An extensive cover of debris has a number of important consequences:
(1) The debris insulates the underlying ice from melting, and so the glaciers can descend to lower altitudes than clean glaciers. For example, in the Khumbu Himal of Nepal, clean glaciers descend to about 5,200 metres, whereas debris mantled glaciers can descend to 4,700 metres.
(2) The thick debris is deposited around the margins of the glacier as moraines. Because of the large volume of debris, such moraines may become very large, hemming in the glacier. This may prevent further advance of the glacier, and during colder periods the glacier may simply thicken up in place, perched higher and higher above the valley floor surrounded by its own moraines. Himalayan moraines may be up to 200 metres high. As can be seen in the image above, these moraines can pond up lakes in side-valleys.
(3) During periods of warmer climate, debris mantled glaciers do not simply melt back, like clean glaciers. Instead, they are protected by the debris cover, and may remain in place as ÔfossilÕ relics of colder times. This situation may rapidly change if the debris cover is broken for any reason, such as the opening of a crevasse. Where bare ice is exposed, it will melt rapidly, allowing the hole to grow. Where meltwater cannot freely drain away, lakes will develop on the glacier surface. Supraglacial lakes can undergo very rapid growth, by melting of the bare ice exposed around their flanks, but also by the process of calving, or the breakaway of ice blocks.
Calving is especially rapid where the water is deep or where the ice is heavily crevassed, and lakes have been observed to grow at over 100 metres per year by this process. Eventually, the situation may arise that the entire lower part of a glacier has become a large lake, ponded up by a moraine rampart. Examples include the Imja, to the south of Mount Everest, and the Tsho Rolpa in the Rolwaling Himalaya ("Tsho" is the Tibetan for "lake". GLOFs are known in Tibetan as "Tsho-scrup").
Tsho Rolpa at the margin of Trakarding Glacier
Such lakes are inherently unstable, and are prone to bursting out in catastrophic Glacier Lake Outburst Floods (GLOFs). Many such floods have occurred in recent years, as glaciers have retreated from the maximum positions they attained during the most recent episode of cold climate, The Little Ice Age (c. 1400 -1850 AD). Floods are triggered when the moraine dam fails for some reason. This may be due to melting of buried ice, seismic shocks, or large waves generated by rock or ice avalanches into the lake. A large enough wave can erode a channel into the moraine, and once a channel has been cut or enlarged, more water escapes, causing more erosion, enlarging the channel, and so on. In other words, a positive feedback process causes the channel to be enlarged catastrophically, and the entire lake can drain in a matter of hours.
An example of the destructive power of a GLOF is given by the Dig Tsho, a lake that burst in August 1985, with a probable peak discharge of 20,000 cubic metres per second. The flood was a thick mixture of water and debris, that swept downvalley in a series of surges, and destroyed a newly completed Hydo-Electric scheme (designed to provide electricity to the Sherpa capital of Namche Bazar, at a cost of over $US 2 million) and swept away large amounts of farmland, bridges, and buildings. Amazingly, only 4 or 5 people were killed, as the flood occurred on a festival day, when most of the population were attending ceremonies at monasteries high above the valley floor. A more serious GLOF occurred in Bhutan on 7th October 1994, when a moraine dammed lake, the Luggye Tsho released about 48 million cubic metres of water. The flood travelled rapidlt downstream causing significant damage and 23 deaths. A more recent event occurred in September 1998, which caught a white-water rafting expedition, carrying them 60 miles downstream, fortunately without loss of life.
Making dangerous lakes safe is difficult, expensive and dangerous. Pipes or siphons can be installed and the water level lowered gradually, but this may destabilise the glacier snout triggering ice avalanches into the lake. Thus, this may bring about the very event the engineers wish to avoid. Thus, lakes have to be drained slowly, by stages, to allow the glacier to find a new equilibrium before draining resumes. Drainage work in progress at the Tsho Rolpa is proceeding slowly for this reason. To date, a concrete overspill channel has been installed, and the lake level lowered by 4 metres. This will delay flood hazards, but will not remove them. Additionally, a flood warning system has been installed in the valley downstream.
Much work has been conducted (mainly by Japanese researchers) on identifying potentially dangerous lakes, and prioritising remedial works. If present climatic trends continue (Nepal is warming at a rate higher than the global average), many more debris-mantled glaciers may enter into a critical state in the coming decades. There is thus a need to identify such glaciers, and to instigate remedial works before such work becomes too expensive - an important consideration for Himalayan countries that do not have many resources to cope with such a widespread hazard.
Details of the Tsho Rolpa flood hazard reduction project:
Earthquakes in the Himalaya:
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