The Bhote Koshi flood is reckoned to have been triggered by a massive landslide in Tibet that blocked the river. This impounded lake subsequently burst, causing the flood. A similar disaster knocked out the Sun Kosi power house and 20 km of the Kodari Highway in 1981 when the Zhangzangbo glacial lake in Tibet burst. In 1985, the Namche hydropower project was badly damaged when the Dig Tsho glacial lake in Khumbu burst.
Floods triggered by the landslide dam outbursts have damaged hydropower plants on Nepal’s three Bhote Koshis, which flow down from Tibet as well as other glacier-fed rivers.
The Kathmandu-based International Centre for Integrated Mountain Development (ICIMOD) has identified 22 glacial lakes — 12 on the Nepali and 10 on the Tibetan side of the Himalaya — that could burst. A World Bank-funded report published by the Nepal Hydropower Association after last year’s earthquake says a significant glacial lake outburst flood could sweep away all hydropower plants built in cascades along trans-Himalayan rivers like the ones on the Rasuwa Bhote Kosi.
Nepal has taken several policy measures to protect the hydropower sector from glacial lake and landslide dam outbursts. The Department of Electricity Development guidelines require hydropower developers to factor in flood risk from the design phase onwards. Banks now require hydropower projects to address the dangers.
However, the scale of future floods on Himalayan rivers could be so catastrophic that no mitigation measures will be adequate. The worst case scenario is a mega-quake hitting the Nepal-China border region in which the shaking will first damage the projects, and then expose them to risks of tsunami-scale floods barrelling down the rivers as lakes burst upstream.
Arun Rajouriya of the Hydroelectricity Investment and Development Company says small hydropower projects could do more to be prepared for floods. “We do not invest money in projects that are not designed to withstand glacial or landslide floods, private investors must do the same,” he says.
Hari Pandit, an Institute of Engineering professor involved in the design of several hydropower projects, including the 400 MW Kaligandaki Koban, says a flood per sec is not a threat to a mega-hydropower project — it is the sediment and debris they bring down that damage equipment and structures.
Pandit says techniques like centrifugal separators could replace the current gravity method for sedimentation. He adds: “To be better prepared for glacial lake and landslide dam outburst floods, we need to focus on advanced designs.”
Source : Nepal Times