wildlife. Self-sown plants, trees or
other deep-rooted plants should not be permitted on the embankment and
spillway. The roots of these types of vegetation can provide a path for leakage
through the dam, and ultimately result in its failure. Trees that are alive and
apparently vigorous, such as poplars, can have dying roots with similar
consequences. Vegetation larger than small shrubs should not normally be allowed
to grow. Once established, a decision on whether trees should be removed, left
undisturbed or severely pruned has to be made on the basis of the particular
circumstances of the site. Rotting tree roots can also form tunnels that allow water
to leak and seep through the dam bank. These tunnels can lead to failure through
piping erosion. Seepage flow through the embankment or abutments can lead to
the development of high water pressures near the downstream face, causing failure
through sliding or erosion. Adequate compaction and the use of appropriate
materials in the design and construction phases may prevent these conditions from
occurring. If trees are required at a dam site, they should be planted at the
upstream section of the water storage, well away from the dam and spillway, in
line with the dam and at right angles to the dominant wind direction.

6.3.8 Total catchment protection

To maintain the required depth and capacity of a farm dam, the inflow should be
reasonably free from sediment. The best protection is to control erosion of the
surrounding catchment area. Land with a permanent cover of vegetation, such as
trees or perennial grass, provides the ideal catchment condition. If the catchment is
not suitably vegetated, you may need to utilise cultivated areas that are protected
by conservation practices, such as contour tillage, strip-cropping, conservation
cropping systems, vegetated desilting areas and other land improvement practices.

Desilting
While water levels are low in a dam, farmers have an excellent opportunity to
remove excess silt and mud from storages. One simple method of desilting is to
use a mud scoop of 1–2 m3 capacity and two tractors. The mud scoop is pulled
through the dam by the cable of a heavy crawler tractor and the mud is deposited

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 116 • Farm Dams

on the downhill side. The empty scoop is returned to the other side by a lighter
tractor and then pulled through as before (Nelson, 1968).
Care should be taken to ensure that the equipment does not expose sandy
seams on the sides or bottom of the dam as these would cause excessive seepage.
If sandy seams are discovered, leave 150 mm of silt to form a water-tight layer.
High levels of silt in a dam indicate soil erosion problems within the catchment.
6.3.9 Weed control
Aquatic weeds in farm dams can block pump and pipe inlets, deter stock from
drinking, and in some cases, taint the water (see Section 8.2). If weeds are treated
when they first appear, dams can be kept relatively free of some of the more
troublesome species. All plants can become a problem and each type may require
a different control method. However, in all situations the same factors should be
considered in deciding what control methods, if any, should be used. In each case:
• determine whether there is a problem, and if so, what it is;
• identify the plant causing the problem;
• find out what control methods are available and which of them could be used
safely;
• investigate whether these control measures could cause any other problems (for
example, toxicity to fish and livestock) and if so, whether they can be avoided;
and
• decide whether control is practical, desirable and worthwhile.

6.4 Dam leakage

There is nothing more frustrating than to find that the dam you built to water the
stock and the house garden, to grow some fish, and to use for fire control becomes
a muddy puddle when you most need it. But many dams do just that. Anecdotal
evidence suggests that approximately 35 per cent of all newly built dams fail,
primarily because of soil type and the construction technique. Dams are built in a
range of soil types from sand to the stickiest of black clays. Each general soil type
has its own characteristics and problems. In building construction you can
designate the quality of the concrete, the timber and the steel, but with soil, you
can’t be so sure. In fact, no dam can be guaranteed against leakage or failure.
However, the risk can be reduced to nearly zero by taking the right precautions.
Dams can leak simply because the construction was faulty. Where the bank is
built on top of the natural surface (that is, it is not compatible with the subsoil),
a line of weakness at the base of the bank allows seepage of water.
How a leaking dam is fixed depends on why it leaks. Therefore, the same
solution will not work every time. Leaking dams are usually fixed by one of four
methods:
1 rebuilding;
2 the use of soil additives;
3 the use of artificial liners; or
4 the use of clay liners.
6.4.1 Rebuilding
Badly built dams will always give problems and no amount of chemicals or liners
will overcome inherent design or construction faults. Some temporary works may

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 Maintenance • 117

help in the short term in some instances but usually these dams eventually need
rebuilding.
Since compaction is one of the most important factors in dam construction, it
should be remembered that wheeled tractors or sheepsfoot rollers are superior to
bulldozers. This is largely because bulldozers are designed to spread their weight
over the tracks with a large surface area and therefore exert low pressure. With
the cost of bulldozing being quite considerable these days, it is important to get
any reconstruction job done by an operator with a good reputation for building
dams (see Section 5.8). The best approach is to ask neighbours with good dams
‘Who built your dam?’ The local water distribution officer is also a good source
of information. Two or three quotes should be obtained for the project, including
details on when construction can be done. Remember contractors, like farmers,
are often at the mercy of the weather and usually cannot guarantee to be at your
property on a particular day.
6.4.2 Use of soil additives
Soil additives are used to stabilise some dispersible clay soils and to clog up other
soils that are highly stable but leak (SRW, 1995).

i Gypsum
Because of its calcium content, gypsum has been recommended to stabilise
dispersible clay and hence reduce the incidence of tunnelling and slumping.
Gypsum should be added to the soil when building new dams and after repairing
failed dams in dispersible clay soil areas.

ii Bentonite or Volclay
Bentonite or Volclay are trade names for a naturally occurring clay that swells up
as a gel, when wet, to about ten times its original size (CETC, 1995). It is this
property that makes it useful for sealing dams. These expanding clays are best
suited to reducing seepage in dams made from sandy or gravelly soils. Where it is
impractical to drain the dam the clay can be sprinkled onto the water surface of
the storage. Although it has a role in emergency repairs, it is only of limited use.
If the leak is too rapid, or the clay is too thinly distributed in the water, the effect
will be minimal.

iii Sodium tripolyphosphate (STPP)

This chemical has the opposite effect to gypsum. It aims to disperse clay particles
that are naturally highly stable (Albright and Wilson, 1990).
STPP is used in red and chocolate soils that are very stable and good for growing
plants, but leak badly. These soils have high pore space (porosity) and require
compaction to make them more impermeable. However, this is very difficult
naturally, and STPP is added to the soil to complement the compaction process.
Laboratory tests are needed to determine the suitability of STPP for individual
soil types because not all red soils react favourably to this treatment.
6.4.3 Artificial liners
Artificial liners are often used when the chance of natural seepage, or the cost of
alternative treatments, is high (Table 6.2). Liners used to hold water can range
from flexible black polythene sheet to air blown concrete (gunite).

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 118 • Farm Dams

The cost of liners varies considerably depending on site preparations,

installation techniques and operator experience. While artificial liners may offer
an obvious solution to a leaking dam problem (particularly in porous soils), it
should be realised that most are subject to damage from stones, sticks, stock and
sunlight. All lined dams should be fenced to exclude stock and other animals that
might damage the liner.
Plastic liner on the upstream side of the dam crest

6.4.4 Using a clay liner

Soil itself can be used to line a dam if it contains enough clay and if it is compacted
carefully when moist. Representative samples of the clay to be used need to be tested
for suitability. The blanket of clay needs to be 300 to 600 mm thick, depending on
its erodability and how much it shrinks and cracks when dry. The cost of making a
clay liner depends mainly on how far the clay has to be transported.
Figure 6.11 Typical cross-section of hillside dam
Protective blanket
Floating breakwater

Freeboard height
Topsoil 50 mm Full supply level
Crest width
to 150 mm deep
10 or more
1
2 1–2 to 3 1m
1
Bank height 3 to 4 3 to 4
1 1

Topsoil

Lower layer ripped

Rip to 50 mm then compacted
below bank Excavate topsoil for
entire foundation area

Treated layer compacted with or without

chemicals. Covered with polythene sheeting
Source: SRW, 1995. if this is used as a mulch

Note: For construction in well structured friable clay soils.

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 Maintenance • 119

Self-installation reduces the costs of a liner or other treatment to seal a dam.

There may be advantages, however, in employing people experienced in dam
design, soil treatment and liner installation. Typical problems that need to be
considered include:
• uplift of liners by groundwater pressures;
• uplift by wind blowing over the liner;
• damage by sunlight;
• drying, cracking, shrinking, swelling or settlement of soil;
• inadequate soil compaction;
• soil erosion; and
• damage by plant roots or animals.

Table 6.2 Cost for lining dams

Materials Typical material Typical labour costs Price range
cost ($/m2) in total cost
Machinery cost Manual labour ($/m2)
($/m2) ($/m2)

Liners

Black Polyethylene Membrane 2 – 0.5 2–5

Woven (reinforced) Polyethylene 3 – 0.5 4–8
Membrane
Vinyl (PVC) Membrane 5 – 0.5 4–8
Hypalon 14 – 1 10–20
Hertalan (EPDM) 10 – 1 10–20
Butyl Rubber 12 – 1 10–20
Chlorinated Polyethylene (CPE) 10 – 1 10–20
Compacted Asphalt 7 2 5 8–16
Reinforced Concrete 10 2 6 12–24
Gunite (Air Blown Concrete) 10 20 – 10–40
Steel 10+ – 10+ 20+
Compacted Soil – 3 – 2–15
Chemical treatment
Bentonite 7 4 1 2–10
Sodium Tripolyphosphate or 1 2 0.5 2–20
Sodium Carbonate
Bitumen/Soil Mix 3 3 – 4–10
Cement Stabilised Soil 0.5 3 1 4–10
Gypsum Stabilised Soil 0.01 2 0.5 2–8
SS-13 Waterborne Dispersion 4 – 0.5 3–10
Underlays for drainage or
protection
Geotextiles 2 – 1 3–20
Sand 0.5 0.5 – 1–3
Source: SRW, 1995.

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 120 • Farm Dams

Notes:
1 Total costs of polyethylene and vinyl liners include a secondary cover of heavy duty sheeting around the top half of the
dam, to protect the liner from sunlight and other causes of damage. Protective covers of soil may be applicable where
gradients are no steeper than 3.5:1 (horizontal:vertical).
2 The range in total costs for bentonite, sodium tripolyphosphate, sodium carbonate and gypsum-stabilised soil includes
allowance for a protective cover of untreated soil 100 mm to 1000 mm thick over the treated layers. Different thicknesses
are necessary in different situations to protect treated layers from varying degrees of erosion, drying and cracking.
3 Costs do not include the provision of a firm compact base onto which liners or other treatments are placed. Dam
construction costs are also additional and are valid at time of writing this book.
4 Not all types of liners or chemicals are listed in this table. Information about other treatments can be obtained from the
suppliers.
5 Material costs do not include allowances for transport or delivery.

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 Section 7
Water
Two aspects of water requirements need to be considered, quantity and quality.
The quantity of water required is one of the factors which determine the size of a
storage (see Section 1). Other factors include:
• losses from the storage;
• practical limits on the size set by topography and materials;
• probable volumes of run-off or pumped inflow available; and
• cost limits set by available finance.
The quality of the water requirement is also important and may lead to a
storage site being unsuitable for the proposed purpose. In other cases, it may be
the reason why a storage is preferred to other, unsuitable sources of supply.
It is important to recognise that the quantity and quality requirements,
especially for irrigation but also for stock and domestic supplies, cannot be
determined absolutely. Consequently, there is no such thing as ‘the water
requirement for irrigated lucerne’. Both the required quantity and the limiting
quality are controlled by economics. It is correct practice to use less water if it is
more expensive, and to then manage with lower production in some years. The
water quality which can be tolerated depends on how much water is available,
what it costs, what the product is worth, and even the method of irrigation (see
Section 1.9). Consumption of water by livestock is subject to considerable
variation depending on the age and condition of the animal, available food,
climatic conditions and water quality. Therefore, ‘standard’ methods of estimating
water requirements, and ‘standard’ tables of suitable water quality provide
guidelines for management and monitoring. This is particularly important in use
of on-farm water supplies, particularly farm dams, where the cost and quality of
the water is extremely variable.

7.1 Water quantity

One of the factors determining the planned capacity of a farm dam is the potential
water demand of stock, domestic services or irrigation areas. Recommended
storage design procedures are based on the provision of sufficient storage capacity
to meet water demands over a selected ‘critical storage period’. This is a period
without appreciable inflow when the reservoir must supply all water demands and
losses without replenishment (see Section 1.2). Rule-of-thumb values of critical
storage period are used for the design of minor storages. However, the critical
storage period must be determined from an analysis of past run-off estimates for

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 122 • Farm Dams

the design of major storages. In both cases, it is necessary to calculate a quantity-

and-time pattern of water use for the critical period, in order to determine the
storage volume needed to meet all demands and losses.
Average consumption figures may be used as a basis for preliminary planning
(see Section 1.5.4, Table 7.2). They may also be used to calculate patterns of
demand for the design of minor storages, provided that these patterns are
computed for the appropriate critical storage period.
Peak consumption figures should be used for the design of pumps, distribution
systems and spray irrigation layouts. They should not be used for storage design,
except in the case of trickle-inflow storages for irrigation use. In storage design the
use of average consumption figures for estimating reservoir demand may lead to
under-design, especially if the critical storage period is more than one year and
includes two ‘rainy’ seasons. A quantity-and-time pattern of demand must
therefore be calculated. This is particularly important in the design of major
irrigation storages, for which a detailed monthly analysis of irrigation
requirements over a critical storage period of known severity is essential (see
Section 1.2.2).

7.2 Water quality

Water quality must be considered in project planning for several reasons. In the
first place, the quality of the water available from a farm water system will
determine the uses of that water and hence govern the overall feasibility of the
system. As many natural waters have impurities that make them directly harmful
to crops, a knowledge of the quality of the water supply is essential (Table 7.1).
Furthermore, water quality may be one of the factors that determine the potential
storage capacity. For example, if saline water is intended for irrigation use,
additional quantities of fresh water must be applied periodically to avoid damage
to the irrigated crops or soils. Finally, the quality of the water to be stored in a
farm dam must be considered in the design and construction of the impounding
embankment. The presence of certain mineral constituents (including calcium,
magnesium, potassium and sodium) in stored waters may be the cause of
tunnelling failures in some soil types.
A variety of harmful constituents may also impair water quality and should be
considered in farm water planning in terms of the intended uses (Table 7.3).

i Water for consumption by humans and animals

• Bacteria and other harmful organisms can cause illness or disease.
• Dissolved minerals can produce an unpleasant odour, or make water unpalatable.
• Silts, algae or other suspended particles can cause water to be unpalatable,
discoloured or odorous.

ii Water for household, dairy or general farm use

• Dissolved minerals can cause hardness, which makes washing difficult and
promotes scaling in pipelines, boilers and hot-water systems.
• Dissolved minerals, acids or gases can corrode bore casings, pumps, pipelines
and other farm infrastructure.
• Minerals or suspended impurities can discolour water and cause staining or an
unpleasant odour.

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 Water • 123

iii Water for irrigation

• Dissolved minerals can inhibit plant growth or alter the character of irrigated
soils.

iv Water to be stored behind farm dams

• Dissolved minerals can affect the structural stability of the soils in the dam and
lead to tunnelling failure.
Water samples should be tested in a laboratory to determine the presence and
likely effect of impurities. Bacteria and other micro-organisms may render water
unsafe for stock or human consumption. In addition, the main factors which
determine the suitability of water for various farm purposes depend on the nature
and quantity of the dissolved minerals or salts.

Table 7.1 Impurities frequently found in water

Common name Chemical name Adverse effect Appears as
Lime Calcium Carbonate Hardness Dissolved Solid
Magnesia Magnesium Carbonate Hardness Dissolved Solid
Gypsum Calcium Sulphate Hardness Dissolved Solid
Epsom Salts Magnesium Sulphate Hardness & Purgative Dissolved Solid
Glauber’s Salt Sodium Sulphate Taste & Purgative Dissolved Solid
Common Salt Sodium Chloride Taste & Purgative Dissolved Solid
Iron Ferrous Iron Rust Stains Dissolved Solid
Ferric Iron Rust Stains Solid or Rust
Manganese Manganese Black Stains Dissolved Solid
Manganese Oxide Black Stains Black Solid
Carbon Dioxide Carbon Dioxide Corrosive Gas Absorbed Gas
Rotten Egg Odour Hydrogen Sulphide Corrosive Gas & Bad Odour Absorbed Gas
Marsh Gas Methane Inflammable Gas Absorbed Gas
Nitrogen Nitrogen Inert Gas Absorbed Gas
Oxygen Oxygen Oxidising & Corrosive Absorbed Gas
Source: SCA, 1983.

Seepage which flows both directly into the storage area and as low flows in
gullies may have very high levels of salinity. This salinity is often associated with
a rising water table caused by clearing of trees, or by unusually wet periods, and
may well occur for the first time after a storage is built. If this saline water is
allowed to accumulate in a dam during dry weather, the overall quality of the
water may deteriorate so that the water is unsuitable for irrigation even after fresh
run-off has occurred.
It is therefore important to check the salinity of any low flows present at the
design stage. If any salinity is present, steps need to be taken to ensure that this
water by-passes the storage. If this is not practicable, the site should not be used
for storage.
Any source of farm water other than rainwater, which is intended for human
consumption should be tested to ensure that no pathogens or toxins are present.

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 124 • Farm Dams

Table 7.2 Average water consumption for farm animals

Type of Stock Daily consumption (Litres/head) Annual consumption

Summer Winter (kL/head)

Sheep

ewes-lactating – dry feed 9 7 2.88

mature sheep – dry land 7 4.5 2.07
mature sheep – wet feed 3.5 2 0.99
Lambs

fattening – dry feed 2.5 1 0.63

fattening – Irrigated pasture 1 0.5 0.31
Cattle

grazing (<550kg wgt) 45 30 13.5

grazing (>550kg wgt) 67.5 45 20.7
feeding lot 94 60 27.2
calves 25 15 7.2
dairy cows in milk 70 45 20.7
cows – dry 45 30 13.5
Horses

working 54 37 16.4
grazing 36 27 11.3
Pigs

sows – lactating 23 18 8.2

sow & litter 45 21 –
growers (70kg) 10 8 3.24
grower (25kg) 5 3 –
Poultry

laying 32L/100 – 12
pullets or broilers 18L/100 – 7
turkeys 54L/100 – 20
Source: Modified ANZECC, 1992 and Hislop, 1998.
Note: The above figures are only average consumption. Higher demands in hot, dry weather conditions and with dry or salty
feed.

