Chennai receives rainfall during North East Monsoon (October – December) and South West Monsoon (June – September). Most of it is received during October – December season. This year is an exception, that during June 1 to July 25, 2007 Chennai received a rainfall of 220 mm which is excess rainfall by 46 %. Chennai also receives rainfall during January – February, but that is rare.

The annual rainfall in Chennai is in the range of 1200 mm – 1300 mm as against the country’s average rainfall of only 800 mm. However the rainfall at Chennai occurs in short spells over a few days – spread over 300 hours throughout the year. The characteristic of rainfall is such that if it is not stored, it will cause flooding in low lying areas or will drain into the sea.

Considering the importance of collecting the rainwater, the previous State Government made it mandatory to have Rain Water Harvesting structures in all the buildings in the Chennai City. RWH structures were installed at residential and commercial buildings.

With a rainfall of 1300 mm per annum, a building with a terrace of 100 sq metre area can harvest about 78000 litres of water per annum at 60 % efficiency. This translates into a per day availability of about 213 litres per day. This much quantity of water is adequate to meet the drinking and cooking needs of a family of 8 to 10 members.

In a multistoried building with a terrace area of 500 sq metre, the rainwater harvesting potential is 3,90,000 litres. Assuming that the building has 16 flats, per day availability of water per flat will be about 67 litres.

Chennai Metropolitan area of 173 sq km under the above conditions can harvest about 370 million litres per day theoretically. Compare this with the 645 million litres of water per day presently supplied through pipelines and tankers.

With another monsoon around the corner, this is the best time to check and prepare the RWH structures for better returns from the monsoon. Though most households in the city have installed such structures, their efficacy remains a question mark.

A simple method suggested was to observe the percolation of water during rains or by pouring water into the structure. If there was an overflowing of water, the silt accumulated in the system needed to be cleared. According to a report most households in the city adopted roof-top harvesting system, which was the simplest and the best harnessing method. The pipes connecting the rooftop and the recharge structures are to be checked. (The other method of harvesting rainwater is allowing the rainwater to pass through filter bed and direct storage in tanks).

In our apartment complex after installing the RWH structures, the quality of the well water and the bore well water has improved and also the water level has risen

The RWH Cell provides free advice about the maintenance of the structures besides monitoring the water level and quality and can be contacted at 28454080.

Let us take up the maintenance of RWH structures and get prepared for harvesting the rain water and welcome the North East Monsoon.

(with inputs from the website of Chennai Metro Water and the online edition of The HIndu)