Groundwater dams are structures that intercept or obstruct the natural flow of groundwater and provide storage for water underground. They have been used in several parts of the world, notably India, Africa and Brazil. Their use is in areas where flows of groundwater vary considerably during the course of the year, from very high flows following rain to negligible flows during the dry season.
The basic principle of the groundwater dam is that instead of storing the water in surface reservoirs, water is stored underground. The main advantages of water storage in groundwater dams is that evaporation losses are much less for water stored underground. Further, risk of contamination of the stored water from the surface is reduced because as parasites cannot breed in underground water. The problem of submergence of land which is normally associated with surface dams is not present with sub-surface dams.
There are two main types of groundwater dam: the sub-surface dam and the sand storage dam.
A sub-surface dam
A sub-surface dam intercepts or obstructs the flow of
an aquifer and reduces the variation of the level of the groundwater table upstream of the dam. It is built entirely under the ground.
The sand storage dam is constructed above ground. Sand and soil particles transported during periods of high flow are allowed to deposit behind the dam, and water is stored in these soil deposits (see figure 2). The sand storage dam is constructed in layers to allow sand to be deposited and finer material be washed downstream.
A sand storage dam
A groundwater dam can also be a combination of these two types. When constructing a sub-surface dam in a river bed, one can increase the storage volume by letting the dam wall rise over the surface, thus causing additional accumulation of sediments. Similarly, when a sand-storage dam is constructed it is necessary to excavate a trench in the sand bed in order to reach bedrock, which can be used to create a sub-surface dam too.
Groundwater dams are built across streams or valleys. A trench is dug across the valley or stream, reaching to the bedrock or other stable layer like clay. An impervious wall is constructed in the trench, which is then refilled with the excavated material.
Various materials may be used for the construction of groundwater dams. Materials should be waterproof, and the dam should be strong enough to withstand the imposed soil and water loads. Dams may vary from 2 to 10 metres high. Materials include compacted clay, concrete, stones and clay, masonry wall or plastic sheets.
Stages of construction of a sand-storage dam
The reservoir is recharged during the monsoon period and the stored water can be used during the dry season. Excess water flows over the top of the dam to replenish aquifers downstream. Water may be obtained from the underground reservoir either from a well upstream of the dam or from a pipe, passing through the dam, and leading to a collection point downstream (see figures 1 and 2). Groundwater dams cannot be a universally applicable as these require specific conditions for functioning. The best sites for construction of groundwater dams are where the soil consists of sands and gravel, with rock or a permeable layer at a depth of a few metres. Ideally the dam should be built where rainwater from a large catchment area flows through a narrow passage.
The Central Ground Water Board has sited and constructed a number of sub-surface dams in Kerala in the 1980s. Presently, Shri Vivekananda Research and Training Institute (SVRTI), under the guidance of K C B Raju is involved in constructing groundwater dams in Kutch district of Gujarat.