Koramangala, Vrishabhavathi valleys lost 50% of drain length: CAG report

Poor maintenance of stormwater drains (SWDs), exposed by the recent floods, have impacted the city’s major valleys and watersheds, which, experts say, need immediate attention.

The September 2021 report of the Comptroller and Auditor General (CAG) on the city’s SWD network has revealed that the Koramangala and Vrishabhavathi valleys have lost close to 50% of their drain length over the years, bringing down their carrying capacity.

A similar reduction in vegetation cover was also observed. “Valleys have a major role in carrying the rainwater out of the city, to avoid flooding. Over the years, the authorities have neglected them leading to such reduction in vegetation cover and open land space,” said V Ramprasad, co-founder of Friends of Lakes. He said authorities allowed construction in the valley zones where maintaining topology was vital. “The Revised Master Plan (RMP) 2015 had stated that the valley zones were to be no-construction areas. The government discarded that and allowed construction on them.”

Also, the Karnataka Tank Conservation and Development Authority (KTCDA) Act was amended only to fuel such construction. “When that is the case, how can the valleys remain intact?” Ramprasad wondered.

Apart from the length, the vegetation cover of the Koramangala Valley has come down by close to 5%. “The vegetation cover decides the rate of infiltration of water. As vegetation cover reduces, the percolation of water decreases and hence, the amount of runoff water increases resulting in flood-like situations,” explained Prof T V Ramachandra from the Indian Institute of Science (IISc).

The reduction in vegetation cover has proven to be a bigger problem than a mere increase in runoff water, he explained. “On one hand, the runoff water leads to flood-like situations. On the other hand, the groundwater levels are coming down drastically. Land use patterns play an important role in preventing such problems,” he said.

The CAG report also revealed that, over the years, the number of waterbodies that existed in the Koramangala Valley came down to 8 from 41, and those in the Vrishabhavathi Valley reduced to 13 from 51, mainly owing to the conversion of lakes.