A recent exhibition was a mine of information to householders and organisations on how to conserve water and help meet one of the City's most acute problems. The exhibition, put up by the C.P.R.Environmental Education Centre, displayed models of rainwater harvesting systems, like 'trench bore pits', 'pebble beds', 'service wells' and 'soak pits' besides numerous other methods of water management by which Chennaiwasis could erase the term 'water scarcity' from their dictionary.

Trench Bore Pits

This method is applicable to properties with large areas. Here, the run off water from the rooftops is diverted into the bare soil or garden in the premises. At the periphery of the plot, a trench 1m deep and 0.5m wide is dug and 2-5 bores 2m deep are drilled. This is filled with broken bricks and the trench covered with a grill. The entire run off water is trapped in the trench and percolates underground through the bore pits. This was the earliest method of rainwater harvesting followed in Chennai, but soon fell out of favour because few knew how deep the trench had to be and also because unscrupulous builders would dig a shallow pit that served no purpose.

Pebble Beds

In houses and complexes with large open spaces, a wide pit is dug along the compound wall and filled with large pebbles (broken bricks, far cheaper and just as effective, are used nowadays). The pit is then covered with perforated RCC slabs. The run off water from the roof and surface are directed into it. The pebbles may have to be cleaned once a year to improve percolation. Experts do not favour this system because here again one cannot be sure to what depth the pit has to be dug. Also this kind of digging was found to damage compound walls.

Service well-cum-recharge well

This method of recharging a service well or an existing well and also the ground water table by directing rooftop and surface run off water into them has proved to be by far the most effective, economical and ideally suited for households. Here any open well within the complex itself is utilized to divert the rainwater from the terrace. The roof top water is clean unlike the surface run off water and hence can be safely let into the well. If an open well is not already there, then a service well is constructed and the water that accumulates in it can be used for immediate purposes.

The surface run off water can be tapped near the gate by providing a gutter with a perforated lid. The collected water can be piped into a recharge well. For this, the cost may range from Rs.5000/= for an independent house to about Rs.30,000 for a complex with, say, 40 apartments. Further, wherever water is found to be stagnating inside the compound, a percolation pit, nine inches in diameter and to the desired depth depending on the soil can be dug and filled with broken bricks. This may cost a maximum sum of Rs.1000/= even for an area like Anna Nagar, where the soil is clayey up to 25 metres (the pit has to be deeper to be functional).

Soak Pits

Ground water can be effectively recharged through soak pits that have sandy layers and Brick jelly. Wastewater from household chores can be diverted into this.

A job for the rulers

Through Watershed Management (WSM), both harvesting of rainwater and recharging of ground water can be achieved. Effective WSM increases the ability of land to hold more water and also conserves soil and water. Wastelands can be converted into productive areas and used for rural development, if only WSM is adopted.

Since the upper soil is clayey in most parts of Chennai, natural infiltration of rainwater is arrested. However beyond 4 metres, the soil tends to be sandy and receptive to rainwater harvesting.