Water can be lost from a farm due to seepage through the base or sides of a farm dam and although very difficult to quantify, seepage losses can be significant in some cases. Seepage loss is likely to be more critical in low rainfall areas or where dams fill infrequently, and consequently some allowance should be made for them when doing water budgets.
The main factors influencing seepage losses will be the site and soil conditions of the dam, and construction techniques used in building the dam. Dams sited in an area with the presence of permeable soils or layers of permeable soils such as sand or gravels or layers of rock may be prone to seepage loss. In these cases lining with a suitable clay may reduce or prevent losses.
The use of unsuitable materials for constructing the base or walls of a dam can also contribute to seepage losses. Certain clays can also be prone to seepage, such as the red or brown clays derived from recent basalts or volcanic material (termed krasnozems or ferrosols). Although containing a high proportion of clay particles, these also contain a lot of iron and are strongly structured, making them quite permeable even when compacted.
Sodic clays can be prone to dispersion or slaking, and although they can be good for sealing the base of dam, if they are used in the dam walls they can be prone to tunnel erosion which can result in failure of the wall. Materials with a low proportion of clay or those with sands or gravels present in the clay are relatively permeable and more likely to experience water to be lost through seepage.
Poor construction techniques such as inadequate or insufficient compaction, too little or too much soil moisture to obtain optimum compaction or incorrect compaction equipment can contribute to seepage losses.
Seepage losses are extremely difficult and expensive to quantify, however it is worthwhile making some allowance for them in doing water budgets, especially in low rainfall zones. Generally an allowance of 10% of total volume is made for seepage losses, however local knowledge and experience with particular soil types can be used to adjust this.
Assume loss as an allowance of storage volume, say 10%
| L seepage (kilolitres) = Volume x 0.1 | |||
| Where | L seepage = Loss due to seepage | ||
| Example | |||
| L seepage | = 1900 x 0.1 = 190 kilolitres |
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| note | 1 kilolitre = 1 cubic metres = 1000 litres = 0.001 megalitres | ||
The net volume of water available from a farm dam is the Total Dam Volume less the amounts lost due to evaporation and seepage.
Net Volume Available (cubic metres) = Dam Volume - E dam - L seepage
Where Edam = Evaporation from water surface of the dam (see Agnote XX Calculating Evaporation Loss from a Dam)
| Example | ||
| net Volume Available | = 1900 - 423 - 190 = 1287 kilolitres |
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Potential seepage losses can be reduced or minimised during dam construction by careful attention to the materials used and compaction techniques employed. It is important to remove any soils that are prone to seepage such those containing sands or gravels and to use impermeable clays for the dam base and walls. In situations where suitable clays are either not present or it is not feasible to import them, consideration can be given to using a plastic dam liner.
During construction it is important to ensure that the soils used for the base and banks have adequate moisture content and that suitable compaction equipment is employed to achieve optimum compaction.
When desilting or cleaning a farm dam, care should be taken not to interfere with the base or sides of the dam that may affect its' integrity and cause it to leak. During times of drought or low rainfall it is important that some water remains in the dam to prevent drying and cracking of the base. This is particularly important on sodic soils as any cracking may eventuate in tunnel erosion that may lead to failure of the dam.