rain water harvesting
Rain water harvesting is one of the most effective methods of water management and water conservation. It is the term used to indicate the collection and storage of rain water used for human, animals and plant needs. It involves collection and storage of rain water at surface or in sub-surface aquifer, before it is lost as surface run off. The augmented resource can be harvested in the time of need.
Artificial recharge to ground water is a process by which the ground water reservoir is augmented at a rate exceeding that under natural conditions of replenishment. The collected water is stored and pumped in a separate pipe distribution. This is a very useful method for a developing country like India in reducing the cost and the demand of treated water and also economising the treatment plants operation, maintenance and distribution costs.
Need:
i. To overcome the inadequacy of surface water to meet our demands.
ii. To arrest decline in ground water levels.
iii. To enhance availability of ground water at specific place and time and utilize rain water for sustainable development.
iv. To increase infiltration of rain water in the subsoil this has decreased drastically in urban areas due to paving of open area.
v. To improve ground water quality by dilution.
vi. To increase agriculture production.
vii. To improve ecology of the area by increase in vegetation cover etc.
Advantages:
i. The cost of recharge to sub-surface reservoir is lower than surface reservoirs.
ii. The aquifer serves as a distribution system also.
iii. No land is wasted for storage purpose and no population displacement is involved.
iv. Ground water is not directly exposed to evaporation and pollution.
v. Storing water under ground is environment friendly.
vi. It increases the productivity of aquifer.
vii. It reduces flood hazards.
viii. Effects rise in ground water levels.
ix. Mitigates effects of drought.
x. Reduces soil erosion.
The New Delhi-based Centre for Science and Environment estimates that merely capturing the rain water and run off on 2 per cent of India’s land area could supply 26 gallons of water per person.
As such much effort is to being made to popularize the concept of rain water harvesting at the grass roots level.
Potential Areas:
i. Where ground water levels are declining on regular basis.
ii. Where substantial amount of aquifer has been de-saturated.
iii. Where availability of ground water is inadequate in lean months.
iv. Where due to rapid urbanization, infiltration of rain water into subsoil has decreased drastically and recharging of ground water has diminished.
Methods and Techniques:
The methods of ground water recharge mainly are:
Urban Areas:
Roof top rain water/storm runoff harvesting through
(i) Recharge Pit
(ii) Recharge Trench
(iii) Tubewell
(iv) Recharge Well
Rural Areas:
Rain water harvesting through
(i) Gully Plug
(ii) Contour Bund
(iii) Gabion Structure
(iv) Percolation Tank
(v) Check Dam/Cement Plug/Nala Bund
(vi) Recharge Shaft
(vii) Dugwell Recharge
(viii) Ground Water Dams/Subsurface Dyke
Urban Areas:
In urban areas, rain water available from roof tops of buildings, paved and unpaved areas goes waste. This water can be recharged to aquifer and can be utilized gainfully at the time of need. The rain water harvesting system needs to be designed in a way that it does not occupy large space for collection and recharge system.
Roof top rain water harvesting can be a very effective tool to fight the problem of water shortage particularly in urban areas. Roof top rain water harvesting depends upon the amount of rainfall and the roof top area.
More the amount of rainfall more is the harvested water from roof top. Similarly, larger amount of roof top rain water is harvested from roofs with large area. Table 16.7 gives the amount of harvested water from roof top in cubic metres in relation to the amount of rainfall in millimetres and the roof top area in square metres.
