The latest spell of heavy rainfall has once again flooded several parts of Mumbai, with the India Meteorological Department (IMD) warning that more intense rainfall could occur as favourable weather systems remain active over the western coast.
The repeated flooding has raised a long-standing question: Why does Mumbai continue to experience severe waterlogging every year despite decades of drainage improvements, pumping stations, desilting operations and monsoon preparedness measures?
The answer lies in a complex combination of geography, weather patterns, urban development challenges and climate change. Mumbai's flooding is rarely caused by rainfall alone. Multiple natural and human-made factors combine simultaneously, creating what urban planners describe as the city's "bathtub effect", where rainwater accumulates faster than it can drain away.
The Western Ghats intensify monsoon rainfall over Mumbai
Mumbai's geographical location makes it one of India's wettest major cities. During every southwest monsoon, moisture-filled winds from the Arabian Sea move towards India's western coastline before encountering the Western Ghats, which run parallel to the coast.
Unable to cross the mountain range easily, the moist air rises upwards. As temperatures fall at higher altitudes, water vapour condenses into dense clouds, resulting in heavy rainfall over Mumbai and the Konkan region.
Meteorologists describe this process as orographic rainfall, which remains one of the primary reasons behind the region's exceptionally high seasonal rainfall levels.
The Arabian Sea continuously supplies moisture
Even after one intense rainfall spell ends, another often follows within hours because the Arabian Sea continuously supplies warm, moisture-rich air during the southwest monsoon season.
Persistent southwesterly winds transport large quantities of water vapour towards Mumbai almost continuously. This constant moisture supply allows new rain systems to develop repeatedly, causing heavy rainfall to continue for several hours or even multiple days.
Offshore troughs create prolonged rainfall conditions
Another major factor behind Mumbai's intense rainfall is the offshore trough that frequently develops parallel to the Konkan coastline during the monsoon.
According to the India Meteorological Department, this elongated low-pressure zone continuously attracts additional moisture from the Arabian Sea while supporting cloud formation. Instead of moving inland quickly, rainfall systems often slow down or remain stationary over Mumbai, resulting in repeated spells of heavy to extremely heavy rainfall.
When cyclonic circulations or low-pressure systems develop over western India or the adjoining Arabian Sea, the offshore trough becomes more active, significantly increasing rainfall across the Mumbai Metropolitan Region.
Mumbai's geography increases flood vulnerability
Heavy rainfall alone does not explain why Mumbai floods so rapidly. The city's geographical structure makes it naturally vulnerable to water accumulation.
Modern Mumbai was developed by connecting seven islands through centuries of land reclamation. Many reclaimed areas are located only a few metres above sea level, causing rainwater to collect naturally in low-lying areas.
Flood-prone locations including Hindmata, Sion, King's Circle, Kurla, Milan Subway, Andheri Subway, Dadar and Bandra experience severe waterlogging almost every monsoon season.
Urban planners state that these are not newly identified vulnerable zones, with most of them remaining on flood maps for decades.
High tides block the city's drainage process
One of Mumbai's biggest flood-related challenges is that its stormwater drainage network ultimately releases water into the Arabian Sea.
During high tide, seawater pushes against drainage outlets, reducing or completely blocking the flow of rainwater into the sea.
When heavy rainfall coincides with high tide, water cannot escape. Instead, it moves back through drains and spreads across roads, railway tracks, underground passages and residential areas.
Several of Mumbai's worst flooding events have occurred when intense rainfall and high tide happened together.
Ageing drainage infrastructure struggles with extreme rainfall
Large sections of Mumbai's stormwater drainage system were originally constructed during the British colonial period.
Although the Brihanmumbai Municipal Corporation (BMC) expanded the drainage network significantly after the devastating July 2005 floods, experts state that parts of the infrastructure continue to struggle during extremely intense rainfall events.
Plastic waste, construction debris, accumulated silt and illegal occupation of drainage areas reduce the capacity of drains, slowing water movement at the time when rapid drainage is most required.
Authorities conduct annual desilting operations before every monsoon, but blocked drains remain a recurring challenge.
