Study estimates 900 square kilometres of land across 5 Indian megacities subsiding

New Delhi: Nearly 900 square kilometres of land across five megacities in India including New Delhi and Chennai could be subsiding, exposing 19 lakh people to subsidence rates of more than four millimetres a year, a study has estimated.

Researchers, including those from the US' Virginia Polytechnic Institute and State University, analysed satellite data gathered during 2015-23 and found over 2,400 buildings are already at high risk of structural damage.

Land subsidence can compound threats from flooding and earthquakes. When the ground beneath a city sinks unevenly, it can weaken foundations, damage utility lines, and amplify structural vulnerability, they said.

More than 23,500 buildings could face very high structural damage in the next 50 years, should current subsidence rates continue, findings published in the journal Nature Sustainability show. Buildings in Mumbai, Kolkata and Bengaluru were also looked at in the study.

Excessive groundwater use is among the major factors contributing to land subsidence in the cities studied, with the weight of buildings in densely built-up areas adding to the observed subsidence, lead author Nitheshnirmal Sadhasivam, a graduate student at Virginia Polytechnic Institute and State University, told PTI.

Further, "Delhi-NCR exhibits the highest susceptibility to differential settlement-related building damage risk, based on our analysis of current subsidence rates and undefended future projection scenarios," he said.

By 2030, New Delhi is projected to surpass Japan's Tokyo as the world's largest megacity and India is expected to see two additional megacities emerging, the authors said in the study.

The country's population, estimated to grow at 0.92 per cent as of 2024, could worsen the severe water scarcity in many urban areas, they added.

The analysis, which looked at more than 13 million buildings and 80 million people across the five megacities, "reveals 878 square kilometres of land subsiding, exposing nearly 1.9 million people to subsidence rates of more than four millimetres per year".

Widespread subsidence was seen in the cities, with the highest rates of up to 51 millimetres per year seen in New Delhi, followed by 31.7 millimetres per year in Chennai and more than 26 millimetres per year in Mumbai.

Kolkata noted the highest subsidence rate of 16.4 millimetres per year, while 6.7 millimetres per year was seen in Bengaluru.

Subsidence hotspots were also identified within a city, with Bijwasan, Faridabad and Ghaziabad in Delhi-NCR estimated to see subsidence of up to 28.5 millimetres per year, 38.2 millimetres per year and 20.7 millimetres per year, respectively.

The authors also found a localised uplift in urban areas, such as those near Dwarka in Delhi-NCR that are rising at a rate of over 15 millimetres per year.

In Chennai, the fastest subsidence rates were seen in the flood plains surrounding the Adyar River and city centre areas, including Valasaravakkam, Kodambakkam, Alandur and Tondiarpet, they said.

"The silent strain we see today could lead to tomorrow's disasters if cities do not adapt their infrastructure and groundwater management policies," Sadhasivam said.