WATER HARVESTING TECHNIQUES :
There are two main techniques of rain water harvestings. Storage of rainwater on surface for future use. Recharge to ground water.
The storage of rain water on surface is a traditional techniques and structures used were underground tanks, ponds, check dams, weirs etc. Recharge to ground water is a new concept of
Rain water harvesting and the structures generally used are:-
Pits: - Recharge pits are constructed for recharging the shallow aquifer. These are constructed 1 to 2 m, wide and to 3 m. deep which are back filled with boulders, gravels, coarse sand.
Trenches: - These are constructed when the permeable stram is available at shallow depth. Trench may be 0.5 to 1 m. wide, 1 to 1.5m. deep and 10 to 20 m. long depending up availability of water. These are back filled with filter materials.
Dug wells:- Existing dug wells may be utilised as recharge structure and water should pass through filter media before putting into dug well.
Hand pumps: - The existing hand pumps may be used for recharging the shallow/deep aquifers, if the availability of water is limited. Water should pass through filter media before diverting it into hand pumps.
Recharge wells: - Recharge wells of 100 to 300 mm. diameter are generally constructed for recharging the deeper aquifers and water is passed through filter media to avoid choking of recharge wells.
Recharge Shafts: - For recharging the shallow aquifer which are located below clayey surface, recharge shafts of 0.5 to 3 m. diameter and 10 to 15 m. deep are constructed and back filled with boulders, gravels & coarse sand.
Lateral shafts with bore wells: - For recharging the upper as well as deeper aquifers lateral shafts of 1.5 to 2 m. wide & 10 to 30 m. long depending upon availability of water with one or two bore wells are constructed. The lateral shafts is back filled with boulders, gravels & coarse sand.
Spreading techniques: - When permeable strata start from top then this technique is used. Spread the water in streams/Nasal by making check dams, nala bunds, cement plugs, gabion structures or a percolation pond may be constructed.
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Ground water recharge
Underground water is one of the important sources of water in urban areas. With increasing urbanization, underground water has been indiscriminately exploited causing depletion in water table and water availability. It is very evident from the number of failing bore wells / open wells that it is unsustainable to pump out water from wells without recharging the same from the rainwater. To reverse the trend or to reduce the effect of over exploitation, ground water recharge needs to be taken up in large scale at residential and institutional buildings.
There are many methods of ground water recharge. Following are few methods for recharging ground water using rainwater from rooftops of buildings.
a. Reused plastic barrels method
b. Direct recharge through Open wells and Bore wells
c. Infiltration Gallery for large buildings
d. How to build an Infiltration Gallery?
e. Roads / Trenches
f. Parks and Open spaces