A few techniques of roof top rain water harvesting in urban areas are described as under:
(i) Roof Top Rainwater Harvesting through Recharge Pit:
a. In alluvial areas where permeable rocks are exposed on the land surface or at very shallow depth, roof top rain water harvesting can be done through recharge pits.
b. The technique is suitable for buildings having a roof area of 100 sq m and is constructed for recharging the shallow aquifers.
c. Recharge Pits may be of any shape and size and are generally constructed 1 to 2 m wide and 2 to 3 m deep which are back filled with boulders (5-20 cm), gravels (5-10 mm) and coarse sand (1.5-2 mm) in graded form— Boulders at the bottom, gravels in between the coarse sand at the top so that the silt content that will come with runoff will be deposited on FIG. 16.7. Roof top rain water harvesting through recharge pit the top of the coarse sand layer and can easily be removed. For smaller roof area, pit may be filled with broken bricks/cobbles.
d. A mesh should be provided at the roof so that leaves or any other solid waste/debris is prevented from entering the pit and a desalting/collection chamber may also be provided at the ground to arrest the flow of finer particles to the recharge pit.
e. The top layer of sand should be cleaned periodically to maintain the recharge rate.
f. By-pass arrangement is provided before the collection chamber to reject the first showers.
(ii) Roof Top Rain Water Harvesting through Recharge Trench:
a. Recharge trenches are suitable for buildings having roof area of 200-300 sq m and where permeable strata are available at shallow depths.
b. Trench may be 0.5 to 1 m wide, 1 to 1.5 m deep and 10 to 20 m long depending upon availability of water to be recharged.
c. These are back filled with boulders (5-20 cm), gravels (5-10 mm) and coarse sand (1.5-2 mm) in graded form—boulders at the bottom, gravel in between and coarse sand at the top so that the silt content that will come with runoff will be deposited on the top of the sand layer and can easily be removed.
d. A mesh should be provided at the roof so that leaves or any other solid waste/debris is prevented from entering the trench and a desilting/collection chamber may also be provided on ground to arrest the flow of finer particles to the trench.
e. By-pass arrangement is provided before the collection chamber to reject the first showers.
f. The top layer of sand should be cleaned periodically to maintain the recharge rate.
(iii) Roof Top Rain Water Harvesting through Existing Tubewells
a. In areas where the shallow aquifers have dried up and existing tubewells are tapping deeper aquifer, roof top rain water harvesting through existing tubewell can be adopted to recharge the deeper aquifers.
b. PVC pipes of 10 cm diameter are connected to roof drains to collect rain water. The first roof runoff is let off through the bottom of drain pipe. After closing the bottom pipe, the rain water of subsequent rain showers is taken through a T to an online PVC filter.
The filter may be provided before water enters the tube-well. The filter is 1-1.2 m in length and is made up of PVC pipe. Its diameter should vary depending on the area of roof, 15 cm if roof area is less than 150 sq m and 20 cm if the roof area is more.
The filter is provided with a reducer of 6.25 cm on both the sides. Filter is divided into three chambers by PVC screens so that filter material is not mixed up. The first chamber is filled up with gravel (6-10 mm), middle chamber with pebbles (12-20 mm) and last chamber with bigger pebbles (20-40 mm).
c. If the roof area is more, a filter pit may be provided. Rain water from roofs is taken to collection/desilting chambers located on ground. These collection chambers are interconnected as well as connected to the filter pit through pipes having a slope of 1: 15.
The filter pit may vary in shape and size depending upon available run off and are back-filled with graded material, boulder at the bottom, gravel in the middle and sand at the top with varying thickness (0.30-0.50 m) and may be separated by screen.
The pit is divided into two chambers, filter material is one chamber and other chamber is kept empty on accommodate excess filtered water and to monitor the quality of filtrated water. A connecting pipe with recharge well is provided at the bottom of the pit for recharging of filtered water through well.
(iv) Roof Top Rain Water Harvesting through Trench with Recharge Well:
a. In areas where the surface soil is impervious and large quantities of roof water or surface runoff is available within a very short period of heavy rainfall, the use of trench/pits is made to store the water in a filter media and subsequently recharge to ground water through specially constructed recharge wells.
b. This technique is ideally suited for area where permeable horizon is within 3 m below ground level.
c. Recharge well of 100-300 diameter is constructed to a depth of at least 3 to 5 m below the water level. Based on the lithology of the area well assembly is designed with slotted pipe against the shallow and deeper aquifer.
d. A later trench of 1.5 to 3 m width and 10 to 30 m length, depending upon the availability of water is constructed with the recharge well in the centre.
e. The number of recharge wells in the trench can be decided on the basis of water availability and local vertical permeability of the rocks.
f. The trench is backfilled with boulders, gravels and coarse sand to act as a filter media for the recharge wells.
g. If the aquifer is available at greater depth say more than 20 m, a shallow shaft of 2 to 5 m diameter and 3-5 metres deep may be constructed depending upon availability of runoff. Inside the shaft a recharge well of 100-300 mm dia is constructed for recharging the available water to the deeper aquifers. At the bottom of the shaft a filter media is provided to avoid choking of recharge well.
