Key features of a well-designed farm dam
Building a new dam, or undertaking significant works on an existing dam, is an opportunity to create an asset for your farm that should last for several decades. However, the value and longevity of your asset depends on a set of factors including: construction quality, water storage capacity, site appropriateness, impact of stock on the dam, vegetation around the dam and biodiversity supported by the dam.
This section highlights the key features of a dam and what needs to be considered to ensure maximum performance from a newly constructed or renovated dam (Figure 6).
Dam wall, bank or embankment
The dam wall is usually built using the material excavated to create the dam storage. The wall is an essential component of the dam’s ability to hold water, and must be compacted during construction. Choosing a site with soils that are appropriate for building the dam wall is particularly important.
The topsoil under where the dam wall will be built must be removed and set aside during construction, to avoid creating a leaky layer of earth under the wall. Once the wall is built, the topsoil should be returned to the top of the dam wall, enabling planting of grasses and other ground covers. Don’t plant trees on the dam wall, as they can destabilise the bank and cause leaks.
The top of the dam wall is the crest, and should be at least 3 metres wide.
The excavated pit that forms the bulk of the dam’s water storage capacity. All excavated areas should be covered by water once the dam is full, to minimise erosion.
Dam borrow pits are vulnerable to silting up over time, reducing the storage capacity of the dam. As well as enhancing overall storage, deeper dams have a better capacity to resist evaporation during dry times, so siltation can pose a significant risk to a dam’s performance. This can be reduced through enhancing fringing vegetation that will filter inflow and minimise sediment flow into the dam, as well as minimising stock disturbance on the dam edge.
A batter is a receding slope – in this case, the slopes that form the inside of the dam storage, as well as the faces of the dam wall. A battered dam wall provides a wide base to support the weight of the water behind it. The batters of most concern in building a dam are:
- Inlet batter – upstream cut of the dam’s excavation area. This is the batter down which water flows into the dam when it is filling. The inlet batter can be vulnerable to erosion if the water level falls sufficiently to expose it (see figure 9).
- Upstream batter – upstream face of the dam wall. This is the batter that holds the water in the dam.
- Downstream batter – downstream face of the dam wall. This batter generally won’t have water against it.
The slopes of the dam wall batters should be 2.5-3 horizontal to 1 vertical – in other words, for every metre of height, the wall should step back 2.5-3 metres horizontally. As dam sizes become larger and the depth of water stored against the wall becomes deeper so the batter slope needs to be shallower – at least 3 horizontal to 1 vertical.
A flat area between the borrow pit and the upstream batter of the dam wall which helps provide structural support to the embankment.
Cutoff trench (keying-in)
A trench below the foundation base of the dam wall that is, like the embankment itself, impervious to water. This helps prevents seepage of water underneath the dam wall. Incorporating a cutoff trench is known as “keying in” – ensuring the dam wall is “keyed in” to the clay beneath it. This is an important part of any dam building project.
Downstream batter toe
The downstream batter toe refers to the edge of the downstream batter where it connects to the original ground surface. The toe is the point where the wall base is widest and is located at the farthest point downslope from the dam. It is often the point where seepage flows from under the wall will exit to the surface. It is a point that should be regularly inspected for signs of the dam leaking. If the seepage is cloudy then advice should be sought for guidance on if the dam is going to tunnel and/or break.
Height from the top water level to the top of the dam wall. The freeboard should be at least 1 metre.
The inflow area of the dam should ideally have a shallow gradient, so that water moves into the dam slowly and the dam does not get too deep too quickly. This shallow gradient also allows for the growth of plants that can handle periodic inundation and will help filter inflow water.
In addition to a shallow gradient, the creation of a shallow shelf in the inflow area will further support the growth of aquatic vegetation and attract a range of wildlife. This area should be less than 2 metres deep at full water level, to support vegetation and wading birds. In drier times when the dam level falls, this area may become ephemeral (temporary), which can also provide habitat for a different group of animals and plants.
A spillway allows the exit of overflow water that would otherwise go over the dam wall or erode a new channel around the embankment. Most farm spillways are an excavated level channel, lined with grass to prevent erosion, but sometimes spillways are lined with concrete or rock, and can be supplemented with a trickle pipe through the dam wall. It is important to avoid woody vegetation growing on spillways, where it can impede the flow of water and cause erosion.
The sill is the level edge over which water overflows from the dam into the spillway. The height of the sill in relation to the structure governs the top water level if pipes are not used, and is therefore important in determining the freeboard.
A wide, shallow area below the spillway enables overflow water to spread out, minimising erosion risk, hydrating a greater area and enabling the growth of plants that thrive with occasional inundation.
A trickle pipe through the dam wall can be helpful for gradual return of smaller amounts of water to the downstream catchment, along the same line as the water would have flowed originally. This can help keep the spillway, overflow and paddock dry in between larger overflow events.
Shallow areas at the edges of the dam will, when the dam is full, provide important habitat for biodiversity. Shallow areas will also support the growth of fringing vegetation that may be suited to a range of water depths or periodic inundation. Supporting the growth of this vegetation in shallow areas during wet times will help minimise evaporation. During drier times, the water may retreat to the deeper parts of the dam.
To improve water quality and ensure longevity of the dam structure, it is of paramount importance that dams be fenced to exclude stock access. Water can then be reticulated to where it is needed. Fencing dams helps in the following ways:
- Protects the structure of the dam (dam wall, spillway and banks) from erosion caused by heavy livestock.
- Allows fringing vegetation to grow, stabilising dam banks and filtering sediment and nutrients from water entering the dam.
- Reduces or removes the negative impacts of livestock on water quality, such as fouling of the water through defecation and disturbance of the dam edge and banks.
Note that a fenced dam should still have an access gate into the fenced area for future management access needs. See section 2 for more information on fencing.
Grassed buffer zone
A grassy buffer zone around the dam, particularly in inflow areas, will help filter the water and capture paddock run-off such as sediment, animal dung and other pollutants. This will improve water quality and help prevent sediment build-up in the dam, minimising the need to de-silt the dam in the future.
The part of the storage volume that exists between the level of a low flow outlet pipe and the spillway level. If a pipe is installed, it is usually set about 300mm below the spillway level. This space fills in flood flows. As soon as water reaches the pipe level, the pipe starts to discharge. If the flood is large enough then the flows will eventually discharge through the spillway. The flows through the dam will drop back to the pipe inlet level and the flow stops. The combination of the pipe and the air space is designed to take flow pressure off the spillway in large flood flows.
Full supply level
The dam’s capacity when it is full. The dam’s maximum capacity is usually determined by the height of the spillway sill.
Figure 6: Key features of a properly constructed farm dam.