The National Disaster Management Authority (NDMA) has published a compilation of guidelines on managing the risk associated with the glacial lake outburst flood phenomenon in Uttarakhand.
A GLOF is a type of flash flood, occurring when a lake dammed by either a glacier or moraine collapses, releasing a huge amount of water. This can be caused by volcanic eruptions, earthquakes, or other kinds of land movements that expose the lake to the atmosphere.
When glaciers melt, the water in glacial lakes accumulates behind loose, natural “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 the dam on top of the glacial lake, which holds large volume of water.
Glacial Lake Outburst Floods, or GLOFs, are caused by the sudden and rapid release of meltwater from the ice-rich regions of the Hindu Kush Himalaya (HKH), a mountain range stretching across northern India, Nepal, Bhutan, China, and Pakistan.
The melting of glaciers due to climate change causes the formation of large lakes which are unable to drain effectively. Sometimes the lakes break their natural ice barrier and flood downstream areas causing death and devastation.
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 GLOFs.
A glacial lake is defined as water mass existing in a sufficient amount and extending with a free surface in, under, beside, and/or in front of a glacier and originating from glacier activities and/or retreating processes of a glacier.
Glacial lakes are bodies of water that are influenced by the presence of glaciers. They are commonly divided into two main groups: ice-contact lakes which are characterized by the presence of glacier ice terminating in lake water and distal lakes that are somewhat distant from, but still influenced by, the presence of glaciers and/or ice sheets.
Glacial lakes are common features around the margins of glaciers and ice sheets, and often evolve from ice-contact lakes into ice-distal lakes as glaciers and ice sheets margins retreat and become spatially separated from the lake. Ice-contact lakes are characterized by rapid deposition, high variability in sedimentary facies and deposition of ice-rafted debris.
Glacial lakes may have a significant impact on an ice flow and therefore can affect rates of movement or change their pattern over time. Glacial lakes do this by decreasing the amount of friction at the ice-bed interface, encouraging basal sliding, which ultimately accelerates the glacier's motion.
Glacial lake outburst floods can be the result of a glacier's shrinking because of climate change. As the water level in a glacial lake rises due to melting glacial ice above, it can overtop the natural dam it is contained by, resulting in a sudden flood.
Proglacial lakes from behind ice dams and proglacial lakes may play an important role in the mass balance of glaciers by causing calving, which affects the surface area-altitude relationship of a glacier.
Most glacial lake outburst floods result from glacier lakes forming behind ice and moraine dams created by the glaciers themselves. Glacial lakes form in valley bottoms as a result of melting glaciers or seasonal variations in snowfall. Sometimes when ponds thaw, water can flow away from them through the surrounding permeable glacial sediments.