With Bengaluru monsoon approaching, predict floods, but design to drain

Trapped in knee-deep water, the motorist revved up his bike engine in despair as the flooded road completely stumped him. Clueless about where the footpath ends and the road begins, the elderly pedestrian just stood there in deep fear. Both had a probing query: What if we knew well in advance tonight’s showers would definitely flood this road?

Monsoon showers are just round the corner, and the torrential downpour last week has already shown how the annual picture of floods will predictably reappear. Road-users are bound to suffer enormously since a long-term, well thought out flood mitigation project is nowhere in sight. The best bet – even if not foolproof – is a good prediction method, warning people well in advance.

So, what do we have now? The city has 132 water level sensors, installed by the Karnataka State Natural Disaster Management Centre (KSNDMC) in areas highly vulnerable to flooding. The sensors are designed to alert users in real-time, just when the water level in storm water drains crosses a pre-defined danger mark.

A part of the Urban Flood Modelling project conceptualised by KSNDMC and the Indian Institute of Science (IISc), the sensors could boost the accuracy of flood alerts to a great extent. But a problem remains: How do you tackle a relatively new pattern in Bengaluru, where rain water that normally hits in 24 hours comes down at enormous force in barely an hour?

Limited impact

Disaster management experts say early warning systems can only have a limited impact in such scenarios. “Rapid urbanisation has dramatically changed the land-use patterns in Bengaluru. Many water bodies have disappeared, storm water drains have been encroached upon. The topography has been changed,” notes an expert, preferring anonymity.

Concretisation has severely reduced the exposed areas for rain to percolate down to the ground. “In the 1980s, the run-off of rain-water did not exceed 40% (run-off quotient). Now the run-off has gone up to more than 80% in some areas. The carrying capacity of our drains is just not enough. So, flooding happens automatically,” he explains.

Remodelling

Remodelling the existing storm water drain (SWD) network has been a much-talked-about way out of the recurring floods. But this project of clearing encroachements, widening drains, building walls and de-silting is in progress for years. Till date, work has been completed only upto a drain network length of 400km. The government has earmarked Rs 1,600 for the project this year.

Early onset of monsoon this year is likely to disrupt the work, aggravating the flooding risk. The government is tasked with the primary drains while the Bruhath Bengaluru Mahanagara Palike (BBMP) has to manage the ward-level drains.

Complicating matters further for both the government and the Palike are the changing rainfall patterns. Weather experts are clear that any work on flood mitigation should keep in mind the shifting rain schedules and plan to complete work accordingly. “Extreme events are taking place more often. There is not much variation in the rainfall volume for the entire season or year. But the percentage of extreme events has increased now. So, 100mm of rain in one hour can generate a lot of water.”

Excess water reuse

The existing water bodies have to be strengthened so that they could store the excess water from the rains. “We also have to reuse this excess water. If not, in one rain, the storage fills up, and the flooding will reoccur. We have to create a dual water system where the additional pipeline can supply this water for purposes other than drinking, maybe after treating. That is the only way,” notes the expert.

As for improved flood prediction, the only long-term solution is acquiring a Doppler weather radar, he points out. “This radar will give real-time data, that is acquired directly from rain-bearing clouds. The Indian Meteorological Department has been telling this for the last ten years. The time to act is now.”