Mumbai needs to restore flood plains of its rivers at the earliest
By Manoj R Nair
On Tuesday, when Mumbai received its highest 24-hour rain in a decade, the Dahisar River flooded shops, entered homes and swept away cars. The 12-kilometer-long Dahisar drains around 35 square kilometers in the northern suburbs – 6% of Mumbai, but the river, constricted by walls and constructions on its flood plain, was overwhelmed by the downpour.
A flood plain is a low-lying area of land along a river which acts as a natural spillway for floodwater during spells of heavy rain. In cities, where the high price of land makes it impossible to leave flood plains vacant, planners need to incorporate flood plains in their blue prints. Many cities which are prone to flooding are now in the process of restoring the flood plains of water ways that run through them.
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Mumbai has to learn to live with floods. A report on BRIMSTOWAD (Brihanmumbai StormWater Drainage) project explained why the city cannot find a permanent solution to floods. The city receives around 2200 mm of rain every year, 70% of which comes down in July and August. The city’s sewage water drainage (SWD), which stretches along a length of 2,000km – equal to the distance between Delhi and Bangalore – and includes underground drains and nullahs (open channels), can handle 25 mm of rain per hour at low tide. Since all of Mumbai’s drains empty into the Arabian Sea, tides affect the discharge. On August 29, Mumbai received 315mm of rain from 8.30am to 8.30pm – an average of 26mm per hour. The flow of rainwater was impeded by high tide in the afternoon.
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