5-11 August 2016 #820

Preparing for the big flood

Last month’s transboundary flash flood on the Bhote Kosi alarms Nepal’s hydropower developers
Om Astha Rai

Gopen Rai
THRICE HIT: The Bhote Kosi hydropower dam last month after it was damaged by a flash flood caused by a landslide dam that burst in Tibet .

The Bhote Kosi has been hit thrice by disasters in the last two years. The Jure landslide of August 2014 brought down a whole mountainside and blocked the river. A year later, the earthquakes struck. And last month, a landslide dam burst in Tibet, causing much destruction downstream in Nepal.

The 45 MW Bhote Kosi private power project was closed for six months after the Jure landslide because the transmission lines were swept away. It had just started generating electricity when the earthquake damaged the penstock pipe and caused the power house to subside. It was being repaired and was supposed to resume operation in September, but the 22 July flash flood damaged the intake and powerhouse.

The multiple disasters on the Bhote Kosi have been a wake-up call for other large infrastructure projects along Himalayan rivers, and mean that Nepal’s ambitious goal of generating 10,000 MW of electricity by 2026 may have to be re-evaluated.

Himalayan rivers have always been prone to floods and landslides but climate change has added another level of danger, with glacial lakes filling up because of melting ice fields. The floods and landslides of the past two years could well be rehearsals for bigger disasters to come.

Khadga Bahadur Bista, President of the Independent Power Producers’ Association, says the Bhote Kosi is a reminder of the threat of floods caused by glacial lake and landslide dam outbursts on hydropower projects. “The Jure landslide and the earthquakes were eye openers, but the recent floods have alarmed us even more,” he said.

The Bhote Kosi powerhouse subsided and was covered with debris for months after last year’s earthquakes.

The Bhote Kosi 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 Kosis, 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.

The Jure landslide that threw the Bhote Kosi plant out of operation for six months in 2014.

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 se 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.”

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