4.Water Harvesting



Chapter – 13

Rainwater Harvesting

Selection of Appropriate Techniques

(Sudhir Kamal Seem, M. Arch. (Landscape), Senior Architect, CPWD)

What is Rainwater Harvesting?

The principle of collecting and using precipitation (rainwater) from a catchment surface is called rainwater harvesting. It takes few days to several centuries to replenish and recharge the rain water in the aquifers in ground.

There are two main techniques of rainwater harvestings:

Storage of rainwater on surface for future use.

Artificial Recharge to ground water.

Artificial Recharge to Ground Water Artificial recharge to ground water is a process by which the ground water reservoir is augmented at a rate exceeding that obtaining under natural conditions or replenishment. Any man-made scheme or facility that adds water to an aquifer may be considered to be an artificial recharge system.

Components of a Rainwater Harvesting System

A rainwater harvesting system comprises of various stage components - transporting rainwater through pipes or drains, filtration, and storage in tanks for reuse or recharge. The common components of a rainwater harvesting are.

Pits

Recharge pits are constructed for recharging the shallow aquifer. These are constructed 1 to 2m, wide and to 3m. deep which are back filled with boulders, gravels, coarse sand.

Trenches

These are constructed when the permeable stream is available at shallow depth. Trench may be 0.5 to 1m. wide, 1 to 1.5m. deep and 10 to 20m. long depending up availability of water. These are back-filled with filter material.

Dug Wells

Existing dug wells may be utilized 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 3m. diameter and 10 to 15m. 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 (1.5 to 2m. wide & 10 to 30m. long depending upon availability of water with one or two bore wells) are constructed. The lateral shafts are 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/Nalas by making check dams, nala bunds, cement plugs, gabion structures or a percolation pond may be constructed.

“ Since June 2001, the Ministry of Urban Affairs & Poverty Alleviation has made rainwater harvesting mandatory

in all new buildings with a roof area of more than 100 sqm and in all plots with an area of more than 1000 sqm, that are being developed”

Selection of Appropriate Rain Water Recharge Structure for Different Areas in Delhi Groundwater is one of the major sources for water supply in many parts of the country. In Delhi too ground water contributes to substantial quantity of supply. The groundwater is largely being utilized as a drinking water and for agriculture in a large area of the city because of the insufficiency of water from the River Yamuna.

Groundwater collects in the aquifers over thousands of years through infiltration and groundwater flow recharge. A particular amount of groundwater is replenished regularly through rainwater infiltration. Sustainable use of groundwater means withdrawal of groundwater at a rate at which it is replenished through recharge. Faster withdrawal rate would lead to fall in water table and finally depletion of groundwater.

The groundwater recharge areas need to be identified so that maximum recharge can be achieved. The recharge areas needs to conserved and preserved for the sustainable management of groundwater and to maintain the potential of the groundwater in Delhi. According to the Central Ground Water Board the recharge area identified is the northernmost part of the city. Areas, where the ponds already exist in the villages, the Najafgarh Lake and its surroundings and Delhi region between the northern ridges can also be used.

The city of Delhi comprises of four distinct physiographic units that influence and control the groundwater occurrence and movement in the city.

i. The ridge,

ii. Alluvial Plains,

iii.River Yamuna flood plains,

iv. Chattarpur Alluvial Plains.

Further the area is divided as per the soil strata and other physiological conditions:

Kohi

Area immediately south of Delhi that includes Ridge, Mehrauli, Toughlaqabad etc. the area is rocky and undulating in nature and is a limited source of groundwater which is confined to fractured planes and weathered zones of the ridge.

Khadar

The low lying area of plains that is liable to inundation during floods. It has light sandy soil.

Bangar

The area north of ridge is characterized by patches of saline efflorescence which is the result of composition of alluvium and gentle slopes of the land.

Dabar

The low lying basin situated west of ridge and consists of low ground or the basin scooped out by their westward drainage.

The Surface Geology, Groundwater Level and Water Quality of various parts of Delhi are shown on the table-1.

Techniques Suitable for Groundwater Recharge for Different Locations A correct understanding of hydrology, geomorphology and geography of the area are important in successful implementation of any artificial recharge scheme. Recharge structures suitable for different area of the city are suggested on the basis of the following study.

References:

i. CGWC, Govt. of India.

ii. National Building Code

iii. Making Water Everybody’s Business iv. Jeyakumar; Rain water Harvest Manual v. cleanwater@aquasure.nl

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