Explainer: What makes Mumbai flood every monsoon? Clogged drains, rivers and receding mangroves likely reasons

The first showers of this year’s monsoon just about hit Mumbai and India’s financial capital went under. If you are a Mumbaikar, the question you would want urgently answered is this: can Mumbai ever welcome the rains when it doesn’t cause flooding and bring the maximum city to a standstill? There are key factors at play that you should know about while waiting for the waters to recede and the city to up its drainage game. Built-up Area As the primary factor behind frequent flooding, one needs to look no further than the frenetic pace of construction in the city. Mumbai is in a constant state of expansion, laterally and vertically, and large-scale construction activity is a constant for the city. This impacts flooding in two ways: first, as more and more natural space gets converted into a built-up area, the natural ability of the land to absorb water and prevent it from collecting in a place is lost. Second, the debris and waste generated by construction activity clog up the drains and nullahs, preventing the run-off water from escaping. It was a couple of years back that protests over the Aarey metro shed had rocked Mumbai. Then there is the coastal road project that is expected to have serious ecological ramifications. Low-lying localities According to a 2019 report by Kanchan Srivastava and Aditi Tandon for Mongabay, the trend of reclamation of land that the city has been historically a witness to has also played a part in choking it up when heavy rainfall strikes. The land area of the city swelled by almost 50 sq km in less than three decades since 1991, the report said. But the real issue is, of course, that most of the reclaimed land is essentially low lying and, hence, flood-prone. The average height from sea level along some stretches is less than one metre, the report said. It doesn’t help when torrential downpours are accompanied by high tides. Clogged rivers Once these would have done the work of stormwater drains, but illegal construction, encroaching, and pollution have turned Mumbai’s rivers into shadows of their former selves. According to the Mongabay report, “one of the major rivers, Mithi, has become a veritable sewer, choked with domestic and industrial waste and overflows every monsoon”. The situation is similar in the other rivers like Dahisar, Poisar and Oshiwara. Not only that, the wetlands along these rivers are now practically non-existent, which means that there is no buffer between the waterline and adjoining localities. When the rivers overflow, they automatically lead to flooding in such localities. Stormwater drainage A 2019 report by the Comptroller and Auditor General (CAG) on the flood preparedness of Mumbai had pointed to “major deficiencies” with the drainage system. One of the factors it had flagged was the delay in upgrades to city’s floodwater drainage system. The Brihanmumbai Storm Water Disposal System, or BRIMSTOWAD project, was conceptualised in 1993 but was reportedly taken up only after the devastating 2005 floods that hit Mumbai. The works to complete the project are said to be ongoing even as the CAG had highlighted serious issues in the report. Among these were heavy silting of the drains and obstructions caused by construction and dumping of waste. An added worry, the report said, was that the major outlets to flush the water away were all below sea level, which means that on high tide or heavy rainfall days, these drains are incapable of clearing out the water. The report had also noted that the capacity of the drains was to deal with rainfall of the level of 25 mm an hour. But the reality is that as of 2019, work was still ongoing to increase that capacity for rainfall of 50 mm per hour. Mangrove forests Alleged deforestation of coastal mangrove belts has also been cited by environmentalists as a big contributor to frequent flooding. The Mongabay report had cited research by Mumbai-based environmental group Vanashakti that the city had “lost 40 percent of its mangrove forest cover between the early 1990s and 2005”. Although that estimate was contradicted by the government’s State of Forest Report 2017 that their spread had actually increased since 2015, activists say that the present mangrove cover is not adequate for a city that is one of the wettest of India’s metros.