Glacial Lake Outburst Flood
in the Hindu Kush-Himalayan Region
Global climatic change during the first half of the twentieth century has had a significant impact on the high mountainous glacial environment. Many big glaciers melted rapidly forming a large number of glacial lakes. Due to an increase in the rate at which ice and snow melted, the accumulation of water in these lakes has been increasing rapidly. Sudden discharge of large volumes of water with debris from these lakes causes glacial lake outburst floods (GLOFs) in valleys downstream. These result in serious death tolls and destruction of valuable natural resources such as forests, farms, and costly mountain infrastructures. The Hindu Kush-Himalayan region has suffered several GLOF events originating from numerous glacial lakes, some of which have trans-boundary impacts.
At least 12 GLOF events have been recorded since 1935 in the Tibetan area of the Himalayas. A GLOF from Sangwang Cho glacial lake at the head-waters of the Nyangqu River in the Yarlung Zangbo basin in Tibet in July 1954 buried the upper valley with 3 to 5 m thick debris.
The flood damaged the city of Gyangze 120 km away, with a peak discharge of 10,000 m3s-1, and the city of Xigaze 200 km downstream. This GLOF released 300 million cubic metres of water and created a 40-metre high surge flood in Nyang Qu River (Xu & Feng 1994). The GLOF that occurred on 11 July 1981 originated from the Zhangzangbo glacial lake in Tibet and caused damage 50 km downstream, destroying three concrete bridges (one in China, one in Nepal, and one at the Nepal-China border) and a long section of the Nepal-China Highway which cost US$ 3 million to rebuild.
Thirty-five destructive GLOF events have been recorded in the Upper Indus River system in the past two hundred years, but there have been few catastrophic floods in the recent past (WECS 1987). A GLOF from the Shyok area in August 1929 in the Indus River system extended 1,300 km downstream to Attock and had a discharge greater than 15,000 m3s-1 (WECS 1987).
GLOF events from the Lunana area of northwestern Bhutan damaged the Punakha Dzong in 1957, 1969, and 1994. The latest GLOF in Bhutan was triggered by partial breaching of Lugge Tsho glacial lake.
The GLOF event that occurred on 4 August 1985 from Dig Tsho (Langmoche) glacial lake destroyed the nearly complete Namche Small Hydropower Plant (estimated cost of US $ one and half million), 14 bridges, cultivated lands, and so on. Five GLOF events occurred in Nepal from 1977 to 1998 according to the records and, based on the study of satellite images, evidence of other occurrences in the past has has been found. Six GLOF events (1935 - 1981) originating from Tibet, China, were reported to have damaging effects inside the Koshi River Basin in Nepal.
A recent inventory carried out by ICIMOD and UNEP/EAP-AP shows that there are 3,252 glaciers covering a surface area of 53,23 sq.km and 2,315 glacial lakes out of which 26 potential dangerous glacial lakes are in Nepal (ICIMOD and UNEP/EAP-AP 2000). Field studies of the six glacial lakes (Tsho Rolpa, Imja, Thulagi, Lower Barun, Dig Tsho, and Tam Pokhari) in Nepal have been carried out by different organisations. Similarly, the inventory carried out shows that in Bhutan there are 677 glaciers covering 1,316 sq.km and 2,674 lakes, out of which 22 are potentially dangerous.
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