Environment

Glacial lakes, a potential threat

The new arched bridge over the Bhote Kosi river at Phulping, near the Nepal- China border, which was built after the old stone bridge, remnants of which can be seen in the left hand side of the picture, was washed away in the floods of 1981. Photo: Meena Menon  

On the way to the China border from Kathmandu, in Sindhupalchowk district, a 19-metre-high rock sits in the middle of the Kosi river shadowed by the canopy of a giant tree growing out of it. It has become a tourist attraction and acquired a reverence of sorts with a temple and steps leading to the top.

This popular rock, believed to have come down by an incredible force of water over 100 years ago, is a stark reminder of the havoc floods can create in the Himalayan region. This region is no stranger to what is known as GLOF or Glacial Lake Outburst Flood.

Downstream you can see a hydroelectric project which was wrecked in a GLOF event in 1981 while under construction. On July 11, 1981, the diversion weir of the Sunkoshi Hydropower project was struck by a large flood, which also destroyed two bridges and extensive sections of the highway linking China and Nepal. It was only later that research indicated that the flood resulted from drainage of the Zhangzangbo glacial lake north of the border in the Tibet Autonomous Region, triggered by an ice avalanche.

The glacial lake is about 27 km away from a stone bridge at Phulping, which was also destroyed and whose remnants are still there just below a massive arched new construction which links the road since 1986. Sharad Joshi, glacial lake and GLOF research associate at the International Centre for Integrated Mountain Development (ICIMOD), which has brought out a new report on the subject, says that before this there was another GLOF in 1964. Both GLOFs did not kill people since there was not much population, but the total discharge from the Zhangzangbo lake was 16,000 cubic metres in 1981 and the damage was pegged at $3 million. Climate change, which is causing glaciers to recede in the Himalayas, is the push factor for GLOFs, he said. Upto 50 km of area downstream was affected. Keeping this in mind, another hydro power project on the Bhote Kosi river installed an early warning system for floods.

The ICIMOD report on Glacial Lakes and Glacial Lake Outburst Floods in Nepal, prepared for the Global Facility for Disaster Reduction and Recovery (GFDRR)/ The World Bank, said glacier thinning and retreat in the Himalayas has resulted in the formation of new glacial lakes and the enlargement of existing ones due to the accumulation of melted water behind loosely consolidated end moraine dams. Because such lakes are inherently unstable and subject to catastrophic drainage, they are potential sources of danger to people and property in the valleys below them. The torrent of water and associated debris that sudden lake discharges produce is known as GLOF. Recent surveys have shown that many glacial lakes in Nepal are expanding at a considerable rate.

Nepal has experienced 24 GLOF events in the recent past, several of which have caused considerable damage and loss of life. Of them, 14 are believed to have occurred in Nepal itself and 10 were the result of flood surge overspills across the China-Nepal border.

The report said that in Nepal, little attention had been paid to this phenomenon until the sudden outburst of the Dig Tsho glacial lake in the western section of the Sagarmatha (Mt. Everest) National Park, Khumbu Himal, on August, 4, 1985. This sudden outburst destroyed the nearly completed Namche small hydel facility, some 11 km downstream, and caused destruction as far as 50 to 60 km downstream. The outburst was triggered by a large ice and rock avalanche that cascaded into the lake from a steep glacial surface. The downstream communities in Khumbu were in disarray for months and damages were estimated at around $3 million.

Perhaps the most famous of these glacial lakes is Imja, located in the Khumbu region close to the Everest base camp. The lake did not exist in photographs taken in the 1950s, but has now rapidly expanded to 1.012 sq.km. over the years. It is also in focus because it lies within one of the top 10 tourist destinations in Nepal, but is considered fairly stable. Imja was one of three lakes studied in the ICIMOD report for its critical situation and a large population downstream.

The report said a 2001 inventory identified 3,252 glaciers in Nepal covering an area of 5,324 sq.km. The new inventory prepared using the Landsat 5 and 7 images mapped 3,808 glaciers with a total area of 4,212 sq.km. Some glacial lakes in Nepal are being monitored; early warning systems have been developed and installed in the Tsho Rolpa and Tamakoshi valleys as well as in the Upper Bhote Koshi area.

Tsho Rolpa is the only glacial lake in Nepal that has been subjected to rather costly mitigation measures. A siphon system installed in 1995 had limited success. It was followed by cutting of an open channel through the moraine dam; the four-metre-deep artificial spillway completed in 2000 succeeded in lowering the lake level by three metres, the report said.

The Nepal National Strategy for Disaster Risk Management (2009) refers to disasters in general: it includes GLOF risk, but does not address it distinctively. National strategies and approaches to disaster risk management pay little attention to GLOF risk management, perhaps because information about it is inadequate. It is essential to develop short and long-term action plans and programmes, the report concluded.


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Printable version | Jan 17, 2022 4:40:34 PM | https://www.thehindu.com/sci-tech/energy-and-environment/glacial-lakes-a-potential-threat/article2042745.ece

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