Mumbai sinking at 2mm per year, new research warns
MUMBAI: The land around Mumbai is sinking at an average rate of 2mm per year, a recent research has found, prompting experts to warn that the city is likely to see increased flooding unless urgent action is taken by urban planners and municipal authorities.
The study by scientists from University of Rhode Island, published in the peer-reviewed Geophysical Research Letters journal in March, analysed land subsidence in 99 countries, finding Tianjin in China to be the fastest-sinking coastal city in the world at a rate of 5.2cm per year.
Land subsidence, the researchers explained, is caused by excessive groundwater extraction, destruction of natural wetlands, development of underground infrastructure, mining and other ecological disturbances. It is irreversible, and can adversely alter local hydrology, causing floods and damaging civic infrastructure like roads, railways, bridges, telecommunications and others.
Out of 46 sq km of land in Mumbai at an elevation of less than 10 metres — which makes it extremely vulnerable to flooding — 19 sq km is subsiding at a rate faster than 2mm per year, with a maximum subsidence rate of 8.45mm a year, the study, Subsidence in Coastal Cities Throughout the World Observed by InSAR, found. Mumbai’s total land cover is 603.4 sq km, it said.
These figures require further investigation, co-author Matt Wei told HT. “The most important thing to resolve this issue is using continuous global navigation satellite (GNSS) data on the ground.”
In India, data from the country’s indigenous GNSS programme, the Indian Regional Navigation Satellite System, is classified, making it hard for researchers to corroborate InSAR data used in the study with field observations.
Although the rate of subsidence in Mumbai is significantly lower than for other countries in South Asia, it can compound the impacts of sea level rise and extreme rainfall events, experts warned.
“A significant portion of the city is subsiding more rapidly than 2mm per year,” noted the study.
Other recent studies have also indicated that the Arabian Sea is rising by 0.5mm to 3mm per year, suggesting that some parts of Mumbai may be sinking faster than sea levels are rising.
“This could pose a double whammy for urban planners and policy makers,” said Sudha Rani NNV, a researcher at Indian Institute of Technology (IIT) Bombay, who studies this phenomenon. “Even a small amount of subsidence could have major implications over a given area.”
A study by researchers at the interdisciplinary programme in climate studies at IIT Bombay, which is awaiting peer review, found a maximum subsidence of 93mm per year in the city, and an average annual subsidence rate of 28.8mm per year.
“With this declining groundwater, high-rise buildings, and metro development projects, it is becoming increasingly vulnerable to subsidence,” the study noted said. “As the city expands and natural areas get built over, this phenomenon will worsen, exacerbating flooding.”