New Indian Express
11th August, 2003
ready recknoner on rainwater harvesting
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
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.