• A portion of the Nanda Devi glacier broke off in Uttarakhand’s Chamoli district on February 7, triggering an avalanche and a deluge in the Alaknanda river system that washed away hydroelectric stations, leaving at least 10 people dead and 143 others missing.

• The sudden flood in the middle of the day in the Dhauli Ganga, Rishi Ganga and Alaknanda rivers — all intricately linked tributaries of the Ganga — triggered widespread panic and large-scale devastation in the high mountain areas.

• Two power projects — NTPC’s Tapovan-Vishnugad hydel project and the Rishi Ganga Hydel Project — were extensively damaged with scores of labourers trapped in tunnels as the waters came rushing in.

• The scenes brought back to the people horrifying memories of the 2013 Kedarnath deluge that killed thousands.

• Two teams of glaciologists are heading to Joshimath-Tapovan to study the causes of the incident. One of the most widely considered scenarios for the incident is the Glacial Lake Outburst Flood (GLOF).

What is Glacial Lake Outburst Flood?

• Flash floods caused by the outburst of glacial lakes, called as Glacial Lake Outburst Flood (GLOF), are well known in the Himalayan region where such lakes had often been formed by landslides. 

• Glacial lakes are common in the high elevation of glacier basin. They are formed when glacial ice or moraines or natural depressions impound water. 

• There are varieties of such lakes, ranging from melt water ponds on the surface of glaciers to large lakes in valleys dammed by a glacier in the main valley. These lakes normally drain their water through seepage in front of the retreating glacier. 

• The moraine creates topographic depression in which the melt water is generally accumulated leading to formation of glacial lake. When this lake is watertight, melt waters will accumulate in the basin until seepage or overflow limits the lake level.

• GLOFs have immense potential of flooding in downstream areas, causing disastrous consequences due to release of large volumes of water in a very short interval of time. 

• Most often, the consequences arising out of such situations are highly unpredictable primarily due to lack of availability of sufficient data regarding rainfall intensity, location of landslide, impounded volume and area and physical conditions of lakes/ water bodies. 

• GLOFs are not a new phenomenon but with the worldwide receding of glaciers and rising temperature the probability of their occurrences has risen in many mountain ranges.

• The frequency of GLOF events is increasing in the Hindu Kush Himalayan (HKH) region since the second half of the 20th century due to the combined effects of climate change and deforestation.

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