Fokus Oha

Building a farm dam is not a case of digging a hole and waiting for it to fill with water. Many farm dams fail because they were not planned or built properly.

Most farm dams are compacted earth structures built at depressions, gullies or hillsides. Farm dams should be planned and built with the same care you would apply to building a new house.

There are several things to think about when planning and building a farm dam:

• which government regulations (if any) apply
• who should build the dam
• how big to make it
• where to put it
• how to prepare the site
• how to build the dam
• how to look after it.

Government regulations

Some Councils require a Development Application for every dam built in their area. Check with your local Council to see if you must comply with any development control plans.

You may also need a licence to build your dam. The Department of Infrastructure, Planning and Natural Resources (DIPNR) issues the licences and can advise you on whether you need one.

Some dams may need to be registered with the Dam Safety Committee, which considers the consequences of dam failure. Although registration is required for very few farm dams, you should check the need for registration by contacting the Dam Safety Committee at the Parramatta office of DIPNR.

The removal of trees within 20 metres of a watercourse may be prohibited. Sometimes, special permits are issued to allow tree removal. If the location of your proposed dam is affected by trees, check with DIPNR before you start building.

Finally, DIPNR considers the suitability of works at or near a watercourse, lake or estuary. A permit is required for any excavations, placement of fill or construction in or near these areas. You should approach DIPNR to find out if your dam is affected.

Who should build your dam?

It is worthwhile to choose an earthmoving contractor who has experience and a good work record. Not every contractor has the experience or the ability to build a dam.

Try to find someone who specialises in dam construction. For example, you could ask neighbours who have had dams built if they are happy with the results.

Once you have found a few dam builders ask them to show you some of their recent work. If possible, talk to the owners of these dams and ask their opinions of the work.

If you choose an experienced dam builder with good references, you can reduce the chance of problems occurring during and after the building of your dam.

How big should the dam be?

Decide how much water you need before you start building. Water requirements vary considerably depending on crop type, geographic location, type of stock and stock numbers. Pastures grown at Bega, for example, need much less irrigation water than pastures grown at Dubbo. Similarly, 100 sheep need less water than 100 cows.

The estimated average annual water requirements for some crops and stock are listed in the following text, and can be used as a basis for calculating storage requirements. Remember, however, these figures are for guidance only and may be quite different for your particular situation.

Crops and pastures:

• pastures: allow 4–6 ML/ha a year*
• vines: allow 2–3 ML/ha a year
• vegetables: allow 3–5 ML/ha a year
• stone fruit: allow 3–4 ML/ha a year

Livestock:

• sheep: allow 1,000 litres per animal per year
• horses: allow 14,000 litres per animal per year
• beef cattle: allow 15,000 litres per animal per year
• poultry: allow 100 litres per bird per year

*1 ML = 1 megalitre = one million litres

Evaporation losses from the dam must also be considered in the size of the storage. Evaporation losses from a storage are not constant and vary from month to month and from location to location. Higher evaporation losses can be expected in summer than in winter and higher losses can be expected inland than on the coast. Average evaporation losses could be as low as 1,400 millimetres per year on the coast and as high as 3,600 millimetres per year inland.

Where to put the dam

Once you have estimated how much water you need, the next thing to do is to establish where to put the dam. Remember the dam site you choose will determine its success.

A gully is usually a good place to site a dam because it reduces the amount of earthworks and the overall cost. However, not everyone has the advantage of a gully site on their property; and many farm dams are built on hillsides.

Two important considerations, the catchment yield and the soil type at the site, will help you to choose the best location for your dam.

The yield of the catchment

Unless your dam fills from a spring or by pumping from a river or bore, the source of water will be rainfall that runs off the catchment. The catchment is the area that collects rainfall run-off and directs it to the dam. The catchment can be natural, like fields and forest, or it can be artificial, like roads and roofs. Generally, the bigger the catchment area, the bigger the volume of run-off water.The amount of run-off also depends on several other factors, including:

• ground slope
• rainfall intensity
• type of groundcover
• soil type
• existing drainage patterns.

If the catchment does not provide the amount of run-off water you need, then you can build catch drains. These drains collect run-off water from outside the dam catchment area and direct it to the dam. The drains need to be built carefully to ensure a grade towards the dam; and they should be sealed with a covering of grass, to avoid erosion.

