Vol. 2
No.
2
April 2000
|
Groundwater
dams
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 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).
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.
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
Glimpses of books
Watershed management, guidelines for Indian Conditions,
EM Tideman, Omega Scientific Publications, New Delhi, 372pp, fifth reprint 1999, Rs
600
The importance of watershed development cannot be overestimated. On one hand is the
need to increase food productivity and hence productivity from the soil, and on the other
is increasing soil erosion and depleting water availability.
Water is almost a dual edged sword. In the form of rain, if allowed to fall and flow
unabated and unchecked it can enhance erosion. If, instead, it can be captured, allowed
percolation time, it can replenish depleting reservoirs and halt soil erosion to a certain
extent. It thus only makes sense to adopt soil and water conservation methods together
through watershed management and development.
This is precisely what the book sets out to do. It attempts to inform about the ‘a
to zee’ of watershed development in the Indian context. It thus describes various
parameters involved in effective watershed development. For instance there are detailed
chapters on soils- the various types, soil ecology, rainfall- various parameters
associated with rainfall like pattern, intensity and duration of rainfall and estimation
of runoff generated from these rains. It also warns that watersheds cannot be developed
successfully on every type of soil. Soil survey and analysis thus becomes a key deciding
factor.
The book then addresses the next step of watershed development: the actual designing
and identifying ideal water harvesting structures suited to a particular topography. How
can a little bit of engineering and a lot of common sense can control erosion of
agricultural lands is also discussed.
There is another factor that plays a vital role in watershed development and that is
agronomy, to what the author fondly refers to as ‘second line of defense,’ the
first being engineering ways to prevent soil erosion. Agronomic measures include the
selection of crop, the cropping pattern, the preparation of the land prior to sowing, the
pattern of sowing, the adoption of organic manure and many others.
The book concludes with a chapter of watershed management that discusses the impact of
a watershed on the people, the need for people’s participation and how this can be
achieved and most considerately, provides a format for watershed planning.
The book is a must have. Even for those even remotely connected with water Management.
The book is an outcome of an Indo-German bilateral project on watershed management. That
it is the fifth reprint speaks about its usefulness. |