Mumbai is receiving more rainfall within shorter periods
The challenge is no longer only about the total amount of rainfall received during the monsoon season. Climate scientists identify rainfall intensity as the growing concern.
Mumbai receives approximately 2,200 mm of annual rainfall, with the majority occurring between June and September. However, increasing amounts of rainfall are now arriving within only a few hours.
When 150 mm to 300 mm of rain falls within a short duration, even improved drainage systems face difficulty because water reaches the surface faster than it can be removed.
The devastating July 26, 2005 floods remain the most severe example, when parts of the Mumbai Metropolitan Region recorded more than 900 mm of rainfall within 24 hours.
Concrete expansion has reduced Mumbai's natural water absorption capacity
Before rapid urban expansion, Mumbai had extensive wetlands, mangroves, salt pans and open spaces that naturally absorbed excess rainwater.
Over time, large portions of this landscape have been replaced by roads, buildings, parking areas and concrete surfaces.
With limited exposed soil remaining, rainwater now flows directly into drainage systems instead of being absorbed naturally into the ground.
Urban planners state that this increased surface runoff places additional pressure on drainage infrastructure during heavy rainfall.
Environmental experts have repeatedly warned that the continued loss of mangroves and wetlands has weakened Mumbai's natural flood protection system.
Mithi River remains a major flood concern
The Mithi River plays an important role in carrying excess rainwater into the Arabian Sea.
However, pollution, illegal occupation, narrowing of the river channel and sediment accumulation have reduced its ability to carry large volumes of water.
As a result, areas surrounding Kurla, Dharavi, Bandra-Kurla Complex, Saki Naka and nearby neighbourhoods remain highly vulnerable during periods of intense rainfall.
Every monsoon, concerns emerge over whether river restoration efforts and desilting operations are sufficient to manage increasingly extreme rainfall events.
Climate change is increasing extreme rainfall events
Scientists increasingly believe climate change is altering rainfall patterns along India's western coastline.
Warmer sea surface temperatures over the Arabian Sea allow the atmosphere to hold greater amounts of moisture. When weather systems such as offshore troughs, cyclonic circulations or low-pressure areas develop, this moisture can be released within a short period, resulting in exceptionally intense rainfall.
This indicates that infrastructure designed decades ago may no longer be sufficient to manage current rainfall extremes.
Changes after the July 26, 2005 floods
The catastrophic July 26, 2005 floods, which claimed hundreds of lives after more than 900 mm of rainfall within a day, transformed Mumbai's flood management strategy.
Since then, authorities have invested in larger stormwater drains, pumping stations, flood gates, automated weather monitoring systems, real-time flood warnings and annual desilting operations.
Studies on urban flooding indicate that these measures have improved Mumbai's response capabilities in several areas. However, experts caution that improved preparedness alone cannot remove the city's underlying vulnerabilities.
The same locations continue to appear in annual flood advisories because many structural challenges remain unresolved.
Flooding creates major economic and human losses
Flooding is often considered a transportation issue, but its consequences extend much further.
Businesses lose productive working hours, schools suspend operations, flights and trains face major disruptions, and public infrastructure suffers repeated damage.
Research by the University of Chicago estimates that extreme monsoon rainfall contributes to approximately 2,300 to 2,700 additional deaths annually in Mumbai through drowning, electrocution, disease outbreaks and worsening health conditions.
The researchers estimate the economic value of these losses at around 10,000 crore annually, excluding additional losses from infrastructure damage, reduced wages and disruption to economic activity.
The study also found that nearly 80 percent of people who die during frequent flood events live in Mumbai's slum settlements, showing that flooding affects the city's poorest communities most severely.
Can Mumbai eliminate its recurring flood crisis?
Experts believe engineering projects alone cannot solve Mumbai's flooding challenge.
Along with drainage improvements, they recommend restoring wetlands and mangroves, protecting natural drainage channels, improving waste management systems, preventing construction on floodplains and incorporating climate resilience into urban planning.
As rainfall intensity increases due to a warming climate, experts emphasise that future flood management will require adapting Mumbai's development approach to its natural water systems rather than attempting to completely override them.
Until these challenges are addressed, Mumbai's annual flood risk is likely to remain one of the most predictable features of every monsoon season.
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