What are Glacial Lake Outburst Floods?
A Glacial Lake Outburst Flood (GLOF) is a catastrophic release of water from a glacier lake, i.e. a water reservoir that has formed either at the side, in front, within, beneath or on the surface of a glacier. The dam that impounds the water body may be composed primarily of debris, bedrock or glacial ice.
Ice-dammed lakes can develop at the margin of an advancing glacier, when side-valleys or depressions at the side of the glacier become blocked. Many such lakes formed in the past during former Ice Ages. Many ice-dammed glacier lakes form and drain repeatedly. Lake Merzbacher in Kyrgyzstan, which drains every year, is one of the most famous and best investigated ice-dammed glacier lakes in the world. Over time, as glaciers retreat, the support of the ice dam is removed and the lake may drain catastrophically, or remain trapped behind piles of debris (moraines) left behind by the former glacier. The recent 2013 GLOF disaster in Kedarnath, India, involved failure of such a lake.
Outbursts from lakes at the surface (supraglacial), within or beneath a glacier, have also been described across most mountain regions of the world, often triggered by heavy rainfall or enhanced melt during warm weather. Recent studies from the Tien Shan have shown that frequent monitoring is required to identify rapidly evolving dangerous situations. In Central Asia, these highly dynamic lakes, that can form and drain rapidly, are termed “non-stationary” lakes, and represent a special challenge for early warning and response strategies.
The widespread retreat of mountain glaciers over the past century resulted in the formation of numerous new glacier lakes trapped behind moraines in many mountain regions of the world. Some of these lakes are spectacularly large with volumes of up to 100 million m3, and depths exceeding 200 m. Large piles of steep glacial moraine are unstable, and can contain slowly thawing ice, meaning they can be weak and prone to failure. Intense rain or snowmelt or the generation of tsunami waves from landslides into the lake are common triggers of GLOFs from moraine dammed lakes. Earthquakes are a concern, because they both trigger landslides into a lake, or directly destabilise a dam.
In rare instances, tsunami waves from large landslides can overtop a moraine dam and cause an outburst event without actually destroying the dam, meaning that the threat of secondary events remains. For lakes dammed by solid rock, tsunami waves are the only mechanism by which a catastrophic flood may be initiated, as the dam structures themselves are considered stable.
Once initiated, GLOFs tend to mobilise large amounts of debris and can transport massive boulders, particularly in the steep river sections. This is particularly true for floods from moraine dammed lakes, which frequently transform into debris or mud flows. Several transitions of these flow types, depending on the steepness and therefore erosive power of the river, are typical for GLOFs . Importantly,GLOFs typically produce discharge values, erosive forces and hence also impacts that are far greater than normal seasonal floods. However, unlike seasonal floods, GLOFs tend to rapidly lose their power downstream which has implications for potential impacts and losses on the lowlands. Nevertheless, flood paths extending up to hundred kilometers and even more have been observed, including events involving more than one country (transboundary GLOFs), while secondary hazards can occur owing to erosion of river banks,blocking of river channels and impacts into downstream lakes.
Given potential future expansion of lakes as the climate warms and glaciers melt, and the rapidly increasing exposure of residential, tourism, transport, and hydropower infrastructure higher into the alpine valleys, a significant increase in future GLOF risk is anticipated globally. Therefore, robust scientific assessments are urgently needed to underpin the design of response and mitigation strategies by national- and regional stakeholders.
This text is a plain-language summary of the paragraph of GLOFs from the GAPHAZ guidelines.
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