The biggest irony of the state is our rivers don’t flow into the sea, but our rainwater does. Although Chennai received 2095mm rainfall in 2015 — enough to meet its water demands for the next few years despite a 60% deficit rainfall in 2016 — the city has its hopes pinned on the next northeast monsoon to replenish its dry tanks and ground water table. Chennai that harvested enough rainwater following the record-breaking rainfall (2566mm) in 2005 couldn’t repeat the feat post 2015.
A major reason for this failure is a prolonged delay in desilting traditional water bodies (eris) that help collect runoff rainwater for agricultural use and recharge groundwater. While the four reservoirs near Chennai — at Poondi, Sholavaram, Red Hills and Chembarambakkam – are yet to be desilted, a few more traditional eris in Thiruvallur and Kanchipuram districts which have the potential of being converted into reservoirs have been encroached upon and filled up with waste. The water-holding capacity of these reservoirs has come down and they tend to overflow even after a small spell of rain.
At the micro level, proper rainwater harvesting (RWH) systems help a city store enough rainwater in its aquifers when it gets a good rainfall. When there was excess rainfall in 1996, the city did not have RWH structures. As a result, there was hardly any improvement in the groundwater table. But in 2005, when Chennai received a record rainfall of 2566mm (as against the average yearly rainfall of 1400mm), the TN government’s RWH mandate (in 2003) was already in place. A survey by Rain Centre in 2006 revealed that the groundwater table went up by 20ft. The city’s water needs were met comfortably at least for a good nine years. However, after the 2015 floods, there was only a 10-15ft rise in groundwater table in south Chennai. The rise was less as the aquifers were already half full due to the impact of the RWH structures over the years. An optimum harvest would have been possible if the city had enough well-maintained RWH systems.
The sluggish pace of RWH in the city results from a lack of awareness and interest among the public about it. There is also no authority to monitor the process.
Although RWH involves both rooftop harvesting and driveway runoff harvesting, an audit carried out by Rain Center in 2013-2015, found that around 98% of apartment buildings in the city did not implement the driveway run-off harvesting. Many had the notion that driveway run-off water is dirty and thus cannot be harvested, which is not true as soil filters out all the suspended impurity.
To resolve these issues, the current RWH model as mentioned in Annexure XIX in Development Regulations (of CMDA) need to be replaced by one that enables both rooftop and driveway runoff harvesting. In RWH systems, recharge pits — slotted PVC pipes inserted into a bore hole — should be replaced by recharge wells with a diameter ranging from 3ft to 6ft and a depth of 15ft. These wells are easy to maintain and have a larger volume.
A separate department for rainwater management should be created under whose ambit all these steps could be taken. Water tariff should also be introduced, which in turn will encourage residents to use their water judiciously and harvest rainwater. Rainwater harvesting and recharge systems of apartment projects awaiting approval of CMDA should be monitored by a third party like an NGO or academics. However, none of these measures will bear fruit unless citizens do their own bit in their neighborhoods.
Vardhman Envirotech
India’s Passionate rainwater company
This article is published on: The Times of India, 18 September, 2017.
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