Seven dead as Glacial Lake bursts in Sikkim


Context: Ten people have been killed and 80, including 23 Army personnel, are missing in Sikkim after the South Lhonak Lake, a glacial lake situated in the state’s northwest at 17,000 ft burst due to incessant rains, leading to the release of water in downstream areas.


According to Sikkim State Disaster Management Authority (SSDMA), this has caused the rise of water levels in Teesta river that flooded at least four districts, including Mangan, Gangtok, Pakyong and Namchi.


Reasons for Glacial Lake burst in Sikkim

  • The lake outburst in portions of Lhonak Lake in North Sikkim caused a rise in water levels with very high velocities near about 15m/sec, and crossed the CWC (Centre Water Commission) Melli site which measured 227 m, near about 3m above danger level.
  • The floods are believed to have been triggered after South Lhonak lake formed from the gradual melting of a Himalayan glacier, suddenly overflowed and inundated the Teesta river basin.
  • This destroyed the Chungthang dam, a key component of the State’s largest hydroelectric project, and washed away highways, villages, and towns.


  • Scientists have previously warned that the lake had been expanding over years, possibly from the melting of the ice at its head. 
  • According to the National Disaster Management Agency (NDMA), nearly half the lake was drained out. This was likely caused by an “avalanche from the ice-capped feature”.
  • The Chungthang dam, a concrete-rock dam, was breached from its centre from the sheer force and speed of the water, estimated at nearly 54 kmph.
  • According to a press statement from the West Bengal government, more than 8,000 cubic metres per second of water has been released from Teesta barrage which has resulted in flood-like situations in the downstream districts of Kalimpong, Darjeeling, Jalpaiguri and Cooch Behar.


What is Glacial Lake Outburst Floods(GLOF)

  • A glacial lake is a body of water with origins from glacier activity. They are formed when a glacier erodes the land and then melts, filling the depression created by the glacier.
  • The growth of glacial lakes becomes more dangerous because they are mostly dammed by unstable ice or sediment composed of loose rock and debris. 
  • In case the boundary around them breaks, huge amounts of water rush down the side of the mountains, which could cause flooding in the downstream areas. This is called glacial lake outburst floods (GLOF).
  • GLOF can be triggered by several reasons, including earthquakes, extremely heavy rains and ice avalanches.
  • These lakes are also often found in steep, mountainous regions, which means landslides or ice avalanches can sometimes fall directly into the lakes and displace the water, causing it to over-top the natural dam and flood downstream.
  • In 2013, one such event took place in Uttarakhand’s Kedarnath when the region witnessed flash floods along with a GLOF caused by the Chorabari Tal glacial lake, killing thousands of people.


How did South Lhonak Lake become susceptible to GLOF?

  • With the rising global temperatures, glaciers in Sikkim Himalayan have been melting rapidly, giving rise to many glacier lakes and expanding the already existing ones in the region. 
  • According to the Sikkim State Disaster Management Authority, there are currently more than 300 glacial lakes in Sikkim Himalayan. Out of these, 10 have been identified as vulnerable to outburst floods.
  • A report published by the Sikkim Forest and Environment Department found that the lake’s area had significantly increased in the past five decades.
    • Lhonak has grown nearly 1.5 times and South Lhonak nearly 2.5 times their initial size in 1989.
    • An earthquake of magnitude 4.9 on Sept 21, 1991 (as reported by United States Geological Survey) near the parent glacier feeding the South Lhonak Lake and the recent earthquake (magnitude 6.9) of Sept 18, 2011 approximately 70 km from the lakes and future earthquakes may trigger the GLOF events.
  • Scientists at the Indian Institute of Science (IISc) in Bengaluru and the Indian Space Research Organisation (ISRO), through multiple studies in 2013 and 2019, had warned that the South Lhonak lake was “potentially hazardous” and cautioned the authorities to keep an eye on it as it was expanding “dangerously”.


Measures taken by the Sikkim government

  • In 2016, the Sikkim State Disaster Management Authority and Sikkim’s Department of Science and Technology and Climate Change, among others, decided to syphon off lake water from the South Lhonak Lake to tackle the expanding South Lhonak Lake.
  • According to the Sikkim SDMA, authorities installed three eight-inch wide and 130-140 metres long High Density Polyethylene (HDPE) pipes in the lake to take out the water. They syphoned off 150 litres of water per sec.
  • The technique was implemented under the supervision of innovator Sonam Wangchuk




Glacial lake outburst floods (GLOF) are a major concern in Bhutan, Tibet, India, Nepal, and Pakistan


The IISc study had recommended measures like early warning systems and mitigation measures to be put in place in potential GLOF areas.



Imp for: UPSC Prelims, UPSC GS Mains Paper I, UPSC GS Mains Paper III

Topic: Geography, Flash Floods, Glacial Lake Outburst Floods, Disaster Management



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