Flooding during the monsoons in Mumbai has worsened owing to a more than 70% increase in concretised surfaces that have reduced the soil’s capacity to absorb rainwater, revealed an analysis by the Veermata Jijabai Technological Institute (VJTI), Matunga.
Researchers said over the past 45 years, concretisation and filling up of wetlands that worked as flood absorbers has led to a 40% increase in the amount of rainwater entering stormwater drains that have not been rebuilt to handle the increased flow.
“To cater to urbanisation, industrialisation and the enormous growth of population, most of the city is covered in concrete,” said Darshan Sansare, lead author and research scholar, civil and environmental engineering department, VJTI. “Flooding in urban areas, particularly in Mumbai, is owing to these haphazard alterations in land use, land cover, escalation in the amount of precipitation (rainfall) by virtue of climate change and related impacts owing to other hydrological changes such as flooding.”
Integrating geographical information system and remote sensing, the analysis found that built-up area in Mumbai increased from 166.95 sq. km to 285 sq. km (72%) between 1973 and 2018, but with a simultaneous decrease in all other types of land use. Open spaces have reduced by 83% from 61.55 sq. km to 10 sq. km, followed by wetlands from 80 sq. km to 50.67 sq. km (48%), forests from 131.93 sq. km to 100.21 sq. km (25%), and water bodies from 29.74 sq. km to 23.47 sq. km (22%).
The study has also mapped approximately 72 flooding spots in the city. “Significant changes in land-use pattern coupled with high-intensity rain and high tide has led to an increase in flooding incidents in the city,” said professor Sumedh Mhaske, co-author and associate professor, civil and environmental engineering department, VJTI. “This is because rainwater cannot percolate into concrete surfaces, and neither do stormwater drains at present have the requisite carrying capacity to channel it into various waterways and the sea.”
The city’s existing stormwater drain system built during the colonial era can handle only 25mm rain an hour. With changing rainfall patterns, VJTI analysis considered 53.44mm rain an hour and found an increase in rainwater discharge from 3,207 cubic metre per second in 1973 (cu. m/sec) to 4,465cu. m/sec last year. The volume of run-off rain water in built-up areas has increased from 2,353 cu.m /sec to 4,029cu.m/sec, while it decreased in open spaces (from 365cu.m/sec to 64cu.m/sec) and forests (489cu.m/sec to 372cu.m/sec).
But here’s the situation on ground – only six of the 186 outfalls in Mumbai are above the hide tide level that can drain water during a heavy rain high-tide situation. Majority of the outfalls lie below 14 metres, which is the average elevation of Mumbai.
With 135 outfalls above mean sea level, but below the high tide level, 46 outfalls are below mean sea level, and the 2,000km roadside open drain system that are mostly clogged with plastic and waste, the study states “flash floods and high tides cause most of the outfalls to submerge under the sea water leaving it useless for disposal of city water.”
As a measure to mitigate flooding and its impacts, the two-member team recommends the use of pervious or permeable concrete on pavements and sidewalks, which will allow rainwater to percolate into the ground.
“Using pervious concrete will also help recharge ground water and reduce the burden on the existing stormwater drains. It can also minimise the impact of urban heat islands, as the material absorbs sun’s radiation,” said Sansare, who is developing his own pervious concrete mix and plans to test it on campus.
Professor Subhankar Karmakar, Centre for Environmental Science and Engineering, IIT- Bombay, said pervious concrete is a practical solution. “Pervious concrete can be used in new cities that are planned. However a feasibility study needs to be undertaken since Mumbai has a very uneven terrain and is also an unplanned city whose sidewalks and footpaths are encroached upon,” said Karmakar, who was not involved in the study.
