The New Indian Express
11th August, 2003

A ready recknoner on rainwater harvesting

CHENNAI: Chennai's geology is so varied that different types of rainwater harvesting (RWH) structures are suited to different areas. Identification of the soil type in your locality would help in installing RWH structures that are both cost-effective and can give you optimum benefits.

Studies by water management agencies show that rain water percolates fastest in the sandy strata of the soil profile. Hence, it is important to ensure that all rain water harvesting structures _ such as percolation pits and recharge wells _ are installed to the depth of the sandy layer. Buildings that have open wells, bore wells or underground wells in the premises can direct rain water through a filter chamber into them. Buildings that lack these water sources can recharge ground water through percolation pits and recharge wells.

A resident of Velachery expressed concern that RWH was impossible in her locality because of the hard rock soil strata. Water experts, however, say that RWH is possible in any region. For Velachery residents, baby wells or recharge wells that allow water to percolate beyond the rocky strata could be the solution.

SANDY AREAS: The coastal stretch from Tondiarpet to Thiruvanmiyur and the stretch adjacent to the river banks of Cooum and Adyar are of sandy strata. These require shallow rain water harvesting structures, says Metrowater data. Percolation pits that are about five to ten feet deep can be installed in buildings with small roof top areas and recharge wells of five feet can be installed in buildings with large roof top areas.

Besant Nagar, Thiruvalluvar Nagar and Valmiki Nagar are located along the coastline and are ideal for RWH. The nature of the soil is sandy and hence permeable. ``The soil will be able to absorb any amount of rainwater. So. construction of percolation pits and recharge wells is simple and need not be very deep,'' says a pamphlet issued by Metrowater.

CLAYEY AREAS: A large portion of Greater Chennai has soil of clayey nature. These include areas that lie between the Cooum and the Adyar rivers such as Mambalam, Nungambakkam, Kodambakkam, part of Saidapet. Clayey soil is also found in areas north of the Cooum, such as Anna Nagar, Ayanavaram, Perambur, Kolathur and Villivakkam. Here, percolation pits with bore holes reaching from 10 feet to 30 feet are required. Recharge wells can be 10 feet deep.

HARD ROCK AREAS: Residents of Guindy, Velachery, Adambakkam and part of Saidapet have to dig the deepest because their houses lie in the hard rock area. Hence, recharge wells of about 10 - 15 feet could be used for RWH.

A Metrowater study published in `Managing Water in Chennai' brought out by the National Water Harvesters' Network stresses the importance of RWH. In almost all areas of the city, ground water levels have receded. Ground water levels fell from 20 feet below ground level to 40 feet in the span of two years (April 1999 to April 2001) in Ayanavaram and Anna Nagar. In Washermanpet and Foreshore Estate the fall in ground water levels during the same period was an average of 10 feet; in Thiruvanmiyur and Besant Nagar a fall of 18 feet; in Taramani a fall of an alarming 24 feet and in Kodambakkam and Saidapet a fall of 22 feet.