Three Reasons for Excessive Water Logging in Mumbai this year By Roshni Udyavar Yehuda

https://www.hindustantimes.com/mumbai-news/mumbai-drowning-under-haphazard-planning-changing-weather-patterns/story-iDcURfCF2m6U4KquqnJSNI.html

This year, Mumbai saw exceptional rains in the beginning of the monsoon season causing water-logging in hitherto newer places. What are the causes? 

There can be multiple reasons for water logging in newer places this:

1)      Excessive concretization – in Mumbai, most of the building construction projects are redevelopment projects. In these projects, there is next to no open or garden space. Every inch of space is utilized for parking. This as compared to a three or four storied building with open ground prior to the redevelopment.

The run-off coefficient of a concretized surface or a hard paved surface is close to 0.9 or 1 whereas gardens and soft scapes have a run-off coefficient between 0.2 to 0.45. In other words, almost 50% of water falling on soft scapes gets absorbed in the ground while almost 100% water falling on concrete surface runs off.

2)      The storm-water system of Mumbai as outlined in the BRIMSTOWAD report is more than 100 years old and is designed for a peak rainfall intensity of 25 mm per hour with run-off coefficient of 0.5. With building and infrastructure construction in independent and individual sites, this storm water system is disrupted. This has lead to huge quantity of water suddently appearing in places where a storm water channel has perhaps been disrupted due to construction work. In addition, the run-off coefficient has also now increased in the highly concretized city.

3)      Lastly, the rainfall intensity in Mumbai has changed (this is my personal observation and should be verified by IMD) – as a reason of global climate change effects as predicted by IPCC. There are longer and intensive spells of rainfall increasing the peak intensity to way above 25 mm per hour. Each and every terrace acts as a concrete surface and as a catchment  generating several hundred thousand liters of rainfall in an hour. The city at present neither has the capacity nor the infrastructure to deal with current and future trends of rainfall.