The increasing frequency of intense rainfall as witnessed in Chennai is changing traditional approaches to drainage with “retention and detention” strategies that reduce peak flows and encourage local storage finding increasing favour with urban planners. Though the 347 mm rain — in contrast to the more usual 53 mm — that inundated Chennai in the past six days can challenge most sewage and drainage systems, mapping the urban needs of an area can help reduce pressure on the drainage system during normal rainfall and well as during surges.
The traditional approach to drainage aims to remove water as quickly as possible from the point of collection but this can prove counter-productive during intense rainfall and in areas that might get wetter due to the effects of climate change.
Countries such as Singapore that have geared their water collection to harvest as much of the runoff as possible are moving to reduce peak flows by as much as 55% by using detention tanks. The city state’s active, beautiful, clean (ABC) waters programme looks to both store and treat water near locations such as residential areas and parks.
Newer constructions are encouraged to harvest water for local use such as cleaning, gardening and air cooling.
Storm water is channeled through parks that use natural media to filter water on its way to collection tanks and eventually reservoirs. This process is, of course, dependent on strong separation of sewage and drainage which is absent in most Indian cities.
But the Chennai experience can give a major push to such solutions that find mention in the smart cities and the Atal mission for urban rejuvenation and urban transformation (AMRUT) programmes.