Experience has taught most Bengalureans that a sprinkle of rain is enough to inundate roads in the city. How much rain, however, is needed to inundate areas en masse?
Not much, apparently, says a study by the Indian Institute of Science (IISc.) — which analysed the capacity of North Bengaluru to withstand flooding.
The analysis points to drains in the area — between IISc and Kodigehalli, covering high-construction areas of Hebbal, Sahakarnagar and Jakkur, among others — being just one-fifth as effective in dealing with storm events as compared to “standard” drains.
Researchers from the Department of Civil Engineering at IISc went about comparing the daily rainfall for more than 20 years, drain width, depth, slope and roughness to determine the reliability of the 18 km drain that is expected to handle excess rainwater. The scientists looked at three possible reasons for failure of the drain and 34 sub-watersheds (which feed into this drain) — one, if the flow rate was high enough to erode the drain; two, if the flow rate was too low causing stagnation and silt accumulation, and three, quantity of water exceeds the capacity of the drain. The results are grim. The analysis finds the network is prone to failure in all three events.
While “ideal drains” have failure risk of 10 per cent annually, North Bengaluru has a risk of 50 per cent of urban flooding each year.
“Internationally, drains must be designed to fail only at “rare” storm events that may occur once in 10 years.
However, the drains in North Bengaluru fail at storm events that have a high probability of occurring once in two years,” said V.V. Srinivas, Associate Professor at the Department of Civil Engineering, IISc., who along with research scholar R.L. Gouri authored the paper that was published in the journal Aquatic Procedia.
The researchers hope to extend the study to all areas of the city to get an idea of the reliability of storm-water drains here.
Presence of lakes in the drain network enhances reliability of the network. Citizens can work towards prevention of encroachment of lakes and maintaining capacity of drains, by ensuring that they are not clogged
V.V. Srinivas,Associate Professor, Department of Civil Engineering, IISc.
