Floodplain zoning, timely dam release key for downstream safety in Assam: NESAC study

Guwahati, Nov 10: A study on downstream impact for three hydropower projects has emphasised the need for floodplain zoning, community preparedness and coordination of dam water release to safeguard downstream communities and agricultural assets.

The study 'Flooding scenario generation and discharge based downstream impact study in Assam for Ranganadi, Doyang & Kurichhu hydro power projects' was done by NESAC in collaboration with ASDMA, taking into account observed peak discharges of dam water from the projects and hypothetical flooding scenarios.

The peak observed water discharge of 760 cumec Ranganadi hydroelectric project could flood 1,391 hectares of land downstream, it noted. Dam water release data from the Bhutan project was not available with the researchers and it had to rely on hypothetical scenarios.

The generated inundation scenarios will enable opera- tors to anticipate village-level impacts, time bound evacuation and warning dissemination.

"Dams can be helpful for producing energy and irrigation, but they can also change river flow or increase the risk of flooding at the downstream. Floods remain the most persistent natural hazard in Assam, primarily driven by the Brahmaputra's dynamic hydrology, high monsoonal rain- fall and trans-boundary inflows. The challenge intensifies with the operation of upstream hydropower re- serviors, whose controlled or emergency releases can trigger downstream inundation," the researchers stated.

Pointing out the gaps in flood mitigating and management, the researchers concluded that the final inundation scenarios observed in the study will become more realistic and authentic once spillage data from hydel sites are further co-related with observed river flows of the channels joining the flood plain downstream of the dam enilluau

"As currently these channels are all ungauged, installation of four state-of-the-art discharge measurement stations are proposed for approval by the Government of Assam. If installed, the data from these stations will compliment the current in- undation scenarios for further improvement in accu- racy and precision of the inundation simulation output," they said

The study also provides a technical baseline for cross- border cooperation, particularly between Bhutan and India for the Kurichhu-Beki system. "Quantifying down- stream response to hypothetical releases establishes a negotiation-ready frame- work for data sharing proto- col and real time coordination between national hydro power and disaster agencies." it added.