Table 1 will help you to estimate the likely yield of your catchment. For example, if your average annual rainfall is around 750 mm and your catchment area is about 4 ha, then you could expect an average 3 ML yield from the catchment annually.

Remember, the tabulated yields have been derived using assumptions regarding:

• slope
• soil type
• ground cover
• catchment geometry
• drainage paths.

An accurate assessment of your catchment yield can only be made after investigation of the characteristics of your catchment.

The type of soil at the site

The embankment must be structurally stable and able to hold water. This means that the soil used to build the embankment must also be structurally stable or impermeable - preferably both.

The soil type that usually satisfies both these criteria is clay. However, not all clays are suitable for dam embankments. For example, some clays disperse when wet and result in tunnels forming in the embankment. Thus, the soil must be tested to determine its behaviour.

To do the tests, samples of the soil must be obtained from the excavation area. The samples can come from auger holes or backhoe trenches. The sampling depth should extend at least to the depth of anticipated excavation. Sampling the site will also give you an idea if there is enough soil in the excavation area to build the embankment.

The samples should be tested by someone experienced with soil behaviour in dam construction.

Preparing the site

The embankment and spillway should be set out with markers, such as painted timber stakes, which are easy to see.

The features of the works that markers should be used to locate are:

• upstream toe of the embankment
• downstream toe of the embankment
• spillway inlet and outlet widths
• top water level
• crest level
• centreline of the embankment
• extent of the excavation area

Before building starts, the area to be covered by the embankment, spillway and excavation should be stripped of all topsoil, roots and vegetation (including trees and stumps). Store the topsoil for use in the completion of the embankment and spillway.

How to build your dam

The successful completion of an earth dam relies heavily on achieving a well-compacted embankment.

Apart from soil compaction, other major construction considerations include:

• the cut-off trench
• exposed rock
• spillways
• clay cores
• embankment batters
• freeboard
• settlement of the embankment
• crest width
• topsoil use

The cut-off trench

Dams lose water through evaporation and seepage. Little can be done about evaporation losses, but with good construction methods seepage losses can be reduced.

One method is to build a cut-off trench along the entire length of the embankment. Usually the trench does not need to extend across the spillway. The trench should be taken at least 600 mm into impervious soil and backfilled with good clay that is thoroughly compacted. All farm dams should have a cut-off trench.

Soil compaction

At the correct moisture content, soil compaction helps to provide structural stability to the embankment and is another way of reducing seepage losses. The soil used to backfill the cut-off trench and to form the embankment should be placed in layers, with each layer thoroughly compacted before the next layer is placed.

Preferably, compaction should be achieved with a sheepsfoot roller. However, a scraper or bulldozer may be satisfactory, depending on the soil behaviour and the layer thickness. A layer not more than 150 mm of loose thickness (for a sheepsfoot roller) or 100 mm of loose thickness (for a bulldozer or scraper) is recommended.

The number of passes that should be made by the compacting equipment depends on the soil type, but it should be at least four. Generally, embankments lower than about 2–3 m may be compacted satisfactorily with a bulldozer or scraper.

The soil used to build the embankment should not be too wet or too dry. If the soil is too dry when it is compacted there is a good chance that air voids will result and the soil will be permeable. Compaction will also be hampered and produce an unsatisfactory result if the soil is too wet.

A good guide to soil moisture content can be obtained from a simple field test. When soil moisture is at the best level for effective compaction, you should be able to roll the soil between your palms into a thread (about the thickness of a pencil) that just begins to crumble on further rolling. If the soil thread crumbles before it reaches pencil thickness, it is too dry. If the thread can be rolled to a thickness much less than a pencil, then it is too wet.

If the soil is too dry a water cart can be used to wet it before it is used in the embankment. The best way to do this is to rip the excavation area, wet the soil, allow it to stand for about 24 hours, check its moisture content using the field test described previously; and if that is right, place the soil in the embankment. Try to avoid wetting the embankment to increase the soil moisture during construction, as this usually causes very uneven soil moisture and uneven compaction.

If the soil in the excavation area is too wet, the drying process can be accelerated by ripping.

