To mitigate Mumbai’s flooding woes, the Brihanmumbai Municipal Corporation (BMC) is set to construct a series of bioswales in Mumbai, as part of its Rs 12,705-crore proposal submitted to the National Disaster Management Authority (NDMA) to re-engineer city’s aging drainage infrastructure and enhance its climate resilience.
A bioswale is a vegetated shallow areas or ditch designed to naturally filter and treat accumulated rainwater or stormwater by removing pollutants such as silt, oil or construction debris. These bioswales comprise a sunken base with slopes through which the accumulated rainwater gets collected in the base and seeps underneath, recharging the ground water levels.
Beside bioswales, the amount will be utilised for construction of a sponge park for Rs 200 crore, setting up of sluice gates at Rs 2,000 crore, construction of permeable pavements at Rs 120 crore, as well as construction of pumping stations at Rs 700 crore.
The move comes as Mumbai continues to grapple with increasingly incidents of intense rainfall. According to BMC data, the city has seen a sharp rise in downpours with the average very heavy rainfall within a 24-hour period climbing from 132 mm to 182 mm over the past six years. This surge in precipitation has led to frequent flash floods, disrupting life and transport across the city. Between March and August this year alone, Mumbai was repeatedly brought to a halt due to relentless rain.
Surface water
Being an island city, Mumbai has an array of chronic flooding spots. Speaking to The Indian Express, Abhijit Bangar, additional municipal commissioner (projects), said the idea behind the project is that if surface water during heavy rainfall reduced, it will reduce waterlogging in low-lying areas.
“Slow rainfall creates maximum percolation, but high rainfall doesn’t allow smooth percolation of water because the surface of Mumbai has become so impervious with time. The rainwater that doesn’t percolate get accumulated and create waterlogging. Therefore, our target is to increase Mumbai’s percolation capacity,” Bangar said.
“Creation of bioswales is a cost-effective measure. Once they become operational, the percolation capacity of Mumbai’s surface will increase. Even if we are able to increase the percolation by 10 per cent, then it will reduce the overall waterlogging in Mumbai by a significant margin,” he added.
In total, the BMC intends to set up 800 bioswales in Mumbai’sparks and open spaces, however, initially they will be set up in chronic flooding areas like Dadar, Matunga, Santacruz, Andheri and Vakola where more waterlogging is reported.
“Our initial plan is to set up the bioswales in traditional flooding spots or low lying areas. We are setting them up in gardens, because in Mumbai there is a scarcity of open land. The idea is to earmark a portion of land in a civic garden and then to modify it to a bioswale so that it increases percolation of water, in a way that the intensity of waterlogging in a given area is reduced or brought under control,” said Bangar.
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He added that the overall project will be funded by the Union Home ministry — the parent department of the NDMA. Bangar said that earlier this week, BMC officials had a meeting with the department officials and early next year, a team is likely to visit Mumbai to verify the plan. Following the visit, a detailed project report for executing the plan would be made.
Likely outcomes
Speaking to The Indian Express, civic officials said that initially Rs 100 crore has been earmarked for this project with the construction of each bioswale costing Rs 12-15 lakh.
“Patches of bioswales will also be created along the roads and traffic medians. This will slow down the rainfall stormwater and direct it into the drains after filtration,” an official said, adding each pit will comprise native vegetation and layered filtration comprising sand, gravel and geotextile.
Civic officials said the bioswales would reduce micro-flooding or waterlogging in a given natural area, increase ground water retention capacity, improve biodiversity pockets, act as natural dust suppression mechanism as well as lead to cleaner footpaths and pedestrian zones.
“Once an interconnected network of bioswale is activated, the city will start seeing bigger results. Firstly, due to improved percolation of ground water, the intensity of waterlogging will reduce, secondly due to improved ground water level the surface temperature may become cooler,” a civic official said.
