Explainer: Why Mumbai Monsoon Ends Up In A Flood-Like Situation Every Year

Images of roads, railway tracks, and even recently inaugurated underground metro stations facing floods due to heavy rain in Mumbai are all over social media once again. While the chaos is familiar, the reasons for such repeated occurrences are not new. Questions are being asked why these problems cannot be resolved and why Maximum City, Mumbai, has to deal with repeated flood-like situations every year on high rainfall days.

The Dependence On Tides

The efficiency of Mumbai's drainage system is largely dependent on tides, and if heavy rains and high tides coincide, then the water must be pumped out through pumping stations that have been built. Mumbai's natural drainage and flow of water work only when the tide is low. During high tides, lock gates are used to stop the sea water from entering the city, and that is why, even with pumps functioning, it takes time to pump out all the water that is collected in low-lying areas.

Topography: A Collection Of Seven Islands

Mumbai was originally a collection of seven islands. The land was reclaimed to form one continuing land mass, but that led to the creation of low-lying areas where rainwater collects during heavy rainfall, which is then pumped out through a network of stormwater drains and pumping stations. But the excess water collects in the low-lying areas before it is pumped out through pumping stations that have been set up.

Changing Rainfall Pattern A Huge Worry

Over the last few years, Mumbai's rainfall pattern has reported sharp changes, and has led to more rain over a shorter period rather than being spread out all through the Monsoon season. The stormwater drains, even with built-in redundancies, cannot cope with heavy rainfall.

With heavy rain in the last 24 hours, Mumbai has also broken a 107-year-old record of rainfall recorded in May. The Colaba weather monitoring station has recorded 135.4 mm of rainfall in the 24 hours ending 8:30 am on Monday, which is categorised as "very heavy rain" (115.6 mm - 204.4 mm).

The recordings from Colaba Monitoring Station also suggest that the city has broken the record for rainfall registered in May, logging 295 mm of rainfall, crossing the old record of 279.4 mm recorded in May in 1918.

The weather department said, "The Southwest Monsoon has advanced to Mumbai today, against the normal date of advancement, June 11. Thus, the monsoon has arrived in Mumbai 16 days earlier than usual. This marks the earliest monsoon advancement over Mumbai during 2001-2025."

Critical Pre-Monsoon Work

An early onset of monsoon can bring its own set of challenges for a city like Mumbai. Unclogging of drains and desilting works follow a fixed schedule, and incomplete work led to a situation where the drainage system is not at optimally.

With monsoon arriving early this year, the BMC said drainage cleaning operations have not been completed yet, leading to a flood-like situation in the city.

Pre-monsoon works are critical for Mumbai city, and this year the schedule has been interrupted with the early arrival of monsoons, officials said.

The Incomplete Drainage Project

The Brimstowad drainage system, which was unveiled after the 2005 Mumbai flood that killed 1094 people that year was aimed at overhauling Mumbai's 19th-century drainage system, is still incomplete. The drainage system has seen considerable additions to augment capacity over the last decade by widening stormwater drains and adding pumping stations.

However, it is yet to be completed, leaving critical gaps in the drainage system. The authorities blame the delay on encroachment and litigation, but the delay in this project leads to Mumbaikars suffering every year.