| 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.
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| 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 (see figure 1).
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 (see figure 3).
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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.
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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.
Source: Dr. K.C.B. Raju, Nanda Gautam, 492 10th Cross Sadashiv Nagar,
Bangalore - 560 080 |