7.2.1 Domestic use

i Problems
The main factor for determining the suitability of water for domestic uses (other
than human consumption) is its hardness. Table 7.3 gives suggested hardness
limits for domestic and general farm use.
Muddy water in farm dams occurs when the water contains suspended matter,
that is fine clays and organic material. Some of the fine clays can remain in
suspension almost indefinitely. Muddy water is undesirable when used for
domestic purposes. If the water is used for trickle irrigation, blockages can occur

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 Water • 125

in the trickle tubes, and bacteria or mud in water supplies can cause reduction in
milk quality. The treatment of muddy water is discussed in Section 7.3.
To minimise the risk of muddy water supplies, domestic household water for
drinking and cooking purposes should be supplied by rainwater collected from the
roofs of houses and sheds.

ii Precautions
Dams intended for household supplies or trickle irrigation should be fenced out
from stock. Ducks and geese should also be denied access. While this is primarily
a hygiene precaution, it also prevents the clay from being stirred up. It is advisable
to check carefully that muddy water is not caused by erosion in the dam
catchment. If this is the case, the erosion should be treated because correcting this
problem is less costly than continually treating the water.
7.2.2 Stock water
General farm use is taken to include spraying, dipping, washing in dairies and
milking sheds, and septic tank flushing. As a consequence of the wide variety of
uses, it is not feasible to lay down specific water quality criteria.
Some properties rely on dams for their stock and domestic water supplies.
They are filled from either local run-off or from water supply channels that flow
only once a year to enable the dams to be filled. The main problems with this
water include the high turbidity or clay colloid content and faecal pollution from
animals with access to the dam. To control this, a proportion of the water supplies
from the dam should be stored at regular intervals in a steel or concrete tank and
treated to flocculate the clay and eliminate bacterial contamination.
The major factor determining the suitability of water for stock use is its total
salinity. In general, stock can tolerate water having total soluble salts (TSS) levels
far in excess of those acceptable to humans. Table 7.3 shows the generally
accepted limits of TSS in water, which can be drunk by stock over long periods.
7.2.3 Irrigation water
Water can be obtained from natural run-off, diversion from streams and rivers or
the development of underground resources by bore or dragline hole, but it must
be of acceptable quality. In general, the quality of water for irrigation can be
assessed in terms of six criteria, which together indicate the potential harm to
crop, soil or equipment. These criteria are: pH, total salinity, sodium adsorption
ratio, residual alkalis, specific ions, and iron (where micro-irrigation is involved).
In addition, type of plant, nature of soil, extent of drainage and climatic
conditions may determine the feasibility of an irrigation proposal.

7.3 Water treatment for human consumption

Threshold values of bacteriological contamination are laid down by State Public
Health Authorities, as are standards for turbidity, taste and odour for town water
supplies. By contrast, the acceptable level of tolerance for farm water supplies
relating to turbidity, taste and odour may depend upon the individual landowner.
There are several methods of treating muddy water, the most common being to
use aluminium sulphate (filter alum or potash alum) and lime, as specified below.
This treatment causes the fine clay particles in the water to aggregate and sink to

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 126 • Farm Dams

the bottom. However, water in the dam will only stay clear until the next run-off
event, when sediments either flow into the dam or are stirred up from the dam
floor. When this occurs, the water has to be treated once more. Therefore, instead
of attempting to treat water in the dam, it is more effective and economical to
pump a required quantity of water into a holding tank. A fully-dissolved basic
solution of 1 kg each of alum and lime per 10 000 L is prepared and tested with
proportionate quantities of solution and water to determine whether the
concentration should be increased or reduced. If the quantities are satisfactory, the
solution is stirred thoroughly into the tank water. It will take from 24 to 48 hours
for the suspended clays to settle. The clean water can then be drawn off and the
tank cleaned in preparation for the next batch.

7.4 Algae in farm water supplies

Although algae are mostly microscopic, in large numbers they can appear as
floating masses or as slimes on waterways, dams, troughs, tanks or on the walls
of water containers (Powling, 1990).
The major requirements for algal growth are sunlight, water, carbon dioxide
and nutrients, for example, nitrogen, phosphorus or organic matter. Algal blooms
can become problematic in catchments where high rates of fertiliser are applied,
or where the parent rock is volcanic (basalt). In each case, high levels of
phosphorus can be delivered to dams, which then provide a rich source of
nutrients to promote algal growth. Most algal problems arise in late summer–early
autumn when water temperatures are elevated and light levels are high. Given
favourable conditions of water temperature and nutrient levels, an algal bloom
can appear literally overnight.
Freshwater algae have been divided into four groups (Powling, 1990).
• The flagellates are of all shapes, sizes and colours and are capable of
independent movement by means of their whip-like projections (flagella). In
large numbers, they are often responsible for the tastes and odours that can
occur in drinking waters.
• Green algae are best known by the long, green ribbons of growth often seen in
rivers and channels, on the sides of tanks and drinking troughs, and in thick
tangled masses in low-lying swampy areas. Green algae too may be of all
shapes and sizes.
• Diatoms consist of silica. They are usually brown or yellow in colour, and often
may be seen attached to the stems of other plants, coating them with a brown
slime. Other members of the group are planktonic, that is, they are suspended
in the water rather than being attached to anything. They are not usually a
problem in farm water supplies.
• The blue-green algae are capable of very sudden, explosive growth and can
appear as a thick green scum on the surface of a lake or dam, more particularly
at the downwind end. They occur either as clumps or filaments (Powling,
1990).

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 Water • 127

Table 7.3 Water quality guidelines for domestic drinking

Common name of contaminant Guideline limits (µg/L unless otherwise stated)

Biological

Total coliforms Should not be detected by any two consecutive 100 mL

samples. 95% of samples in a year should not contain any.
Faecal coliforms No 100 mL sample should contain any.
Algae Up to 5 000 cells/mL can be tolerated.
1 000–2 000 cells of cyanobacteria can cause problems.
Toxic material – inorganic

Arsenic 50
Barium 1 000
Cadmium 5
Cyanide 100
Nitrate-N 10 000
Nitrite-N 1 000
Selenium 10
Organic

Benzene 10
Carbon tetrachloride 3
1,1–Dichloroethene 0.3
1,2–Dichloroethane 10
Pesticides

Polychlorinated biphenyls 0.1

Tetrachloroethene 10
2,4,5–Trichlorophenol 1
Chemical

Aluminium 200
Ammonia (as N) 10
Chloride 400 000
Copper 1 000
Hardness (as CaCO3) 500 000
Iron 300
Manganese 100
pH 6.5–8.5
Sodium 300 000
Total dissolved solids 1 000 000

Source: Modified ANZECC, 1992 and Hislop, 1998.

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 128 • Farm Dams

7.4.1 Problems
The problems caused by algae may be physical or biological:

Physical
• clogging of filters, meters, valves and trickle irrigation lines;
• corrosion of metal tanks and other structures; and
• de-oxygenation of the water during decay of a heavy growth,

Biological
• tastes and odours, which may be particularly noticeable in water used for the
house and garden;
• stock poisoning;
• death of fish and animals; and
• unsightly slimes on channels, tanks and troughs, and scums on the surface of
dams.
Not enough is known yet about the mechanisms of algal poisoning, or for
instance, when a scum becomes dangerous to humans or animals, although in
some cases the toxin has been isolated. The toxicology of algae is very variable;
research into the underlying mechanisms is ongoing. Physical symptoms in
humans can vary from loss of appetite, skin sensitisation to convulsions, paralysis,
and death.
Wind movement can concentrate the algal scum into a corner of the dam. If
animals consume a large amount of this material, they may become very sick.
However, it has been observed that these algae do not always affect animals and
it is believed that only under certain conditions do the algae release a toxin
(poison) into the water. A symptom of algal poisoning in diary cattle is loss of
appetite and the consequent decline in the milk yield. An animal can die as rapidly
as fifteen minutes after drinking contaminated water.
Prevention or reduction of algal blooms requires minimising the requirements
of algal growth. Although covering the water surface to lower sunlight is only
practical for small dams, trees may be located to provide appropriate shade.
The bed of the dam should be cleared before it is filled to reduce the quantity
of organic matter. Nutrient-rich sediments in the beds of farm dams have been
treated only on an experimental basis. Reeds and rushes along the dam margins
will compete with algae for nutrients and will provide shelter for wildlife.
Maintenance will be required to control the reeds and rushes. Cutting and removal
of reeds and rushes also lowers nutrient levels (see Section 8.1.5).
Waterborne sources of nutrients include phosphorus and nitrogen in fertilisers
and organic residues from stockyards, feedlots and dairies. Drainage from these
areas should be diverted away from the waterway and utilised elsewhere.
If any form of algae is found in a water storage, you should still seek the advice
of a person experienced in the area of water science immediately, before
attempting to treat the problem.
7.4.2 Identification
The type of algae should be identified as an initial step to defining optimal
treatment strategy. Identification is important if stock deaths have occurred. A
sample of the algae plus water should be collected in a clean drink container of

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 Water • 129

one litre capacity. The container should be filled, with a small air gap and tightly
sealed. An explanatory note giving details of the dam and the problem should be
attached. Samples should be sent to an accredited water laboratory for
identification.

7.5 Salt in dam water

Salinity is a widespread form of land degradation in many parts of Australia. Not
only can soil become saline, but groundwater and surface water can be affected
too. This section outlines ways of minimising the chances of water in your dam
becoming salt-affected. If your dam has already become salty, a ready reckoner for
measuring the suitability of the water for stock use is provided (see Table 7.4).
With the intense development of farms and associated difficulties in finding
suitable dam sites, poor quality (saline) water is increasingly likely to be found.
There is no point in building a dam which will collect saline water, particularly as
high rates of evaporation may increase the concentration of salt in the storage.
Before building a dam, check for saline ground in the catchment area along
drainage lines, around the site and in areas where the water from the catchment
gathers or disperses.
Under certain conditions, it is possible to design works in dams which enable
saline surface and sub-surface flows to be intercepted and passed through or
around the dam before affecting the water in storage. These works are ideally
included in the original dam design but can be added at a later stage. Techniques
for diverting saline water away from dams are outlined in Section 7.5.1.
Where salinity threatens to affect an existing dam, the vegetation cover should
be increased within the catchment by planting trees and deep-rooted perennials.
This will help to minimise recharge to the groundwater system. High groundwater
flows can upset the existing salt balance by flushing large quantities of soluble
salts, previously stored in the soil profile, to dams through the stream system or
directly from the underlying soil. Water from over-irrigation and residual salts
from fertilisers, may also contribute to the problem.
7.5.1 Minimising salinity in dams
Surface interception techniques to improve water quality are suitable for
protecting new works or when modifying an existing storage. Surface water
interception relies on diversion works being installed in a waterway or gully
upstream of the dam. Flows of saline surface water are intercepted either by a
suitably protected low embankment or diversion drain. The drain needs to be
constructed slightly off the contour to carry flows around the storage to be
discharged into the waterway downstream of the embankment or an adjacent
gully.
A limitation of this technique is that the diversion is not effective for permeable
soils where flows will eventually reach and pollute the storage. This can be
overcome by collecting the low flows in a sump and piping them either around or
directly through the storage, before discharging into the drainage line. Where a
diversion pipe is used and is laid around the storage, the available head will be
limited. Care will be needed when installing the pipe to maintain a uniform grade
and to prevent the formation of airlocks in the line.

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 130 • Farm Dams

7.5.2 Interception of sub-surface saline water

Sub-surface interception works involve a cut-off trench below ground level. This
extends into permeable material to intercept saline flows. Upstream of, and
adjacent to this cut-off, a filter drain helps to collect the flow and deliver it into a
diversion pipe. This pipe passes either around or through the storage. Since the
diversion pipe functions as a siphon, it must be periodically primed before
operation.
When sub-surface saline flows are found in your catchment and you are
considering the most suitable technique for interception, further investigation
should be carried out to determine the source of these flows. If a particular gully
or hillside is identified as the source, diversion works should be installed in the
specific area. However, if no point source can be determined, it may be necessary
to consider an installation beneath the storage.
Interception beneath the storage has the advantage of requiring a much shorter
diversion pipe. It also allows the main cut-off to perform the dual function of
preventing seepage beneath the embankment, and intercepting saline flows.
However, this technique should be viewed with caution, as there is a possibility of
the saline flows mixing with water from the storage before reaching the
interception works. An ideal situation exists where (a) the bed of the waterway is
covered by a clay layer that overlies rock, and (b) where salt inflow from the
surrounding topography occurs above the rock. The clay membrane helps to
suppress the poor quality flow, which is then removed by the diversion.
7.5.3 Maintenance
Maintenance of sub-surface and surface interception works is essential for their
continual successful operation. The long-term reliability of these works is
particularly important when it is necessary to reprime siphons or to periodically
check flows. There is a real danger that maintenance can be overlooked if the farm
changes ownership, particularly where the incoming landowner or share farmer
may not fully understand the significance of maintaining the works in operating
condition.
7.5.4 Downstream effects
Consideration should be given to the discharge of poor quality water downstream
of the dam or storage. In most cases, this would be the natural destination of the
water if the storage had not been constructed. The installation of interception
works does not change the situation with respect to downstream properties.
However, as the saline flow will have been diverted to the surface by the works,
you must consider your common law obligation or ‘duty of care’ (see Section 10.2)
not to harm another landowner. You may also need to consider and obtain permits
under other local legal requirements such as Proclaimed Catchments or Planning
Schemes (Local Government). If, for some reason, you are unable to discharge to
a waterway, then a natural or artificial evaporation basin should be considered.
7.5.5 Minimising the effects of evaporation
Evaporation concentrates salt in water. The surface area of the stored water and
local climatic conditions determine the evaporation rates. A narrow, deep storage
will lose less water by evaporation than a broad, shallow storage.

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 Water • 131

When building a new dam it is important to make it a minimum of 2 m deep

to avoid it becoming a holding basin for salt every time the water evaporates.
Constant evaporation leads to a build-up of salt on the base of the storage. Each
time new water flows into the dam, this salty base dissolves into the clear water,
polluting it with salt. To minimise this effect where possible, skim off the top 100
mm of salt residue using a grader blade. Remember to deposit the resulting sludge
in a place where it cannot leach back into the storage.
7.5.6 Stock and salty water
Where water is salty, as is often the case with groundwater, you may have
difficulty deciding whether your stock can drink it without harmful effects.
Although cattle and sheep can survive on quite salty water, better quality water is
required for poultry and pigs, and for animals that are lactating or growing. The
suitability of water for stock is usually assessed in terms of the concentration and
composition of dissolved salts. The limits adopted Australia wide for total soluble
salts in drinking water for stock are shown in Tables 7.4 and 7.5.

Table 7.4 Water quality standards

EC units Use

0–800 • Good drinking water for humans (provided there is no organic pollution and not too much
suspended clay material).
• Generally good for irrigation, although above 300 EC some care must be taken, particularly
with overhead sprinklers that may cause leaf scorch on some salt-sensitive plants.
• Suitable for all stock.
800–2 500 • Can be consumed by humans, although people prefer water in the lower half of this range,
if available.
• When used for irrigation, requires special management including suitable soils, good
drainage and consideration of soil tolerance of plants.
• Suitable for all livestock.
2 500–10 000 • Not recommended for human consumption, although water up to 3 000 EC could be drunk
if nothing else is available.
• Not normally suitable for irrigation, although water up to 6 000 EC can be used on very salt
tolerant crops with special management techniques. Over 6 000 EC, occasional emergency
irrigation may be possible with care or if sufficient low salinity water is available, this could
be mixed with the high salinity water to obtain an acceptable supply.
• When used for drinking water by poultry and pigs, the salinity should be limited to about
6 000 EC. Most other livestock can use water up to 10 000 EC.
Over 10 000 • Not suitable for human consumption or irrigation.
• Not suitable for poultry, pigs or any lactating animals, but beef cattle can use water to 17 000
EC and adult sheep on dry feed can tolerate 23 000 EC. However, it is possible that water
below these levels could contain unacceptable concentrations of particular ions. Detailed
chemical analysis should therefore be considered before using high salinity water for stock.
• Water up to 50 000 EC (the salinity of the sea) can be used to flush toilets provided
corrosion in the cistern can be controlled, and for making concrete provided the
reinforcement is well covered.
Source: Modified from SR&WSC, 1970.

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 132 • Farm Dams

Table 7.5 Range of total saline matter in water supply

Human Livestock Preferred maximum Upper limit dry conditions
Horticulture (ppm) (ppm)

mS/cm EC ppm mS/cm EC ppm

Drinking (human) 0.83 830 500 2.5 2 500 1 500

Biological effects – – – 1.3 1 300 780
Fruit trees – – – 3.0 3 000 1 800
Poultry 3.3 3 300 2 000 6.7 6 700 4 000
Pigs 3.3 3 300 2 000 7.5 7 500 4 500
Ewes – lambing 5.0 5 000 3 000 10.0 10 000 6 000
Dairy cows 5.0 5 000 3 000 10.0 10 000 6 000
Horses 6.7 6 700 4 000 11.7 11 700 7 000
Cattle grazing 6.7 6 700 4 000 16.7 16 700 10 000
Sheep (wet feed) 6.7 6 700 4 000 20.0 20 000 12 000
Sheep (dry feed) 8.3 8 300 5 000 25.0 25 000 15 000
Yabbies 23.3 2 330 14 000 41.7 41 700 25 000
Pacific Ocean – – – 58.3 58 300 35 000
Dead Sea – – – 550 550 000 330 000
Source: Modified SR&WSC, 1970.
Notes: Relationship between units: 1dS/m = 1 mS/cm = 1000 EC = 1000 µS/cm = 600 mg/l = 600 ppm.
EC units means electrical conductivity, in micro-siemens per centimetre (µS/cm).
To find the approximate mass of salt in milligrams of salts per litre of water (or kilograms per megalitre of water) multiply the
EC unit by 0.6; e.g. water of 2500 EC has 1.5 tons of salt per megalitre of water. A megalitre of water has a mass of 1000
tonnes.