Rural Areas:
In rural areas, rain water 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.
(i) Rain Water Harvesting through Gully Plug:
a. 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.
b. Gully plugs help in conservation of soil and moisture.
c. 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.
(ii) Rain Water Harvesting through Contour Bund:
a. Contour bunds are effective method to conserve soil moisture in watershed for long duration.
b. 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.
c. Flowing water is intercepted before it attains the erosive velocity by keeping suitable spacing between bunds.
d. Spacing between two contour bunds depends on the slope of the area and the permeability of the soil. Lesser the permeability of soil, the close should be spacing of bunds.
e. Contour bunding is suitable on lands with moderate slopes without involving terracing.
(iii) Rain Water Harvesting through Gabion Structure:
a. This is a kind of check dam commonly constructed across small streams to conserve stream flows with practically no submergence beyond stream course.
b. 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.
c. The height of such structures is around 0.5 m and is normally used in the streams with width of less than 10 m.
d. 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 runoff for sufficient time after rains to recharge the ground water body.
(iv) Rain Water Harvesting through Percolation Tank:
a. 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.
b. Percolation tank should be constructed preferably on second to third order streams, located on highly fractured and weathered rocks which have lateral continuity downstream.
c. The recharged area downstream should have sufficient number of wells and cultivable land to benefit from the augmented ground water.
d. 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 dam’s 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.
(v) Rain water harvesting through Check Dams/Cement Plugs/Nala Bunds:
a. 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.
b. 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.
c. To harness the maximum run off in the stream, series of such check dams can be constructed to have recharge on regional scale.
d. Clay filled cement bags arranged as a wall is 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.
(vi) Rain Water Harvesting through Recharge Shaft:
a. This is the most efficient and cost effective technique to recharge unconfined aquifer overlain by poorly permeable strata.
b. Recharge shaft may be dug manually if the strata are of non-caving nature. The diameter of shaft is normally more than 2 m.
c. The shaft should end in more permeable strata below the top impermeable strata. It may not touch water table.
d. The unlined shaft should be backfilled, initially with boulders/cobbles followed by gravel and coarse sand.
e. In case of lined shaft the recharge water may be fed through a smaller conductor pipe reaching up to the filter pack.
f. These recharge structures are very useful for village ponds where shallow clay layer impedes the infiltration of water to the aquifer.
g. 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 tubewells and dugwells located nearby remain dried up. The water from village tanks get evaporated and is not available for the beneficial use.
h. 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.
i. In upper portion of 1 or 2 m depth, the brick masonry work is carried out for the stability of the structure.
j. 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.
(vii) Rain Water Harvesting through Dugwell Recharge:
a. Existing and abandoned dug wells may be utilized as recharge structure after cleaning and desilting the same.
b. 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.
c. 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.
d. Periodic chlorination should be done for controlling the bacteriological contaminations.
(viii) Ground Water Dams or Sub-surface Dykes:
a. 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 rises saturating otherwise dry part of aquifer.
b. The site where sub-surface dyke is proposed should have shallow impervious layer with wide valley and narrow outlet.
c. After selection of suitable site, a trench of 1-2m wide is dug across the breadth of stream down to impermeable bed. The trench may be filled with clay of brick/concrete wall upto 0.5 m below the ground level.
d. For ensuring total imperviousness, PVC sheets of 3,000 PSI tearing strength at 400 to 600 gauge or low density polythene film of 200 gauge can also be used to cover the cut out dyke faces.
e. 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.
