19-06-2025
Study warns loss of water bodies in Chennai significantly increases flood risk
Chennai: Ten years ago, a relentless downpour of 50 cm of rainfall in Chennai on December 1, 2015, followed by the release of water from the Chembarambakkam reservoir, flooded most areas and brought the city to a standstill.
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A new study now warns that if the city loses its existing upstream water bodies, which currently have a capacity of 174.7 million cubic metres, to future urbanisation, flood damages could increase by 44%, deaths by 60%, and risk to the population by 40.5% if a 2015-like flood repeats.
Researchers said the 2015 flood could have caused less damage if the city retained its traditional tanks, man-made water bodies designed to store rainwater.
The study found flood losses could have dropped by 17% and fatalities by 12% if the lost urban tanks were still functional, with flood levels dropping by up to 0.8 m in areas like T Nagar, Ashok Nagar, Virugambakkam, and Saidapet. The population at risk would have decreased by 25.3% compared to the 2015 flood.
Since the early 1900s, urbanisation has led to the loss of 13.6 million cubic metres of tank storage within the city.
The remaining 174.7 million cubic metres of tank storage outside the city are also at risk. Though the urban tanks held a smaller volume than the upstream ones, their proximity to densely populated areas meant they could have intercepted rainwater before it flooded streets and homes.
"The 2015 flood was primarily due to excessive rainfall. The water release from Chembarambakkam reservoir played some role, but it was only 25% of the total flow from upstream," said researcher Abinesh Ganapathy.
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"The water bodies provide extra cushion to store water and reduce inflow into the Adyar River. The lakes that disappeared should have provided even more storage and moderated the flood in the city."
Researchers simulated three scenarios: the actual 2015 flood (baseline), a version where lost urban tanks still existed (with tanks), and one where even upstream tanks are lost (no tanks). Compared to the 2015 scenario, if the traditional tanks were present, flooding in the lower flood hazard zones in parts of Vadapalani, Mylapore, Alwarpet, Adyar, Ashok Nagar, Ekkattuthangal, and Guindy could have dropped by 23% to 37%.
When encroached tank areas are included, this reduction rose to over 60% in parts of Vadapalani, Mylapore, and Alwarpet.
In contrast, if upstream tanks are also lost, flooding in the high-risk flood zones like parts of Koyambedu, Adyar, Porur, T Nagar, Nandanam, St Thomas Mount, parts of Mylapore, RA Puram, and Nungambakkam would expand by 45% to 54%.
About 80,000 people and 15,000 buildings were spared from flooding in the 'with tanks' scenario, while over 1.7 lakh people and 30,000 buildings were newly affected in the 'no tanks' scenario.
The study, conducted by institutions including GFZ Potsdam, IIT Roorkee, and IIT Madras, was published in Urban Climate. An earlier study led by corresponding author Nithila Devi Nallasamy projected that urban areas in the Adyar Basin will expand from 20% to 46% by 2050, putting the upstream tanks at high risk.
Nivedita Sairam, a researcher from GFZ Potsdam, said, "The study shows that in addition to flood defences, we need to give room for the river and controlled urban development.
Protecting water bodies plays a role in controlling weather extremes."
The study supports using nature-based solutions for flood control in cities like Chennai, which face both flood risks and water scarcity.
T Kanthimathinathan, coordinator, Tamil Nadu State Disaster Management Authority, said the only way to reduce flooding in the city is for urban planners to ensure the clearing and protection of the flow paths of existing water bodies to downstream systems from the impact of urbanisation.
He said that between 1975 and 2008, many water bodies were lost to city expansion, including those in Nungambakkam, Mambalam, and Mogappair.
Only 49, now significantly shrunk, remained.
"We must give room for the water to flow," he said. "The water bodies in the upstream may shrink in size, but not disappear. The Thiruppugazh committee recommended restoration of these upstream water bodies to their original condition. If possible, they can be deepened to increase holding capacity, without affecting groundwater or the environment," he added.
