Bengaluru:
An expert committee appointed by the Karnataka government to review the Detailed Project Report (DPR) of the proposed Tunnel Road project has found the planned diversion of the Hebbal nala (stormwater drain) “unrealistic” and lacking key engineering details.
The committee raised red flags over missing hydraulic design calculations and cross-drainage structure drawings for the downstream section near the Hebbal tank weir, calling for a comprehensive reassessment before any implementation.
Committee flags major design gaps
In its report, the committee noted that the existing nala downstream of Hebbal tank was proposed to be diverted through a U-turn, taking a long detour toward the airport — an alignment it described as unfeasible.
It observed that crucial engineering data — such as hydraulic flow computations and cross-drainage structure drawings near the Hebbal Tank embankment — were missing from the DPR.
The panel recommended that detailed hydraulic calculations be conducted, factoring in a 100-year return flood period, to assess the project’s true feasibility and potential flood risk.
Experts warn of ecological consequences
Environmental experts have strongly criticised the plan, cautioning that altering natural drainage patterns could trigger long-term ecological and geological damage.
Veteran environmentalist A.N. Yellappa Reddy told TNIE that diverting natural stormwater channels disrupts groundwater recharge systems and soil biology.
“When you impose changes on nature, you alter the land surface, use heavy machinery, and risk infiltration that contaminates groundwater. The existing nala already filters and recharges aquifers naturally,” he said.
Reddy explained that moving a drain destroys the biota — living organisms in the soil — that naturally filter and replenish groundwater.
“It takes years for this biota to rebuild. A diverted nala loses this balance, allowing untreated sewage to leak vertically or laterally. Before any diversion, the soil’s porosity, texture, and structure must be studied — these vary every few hundred metres,” he added.
Legal and environmental violations flagged
Sandeep Anirudhan, convenor of Bengaluru Townhall, said that the diversion of natural drains violates multiple laws and judicial directives.
“This step breaches the Karnataka Town and Country Planning Act, municipal bye-laws, and NGT and High Court orders protecting rajakaluves (SWDs),” he said.
Anirudhan warned that such actions could lead to flooding, erosion, and contamination of downstream lakes, apart from habitat loss for local flora and fauna.
“It can also cause backflow, pollution, and infiltration into aquifers. Essentially, it invites an ecological disaster,” he said.
Call for caution before execution
Urban experts say the Tunnel Road project must undergo comprehensive hydrological and environmental impact assessments before any further work begins. They stress that restoring and strengthening natural drainage channels is a more sustainable alternative to diversion.
With the expert committee’s findings now under government review, the future of the Hebbal nala diversion — a crucial component of the Tunnel Road plan — remains uncertain.