Finding a dam site
These are the requirements for a good dam site and advice on the design required to make the most of a possible site. The information relates to earth dams of up to 20 ML water holding capacity. For larger dams it is strongly recommended that professional services be employed as incorrect siting is frequently a cause of dam failure.
Why build a dam?
Why do you want the water? When is it needed? How long do you want the water to last?
How much is needed?
Having answered these questions and subsequently decided on the storage capacity needed, the next step is to find a site that can successfully collect and hold that required volume of water.
To be successful, it has to be a site that:
- will allow the construction of an economic and safe dam of appropriate size;
- has a catchment of adequate size to reliably fill the dam;
- will allow safe disposal of excess water flows;
- fits in with the farm plan for water availability and distribution; and,
- can meet any legal requirements.
Choosing the site
It is critical that any site can support a flood bypass system. The water yield characteristics of the catchment along with the rainfall patterns (see later) will determine the size of the spillway needed. The site must be topographically suitable to enable the spillway to be constructed as an integral part of the dam.
It is necessary for spillway flows to be returned to the normal drainage route before they leave a property.
The site with the best utility and economic advantage is where the most water is stored for the amount of soil material moved. This is the storage to excavation ratio. A first class site may have a ration of 10 or higher. Such a site is most likely in a broad flat drainage line (possibly at the confluence of two drainage lines) in gently undulating country. Low ratios occur in steeper country and in narrow drainage lines.
Where drainage lines are not available for dam construction, a hillside storage is an option. These may have storage to excavation ratios of 1:5 or so. For more information see A gully dam or a hillside dam?
Storage size potential
Landscape shape will very much determine how much water may be retained at any given site. The longitudinal slope of the site, the cross section shape of the site, the height of the embankment, the area of the full supply level and the volume of the excavation will all influence the outcome.
A simple rule of thumb method to get a rough estimate of storage potential is as follows
(0.4 x A x d)
Vg = total volume of water stored behind bank (megalitres)
d = depth of water adjacent to bank (m) but not including excavation depth
A = surface area of water behind embankment when full (m2)
Ve = Volume of excavation below full supply level (megalitres).
Soil materials available
Three questions need to be answered about all possible sites:
- Is there suitable material available which potentially could build a water-tight bank?
- Is there enough material available to build the bank required?
- Can the bank be tied-in with a stable underground base which is also impervious?
A proper view of the underground materials is essential to answer these questions. This requires either auger hole drilling or, better still, digging of test pits with either a back-hoe of excavator. Both the excavation zone and the bank centre line need evaluation.
- sand or gravel deposits
- excessive thickness of topsoil or organic rich materials
- well structured clays which look permeable or may be hard to compact
- soil material which can be moulded when moist for bank construction.
- dense and impervious material below the centre-line of the bank for tying the core-trench into.
More detailed information can be obtained from Agriculture Note AG1397: Soil materials for farm dam construction
There is no point in building a dam which will collect salty water, particularly as progressive evaporation will further increase salt concentrations in the storage. Check for salt patches in the dam catchment, along the drainage lines and around the site.
Test the salinity of runoff water if in doubt and seek further advice if salt levels (as electrolyte conductivity) are above 800 microSiemens/cm @ 25 C°.
Avoid building dams in a saline discharge zone.
Estimating catchment water yields
Just what total amount of water can be expected on average from a given catchment? A catchment with shallow soil and poor vegetation cover will have a greater water yield from the same catchment size and rainfall than one with deep soils and a sound vegetative cover.
Runoff is estimated from the formula:
A x R x Y
A = catchment area in hectares
R = rainfall in mm
Y = yield as % of rainfall converting to runoff
The yield factor (Y) for individual small rural catchments is hard to determine. Not only does it vary from place to place, and with land use, but it also varies from year to year. Various rules of thumb have been proposed over the years, based on yearly rainfall, and soil type. We will not reproduce these here, but provide the following relationship to demonstrate how the % yield is related to annual rainfall. The comments underlying suggest how various conditions will cause deviations from this basic relationship
A soil of low permeability will have a yield coefficient above this line. A soil of high permeability will have a yield coefficient below. A timbered or well vegetated catchment will have values below this line. A burnt or denuded catchment will have values above this line. If annual evaporation is greater than about 1300 mm then values will be above the line, if less, values will be below. Further, a year containing a number of large rainfall events will have values above this line. Conversely, a year with well distributed moderate rainfall events will have values below the line.
There are more accurate and detailed methods available for predicting catchment yield based on long term assessments of daily rainfall records
Heavy rain produces flood flows from a catchment. A dams needs a spillway which will allow these flows to traverse the dam and return to the original drainage line without any damage to the dam.
If the spillway is not designed to cope with such flows the dam bank may be overtopped and spillway badly eroded. Either may then lead to undermining of the dam bank.
In some cases anticipated flood flows are so large that provision of an adequate spillway is not economically feasible. In these cases, an off-stream storage is the only option.
Freeboard should be a minimum of 1 metre (vertically) above the full supply level. It is necessary to allow for bank settlement following construction. Add an extra 10% to the bank height to allow for this and ensure an adequate freeboard.
Design of the spillway depends on:
- catchment size, shape and condition
- rainfall characteristics
- site shape, soil and vegetation
A very simple rule-of-thumb for calculating the top width of the spillway is:
w = width of spillway inlet in m
A = catchment area in ha
Hence a 9 ha catchment probably would require a 3 m wide spillway inlet.
Some farm dam sized catchments produce small but consistent trickle flows especially during winter and spring. If these continually trickle down the spillway they will cause deterioration of its vegetative stability.
The problem can be overcome with the installation of a trickle flow pipe to intercept this flow and deliver it directly to the floor of the drainage line downstream of the dam.. More information is obtained in Agriculture Note AG1403 Trickle flow pipes for farm dams.
An eroding catchment will soon fill a dam with sediment. Control sediment movement before dam building.
High levels of nutrients in catchment runoff water can readily set up a dam for algal blooms. Isolate runoff from intensive animal industry areas and use fertilizers efficiently on intensively cropped areas (see Minimizing algal growth in farm dams)
A useful precaution is to develop a vegetative filter area of 10m width or more immediately upstream of the dam to trap and utilize small loads of sediment and nutrients.
Many planning schemes require landowners to obtain a permit for the construction of farm dams. Check first.
Dams on waterways
Any dam to be constructed on a waterway requires the specific approval of the appropriate rural water authority. Depending on the size of the dam proposed the authority may require design, certification and construction supervision by an experienced farm water supply engineer.
The builder or land owner of a dam must notify the rural water authority if:
- the bank is 5m or more in height with a storage capacity of 50ML or more, or
- the bank is 10m or more in height with a storage capacity of 20ML or more.
- the bank is 15m or more
Water distribution to stock
Direct access to the dam water by stock has undesirable aspects.
- vegetation removal; and,
- water contamination.
Fencing of the dam in combination with a reliable reticulation system is a sound investment
- Widely varying circumstances apply on different properties and thus the information contained in this note should be used for general guidance only. It is advisable to seek expert assistance with detailed planning when a decision to construct is made.
- Your local water authority (ie either Gippsland and Southern Rural Water, Goulburn-Murray Rural Water, Lower Murray Urban and Rural Water, Grampians Wimmera Mallee Water or Melbourne Water) is able to supply additional information.
- The following 2007 publication is available from us: Your dam your responsibility. A guide to managing the safety of farm dams.