Work to construct rainwater harvesting structures in stormwater drain network to begin next month

Work on construction of rainwater harvesting structures in stormwater drains by the Chennai Corporation is expected to begin in May when the water table is the lowest in the city. The civic body had announced that over 5,000 rainwater harvesting structures would come up across the city.

“The number of RWH structures can be increased if needed. The structures would come up at 30-metre intervals at every third stormwater drain inlet. A borewell pipe would be placed inside the inlets. Sand and blue metal would be filled in the pipe to filter the water,” said a Corporation official.

“As the pipe would reach the sandy area below ground level, we are taking the assistance of the Chennai Metrowater to know the water table and the kind of soil in each area,” the official explained. These structures would not be placed inside the stormwater drains as even now, in many places, they are being used as sewage carriers.

Metrowater now monitors the water level and quality from 145 observation wells spread across the expanded city. At present, the average water level in the city ranges between 5 and 6 metre. According to sources, the water level dips to its maximum during June as there is not much recharge. Groundwater is available at a shallow depth of 1.5 m to 2m in January, just after the northeast monsoon.

The water level now in sandy areas such as New Washermenpet, George Town, Manali, Porur and Besant Nagar is between 5 m and 6 m. The level in clay areas, including Kolathur, Pulianthope, Ambattur, Sholinganallur, K.K.Nagar and Virugambakkam, was at 5.5 m to 6 m last month compared to 2.2 metre during the same period the previous year. In hard rock areas such as Guindy, Perungudi, Taramani and Velachery, the level stood at 6.5 metre. Unlike sandy area, recharge and dip in water level is fast.

The water quality also varies across the city. The level of total dissolved solids presently ranges between 600 parts per million (ppm) and 1,500 ppm across the city. The permissible limit of TDS is 500 ppm.

Mayor Saidai S. Duraisamy said that apart from helping recharge groundwater, this would also help in areas where stagnation is heavy. “Rainwater runs into the sea and is wasted. If our aquifers are recharged methodically, we need not depend on any other source,” he said. Model RWH structures have already come up on Sydenhams Road.

Sekhar Raghavan, director, Rain Centre, said bore pits could get clogged due to silt and water would only run through the SWD. “We had installed harvesting structures with bore pipes inside SWD on G.N. Chetty Road in 1990s but after a year they did not work as the pipes got clogged. Instead, the civic body could look at gaps in the cement flooring of SWD. We tried it out in 1999 in Besant Nagar from Velankanni Church to Vannandurai and it still works,” he said.

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Deepa H. RamakrishnanJune 28, 2012

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