Mumbai monsoon: Understanding BMC’s flood mitigation measures and what to expect

Every June, Mumbai prepares itself for floods. Measures by BMC have reduced flooding in several areas but the problem still persists. Why?

“It used to flood a couple of years ago. Water would get into the bank on the ground floor of my building. But now there is a pump to clear out the water. So it doesn’t flood, unless there is a lot of rainfall,” says Shubhalaxmi Burde, a resident of Sleater Road in Grant Road. But she remembers a time, when she moved as a new bride in 1978, she discovered that people in the area had to use small boats to get around because of the flooding.

Nana Chowk  at Grant Road and the area around it, has been a waterlogging hotspot for several decades. And Mumbai had such hotspots all over the city, which would flood every monsoon. However, according to the civic officials, the number of hotspots has reduced to 84 from 440 over time.   

In this two-part series Citizen Matters explores the factors that cause flooding, various measures adopted by the BMC, their efficacy and information that people need in case of waterlogging and floods in their neighbourhoods. This is the first part.

Why does it flood in Mumbai?

The answer to this is complex and multilayered. Mumbai is made of seven groups of islands and the suburban district comprises four islands. This reclamation, which started in the 17th century, continues to date. These islands have been joined via reclamation projects and the building of causeways.

That’s why Mumbai has a low lying coastal tract, with some areas lying on an average 1 metre above sea level, or 1.5 metres below the high tide level. Such areas are particularly vulnerable to floods, especially during high tides.

According to the Flood Preparedness Guidelines 2023 brought out by the Disaster Management Department of the Brihanmumbai Municipal Corporation (BMC) there are three types of floods:

  • Localised flooding due to inadequate drainage
  • Flooding due to overflows from Mithi river
  • Flooding due to a combination of high tides and high river flows

Many factors are responsible for the inadequacy of the stormwater drains to handle the rainfall. According to an official of the Storm Water Drains (SWD) Department of the BMC, the drains had been built for 25 mm per hour of rainfall. He also said that instead of moderate rainfall over a longer period, we often see a heavy downpour for a short period. This fills up the stormwater drains that much more quickly.

Changes in climate patterns and increase in the number of extreme rain events have rendered the drains insufficient to tackle the overflow of water.

Following figures show the number of days when Mumbai received more than 65 mm rains.

YearColaba rainfall greater than 65 mmSantacruz rainfall greater than 65 mmAverage

Mumbai Climate Action Plan, a report prepared by the Mumbai Metropolitan Area Development Authority and published by the BMC, says extremely heavy rainfall events are on the rise since 2017.

It also states that 35% of Mumbai’s population is at flood risk, with H/E, H/W and F/N wards being the most susceptible with 60% of the people here exposed to the risk of flooding. The H/E ward has areas such as the Milan subway, the Kala Nagar Creek, Mithi River and Santacruz/Bandra station. The H/W ward extends north-south from Milan Subway and SNDT college to Mahim Causeway. F/N ward has areas such as NS Mankikar Road in Sion, Mumbai Marathi Granth Sangrahalay Road in Dadar east.

Increased concretisation

Experts say that water can percolate at fewer places than before due to persistent and rampant urbanisation. Moreover, roads are also being concretised and concrete is waterproof. Even playgrounds, which allow water to seep through are being covered with astroturf. Societies and apartment complexes are trading open, permeable and unpaved areas for parking spaces and other amenities.

All this results in more pressure on storm water drains to carry water into the sea. Furthermore, if the nullahs have not been de-silted adequately, it hinders the flow of water, aggravating the issue.

Read more: Climate change: Rising sea levels ring alarm bells for Mumbai

When do floods take place?

The deluge which hit Mumbai on 26th July, 2005 is etched in the memories of Mumbaikars. On that day the Santacruz Observatory recorded 944 mm of rainfall. Compare this to the annual average rainfall in India is 1200 mm. Along with the cloud burst, it was also a day of high tide, which resulted in unimaginable loss of lives and property.

Although that was rare tragedy, the city grapples with floods on several days in monsoon. If two events coincide it creates a potential flood situation—high tide over 4.5 m and very heavy to extremely heavy rainfall.