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 Section 8
Ecology
8.1 Wildlife and plants in dams
Most farm dams are designed and constructed primarily for the conservation of
water for stock, domestic and irrigation purposes. However, while a dam can
seldom be devoted wholly to the conservation of wildlife, many farmers regard
this as an important secondary use.
If attracting wildlife is desired, the actual siting and shape of the dam are
important considerations. Unfortunately, the basic rules of design relating to
stock, domestic and irrigation dams vary in some ways from that of an ideal
wildlife dam. Furthermore, the characteristics of a dam designed for fish will vary
from one designed for waterbirds. However, with careful planning it is generally
possible to reconcile the various uses required of the dam and to modify the
design, siting and construction of most dams to suit all purposes. In most
Australian States, the wildlife associated with farm dams is almost exclusively
waterbirds, with other forms using the margins if these are suitable. Waterbirds,
and particularly wild ducks, are attracted to almost any stretch of water, whether
it be natural wetland or land deliberately flooded for a specific purpose. However,
birds will only remain if these water areas provide food, shelter, breeding sites or
other basic necessities.
The key to food production for waterbirds is water, and most areas of wetland
which are dry for a part of the year and then flooded, can produce bird food as a
regular part of the soil and water conservation plan for a farm. The level of this
supply depends upon a number of factors, including quality of the soil, vegetation
type, flooding frequency and depth, and present climatic conditions.
In general, farm dams require a certain aesthetic value to attract waterbirds.
They do not support a waterbird population in proportion to their area, nor do
they provide suitable habitat throughout the year. There are exceptions to this, but
in most places the importance of farm dams to waterbirds is a reflection of the
huge number of farm dams in existence rather than the quality of habitat provided
by them.
These dams could make a very real contribution to waterfowl conservation if
made into a more suitable habitat. In addition, farm dams can provide very
important habitat niches to frogs, which are an essential component of healthy
ecosystems.
Irrigation schemes can benefit wildlife in many ways, particularly in the drier
areas of the country, by increasing the food supply and sometimes the amount of

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 134 • Farm Dams

cover. Food sources include unharvested grain and insects in irrigated pasture.
Furthermore, the waterways and drainage pondages are sites for breeding, feeding
and resting. Balanced against this is the drainage of natural swampland, and the
clearing and channelling of waterways, all of which decrease or fragment habitats
for wildlife. Any wildlife management program in irrigated regions must provide
adequate breeding facilities, which are associated with an ample food supply as
well as shelter. Government agencies are endeavouring to provide this on State-
owned land but the assistance of the general farming community is necessary if the
land is to support sufficient numbers of waterbirds.
If the present waterfowl populations are to be maintained, we must not only
develop existing wetlands, but create new areas of suitable habitat. The
characteristics of a farm dam, which are relevant to wildfowl conservation, are
discussed in Section 8.1.7. It should be noted that many of these characteristics
conflict with the requirements for fish production, and it is often difficult to
reconcile these in the management of any one dam. However, dams of both types
occur, and there is a sufficient number to allow management for both fish and
wildlife conservation.
8.1.1 Water regime
The amount of wildlife attracted to a farm dam is determined by the area of the
water surface. A surface area between 0.2–1.0 ha has been found to be the most
practical. The feeding depth for most ducks and many of the wading birds in
eastern Australia is 25 to 300 mm, and therefore a large area of shallow water is
required. This is often only provided when a farm dam overflows, but could well
be a feature in the construction of new dams. Periodic drying out and flooding of
this shallow water environment, with the subsequent growth of rank, ungrazed
grasses and herbs provide ideal conditions for the maximum production of duck
and more importantly, duckling food.
8.1.2 Basic topography
The edges of dams should be long and shallow, and retain as much topsoil and
vegetation as possible in an undisturbed condition. In addition, the ideal
waterfowl pondage should have a shelving margin. Waterbirds prefer to walk out
of the water to feed, rest, or reach a nesting site rather than to fly onto dry land.
The edge of the dam should be as irregular as possible, as the maximum use of the
margins is made by waterbirds for feeding and resting. Consequently, the longer
margin provided by an irregular edge to a dam may outweigh considerations of
the acreage of the dam.
A desirable feature of any waterbird pondage is the inclusion of a number of
islands, which provide excellent breeding and resting sites, even if they have
relatively steep edges. Islands afford protection from disturbance, particularly
from predators in the breeding season. Half-submerged logs, mud banks, or open
flat margins can provide other useful nesting sites.
8.1.3 Vegetation
Trees and shrubs can be used around farm dams to attract wildlife, especially
birds. To be fully effective, planting for habitat should be done in combination
with the provision of feeding and shelter sites. Shelterbelts near dams can also
provide shade for stock and reduce evaporation by lowering the wind speed at

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 Ecology • 135

ground level and decreasing surface temperatures. In addition, frog species favour
dam sites characterised by a high percentage of emergent vegetation in the dam,
low levels of bare ground in the immediately adjacent area, and a high percentage
of tree cover within an approximate 200 m radius of the dam site (Hazell et al.,
2001).
Trees and shrubs should be placed carefully to avoid problems occurring in the
future. Trees should never be planted on the dam embankment and spillway, or
near the outlets. Trees, shrubs and grasses should be established around the
margins, and up to the waterline, using native species wherever possible. River red
gum (E. camaldulensis) can be planted in shallow water or on land subject to
flooding, to provide shade, and eventually nesting habitat and shelter for some
species of waterbirds. However, care should be exercised in the selection and
placement of tree and shrub species. Too many tall trees around a small dam will
make it unattractive to waterbirds, particularly if these are across their normal
flight lane on and off the water. Low shrubs will afford birds better cover and
minimise interference with their flight pattern, although a few tall trees are still
needed.
The planting of food crops for waterfowl has proved successful overseas, but
there has been little investigation into this aspect of waterfowl management in
Australia (Cowling, 1967). Anecdotal evidence suggests that sufficient food
sources are provided from pasture and grain crops and by the flooding of most
native vegetation.

8.1.4 Grazing by stock

Grazing the margins and shallow water zones of farm dams reduces food supply
through trampling and manuring, and is therefore detrimental to the provision of
waterbird habitat. The exclusion of stock by fencing the dam, including a
reasonable dryland margin, allows the area to provide sufficient food, shelter and
nesting sites. Many species of waterbirds nest in grass or other ground vegetation.
Stock do not need to be excluded for the entire year. An alternative means of
watering, or a fenced lane can be provided.

8.1.5 Waterways and swamps

Farm dams in irrigation areas have the potential to support whole populations of
waterbirds. Under these conditions, there is a significant acreage of waterways (for
both irrigation supply and drainage), and swamps which are too low to drain,
salty or receive drainage water. These areas can also be developed to produce
wildlife. In particular, those pockets of unusable land along creeks, or low-lying
swamps, can be fenced and planted with native vegetation, or even cultivated and
sown with grass or food crops.
A number of the more important duck foods are relatively salt tolerant, and
salty land can be converted to waterfowl feeding areas by flooding with excess
irrigation water, or drainage water from irrigation. In times of flood, water often
reaches these lower, saltier areas with comparative ease. Often these areas can be
planted with red gum and used to provide shelter for stock, or rough grazing if the
farmer does not wish to devote the entire area to wildlife production. While these
areas may be small in relation to larger and more spectacular natural swamps,
they would be large in relation to the more permanent water of the farm dams.

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 136 • Farm Dams

Often swamps carry a dense growth of cane-reed, cumbungi or rushes (see

Section 8.2.1). These provide ideal shelter, but little food, and often waterbirds are
deprived of sufficient open water to land or take off. These areas should be partly
cleared to provide sufficient landing space and food production, but some non-
invasive reed should be retained for shelter. Research currently under way indicates
that farm dams offer a significant array of niches for wildlife that may maintain
certain species in otherwise hostile environments. Dams appear to provide a chain
or corridor of opportunities. Hunting within this setting is not an option.

8.1.6 The role of hunting

Currently, pressures for public recreation are being exerted from intensively
developed urban areas, and from the rural population. However, the effect of this
additional pressure on natural resources has been disastrous for many species of
wildlife. Hunting is one of many pressures acting upon wildlife populations, and
often it accelerates, or even completes, a process of extermination of a species that
commenced with environmental change. These changes and associated impacts
include:
• drainage of breeding areas;
• competition with introduced animals for food;
• clearing;
• the insidious effects of pesticides or effluent; and
• altering predator–prey relationships.
Hunting reduces wildlife resources immediately, but there are also longer-term
impacts imposed by loss of biodiversity and vulnerability in reduced, fragmented
populations.
Hunting pressure on a farm dam will drive wildlife to less disturbed habitats,
which may not be as suitable for sustaining populations. This is particularly
important given the tendency for some duck species to stay close to the dams
where they hatched, and their preference of dam sites within 400 m of tree
hollows (Kingsford, 1992). The response to seek less disturbed habitats under
these conditions may also have longer-term impacts on ecosystems since the
interactions between species will be modified. Therefore, it is imperative that
hunting on or near farm dams should not be practiced.

8.1.7 Waterfowl management

The basic points for siting and construction (Cowling, 1967 and see Figure 8.1),
development and maintenance of farm dams for wildlife purposes include:

i Siting and construction

• Ensure that there is sufficient shallow water to produce food and allow birds
to feed.
• Provide at least one island as a resting and breeding site.
• Endeavour to provide the maximum amount of edge by an irregular margin.
• Provide a reasonable area of shelving edge to allow birds access to dry land.
• Place old logs, beams, in the shallow water as resting sites.
• Ensure adequate stabilisation of banks and waterways.

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 Ecology • 137

Figure 8.1 Dam layout for wildlife conservation

Grassed spillway (no trees)

Trees in clumps
Areas planted to trees
should be fenced out
from stock

Raft
Log (floating island)
Flight path clear of trees
Low watermark
High watermark

Island
A variety of vegetation
large trees, small trees,
shrubs, herbs and No trees
grasses. A variety of on dam wall
species as well as size
is useful
Trees set back
from high watermark.
Only a few overhang
the water

Shoreline
vegetation
Full supply level Floating island
Fill embankment

Shallow water provides food Minimum depth level (summer)

for wading birds
Depth approx. 2 m
Hollow logs or pipes
give protection from
predators At least 30% of the storage surface area should cover
deep water to enable fish to escape summer heat

Source: Modified from Hill and Edquist, 1982.

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 138 • Farm Dams

ii Development and maintenance

• Fence at least part of the water’s edge to exclude stock, particularly the
shallower parts of the dam.
• Remove any vermin and noxious weeds from around the dam. (These cut
down the production and availability of food.)
• Ensure that ground vegetation will develop, either naturally or by cultivation
and sowing.
• Plant the margins with native trees and shrubs, limiting tall growing species to
a few trees.
• If necessary, provide artificial nesting sites in the form of ‘rafts’ or nest boxes.
Remember that birds will not breed if there is no food for themselves, or the
young they will produce, regardless of the quality of nesting.
• Regulate the hunting pressure to a moderate level to retain the attractiveness
of the area for waterfowl.

8.2 Water plants in dams

Aquatic plants in farm dams can block pump and pipe inlets, deter stock from
drinking and, in some cases, taint the water. If plants are treated when they first
appear, the dam can be kept relatively free of the more troublesome species. Dense
weed growth or outbreaks usually indicate that the water is high in nutrients.
All plants can become a problem if allowed to spread and disrupt water flow,
and each species may require a different control method. When considering
control measures for aquatic plant problems, it is recommended that the following
steps be taken:
• correctly identify the plant, using a good reference guide or by consulting your
local naturalist or botanist.
• decide whether you will use mechanical methods (such as raking, slashing,
mowing, grazing and burning) or chemical methods, or a combination of both.
• if using chemicals, use only the recommended quantities and follow all safety
precautions listed on the container labels. Remember that greater quantities of
herbicide do not produce better results.
• avoid spray drift by spraying only on calm days.
• use of chemicals should be avoided if possible in or near waterways or drains
flowing into rivers or creeks.
8.2.1 Aquatic plants
The aquatic plants that are best known to most of us are the varieties of water
lilies we see either in urban lakes and farm dams or in fish ponds in home gardens,
and eel grass associated with aquariums. These plants rarely cause problems.
In the management of water resources in Australia, the aquatic plants of most
concern are duckweeds (Wolffia spp.) and azollas (Azolla filiculoides), arrowhead
(Sagittaria montevidensis), cane grass (Eragrostic australasica) and cumbungi
(Tyha domingensis), alligator weed (Alternanthera philoxeroides), water hyacinth
(Eichhornia crassipes), salvinia (Salvinia molesta) and lagarosiphon
(Lagarosiphon major). These plants have been declared noxious throughout
Australia because of their potential to disrupt the proper utilisation of water
resources through the blocking of streams and irrigation channels. Being noxious,
these plants must be destroyed wherever they occur.

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 Ecology • 139

These species have the potential to cause problems in water supplies. An

infestation that remains unrecognised for several years will spread and become
increasingly difficult or impossible to eradicate. Consequently, early recognition of
a potential problem is essential.
Reporting the occurrence of these species to the relevant State agency or water
authority is recommended even when correct identification of the plant may be in
doubt.

a Duckweed and azolla

Duckweed and azolla are free-floating aquatic weeds that grow and flourish on
slow-moving or ponded water, including farm dams and irrigation channels. These
plants can cover the water surface completely. As a result they may block pump-
inlets and deter stock from drinking the water. Both weeds use nutrients in the
water for their growth and, in most cases, wash-off from heavily stocked or
over-fertilised areas of the farm contribute to the problem.
The two weeds are easy to identify. Duckweed has round green leaves about
half the size of a match head. Azolla has slightly larger fern-like leaves, which are
usually green in their early stage or when growing in shaded areas, but redden on
maturity or in full sunlight.
These plants should not be confused with algae, which are fibrous like hemp
or cotton wool, or appear as scum on the water surface.
Duckweed is commonly spread by wild ducks carrying the seed on their
feathers from dam to dam, or by flushing downstream during heavy stream flows.
Duckweed and azolla are best controlled by:
• lowering the nutrient level in the water, by stopping inflows to a dam from
heavily stocked or fertilised areas;
• re-filling the dam, as often as possible, with good quality water; and
• cleaning out silt from the bottom to remove built-up organic matter.
In case of dense growth, as much of the weed as possible should be removed
by raking or ‘skimming’ it off the top of the water, and disposing of it
appropriately. This job is easier when there is a strong wind that helps blow the
weeds to one side of the dam. Raking reduces the amount of herbicide required to
kill the plants, and helps reduce the likelihood of an algal bloom caused by the
breakdown and decay of sprayed plants. Any remaining weed can be treated with
herbicides available from agricultural chemical suppliers, stock and station agents
and cooperative trading stores.
After spraying, water from the dam should not be used for domestic purposes
or sprinkler irrigation for at least 10 days, or for stock for at least a day. This
precaution is necessary as most chemicals take some time to break down in water.
However, they may not be harmful to fish or wildlife. It is recommended that
safety precautions listed on the container label are closely observed when using
any farm chemical. Follow-up treatments are also recommended to prevent
further infestations.
The entire dam does not need to be treated with herbicides as the plants will
settle to the bottom of the dam and decay, releasing more nutrients to continue the
cycle of plant and algal growth. If these treatments are unsuccessful, it may be
necessary to empty the entire dam, clean out the plants, remove silt, and re-fill
with clean water. This will help remove the nutrients that contribute to plant
growth.

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 140 • Farm Dams

b Arrowhead
Arrowhead is becoming increasingly prevalent in northern Victoria. Although it
has not spread as extensively as some plants, it has the ability to spread rapidly,
resulting in restriction to the flow of channels, drains and creeks. Arrowhead is an
erect plant which grows up to 1 m high. Its leaves are arrow shaped and are a
distinctive dull-green colour. Its flowers are white with yellow centres carried on
fleshy stems.
This plant is commonly found growing in shallow, slow-flowing creeks,
channels and drains. It spreads rapidly, forming dense clumps which slow water
flow significantly. Arrowhead reproduces by seed germination, underground
rhizomes (horizontal stems that put out roots and shoots), and bulbs which
remain viable in the soil for many years. Due to these various methods of
reproduction, it can be difficult to control this plant mechanically. Any rhizome or
root fragments left in the soil will result in a recurrence of the infestation.
Herbicides appear to be the only effective way to control this plant. Repeated
treatments are usually necessary because of the long germination period from
September to December. For herbicide rates, contact your nearest chemical supplier.

c Cane grass and cumbungi

Cane grass and cumbungi (bulrush) can usefully provide a habitat for fish and
wildlife, and help prevent riverbank erosion. However, if these plants are allowed
to spread across a stream, water flow may be restricted and the banks may
deteriorate, leading to flooding on either side. You must be careful when
controlling these plants that you do not completely denude the area.
Control measures include the use of a dragline in combination with an
appropriate herbicide. Using a dragline to pull out cumbungi and cane grass gives
immediate results but is often impractical because of poor tractor access, and weed
regrowth from broken rhizomes and roots. If weeds need to be cleared
immediately, the use of a herbicide followed by excavation with a dragline, no
later than two months after spraying, is recommended.
Where access along banks or embankments is difficult, spraying from a small
boat may be feasible. Chemical control used on its own does not result in
immediate clearance of the weeds, as they can take several years to decay
completely. In the case of dense, mature infestations, burning old foliage before the
new season’s growth starts is an effective way of reducing the amount of plant
matter. Ask your chemical supplier to recommend an effective herbicide.

d Alligator weed
This perennial creeper has shiny, green leaves and white clover-like flowers. It
forms dense, floating or rooted mats up to 1 m depth, and also thrives on dry land
sites. It spreads by stem fragmentation.
Alligator weed is arguably the most destructive aquatic plant in Australia. At
present, it covers several thousand hectares of coastal New South Wales.
Biological control has been successful only in isolated situations. Overall there is
no control in river-banks or on other dry land sites.

e Water hyacinth
Water hyacinth continues to be a serious threat to waterways and storages
throughout Australia, but control measures and inspection programs undertaken

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 Ecology • 141

by State authorities have prevented any significant increase in its distribution in

recent years. It is a free-floating perennial plant that grows to almost 1 m in
height. The leaves have distinctive swollen stems and the flowers are mauve in
colour. The plant spreads both by seed and by stem fragments.
All infestations that have developed in some States to date have been
eradicated. Herbicides have been used as a control measure in a number of
situations and biological control agents have led to a substantial reduction in
levels of infestation in Queensland and to a lesser extent in northern coastal New
South Wales.
f Salvinia
Salvinia is a threat to Australian waters which rivals water hyacinth. At some
locations, where water hyacinth has been brought under control, salvinia has
replaced it. Salvinia is a free-floating and hairy aquatic fern. It spreads by
fragmentation and grows rapidly, the area of infestation doubling in two to three
days under warm conditions.
Herbicides have been used to control some infestations of salvinia. Biological
control agents have proved successful at some locations in the Northern Territory
and Queensland.
g Lagarosiphon
Lagarosiphon has the capability to cause major problems in water storages and
waterways, particularly in southern Australia. It has been found amongst plants
being sold in the aquarium trade. Lagarosiphon is a submerged perennial with
narrow, recurved leaves on branched stems up to 5 m long. It prefers cool
conditions and grows well in water with low nutrient levels on a silty or sandy
bed.

