Mumbai’s flood management infrastructure a boon for city – Hindustan Times

Mumbai ranks fifth among the world’s cities which are prone to flooding, recording annual losses amounting to USD 284 million. As a tropical coastal city, Mumbai has always received torrential rain during the monsoon and historically had a dense stormwater management network. However, this network, which is a colonial legacy, is largely restricted to the island city. The suburban district is largely dependent on surface drainage along roads that leads to a system of nallahs that drain into one of four rivers or directly into the Arabian sea.

Mumbai’s stormwater network is structured by a hierarchy of large to small drains. Of the 2,770 kilometers of arched or boxed drains, that form the major stormwater channels, 80% are in the city and 20% in suburbs. These are fed by a combination of closed pipes and road-side drains. Of the total 565 kilometers of closed pipes, 78% are in the city, and 22% in suburbs, while 98.6% of all road-side drains are in the suburbs and only 1.4% are in the island city.

Mumbai’s drainage system is designed on gravity, hence changes in the natural topography create isolated pockets of waterlogging, which during an extreme event can be quite severe. To assist outflow during the high-tide or on stormy days, when the tide is unusually high, there are four pumping stations built along the coast, and four under-construction.

The Brihanmumbai Municipal Corporation is investing heavily in flood management infrastructure like the Hindmata holding tank and smaller interventions like mini pumping stations to better manage flow into the city’s drains. The city’s stormwater drainage department is one of the best and heavily funded entities in the BMC, with a committed staff of engineers.

The four major rivers in the city – Mithi, Dahisar, Poisar and Oshiwara – are all engineered into functional stormwater drains with high retaining walls and modern technologies for desilting the riverbeds and ensuring steady flow and capacity. All natural edges and landscapes, that perform a critical role of buffering and retaining water, have been disconnected from the river. Any open areas along the river are then taken up for beautification to avoid further encroachment, but these landscapes are passive; they do not allow the river to breathe, swell and contract as needed during heavy rain days.

Historically, the city’s natural infrastructure has always played a critical role in flood management and assisting stormwater run-off. These comprise of the rivers, nallahs, marshy lands and mangroves along the coast, and the city’s forests and open spaces that perform the much-needed function of retention. Over the years large areas of natural landscape have been taken over for development, resulting in a run-off coefficient of almost one, which means almost all the stormwater of Mumbai must be drained into the sea. As a result, Mumbai’s natural absorptive capacity is highly compromised.

For tropical cities like Mumbai, at risk of coastal inundation and annual rainfall extremes, building hybrid infrastructure is critical. Engineered solutions, that are hard edged, have limited capacities and perform no ecological functions. Instead, hybrid solutions that are engineered sensitively, can additionally support key ecosystem functions like protecting biodiversity, creating alternate livelihoods, connecting people to natural spaces, and buffering the city from the frequent, ‘unprecedented climate events’.

Lubaina Rangwala is program head, urban development and resilience, World Resource Institute, India and Sahil Kanekar is senior associate, urban development, WRI india