Excavated tanks (farm dams)

Page last updated: Tuesday, 15 January 2019 - 3:48pm

Excavated tanks (farm dams) provide effective water storage wherever surface water run-off can be harvested for livestock, crop spraying, irrigation and domestic use on rural properties. Excavated tanks for rainfall storage are common in the southern agricultural areas of Western Australia.

Well-designed and constructed dams are farm assets, safe and require little maintenance. Poor design and construction can lead to excessive costs, downstream erosion and risks to property and people.

Excavated tanks should be part of an integrated farm water and salinity management program.

What is an 'excavated tank' or 'farm dam'?

Dams block an existing waterway, and earth tanks are an excavation into which water is diverted. This page only deals with earth tanks.

An excavated tank is an earth structure on near-level land used to store water, in which part of the storage capacity is below ground level. On sloping land, the tank is often called a hillside dam or just a farm dam. The excavation can be rectangular or square – with 3 or 4 walls – or circular.

The following guidelines are for rectangular, square and circular excavated tanks that do not require a civil engineer.

Why use hillside dams?

Hillside dams are a cost-efficient way of collecting and storing run-off water from winter rainfall where there is no easily accessible and suitable groundwater for pumping. Also, hillside dams avoid shallow saline watertables in the valley floors. Many of the soils in the south-west agricultural areas have sufficient clay content to provide a good seal. A disadvantage of hillside dams is that they have relatively large surface areas for the dam volume, and net evaporation rates in the dry summer are often greater than 1500mm.

How farm dams are used

Farm dams are used:

  • where filling is from a natural or improved catchment (grade banks or roaded catchment)
  • where the front wall of the structure is open or continuous with piped inlets
  • for supplying single paddocks or drought-proofing whole farms
  • for livestock water supplies
  • for crop spraying water
  • for domestic use on-farm
  • for fire-fighting water supplies
  • for aquaculture.

Back to top

How farm dams work

Farm dams are most effective at holding water when the base and inside walls of the dam are sealed with clay of low permeability to minimise leakage, and when catchments are improved to increase and collect run-off. In many agricultural areas of Western Australia (WA), run-off from crop land and pasture is not adequate to reliably fill farm dams. Run-off can be improved by using grade banks and roaded catchments (Figure 1).

Showing how side roads or grade banks can increase catchment water harvesting into a farm dam
Figure 1 Side roads or grade banks increasing water harvesting into a farm dam

Conditions where these guidelines apply:

  • on gently to moderately sloping land, for excavated tanks and hillside dams in agricultural and pastoral areas
  • anywhere there is a requirement for initial or additional water supplies
  • near to, but clear of, streams, creeks and rivers; positioned so that the overflow from the structure can be safely diverted to a stream, creek or river
  • where a natural or improved (roaded) catchment is of sufficient area to fill the farm dam.

Types of dams

Rectangular or square excavated tanks with three or four walls

These are the most common type of farm dams in the agricultural areas of WA. Excavated tanks need to be constructed of clay or have a clay lining that is 0.7 to 1.0m thick. A 3-walled excavated tank is open at ground level on the uphill, and the excavated soil is used to construct the 3 walls above ground with freeboard above the full supply level (Figure 2). Four-walled and round excavated tanks optimise capacity and minimise siltation.

Plan and cross section of a four walled dam with inlet and outlet pipes
Figure 2 Plan and cross section of a four walled dam with inlet and outlet pipes

Double dams

Double dams are useful where watertables limit depth of a dam. The design reduces the loss of water to evaporation by having smaller and deeper excavations to limit the surface area relative to the volume.  Evaporation reduction is particularly effective when the smaller dam is kept topped up by pumping from the larger dam.

Line drawing of a double dam system
Figure 3 A double dam design showing water movement between dams

Ring and turkey nest tanks

Ring and turkey nest tanks (Figures 4 and 5) are used on flatter sites where shallow saline watertables may be present. The base of the dam can be near ground level. Ring tank (Figure 4) walls are usually constructed from earth 'borrowed' from a ring inside the tank, with a centre 'island' at normal ground level. This type of tank is prone to leakage (there can be low levels of clay in the excavated soil) and high evaporation loss relative to hillside excavated tanks (the surface area relative to water volume is high).