8.3 Using herbicides near water

It is important to remember that the landowner is legally responsible for injury or
damage caused by the careless use of herbicides (see Section 10). Users of
herbicides have the responsibility to select the most suitable herbicide for the
intended purpose and to use it in a way that does not have adverse effects.
Herbicides are chemicals that have been developed for the purpose of killing
plants or controlling their growth. When herbicides are used near waterways,
storages and other water bodies, several aspects must be considered that are of
special significance. Only a few herbicides are effective against plants that grow in
or near water. These herbicides are toxic to some extent, and consequently, care
must be taken to ensure that their use does not cause injury to people, stock, fish
and wildlife, or damage crops and other useful plants.
Amongst the more important requirements are the correct rate and time of
application, and depending on the particular herbicide being used, whether it is
applied to the foliage or the soil. To ensure maximum effectiveness, herbicides
must be applied in the way recommended on the labels. Where submerged species
require treatment, the herbicide may contaminate the water.

8.3.1 Hazards
Herbicide residues must be considered in relation to the use made of the water.
There are approved residue limits for most of the herbicides used near water, and

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 142 • Farm Dams

for a particular herbicide, the limit may vary depending on whether the water is
used for domestic purposes, agriculture or recreation, or if fish or their habitat are
of special significance to the area. In addition to choosing the herbicide that will
achieve the desired result, the user must ensure that it is applied in such a way that
the appropriate residue limit is not exceeded (Bill, 1985).
The degree of risk from herbicide residues in the water is determined by the
toxicity of the herbicide and by the concentration and persistence of herbicide
residues in the water. Plant decomposition following the use of a herbicide, and
de-oxygenation of the water which kills fish are indirect impacts of herbicide use.
Therefore, it is essential that the user be aware of these considerations.
Important points that should be observed when a herbicide is used near water
are summarised below.
• Use only a herbicide that is approved for the purpose and apply as directed on
the label and in accordance with conditions specified by the approving
authority.
• Identify the uses made of the water and ensure that the proposed herbicide
treatment does not affect those uses adversely, and that any residues of
herbicides in the water do not exceed approved limits.
• Where the water is used by the public, obtain approval for the proposed
herbicide treatment and notify people nearby of the nature of the treatment
and any restrictions on the use of the water.

8.4 Vegetation on and around dams

Clumps of tall trees that shade the water’s edge in summer are important in
providing shade and shelter (see Section 8.1.3).
The quantity of water used by trees growing at a dam edge will vary greatly
depending on factors including the size, number, location and species of the trees,
and the soil and weather conditions. Water use by trees at a dam edge is not
usually a problem where a dam is large, the ratio between catchment size and dam
size is favourable, or rainfall is high and reliable. Even with small dams, or when
rainfall is less reliable, nearby trees can still provide overall benefits providing they
are not established too close to the dam edge.

8.5 Yabbies
There are many species of yabby or freshwater crayfish that can damage dam
walls by burrowing through the clay. Although it is commonly believed that
yabbies are responsible for damage to dam walls, they should not be blamed
automatically for all leaks or damage. Tree roots may also weaken the banks of
dams, and yabby holes seen outside the water are probably due to land yabbies,
which are different from the aquatic species. Increasingly, people are regarding
yabbies as a recreational and food resource and are glad to have them in their
dam. If you are sure aquatic yabbies are causing damage, you can take several
steps to reduce their numbers.
8.5.1 Physical removal
Constant yabbying may reduce the numbers sufficiently to prevent damage.
‘Dragging’ is probably the quickest method. This involves throwing a few pieces

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 Ecology • 143

of red meat into the dam to draw the yabbies out of their holes, then, with the help
of another person on the other side of the dam, slowly dragging a net through the
water.
8.5.2 Biological removal
Yabbies form part of the diet of some native fish. Stocking a dam with fish may
help reduce the numbers of yabbies but is unlikely to completely eradicate them.
Stocking with larger fish is preferable, as they are immediately physically capable
of eating the yabbies. Furthermore, their survival rate is proportionally higher
than smaller fish and as such likely to be ultimately more cost efficient. The
number of fish required is determined by the size of the fish and the area of water
surface, rather than the volume of water in the dam.
The best fish to use for reducing yabby numbers are golden perch (also called
callop or yellow belly), silver perch, or Murray cod. These fish are readily
available commercially. Redfin are no longer recommended as they have been
found to carry a disease harmful to trout, and their tendency to over-breed results
in too many small fish. However, they have a great preference for yabbies and can
tolerate a wide range of environmental conditions.
Exotic fish species such as brown and rainbow trout are also suitable, although
conditions in farm dams are usually unsuitable for trout to spawn. This means you
will have to re-stock at regular intervals to maintain the fish population. Trout
have a relatively fast growth rate and are a good sporting fish, but conditions for
their survival are critical. They need clear, cool water with a summer temperature
not exceeding 19°C, and at least one-third of the dam should be 2 m deep.
When only fry are available, a stocking rate of about 2500 per hectare is
necessary to allow for their expected high mortality. The stocking rate
recommended for yearling-size perch is about 250 to 350 per hectare.
More fish can be supported when the water quality, natural cover and
vegetation are all in good condition. However, for larger fish the stocking rate is
lower. An adequate natural food supply is needed to enable the fish to grow to a
size where they can eat the larger yabbies. It is therefore helpful to allow some
aquatic plants to grow around the water’s edge which, in turn, encourages small
invertebrates, such as mud eye, on which fish feed. The survival rate of fish will
be higher if a dam is managed for fish-eating birds such as cormorants.
It is important to remember that a permit is required from a State agency office
before purchasing the fish.
8.5.3 Chemical control
Chemical methods of eradicating yabbies, including manipulating water pH and
adding chlorine, are no longer recommended because of possible long-term
damage to aquatic ecosystems, and because they are not proven to be effective
control measures.

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 Section 9
Commercial
Some of the information provided in this section is from Notes and Infosheets
from the Department of Conservation and Environment, Victoria, Fisheries
Management Division (DC&E, 1990).
This Section of the book is not intended as a complete guide to freshwater
aquaculture or investment in commercial aquaculture, but provides a broad
overview. A commercial venture into aquaculture deals with the same basic
principles, and even some of the same species as small-scale operations. The design
of a pond or a dam for commercial production must be different in large-scale
enterprises for ease of maintenance and harvesting.
Aquaculture is now an important industry with increased future export
potential (Lewis and Branson, 1996). There are various definitions of aquaculture,
but for the purpose of this book it has been defined as the activity of husbandry
of aquatic organisms. Most commonly this is regarded as the value-adding to fish,
shellfish and plants living in water. Generally, few species of fish have been found
suitable for farming and, in Australia, most fish farming enterprises are in the
early stages of development with an associated moderate economic risk.
Inland salmonid production, which includes rainbow trout and some Atlantic
salmon, produces the most economic wealth for some Australian States.
Production from this type of aquaculture has steadily increased over the last few
years. The second largest aquaculture sector is aquarium fish. This consists of still
water goldfish and the on-growing of imported stock in quarantine rooms. Eel
production is the next most common form of aquaculture. Over the last ten years
this form of aquaculture has gradually increased as the result of improved
technologies. Inland yabby farming is another popular form of aquaculture.
However, although potential for profitability exists, yields have generally not been
high and production has been fairly static over recent years. Finally, there is some
production of the fry of native species such as silver perch and Murray cod.
Although not commercially produced at present, inland barramundi production
shows some potential.
The attitude of the public to aquaculture tends to vary between optimism and
scepticism. Aquaculture is a moderately risky form of enterprise that is capable of
fair but sustained growth if undertaken sensibly. Overseas experience has shown
that poor planning and management of the aquaculture industry lead to a boom
or bust cycle and associated environmental degradation. Freshwater aquaculture
developments are non-consumptive users of water because it is returned to the

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 146 • Farm Dams

waterway after use. However, the discharge from these enterprises contains
nutrient-rich effluent which has water quality implications for waterways.

9.1 Fish farming

In theory, there are many species of fish that could be farmed commercially, but
few have been found to be suitable. Most species require particular sets of water
conditions, which are affected by climate, and are therefore often best suited to
specific areas.
Before deciding on a species to grow, it is important to first examine existing
ventures in the proposed area. If your choice of fish is not currently being farmed,
find out why, as it is probable that there are good reasons to avoid it.
9.1.1 Established freshwater species
Today there are, at least, three freshwater species that have formed the basis of a
successful aquaculture in Australia with a fourth showing some potential. The
farming of rainbow trout is a proven success of long standing. However, most of
the more suitable sites are occupied and there is little room for expansion. For a
commercial farm, a large quantity of high quality, cool water is needed every day
of the year and there are few places remaining where this is available.
Figure 9.1 Oncorhynchus mykiss (Rainbow Trout)

Source: DC&E, 1990.

Eels are successfully and extensively farmed. Natural waters are stocked and
harvested using traditional commercial fishing methods. Most of the suitable
waters are now committed and there are possible limits on the supply of seed
stock. The opportunity for new enterprises is limited. There may be potential for
intensive eel farming on private land but there are many problems that have to be
solved before this method is viable.
Figure 9.2 Anguilla australis (Short Finned Eel)

Source: DC&E, 1990.

Goldfish are grown for supply to the aquarium trade. Ventures in this area
have been amongst the most financially rewarding forms of fish farming, but the
market is now probably close to saturation. However, there may be room for
limited expansion in production of high quality, fancy strains.

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 Commercial • 147

9.1.2 Dam conditions that control productivity

The major factors in determining which species of fish are best suited to a dam
and what level of productivity can be expected include:
• size, shape and depth of the dam;
• the depth of water remaining during the summer;
• the amount of overflow; and
• the water quality and temperature.
In general terms, the ecosystem of a dam is dependent on the following
conditions:
• productivity, which is usually more dependent on surface area than volume;
and
• depth, which provides refuge from predation and cooler temperatures for
species. However, depth does not contribute directly to the productivity of the
dam.
Figure 9.3 Stocking rate for trout

800

700

600
Number of fish

500

400
Two-year-old
300 Mature yearling
200 Yearlings
Fingerlings
100 Fry

0
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Dam surface area (hectares)

Source: DC&E, 1990; Hill et al., 1982.

Natural food supplies for fish within a dam come from two basic sources:
• terrestrial animals that find their way into the dam and are eaten by the fish;
and
• aquatic organisms that are part of the basic food chain within the dam.
The basis of the food chain in a dam is algae, which utilise available sunlight
and nutrients, such as phosphorus and nitrogen, to grow and breed. These algae
are consumed by other organisms in the dam that are in turn consumed by the
fish. The aim of sound aquaculture practices is to balance the numbers and size of
fish in a dam to optimise productivity. Unwanted algal growths, which can
produce low dissolved oxygen levels, can result from excessive nutrient inputs
from agricultural fertilisers, run-off from dairies or piggeries, or natural run-off
from basaltic catchments. High nutrient levels will ultimately lead to an
insufficient food supply, reduced growth rates and poor fish survival. High
stocking rates will also result in poor growth rates and poor survival.

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 148 • Farm Dams

Apart from embankment failure or loss of water through leakage, the biggest
threat to any fish dam is input of sediment. Erosion in the catchment from gullies,
over-cultivated or over-grazed fields and poorly vegetated hillslopes/valley
bottoms are all sources of sediments that are delivered to a dam. When sediments
flow into a dam, they settle to the bottom, thereby decreasing its depth and
capacity over time. In addition, sediment build-up within a dam may produce
increased pressure on the spillway, increased water temperatures in summer and
decreased productivity of the fish farm.
Some operators of fish farms suggest that trout require at least one-third of the
dam to be at least 2 m deep during summer, whilst native fish can survive when at
least one-third of the dam is 1.5 m deep. These numbers are a guide only, and can
be varied depending on climatic zone. The fish that thrive in similar waters in your
area are a good guide in selecting the most appropriate species for your dam.
9.1.3 Feeding
In correctly stocked dams, the natural food supply should be enough to support
the fish population. There are, at present, no adequately formulated artificial diets
for native fish, and trout will only eat floating or sinking pellets. Once pellets
reach the bottom they become, in the long term, very expensive fertiliser. However,
some dam owners like to feed pellets to trout which enables a rapid appraisal of
the numbers and size of the fish present in the dam. This approach needs to be
consistently managed to obtain meaningful results, so the fish need to be fed small
amounts at the same time and place.
9.1.4 Aquatic vegetation
Vegetation may be a food source for some of the organisms on which fish feed. It
can also provide shelter for both food organisms and fish. Most dams will develop
aquatic vegetation naturally, but shallow dams can be completely taken over.
Generally, no more than a third of the dam’s surface should be covered by
vegetation. When this is exceeded, control by physical removal is effective, but
care must be taken not to kill fish.
9.1.5 Fish loss by escaping
Migration of rainbow trout, golden perch and silver perch is more likely than
other species. If a dam overflows then it is very likely to lose fish, particularly once
they reach sexual maturity. It is, therefore, worthwhile to install an upward-
sloping screen, which extends well above maximum water level at the dam
overflow, and sloped in the direction of the water flow. This allows debris to be
pushed up onto the screen for regular cleaning off, or over the top, but prevents
fish from escaping. It is necessary to ensure that the size of the screen apertures are
small enough to prevent the smallest fish from passing through, that is, screen
apertures one-third the size of the cross-section of the fish’s body. Screens should
also be installed around all outlet pipes used to supply water for other purposes
such as irrigation or stock.
9.1.6 Muddy water
While most bodies of water will become turbid (cloudy) when they are stirred up
by wind or flowing water, they usually clear when conditions are stable. If a dam
remains turbid, it is most likely because of the type of clay present in the dam base

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 Commercial • 149

and wall. Some clay types produce what are known as colloidal particles in the
water. These are very small particles, which have similar electrical charges and
repel each other causing them to remain in suspension (see Section 6.3.1). There
are a number of chemicals that will neutralise these charges, allowing the particles
to settle (see Section 7.3).
9.1.7 Algae
Excessive algal growth can taint water, clog filters and valves, deplete oxygen
levels and kill stock. Algae increase in number by the action of sunlight on
nutrient-rich water. In the long term, control is achieved by reducing nutrient
input. This can be achieved by:
• diverting effluent from a dairy away from the dam;
• planting vegetation that will shade the dam in summer; or
• reducing run-off and associated erosion within the catchment.
Most chemical treatment for algae will kill fish and expert advice is required
before treating any water in the dam (see Section 7.4).
9.1.8 Other species
At different times, a number of species have been suggested as being suitable for
culture. Currently, it would appear that one or more factors make farming of them
uneconomical. Interstate or tropical species may be popular in some areas, but
they are not adapted to colder climates and cannot be economically grown in some
places. For example, tropical barramundi and prawns die, or grow extremely
slowly under cooler temperature conditions. Artificial heating is yet to be an
economical solution. Native fish that occur naturally in streams and lakes can
have traits that make them unsuitable for farming. Despite much interest and
effort, nobody, has developed an economical system of feeding and harvesting
table-sized golden perch, silver perch, freshwater catfish and Murray cod. Redfin
(English perch) carry a disease that also infects other species of fish. To ensure
quarantine, the movement of redfin stocks has been limited. They also have a
natural propensity to over-breed and over-stock, thereby producing under-size fish.
As a final disincentive to potential fish farmers, redfin attract only a poor price at
market.
Many species of freshwater crayfish are native to Australia but, while yabbies
show some potential for farming., other species of freshwater crayfish are not
suitable. Murray spiny crayfish, Gippsland spiny crayfish, Glenelg River spiny
crayfish and the giant Tasmanian crayfish grow too slowly to be commercially
attractive. It may take a decade for a crop to reach market size. The Western
Australian Marron is declared a noxious fish in Victoria and the tropical red claw
crayfish carries diseases.
If some Asian fish species were to escape, they could have the potential to
become pests. To conserve freshwater ecology these species have also been made
noxious in most States of Australia and cannot be farmed.

9.2 Yabby farming

In Western Australia, yabbies are raised with minimal feeding and maintenance
in farm dams used for stock watering (Romanowski, 1994). In some cases, the
owner of the dam catches and delivers the yabbies to a processing facility, but a

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 150 • Farm Dams

reasonable profit for very little effort can still be expected even if they are
harvested by the reseller.
While it seems possible that yabby farming may become a viable industry, it is
unlikely that the potential financial returns are as great as some promoters state.
The technology is simple but yabby farming can be labour intensive and requires
good fish husbandry skills. To make a profit, good business skills are also required.
Landowners that grow yabbies for sale are required in most States of Australia
to have a Fish Culture Permit/Licence. Anyone contemplating the farming of
yabbies should check this out with the responsible State authorities that manage
fisheries and/or wildlife.
Figure 9.4 Cherax destructor (Yabby)

Source: DC&E, 1990.

9.2.1 Types of dams for yabby production

Yabby dams can be either specially constructed ponds or located in large areas of
unimproved wetland. For a given area of land, specially constructed ponds have
the highest potential return but cost more to establish and maintain. Crowding
yabbies also increases the likelihood of disease and slows growth.
Dam dimensions may vary markedly according to the site but ideally have a
depth of 1 m, and a surface area of 0.25 ha. Long, narrow ponds are considered
to be more practical than round or square ponds.
There is a difference of opinion as to whether dams should be lined with
plastic, gravel and limestone or natural topsoil. Farmers using all types of linings
report similar growth rates so the bottom type may not be critical. High water
temperatures are essential for the fast growth of yabbies, so farms that are located
in warm, sunny areas are likely to get the best returns.
9.2.2 Water properties
Mature aged yabbies can withstand salinities as high as 14 000 parts per million
(2330 µS/cm). However, they thrive better in lower salinities. As a rule of thumb,
if stock can drink the water, yabbies can survive and grow in it (see Section 7.5).
Yabbies start to feed and grow when the water temperature exceeds about 13°C.
As cold blooded animals, their feeding activity and growth rate increases as the
temperature rises. At about 28°C, yabbies reach an optimal growth rate and
higher temperatures can cause a decline in growth and death will occur when the
temperature reaches 34–36°C. Increased water temperatures impose indirect
impacts on yabbies through an associated decrease in dissolved oxygen and an
increase in the rate of organic decomposition. Ideal water temperatures are
24–27°C.

