Parts of Delhi, Mumbai, Chennai, Kolkata and Bengaluru, India's largest cities, are slowly sinking, mainly due to overextraction of groundwater, according to a recent study, reports Mongabay India's Manish Chandra Mishra.
Researchers used eight years of satellite radar data and found that 878 square kilometers (339 square miles) of land across the five megacities show signs of subsidence. This leaves more than 2,400 buildings at high risk of structural damage. If current trends continue, the number could rise to more than 23,000 buildings in the next 50 years, the study found.
"Our motivation to study land subsidence and building damage risk in Indian megacities stems from the absence of prior research that explicitly investigates land subsidence and links differential settlement with observed structural damage," Nitheshnirmal Sadhasivam, study co-author from Virginia Tech, U.S., told Mongabay India. "While the impact of land subsidence on infrastructure is a well-recognised geohazard globally, in cities such as Jakarta, Mexico City and Tehran, its implications for building stability in India have not been systematically assessed."
The study found that roughly 1.9 million people across the five cities live in areas that are sinking at a rate of more than 4 millimeters (0.16 inches) per year.
Delhi, Chennai and Mumbai face the highest levels of subsidence: the annual rates of subsidence reach 51 mm (2 in) for parts of Delhi, 31.7 mm (1.25 in) for Chennai, and 26.1 mm (1 in) for Mumbai.
"Across all five megacities, groundwater dependence and overexploitation emerge as the dominant local drivers of subsidence," Sadhasivam said. "As groundwater is withdrawn, the resulting loss of pore pressure in compressible aquifer layers causes gradual compaction, leading to measurable land surface sinking over time."
Vikas Kanojia, an urban designer not involved in the study, told Mongabay India that Delhi's rapid growth has outpaced groundwater recharge. "Many zones in NCR [National Capital Region] are facing water shortage due to deficient ground water recharge versus demand," he said. "The ground water table has drastically dropped in many areas of NCR leading to change in the density of the sub-soil structure."
However, some areas show signs of recovery, Mishra writes. Parts of Dwarka in southwest Delhi, for example, are rising, thanks to groundwater recharge programs like rainwater harvesting and restrictions on extraction.
"Regions like Dwarka in Delhi demonstrate that policies promoting groundwater regulation and artificial recharge can help stabilise the ground and partially reverse subsidence trends," Sadhasivam said. "However, recovery is not always uniform. When some areas experience uplift while adjacent zones continue to subside, sharp gradients in ground motion result, creating transitional stress zones."
Researchers say the cities' land subsidence can amplify impacts of flooding, earthquakes and other disasters on the already overburdened infrastructure.
Read the full story by Manish Chandra Mishra on Mongabay India here.
Banner image: Land subsidence is putting buildings in five Indian megacities, including Delhi, pictured here, at high risk of structural damage. Image by Pinakpani via Wikimedia Commons (CC BY-SA 4.0).