Plan of a circular excavated tank with guiding wing banks and a silt trap
Figure 4 A circular excavated tank with guiding wing banks and a silt trap.

Turkey nest dams consist of a completely enclosed earth embankment, which is filled by pumping from an alternative water source (i.e. a creek, groundwater or other smaller dams). These dams are usually sited as high as possible in the landscape so that water can be reticulated from them to other parts of the property.   

The dams built in WA do not usually retain a mound of soil in the centre and are smaller than those in New South Wales and Queensland. Turkey nest dams require a site that is reasonably flat with good dam building clay not more than a metre below the surface; otherwise, the cost of overburden removal reduces the cost effectiveness of construction.

Turkey nest tank walls are constructed mostly from earth borrowed from outside the tank. The tank in Figure 5 has used this excavated area as a catchment, and the centre of the tank has been excavated to some degree. The milky blue colour is caused by suspended clay in the very fresh water. Water from this tank is pumped and reticulated to other parts of the property near Esperance.

Photograph of a turkey nest tank
Figure 5 A turkey nest tank excavated on level ground over a saline watertable. As the saucer catchment fills, water enters the tank through a piped inlet. The piped inlet can be closed on the outside, and fresh water pumped into the tank, allowing the water level in the tank to be higher than the surrounding land.

Back to top

Planning considerations

The Occupational Safety and Health Act 1984 sets objectives to promote and improve occupational safety and health standards. The Act sets out broad duties and is supported by more detailed requirements in the Occupational Safety and Health Regulations 1996.

The legislation is further supported by guidance material, such as approved Codes of Practice, through WorkSafe Western Australia. ‘Code of Practice – Excavations’ applies to all workplaces where excavation occurs and particularly when “a person is required to work in an excavated area or other opening in the ground that is at least 1.5m deep”. This is especially relevant to soil test pits and pipes in trenches.

Local government may have limitations on dam placement, especially in relation to roads and other infrastructure. For example, dams are not to be sited within 100m of a watercourse or within 200m of roads. Other planning considerations include:

  • avoid sites close to and above buildings, work sites, roads, and valuable infrastructure
  • avoid valley floors or drainage lines with shallow watertables; check options for reducing salt build up in dams
  • allow space above the dam for improved catchment structures (grade banks or roaded catchments)
  • calculate water needs for a full year in dry conditions
  • calculate water flows and storage based on dry years, and for the intended improved catchment structures
  • design inflow (silt traps, piped inlets) and outlets (outlet pipes, emergency spillway) to safely handle peak flows from intense storms
  • design dam and water access to reduce erosion and allow for maintenance; direct access by livestock is not recommended
  • consider fewer, larger dams and reticulation to reduce evaporation losses and increase flexibility of water distribution.

Design criteria

Site characteristics are accurately measured with particular attention to site slope, topsoil depth, overflow disposal and available area of catchment/improved catchment.

Structure capacity is determined by assessing water requirements and catchment yield and with consideration for evaporation and seepage.

Excavation shape is round, rectangular or square with consideration to having the embankment (wall/s) constructed continuously around the structure to reduce evaporation, control inflow and exclude paddock debris generated by severe storms. Consider using round dams because this shape has the smallest surface area for evaporation to affect.

Catchment peak flow run-off – where natural catchment is diverted into the dam, the peak flow run-off is determined using a recognised method, such as the Flood Index Method or the Rational Method. Usually 10 hectares of natural catchment, directed by grade banks, is appropriate for each 1000m3 of storage. The peak flow run-off can be used to determine spillway dimensions. See Australian Rainfall and Runoff for more information.

Improved (roaded) catchment area is determined using daily rainfall, daily water requirements (demand), catchment threshold and monthly evaporation rates.

Depth is adequate for providing sufficient water supplies and allowing for evaporation loss. Depth should be greater than annual evaporation, or greater than total evaporation for the chosen design period if the dam is for drought-proofing.

Sideslope (batter) ratio is usually 3:1.

Freeboard is a minimum of 1m above the maximum water level.

