Chennai all set to tackle inundation during this year’s monsoon – The Hindu

While the quantum of rainfall that the city will get from cyclonic storm Nivar is anyone’s guess, water managers point out that compared to the 2015 scenario, Chennai is now better placed in terms of infrastructure to tackle particularly heavy spells during the Northeast monsoon. Additional infrastructure has been created over the past five years to handle the probability of 50-year floods.

With Cyclone Nivar expected to bring heavy to very heavy rain over some places in the city and neighbouring districts, the Water Resources Department is closely monitoring the level in city reservoirs, particularly the one in Chembarambakkam that has touched 21.18 ft against its full level of 24 ft.

Each of the three other reservoirs need 1,000 million cubic feet of water to reach capacity.

Officials of the WRD noted that unlike in 2015, when the Adyar received the surplus of nearly 186 waterbodies upstream, only 30% of the tanks have been filled now.

Feeder waterbodies

Moreover, the group of 40 tanks contributing to two major waterbodies in Pillaipakkam and Sriperumbudur that drain the surplus into the Chembarambakkam reservoir also need to fill up.

“We plan to maintain the water level at 22 ft and expect to release up to 5,000 cusecs (cubic feet per second) from Chembarambakkam reservoir if it touches the level and depending on the inflow during the rain expected this week. A capacity of 2 ft will be left unfilled for the next spell. Another 5,000 cusecs of floodwater may flow into the Adyar if the tanks upstream in Kancheepuram fill up,” an official said.

Over the past few years, the carrying capacity of the Adyar was doubled from about 8,000 cusecs in the upper stretch to nearly 1 lakh cusecs in the tail end near Maraimalai Adigal bridge. Besides desilting, flood protection walls were built in vulnerable areas, including Jaffarkhanpet, which was a disaster zone in the 2015 floods.

“The current infrastructure can handle heavy rain up to 30 cm in a day. Even in Tiruvallur district, only 25% of the tanks have filled up so far. There are no chances of a repeat of 2015 floods this year,” the official added.

The completion of the Thervoy Kandigai Kannankottai reservoir will help store additional Krishna water this year.

The city continues to get about 650 cusecs of Krishna water. If the waterbody in Poondi receives rainwater this week, Krishna water will be diverted for 10 days and stored in Thervoy Kandigai.

Officials of the Tamil Nadu Disaster Risk Reduction Agency noted that the obstructions in Adyar river that led to the spillover in the neighbouring areas during 2015 have been cleared. Waterbodies have been deepened under the Kudimaramathu scheme.

Many tanks in the southern suburbs now have regulators to streamline surplus water, and their bund levels have been raised to 1.5 m above full tank level to avoid breaches.

Long-term measures

“We have executed long-term flood mitigation works in Chennai and its neighbouring districts to decrease the severity of inundation by 70%. Cut and cover drains emptying into Pallikaranai swamp have been provided to connect the tanks. These measures were taken without much land acquisition,” said an official.

More checkdams across the Adyar and reservoirs formed in the river’s tributaries would also help divert floodwater. “We are confident that the time frame of inundation in localities would be largely reduced compared to previous years,” the official added.