BMC to adopt ‘sponge city’ initiative to tackle urban flooding during extreme rainfall

Every year, after a downpour, the BMC flushes rainwater equivalent to the capacity of Tulsi lake — one of the seven lakes supplying potable water to the city. Last year, on September 25, after “extremely” heavy rain, 8,000 million litres of rainwater — almost equivalent to the supply capacity of Tulsi lake (8,046 million litres) — was flushed out into the sea. In 2019, in two consecutive days in June, the BMC flushed out 13,694 million litres of rainwater into the sea.

The BMC’s Development Plan is currently studying the regulations for new buildings coming up in the city, including green/grassy rooftops and vertical gardens, among others. Some other measures that the civic body is studying include permeable footpaths that store excess runoff water, water-absorbing concrete, wetlands restoration and rainwater harvesting.

“We have accepted the sponge city as a principle. There are many components in the plan, including water-absorbent concrete for construction of road and footpaths and honeycomb water storage structures below the road and footpath… buildings also have to be adopted according to the concept. The basic idea is to re-use or retain the rainwater run-off locally, rather than being carried by extensive drainages to the outfall,” said P Velrasu, additional municipal commissioner (projects) at BMC.

One of the measures of the sponge city initiative — rainwater harvesting — has been mandatory since 2002, for all new building constructions in Mumbai covering more than 1,000 square metres. However, the implementation has been abysmal.

To reduce the dependence on reservoirs and use the rainwater runoff for non-essential purposes, in 2007, the rule was made even more stringent: all new constructions covering more than 300 square metres were required to have in-built rainwater harvesting facilities, without which buildings would not be given occupation certificates.

The idea of the sponge city was mooted by state Environment Minister Aaditya Thackeray last month. Explaining the initiative, he had said, “We have given the go-ahead to the BMC to prepare futuristic plans”.

He also said that a consultant has been appointed to plan and design porous and water-impervious structures wherever possible