In the blog “Chennai going down the drain” our interns Sneha and Sripad highlighted the standards and misuse of storm water drain. I would like to take this further by discussing the effectiveness of building such an infrastructure. The Corporation of Chennai had created the storm water drain (SWD) department to prevent water stagnation, which Chennai is prone to due to its flat terrain. Sometimes there are heavy rains during high tide which prevents water from draining into the sea, rendering SWD useless. Additionally, most roads in the city are badly maintained and have potholes, and the slope of the carriageway is not always appropriate for rain water to be drained.
Rain water has little impurity and, which if passed through primary filters, can be easily purified into drinking water. However, most of it is drained through the network of SWD and emptied into water bodies in the city. In many parts of the city sewage is emptied into SWD, which then carry this polluted water resulting in environmental degradation and loss of precious resource. While the government has made rain water harvesting mandatory for all the buildings, it has resisted the idea of creating sand filter pits on the roads for recharging the depleting ground water. Considering most of Chennai is on a flat terrain, the SWD network has a gradient that to allow them to drain into the sea or in existing water bodies, which prevents water from flowing to natural catchment areas.
One of the most interesting cases of draining rain water to improve the depleting ground water table is in Mylapore. All the rain water that falls in the PS Higher secondary ground gets sand filtered and is drained in the nearby Kapaleeshwar temple tank, which is generally kept clean and the access is restricted to festive occasions. However, this method of storing the surface run off in a static water body could lead to other problems like breeding of mosquitoes. Prof. Madhavi Ganesan from the Centre for Water Resources in Anna University cautioned about the use of such wet detention methods since they are precarious in nature. She stressed the need to leave temple tanks un-cemented since that would prevent percolation of water from high water table areas to low ones which is critical for maintaining a balance in the level of ground water in the vicinity.
Bio-retention and vegetated bioswale are other alternative storm water management practises which are cost-effective, maintenance-free and environment friendly. A citizen-led group SWARAN (Save Water and Recharge Aquifier Network) has been proposing alternatives to SWD department. They have successfully advocated for the Corporation to experiment with laying a harvesting pit in appropriate locations on a road in Besant Nagar. If this yields intended results the recharged water table will help in supplementing the water available to local communities. In the long run it could help the city be less dependent on the monsoons, which have been unreliable in recent years.
Written by Roshan Toshniwal, researcher, Transparent Chennai