Different Methods To Recharge Ground Water - Rain Water Harvesting in Spacious Areas.
Fresh water on earth is a scarce resource. Fresh water constitutes only 3% of total water on earth. Rest is saline water in the oceans. 11% of the total freshwater available on earth is ground water. It is available upto a depth of 800m which can be bored for everyday usage. Exploitation and mindless extraction of ground water resources has caused a rapid depletion and deterioration of ground water.
In rural areas surface spreading techniques are common since space for such methods is available in plenty. Quantity of recharged water is also large. Some of the methods listed below may be adopted for ground water recharging and can control rain water wastage flowing through slopes, rivers, rivulets and nalas.
Gully plugs are built using local stones, clay and bushes across small gullies and streams running down the hill slopes carrying drainage to tiny catchments during the rainy season. Gully Plugs help in conservation of soil and moisture. The sites for gully plugs may be chosen whenever there is a local break in slope to permit accumulation of adequate water behind the bunds.
Contour bunds are effective methods to conserve soil moisture in watersheds for long duration. These are suitable in low rainfall areas where monsoon runoff can be impounded by constructing bunds on the sloping ground all along the contour of equal elevation.
Flowing water is intercepted before it attains the erosive velocity by keeping suitable spacing between bunds. Spacing between two contour bunds depends on the slope, the area and the permeability of the soil. Lesser the permeability of soil, the close should be spacing of bunds. Contour bunding is suitable on lands with moderate slopes without involving terracing.
This is a kind of check dam commonly constructed across small streams to conserve stream flows with practically no submergence beyond stream course.A small bund across the stream is made by putting locally available boulders in a mesh of steel wires and anchored to the stream banks.
The height of such structures is around 0.5 m and is normally used in the streams with width of less than 10 m. The excess water overflows this structure storing some water to serve as a source of recharge. The silt content of stream water in due course is deposited in the interstices of the boulders. With the growth of vegetation, the bund becomes quite impermeable and helps in retaining surface water run off for sufficient time after rains to recharge the ground water body.
Percolation tank is an artificially created surface water body, submerging in its reservoir a highly permeable land, so that surface runoff is made to percolate and recharge the ground water storage. Percolation tank should be constructed preferably on second to third order streams, located on highly fractured and weathered rocks, which have lateral continuity downstream. The recharge area downstream should have sufficient number of wells and cultivable land to benefit from the augmented ground water.
The size of the percolation tank should be governed by the percolation capacity of strata in the tank bed. Normally percolation tanks are designed for storage capacity of 0.1 to 0.5 MCM. It is necessary to design the tank to provide a ponded water column generally between 3 & 4.5 m.
The percolation tanks are mostly earthen dams with masonry structure only for spillway. The purpose of the percolation tanks is to recharge the ground water storage and hence seepage below the seat of the bed is permissible. For dams upto 4.5 m height, cut off trenches are not necessary and keying and benching between the dam seat and the natural ground is sufficient.
Check Dams - Cement Plugs - Nala Bunds
Check dams are constructed across small streams having gentle slopes. The site selected should have sufficient thickness of permeable bed or weathered formation to facilitate recharge of stored water within a short span of time.
The water stored in these structures is mostly confined to stream course and the height is normally less than 2 m and excess water is allowed to flow over the wall. In order to avoid scouring from excess runoff, water cushions are provided at the downstream side.
To harness the maximum run off in the stream, series of such check dams can be constructed to have recharge on regional scale. Clay filled cement bags arranged as a wall are also being successfully used as a barrier across small nalas. At places, shallow trench is excavated across the nala and asbestos sheets are put on two sides. The space between the rows of asbestos sheets across the nala is backfilled with clay. Thus a low cost check dam is created. On the upstream side clay filled cement bags can be stacked in a slope to provide stability to the structure.
Recharge shafts may be dug manually if the strata are of non-caving nature. The diameter of the shaft is normally more than 2 meter. The shaft should end in more permeable strata below the top impermeable strata. It may not touch the water table.
The unlined shaft should be back filled, initially with boulders/ cobbles followed by gravel and coarse sand.In the case of a lined shaft the recharge water may be fed through a smaller conductor pipe reaching up to the filter pack. These recharge structures are very useful for village ponds where shallow clay layer impedes the infiltration of water to the aquifer.
It is seen that in the rainy season village tanks are fully filled up but water from these tanks does not percolate down the ground due to siltation and tube well and dug wells located nearby remains dried up. The water from village tanks evaporates and is not available for beneficial use.
By constructing recharge shafts in tanks, surplus water can be recharged to ground water. Recharge shafts of 0.5 to 3 m in diameter and 10 to 15 m deep are constructed depending upon availability of water in the rainy season. The top of the shaft is kept above the tank bed level preferably at half of full supply level. These are back filled with boulders, gravel and coarse sand.
In an upper portion of 1 to 2 meter depth, the brick masonry work is carried out for the stability of the structure. Through this technique all the accumulated water in the tank above 50% full supply level would be recharged to ground water. Sufficient water will continue to remain in the tank for domestic use after recharge.
Existing and abandoned dug wells may be utilized as recharge structure after cleaning and desilting the same. The recharge water is guided through a pipe from the desilting chamber to the bottom of well or below the water level to avoid scouring of bottom and entrapment of air bubbles in the aquifer.
Recharge water should be silt free and for removing the silt contents, the runoff water should pass either through a desilting chamber or filter chamber. Periodic chlorination should be done for controlling the bacteriological contaminations.
Ground Water Dams or Sub Surface Dykes
Subsurface dyke or underground dam is a subsurface barrier across streams which retards the base flow and stores water upstream below ground surface. By doing so, the water levels in the upstream part of the ground water dam rises, saturating the otherwise dry part of the aquifer.
The site where sub-surface dyke is proposed should have a shallow impervious layer with wide valley and narrow outlet. After selection of suitable site, a trench of 1-2 m wide is dug across the breadth of stream down to impermeable bed. The trench may be filled with clay or brick/ concrete wall upto 0.5m below the ground level.
For ensuring total imperviousness, PVC sheets of 3000 PSI tearing strength at 400 to 600 gauge or low-density polythene film of 200 gauges can also be used to cover the cut out dyke faces. Since the water is stored within the aquifer, submergence of land can be avoided and land above the reservoir can be utilized even after the construction of the dam. No evaporation loss from the reservoir and no siltation in the reservoir takes place. The potential disaster like collapse of the dams can also be avoided.
Source : vikaspedia.in