Chennai - A Success Story: Rainwater Harvesting

Mr. Shiv Das Meena
Joint Commissioner (Education), Chennai Corporation, India

Introduction

Drinking water is one of the essentials for human existence. Surface and the ground water are the two major sources of drinking water. Drinking water supply in urban areas is mostly centralized and is primarily from surface sources such as natural or impounded reservoirs. As the population density and usage levels are relatively high in urban areas, water agencies construct, operate and maintain huge surface water dams and reservoirs for meeting their water demands. However individual households in urban areas also have their own bore wells/open wells to cater to needs other than potable water.

Chennai city depends on a few surface reservoirs outside the city limit which are fed by rainwater. However, on one hand due to poor monsoon rains these reservoirs rarely receives sufficient inflows resulting in water scarcity, and on the other hand the city is fast growing. Therefore, due to excessive pressure on ground water,  the ground water table is fast depleting, and in some cases saline water intrusion is also taking place.

What Rainwater Harvesting is

The concept of rainwater harvesting lies in tapping the rainwater where it falls. The rainwater collected can be stored for direct use or can be recharged into the ground water. A major portion of the rainwater that falls on the earth’s surface, runs-off in streams to rivers and finally to the sea. On an average only 8 to 10 % of the total rainfall recharges the ground water aquifer and most of the remaining rainfall is wasted in the form of surface runoff or evaporation. The technique of rainwater harvesting involves catching the rains from localized catchments such as the roof of a house or plain and slopping ground surfaces, etc. The rainwater that falls on these catchments is diverted into dugout ponds, vessels or underground tanks to store for a longer period. The annual rainfall in Chennai is in the range of 1200-1300mm. This is much higher than India’s average rainfall of 800mm. However, this rainfall occurs in short spells of a few days. On an average the city receives rainfall for only 300 hours throughout the year. The characteristics of this rainfall demands not only that the city conserve large quantities of rainwater during these few days but also that it stores water whenever it rains preferably for direct use and alternatively to recharge ground water.

The following illustration gives an idea of the quantum of water which can be harvested in Chennai city.

How rainwater is harvested

From the rooftop water is collected through drain pipes (the first runoff is allowed to drain away as it may contain impurities) and diverted to a storage tank through a filter unit. The surplus water from this tank is connected either to an existing well or recharge pits constructed for this purpose. In case of open space areas such as grounds, roads, open areas in houses, etc. recharge pits or trenches are constructed to harvest the rainwater for ground water recharge. Existing open wells and bore wells, which have become dysfunctional either due to lack of water or saline water intrusion, are also used for rainwater harvesting to recharge the ground water.

Rainwater can be stored either in containers above or below ground level or it can be charged into ground as indicated below:

How the rainwater harvesting has been promoted

Chennai city faced severe water scarcity during the year 2001 and that acted as a major stimulus for rainwater harvesting. A special campaign was launched as a people’s movement during July 2001 to popularize rainwater harvesting by the institutions as well as individual households. Simultaneously steps were also taken to provide rainwater harvesting in public buildings. Technical assistance cells were established in various organizations such as the City Corporation office, city water agency’s office etc. An campaign was launched through mass media, seminars, exhibitions, rallies, mobile publicity vans, etc. Rain canters demonstrating various methods of rainwater harvesting, were also established in different parts of the city.

 

Rainwater harvesting was made mandatory by promulgating an ordinance during July 2003. All the citizens were directed through this ordinance to provide Rainwater Harvesting structure in all the buildings before 31 August.2003.

Today Chennai city is a rainwater friendly city. All the buildings, commercial as well as residential, are provided with rainwater harvesting structures. This has helped in improving the ground water table as well as in improving the quality of ground water. Moreover, during rainy season the dependence on the municipal water agency has come down.

Editor's Note: This article was contributed by Mr. Shiv Das Meena specially to this newsletter to provide more detailed information on the Chennai's water harvesting practice after the workshop.