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 Commercial • 151

9.2.3 Stocking and feeding

The stocking rate for yabbies is a function of farming intensity. Very high stocking
rates cause problems, including fouling of water, disease and slow growth. The
recommended maximum initial stocking rate is about 5 or 6 juvenile yabbies per
square metre of pond bottom. As they grow, a few will die and crowding will
naturally reduce.
Yabbies will eat almost anything, so it is possible to provide adequate feed at
a reasonable cost. Meat and fish are acceptable, but be cautious of red meat and
any processed meat that contains nitrates. Contrary to popular opinion, yabbies
do not prefer rotten meat. Stock feed firms manufacture pellets specially
formulated for yabbies. These are expensive, but are convenient and produce good
growth. Lucerne is also a suitable food.
Yabbies will not consume excessive food, which will lie on the bottom of the
pond and decompose, with consequent deterioration of water quality. To avoid
this, some operators of yabby farms place food on a tray in the water which is only
replenished when empty.
9.2.4 Harvesting
There are several methods for harvesting yabbies.
• The quickest and easiest method is to use a net. A disadvantage is that yabbies
may be damaged in dams with irregular bottoms.
• A more expensive but popular method of harvesting is to use pots or traps.
• Another method is to drain the dam and remove the yabbies by hand. If this is
done, it is best to drain the dam or pond very quickly. Lowering the water level
gradually may stimulate the yabbies into burrowing, which then makes their
harvesting very difficult.

9.3 Native fauna and total ecosystem

management
Native fauna can apply pressures on aquaculture farming, which, if significant,
can compromise the viability of a commercial enterprise. Therefore, prior to
establishment, landowners or managers should liaise with professional
associations, industry contacts and people with local knowledge to develop an
understanding of all the risks and uncertainties of aquaculture in the area. Using
this approach, an operator will be able to avoid sites that are unsuitable, calculate
possible costs, and determine the likelihood of wildlife problems. Rather than start
full-scale production it is better to initiate a small-scale project. The pilot project
will give an insight into problems and may enable solutions to be found before a
large amount of capital has to been committed to the project. When an enterprise
is considered to be potentially viable, management of the total ecosystem in the
initial planning stages of development is recommended. Survival rate can be
increased through strategic management.
9.3.1 Native fauna
A diversity of species of native fauna will eat fish and crayfish. Birds that consume
fish and crayfish/yabbies include cormorants and darters, which are divers, and
waders such as herons. Other native species that consume fish include the nocturnal,

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 152 • Farm Dams

elusive water rat (Hydromys chrysogaster) and eels. However, it is important to

note that water rats will only be present where the aquaculture enterprise is in very
close proximity to a stream.
9.3.2 Managing predation by native fauna
Aquaculture has a prime focus on commercial production. This can be sustained
without compromising conservation of wildlife through an holistic approach to
management. Appropriate strategies can include those shown below.
• Covering ponds with netting to exclude birds from ponds. Bird netting is
usually made of plastic and must give total coverage of ponds including the
sides. Plastic netting can last many years and is expensive but its cost must be
viewed in the context of the losses it minimises.
• Designing ponds to discourage waders, by having steep, deep margins.
• Increasing the numbers of fry slowly when first introduced to a pond.
• Including yabbies in the pond population when possible, because cormorants
find them easier to catch than fish.
• Providing protective cover for fish and fry with water plants along the margins,
and irregularly shaped objects, such as car bodies and tyres on the pond floor.
• Developing the adjacent areas to support alternative food sources that are
adequate and attractive.
Studies have shown that more than 50 per cent of fish can be consumed in a
farm dam by cormorants unless alternative prey (as yabbies) are provided, and
dams are stocked with few fish (around 150 per hectare) (Barlow and Bock, 1992).
High stocking rates (more than 450 per hectare) are associated with high predation
rates. Ideally, management should combine a number of strategies simultaneously.
All native bird and mammal predators of fish and crayfish are protected
wildlife in most States of Australia. People found killing protected wildlife face
fines and the possibility of jail terms. Where it can be demonstrated that wildlife
damage to fish and crayfish is causing economic hardship, application can be
made to obtain a licence to cull from the appropriate State agency. The application
process includes field inspection and consultation with agency staff. Licences are
viewed as a short term remedy, and State agencies support landowners with advice
on alternative, longer-term solutions.

9.4 Licensing process

Aquaculture requires approval from several authorities to help safeguard the
water resource and existing water users’ entitlements. This section outlines some
of the steps required to obtain a licence from a responsible authority to take and
use water for such purposes.
9.4.1 Licences required
In most States, if you are interested in establishing an aquaculture enterprise or
fish farm (SRW, 1995), you will need to obtain licences or approvals from the
following authorities:

a Rural Water Authority (RWA)

You will need a licence from your nearest RWA office, as most aquaculture
enterprises require a constant supply of fresh water. If you need to divert water

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 Commercial • 153

from a nearby waterway you must obtain a Diversion Licence from the
responsible authority, as stated under the legislation of the State. A separate
licence may also be required for any necessary in-stream works (Lewis and
Branson, 1996; Lewis and Beavis, 2001).

RWA licence requirements

In order to obtain a RWA diversion licence you will need to supply the
information shown below.
• The proposed off-take works (including an appropriate method for measuring
and monitoring water flows).
• The proposed discharge works (including an appropriate method of measuring
and monitoring flows).
• The farm layout showing the number of ponds and settling ponds, and their
capacities (including levels for bed and crest of ponds and internal channels).
• Surveyed cross-sections of the river and floodplain at the off-take, ponds and
discharge points. The survey should also indicate on a locality plan the edge of
the flood plain (that is, the edge of the high land) and any known recorded
flood levels or other flooding information.
• Stream distance (in metres) between off-take and discharge points.
• Maximum diversion (in megalitres per day) required for the farm’s operation.
Based on this information, the RWA will make an evaluation, taking into
account water availability in the stream, off-take and discharge structures
required, the likely effect on flooding and any possible detrimental effects to the
water quality of local waterways.
A diversion licence will be issued on the basis of ‘non-consumptive use’ of the
water. This means that all flows that are diverted must be returned to the stream.
Charges will be determined according to current rates and the volume of water
diverted.

b Environment Protection Authority (EPA)

You will need to obtain a waste water discharge licence from your local EPA
office, to enable you to discharge effluent from your aquaculture farm. A dual
process may exist between the RWA and EPA. However, you will need to obtain
EPA approval before the RWA will issue a licence. To save time, make sure you
have estimated your expected water volumes and other criteria listed below before
you apply for any approval or licence.

c EPA Waste Water Discharge Licence

When issuing a waste water discharge licence, the EPA sets conditions for
monitoring water quality and minimum acceptable levels for the waste water.
Approvals for the volume of water discharged are based on the ratio of water
diverted to water passed downstream to ensure adequate dilution flow in the
stream. The EPA recommends a minimum dilution ratio, so that only a certain
percentage of flows may be available for diversion.

d Other authorities
Other approvals in regard to the location and operation of a commercial
enterprise will be needed from relevant authorities including the local shire
council, conservation authorities, and any river management authorities. Once all

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 154 • Farm Dams

these authorities have been contacted, you must arrange a joint inspection of your
proposed site.
9.4.2 Other steps that may need to be taken
In addition to the permits and licences already described, it may be necessary to
obtain the following:
i A planning permit from the local shire council;
ii A fish culture licence from your local conservation or wildlife authority;
iii An occupancy licence from the appropriate authority, if any part of the
proposed works is to be located on Crown Land; and
iv Approval for in-stream works from any local river management authority, if
such an authority exists in your area (check with your shire office).

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 Section 10
Legal
This section is not meant to be an interpretation of the law or State and Federal
legislation. It has been added to make farm dam owners, designers and contractors
aware of some issues that could create future problems.
Liabilities that are determined in response to a dam failure affect society,
governments and dam owners. The determination of the liability is the legal
mechanism developed by society to recover damages due to a ‘wrong’ (in this case,
lack of dam safety) and is another aspect of the dam safety problem. A better
awareness of this legal process may help dam owners determine the steps to be
taken to reduce their liability.
Asset and third party insurance is considered in some quarters to be a
fundamental need and a prime responsibility of all dam owners. Therefore, the
purchase of an insurance policy may be the most important measure a dam owner
can take to reduce the consequences of dam failure.

10.1 Legal and policy aspects in Australia

Assessment of the environmental impact of farm dams needs to consider not only
the scientific, but also the legal/policy framework within which farmers manage
their water resources. Examination of this broader decision-making context
reveals current legislation that impacts upon farm dam management.
First, each State or Territory has a Water Act, which controls the utilisation of
water from streams including the licensing of on-waterway dams. Upon
examination, an overall lack of specificity is a common characteristic of these
Acts. Unlicensed dams are not subject to formal allocation processes. Moreover,
provisions relating to on-waterway dams rely on definitions of what constitutes a
waterway, stream or river, which can vary between States and are open to wide
variation in interpretation.
Second, a series of Acts and policies exist which impact on farm dam
management and which vary between States and Territories. Table 10.1 provides
an overview of key legislation and policies that have been made in relation to the
construction and management of farm dams at a Federal, State and Local
Government level.

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Table 10.1 Key Federal, State and Local Government legislation on farm dams
Act/Policy/Program Impact on farm dam management

Federal

Income Tax Assessment Act, 1936 Provides incentives for dam development through
tax write-offs for investment in bona fide storages,
and drought investment allowances.
National Landcare Program (NLP) Grants provide proportion of funds for projects,
including dams for drought proofing and erosion
control.
State

Tasmania Water Act, 1983 Licenses on-stream dams and controls water
diversion and use.
Environmental Management and Environmental Impact Assessment required for
Pollution Control Act, 1996 large on-stream dams.
Farm Dam Working Group Assessment of licence applications on basis of
impacts on fisheries and ecosystems.
Farm Dam Development Plan Provision of financial incentives to farmers
(low interest loans and subsidies for consultants’
advice).
Victoria Water Act, 1989 Right to flow of water, applications for licences and
authority, and water allocation.
Special Water Supply Catchment Approval for farm dams within Special WSC Areas
Areas required from Dept Natural Resources and
Environment.
South Australia Water Resources Act, 1990 Riparian rights, applications for licences for dams
(currently being revised as Water on proclaimed waterways, and water allocation.
Resources Bill, 1996)
Development Act, 1993 Development authorisation required for specific
dam applications according to size and location
(including ‘local heritage places’, waterway and
flood-zones, and flood plains).
Waterworks Act, 1932 Consent of the Minister is required for dams in the
Adelaide Hills catchment.
Draft Surface Water Allocation Addresses environmental issues relating to farm
Policy dam development. Will include a Surface Water
Management Plan.
Queensland Water Act, 2000 Replaces the 1994 Dam Safety Management
Guidelines.
Environmental Protection Act, 1994 Applies to dams containing ‘hazardous waste’ such
as tailing dams and contaminated water
containment dams.
Water Resources Act, 1989 Relates to permits that originated from existing
dams, waterworks licences.
Western Australia Rights in Water and Irrigation Act, Right of flow of water, applications for licences and
1914 allocation of water.

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Act/Policy/Program Impact on farm dam management

Farm Water Plan Farmers are encouraged to become self-reliant with
their farm water supply, in the context of the
development of a water pipeline system.
Farm Water Grants Scheme Financial incentives for farm dam construction (up
to $12 500).
Environmental Protection Act, 1994 Limited constraints applied to on-stream dams to
preserve flora and to protect stream banks.
New South Wales Water Act, 1912 Applications for licences and authority, and
allocation of water between users of a river system
according to specified criteria.
Farm Dams Policy, 1999 Landowners can harvest up to 10% of the mean
annual run-off for their property as a harvestable
right. A licence is needed for building dams to
store water in excess of the harvestable right.
Licence fees vary according to whether the dam will
be used for stock/domestic or irrigation purposes.
Farm Dam Safety Act, 1978 Prescribed dam as listed in the Act which come
under a Dam Safety Committee. For more details
look at References (DSC, 1996).
Mining Act, 1992 Dams under this Act have the same conditions as
Prescribed dams, but tend to be related only to
tailings and mining operations.
Soil Conservation Act, 1989 Subsidies for soil conservation works, including
farm dams as erosion control structures, are
available under Section 10 of the Act.
ACT Land (Planning and Environment) Approval required for any action, which disturbs
Act, 1991 soil and/or is within 20 m of a stream.
ACT Rural Policy At sale of property, landowner required to provide
an environmental audit including details on dams,
vegetation, erosion and any environmental
problems. New owner required to provide a
Property Management Agreement addressing
degradation problems listed in audit and
management proposals for development, including
farm dam construction.
Local Government
Local Government Act (Vic),1989 Approval may be required through Planning
Scheme. Varies according to Shire Council.
Local Government Act (SA),1934 Permit required for dam building on an
unproclaimed waterway.
Local Government Act (NSW), 1993 Farmers required to submit a Local Environmental
Plan (LEP) with their application to build a farm
dam. Varies according to Shire Council.
Local Government Act (Qld), 1994 Local regulations may require approval for siting
and design.
Source: Modified with additions to Beavis and Howden, 1996.

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The legislation and policies that exist are specific in the level of control on
further development and the environmental requirements during design and siting.
Increasingly, farmers cannot construct dams in an ad hoc manner, particularly in
rural residential areas, and where environmental considerations are enforced.
However, decisions are at the discretion of local officers of State agencies and local
governments without standardised procedures on a broad scale (Beavis and
Howden, 1996).
This outline demonstrates that the legal and policy framework, which applies
to farm dam management, is variable. While there is considerable scope for
interpretation by farmers on the construction, design, siting and management of a
dam, there are also specific associated tax benefits, costs, and constraints, which
must be considered in order to profit from such an investment. Furthermore, it is
important to note that in their present form the Acts listed above do not appear
to have had significant impact on the actual management of farm dams.
The immediate questions that arise are those which address the relevance and
effectiveness of such legislation, policies and regulations in ensuring that future
farm dam development meets standards set to preserve water quality and river
health. Clearly, policy development needs to address the mismatch between
policies for dam construction and those relating to water for the environment.
This approach will optimise policy alignment and disable the current antagonistic
system of incentives and constraints on farm dams.

10.2 Liability
Liability in specific instances very much depends upon the dam, the accident, the
owner and the State jurisdiction in which the dam is located (US FEMA, 1987).
The liability of an owner, designer or contractor of a dam is considered general
civil liability (tort). A tort is simply a civil wrong for which an injured party may
recover damages from the responsible party. In most circumstances, simply
causing damage is not a sufficient basis for the imposition of liability. An element
of negligence must accompany the injury before liability is incurred. However,
negligence is not a fixed concept. It has been modified and changed by legal
precedence over time. In the simplest terms, it has been described as the violation
of a duty to act as a reasonable and prudent person would act; a violation which
directly results in damage to another.
The question of what ‘duty of care’ is imposed by society and what standard of
reasonable care is imposed by the duty has undergone scrutiny and changed over
time. In many instances, the duty to make a dam safe or the duty to ensure that
one’s property does not pose a danger to others, has been significantly increased.
While the concept of negligence has been broadened, changes in the limits of
negligence do not directly affect dam owners because a separate basis of liability
has long been imposed upon them. This standard is one of ‘strict liability’. Strict
liability is not based upon fault or negligence, rather it is based solely upon
resulting damage, regardless of fault. Strict liability is generally applied to those
activities that are deemed ‘ultra-hazardous’ and not capable of being rendered
reasonably safe.
The situation of strict liability was first established in a case involving a dam
in 1866 in England, Fletcher v. Rylands (L.R.1 Ex 265, 279–280). A dam was built
in the vicinity of abandoned coal mines. The water from the dam found its way

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 Commercial • 159

into the abandoned shafts and from there into active shafts and resulted in
damage. Under present legal thought, the basis of liability for such an occurrence
may well be negligent design because of failure to adequately investigate the
surrounding circumstances when the dam was built. In this decision, it was
assumed that no one could have known the abandoned mine shafts existed and
specifically decided that the owner was not negligent. Nonetheless, the court at
that time, established the concept of strict liability for dam owners, and the owner
of the dam was found to be liable for the escape of water from the dam, regardless
of fault.
Thus, with a very limited number of exceptions, the general statement of
liability for the owner or operator of a dam can relate back to a statement by
Henry Manisty QC, who addressed the English Court of the Exchequer in 1866:
“A large collection of water is a thing pregnant with dangers, and it
behoves anyone who makes a collection for his profit, to be aware
how he may prejudice his neighbour by mismanaging it.”
Since that time this was known as the Fletcher v. Rylands rule. This is now held
by the High Court to have no place in Australian law, based on the precedent:
Burnie Port Authority vs. General Jones (1994) HCA.
Strict liability has two exceptions:
• acts of God—natural occurrences over which the owner has no control. While
acts of God are recognised as a defence, this is limited to those events over
which the owner had no control and also which the owner could not, using
available expertise, have anticipated.
• intentional acts of third parties—was established in the United States of
America, Wyoming Supreme Court in the Wheatland case. In this case it was
asserted that the dam had been damaged by saboteurs, and the Court
recognised that illegal, intentional acts by third parties which the owner could
not protect against or reasonably foresee were a viable defence to strict liability
(FEMA, 1987).
In summary, existing law holds a dam owner to the highest standard of care.
Pending legislation may limit liability in certain circumstances, but the general
statement remains unchanged. The owner is liable for all damages caused by water
escaping from a dam—despite the best efforts of the owner.

10.3 Responsibility of dam owners

Some State authorities provide information on acquiring a permit or licence for
dam construction, dam ‘hazard’ and ‘risk’ categories, and liability in the event of
dam failure. The information is intended only as a guide, and it is recommended
that you seek legal advice if you have any concerns about your potential liability
as a private dam owner. Also there are other issues that need to be considered.
• If you build a dam without permission it can create problems at a later stage
when repairs are needed, or perhaps when you are selling the land. Nobody
wants to buy someone else’s mistakes or legal problems.
• Building a farm dam is not simply a matter of walling off a gully or waterway.
It is a task that requires time, effort and money to plan and complete
successfully. However, no matter how well planned or constructed your dam
is, there is always the possibility that it may fail.

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 160 • Farm Dams

• Dam failure can injure people or livestock, damage residences or industrial

buildings, railways or highways, interrupt the service of public utilities such as
electricity, or even cause loss of life. Dam failure can also result in loss of
income for the dam owner, due to lack of water for irrigation or stock.
• Dam owners are ultimately responsible for the safety of their dams. Under
legislation, the owner of the land on which a dam is situated may be liable for
all damage caused by the escape of water from the dam. Therefore, it is
important to know beforehand if your insurance policy covers this situation.

