National Disaster Management Authority Guidelines: Management of Glacial Lake Outburst Floods (GLOFs), October 2020

Manual and Guideline
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Glacial retreat due to climate change occurring in most parts of the Hindu Kush Himalaya has given rise to the formation of numerous new glacial lakes, which are the major cause of Glacial Lake Outburst Floods (GLOFs). A GLOF is a type of flood occurring when water dammed by a glacier or a moraine is released suddenly. When glaciers melt, the water in these glacial lakes accumulates behind loose naturally formed 'glacial/moraine dams' made of ice, sand, pebbles and ice residue. Unlike earthen dams, the weak structure of the moraine dam leads to the abrupt failure of moraine dam on top of the glacial lake, which holds large volume of water. A catastrophic failure of the dam can release the water over periods of minutes to days causing extreme downstream flooding. Such outbursts known as GLOF have the potential of releasing millions of cubic metres of water in a short period causing catastrophic flooding downstream. Peak flows as high as 15,000 cubic metre per second have been recorded in such events. As a result, the threat of GLOFs is receiving increased attention and awareness for glacial lake monitoring and hazard mitigation has increased recently.

Since glaciers in the Himalayas are in a retreating phase, glacial lakes are growing and pose a potentially large risk to downstream infrastructure and life. As glaciers retreat, the formation of glacial lakes takes place behind moraine or ice 'dam'. Different types of lakes may have different levels of hazard potential. For instance, moraine-dammed lakes located at the snout of a glacier have a high probability of breaching with high hazard potential whereas erosion lakes have little chance of breaching.

These floods pose severe geomorphologic hazards and can wreak havoc on all manmade structures located along their path. Much of the damage caused during GLOF events are associated with large amounts of debris that accompany the floodwaters. GLOF events have resulted in many deaths, as well as the destruction of houses, bridges, forests, and roads. Unrecoverable damage to settlements and farmland can take place at large distances from the outburst source with longer term disturbance to the livelihoods.

The potentially dangerous lakes can be identified based on the condition of lakes, dams, associated mother glaciers, and topographic features around the lakes and glaciers. The criteria used to identify these lakes are based on field observations, processes and records of past events, geomorphologic and geotechnical characteristics of the lake/dam and surroundings, and other physical conditions.