A recent study published by the Indian Institute of Science Education and Research (IISER) — a public research institute under the Government of India — revealed 13 of Mumbai’s 24 municipal wards are highly vulnerable to flood. This is up from 11 in 2005. These zones span from coastal areas like Colaba and Marine Drive to dense suburban pockets like Govandi, Andheri, and Kandivali.As per the report, these 13 municipal wards house nearly 78.69 lakh people or 61 per cent of Mumbai’s total population of 1.29 crore people. These municipal wards cover almost half of the city’s landmass. In contrast, only 11 wards were designated as highly vulnerable in 2005.The IISER study also solidified the BMC’s 2021 Mumbai Climate Action Plan (MCAP) report, which stated that 37 per cent of Mumbai’s population resides in flood prone areas.Researchers and reports attribute this heightened risk to climate change, loss of mangrove cover and current land-use patterns.Between 16 and 19 August, Mumbai was brought to its knees by an extraordinary downpour, with the city receiving 944 mm of rain in 96 hours. The resulting inundation crippled public services. On May 26 this year, the city experienced its earliest monsoon onset ever, marked by 145 mm of rainfall in a 24-hour period.While such extreme weather is familiar to Mumbai, recent studies and analyses by the Brihanmumbai Municipal Corporation (BMC) revealed the city’s vulnerability to floods has significantly increased over the past two decades. The Inter-governmental Panel on Climate Change (IPCC) defines vulnerability as “the degree to which a system is susceptible to, or unable to cope with, adverse effects of climate change, including climate variability and extremes”.Increased flood risk, extreme rainfallThe IISER study, titled “Analysis of urban flood vulnerability at the sub-city administrative unit level in Mumbai: A comparison over two time periods”, was authored by Dr Sanjeev Kumar Jha and PhD students Pragati Prajapati and Akshay Singhal. The study introduced a combined flood vulnerability index (CFVI) for all 24 wards, comparing vulnerability indicators from 2005 and 2025 to assess the city’s exposure and adaptability, following the infamous 26 July 2005 deluge, which dumped over 900 mm of rain within 24 hours, killed 419 people, and caused widespread destruction.Story continues below this adWards were categorised as high (H), moderate (M), or low (L) in terms of vulnerability. High-vulnerability zones include Colaba, Marine Drive, Cuffe Parade, Nariman Point, Worli, Prabhadevi, Dadar, Matunga, and suburban areas such as Govandi, Bandra, Andheri, Kandivali and Dahisar.The study revealed that five wards have become more vulnerable since 2005: A ward (Colaba, Churchgate, Nariman Point) which were classified as “low” have now moved to high vulnerability; F North (Matunga) jumped from moderate to high; and M East (Mankhurd, Govandi) shifted from low to high.On the other hand, K West (Andheri West, Juhu, Versova), H West (Bandra West), and G North (Dadar) have remained consistently highly vulnerable, whereas P South (Goregaon) and M West (Chembur) improved from moderate to low vulnerability.As of 2025, 13 wards are highly vulnerable, seven moderately, and four less susceptible. In 2005, it was 10, 10, and four respectively.“The report is being made on the basis of the existing data related to flooding, civic amenities, and census among others. The fact that the larger number of wards are highly vulnerable indicates that Mumbai’s overall vulnerability against flooding has only increased with time. This has mainly happened due to factors like climate change, change in overall demography and landmass usage,” Dr Sanjeev Kumar Jha told The Indian Express.Story continues below this adThe CBFI was constructed using IPCC 2007 methodology, assessing indicators such as stormwater drain outfalls, dewatering pumps, population density, slum population, waterlogging hotspots, road length, and fragile buildings. These indicators were quantified and analysed over a 20-year span using a three-tier aggregation method.“This study is absolutely based on the data sets recorded by the indicators. For example, if we have noticed that the population in slum areas have increased and accessibility to amenities have reduced, then it is established that the overall vulnerability of a ward has reduced. Similarly, if we see that factors like road quality and drainage outfall have become better, then the vulnerability of that ward has reduced,” Dr Jha said.BMC data further showed that instances of very heavy rainfall surged in the past six years. According to IMD standards, rainfall between 115.6 mm and 204.4 mm in 24 hours is considered “very heavy” and that over 204.5 mm is labelled extreme.BMC data shows over the past six years, the average quantity of very heavy rainfall climbed from 132 mm to 182 mm. In the past 10 years, intense spells of very heavy rainfall lasting for a prolonged four-hour period occurred 28 times.Story continues below this adBetween 2016 and 2024, Mumbai experienced at least 16 days a year with over 100 mm rainfall in 2017, 2018, 2019, 2021 and 2024. The city recorded 66 such days from 2021 to 2024, compared to 52 between 2018 and 2020.In tandem, the number of flooding hotspots rose to 453 in 2025, up from 386 in 2024 and 306 in 2023. On average, 65 new spots are identified annually.