Exposed rock: maximise the storage by obtaining as much soil as possible to build the embankment from below top water level. Remember that some soil will also be available from the spillway excavation. Rock, sand or gravel exposed below top water level should be covered with at least 300 mm of compacted clay to prevent seepage.

Spillways reduce erosion damage to the embankment and help prevent dam failure. They are used to pass floodwater around the dam that would otherwise go over the embankment or erode a new channel around the embankment. Most farm dams have a grass-lined earth spillway. Spillways can also be concrete-lined, cut into stable rock or made of gabions (rock-filled wire baskets). Sometimes, the spillway is supplemented by a pipe built through the embankment. If you don’t know what size spillway to build, seek expert advice.

A clay core may be required if there is not enough suitable material at the excavation area to build a homogeneous clay embankment. In these cases, the clay core is used to provide the impermeable barrier and the balance of the material in the embankment provides the structural stability.

The core can be located between more permeable material or it may be constructed at either the water or non-water face. Typical core arrangements are illustrated in figures 2, 3 and 4.

The batter (slope) of the embankment needed for dams depends on soil type, embankment height, stored water depth and other factors.

If the embankment is more than 4 m high at its highest point, seek engineering advice on appropriate batters. If the embankment is less than 4 m high and is sitting on a stable foundation, batters 3:1 upstream and 2.5:1 downstream will cater adequately for most soil types.

The freeboard is the height of the embankment above top water level (or spillway level). The freeboard must be enough to prevent water overtopping the embankment when the spillway is working and/or when winds cause waves on the storage. The freeboard must be at least 750 mm; and sometimes a freeboard of more than 2 m is required.

Settlement of soil banks is common and an allowance must be made for settlement of the dam embankment.

The embankment could settle to a level where it is overtopped by water and failure will result. To avoid the likelihood of this type of failure the crest level must be built higher than the design level.

Clay soil can settle more than 10%, but well-constructed clay dam embankments are not likely to settle more than about 5%. You should allow 5% of the height of the embankment (along its length) to cater for settlement. For example, if the intended maximum height of the crest is 5 m, the embankment must be built to a height of 5.25 m (an additional 5%) to allow for settlement to a design crest height of 5 m. A typical allowance for settlement is shown in figure 5.

Crest width. The required crest width is a function of the stability requirements of the embankment. At the same time, the minimum crest width must allow the safe operation of construction equipment.

In the absence of engineering design, a good guide to estimating the required crest width of the embankment is to adopt a minimum width of 2.5 m for embankments up to 5 m high. For embankments higher than 5 m, allow an additional 0.2 m for every metre in height greater than 5 m. For example, if the maximum height of the embankment is 7 m, the crest width would be 2.9 m (the sum of 2.5 + 0.2 + 0.2).

Topsoil. The embankment should be completed with about 150 mm of compacted topsoil and the spillway should be cut about 100 mm below top water level and built back to that level with topsoil. The embankment, spillway and the spillway outlet should then be planted with a good holding grass such as kikuyu. If the stripped areas do not provide enough topsoil, then it should be imported.

Figure 6. The plan view of a typical earth dam

How to look after the dam

To allow the storage to fulfil its function it is important to carry out regular maintenance. It also helps to observe the following:

• Ensure topsoil cover of about 150 mm.
• Establish a grass cover on the embankment and spillway as soon as possible. A grass that mats, such as kikuyu, is preferable to a grass that tufts.
• Do not let trees or shrubs grow on the embankment, spillway or spillway outlet slope. Roots might disturb the compacted soil and provide a seepage path for water, while trees or shrubs in the spillway area will restrict the flow of flood water.

Some maintenance tips:

• Topdress areas that become bare of topsoil as soon as possible.
• Fertilise and water grass cover regularly.
• Slash grass regularly on the spillway and outlet slope to promote a dense groundcover.
• Avoid using the spillway and outlet slope for vehicular access, to reduce potential erosion.
• Maintain a sealed catchment to minimise the potential silting-up of the dam. A permanent pasture is ideal.

Further information

Each dam site is different and this Agfact is not intended to replace the advice of qualified consultants. For further information, you can also contact your local office of NSW DPI.