Sinking land and rising seas could push an additional one million people in Chennai into high-risk flood zones within 25 years, eventually threatening a third of the city’s low-lying coastal areas through worsening storm surges.
Scientists have found that the land beneath Chennai is sinking by up to 1.5cm a year while coastal sea levels rise by 2.8mm annually. Together, the two trends could turn localised coastal flooding into widespread inland inundation.
Their study, the most detailed yet of how subsidence and sea level rise might affect an Indian coastal city, estimates that the population at risk of severe flooding in Chennai will increase from 5.25 million in 2030 to 6.56 million by 2050.
“Several prominent residential and commercial localities will likely be at risk of catastrophic flooding,” said Arpan Shastri, a postdoctoral geoscientist at the Indian Institute of Science Education and Research in Mohali (Punjab) who led the study.
Chennai’s coast faces on average five storms a year, making it one of the world’s most storm-prone regions.
The city has been hit by six cyclones and repeated urban floods in the past two decades, including the devastating 2015 floods that inundated the airport nearly 30km inland.
“Local subsidence can exacerbate the impacts of sea level rise -- this is likely to happen along Chennai’s coastal region,” said Chandrakanta Ojha, a satellite radar specialist who leads the satellite remote sensing lab at the institute.
Low-lying coastal areas are highly vulnerable to storm surges — giant waves driven by storms or cyclones — and the proportion of Chennai’s coastal area at risk of severe flooding could rise to 33 per cent by 2100.
In a severe scenario, a six-metre storm surge — comparable to those seen in major cyclones — could inundate several low-lying neighbourhoods such as Adyar, Tondiarpet and Sholinganallur.
The risk is not limited to the coast. Subsidence is also extending flood-prone zones inland, particularly in rapidly growing southern areas like Sithalapakkam, T. Nagar and Chennai’s information technology corridor, where poor drainage and paved surfaces have worsened waterlogging.
Similar patterns have been reported in cities such as Bangkok, Jakarta, New Orleans and Kochi in India, where sinking land has intensified the impact of rising seas.
The number of people at risk of severe flooding in Chennai is expected to rise sharply. Between 2030 and 2050, an estimated 20,000 additional people could be at risk in Adyar, 90,000 in T. Nagar, 710,000 in Sithalapakkam, and 90,000 in Sholinganallur.
Shastri combined three decades of satellite imagery and tide-gauge data with 10 years of satellite radar observations for the study, published last week in the journal Scientific Reports.
“We need more comprehensive assessments of how subsidence and sea level rise together could affect India’s coasts,” said Jatisankar Bandyopadhyay, a remote sensing specialist at Vidyasagar University who was not involved in the study.
Bandyopadhyay, who has been studying similar processes in the Sunderbans using satellite radar data, said the dynamics there were more complex.
“We see subsidence in parts of the Sunderbans, but river-borne sediments can partly offset it,” he said.
A preliminary study by his team had earlier this year found expanding inundation between 2015 and 2024 in the Sunderbans, though he cautioned that detailed long-term analyses were needed to better assess the possible impacts on people and the land.
The Chennai and Sunderbans findings underscore the need for measures such as mangrove restoration and stronger coastal defences.