To solve urban flooding, start with watershed areas

For years, the city’s civic agencies have tried tackling the flooding problem through eleventh hour, last ditch efforts that often does not go beyond desilting and clearing the storm water drains (SWDs). The drain redesign project has been going on for years, even as lake rejuvenation programmes are going nowhere.

So, what is the alternative? “To ensure visible outcomes of interventions in short term and long term, a comprehensive and integrated design, planning and management agenda is critical across scales, sectors and time,” says a study that went deep into Bengaluru’s urban flooding issues from a design perspective.

First, the basics. An estimated 940 Million Litres Daily (MLD) of water is pumped into the city from Cauvery. Another 70 MLD of ground water is extracted every day. Eventually, 1,040 MLD of sewage is generated daily. All of this is discharged locally, both treated and untreated, mostly in the lakes / tanks and Rajakaluves (SWDs). The sewage is an additional load, something that considerably reduces the capacity of the drains. When it rains, the drains naturally overflow flooding roads, low-lying areas and lakes.

Taking Madiwala lake as an illustration, the study recommends cleaning up of the network of SWDs and restoration of disconnected drains in the immediate micro-watershed of the lake. This should also be done in the primary and secondary valley system of Madiwala.

Cleaning / dredging the lake to increase its lost capacity due to inflow of tons of untreated sewage is another priority intervention. “Create water swales around the tank which can carry/ hold water along with the tank itself.

Remove the cemented / hard surfaced floor of the storm water drains at regular intervals to allow penetration of water into the soil while protecting the soil from eroding (by means of laying net and by creating gully plugs) Swale system at intervals.”

The study wants regulated physical links from the primary and secondary valleys of Madiwala to all open spaces / parks near the valley line. This will redirect the flood waters into these open parks, which can act as sinks if the Madiwala lake / valley cannot carry the water.

Undertaking the study, environmental architect, Mohan S Rao and civic evangelist V Ravichandar had a clear objective: To find alternative short and long term solutions to the problem while minimising demolitions and having zero tolerance for any future violations that impact water resilience. At the heart of the idea was to respect nature’s lines, realising that low-lying areas will always be at greater risk.

Explains Mohan Rao, “The solution cannot be exclusively about engineering. You need to take into account urban planning, natural system of lakes and drains. So, if Madiwala flooding issue is to be solved, you need to start from Puttenahalli.”

What he implies is this: “You cannot look at scarcity of water and urban flooding in isolation.

They are related. We need an integrated plan to combat flooding, water scarcity. We have a history of closing down lakes not just in Bengaluru, but in cities across the country.”