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Appearances of GLOF Disaster in Nepal

Though there were many GLOFs in Nepal in the past as mentioned hereunder the detailed features of a GLOF itself were not well investigated because of its occurrence in the remote areas. Among the GLOFs which have occurred in Nepal, only two were investigated in detail. One was the flood of 1981 generated from the glacier lake on the Zhangzangbo glacier located at the head of a tributary of Boqu river (Sun Koshi in Nepal) in Tibet. The other was Dig Tsho GLOF occurred in 1985. The Lake was formed on the Langmoche glacier in Khumbu region of East Nepal. Both are typical moraine-dammed lakes. To understand a GLOF impact let us review the Dig Tsho GLOF as an illustration.

Early morning on 4th August 1983, on a fine day of the monsoon season the nearly completed Namche Hydropower plant was fully destroyed by a flood which occurred unexpectedly. No heavy rain had fallen on the previous days. Investigations in the upstream  reaches to identify the cause of the flood revealed that outbrust of the Dig Tsho glacier lake demand by the moraine was responsible for the flooding. The Dig Tsho lake had been separated from the mother glacier. Langmoche, by the glacier retreat and may have been formed within the last 25 years. One-third of the lake was covered by the floating ice like Ice-shelf on the upstream end of the lake, which is formed by the debris of the avalanches falling along the steep glacier. The lake was dammed by the end moraine of 60m height from the valley bed. The volume of water store in the lake was estimated to be about 6.75 x 10^6 m^3. The lake size was approximately 1.5 Km in length, an average of 300m in width and 18m in maximum depth. The moraine- dam was assumed to be broken by an over-stopping of 4-6m high wave. Which was generated by the Ice avalanche from Langmoche glacier with an estimated volume of 1-2 x 10^5 m^3 falling down into the lake. Finally lake water amounting to 5x 10^6 m^3 in volume was released through the breach during 5 hours and 8.8 x 10^5 m^3 of the moraine materials was eroded at the breach. The maximum discharge was estimated 1.600m^3/s along the river in the reach between 25km and 40km in distance from the flood source. Where the average gradient of the river bed is 0.06. The trace of the flood is still clearly seen in the satellite imagery.

The surge including heavy debris inflicted serious damages along the rivers of the Langmoche Khola. Bhote Kosi and Dudh Kosi within a distance of 40 km from the flood source. The Namche hydropower plant at 12 km downstream, 30 houses, 14 bridges, trails and cultivated lands were washed away as well as three human lives and several livestock were lost. In addition the GLOF also destroyed the fragile Himalayas nature by both erosion and sedimentation along the river course. The direct action of rushing water excavated river banks by lateral erosion and undercutting. Which damaged also the forest adjoining the river banks. After suffering the destructive GLOF impact many researchers had investigated the Dig Tsho GLOF. Nepal Government also directed to study general features of a glacier lake and the GLOF phenomena in Nepal as well as in the upstream area of the international rivers, Bhote Kosi coming from tibet. No one had paid any attention to the existence of the GLOF phenomenon in the Nepal Himalayas before the Dig Tsho GLOF because the Zhangzangbo GLOF which had occurred four years ago in 1981 was considered to be a normal flood and not yet recognized as a GLOF then. The Dig Tsho GLOF was an eye- opening event for high rank officials, engineers, planners and technicians of His Majesty's Government of Nepal and concerned agencies of the water resources development sector. Since the Arun 3 Hydropower project had advanced to a feasibility study level at that time. GLOF related anxiety crept into the Project.

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