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					Earth Dams


 Updated: February 2000




                          Caring for the Wollondilly Shire
1.       INTRODUCTION

1.1      Name and Scope of the Plan

This plan shall be known as “Development Control Plan No.21 – Shire of Wollondilly”. It outlines
Council’s requirements regarding the construction of earth dams. The plan applies to all lands within
Wollondilly Shire.

1.2       Operation of the Plan

This plan has been prepared in accordance with Section 72 of the Environmental Planning and Assessment
Act, 1979.

This plan came into force on 5 July 1989 in accordance with Clause 24(2) of the Environmental Planning
and Assessment Regulations, 1980. It was adopted by Council on 26 June 1989. This plan may be varied in
the manner provided for in the Environmental Planning and Assessment (E.P.A.) Act, 1979.

1.3       Application of the Plan

This plan applies to the construction of earth dams on land in conjunction with agriculture and domestic
water storage. Council shall take the controls outlined in this plan into its consideration in determining an
application.

Compliance with the provisions of this plan does not necessarily imply that Council will consent to an
application. Council must also take into consideration those matters listed under Section 79C of the E.P.A.
Act.

In special circumstances, Council may consent to an application which departs from the provisions of this
plan provided the objectives of the plan area met.

1.4       Relationship with other Plans

Where there is an inconsistency between this plan and any environmental planning instrument applying to
the same land, the provisions of the environmental planning instrument shall prevail. An environmental
planning instrument includes a State Environmental Planning Policy, a Regional Environmental Plan and
Local Environmental Plan.

Where there is an inconsistency between this plan and any other development control plan in force, the
provisions of this development control plan shall prevail.

1.5       Disclaimer

The controls outlined in this plan incorporate current technical information and advice. Council however
takes no responsibility for the failure of any dam which is constructed according to the standards that are
outlined in this plan.

2.       GLOSSARY OF TERMS

The following are definitions of terms used in this plan:

Batter            - is the slope of the dam wall

Bywash            - is a depressed area adjacent to the dam wall used for the dispersion of overflow water.

Crest             - is the top of the dam wall.

                                                                          Caring for the Wollondilly Shire
Cut of trench     - is a trench dug below the ground level of the dam parallel to the crest. The dam wall is
                    built into the cut off trench.

Earth dam         - is compacted earth dam generally built in or near a drainage line which has the primary
                    purpose for impounding water for storage. These dams are usually used for water
                    conservation on properties with an agricultural usage.

Freeboard         - is the distance between the crest and the top water level.

Spillway          - refers to pipes, bywashes or other devices used to divert water from a dam.

Top water level - is the height of the water level of the dam determined at its maximum capacity.

3.    PLAN OBJECTIVES

The objectives of this plan are:

(a)   To provide clear guidelines in relation to the construction of earth dams.

(b)   To ensure that earth dams are constructed to a safe standard.

(c)   To ensure that dam construction does not have an adverse impact on neighbouring properties.

(d)   To ensure that earth dams do not adversely effect the surrounding environment.

4.    DEVELOPMENT APPLICATION REQUIREMENTSF

4.1   Introduction

Earth Dams have been defined in the previous section and they will be referred to as “dams” throughout this
development control plan.

Council’s development approval is required before the construction of a dam may commence. In order to
seek development approval a fully completed development application must be lodged with Council.
Information requirements for development applications are outlined below. In some cases details from a
suitably qualified structural engineer may be required.

4.2   Information Requirements

Development applications for dams are to include the following information:

     Locality sketch of dam to help establish catchment area.

     Cross section of dam showing:

            Crest width
            Batter – upstream and downstream
            Freeboard
            Cutoff trench dimensions

     Soil type

     Equipment to be used for construction


                                                                            Caring for the Wollondilly Shire
     A plan view of the dam showing the location and dimensions of the bywash(es) – both inlet and
      outlet.

     Method of stabilising the bank/bywash.

     Storage area of dam at top water level.

An example to the type of plans and cross sections required for a dam proposal is shown attached.

Development guidelines for dams are outlined in the following section.

5.    DEVELOPMENT STANDARDS

5.1   Introduction

This section outlines development standards regarding dam construction. These standards have been largely
developed by the NSW Department of Agriculture and Fisheries, and the NSW Soil Conservation Service.
They are to be complied with in relation to dam construction.

5.2   General Considerations

Outlined below are general considerations that should be taken into account when planning a dam. Before
building, advice should also be obtained from the Department of Agriculture and Fisheries and the Soil
Conservation Service.

     A gently sloping site stores more water for earth moved than does a flat site or a steep site.

     A very steep site is not advisable – there is seldom enough material to build a satisfactory wall.

     Ensure there is sufficient good dam building material. Rock and sand are common problems as they
      are generally unsuitable.

     A really good site should not be wasted with a small dam. Either build a larger dam or save the site
      until its full value can be realised.

