You leave home on a perfectly ordinary morning. An hour later, the road outside is underwater. Cars are moving slowly, two-wheelers are stranded, and people are rolling up their trousers to cross the street.

It can feel strange. How can one hour of rain turn a familiar road into a river?

The answer lies in how fast the rain falls and what happens to it once it reaches the ground. 

On 24 June 2026, parts of Mumbai received more than 300 mm of rain in 24 hours. South Mumbai recorded 248 mm, making it the city’s wettest June day in more than a decade. Several low-lying areas were waterlogged, disrupting roads and public transport.

A city street, however, can flood even on a day that breaks no rainfall record. 

1. Rain can fall faster than drains can carry it

Think about pouring water into a sink. If you pour slowly, the drain can carry the water away. Pour too quickly, and water begins to collect in the basin.

A city works similarly.

Mumbai’s stormwater drainage system was designed to handle rainfall of around 25 mm per hour in many areasurb, while some newer systems and upgraded drains can handle higher intensities. During exceptionally heavy downpours, rain can reach the streets faster than the drainage network can carry it away. 

That difference matters.

If 50 mm of rain falls over several hours, the drainage system gets time to carry the water away. If much of it falls within a short spell, a huge volume reaches roads and drains almost at once. 

Researchers studying Mumbai’s flooding have repeatedly identified rainfall intensity as one of the major factors behind sudden urban flooding.

2. The city has replaced soil with concrete

Now imagine two surfaces. Pour water onto soil, and some of it sinks underground. Pour the same amount onto a concrete road and almost all of it runs somewhere else.

Cities are........

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