What Caused GLOF 2023 in Sikkim Killing Many? Glaciologist from Assam, Dr Ashim Sattar and Team has the Answer

The study noted that the South Lhonak outburst demonstrates the vulnerability of Himalayan hydropower to extreme natural events

A glaciologist from Assam, who is currently a faculty at IIT Bhuvaneshwar, has solved the trigger for the devastating October 3, 2023 glacial lake outburst flood in Sikkim that killed 55 people besides leaving 70 others traceless.

A year-long study by Dr. Ashim Sattar and his colleagues of 34 global experts from 9 countries, published in the international journal ‘Science’, has now pin-pointed that the GLOF was triggered by a collapse of nearly 15 million cubic meters of frozen mass of rocks and sediment or moraine material into the South Lhonak Lake, which created a 20 m high tsunami-like wave that eroded the frontal moraine dam holding the lake’s water.

The study precisely calculated the amount of water released from the lake during the GLOF event, which was 50 million cubic meters, a volume equivalent to 20,000 Olympic sized swimming pools. The enormous volume of water made its way down the valley causing severe damage to downstream infrastructure. A 28 m drop in the lake level was observed after the GLOF event by analysis of very high-resolution satellite data and their derivatives.

Surprisingly, the study reveals that the part of the lateral moraine that collapsed into the lake was unstable and was showing maximum displacement of over 15 m per year between 2016 to 2023, years before the actual collapse. This finding shows that the surroundings of the South Lhonak Lake have been unstable now for a long time.

The study also quantified the enormous amount of sediments eroded by the GLOF, a volume that would fill 108,000 Olympic-sized swimming pools, that was deposited downstream, causing severe devastation. The flood debris impacted the other hydropower projects including the Teesta V, Teesta VI and the Teesta Low Dams III and IV.

The study noted that the South Lhonak outburst demonstrates the vulnerability of Himalayan hydropower to extreme natural events. Hydropower projects exposed to glaciers and glacial lakes increase the risks of being impacted by outburst floods. With changing climate and global warming, similar disasters are likely to occur in the future and energy policies of Himalayan states should be more considerate of these potential risks and act towards mitigating glacier-related risks.

The study also shows how a GLOF event’s severity was worsened by climate change and heavy rainfall, which increased landslide vulnerability and intensified downstream impacts like erosion leading to a multi-hazard cascade.

The need for early warning systems for vulnerable glacial regions like the Himalayas that are constantly facing the threat of climate change and risks posed by hydro-electric projects near glacier lakes in the Himalayas amplifying GLOF risks, were highlighted in the study.

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