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RURAL AREA |
Depending on the terrain, geology and gradient different method are adopted in the farms and irrigation lands. The maneuver involves storing, spreading, percolating and blocking. Due to severe depletion of groundwater level many open wells, bore wells and hand pumps are being dried. Instead of discarding them, they can be converted into useful recharge wells.
In Gradient lands where there are slopes, construction of bunds will slow down the run of water, enabling stagnation and slow percolation. Where there are deep slopes, trenches and a series of ponds are made. In this method, water is stored in ponds and the overflowing water spreads and percolates saturating the entire area.
In plain flat terrain, storing ponds are built along with canals. Canals have trenches. Recharging wells are built with proper desilting arrangement in between. In a rocky region, thorough study must be made to locate the potential aquifers. Infiltration is possible to enhance the aquifers by recharge wells. In a totally dry land, sieve plantation is adopted to convert it into a fertile land. Pitting, bunding and planting are done together in this system. On riverbeds, construction of Bandharas helps to enhance recharging. Bandharas are nothing but concrete walls or impermeable soil, built across the river but below the riverbed at regular intervals. This will act as underground reservoirs and recharge the surrounding area.
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Specifically Rain Water harvesting in rural area can be done through |
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(I) |
Gully Plug |
(ii) |
Contour Bund |
(iii) |
Gabion Structure |
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(iv) |
Percolation tank |
(v) |
Check Dam/ Cement Plug/ Nala Bund |
(vi) |
Recharge shaft |
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(vii) |
Dug well Recharge |
(viii) |
Ground Water Dams/Subsurface Dyke |
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In rural areas, rainwater harvesting is taken up considering watershed as a unit. Surface spreading techniques are common since space for such systems is available in plenty and quantity of recharged water is also large. Following techniques may be adopted to save water-going waste through slopes, rivers, rivulets and nalas. |
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(I) RAIN WATER HARVESTING THROUGH GULLY PLUG
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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 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. |
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(ii) RAIN WATER HARVESTING THROUGH CONTOUR BUND
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Contour Bunds are effective methods to conserve soil moisture in watershed for long duration. These are suitable in low rain fall areas where monsoon run off 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 as 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.
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(iii) RAIN WATER HARVESTING THROUGH GABION STRUCTURE
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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 over flows this structure storing some water to serve as source of recharge. The silt content of stream water in due course is deposited in the interstices of the boulders in due course and with 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.
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(iv) RAIN WATER HARVESTING THROUGH PERCOLATION TANK
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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 steams, located on highly fractured and weathered rocks, which have lateral continuity down stream. The recharge area down stream should have sufficient number of wells and cultivable land to benefit from the augmented ground water. The size of percolation tank should be governed by 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 up to 4.5 m height, cut off trenches are not necessary and keying and benching between the dam seat and the natural ground is sufficient.
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(v) RAIN WATER HARVESTING THROUGH CHECK DAMS / CEMENT PLUGS / NALA BUNDS
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Check dams are constructed across small streams having gentle slope. The site selected should have sufficient thickness of permeable bed or weathered formation to facilitate recharge of stored water within 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 run off, water cushions are provided at 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.
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(vi) RAIN WATER HARVESTING THROUGH RECHARGE SHAFT
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This is the most efficient and cost effective technique to recharge unconfined aquifer overlain by poorly permeable strata. Recharge shaft may be dug manually if the strata is of non-caving nature. The diameter of shaft is normally more than 2 m. The shaft should end in more permeable strata below the top impermeable strata. It may not touch water table. The unlined shaft should be backfilled, initially with boulders/ cobbles followed by gravel and coarse sand. In case of 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 rainy season village tanks are fully filled up but water from these tanks does not percolate down due to siltation and tube well and dug wells located nearby remains dried up. The water from village tanks get evaporated and is not available for the beneficial use. By constructing recharge shaft in tanks, surplus water can be recharged to ground water. Recharge shafts of 0.5 to 3 m. diameter and 10 to 15 m. deep are constructed depending upon availability of quantum of water. The top of shaft is kept above the tank bed level preferably at half of full supply level. These are back filled with boulders, gravels and coarse sand. In upper portion of 1 or 2 m depth, the brick masonry work is carried out for the stability of the structure. Through this technique all the accumulated water in village tank above 50% full supply level would be recharged to ground water. Sufficient water will continue to remain in tank for domestic use after recharge. |
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(vii) RAIN WATER HARVESTING THROUGH DUGWELL RECHARGE
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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 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. |
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(viii) GROUND WATER DAMS OR SUB-SURFACE DYKES
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Sub surface dyke or under-ground dam is a subsurface barrier across stream which retards the base flow and stores water upstream below ground surface. By doing so, the water level in upstream part of ground water dam raises saturating otherwise dry part of aquifer.
The site where sub-surface dyke is proposed should have shallow impervious layer with wide valley and narrow out let. 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 up to 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 take place. The potential disaster like collapse of the dams can also be avoided. |
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