10.3.1 Liabilities of dam owners

Every owner of a dam has a legal liability under various State legislation (see Table
10.1). In Victoria, owners of private dams may be liable for damage caused by the
failure of a dam on a variety of different grounds. These grounds may arise under
the Water Act 1989 (Vic) or at common law. The relevant sections of the Water Act
1989 are sections 16 to 20. Section 16 (1) of the Water Act 1989 provides that:
If—
(a) there is a flow of water from the land of a person onto any other
land; and
(b) that flow is not reasonable; and the water causes-
(i) injury to any other person; or
(ii) damage to the property (whether real or personal) of any
other person; or
(iii) any other person to suffer economic loss—
the person who caused the flow is liable to pay damages to that other
person in respect of that injury, damage or loss.
Section 18 provides that the Act does not extinguish the common law liability
of a private dam owner for damage caused by the escape of water from a dam.
The Act gives the Administrative Appeals Tribunal jurisdiction to hear
complaints about common law claims for dams failure as well as claims brought
under the Act itself. In some cases claims may also be brought in the Supreme
Court.

10.3.2 Permission to build a dam

In most Australian States, a permit or licence to build a private farm dam is
required in a range of situations.
• If you are planning to build the dam on or over an existing waterway you may
need a licence from your local Rural Water Authority (RWA). If you are
uncertain about whether your proposed dam site is on a waterway, then
contact your local RWA for clarification.
• Your local shire or council may require you to obtain a planning approval permit.
• You may also need to contact a government department if you plan to build
your dam in a ‘Proclaimed Catchment Area’ that is, town water supply source,
or if any stored water will intrude onto Crown Land road reserve.
• In some States there is a requirement to conform with legislation for dams
described as ‘Referable’ (Victoria) or ‘Prescribed’ (New South Wales) dams, i.e:
i A wall that is five (5.0) metres or more high above ground level at the
downstream end of the dam, and a capacity of 50 ML or more, or

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 Commercial • 161

ii A wall that is ten (10.0) metres or more high above ground level at the
downstream end of the dam, and a capacity of 20 megalitres or more.
(1 megalitre = 1000 cubic metres).
For further details see DWR, 1987 Reports No. 3, 4 and 5; DWR, 1989 Report
No. 43; DSC, 1996 and ANCOLD, 1998.
In New South Wales, a new Farm Dams Policy was introduced in 1999. If a
landowner intends to construct a farm dam, the Farm Dams Assessment Guide
(accessible on the NSW Department of Land and Water Conservation (DLWC)
website) provides a method to estimate 10 per cent of the mean annual run-off for
the region in which the land is located (see references L&WC, 1999). This is the
harvestable right, and refers to the amount of run-off generated from a property
that the landowner has a right to harvest without a licence.
If a licence is required (because the landowner needs to use more than the
harvestable right) for water use other than for stock or domestic purposes, the
landowner needs to find out if an embargo is in place within the district or region.
If so, then a licence can only be obtained by purchasing someone else’s entitlement.
Under these conditions, the landowner needs to contact the DLWC for advice.
If the area is not embargoed then the procedure of obtaining a licence is
straightforward. Application forms, including an environmental questionnaire,
from the DLWC need to be completed and submitted with a lodgement fee.
Processing of the licence by the DLWC involves:
• reviewing environmental impacts;
• advertising the application in the local area; and
• resolving objections, if they arise.
If approved, a fee is required, which will vary according to the intended use.
The licence may have conditions applied in relation to environmental impacts
and/or objections.

10.4 Dam failure

The construction of farm dams increased significantly from the 1960s onwards,
with the greatest increases in numbers occurring in areas dominated by
agricultural, pastoral and rural residential development. However, many of the
older dams do not meet current design and construction standards, or have
deteriorated with age. Surveys show that of the new dams that fail, the failure is
due to lack of maintenance in the first few years after construction (see Section 6.3).
Owners of private dams may be liable for damage caused by the failure of their
dam on a variety of grounds arising under legislation, or at common law:
• if there is a flow of water from the land of a person onto any other land;
• if that flow is ‘not reasonable’; and
• the water causes either injury to another person, property damage, or
economic losses to another, then, the person causing the flow is liable to pay
damages to the affected person.
This means that if water from your dam escapes onto your neighbour’s
property and causes damage or injury, then you may be liable to pay for the repair
of that damage (see Section 10.2).
Most State agencies responsible for farm dams have a policy to inform you of
your responsibility to take ‘all reasonable care’ when constructing your dam, so as
to safeguard downstream landowners and others against damage in the event of a
dam failure.

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 162 • Farm Dams

10.4.1 Potential hazard and risk of farm dams

Potentially hazardous dams are those which, due to their size and location, may
threaten life or property if they failed. Engineers categorise dams as being ‘high’,
‘significant’ or ‘low’ hazard according to the potential effects of their failure
(ANCOLD, 1983; DWR, 1989; ANCOLD, 1992; DC&NR, 1992 and DSC,
1996).
The term ‘risk’ is used to describe the possibility of a dam failing. A ‘low risk’
dam would therefore be less likely to fail, whereas a ‘high risk’ dam is considered
more likely to fail. Classifying a dam as ‘high risk’ would not necessarily mean
that it would fail. The classification may mean that the dam is constructed from
materials that are likely to deteriorate over time, (for example, timber), or that its
spillway has inadequate capacity. A qualified engineer is required to classify the
hazard and risk of a farm dam.
In Queensland the classification system for referable dams is based on the
population numbers at risk. A referable dam has a dam failure impact of 1 (if
between 2 and 100 people are at risk) or 2 (if over 100 people are at risk). The
classification system (including the determination of population at risk) for
referable dams can be obtained from the Guidelines for Failure Impact
Assessments for Water Dams published by the Department of Natural Resources
and Mines (NR&M). These guidelines do not apply to dams containing
‘hazardous waste’ such as tailings dams and contaminated water containment
dams. Those dams are now regulated by the Environmental Protection Agency
under the Environmental Protection Act 1994.
10.4.2 Minimising the risk of dam failure
To minimise the risk of dam failure, it is recommended that private dam owners:
• inspect and maintain their dams regularly. Maintenance may include de-silting,
checking for cracks along dam walls, opening the compensation pipe regularly.
• monitor conditions that may affect the safety of their dams, e.g. heavy rain
after dry conditions.
• repair problems as soon as they arise, and ensure the repairs meet current
standards.
• call in an engineer to investigate conditions that may result in partial or total
failure of a dam, for example, cracks appearing on the dam wall.
10.4.3 In case of dam failure
Owners can play an important role in ensuring the safety of farm dams by having
proper operating procedures and adequate inspections, maintenance and safety
surveillance. However, there should be a plan of action in case the dam fails or is
threatening to do so.
The Emergency Action Plan should be directly related to the specific dam’s
structure and its immediate environment, and will depend on the dam owner’s
knowledge of the dam and its operation. It should be reviewed and if necessary
updated annually. This is especially true for dams with a history of leakage,
cracking, settlement, misalignment or erosion from wave action.
Since dam owners may be liable for damages associated with the failure of their
dam, it is imperative to issue an effective and timely warning to downstream
residents if a dam is about to fail.

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 Commercial • 163

In populated areas calls should be made first to the police or the State Emergency
Service (SES) who will then warn and if necessary, evacuate downstream residents.
In rural areas, the warning will usually be given by telephone or direct contact
with the nearest downstream residents. When telephone conversation is not
possible, the person observing the dangerous condition may have to personally
warn the nearest downstream residents, campers, and so on. Therefore, the owner
should keep a listing of the nearest downstream residents and their phone
numbers.

a Notification of a threatening situation

The following situations require immediate action:
• failure imminent (for example, water is rising and approaching the top of the
embankment, or heavily coloured water is issuing from the embankment or
foundation)—assess whether the structure may be saved with immediate
remedial action—engineer required, release water, inform neighbours.
• failure in progress (for example, water is spilling over the embankment, or
erosion of the embankment, spillway or foundation is occurring)—no chance
to save the dam—contact neighbours, police, State Emergency Services.
• flooding expected or in progress upstream from dam site—contact neighbours,
police, State Emergency Services (SES).
Notification and action should be planned in advance.

b Individuals and organisations who could be contacted

Addressing an emergency situation could require many people and organisations
to be informed at the earliest opportunity. Keeping in mind that preservation of
human life must always have first priority a contact list should be prepared and
kept accessible and up to date. It might take the form shown in Table 10.2.

Table 10.2 Emergency contact form

Emergency contacts Phone number

Neighbours
1 ………….
2. …………
3. ………….
Local Organisations
1. Police
2. Shire or Council
3. State Emergency Service (SES)
4. RWA Regional Office
State Organisations
1. State Emergency Service (SES)
2. Department of Conservation and Natural Resources
3. Rural Water Authority
4. State Roads Authority
5. State Electricity Authority
Source: DC&NR, 1992.

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 164 • Farm Dams

c Contact organisations for advice

There are a number of potential problems that may threaten the safety of a farm
dam. These can be expensive to remedy. Consequently, it is imperative that dam
owners seek advice of an expert when in doubt.
Private and government organisations that may be able to provide advice or a
service are:
• State Water Authorities;
• Rural Water Authorities;
or a member of the:
• Association of Consulting Engineers, Australia, State Chapter; or
• Local Government Engineers Association.
In any particular area, the best advice can usually be obtained from a
consulting engineer with local experience in design, construction, maintenance
and repair of farm dams. A consulting engineer in general practice will not always
have an appropriate background, and local knowledge is usually the best guide to
firms or individuals with proven record of experience.
10.4.4 Abandonment of farm dams
To abandon a farm dam which may have outlived its usefulness or economic life
can sometimes mean more than simply walking away from it. When an owner
chooses to abandon a farm dam because it is no longer useful or is too costly to
manage or rehabilitate, the dam should be made incapable of storing any water
(either temporarily or permanently) which may constitute a risk to life or property.
The dam owner is still responsible for ensuring the safety of residents and
development downstream, and for the dam itself, while it is in the process of being
abandoned. Breaching a dam is a complex operation. First, the un-breached
sections must be left in a permanently stable condition. The breach must be wide
enough not to impound significant quantities of water under flood conditions.
Finally, the short and long-term stability of any sediment deposits within the
storage area must be considered before commencement of the breaching
operation.
Breaching should not be attempted while there is any water in storage, unless
expert advice is first obtained. Environmental aspects downstream must also be
considered prior to a dam being breached. It should be noted that the costs of
rendering an abandoned dam safe (particularly against storm and flood events)
can often be quite considerable. For any dam being abandoned or breached,
professional engineering advice should be obtained.

10.5 Designer and earthmoving contractor(s)

The trend in recent negligence cases in Australia indicates that, in the case of
extremely hazardous activities, professional engineers, statutory bodies and others
are required to give a guarantee that no physical harm results from the conduct of
their activities (Wensley, 1995). However, it is clear that it takes relatively extreme
circumstances to require a guarantee of safety, as the case of Cekan v. Haines
shows, with particular reference to the question of how far a government is
required to provide resources to reduce a foreseeable risk of harm. In that case,
the plaintiff received self-inflicted injuries while in a police cell while intoxicated.
He sued for damages. The trial judge found that the Government, which

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 Commercial • 165

conducted the establishment, did owe a duty of care to the plaintiff but that that
duty had not been breached. All members of the Court of Appeal took into
account the fact that it would have been very expensive to redesign the jail in
question—and by extension of the argument, all the jails in New South Wales—to
prevent the type of injury which had occurred. They emphasised that even
governments have limited resources and must make choices on the basis of the
overall common good.
The proprietors and designers of large dams are not the insurers at law of those
who may suffer physical or economic harm should something go wrong.
Nonetheless, the boundaries of the law of negligence are being pushed wider all
the time, and because of the extreme consequences which would follow a dam
failure or over-topping, a very high standard of care will be imposed upon such
persons (McMullan, 1995). Therefore, dam owners and designers must assess
carefully the risks involved in the design and maintenance of such structures, and
take particular care to ensure that risks, even if mathematically quite small, are
minimised or managed.
The law of contract is also largely concerned with the enforcement of duties
that one person has, by agreement, bound himself or herself to perform for the
benefit of another. The law of torts (civil wrong) may also be seen to be concerned
with breaches of duties. Those duties are not established by any agreement
between persons but rather by the law itself. Thus there are, for example, duties
not to assault another person, not to trespass on another’s land, not to take
another’s goods, and to take care not to injure one’s neighbour. Some duties are
laid down by legislation; others are found in what is known as the common law,
that is the rules that judges recognised in the past as being necessary to enable
society to function as harmoniously as possible and which judges still recognise.

10.6 Property insurance

A dam owner can directly and indirectly influence the consequences of dam failure
by purchasing insurance to spread costs from a single dam owner to others.
Insurance may provide liability and asset protection and is important for dam
owners. The level of insurance carried should be based on value of facilities at risk,
potential downstream impacts, condition and age of the dam, likelihood of an
incident occurring and the cost of available insurance. Insurance spreads risk
among a large group of people and provides a measure of protection for the
person or organisation owning the dam, which may be held personally liable.
Types of coverage, availability and cost will vary from time to time, so it is
advisable to seek professional advice when considering the purchase of insurance.
Some insurance companies and brokers specialise in issues related to dam failure.
Recommendations of insurers can normally be obtained from insurance industry
representatives. Not only can damage and liability be covered but also the cost of
business interruption and lost income. Insurance can spread and reduce potential
loss and therefore should be an accepted cost of doing business. Many people have
avoided this cost and have paid severely for their short-sightedness (USDA, 1969;
FEMA, 1987).

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 Appendix 1

A glossary of terminology
The water industry has embraced engineers, scientists, educationalists, farmers as
well as people from other disciplines. A range of terminology is used to describe
some of the physical and scientific processes required to deliver water to farms.
While some are self-explanatory or are commonly used within the water industry
worldwide, others have evolved locally.
Abandonment Indicates that the dam is no longer used and no longer stores
water. It has been modified hydraulically and/or structurally
to ensure complete and permanent safety to life, property
and the environment. It should require no further operation,
maintenance, surveillance or remedial work.
Abutment The natural ground formation between the base of the dam
and its crest. The natural material below the excavation
surface and in the immediate surrounding formation above
the normal river level or flood plain against which the ends
of the dam are placed.
Appurtenant Works Include, but are not limited to, such structures as spillways,
either in or beside the dam and its rim; low level outlet works
and water conduits such as tunnels, pipelines, either through
the dam or its abutments.
Aquifer A water-bearing layer of rock or material or sediment below
the natural surface within which water is transmitted and
can be removed by pumping.
Base of Dam The foundation area of the lowest part of the main body of
the dam, the portion excluding the abutments. The base
elevation is considered to be at the lowest foundation level of
a substantial section of the dam. It excludes isolated pockets
of excavation which are not representative of the base
extending from heel to toe.
Beaching Rocks placed to dissipate the erosive force of waves on
banks. The term can mean dumped rocks, usually on a
prepared filter bed.
Bed The part of a waterway or channel that is usually or
normally covered with water when it is flowing.

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 168 • Farm Dams

Berm A horizontal ledge on a dam wall used to help stabilise the

slope.
Borrow Pit The source area for natural materials used in dam
construction. This is often located within the storage area of
the dam. Material from the borrow pit is used to build the
embankment.
Catchment Yield The volume of water which flows from a catchment past a
given point (such as a stream gauging station) and is
generally calculated on an annual basis. It comprises surface
run-off and base flow (discharge from shallow and deep
ground water).
Catchment The area drained by the streams or waterways down to the
point at which the dam is located.
Cohesion Degree of bonding between individual soil particles. Loose
sands and gravels are cohesion-less, while sticky clays are
cohesive.
Contractor A person employed to carry out earthmoving works from
plans and specifications in the construction of a dam and
other works for the storage of water.
Crest of Dam Used to denote the top of a dam. However, the term ‘Crest’
is sometimes applied to the level at which water may
overflow the spillway section of the dam. Term ‘Top of Dam’
is preferred to denote uppermost surface of the dam.
Cut-off An impervious barrier of material or concrete to intercept
seepage flows through or beneath the structure.
Dam A barrier constructed for storage, control and diversion
purposes. A dam may be constructed across a natural
waterway or on the periphery of a reservoir. When water is
stored behind the dam for irrigation or other water supply
purposes, the whole complex becomes a ‘reservoir’.
Design Flood The maximum flood for which the dam is designed.
Disused Dam A dam which is no longer needed for any current, definite or
essential purpose, but which is allowed to store water and
discharge floods while subject to routine operation,
maintenance, surveillance and remedial work as may be
required.
Diversion Any natural or artificial method or means by which part of a
waterway is taken from its natural course.
Downchute An inclined open channel through which water flow is
directed. Surface may be grass, concrete or beached.
Embankment An artificial bank built across a waterway to either protect
adjacent land from inundation by flooding or to store water.
Engineer A person who is professionally qualified and suitably
experienced in relevant aspects of dam engineering to allow
him/her to engage in some or all of the investigation, design,
construction, repair and remedial work, operational,
maintenance and abandonment activities.
Fetch The free distance which the wind can travel to any point in
raising waves, i.e. distance from furthermost full supply level
in the direction of the wind.

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 Appendix 1 • 169

Foundation The material of the valley floor and abutments on which the
dam is constructed.
Freeboard The vertical distance between the design flood level and the
top of the dam.
Full Supply Level The level of the water when the dam is at maximum
operating level, excluding times of flood discharge. When a
controlled spillway is provided, it is the spillway crest level.
Groundwater The segment of water below the natural surface of the
ground at a pressure equal to or greater than atmospheric.
Height of Dam The difference in level between the natural bed of the stream
or waterway at the downstream toe of the bank and the top
of the dam. If it is not across a stream, channel or waterway
it is the difference in level between the lowest elevation of the
outside limit of the bank and the top of the dam. See
definition ‘Top of Dam’.
In some instances where a dam has a free-overflow spillway
only or has a controlled spillway, it may be difficult to define
the top of dam level as the normal abutment sections may
not exist. In such cases the height is to be measured to the
level arrived at by adding the design flood rise in water level
to the level of the spillway crest, or to the full supply level.
Large Dam The minimum requirements adopted for determining
whether a large dam qualifies for inclusion in the ICOLD
World Register of Dams are as follows:
All dams above 15 metres (50 feet) in height, measured from
the lowest portion of the general foundation area to the top
of the dam.
Dams between 10 metres (33 feet) and 15 metres (50 feet) are
included, provided they comply with at least one of the
following conditions:
(a) The length of the crest, i.e. the top of the dam, to be not
less than 500 metres (1600 feet).
(b) The capacity of the reservoir formed by the dam to be not
less than 1 000 000 cubic metres (35 million cubic feet or 800
acre-feet).
(c) The maximum flood discharge dealt with by the dam to
be not less than 2000 cubic metres per second (70 000 cusecs).
(d) If the dam has specifically difficult foundation problems.
(e) If the dam is of unusual design.
Maintenance The routine work required to maintain existing works and
systems (mechanical, electrical, hydraulic and civil) in a safe
and functional condition.
Monitoring The observation and recording of data from measuring
devices to check the performance and behavioural trends of
a dam and appurtenant structures.
Outlet Works The combination of intake structure, screen, conduits,
tunnels and gate-valve meters that permit water to be
discharged under control from the dam.

