Increasing erratic rains posing significant challenge to flood mitigation before every urban area

Urban flooding is a phenomenon which is being observed around the world. It has many contributing factors and it is also crucial to look at how this flood water gets discharged. Following showers, the rainwater will try to discharge by percolating through the ground or it will run off through drains.

In Mumbai, currently the drains have a capacity to accommodate rainfall up to 55 mm per hour. This means that if the drains are absolutely clean, they will accommodate up to 55 mm of rain per hour. However, once the rain crosses the threshold of 55 mm per hour, water from the drain will start spilling outside and trigger flooding.

In Mumbai, flooding also has several other factors, one of which can be inadequate desilting. Further, despite desilting, people keep dumping garbage into the drain, choking the drains in some points which reduces the carrying capacity of the drain.

Another flooding factor in Mumbai are low-lying areas, as in these areas, rainwater cannot get easily discharged into a drain. In Mumbai, there are nearly 10-15 percent areas which are recurring flood spots like Hindmata and these encompass low-lying areas. The best solution in these pockets is pumping. Having identified such recurring flood-prone spots, the BMC has taken steps in some places through pumps to control the problem. However, in cases of extreme weather events, these areas still end up experiencing waterlogging, raising questions.

Between August 18 and 19, the city essentially came to a standstill. Besides these low-lying areas, several other areas were flooded while local train services were also disrupted. What triggered this intensive flooding?

During the August 16 to 19 period, all BMC teams were on field. We were responding in real time and all our pumps were deployed and 100 percent operational. Unlike May 26, when we were caught offguard, we were well prepared this time as we had received forecasts from IIT Bombay. Further, the intensity of high tide was also much lower last week, as it remained under four metres.

Despite this preparedness, one of the biggest causes for waterlogging on August 18 and 19 was extremely heavy rain with some areas even observing 400 mm in a single day. As per our data, on August 18 and 19, we received over 487 mm in Chincholi station, followed by 481 mm rain in Vikhroli West. Apart from this, 427 mm rain was recorded in Dadar, 397 mm in F/North. This was followed by another intense spell on the next day when more than 200 mm rain was recorded across the city between August 19 and 20 morning.

Such a heavy spell is very unprecedented, as we even observed over 300 mm rain within six hours in some areas. This intensity is massive, by any standards.

In case of such heavy intensity rain, no matter how many pumps are installed, it may prove insufficient. Even our drains, which normally have a capacity of 55 mm rain per hour, were receiving over 400 mm rain, triggering waterlogging on these two days. Therefore, when we look at flooding that was reported during those days, it is also crucial to look at the data of rainfall and realise the severity of the weather event.

This year, we also experienced extreme flooding in the low-lying areas along Mithi River, where the levels nearly breached the danger mark. What triggered flooding at Mithi River ?

Even though our systems worked efficiently in Chunabhatti and Bhandup, railway services were affected severely and one of the key causes was flooding at Mithi river. We observed that water did not recede from the railway tracks and the slum areas nearby for at least six hours. The reason was that on that day, the Vihar lake was flooding and flowing over 2 feet above its wall. Vihar is at the mouth of Mithi River and it overflows into Mithi River. That day, even though the rain subsided over Mithi River’s catchment area in Kurla, the rain at Vihar was so heavy that its overflow kept flowing continuously.

As the lake continuously overflowed into the river, the level of Mithi River kept on rising and it reached up to 3.9 metres on August 19. After this, it receded up to 3.4 metres but again, it rose up to 3.6 metres.

Since LBS Road runs along Mithi river at a turn, water from the river first flooded onto LBS Road. From here, the water flooded onto the railway tracks, which have three stations next to it–Kurla, Sion and Chunabhatti. Therefore, the flooding from Mithi affected both Central and Harbour railway network. Moreover, the water did not recede owing to the surge at Mithi River. It was only after the level of Mithi River came down below 3 metres that the water started subsiding.

In the past few years, the average rains recorded during monsoons in Mumbai has increased considerably. Is the city equipped to handle the pressure of these extreme rain events and what lessons has the BMC taken from the floods reported this monsoon season?

Empirically, we have seen that the occurrence of extreme events is much more frequent now. Until now, we have devised our systems on the basis of the average as the probability of extreme events has been much less. Everything comes at a cost and therefore we plan our infrastructure upgrade as per the data backed by historical evidence.

Due to the increasing erratic rains, there is a significant challenge of flood mitigation before every urban area.

To be able to mitigate these challenges, we aim to modify our systems and therefore we are looking at what is happening world over and then also take expert opinion to devise future plans.

At present, the BMC has plans to increase the capacity of the Storm Water Drain (SWD) network, but one intervention is simply not enough.
To mitigate flood risks, the BMC is looking at two types of measures, structural and non-structural. The structural measures include SWD augmentation, creation of robust pumping infrastructure at flooding points like in Hindmata and underground tunnels for water discharge. At present, BMC is working on a similar tunnel along Mithi River from Safed pool nalla to Dharavi.

Meanwhile, we are also exploring non-structural interventions which are less expensive and eco-based. In Mumbai’s context, Mangrove restoration is significant, while other non-structural interventions include bioswales and sponge parks which have been implemented in places like Japan.