Mumbai ‘sinking’ by an average of 2mm annually: Study – Hindustan Times

The city is sinking at an average rate of 2mm per year, a recent research has found, due to a geological phenomenon known as land subsidence, with experts warning that the city is likely to see increased flooding unless urgent remedial action is taken by urban planners and municipal authorities.

The study, published in the peer-reviewed Geophysical Research Letters journal in March, analysed land subsidence in 99 countries globally, finding Tianjin in China to be the fastest sinking coastal city in the world at a rate of 5.2cm per year.

‘Land subsidence’ refers to the downward, vertical movement of the earth’s surface, which can be caused by groundwater extraction, mining, reclamation of natural wetlands, infrastructure projects and ecological disturbances.

The study titled Subsidence in Coastal Cities Throughout the World Observed by InSAR by researchers at the University of Rhode Island, USA, published earlier this year in the international peer-reviewed journal Geophysical Research Letters notes that out of 46km2 of land in Mumbai which is at an elevation of less than 10 metres, which makes it more vulnerable to flooding, 19km2 is subsiding at a rate faster than 2mm/yr, with a maximum subsidence rate of 8.45mm/yr. The study analysed land subsidence in 99 countries globally. Apart from Tianjin in China, other coastal cities ‘sinking’ include Semarang (3.96cm per year) and Jakarta (3.44cm per year) in Indonesia, Shanghai (2.94cm year) in China, and Ho Chi Minh (2.81mm per year) and Hanoi (2.44cm per year) in Vietnam.

Though the rate of subsidence in Mumbai is significantly lower than other countries in South Asia, it can over time compound the impacts of sea level rise and extreme rainfall events, experts warn. Land subsidence is irreversible, and can adversely alter local hydrology, causing floods and damaging civic infrastructure like roads, railways, bridges, telecommunications and others.

Recent studies indicate that the Arabian Sea is rising by 0.5 to 3mm per year, suggesting that some parts of Mumbai may be sinking faster than sea levels are rising. This could pose a double threat for urban planners and policy makers.

With the BMC having launched an ambitious Climate Action Plan earlier this year, experts say that authorities need to pay urgent attention to land subsidence as a scientific and socio-economic issue, as areas prone to subsidence may experience the worst impacts of flooding and property damage.

“Mumbai is the second-most populous city in India after Delhi. It is the seventh-most populous city in the world with a population of roughly 20 million. A significant portion of the city is subsiding more rapidly than 2 mm/yr,” notes the study, which measured subsidence rates in 99 coastal cities globally between 2015 and 2020. The researchers did so by analysing satellite data using the InSAR method, short for Interferometric Synthetic Aperture Radar, a commonly used technique for measuring deformations of the earth’s surface.

Matt Wei, corresponding author, clarified that these figures require further investigation. “Our data shows that much of Mumbai is sinking at a rate over 2 mm/yr. However, InSAR data is affected by water vapor content in the atmosphere. Mumbai is in a wet region and might have lots of noise in the data.”

A recent study by researchers at the inter-disciplinary program in climate studies at IIT-Bombay, which is currently awaiting peer-review, found a maximum subsidence of 93 mm/yr in the city, and an average annual subsidence rate of 28.8 mm/yr, with the highest subsidence recorded in Byculla, Colaba, Churchgate, Kalba Devi, Kurla, Andheri East, Mulund, Nahur East, Dadar, Wadala, and parts of Tardeo, Bhandup, Trombay and Govandi.

“Due to a prolonged decline of groundwater in the area that plays a crucial role in land subsidence along the coast, a considerable relationship between land subsidence and groundwater decline can be established over the region. With this declining groundwater, high-rise buildings, and metro development projects, (Mumbai) is becoming increasingly vulnerable to subsidence, leading to increased inundation,” notes the study.

“With sea levels projected to rise by around 1 to 1.2 metres in a high emissions scenario, we found that nearly 38% of Mumbai’s land may be inundated during a normal rainfall. This is a severe issue that has to be addressed immediately,” said Sudha Rani NNV, lead author.

A 2021 conference paper, ‘Mapping land subsidence in Mumbai by Sentinel-1 InSAR time-series’, authored by researchers at the Centre of Studies in Resources Engineering at IIT-Bombay, identified two major subsidence areas in the Mumbai Metropolitan Region, namely Vasai and Wadala.

Using a similar methodology, researchers found that an area in Wadala west, close to mangrove plantations on the western shore of Thane Creek, showed a subsidence of 80mm in three years between 2016 to 2019. More worryingly, a small portion of the Sion-Panvel Expressway, which leads to an over bridge above the creek, also showed a measure of subsidence, the authors noted, suggesting a strong correlation between land subsidence, groundwater extraction, and proximity of the area to mangrove plantations and the coastline.

“The extent of the expressway undergoing subsidence lies close to the mangrove plantation zone surrounded by salt pans along the road,” the study said. “A densely populated and irregularly constructed slum area along the Ghatkopar-Mankhurd Link Road merging with the Sion-Panvel Expressway also shows subsidence in the range of 60mm over the same period.”

BOX:

Methodology

–> Researchers used satellite images of 99 coastal cities captured (between October 2014 and January 2021) by the C-band Sentinel-1 A/B satellite, commissioned by the European Commission and the European Space Agency (ESA).

–> The images were analysed using the Interferometric Synthetic Aperture Radar, which is a commonly used technique for measuring deformations in the earth’s surface through remote sensing.

–> For each city, researchers processed one satellite image every two months. A total of six images per year were analysed to establish the rate of land subsidence over a five year period.