Sea of plastic: Mumbai’s flood risk cannot be managed without treating sewage and stormwater, says Shankar Deshpande

Mumbai’s 2005 floods changed how the city understood the link between extreme rainfall, high tide, river capacity and waste management, says Shankar Deshpande, chief of the Mithi River Development and Protection Authority. 

In this interview with Down To Earth, he explains why the authority was created, how solid waste and silt can worsen flooding, and why desilting, widening, sewage treatment, community rehabilitation and source segregation are all central to protecting the Mithi river and reducing flood risk.

Akshit Sangomla (AS): Why was the Mithi River Development and Protection Authority created?

Shankar Deshpande (SD): The 2005 flood changed the way we looked at flooding, stormwater disposal and other such extreme climate events in the Mumbai Metropolitan Area. On 26 July 2005, unprecedented rainfall of 944 mm in 24 hours coincided with a high tide of 4.48 metres in Mumbai. The alarm this created was that we needed to look at our rivers differently.

The Maharashtra government then constituted the Mithi River Development and Protection Authority (MRDPA), initially to look after the Mithi river, and later expanded it to the four other rivers that drain Mumbai, covering around 8,000 hectares. A separate authority was required to deal with flooding, as we understood that extreme rainfall and high tide had caused the damage in Mumbai.

The MRDPA was responsible for the 6 kilometre stretch of the Mithi river under the Mumbai Metropolitan Region Development Authority (MMRDA) and the 11 km under the Municipal Corporation of Greater Mumbai (MCGM).

The Mithi river starts with the overflow of Vihar lake coming into Powai lake and then meets Mahim Bay downstream. The river passes through the most densely populated areas of Mumbai. It also passes beneath the runway of Mumbai airport. It collects sewage and stormwater from the Jogeshwari-Vikhroli Link Road, the Maharashtra Industrial Development Corporation area, Kurla and the Bandra Kurla Complex.

The MRDPA undertakes master planning for the river to ensure water quality and flow, the velocity of discharge, enhancement of carrying capacity by widening and deepening the river, and prevention of encroachment, so that the flooding event of July 2005 is not repeated. This is why it was constituted.

AS: What role does waste management play in flood management of the Mithi river?

SD: If you look at the topography of the Mithi river, it has a steep slope from its origin near Vihar lake up to Mumbai airport. From there to Mahim Bay, the slope is gradual. Water flows with greater velocity when there is a steep slope. Whatever debris the river carries comes quickly to the airport and then slows down as the slope eases.

The other factor is the high-tide and low-tide phenomenon from the sea. The high tide can reach up the gradual slope up to the airport, which is a length of about 6 km. In the remaining 11 km, there is no tidal effect.

The river is not perennial, but during the four months of the monsoon it collects the overflow from Vihar lake and Powai lake. At that time, there is water from overflow, water from rainfall and, when there is also a high tide, it leads to what we call level rising of the river.

Mumbai’s height above mean sea level is not much higher than the level of the high tide. The Bandra Kurla Complex is about 5-6 metres above mean sea level. There is floating material on top of the rising river, including soil debris, plants, trees and discarded waste. The soil and other material lead to siltation in the river, which further adds to the level.

Waste also clogs stormwater chambers and blocks the flow of water. During low tide, floating waste gets deposited on the riverbed. So we desilt the river and clean the chambers so that water can flow freely and disaster can be averted.

AS: What steps were taken to manage the waste entering the Mithi river and avoid flood disasters?

SD: After the 2005 floods, the Government of Maharashtra appointed multiple committees and institutions to look into the problem. One of them was a fact-finding committee, known as the Chitale Committee, to understand the causes of the floods and what could be done to avoid such a situation in the future.

The state government also appointed the Indian Institute of Technology Bombay to look at the environmental improvements needed in the Mithi river catchment area. The government appointed the Central Water Commission and the Central Water and Power Research Station to carry out actual modelling of the river for different combinations of rainfall intensity, rainfall duration, high and low tides, and riverbed slopes.

The findings of these committees and institutions helped us determine the most appropriate parameters for deepening, widening and riverbed slopes, so that any combination of high tide and rainfall could be planned for across the entire topography of the Mithi river.

We also realised that when you widen the riverbed, you need to remove encroachments, and sewage and stormwater entering the river need to be treated. If this water is not treated, there is less dissolved oxygen and higher biological oxygen demand and chemical oxygen demand. This leads to the absence of flora and fauna in the river.

AS: Did these steps help avoid flooding in the Mithi river?

SD: The Central Water and Power Research Station carried out another study after we deepened and widened the river to assess the results of our efforts. They found that the results were very good.

The carrying capacity of the river increased by two times, the holding capacity increased by three times, and the velocity from the bed slopes increased, which helped water flow more easily into the sea. Overall siltation of the river also came down, and flora and fauna flourished in the river. The density of mangroves near the Mahim causeway increased because of the rise in dissolved oxygen.

Another challenge was convincing communities not to litter in the river and also rehabilitating many families. We took the help of the social sciences department of IIT Bombay to reach out to communities and rehabilitated around 10,000 families from close to the river.

We sensitised them to waste management as a critical part of flood regulation. The best way forward was segregation of waste at source, which would make the effective discharge of floodwaters possible.

AS: How is coordination ensured between the different organisations responsible for waste management and flood control of the Mithi river?

SD: The MRDPA is chaired by the chief minister of Maharashtra. We had frequent meetings after the flood to design the master plan.

The four organisations involved in the management of the Mithi river are the MMRDA, MCGM, the civil aviation authority and the pollution control board, apart from the various organisations that prepared reports and the courts.

We had an empowered committee at the chief secretary level, which was responsible for coordination between all these organisations.

AS: What is known about the major sources of litter entering the Mithi river?

SD: Liquid waste and solid waste both enter the river. Liquid waste mainly comes from sewers carrying domestic waste, which could be treated or untreated, surface runoff during the monsoon and industrial effluents.

Solid waste in the river comes from whatever is left unattended or discarded along the runoff. From there, it flows to the nearest stormwater chamber, then to the nala and from the nala to the river.

Solid waste gets collected everywhere for seven to eight months, and when it rains, most of it flows into the river, which eventually drains into the Arabian Sea.

AS: How can the discarding of solid waste be prevented?

SD: Sensitising society not to throw solid waste anywhere becomes important for its management, and we have received encouraging results from communities.

Even in informal settlements, bins are now placed for people to dispose of their solid waste, and collection is very effective. The MCGM collects around 8,000-9,000 metric tonnes of solid waste from the city efficiently. As efficiency increases further, rivers will become cleaner.