KRG INDIA RESEARCH CENTRE

Background

What is RWH

Methods

Technology Urban

Technology Rural

Advantages

URBAN AREA

The run off water generated in monsoon within an area can be well utilized for ground water recharging by diverting it into suitably designed recharge structures in public parks, play grounds, stadiums, airports, station, temple tanks etc. storm water drains should be designed in such a way that two separate segments are made so as to accommodate water coming from houses and water coming from roads. The segment on the side of the road should be covered with perforated slabs and should have percolation pits of depth 20 to 50 ft. depending on the soil condition at regular intervals. Huge quantity of sewage water generated from the domestic segment can be separated and reclaimed through soil Aquifer Treatment (SAT). This treated water can be used for recharging dry rivers for irrigation purpose.

Roof top Rain Water /Storm run off harvesting through

(i) Recharge Pit (ii) Recharge Trench (iii) Tube well (iv) Recharge Well

In urban areas, rainwater available from rooftops of buildings, paved and unpaved areas go waste. This water can be recharged to aquifer and can be utilized gainfully at the time of need. The rainwater harvesting system needs to be designed in a way that it does not occupy large space for collection and recharge system. A few techniques of roof top rainwater harvesting in urban areas are described below.

(i) ROOF TOP RAIN WATER HARVESTING THROUGH RECHARGE PIT

In alluvial areas where permeable rocks are exposed on the land surface or at very shallow depth, roof top rainwater harvesting can be done through recharge pits. The technique is suitable for buildings having a roof area of 100 sq.m. and are constructed for recharging the shallow aquifers. Recharge Pits may be of any shape and size and are generally constructed 1 to 2 m. wide and 2 to 3, deep which are back filled with boulders (5-20 cm),Gravels (5-10mm) and coarse sand (1.5- 2mm) in graded form. Boulders at the bottom, gravels 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 coarse sand layer and can easily be removed.

For smaller roof area, pit may be filled with broken bricks/ cobbles. 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 desilting /collection chamber may also be provided at the ground to arrest the flow of finer particles to the recharge pit. The top layer of sand should be cleaned periodically to maintain the recharge rate. By-pass arrangement be provided before the collection chamber to reject the first showers.

(ii) ROOF TOP RAIN WATER HARVESTING THROUGH RECHARGE TRENCH

Recharge trenches are suitable for buildings having roof area of 200-300 sq. m. and where permeable strata is available at shallow depths. Trench may be 0.5 to 1 m wide, 1 to 1.5m. deep and 10 to 20 m. long depending upon availability of water to be recharge. These are back filled with boulders (5-20cm), gravel (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 coarse sand at the top of the sand layer and can easily be removed. A mesh should be provided at the roof so that leaves or any other solid Waste/debris is prevented from entering the trenches and a desilting/collection chamber may also be provided on ground to arrest the flow of finer particles to the trench. By-pass arrangement be provided before the collection chamber to reject the first showers. The top layer of sand should be cleaned periodically to maintain the recharge rate.

(iii) ROOF TOP RAIN WATER HARVESTING THROUGH EXISTING TUBEWELLS

In areas where the shallow aquifers have dried up and existing tube wells are tapping deeper aquifer, roof to rain water harvesting through existing tube well can be adopted to recharge the deeper aquifers. PVC pipes of 10 cm dia are connected to roof drains to collect rainwater. The first roof runoff is let off through the bottom of drainpipe. After closing the bottom pipe, the rainwater of subsequent rain showers is taken through a T to an online PVC filter. The filter may be provided before water enters the tube wells. 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 (6-10mm), middle chamber with pebbles (12-20 mm) and last chamber with bigger pebbles (20-40 mm). If the roof area is more, a filter pit may be provided. Rainwater 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 slop of 1:15. The filter pit may vary in shape and size depending upon available runoff 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- 0.50m) and may be separated by screen. The pit is divided into two chambers; filter material in one chamber and other chamber is kept empty to accommodate excess filtered water and to monitor the quality of filtered 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

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.

This technique is ideally suited for area where permeable horizon is within 3m below ground level. Recharge well of 100-300 diameter is constructed to a depth of at least 3 to 5m below the water level. Based on the litho logy of the area well assembly is designed with slotted pipe against the shallow and deeper aquifer. A lateral trench of 1.5-to 3m width and 10 to 30 m length, depending upon the availability of water is constructed with the recharge well in the centre. The number of recharge wells in the trench can be decided on the basis of water availability and local vertical permeability of the rocks. The trench is backfilled with boulders, gravels and coarse sand to act as a filter media for the recharge wells. I f the aquifer is available at greater depth say more than 20 m, a shallow shaft of 2 to 5 m diameter and 3-5 meters deep may be constructed depending upon availability of runoff. Inside the shaft a recharge well of 100-300 mm diameter 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.