At such times, floodgates, which allow water from the drains to drain out into the sea, get reverse flow of water. Water from the sea flows into the drains, thus exacerbating the water logging, explained an official from BMC’s Disaster Management department. In such a situation, the floodgates are shut to prevent seawater from entering into the drains and rainwater collecting in the city is pumped into the sea.

The India Meteorological Department has given the following classification as per the amount of rainfall received in a 24 hour period:

Sr. noTerminologyAmount of rainfall
Colour of alertMeaning of the colour
1Very light rainfall0.1 – 2.4Green alert (less than 64 mm rainfall per)
No warning (No action)
2Light rainfall2.5 – 15.5
3Moderate rainfall15.6 – 64.4 
4Heavy rainfall64.5 – 115.5
Yellow alertWatch (Be updated)
5Very heavy rainfall115.6 – 204.4 Orange alertAlert (Be prepared)
6Extremely heavy rainfall Greater or equal 204.5 mmRed alertWarning (Take action)

How is water drained out of the city?

A network of stormwater drains, which is more than a hundred years old, is in place in Mumbai to carry surface runoff into water bodies such as oceans, rivers and creeks. A total of 183 outfalls drain water into the Arabian Sea, Mahul Creek and Mahim Creek.

BMC’s Flood Preparedness Guidelines say, “The southern city area comprising relatively large low-lying areas has long complex networks, while short drains from small areas discharge directly to the sea. In the city, the drains are covered by roads whilst there are open drains in the suburbs.”

Pramod Mahajan Kala Udyan
Huge pipes carry water from areas like Parel and Dadar to the holding pond at Pramod Mahajan Kala Udyan and then it is pumped into the sea. Pic: Shruti Gokarn

The topography of Mumbai combined with the construction activities has led to some chronic waterlogging hotspots such as Parel, Hindmata cloth market and the surroundings of Milan subway at Vile Parle. Recognising the need for extra steps to curb flooding in this area, the BMC has set up holding ponds here.

In the first phase, they set up a holding pond of 1.05 crore litres below the ground of St. Xavier’s school at Parel and another pond of 1.62 crore litres at Pramod Mahajan Kala Udyan at Dadar.

Excess rainwater is pumped into the ponds and then slowly released into nullahs. The holding pond at Milan subway can hold 2 crore litres of water.

Besides this, the BMC has six big and 10 small pumping stations. It also installs temporary portable pumps wherever required.

Read more: Time to seriously implement already-existing solutions to make monsoons bearable, says a young citizen

Are BMC’s flood prevention measures effective?

At the Milan Subway in Vile Parle, Imran Bhati, the owner of a marble shop right outside the subway said, “The subway does not flood since they have set up the pump and the pond. Earlier they had to put up a bridge-like thing in the subway for the vehicles to cross to the other side.”

A cloth merchant at Hindmata market echoed saying that the extent of flooding has reduced, but added that water entered his shop just two weeks ago. He also pointed out that last year rainfall was less, and wondered how the tanks will hold up if it rains heavily this monsoon.

Milan Subway
A holding pond at the Milan Subway has tackled the problem of chronic water logging, say shopkeepers in the area. Pic: Shruti Gokarn

Localised flooding, especially in informal settlements continues to be a problem. Lubaina Rangwala, the Programme Head of Urban Development at WRI, praises the BMC engineers for their ground intel. However, she is sceptical about the hotspot strategy, where the engineers inspect hotspots, identify the cause for the flooding and fix that problem. It could be less percolation due to new construction, blocked gutters or inadequate drainage capacity of the area and so on.

Lubaina says, “The hotspot then gets relocated to some place else, maybe 10 -15 metres down the line. It doesn’t holistically solve the problem at that catchment level, because we don’t have the data to actually support the understanding of the catchment. We don’t really know how the topography has changed over the years in the city. And the stormwater drainage network is largely unmapped in many places.”

She says that better data collection and mapping of drains will equip city administrations to manage localised flooding and waterlogging better.

With more and more infrastructure projects coming up in Mumbai, the city will need a visionary approach to resolve the problem of waterlogging and floods.

(Part two of the series will focus on information that citizens can use in the event of waterlogging and floods.)

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