Mumbai: Number of days with 200-mm rainfall rising, city has no answers

Vehicles navigate a flooded LBS Marg in Mulund on Monday. Pic/Sameer Markande

Even as authorities give the same excuse of ‘unprecedented rain' whenever the city witnesses heavy rainfall, data shows that torrential downpours in a short period of time are not a rare occurrence anymore. There have been at least two instances of very heavy rain every year in the past decade and their frequency increased in the past three years. With the breaking down of city systems becoming a common sight every monsoon, experts have said that it is time for an infrastructure overhaul.

The heavy to very heavy rain may or may not be due to global warming, but it is certainly a regular affair. In the past 10 years, the city was lashed with more than 200 mm of rain every year except in 2012 and 2016 when there was a drought-like situation. The heavy rainfall occurs especially in July and August, though there are extreme weather days in June and September, too. The Santacruz observatory has recorded more such events than the one in Colaba. Sometimes, both have recorded more than 200 mm rain on the same day.

Pedestrians amid heavy rainfall at Western Express Highway at Kandivli on Monday. Pic/Satej Shinde

The frequency of such events has increased since 2019 when the city saw more than 200 mm rain on a single day in every month of monsoon. In 2020, the city saw a heavy downpour on consecutive days in August. This year, there was more than 200 mm rain on Thursday and Saturday night. Though the rain registered was for a 24-hour period, most of it occurred in a three- to four-hour period.

Also read: Mumbai Rains: Lakes get a full month's water stock in two days

Scientific view

A senior scientist with the IMD said that they need data of more than 50 years to make a statement on climate change. IIT Bombay professor, Subimal Ghosh said, "We have studied the precipitation over Asia and concluded that moisture level in the air has increased due to warming and resulted in the high precipitation in sub-daily scale. But there isn't much impact on daily rain as the moisture isn't enough for 24-hour rain." What this means is that there is enough moisture in the air for it to rain heavily for three hours, but not enough for it to rain heavily for 24 hours. The heavy downpour of a few hours results in flash floods in cities, which are difficult to handle.

Change approach

"There is a substantial change in the pattern of rain and we have to accept it. The events which occurred once in four months are now happening once every month. Though the BMC is improving stormwater networks and pumping stations, passive measures also need to be incorporated with these active measures," said Pankaj Joshi, principal director, Urban Centre Mumbai. He added that no city can drain off 100 per cent of rain but with towers and large parking spaces, the city is paving every inch of the land and making rainwater percolation difficult. "We need to improve open spaces on the ground and not on the upper floors. The change cannot happen within a year but the city has to change the approach to deal with the situation," he said.

Activist Nikhil Desai told mid-day, "The British designed the stormwater drainage system for 25 mm of rain per hour, which took care of our city for 50 years. The amount of rain has increased but the BMC has not been able to increase the capacity to 50 mm per hour in 15 years."

‘Collect all water'

"BMC must make rainwater harvesting system mandatory. All gardens, playgrounds, schools, colleges, markets, courts, municipal offices, government offices, towers should be made to absorb rainwater. There should also be underground water storage under all flyovers by digging deep tunnels as they did for the underground Metro. Third, all the garbage ends up in nullahs choking everything. All illegal hutments must be removed on a war footing," said Desai.

"Extreme rainfall events and the quantity of rainfall in short durations have increased. When such extreme rainfall happens and unfortunately, if the timing coincides with high tides, low-lying areas will face waterlogging. The existing systems cannot handle those events in a short duration," said P Velrasu, additional municipal commissioner. He added, "We should try to collect and retain as much rainfall as possible in the same place where it falls. The Sponge City concept is needed to reduce the runoff and to avoid the collection of huge volumes of water. These methods require large amounts of capital expenditure and a trade-off of benefits versus cost has to be understood and decided by policymakers."

Wall to protect filtration plant

The BMC has decided to construct a wall around the Bhandup water treatment plant to prevent flooding in the future. The plant was flooded on Sunday and led to a disrupted water supply. Additional Municipal Commissioner P Velrasu visited the spot on Monday and instructed the hydraulics department to construct a flood protection wall around the complex and also suggested making a flow path to divert flood water coming from Tulsi lake in Sanjay Gandhi National Park. The Bhandup Water Treatment Plant is the largest plant in Asia and about 65 per cent of the total water supplied to the city is treated here.

No. of days Mumbai had more than 200 mm rainfall

>> 2011: 31 July (210 mm), 29 August (233 mm)
>> 2013: 17 June (203 mm), 24 July (218 mm)
>> 2014: 3 July (207 mm), 16 July (228 mm)
>> 2015: 19 June (283 mm)
>> 2017: 30 August (331 mm), 20 September (303 mm)
>> 2018: 25 June (231 mm)
>> 2019: 29 June (234 mm), 2 July (375 mm), 4 August (204 mm), 5 September (242 mm)
>> 2020: 4 August (268 mm) 6 August (331 mm), 23 September (286 mm)
>> 2021: 9 June (222 mm), 16 July (253 mm), 18 July (235 mm)