One might wonder why our urban centres — Mumbai, Delhi, Kolkata, Hyderabad, and several other cities — are getting flooded more often in the recent past. The reasons are many, ranging from inefficient drainage systems to the implications of climate change.

Historically, when cities were developed, artificial drains were designed to carry the stormwater from rooftops, streets, and roads to natural drains or water bodies so as to avoid flooding of local areas. It is important to understand that the volume of runoff that is generated in an area depends on land use. Another important aspect to understand is that drainage systems are designed for a specific magnitude of rain. In case a city experiences more than the estimated rainfall, some areas will inevitably be flooded. In our country, the return period for drain design with respect to flooding has traditionally been one in two years — this means that inundation of some areas in a city is inevitable every alternate year. The extent of flooding would depend on how much the actual rainfall exceeds the estimates made while designing the drainage system.

Another important fact is that traditionally our cities had a large area which was capable of absorbing a considerable part of the runoff. But over the years, every city has, by design or illegally, allowed more area to be paved, concretised, or consolidated. This has resulted in the generation of more stormwater than what was factored in while designing drains — this strains........

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