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Glacial lake outburst, a real threat

31 October 2008

Preliminary reports of a study on one of the seven potentially dangerous glacial lakes at the sources of Mangdechhu said that it holds high potential for glacial lake outburst flood (GLOF) in the future.

Engineers from Yachiyo Engineering Co. Ltd (YEC), a private firm in Japan, who undertook the survey, said that “the future means any time”. It could be in a day, a week, a month, a year or 100 years. The study was undertaken on Mang-gl-106. Located at an elevation of 5,066 metres, the 970,000 square-metre lake holds 26,000,000 cubic metres of water. It is estimated to be 40 metres deep.

Reports said that the GLOF could be triggered either by the collapse of hanging glaciers, slope failure or moraine dam failure. Subsidence and breaching of terminal moraine was observed. The study found out that there are two hanging glaciers of 7,500,000 cubic metres and 5,600,000 cubic metres in volume respectively.

If there is a glacial lake outburst disaster, Mangdechhu hydropower project and Bjeezam will be affected. The police quarters at Bjeezam and forest office below it will be submerged and paddy fields at Mangdechhu basin will be washed away.

Six officials, three from YEC and three from Bhutan, surveyed the glacial lake earlier this month and presented their findings to the relevant agencies in Thimphu last week. Naoaki Nanbu, Section Manager in YEC, said the findings will be shared with a university in Japan which specialises in glacial lake studies. The final report will be ready by early next year. Once the report is ready, it will be discussed with both the governments.

YEC will provide the necessary technical support to Bhutan. The impact of global warming was felt in the surveyed lake, Naoaki Nanbu said. He said, at the time of the survey, calving of snout of glacier was observed. “It indicated that retreat of glacier has been progressing due to global warming.” Yoshio Yabe, Senior Manager, YEC, said there was need for periodical survey of innermost moraines for which installation of temporary benchmark and periodical survey point was important. He said that the importance of such a benchmark was immediate in order to check the progress of subsidence of particular moraine.

The study found out that although the households were located far away from the river, some of the paddy fields are located near the river. Cattle go to the river to drink, farmers go to their fields for plantation and harvest, and the main source of irrigation for fields was found to be the river. The team suggested some countermeasures for mitigation of GLOF – construct sediment and flood control facilities such as Sabo dam downstream and install an early warning system which could be connected with FM radio stations. The Sabo dam will hold back all the debris and let only the water flow downstream. The team suggested the Sabo dam be located some kilometres upstream at Bjeezam.

Yoshio Yabe said if there is any disaster, the first option is to have minimal casualties. “Our aim is to have least effects on the human settlement.”

For this, they suggested hazard zonation, disaster management plan, training for evacuation, public awareness and capacity development.

Japan government is known for its commitment to mitigation of global warming which inspired the YEC to contribute to surveying the lake.

By Rabi C. Dahal

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