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Sunday, Sep 22, 2019

Mumbai needs to restore flood plains of its rivers at the earliest

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

mumbai Updated: Sep 03, 2017 21:35 IST
Manoj R Nair
Manoj R Nair
Hindustan Times
People and motorists wade through knee-deep water at Andheri  during the downpour on August 29.
People and motorists wade through knee-deep water at Andheri during the downpour on August 29. (HT Photo)

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.

One small city in US’s Washington state, which faced routine floods in the creek that runs through it, bought homes and properties in the low lying areas of the city, tore down the buildings and created a basin for flood waters to disperse. Another town excavated the flood plain of its river to create a channel for flood waters and ground water recharge. The restored flood plain was planned to be converted into a natural wetland, with local species.

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.

Environment experts have said that Mumbai can reduce the impact of heavy rains if its restores sections of the flood plains of its rivers. Apart from the Dahisar, the Mithi, Poinsar and Oshiwara flow through the city. The Mithi has a catchment area of 73 square kilometres, which means it has to carry rain water run-off from 12% of Mumbai’s area. In Mumbai, where land is expensive, buying properties from private owners to create flood plains can be difficult. Environment experts said there are solutions to this problem.

Dr Rakesh Kumar, environment scientists from the National Environment Engineering Research Institute, said, “The National Green Tribunal and the Supreme Court have given orders against construction in flood plains. If there is construction in the flood plain then do engineering to remove the problem or remove the obstruction,” said Kumar. “Anything else is a compromise.”

Kumar said that local planning authorities should define the course of rivers passing through the river. “Definition of flood plains and demarcation is required if land planning is to be done,” said Kumar, The problem, he added, is when, after demarcating the course of the river, local authorities build retaining walls along the course.

Both the Mithi and Dahisar have been constricted by walls. Environmentalists have reported that construction of a retaining wall along the lower reaches of the Mithi at Bandra-Kurla-Complex by the Mumbai Metropolitan Regional Development Authority – the regional planning authority – is destroying aquatic vegetation. Walls have been constructed at the upper reaches in Aarey. Though Dahisar has fewer such barriers, there are concrete walls along some sections. “Building a retaining wall is constricting a river. The retaining walls are not to be constructed. Every river has its own character and one formula does not fit every river,” said Kumar. “If it has to be rejuvenated please revert it back (to its natural state). If there are some difficulties engineering methods can be used.”

First Published: Sep 03, 2017 21:04 IST