Glacier Lake Outburst Floods (GLOFs)
In mountain regions, potentially unstable lakes can form when advancing glaciers block drainage routes, or when basins open up between retreating glaciers and abandoned ter- minal moraines. Glacier lake outburst floods (GLOFs) have caused loss of life, and loss of agricultural land and infrastructure. There is some indication that the frequency of GLOFs has increased due to climate change, and the impacts of GLOFs are very likely to increase in the coming decades as glacier retreat continues. GLOFs can have a very large impact locally, but the number of people affected by potential GLOFs is far less than those impacted by other types of floods.
Fluctuations of mountain glaciers, and other changes in the mountain cryosphere, lead to shifts in the distribution and frequency of hazards. During glacier advances, lakes can form due to the formation of cross-valley ice dams, and during glacier retreat lakes can develop in the gap between frontal or side moraines and the receding ice. GLOFs occur when the ice or moraine dam fails, and can be triggered by seismic activity, landslides or avalanches from moun- tains or hanging glaciers (Fig. 8 and 9). In the Hindu-Kush Karakoram, GLOFs are normally linked to the weakening of ice dams due to recent glacier thinning (Bajracharya et al. 2007). In high mountain regions, numerous moraine- dammed lakes have formed recently due to the retreat of de- bris-covered glaciers, leading to widespread GLOF hazards. Particularly affected areas include the central and eastern Himalaya and the Cordillera Blanca in Peru (Carey, 2005). At least 30,000 people have been killed by over 30 glacier disasters in the Cordillera Blanca region of Peru since 1941 (Carey, 2005).
Many of the Himalayan rivers originate from glaciers, and since most glaciers in this region are retreating a large number of proglacial lakes located between the gla- cier tongue and the frontal moraines are growing. These are best described as dynamic glacial lakes systems since smaller lakes in some locations merge into larger water bodies as they grow. Focusing simply on the number of
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lakes may be misleading as the number of lakes may be reduced while the volume of stored water increases. There have been at least 35 GLOF events in Nepal, Pakistan, Bhu- tan and China during the last century. Nepal alone has already experienced 15 GLOFs (Fig. 10) (Richardson and Reynolds, 2000).
GLOFs have also been recently reported in 2008 and 2009 from the Northern Patagonia Icefield. In these occasions, an estimated 200 milion tons of water spilled into the Co- lonia river, Chile (Dussaillant et al., 2010). More recent near doubling in the frequency of GLOFS in the Yarkant region of Karakoram, China, from 0.4 times annually 1959–1986 to 0.7 times annually in 1997–2006 has been attributed to the rise in warming (Chen et al., 2010)
Although infrequent, largely unpredictable and localised, impacts can be devastating as shown by the GLOF in Lang- moche valley in eastern Nepal in 1985, which washed away bridges, agricultural land, homes and people and impacted areas 90 km downstream (Vuichard and Zimmerman, 1987). An estimated four million cubic meters of sediment-laden water destroyed a hydropower station, 14 bridges, 30 houses and farm lands worth four million US dollars. Loss of life was remarkably small, because a religious festival was in progress and few people were in the valley bottoms. To date, GLOF impacts have been largely confined to mountain communi-
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