     The size of the catchment must be considered when planning dam capacity. For instance a five
      hectare steep rocky catchment will be sufficient to fill a small but not a large dam.

     If a dam is to be built near a boundary, any water which bypasses the dam should flow from the
      property in the same place it did before the dam was built.

     Freeboard (the height from water level to the top of the dam wall) must be sufficient to prevent
      overtopping in a storm.

     When surplus water bypasses the dam, ensure there is a suitable place to return it to its normal
      flowline without causing erosion.

     Dams should not be sited near roads, utilities or near common property boundaries because of the
      adverse affect they may have, especially concerning seepage and bywash/spillway overflow.

     Dam spillways must be designed to handle storm flows.

As a general rule dams should be as deep as practical to prevent excessive evaporation during the dry period
of the year. Three metres should be considered a minimum depth as the evaporation in many areas will
lower the water level by about 1.5 metres.

                                                                          Caring for the Wollondilly Shire
A reasonable size for a stock water dam is about 1000 cubic metres (one million litres). A dam intended for
house and garden will need to have a capacity of at least 3000 cubic metres.

5.3   Crests

The width of the dam crest is to be a minimum of 3 metres for 3 metre high dam wall. The crest should
increase in width 500m from every metre above a 3 metre high dam. The crest is to be slightly rounded to
prevent water ponding.

5.4   Freeboard

A minimum of 1.0 metre is to be established for freeboard. This should increase by 10% for every metre
over a 3 metre high wall.

5.5   Embankments

The material used to construct an embankment should be sufficiently impervious to keep seepage low and to
be stable. A soil with 25% clay content is ideal to form an impervious barrier.

The following soil types should not be used for dam construction or batters.

     Gravels
     Organic soils
     Peat

The embankment is to be completed with at least 200mm of compacted top soil. It is to be planted with a
good holding grass such as kikuyu or couch. Trees or shrubs are not to be planted on the embankment as
roots may provide a seepage path for water.

The slope of embankment batters should conform with the ratio of 3.0 horizontal to 1.0 vertical for both
upstream and downstream slopes.

5.6   Bywash

An earth bywash is required on all dams in order to pass surplus runoff around the dam which would
otherwise pass over the embankment. The bywash should be generally 6 metres in width.

The width of the outlet from the bywash is not to be less than the inlet width. The bywash also is not to
direct flows onto the downstream toe. The bywash cut batter is to have a maximum steepness of 1.5:1.

The bywash is to be excavated 75mm below the top water level and backfilled with compacted topsoil and
planted with a suitable holding grass such as kikuyu or couch. No trees or shrubs are to be planted in the
bywash area.

In some instances Council may require a pipe spillway through the embankment to act as an outlet. This is
especially applicable where spring flows or small flows of long duration are known to occur.

The spillway is to have an inlet of at least 100mm below the level of the bywash. Generally a 150mm
diameter pipe is suitable for this purpose.

The bywash or spillway water from a dam should not have an adverse effect on neighbouring properties.
Dams are to be sited so that excess water is contained on the property on which they are located before
meeting with a watercourse downstream.



                                                                        Caring for the Wollondilly Shire
5.7   Cutoff Trench

All farm dams are required to have a cutoff trench. The cutoff trench is to be constructed along the entire
length of the embankment. It does not need to extend across the bywash. The trench should be taken down
at least 300mm into the impervious soil and back filled with impervious material.

5.8   Licencing

Dams constructed on a watercourse may require licencing with the Department of Water Resources.
Officers of that Department will be able to inform you of particular licencing requirements.

The Dam Safety Committee of this Department also requires that generally dams over 5 metres high with a
storage of greater than 50 megalitres must be referred to it to determine the need for prescribability under the
Dams Safety Act of NSW 1978.

6.    ACKNOWLEDGEMENTS

The assistance of Officers from the Department of Agriculture and Fisheries, Department of Water
Resources and the Soil Conservation Service in providing information and advice in the preparation of this
plan has been greatly appreciated.




GRAHAM TAYLOR
GENERAL MANAGER.




                                                                          Caring for the Wollondilly Shire
Bywash: this, together with batter slopes is probably the hardest variable upon which to place a general rule
or table. However, when checking submissions the following could be useful:

     The outlet should not be less than the inlet width
     The bywash should not direct flows onto the downstream toe
     The bywash cut batter should be no steeper than 1.5:1

If a spring flow or small flows of long duration are known to occur, it would be advisable to incorporate a
pipe spillway through the embankment which would cater to these flows; this spillway would have an inlet
at least 100mm below the level of the bywash; 150mm diameter GWI pipes are commonly used for this
purpose, however, the pipe diameter would depend on flow/slope relationships.




                                                                        Caring for the Wollondilly Shire
Caring for the Wollondilly Shire

				
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