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 170 • Farm Dams

Owner Any person, company or authority owning, leasing or

occupying the land on which a dam is constructed or
proposed to be constructed.
Permeability Property of a soil which allows the movement of water
through its connecting pore spaces.
Phreatic Line An imaginary line which defines the hydrostatic pressure
through homogenous or zoned embankment soils. It is used
to indicate the stability of the embankment at full supply and
when drawdown occurs, i.e. the storage is suddenly emptied
leaving the embankment soils saturated with water.
Piping Sub-surface tunnel erosion which is caused by soil and water
movement.
Plastic Limit The water content at the lower limit of the plastic state of a
clay. It is the minimum water content at which a soil can be
rolled into a thread without crumbling. In compaction of
soils within the embankment this is an important element of
construction.
Pore Pressure The pressure of air and water in the voids or pores of a soil
Referable Dam Any artificial barrier, temporary or permanent, including
appurtenant works which does or could impound, divert or
control water, other liquids, silt, debris or other liquid-borne
material and which:
either is 10 metres (33 feet) or more in height and has a
storage capacity of more than 20 000 cubic metres (16 acre-
feet);
or has a storage capacity of 50 000 cubic metres (40 acre-
feet) or more and is higher than 5 metres (16 feet).
Remedial Work The work required to repair, strengthen, re-construct,
improve or modify an existing dam, appurtenant works,
foundations, abutments or surrounding area to provide an
adequate margin of safety (e.g. drainage, grouting,
buttressing, post-tensioning, spillway or outlet works
modification.
Reservoir A dam or an artificial lake, pond or basin for storage,
regulation and control of water, silt, debris or other liquid or
liquid-carried material.
Reservoir Capacity The total storage capacity of the reservoir or dam up to Full
Supply Level (not up to flood level).
Safety Review The review procedure for assessing the safety of a dam,
comprising a detailed study of structural, hydraulic and
hydrologic design aspects and of the records and reports
from surveillance activities.
Seepage The escape of reservoir water by percolation through, under
or around the dam.
Shear Strength The resistance to deformation in soil, either from frictional
or structural resistance of the soil grains or by cohesion
between the surfaces of the soil particles.

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 Appendix 1 • 171

Spillway An open earth channel, weir, conduit, tunnel or other

structure designed to allow discharges from the dam when
water levels rise above the crest controlling flow down or
into the spillway structure. The spillway is principally to
discharge flood flows safely past a dam but may be used to
release water for other purposes. The spillway may be
uncontrolled (a free-overflow spillway) in which case
discharge occurs when the dam rises above the crest. If a
barrier is used to control the uppermost level of the dam it is
referred to as a controlled spillway.
Spillway Crest The uppermost portion of the overflow section which is
below the ‘Top of the Dam’.
Subgrade Material Material in cuts, fills and embankments immediately below
the foundation material.
Surveillance The continuing examination of the condition of a dam and
its appurtenant structures and the review of operation,
maintenance and monitoring procedures and results in order
to determine whether a hazardous trend is developing or
appears likely to develop.
Tailwater The Full Supply Level of water that inundates upstream land
within a storage.
Top of Dam The elevation of the uppermost surface of the dam proper
not taking into account any camber allowed for settlement or
kerbs, parapets, guardrails or other structures that are not a
part of the main water retaining structure. This elevation is
usually the roadway or walkway or the non-overflow section
of the dam.
Waterway A general term for any stream, river, watercourse or creek.
This may be flowing but not necessarily continuously, so that
it may be a dry bed (depression along which water flows)
periodically and also includes man-made artificial cut
channels. The legal definition may vary between States.

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 Appendix 2

Engineering specification for an earth-fill farm dam

Description of work
1 This specification is for the construction of an earth-fill farm dam and
incidental works on the property of:-
Landowner(s) Name:
Crown Allotment No:
Parish of:
2 This Specification shall include all operations necessary for the construction
and completion of the works including the supply of all plant, labour,
materials, tools and everything whatsoever necessary to carry out the works
in accordance with Plan number____________, this Specification, and to the
satisfaction of the Engineer.
Nature of work
3 Notwithstanding any description contained in the Plans or Specifications, the
Contractor shall be responsible for satisfying himself as to the nature and
extent of the specified works and the physical and legal conditions under
which the works will be carried out, including means of access, extent of
clearing, nature of material to be excavated, type and size of mechanical plant
required, location and suitability of water supply for construction and testing
purposes, and any other like matters affecting the construction of the works.
4 Provision shall be made for diverting water clear of the works during
construction unless approval is given to the contrary by the Engineer. The
Contractor shall take all necessary care to protect the works against flood
conditions and is to provide, install, maintain, and operate all pumping
equipment necessary for de-watering the site where required during progress
of the works.
5 The Contractor shall be entirely responsible for arranging water supply to
increase the soil moisture of the embankment material and for other
construction purposes.
Engineer
6 The Engineer means the person appointed by the landholder to supervise the
construction of the dam. The Principal requires that the Engineer should be

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 Appendix 2 • 173

well experienced in the field of small dam engineering, who has academic
qualifications acceptable for corporate membership of the Institution of
Engineers Australia and who is considered experienced in small dam
engineering.
Plans
7 The following plans (to be attached) shall form part of this Specification.
Survey marks for setting out works
8 A level benchmark has been established close to the site of the works and is
shown on Plan Number___________. The reduced level of this benchmark is
E.L__________, this being an arbitrary datum and shall be used as a
permanent reference for levels during the construction and maintenance of the
works and for calculating the accuracy of the works.
9 The Contractor must not move or disturb the benchmark in any way prior to
or during the construction of the works unless prior approval is given by the
Engineer.
10 The location of pegs and other reference features for the setting out of the new
embankment centre-line and for the location of ancillary works are shown on
Plan Number _________. The new embankment centre-line and ancillary
works are to be located relative to these pegs and other reference features in
accordance with the dimensions shown on the Plans.
Clearing and grubbing
11 The area to be covered by the embankment, borrow pits and incidental works,
together with an area extending beyond the limits of each for a distance of
seven (7) metres all round shall be cleared of all trees, scrub, stumps, roots,
dead timber and rubbish and the same shall be removed from the vicinity of
the work and burned or otherwise disposed of in a manner approved by the
Engineer.
12 The area to be covered by the stored water outside the limits of the borrow
pits shall be cleared of all scrub and rubbish. Trees shall be cut down stump
high and removed from the vicinity of the work.
13 All trees near the site which are likely to damage or obstruct work in any way
shall be cut down stump high or root felled as directed, and removed from the
vicinity of the work.
14 All holes made by grubbing within the area to be covered by the embankment
shall be filled with sound material and well compacted to finish flush with the
natural surface.
Removal of topsoil for use in the embankment
15 Before construction of the cut-off trench or of any ancillary works within the
embankment area, all grass growth and topsoil shall be removed from the area
to be occupied by the embankment and shall be deposited clear of this area
and reserved for use in completing the embankment and spillway.
16 All topsoil within the borrow pit excavation area, including the spillway, shall
likewise be removed clear of the excavation and embankment areas and shall
be reserved for use in completing the embankment and spillway.

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 174 • Farm Dams

Cut-off trench
17 Where directed, because of any doubt as to the nature of existing materials
forming the foundation of the embankment, a cut-off trench extending down-
wards into a sufficiently impervious formation, shall be excavated. The cut-off
trench shall extend for the full length of the embankment and beyond where
shown on plan, until a continuous impervious zone is encountered extending
to above top water level. The minimum width of the cut-off trench shall be
three (3) metres unless otherwise indicated.
18 All water, loose soil, and rock shall be removed from the trench before
backfilling commences. The cut-off trench shall be backfilled with selected
earth-fill of the type specified for the embankment, and this soil shall have a
moisture content and degree of compaction the same as that specified for the
selected core zone.
19 Material excavated from the cut-off trench is to be used in the embankment if
suitable, provided it is placed in the correct zone according to its classification.
Otherwise unsuitable material shall be disposed of as directed by the Engineer.
Selection and placing of material for embankment zones
20 Suitable material from within the storage area, or from borrow pit if sufficient
suitable material is not available from within the storage area, shall be used to
construct the embankment. Earth-fill shall be placed longitudinally in
progressive horizontal layers of uniform thickness of not more than two
hundred (200) millimetres before compacting, inclusive of the depth of loose
fill at the top of the preceding layer.
21 Where the earth-fill materials vary in permeability and/or moisture content,
the embankment shall be constructed with an impervious core zone flanked by
outer zones of semi pervious earth-fill as shown on the plans.
22 The core zone shall extend for the full length of the embankment and shall be
built up and compacted to the dimensions shown on the plans. Where a cut-
off trench is used the core zone shall be continuous with the compacted fill in
the cut-off for the full width of the top of the cut-off fill.
23 Where both dispersive and non-dispersive classified earth-fill materials are
available, non-dispersive earth-fill shall be used in the core zone. The outer
zones shall be built of the remaining classified earth-fill materials with the
most permeable materials being placed in the downstream outer zone.
24 No silt, sand, stones over 75 millimetres diameter, surface soil, tree roots,
organic matter, or other material, which will not compact properly, shall be
used in the embankment except where shown on plans for special purposes.
25 Excavated material which, in the opinion of the Engineer, is not suitable for
use in the embankment, shall be dumped in spoil banks clear of the works as
directed.
26 During construction the Contractor shall maintain the embankment in a free
draining condition in a manner satisfactory to the Engineer and no part of the
work shall be carried up more than 0.6 metres higher than any other part.
Work shall be suspended whenever, in the opinion of the Engineer, it cannot
be carried out satisfactorily owing to the fill becoming too wet.
27 When outlet pipes pass through fill material, this material shall be compacted
up to a level of 0.6 metres above the intended crown of the pipe. If the pipe is
to be laid in undisturbed foundation material and will be less than 0.6 metres

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 Appendix 2 • 175

below the level of the stripped site, the laying of the pipe shall be delayed until
the fill material above the pipe reaches a level at least 0.6 metres over the
crown of the pipe. The trenches for the pipes will then be excavated as
described in the relevant clauses of the Specification and the pipes and any
protective concrete placed in position. The trench shall then be backfilled with
approved material and satisfactorily compacted by pneumatic rammer or
mechanical hand tampers in not more than 100 millimetres (loose) layers to
the satisfaction of the Engineer until the cover over the pipe is at least 0.6
metres. The placing of backfill in trenches, when protective concrete
surrounding the pipe has been placed, shall not commence until the concrete
has cured for such times as may be determined by the Engineer. When no
concrete surround is specified particular care shall be taken to avoid damage
to the pipes or protective pipe coating.

Rollers and rolling

28 The whole of the material in the embankment shall be compacted to the
approval of the Engineer by eight (8) passes of sheepsfoot rollers exerting a
pressure of not less than thirty-five (35) kilopascals of projected area of
tamping feet actually in contact with the ground at one time and with these
feet projected for a length of not less than 175 millimetres.
29 If the Contractor desires to use a roller other than that specified in the
previous clause, variation of depth of layers and/or number of passes may be
required by the Engineer so as to effect the same degree of compaction.
30 The Contractor shall, at the direction of the Engineer, make a greater or lesser
number of passes than the eight specified above, if, with the required moisture
content, such change is necessary to obtain the desired compaction. In the
event of a different roller or procedure being used, any adjustment required
will be made by the Engineer to conform with the intent of this section of the
Specification.

Compaction tests
31 Before the commencement of compaction and from time to time thereafter, the
Contractor will receive notification from the Engineer as to the maximum dry
density of the fill material as determined by the laboratory compaction tests.
The desired degree of compaction is 95% of the maximum dry density as
notified by the Engineer.
32 When the first layer of fill material has been spread and rolled the Engineer
may, in order to carry out compaction tests, direct the Contractor to
discontinue placing material over this compacted layer for a period not
exceeding 24 hours.
33 The Contractor will subsequently be called upon to discontinue work, for
reasons associated with testing of materials, only if other excavated material
differs significantly from this tested first layer of fill.

Moisture content
34 Before commencement of compaction, and from time to time during
construction of the bank, tests for moisture content will be carried out by the
Engineer on the basis of the Standard Drop Test or other tests.

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 176 • Farm Dams

35 The Contractor will receive notification from the Engineer as to the optimum
moisture content for compaction for each type of material as determined by
the tests. The allowable tolerance of moisture content shall be plus or minus
two per cent (+ or –2%) of the optimum moisture content as established by
tests carried out in accordance with Australian Standard 1289—‘Method of
Testing Soils for Engineering Purposes’.
36 Material, which in the Engineer’s opinion is too damp for compaction, shall
be allowed to dry out to a moisture content satisfactory to the Engineer before
rolling is commenced or continued, or such material shall be removed at the
Contractor’s expense.
37 If material is too dry for compaction, it shall be watered at the direction of the
Engineer. This shall be by sprinkling in place on the earth-fill, or, if practicable,
by sprinkling the material in the excavation.

Placement of topsoil cover

38 After completion of the embankment all loose uncompacted earth-fill material
on the upstream and downstream batters shall be removed prior to spreading
of topsoil. This may be carried out progressively during construction so as to
make use of these materials in the appropriate zone of the embankment.
39 Topsoil shall be spread on the batters and on the crest of the embankment to
give a uniform thickness of 150 millimetres measured after rolling with a
crawler tractor. The purpose of this topsoil is to assist in the establishment of
a suitable grass cover to minimise erosion caused by wind, rain and/or wave
action. A suitable holding grass cover shall be established as soon as possible.
40 The finished dimensions of the embankment after spreading of topsoil shall
conform to the drawings with a tolerance of 75 millimetres from the specified
dimensions.

Spillway and spillway return slope

41 The spillway shall be excavated as shown on the plans, and the excavated
material if classified as suitable shall be used in the embankment, and if not
suitable it shall be disposed of into spoil heaps. Special care shall be taken to
ensure that the excavation is level at the lip of the spillway return slope and to
ensure that no spoil spills over onto that slope.
42 The spillway and spillway return slope shall be protected against erosion by
planting and maintaining a suitable holding grass which shall be established
as quickly as possible using fertilisers and watering if necessary.
43 To prevent natural run-off discharging over spillway batters and causing
erosion, catch drains shall be constructed to collect this flow and discharge it
away from the batters.

Installation of outlet pipe

44 A pipe outlet shall be placed under the embankment complete with outlet
valve and cut-off provisions all in accordance with the plans.
45 The outlet pipe, after joining and before backfilling or encasing with concrete,
shall be tested by the Contractor to the satisfaction of the Engineer.

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 Appendix 2 • 177

46 When encasing the outlet pipe in concrete consisting of 1 part by volume of

cement, 2 parts by volume of sand and 4 parts by volume of 20 millimetres
(maximum) screenings, with sufficient water to give 18 MPa (minimum)
concrete at 28 days, care should be exercised to see that the concrete
completely surrounds the pipe and that no air pockets are formed beneath the
pipe. The Contractor may support the pipe on approved concrete chairs to
give the minimum thickness of one hundred (100) millimetres of concrete
below the bottom of the pipe. The concrete should then be placed on one side
of the pipe and vibrated under and up the other side, thereby helping to
remove any air pockets, which would otherwise tend to form. Placing of
concrete must commence at the lowest level of the pipe and work towards the
higher level. Care must be taken to place the concrete encasing in lifts, which
will not cause the pipe to ‘float’ in the trench.
47 When a concrete encasement of the outlet pipe is not specified in the plans,
then the pipe trench shall be backfilled, when specified by the Engineer, with
selected earth-fill material suitably moistened and carefully placed in 100
millimetres loose layers compacted with pneumatic or mechanical tampers.

Completion
48 The embankment and spillway shall be completed in every respect including
neat trimming to correct lines and levels in accordance with this Specification
and the accompanying plans. The works area shall be left in a clean and tidy
condition at the completion of the work.

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 Appendix 3
Metric and imperial conversion tables
Dimensional conversion table—imperial to metric and back to imperial
Imperial Multiply by Metric Multiply by Imperial
dimensions dimension dimensions
(known) (unknown/known) (unknown)
Area
inches square (in2) 645.160 millimetres square (mm2) 0.001 550 inches square (in2)
feet square (ft2) 0.092 903 metres square (m2) 10.763 900 feet square (ft2)
yards square (yd2) 0.836 127 metres square (m2) 1.195 990 yards square (yd2)
acres (acres) 0.404 685 640 hectares (ha) 2.471 053 800 acres (acres)
miles square (miles2) 2.589 990 kilometres square (km2) 0.386 102 miles square (miles2)
Length
inches (in) 25.400 millimetres (mm) 0.039 370 100 inches (in)
feet (ft) 0.304 800 metres (m) 3.280 840 feet (ft)
yards (yd) 0.914 400 metres (m) 1.093 600 yards (yd)
miles (miles) 1.609 344 kilometres (km) 0.621 371 miles (miles)
chains (ch) 20.100 metres (m) 0.049 751 200 chains (ch)
Volume
feet cubed (ft3) 0.028 316 8 metres cubed (m3) 35.314 7 feet cubed (ft3)
yards cubed (yd3) 0.764 555 metres cubed (m3) 1.307 950 yards cubed (yd3)
inches cubed (in3) 16.387 100 centimetres cubed (cm3) 0.061 023 700 inches cubed (in3)

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 Appendix 3 • 179

Mass
pounds (lb) 0.453 592 370 kilogram (kg) 2.204 620 pounds (lb)
ton (ton) 1.016 046 900 tonne (t) 0.984 207 ton (ton)
pounds per foot (lb/ft) 1.488 160 kilograms per metre (kg/m) 0.671 969 pounds per foot (lb/ft)
Flow rates
feet cubed per second (ft3/s) 0.028 316 800 metres cubed per second (m3/s) 35.314 700 feet cubed per second (ft3/s)
feet cubed per second (ft3/s) 2.446 575 500 megalitres per day (ML/d) 0.408 734 600 feet cubed per second (ft3/s)
gallons per minute (gal/min) 0.075 768 200 litres per second (L/s) 13.198 155 gallons per minute (gal/min)
gallons per minute (gal/min) 0.006 546 400 megalitres per day (ML/d) 152.756 420 gallons per minute (gal/min)
million gallons per day (mgd) 52.616 782 400 litres per second (L/s) 0.019 005 300 million gallons per day (mgd)
million gallons per day (mgd) 4.546 090 megalitres per day (ML/d) 0.219 969 millon gallons per day (mgd)
acre feet per day (ac.ft/day) 1.233 481 800 megalitres per day (ML/d) 0.810 713 acre feet per day (ac.ft/day)
megalitres per day (ML/d) 0.011 574 metres cubed per second (m3/s)
metres cubed per second (m3/s) 86.400 megalitres per day (ML/d)
Force
tons per inch square (ton/in2) 15.444 300 megapascals 0.064 748 800 tons per inch square (ton/in2)
pounds/inch square (lb/in2) 6.894 760 kilopascals 0.145 037 700 pounds/inch square (lb/in2)
tons per foot square (ton/ft2) 107.252 kilopascals 0.009 324 tons per foot square (ton/ft2)
pounds/foot square (lb/ft2) 47.880 300 pascals (newtons per metre2) 0.020 885 pounds/foot square (lb/ft2)
Velocity
feet per second (ft/s) 26.334 720 kilometres per day (km/day) 0.037 973 feet per second (ft/s)
Power
horsepower (hp) 0.745 700 kilowatt (kW) 1.341 020 horsepower (hp)
foot pound per second (ftlb/s) 1.355 820 watt (W) 0.737 562 foot pound per second (ftlb/s)
Source: Modified from SR&WSC, 1973.

