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Hand to hand battle against disaster

News

ENGINEERS ARE battling against time and extreme Himalayan weather conditions to prevent collapse of natural dam of a glacial lake in central Nepal that endangers thousands of lives.

A 5m deep channel is now being excavated through the top of the 150m high terminal moraine holding back the rising waters of Tsho Rolpa, Nepal's largest glacial lake. A 100m long concrete lined spillway with three specially designed sluice gates will be built to reduce the lake's water level.

Sixty site staff, equipped only with handtools and one backhoe excavator, are aiming to reduce the water level by 3m before the onset of winter in November.

But work has already been delayed a month by bad weather and logistical problems caused by the Nepalese general election.

John Reynolds, principal engineer for Welsh consultancy Reynolds Geo- Sciences, and international technical adviser to the Nepalese government for the project, said the lake was increasingly likely to burst through the moraine.

Flooding would cause widespread damage, he said, threatening an estimated 7,000 lives and the Khimti HEP scheme 80km further down the valley.

'Without intervention, it is not so much a case of will Tsho Rolpa fail but when,' he said.

The work is seen only as a temporary measure. Water level will have to be lowered by a further 15m to make the lake safe. Reynolds estimated there was 'only a few years at the most' for remediation to be implemented.

Tsho Rolpa is 4,450m above sea level, about 110km north east of Kathmandu at the eastern end of the Rolwaling Valley. The lake is thought to have started forming in the 1950s and is now 3.5km long, 0.5km wide and up to 131m deep, holding some 110M.m3 of water. The lake is continuing to grow, fed with more meltwater from the retreating Trakarding Glacier behind.

With freeboard at just 1.5m, overtopping is increasingly likely, through waves caused by glacial ice avalanching off the front of the glacier.

The problem is compounded by the subsidence of the moraine as its 30m thick ice core melts. Fractures in the core could also provide pathways for water to penetrate the moraine and as hydrostatic pressure builds, the risk of explosive failure increases.

Reynolds said that more than 50% of catastrophic lake bursts in Nepal this century have been triggered in this way and the problem is set to increase with further climatic warming.

The £1.8M remediation project is being carried out by the Department of Hydrology and Meteorology, the Government of Nepal and is funded by Netherlands Development Assistance.

Initial studies began in 1994 and in 1996 a trial siphon was installed to reduce water levels. Following rapid deterioration of the moraine in 1997, it was decided that an open channel built in the top of the moraine would rapidly lower water levels, reducing the amount of water available for a flood and relieving hydrostatic pressure on the dam.

Global warming is putting pressure on dozens of glacial lakes in Nepal, Bhutan, Northern India, China and South America. In 1994 a moraine failure in Bhutan caused flooding 200km downstream, killing 27 people.

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