An avalanche/landslide in conjunction with breakage of rock/ice glacier could be the reason.
However, an avalanche/landslide and/or breakage of rock/ice glacier could not be the sole reason for sudden deluge of water volume leading to flash flood. There must be trapped water at surface or sub-surface, leading to an assessment if a glacial lake is formed simultaneously.
In high mountainous terrains, melting of glaciers due to global warming leading to higher warming rates over higher elevation than lower elevation is seen. This situation increases risk of glacial-related hazards.
One of these risks is Glacial Lake Outburst Floods (GLOFs)—formed due to glacier(s) retreat, basically forming water body behind moraine or ice ‘dams’. These ‘dams’, which act as an embankment, once broken and/or weak, will lead to glacial lake breaching. It will suddenly discharge huge volumes of water, debris, slurries, etc. Such outbursts have the potential of releasing millions of cubic meters of water in a few hours, causing catastrophic flooding downstream with serious damage to life and property. GLOFs evolve as a consequence of a series of different processes, for example, mass movement into lakes, glacier/ice front calving into lake, progressive enlargement of lake, rising lake levels leading to overflow, mechanical rupture/failure of dam, hydrostatic failure, degradation of dam or melting of ice cores in dam, earthquakes, a flood wave from lake located upstream and intensive rainfall or snow melt.
GLOFs have emerged as a serious hazard in the mountain region in recent decades due to increased human settlements, anthropogenic and other developmental activities into areas which were inhabited and were not developed previously. There are hundreds of glacial lakes in Indian Himalayan region (IHR).
A soon to be published study (in press) by a group of scientists including me, examines the issue of GLOFs in the Indian Himalayan Region. A comprehensive inventory reveals a total of 4,418 glacial lakes within the Indian Himalayan Region. Besides, a total l of 636 Trans Boundary glacial lakes, which can potentially flood the IHR, were also identified.
Glacial lakes in the IHR cover an area of 428.71 sq km. Of the four major types of lakes, bedrock (67%) and morainedammed (25%) are most frequent. In Jammu & Kashmir, Sikkim, and Arunachal Pradesh 57-84% of the lakes are dammed by bedrock, while in Uttarakhand and Himachal Pradesh 76-80% of the lakes are moraine-dammed. Ice-dammed and other lakes are not so common, when compared with other lake types.
To the best of our understanding, this is the latest and most complete inventory of glacial lakes in IHR, and the first study to systematically include the lakes from which Trans Boundary GLOFs could originate and influence the downstream regions of IHR.
The writer is Professor, School of Environmental Sciences, Jawaharlal Nehru University.