Despite heavy rains lashing in the past month, the groundwater table level hasn’t gone up
Chennai: Despite widespread waterlogging witnessed in the city following heavy rains, the groundwater table has not gone up. Experts say that simply channelling run-off water on road surfaces into stormwater drains is one of the reasons why there has not been a considerable increase in the water table and suggest construction of rainwater harvesting (RWH) structure on stormwater drains as it will help recharge groundwater.
RWH in buildings collects only a part of the rainwater, whereas more than double this quantity runs off on the road surface in front of buildings without being harnessed, they said.Rain centre director, Dr Sekhar Raghavan, said in the city, the urban run-off is collected in what are called stormwater drains constructed by the corporation, either on one or both sides of the roads. “These drains finally end up in one of the three waterways running within the city, which in turn are connected to the ocean. Thus, a large quantity of rainwater is dumped into the sea and wasted almost every year. It is said a large proportion of water required to meet the yearly demand of Chennai is dumped into the sea every year through these drains,” he said, adding that it should be clearly understood that stormwater drains are only a cure to flooding problems and that prevention is better than cure.
Dr Raghavan said the corporation should leave unpaved area on both sides of the road to allow groundwater recharging. In areas where soil is very sandy, particularly coastal suburbs, if enough area is left unpaved on both sides, any amount of rain can percolate into the soil on its own, he said.In the existing stormwater drains, he said a number of recharge wells of minimum 3 feet diameter or more should be constructed within the drain or adjacent to it, depending on the availability of space.
“Drains close to temple tanks can be straightaway discharged into the tank through a desilting/drop chamber of suitable dimensions, as has been done in Mylapore, Purasawalkam, Tiruvanmiyur and Triplicane,” he noted.Echoing his views, Mr D. Gajarajan, former chief engineer of the public works department, said a series of recharge wells of 2-3 ft diameter should be constructed across drains at 100 to 150 ft intervals.
“These wells will collect almost the entire water that flows on the road and help recharge subsoil water if proper weed holes are provided at appropriate depths in the wells,” he said. He added that facilities should be provided to remove garbage and silt that accumulate in the wells.
A senior corporation official said a number of rainwater harvesting structures installed on roads in Besant Nagar and Shastri Nagar are working well. “We are planning to set up 50,000 RWH structures near stormwater drains in city roads at an estimated cost of '25 crore by the next monsoon,” the official said.
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