With Cyclone Nivar advancing towards Tamil Nadu and expected to make landfall between Mamallapuram and Karaikal on the night of November 25, residents of Chennai and surrounds are worried that there could be a repeat of the nightmarish 2015 floods.
On Wednesday, authorities announced the release of 1000 cusecs of water from the brimming Chembarambakkam reservoir which is the largest drinking water supply for Chennai city.
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This announcement has stoked fear once again in the city because it was the release of enormous amounts of water from this same reservoir into the Adyar river, that led to large swathes of Chennai being flooded in 2015.
2015 Versus 2020
Officials say the likelihood of a repeat of 2015 is small. This is mainly due to the fact that the rains have not been as severe as in 2015.
With 286 mm of rainfall on average over seven days, Chennai was battered by rains in December 2015. The city also recorded the highest single-day rainfall in a century with 494 mm in Tambaram. In November 2015, the total rainfall recorded was 1018 mm, the highest since the year 1918.
In contrast, the average per-day rainfall brought in by Cyclone Nivar is estimated to be around 110 mm. Rains are heavy but they are also intermittent, unlike in 2015.
As on November 25, the Chembarambakkam reservoir is at 21.55 feet of storage. The total storage capacity is 24 feet. Public Works Department officials have decided to begin releasing water from the reservoir on the afternoon of November 25 at the rate of 1000 cusecs.
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The Adyar river’s carrying capacity is 60,000 cusecs. Currently, the river is carrying about 4000 cusecs of water and from noon, will carry an additional 1000 cusecs of water which will be released from Chembarambakkam.
Contrast this with 2015, when the level of water in the reservoir was at 22.08 feet and outflow was steadily increased to 29,000 cusecs by the evening of December 01.
The Adyar, in 2015, carried a massive one lakh cusecs of water, as against its capacity of 60,000 cusecs. Inflows from other drains, tanks and lakes as well as rainfall made the Adyar swell and inundate its flood plains which have been built upon.
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Seeking to allay the fears of Chennai’s residents, Commissioner for Revenue Administration, Atulya Misra told The Lede, “The outflow is being closely regulated considering inflow, rainfall in catchment areas, water levels in feeding water bodies and capacity of Adyar river. Zones 11, 12, 13, 9 and 5 are on high alert and people living in vulnerable areas are being shifted to relief camps.”
The Other Rivers
While the Adyar river’s flood plains fall largely in southern Chennai, the rivers Kosasthalaiyar and Cooum flow through the northern parts of the city and the adjoining district of Tiruvallur.
The Poondi and Cholavaram reservoirs drain into the Kosasthalaiyar river via Redhills. A branch of the Kosasthalaiyar drains into the river Cooum.
At present, the water level at Poondi is at 30.43 feet against the full storage capacity of 35 feet. The water level at the Cholavaram reservoir is at 4.71 feet against its full storage capacity of 18.86 feet. In Redhills, 11.5 feet of water is filled up as opposed to storage of 21.2 feet. All of these reservoirs are yet to be filled completely.
Current status of reservoirs - Source: Public Works Department of Tamil Nadu
As for the rivers, the Kosasthalaiyar has a carrying capacity of 1.1 lakh cusecs. In 2015, the Kosasthalaiyar flooded Tiruvallur district as it was carrying 1.4 lakh cusecs. At present, the flow in this river has just started and is not measurable as yet, according to officials in the Public Works Department.
The flow into the Cooum too has only just begun. The river has a carrying capacity of 19,000 cusecs. In 2015, the Cooum too flooded parts of north Chennai as it was carrying 24,000 cusecs of water.
Data on rivers - Source: Public Works Department of Tamil Nadu
The state government has so far evacuated 24,166 people from 13 districts to 315 relief camps ahead of the cyclone. There are 4733 camps across the state that can accommodate a total of 12.98 lakh people if needed.
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Sixteen NDRF teams are in place in Cuddalore, Nagapattinam, Mayiladuthurai, Villupuram, Chengalpet and Thanjavur.
“5000 personnel of the Fire and Rescue services have been deployed. Chennai, Mamallapuram to Cheyyur and Cuddalore coastlines are under focus,” said an official with the Fire Services team.
Eight teams of the Indian Army will reach Chennai on Wednesday with rescue boats. Similarly, six teams with a rescue boat are slated to reach Trichy.
Read the original article published in The Lede here