Overflow – the crest is set at the maximum water level for the dam. Where a silt pit is installed, overflow can be set out from the silt pit. Overflow is to discharge clear of the dam walls. Where there is a risk of crest erosion, materials other than earth can be used; flumes and chutes are potential applications.

Use the Weir formula to calculate the crest width for the chosen overflow depth for a 1-in-20 to 1-in-50 year (return period) peak flow discharge. Calculations based on the greater return period are recommended for larger dams with large, natural or improved catchments. Chosen flow depth should be small enough that the dam freeboard is not compromised. Consult a specialist engineer or trained contractor for using this formula:
w = Q/(1.7h1.5)
where:
w = drop structure width across stream (m)
Q = design flow or crest capacity (m3s-1)
h = design depth of flow at crest (m)

Mechanical spillways, such as a pipe with hooded inlet or a riser, can be used to overflow the dam where frequent low flows could damage the constructed overflow. Top of the hooded inlet or riser is set at the maximum water level of the dam and to discharge clear of the walls. Without compromising the dam's freeboard, the main overflow crest can be set 0.1–0.15m higher.

Pipes are 150 or 200mm nominal bore PVC and have a debris shield, rack or strainer fitted to the inlet end of the pipe.

Piped drop inlet, sump inlet and headwall with inlet pipes are set out above the dam. Top water surface of the inlet is set at the maximum water surface of the dam. Inlet pipes are set just above the floor of the inlet and on a fall into the dam.

Inlets are designed in such a way as to regulate surges of flow from high run-off events that would otherwise damage the dam. Trash rack or strainer screen should be installed to exclude paddock debris.

Inlet structures are constructed of concrete, sand bags, gabions or large diameter concrete pipes (up to 900mm diameter installed vertically over inlet pipe ends). Inlet pipes are 150 or 200mm nominal bore PVC.

To contain the approximate amount of run-off from a 10-year average recurrence interval (ARI) storm, one inlet pipe is required per hectare of natural and improved catchment. Up to 3 pipes are required to contain a 50-year ARI. The larger (200mm) nominal bore pipes are used in agricultural areas with higher average annual rainfall.

Silt pit with inlet pipes is required where the catchment will yield debris or other eroded materials. Pit volume is determined by the amount of suspended or floating debris required to be settled out.

Silt pit volumes ranging from 100 to 1000m3 may be appropriate. Substantial losses of run-off through evaporation of water retained in the pit may occur as inlet pipes must be set above the settled debris and silt.

Pipe sizes and numbers are to be matched to inflows similar to inlets (above) and to be fitted with a trash rack or strainer screen.

Planning methodology – identify sites that may be suitable for a dam. Given water requirements, catchment/improved catchment run-off/yield, evaporation and leakage over a design period, calculate the structure capacity, shape and dimensions. Choose the layout of structure, catchment, inlet, outlet and safe overflow disposal.

Determine catchment/improved catchment dimensions and confirm site suitability by pegging layout.

Excavation site should be drilled, or test pits dug, to 1m below the proposed maximum depth of the excavation. Drill 1 hole through the centre and 4 holes – one in each corner – through the floor of the proposed structure. Take soil samples and note the presence of subsurface water, rock and gravel or sand seams.

Soil types at the construction site are field-tested and classified to ensure the stability of the proposed structure. Soil is field classified for engineering properties using the Unified Soil Classification System. Test with particular attention to dispersion, aggregation, cracking and grading.

Clay content is to be at least 25%.

Volume of completed structures needs to be confirmed by measuring and calculating the top, middle and bottom areas as well as measured depth. The formula for all excavated shapes is the prismoidal formula expressed as:

V = (A+4M+B)d/6

where:
V = volume
A = top area of excavation (area of water surface when full)
B = bottom area of excavation (area of floor)
M = area at ½ depth
d = depth

For convenient calculating, the following derivations of the prismoidal formula can be used for each excavated shape.

