Earth Dams (Artificial Waterbody)

Objectives

1. Ensure dams are stable, have minimal environmental impact and do not adversely affect surrounding properties, either by ponding water back onto upstream properties or by concentrating the flow of water to any downstream properties; and
2. Ensure riparian right of the water users are not affected by the construction of a dam.

Controls

1. Dams should not be sited near roads, utility installations or neighbouring dwellings due to potential adverse impacts of seepage and bywash/spillway overflow and potential breaks of the embankment.
2. If a dam is to be built near a boundary, any water which bypasses the dam or spillage discharges should flow from the property in the same place it did before the dam was built.
3. Dam spillways must be designed to handle storm flows and freeboard sufficient to prevent overtopping in a 1:100 year (1% Annual Exceedance Probability) storm when a by-wash is provided.
4. No dams must not be constructed within 15 metres (to top water level) of a public road. The toe of the embankment batter or top water level must not be closer than 3 metres of a private property.
5. The width of the dam crest must be a minimum of 3 metres for dams having up to a 3 metre high dam wall. Crests should increase in width 0.5 metres for every metre above a 3 metre high dam.
6. A minimum 1 metre freeboard is required over the top water level. This should increase by 10% for every metre over a 3 metre high wall. Deviations to this may be accepted depending on the particular hydrologic, operational or dam circumstances and the accuracy of engineering design, in accordance with NSW Dam Safety Committee’s Guidelines for earth dams.
7. The height of the outlet in relation to the dam governs the top water level if pipes are excluded. The outlet must be level and at least six (6) metres wide. The width of the outlet should not be less than the inlet width.
8. Natural vegetation below the spillway outlet and on the inflow areas must not be disturbed by machinery, vehicles or livestock.
9. The spillway size must be as follows:
   

   Table 6-2: Spillway size

   Catchment Area (ha)	Outlet length(m)	Channel Width(m)
   less than 20	7	3
   20-40	12	6
   greater than 40	Subject to detailed design

10. An earth bywash is required on all dams in order to pass surplus run-off around the dam which would otherwise pass over the embankment. The bywash should be generally 6 metres in width.
11. The width of the outlet from the bywash is not to be less than the inlet width. The bywash must not direct flows onto the downstream batter toe. The bywash cut batter is not to exceed a maximum steepness of 1.5:1.
12. If the bywash is required to be vegetated, then 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 and shrubs are to be planted in the bywash area.

    Note: 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. Rock baskets and geotextile fabric may be required to prevent erosion where velocities are high. Outlet and/or spillway pipes in earth basins must be encased in concrete.

13. The pipe spillway invert is to be at least 100mm below the level of the bywash.
14. The bywash or spillway water from a dam should not have an adverse impacts 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 natural watercourse downstream.

    Note: Continuous trickle flows kill vegetation, keeps the soil wet and encourages spillway erosion. A small diameter sewer class pipe or stronger (usually 150mm. but varied according to flow rate) can be built into the wall during construction to accommodate trickle flows. The pipe is normally installed with a fall in gradient between its inlet and outlet, with the outlet 300mm, below the flood spillway.
    
    The trickle pipe is to be extended beyond the toe of the batter of the embankment to prevent any potential instability problem to the embankment.

15. Three metres should be considered the minimum depth of a dam, as evaporation in many areas will lower water level by approximately 1.5 metres during dry periods. Batter slopes should be retained at 1:3, therefore the depth will be dependent upon surface area measurements in small dams.
16. 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.
17. The following soil types should not be used for dam construction or batters:
    1. sand
    2. gravels
    3. organic soils
    4. peat
18. The slope of upstream embankment batters should conform with the ratio of 3(horizontal):1(vertical) and downstream embankment batters no steeper than 2.5(horizontal):1(vertical).
19. All dams must have a cut-off trench to be constructed along the entire embankment length a minimum depth of 300mm. Impervious material from the excavation must be placed into the trench and compacted forming a watertight barrier preventing seepage past the structure.

Backfilling of Earth Dams

1. A dam fill plan must be prepared by a suitably qualified person and should indicate the extent of filling, original and final contours, and depth of filling in maximum 0.5m increments. The dam fill plan must accompany a report prepared by a suitably qualified engineer, detailing the type of fill material used, the compaction levels achieved, and classification in accordance with the provisions of AS 1289, Methods of testing soils for engineering purposes Soil strength and consolidation tests.