As rains intensify, seas rise and land sinks, more people exposed to flood risk in city than ever before

Mumbai is unprepared for future deluges, with a booming population and increasing frequency of heavy rain days, according to studies. Over 35% of the city's population lives within 250 meters of chronic flooding hotspots, while less than half have access to flood shelters. The intensity of monsoon rainfall in Mumbai is intensifying, and the city is also experiencing land subsidence. Recent analyses have found that certain areas of Mumbai are at risk of submergence due to sea-level rise. Around 998 buildings and 24km of road length will be affected by potential sea-level rise by 2050.

Mumbai: The deluge of July 26, 2005, may be eighteen years behind us, but multiple studies since — including the Brihanmumbai Municipal Corporation’s own Mumbai Climate Action Plan (MCAP) — reveal a worrying future.

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Not only is the city unprepared for a similar event, but its population has also boomed. Add to this the increasing frequency of heavy rain days, sea-level rise and land subsidence (sinking), and you have all the ingredients for a disaster.

As per the MCAP, more than 35% of the city’s population lives within 250 metres of chronic flooding hotspots, while less than half have easy access to flood shelters.

Nearly 43% of Mumbai houses 75.9% of the city’s population, which has access to a flood shelter within 1km walking distance, but the catch is that during periods of heavy rain — when flood shelters are most needed —only 46.5% of the population can access them easily.

Moreover, accessibility to flood shelters is lacking in those very wards where larger sections of the population are exposed to chronic waterlogging. For example, in M/East Ward (Deonar, Mahul, Chembur, Trombay) — where an estimated 2,75,491 people live at risk of chronic flooding — only 13.3% of the population has easy access to a flood shelter during an extreme rain event, the least in the city.

“Deonar, Mahul, Kandivali East, Saki Vihar Road in Powai, Four Bungalows in Andheri, and Versova are some of the underserved high-population density areas that do not have access to a flood shelter within 1 km,” the MCAP notes.

The intensity of monsoon rainfall — June to September — in Mumbai is intensifying at the rate of 5.18mm per year, while the intensity of annual rainfall has gone up by 22 mm per year, according to a recent study lead by Rohit Mann, from the University of Kurukshetra.

The findings indicate two inflection points in Mumbai’s weather data: the frequency of heavy (more than 120 mm/day) and extreme heavy rainfall events (more than 250 mm/day) increased over Santacruz after 1994, while in Colaba, they increased after 2005, when Mumbai received 944mm of rain in a single day. A subsequent extreme rainfall event in 2011 saw 1,058mm of rain in a single day over Colaba.

“The period between 2001 to 2005 saw a significant change in the way Mumbai experiences rainfall, with general patterns becoming more erratic and individual events becoming more severe. The July 2005 floods are not an isolated incident. It is reflective of broader changes in the Indian Summer Monsoon Rainfall due to climate change,” said Mann.

The MCAP, too, warns of this, pointing out Mumbai experiences, on average, six heavy (64.5 – 115.5mm), five very heavy (115.6 – 204.4 mm), and four extremely heavy (> 204.5 mm) rain events per year.

More recently, “The four-year period between 2017 and 2020 has seen a steady increase in the extremely heavy rainfall events. Spatially, most ERE tend to occur as localized clusters in central and western areas like Worli, Dadar, Kurla and Andheri,” the MCAP cautions.

“Such events will be more frequent as the planet warms further. To whatever extent possible, the capacity of the city to handle such events needs to be upgraded. This hasn’t happened. Every year, the monsoon in Mumbai claims multiple lives, either through floods, drownings or local landslides. That should be enough to tell us how dire the situation is,” said Akshay Deoras, research scientist at the National Centre for Atmospheric Science & Dept. of Meteorology, University of Reading, UK.

Mumbai is also experiencing a geological phenomenon known as land subsidence and is said to be ‘sinking’ at the rate of about 2mm per year.

A study — Subsidence in Coastal Cities Throughout the World Observed by InSAR -- by researchers at the University of Rhode Island, USA, published in 2022 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.

Land subsidence can adversely alter local hydrology, causing floods and damaging civic infrastructure like roads, railways, bridges and others. With recent studies indicating that the Arabian Sea is rising by 0.5 to 3mm per year, some parts of Mumbai may be sinking faster than sea levels are rising.

A 2022 analysis by RMSI, a global risk management firm, found that Haji Ali, the Western Express Highway, Bandra-Worli Sea-Link, and Queen’s Necklace on Marine Drive are at risk of submergence.

In Mumbai, around 998 buildings and 24km of road length will be affected by potential sea-level rise by 2050, and approximately 2,490 buildings and a road length of 126 km will be affected by potential sea-level rise during high tide.