Circular:

V = π [R2+(R.r)+r2]d/3

Rectangular:

V = [(L.W)+(l.w)+[(L+l).(W+w)]]d/6

Square:

V = L2+(L.l)+l2]d/3

where in all formulae:
V = volume (m3)
R = radius of water surface (m)
r = radius of floor (m)
d = depth (m)
π = Pi or 22÷7 or 3.14159
L = length of water surface (m)
W = width of water surface (m)
l = length of floor (m)
w = width of floor (m)
d = depth of water from surface to floor (m)

Volume of water in older dams

See calculating farm dam water volumes for a guide.

Back to top

Safety and environmental aspects

Before construction, consult the local government authority, neighbours and, where needed, a specialist engineer or trained contractor. Take care in siting and constructing farm dams to avoid the risk of injury to people and damage to property or infrastructure. Failure of dam walls can lead to flash flooding. Eroded material from poorly planned, constructed or maintained structures can reduce flow capacities when deposited in downstream channels.

Legal aspects – Statute and common law

Interference with a watercourse in a proclaimed surface water management area is controlled under the provisions of the Rights in Water and Irrigation Act 1914 (WA). In certain circumstances excavated tanks or hillside dams may need a licence so check with the Department of Water and Environmental Regulation to see if your excavated tank or hillside dam needs a licence.

Local government councils often require their Chief Executive Officer is notified of proposals to construct farm dams near road reserves or land vested in the shire council.

Common law rules govern the flow of surface water discharged into watercourses. To reduce the likelihood of cross-boundary disputes:

  • construct water-impounding structures so they do not have a detrimental effect on streams further down the catchment
  • take reasonable steps to ensure the safety of another person and another person's property
  • consider what effect planned earthworks will have on other people and seek consent from any person that may be affected
  • take care during construction and maintenance to stop the loss of disturbed material from the site.

Construction

  • Prepare site by pegging and referencing corners (square and rectangular shapes) or centre (circular shape). Measure fall across the site for calculating any storage volume above the excavation. Install a temporary bench mark (TBM) in a protected location.
  • Remove topsoil and stockpile clear of the embankment (wall) location.
  • Excavate core trench under the embankments if pervious materials are present under the topsoil. Core trench must extend 1m into impervious material.
  • Build the embankments by excavating in ‘floors’ and pushing material to the correct location. Compact the embankments with the bulldozer weight in 50 to 75mm layers or compact 150mm layers with a sheepsfoot roller. Embankment sideslope ratios can be confirmed by using an electronic builders slope finder or battometer.
  • Install inlet and outlet pipes early in construction of the embankments.
  • Construct the overflow and ‘final trim’ structure.
  • Spread the stockpiled topsoil on the outside batters and embankment top. Topsoiling encourages vegetation and helps retain embankment moisture and resist cracking.

Operation and maintenance

  • Inspect embankments for safety and seepage. Check for cracking and piping and movement cracks. Look for erosion of sideslopes, inlets and outlets. If present and attended to early, most of these problems can be treated.
  • Clear drop inlet, sump inlet, inlet pipes and silt pits of debris and eroded material. Vermin can burrow into inlets, outlets, embankments and improved catchment. Burrows should be dug out and repacked with clay. Vermin around the structure should be eradicated.
  • Consider fencing the dam and catchment where supply is an important or drought-proofing structure.

References

  • Bligh, K J 1989, Soil conservation earthworks design manual, Department of Agriculture and Food, Western Australia.
  • Clement, JP, Bennett, M, Kwaymullina, A & Gardner, A 2001, The law of landcare in Western Australia, 2nd edn, Environmental Defender’s Office WA (Inc), Perth, WA.
  • Huffman, RL, Fangmeier, DD, Elliot, WJ & Workman, SR 2013, Soil and Water Conservation Engineering, 7th edn, American Society of Agricultural and Biological Engineers, St. Joseph, Michigan.
  • Keen, MG 1998, Common conservation works used in Western Australia, Agriculture Western Australia, Western Australia.
  • Keen, MG 2001, Field pocket book of conservation earthworks formulae and tables, Department of Agriculture, Western Australia.
  • Lewis, B 2002, Farm Dams, Land Link Press, Collingwood, Victoria.
  • WorkSafe Western Australia 1996, Code of practice: excavation, WorkSafe Western Australia, Perth.

Contact information

Tilwin Westrup
+61 (0)8 9780 6165