Flooding occurred in Mumbai on 29th August 2017 as the metro city recorded nearly 300mm of rainfall

during that day. This event comparable to the 2005 floods (944mm rainfall on 26th July 2005) in Mumbai resulted in water-logging, halting of all three modes of transport, power-cuts and some casualties as well. Big cities like Delhi, Chennai and Bengaluru also have slowed down during heavy rains.

How does Mumbai’s drainage network look?

• Massive underground sewers built during the colonial era dating back to 1860-1900 are impressive with some of it still in use, but is limited to South Mumbai.
• The suburbs of Mumbai are served mostly by open drains, into which tonnes of garbage are dumped each year by citizens.
• Pre-monsoon ‘desilting’ exercises are carried out every year, which has proved to be ever-inadequate. 
• Much of these sewer networks flow several kilometers through congested localities before discharging into the sea, often through outfalls located below mean sea level.
• This means if it’s raining heavily when the tide is sufficiently high, drainage is impossible.

How to deal with the Tides?

• The problem is that Mumbai’s flood waters are simply discharged in the Arabian Sea through gravity.
• There are international examples of balancing reservoirs and deep tunnel systems for underground storage of flood waters to be pumped out later when the tides are low.
• Best practices also involve adopting and conserving rainwater within a catchment area itself, through local storage and recharging alongside filtration systems.

What is unusual about the current floods?

• Although, the rainfall was only one-third of the rainfall that caused the 2005 deluge, flood water failed to recede and the reasons remain much the same.
• Dozens of de-watering pumps for localised relief also suffered technical glitches, as conceded by the municipal commissioner.
• While, even after the highest tide had subsided, flood waters in many places did not recede, which is clear validation of clogged drains.

What are the problems plaguing Mumbai’s drainage?

• Insufficient Drains – Although the drainage capacity of the entire city isn’t impressive as such, experts have explicitly conceded that at least major roads and traffic junctions fall far short of the required. 
• Brimstowad Project – After much delay this project got a go ahead, with resources being spent on measures such as standard operation protocol, better-equipped disaster cells and forecasting systems.
• Yet, incredibly, a range of long-term measures, accepted in principle, were left incomplete.
• Contour mapping - Although contour maps for drains were prepared it remains incomplete for the rest of the city.
• This means, although the prediction of the drainage flow is possible, there is no data point to predict how a river’s flood waters will rush out.
• Other Issues - Recommendations to protect the Sanjay Gandhi National Park within the city limits, construct detention basins for flood water, demarcate flood-prone zones were never undertaken
• Also, recommendations to empower the Maharashtra Pollution Control Board to ensure compliance of environmental regulations by municipalities were never undertaken.

What is the impact of river overflows?

• River floods are different from floods caused by undue, sudden pressure on local drainage networks.
• Mumbai’s Context - Mumbai and its immediate suburbs are home to at least four separate river systems.
• Extensive encroachments have reduced their width to barely a couple of metres in some places.
• The arterial Western Express Highway and Lal Bahadur Shastri Marg were both inundated recently, partially because of the overflowing rivers.
• Recommendations - One of the key recommendations of an extensive report submitted after the 2005 deluge was to restore degraded rivers and river-banks and to probe pollution.
• The report also stressed on the encroachment problem, identifying specific boundaries for each river and establishing buffer zones.
• Current Status - Sadly, a grand plan for the rejuvenation of the Mithi, has been abandoned midway.
• A holistic revival of the river systems and early warning systems for has been long ignored.

How has the disaster response been?

• The Positives - Casualties were fewer than in July 2005 and it was apparent that the state’s response has come a long way since then.
• The hotline for connecting various agencies, introduced after 2005, proved critical.
• Also, the depth and reach of social media for dissemination of advisories and real-time updates was found to be tremendous.
• Areas Needing Betterment - Although ‘Doppler radar-based weather forecasting system’ was installed, advisories to stay indoors came too late.
• Also, the CCTV camera network, which can monitor areas of traffic congestion and rising floods, seems to have been activated a little late.
• Improvisation is needed for drafting detailed but precise advisories that are accurate, localised and accessible to everybody.