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 180 • Farm Dams

Acres to hectares (approximately)

Acres 0 1 2 3 4 5 6 7 8 9
(Ac)
Hectares (ha)
0 0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6
10 4 4.5 4.9 5.3 5.7 6.1 6.7 6.9 7.3 7.7
20 8.1 8.5 8.9 9.3 9.7 10.1 10.5 10.9 11.3 11.7
30 12.1 12.5 12.9 13.4 13.8 14.2 14.6 15.0 15.4 15.8
40 16.2 16.6 17.0 17.4 17.8 18.2 18.6 19.0 19.4 19.8
50 20.2 20.6 21.0 21.4 21.9 22.3 22.7 23.1 23.5 23.9
60 24.3 24.7 25.1 25.5 25.9 26.3 26.7 27.1 27.5 27.9
70 28.3 28.7 29.1 29.5 29.9 30.4 30.8 31.2 31.6 32.0
80 32.4 32.8 33.2 33.6 34.0 34.4 34.8 35.2 35.6 36.0
90 36.4 36.8 37.2 37.6 38.0 38.4 38.8 39.3 39.6 40.1
100 40.5 40.9 41.3 41.7 42.1 42.5 42.9 43.3 43.7 44.1
Source: Modified from SR&WSC, 1973.

Acre feet to megalitres (approximately)

Acre feet 0 1 2 3 4 5 6 7 8 9
(Ac ft)
Megalitres (ML)
0 0 1.2 2.5 3.7 4.9 6.2 7.4 8.6 9.9 11.1
10 12.3 13.6 14.8 16.0 17.3 18.5 19.7 21.0 22.2 23.4
20 24.7 25.9 27.1 28.4 29.6 30.8 32.1 33.3 34.5 35.8
30 37.0 38.2 39.5 40.7 41.9 43.2 44.4 45.6 46.9 48.1
40 49.3 50.6 51.8 53.0 54.3 55.5 56.7 58.0 59.2 60.4
50 61.7 62.9 64.1 65.4 66.6 67.8 69.1 70.3 71.5 72.8
60 74.0 75.2 76.5 77.7 78.9 80.2 81.4 82.6 83.9 85.1
70 86.3 87.6 88.8 90.0 91.3 92.5 93.7 95.0 96.2 97.5
80 98.7 99.9 101.1 102.4 103.6 104.9 106.1 107.3 108.6 109.8
90 111.0 112.3 113.5 114.7 116.0 117.2 118.4 119.7 120.9 122.1
100 123.3 124.6 125.8 127.1 128.3 129.5 130.8 132.0 133.2 134.5
Source: Modified from SR&WSC, 1973.
Notes: Above tables are only approximate and have been rounded to the nearest 0.1.
For the conversion of acres to hectares, e.g.: 25 acres = 20 +5 = 10.1 hectares. The same applies to acre feet to
megalitres, for example, 72 acre feet = 70 + 2 = 88.8 megalitres.

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 References
and suggested
further reading
Other publications which have been used to compile some of the information in
this book or which are relevant to large dams or dams with large capacities or
catchment areas are listed below. It should be noted that they are primarily
addressed to engineers with specialist expertise in dam design and construction.

References
Albright and Wilson (1990). Technical bulletin on ‘Albrite’, a range of sodium
tripolyphosphates, STPP, which is used in certain types of soils as an
additive to minimise dam leaks.
ANCOLD (1983). Guidelines on assessment of the consequences of dam failure.
Australian National Committee on Large Dams, AGPS, Canberra.
ANCOLD (1992). Guidelines on dam safety management. First draft, May 1992.
Australian National Committee on Large Dams, AGPS, Canberra.
ANCOLD (1998). Guidelines on assessment of the consequences of dam failure.
(July 1998), AGPS, Canberra.
ANZECC (1992). National water quality management strategy. Australian and
New Zealand Environmental and Conservation Council.
Barlow, C. C. and Bock, K. (1984). Predation of fish in farm dams by cormorants,
Phalacrocorax spp. Australian Wildlife Research, 11: 559–566.
Beavis, S. G. and Howden, S.M. (1996). Effects of farm dams on water. National
Land Care Program funded project, Bureau of Rural Sciences, unpublished
report.
Beavis, S. G. and Lewis, B. (1999). The impact of farm dam development on water
resources due to catchment yield. Proceedings of the Water 99 Joint
Congress of the Hydrology and Water Resources Symposium and Second
International Conference on Water Resources and Environment Research,
6–8 July 1999, Brisbane, pp. 465–469.
Beavis, S. G. and Lewis, B. (2001). The impact of farm dams on the management
of water resources within the context of total catchment development.
Proceedings of the Third Australian Stream Management Conference: The
value of healthy streams, 27–29 August 2001, Brisbane, Queensland,
pp. 23–27.
Bill, S. M. (1985). Using herbicides near water. Water Talk, No. 53, Rural Water
Commission, Victoria.

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 182 • Farm Dams

Burton, J. R. (1965). Water storage on the farm. A manual of design, construction

and operation for Australian conditions. Bulletin, No 9, Vol. 1, Water
Research Foundation of Australia.
CDWR (1986). Guidelines for the design and construction of small embankment
dams. Division of Safety of Dams, California Department of Water
Resources, Sacramento, California, USA.
CETC (1995). Technical bulletin and manual on the use of Bentomat/Claymax, a
soil additive to minimise dam leaks. Colloid Environment Technologies
Company.
Cowling, S. J. (1967). How to encourage wildlife on an irrigation farm. Water in
Australia. March/April 1967, pp. 43–45.
DC&E (1990). A series of pamphlets on aquaculture in Victoria. Department of
Conservation and Environment, Fisheries Management Division, Inland
Fisheries Management Branch.
DC&NR (1992). Your dam—an asset or a liability. By B. Lewis. Department of
Conservation and Natural Resources.
DoA SA (1977). Storing water in farm dams. By J. A. Edwards. Extension
Bulletin, No. 29/77, AGDEX 583. Department of Agriculture, South
Australia.
DoA VIC (1978). Farm water supplies, Speakers’ notes presented at one-day
seminar at the McMillan Rural Studies Centre, Drouin, 8 March 1978.
Department of Agriculture Victoria.
DSC (1996). A review of major dams, i.e. prescribed dams and statutory functions
under both the Dam Safety Act 1978 and Mining Act 1992 in NSW,
Australia. Dam Safety Committee, New South Wales.
DWR (1987). Water law review, Water law and the individual riparian rights and
dam safety issues papers. Report Nos 3, 4 and 5. Department of Water
Resources, Melbourne.
DWR (1989). Inventory of potentially hazardous dams. Report, No. 43, 1989,
Department of Water Resources, Victoria.
FEMA (1987). Dam safety: an owner’s guidance manual. July 1987, Colorado
Division of Disaster Emergency Services, Federal Emergency Management
Agency, USA.
Fietz, T. R. (1969). Water storage of the farm: soil, structural and seepage aspects.
Bulletin, No. 9. Vol 2, Parts 1 & 2. Water Research Foundation of
Australia, Canberra.
Greening Australia (1990). Trees at work—improving your farm dam. A booklet
on how trees help water quality, ecology. Greening Australia, Canberra.
Hazell, D., Cunningham, R., Lindenmayer, D., Mackey, B. and Osborne, W.
(2001). Use of farm dams as frog habitat in an Australian agricultural
landscape: factors affecting species richness and distribution. Biological
conservation (in press).
Hill, D. and Edquist, N. (1982). Wildlife and farm dams—a guide. Soil
Conservation Authority and Fisheries and Wildlife Division, Victoria.
Hislop, D. (1998). Farm water. CB Alexander Agriculture College, Tocal, NSW.
Hollick, M. (1975). The design of roaded catchments for farm water supplies.
Civil Engineering Transactions, Parts 1, 2 and 3, pp. 83–96.
IEA (1987). Australian rainfall and run-off, a guide to flood estimation.
Institution of Engineers, Australia, Canberra.

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 References • 183

Ingles, O. G. (1984). A short study of dam failures in Australia, 1857 to 1983.

Civil Engineering Systems, Vol 1, June 1984.
Kingsford, R. T. (1992). Maned ducks and farm dams—a success story. Emu, 92,
pp. 163–169.
L&WC (1999). Rural production and water sharing: farm dam assessment guide,
adding value to the natural assets of New South Wales. NSW Dept of Land
& Water Conservation, Sydney.
Lewis, B. (1993). It pays to plan your dam. Aqua, Vol. 1, Issue 6, Autumn 1993,
pp. 12–13.
Lewis, B. (1995a). Farm dam education program: training notes for RWA staff. A
course covering farm dam site evaluation, administration, design and
documentation. Circulation of notes limited to Rural Water Authorities in
Victoria.
Lewis, B. (1995b). Farming for the future: avoid costly farm failures. Farming
ahead with the Kondinin Group, July 1995, No. 42, pp. 14–16.
Lewis, B. (2001a). Minimum energy design using grassed spillways. Sixth
Conference on Hydraulics in Civil Engineering: The state of hydraulics,
Hobart, Tasmania, 28–30 November 2001. Institution of Engineers,
Australia.
Lewis, B (2001b). The impact and issues of increased farm dam development on
Victorian Water Resources. Sixth Conference on Hydraulics in Civil
Engineering: The state of hydraulics, Hobart, Tasmania, 28–30 November
2001. Institution of Engineers, Australia.
Lewis, B. and Beavis, S. (2001). Licensing aquaculture development in Victoria.
Proceedings of the Third Australian Stream Management Conference: the
value of healthy streams, 27–29 August 2001, Brisbane, Queensland,
pp. 373–377.
Lewis, B. and Branson, G. (1996). Licensing of aquaculture in Victoria.
Proceedings of Twenty-third Hydrology and Water Resources Symposium,
Hobart, Tasmania, May 1996, pp. 343–349.
Lewis, B. and O’Brien, T. (2001). Providing for fish passage at small instream
structures. Sixth Conference on Hydraulics in Civil Engineering: The state
of hydraulics, Hobart, Tasmania, 28–30 November 2001. Institution of
Engineers, Australia.
Lewis, B. and Perera, S. (1997). The impact of increased farm dam development
on Victorian water resources. Proceedings of Twenty-fourth Hydrology and
Water Resources Symposium, Auckland NZ, Nov. 1997, pp. 297–303.
Lewis, B., O’Brien, T. and Perera, S. (1999). Providing fish passage for instream
structures. Proceedings of Second Stream Management Conference,
Adelaide, February 1999.
McMullan, J. (1995). Engineers in the water industry—legal issues. Published
with the support of the Institution of Engineers Australia and Chambers &
Company Solicitors and Attorneys.
Nelson, K. D. (1968). Let’s look at those dams. Reprinted 2601/68, from the
Journal of the Department of Agriculture, (Victoria), A. C. Brooks,
Government Printer, Melbourne.
Nelson, K. D. (1973). The whole trouble with dams. The Journal of Agriculture,
Nov 1973, pp. 391–395.

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 184 • Farm Dams

Nelson, K. D. (1983). Farm water supplies: a survey. Water Talk, No. 49, January
1983, p. 3.
Nelson, K. D. (1985). Design and construction of small earth dams. Melbourne,
Inkata Press, Melbourne.
O’Brien T., Perera, S. and Lewis, B. (1999). Providing for fish passage at small
instream structures. Proceedings of Second International Conference on
Water Resources & Environment Research, July 99, Brisbane. Queensland.
Powling, I. J. (1990). Algae in water. Water Talk, No. 39 Rural Water
Commission, Victoria.
Romanowski, N. (1994). Farming in ponds and dams—an introduction to
freshwater aquaculture in Australia. Lothian, Melbourne
SCA (1979). Guidelines for minimising soil erosion and sedimentation from
construction sites in Victoria. Compiled under the guidance of R. J. Garvin,
M. R. Knight and T. J. Richmond. Soil Conservation Authority.
SCA (1983). Farm water supply manual collated by B. Garrett (unpublished). Soil
Conservation Authority.
SCS USDA (1967). A supplement to the Soil Classification System that was
adapted by the U.S. Bureau of Reclamation. Soil Conservation Service,
United States Department of Agriculture.
SCS USDA (1969). Engineering field manual for conservation practices. Soil
Conservation Service, United States Department of Agriculture, USA.
SR&WSC (1970a). Farm water supplies: practical advice on making the most of
available water. State Rivers and Water Supply Commission, Victoria.
SR&WSC (1970b). Series of pamphlets on water quality. State Rivers and Water
Supply Commission, Victoria.
SR&WSC (1973). Metric conversion tables. State Rivers and Water Supply
Commission, Victoria.
SRW (1995). A series of farm dam pamphlets by B. Lewis that provides
information advice on building and maintaining a farm dam. Each
pamphlet is numbered 1 to 21. Southern Rural Water, Victoria.
Stephens, T. (1991). Handbook on small earth dams and weirs: a guide to
sighting, design and construction. Cranfield Institute of Technology,
Cranfield Press, UK.
USDA (1969). Engineering field manual for conservation practices. United States
Department of Agriculture, Soil Conservation Service.
US-FEMA (1987). Dam safety: an owner’s guidance manual. United States Federal
Emergency Management Agency.
WAWA (1991). Guidelines for the design and construction of small farm dams
(Category 2) in the Warren Lefroy Rivers Area. Water Authority of Western
Australia.
Wensley, R. (1995). Dam safety management towards 2000, legal implications.
Gutteridge Haskins & Davey Pty Ltd Seminar on 16 August 1995.

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 Further reading • 185

Further reading
ANCOLD (1976). Guidelines for operation, maintenance and surveillance of
dams. Australian National Committee on Large Dams.
ANCOLD (1986). Guidelines on design floods for dams. Australian National
Committee on Large Dams.
Andrews, C. and Carrington, N. (1988). The manual of fish health. Salamander
Books.
Bell, F. G. (1980). Engineering geology and geotechnics news. Butterworth,
London.
Cadwallader, P. L. and Backhouse, G. N. (1983). A guide to the freshwater fish of
Victoria. Government Printing Office on behalf of the Fisheries and Wildlife
Division, Ministry for Conservation, Melbourne.
Caterpillar (1980). Caterpillar performance handbook, Edition 11. USA.
Chanson, H. (1999). Hydraulics of open channel flow. Butterworth Heinemann,
UK.
DWR (1989). Water Victoria—an environmental handbook. Department of Water
Resources, Victoria.
GH&D (1987). A report to the Department of Water Resources Victoria: farm
dams in catchment study. Gutteridge, Haskins & Davey.
Lewis, B. (1991). Inspecting a dam with the experts. Aqua, Vol. 1, Issue 1,
Autumn 1991, pp. 10–11.
Lewis, B. and Branson, G. (1996). Licensing of aquaculture developments in
Victoria. Twenty-third Hydrology and Water Resources Symposium
Proceedings, Hobart, May 1996, pp. 343–349.
Merrick, J. R. and Lambert, C. N. (1991). The yabby, marron and redclaw.
Merrick Publications.
NR&M QLD (2001). Safety management guidelines for referable dams (draft).
Natural Resources and Mines, Queensland Government.
Papworth, M. and Lewis, B. (2001). Some thoughts on humans and waterways.
Proceedings of the Third Australian Stream Management Conference: the
value of healthy streams, 27–29 August 2001, Brisbane, Queensland,
pp. 501–506.
QWRC (1980). Farm water supplies. Design charts for: earthworks, pipes,
pumps, channels, spillways, drainage, irrigation and water quality.
Queensland Water Resource Commission.
QWRC (1984). Farm water supplies design manual, Vol. 1, edited by A. J. Horton
and G. A. Jobling. Farm Water Supplies Section, Irrigation Branch,
Queensland Water Resource Commission.
RWC (1988). Irrigation and drainage practice. A text on the design practices that
should be used in the management of water supplies. Rural Water
Commission, Armadale, Victoria.
Sainty, G. R. and Jacobs, S. W. L. (1994). Waterplants in Australia. 3rd edn.
CSIRO, Australia, Division of Water Resources.
SCA (1972). Course notes for training course no. 38 at Mildura Victoria on Farm
water reticulation. 26 June 1972. Limited circulation. Soil Conservation
Authority.
SCA (1974). Compaction of soils. Soil engineering training course. Unpublished
report. Soil Conservation Authority.

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 186 • Farm Dams

Southorn, N. (1995). Farm water supplies: planning and installation. Inkata Press,
Reed International Books, Australia.
TADS (1990). Training aids for dam safety. Module on inspection of embankment
dams. Manual developed by the United States Interagency Steering
Committee of Federal and States.
USDA (1980). Guide for safety evaluation and periodic inspection of existing
dams. United States Department of Agriculture, Forest Services and Soil
Conservation Service, Washington, D.C.
Williams, W. D. (1980). Australian freshwater life. Macmillan.

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