“During the July 26 rain in 2005, Mumbai recorded 900 mm of rainfall in 24 hours. It was the first time in several decades that the city experienced such a heavy rainfall within such a short period. However, our records show that in the last decade, the frequency of heavy rainfall within a short period has only increased, making us more vulnerable to calamities like flooding. This is occurring mainly due to climate change and we can’t reverse this process immediately. The only way to tackle vulnerability is by adapting long term resilient measures,” Abhijit Bangar, additional municipal commissioner (Projects) told The Indian Express.The 2021 MCAP report echoed these concerns — “Projections made as part of recent research by Climate Central projects indicate that a large part of Mumbai city is at risk of being submerged by 2050 and that by 2080, the likelihood of urban floods, such as the 2005 event, is more than double”. It added that by the 2070s, Mumbai could be among Asia’s most exposed cities alongside Kolkata, Shanghai, and Bangkok to coastal flooding.Story continues below this adFactors making Mumbai more vulnerableAlong with extreme weather events, Mumbai’s flood vulnerability is also driven by the city’s geography and ecology. Mumbai was formed by reclaiming seven islands between 1784 and 1845, creating naturally low-lying areas such as Dadar, Parel and Hindmata, where water accumulates easily.Bangar said, “The reclamation of the islands had resulted in the formation of several low-lying areas. Therefore, areas like Dadar, Parel and Hindmata in south Central Mumbai automatically became low-lying, due to which, water continues to accumulate there because of gravitational force.”Speaking to The Indian Express, Raghu Murtugudde, earth system scientist and professor emeritus at University of Maryland’s (UMD) department of atmospheric and oceanic science, said that while global warming has led to an overall increase in the rainfall pattern over Mumbai, local factors amplify the city’s vulnerability to flooding.“The entire Middle East region starting from Western Asia, Iran, Saudi Arabia, Syria and Egypt, is warming up rapidly due to global warming… This high pressure is triggering extreme rainfall… Mumbai is not only witnessing above normal rainfall, but also higher temperature during the months of October and November,” said Murtugudde.Story continues below this adHe added, “Local factors such as an inadequate drainage system and haphazard disposal of solid waste into the drains and water bodies amplifies Mumbai’s overall vulnerability. When the drains get choked due to floating waste, water cannot flow through them during heavy rainfall, thus resulting in overflow.”Loss of mangroves and rampant urbanisation have compounded the risk. Mangrove forests that are vital natural buffers against coastal erosion, storm surges, and flooding have dwindled due to development and erosion. The dense root systems stabilise sediments and reduce the impact of waves, however, increased encroachment and infrastructure development around Mumbai’s eastern and western shoreline mangrove patches have led to reduction in its density. MCAP estimates that from 2008 to 2021, 325 hectares of dense mangroves were degraded or converted into mudflats.Professor Shubhankar Karmakar of IIT Bombay said, “As Mumbai experiences more and more urbanisation over decades, the surface has become more impervious. As a result, the accumulated rainwater on the surface of the top soil is unable to percolate into the ground and flows through the surface resulting in flooding. In addition to this, factors like loss of mangrove cover also affect the vulnerability of the city as mangroves are the first line of natural defence.”Tackling the challengeFaced with rising challenges, the BMC and IIT Bombay formulated a risk assessment with 21 mitigation projects totaling Rs 12,705 crore and submitted it to the National Disaster Management Authority. These projects include structural and non-structural solutions. Structural measures include floodgates and pumping stations, while non-structural solutions focus on bioswales and sponge parks to enhance water infiltration.Story continues below this adBangar said, “Slow rainfall creates maximum percolation, but high rainfall doesn’t allow smooth percolation… even if we are able to increase the percolation by 10 per cent, then it will reduce the overall surface runoff by a significant margin.”“After the NDMA accepts our proposal, we will carry out an overall hydrological mapping of Mumbai to mitigate the risk of climate change through routine interventions,” he added.As a preventive measure, the BMC has also initiated a project to widen the existing drains of island city to be able to hold 120 mm of rainfall in an hour — currently they are able to handle 55 mm of rainfall water in an hour.“Since the population and demography of the city have changed drastically in the past decades and even the pattern of rainfall has recorded a massive change, there is a need to now widen the existing width of the drains to improve their carrying capacity,” a BMC official said.