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Glacial Lake Outburst Flood (GLOF)

Why is it in the news?

  • Recently, ten people have been killed, and 80, including 23 Army personnel, are missing in Sikkim after the South Lhonak Lake burst due to incessant rains.
  • The South Lhonak Lake, located at 17,000 ft in Sikkim’s northwest, released water downstream, causing flooding in Teesta River and several districts.

More about the news

Rising Risk due to Glacial Melting

  • Glacial lakes, like South Lhonak Lake, form in front of, on top of, or beneath melting glaciers.
  • Global temperature rise has accelerated glacial melting, leading to the expansion of glacial lakes.
  • More than 300 glacial lakes exist in Sikkim Himalayas, with 10 identified as vulnerable to outburst floods.
  • The South Lhonak Lake’s area significantly increased in the past five decades.
  • It was identified as potentially hazardous, especially following earthquakes and expanding size.
  • Scientists from the Indian Institute of Science (IISc) and ISRO had warned about its danger in 2013 and 2019.
  • Himalayas are estimated to have about 7,500 lakes, with nearly 10% in Sikkim.


Government Response

  • In 2016, authorities initiated a water-siphoning technique under Sonam Wangchuk’s supervision to reduce the lake’s water level.
  • Three eight-inch wide and 130-140 meters long pipes were installed to syphon off water.
  • Despite mitigation efforts, the lake burst due to continuous rain and other factors.


Impact and Rescue efforts

  • The GLOF caused significant damage, destroying the Chungthang dam and leading to flash floods.
  • Roads, bridges, villages, and towns were washed away.
  • The flooding affected multiple districts in Sikkim and West Bengal.


Similar Events in the Region

  • The South Lhonak Lake overflow event is reminiscent of the 2021 deluge caused by an avalanche involving the Nanda Devi glacier in Uttarakhand, leading to the flooding of the Rishiganga River and destruction of hydroelectric power projects.


About Glacial Lakes and GLOFs


  • Glacial lakes include ice-dammed, moraine-dammed, and bedrock-dammed lakes, formed by trapping meltwater from glaciers. Global warming-induced glacier retreat is expanding the size and number of glacial lakes.
  • Glacial Lake Outburst Flood (GLOF) is a sudden release of a significant amount of water from a glacial lake, regardless of the cause.
  • Major concerns regarding GLOFs exist in countries like Bhutan, Tibet (China), India, Nepal, and Pakistan.
  • GLOF formation is a major concern in India’s Himalayan states as states such as Uttarakhand, Himachal Pradesh, and Jammu and Kashmir, are surrounded by about 200 potentially hazardous glacial lakes.
  • The formation and impact of GLOFs can be influenced by factors like increased debris cover and black carbon (soot) transport in glacier accumulation areas.


  • Moraines areĀ long ridges of deposits of glacial till.
  • Terminal morainesĀ are long ridges of debris deposited at the end (toe) of the glaciers.
  • Lateral moraines form along the sides parallel to the glacial valleys.
  • Many valley glaciers retreating rapidly leave an irregular sheet of till over their valley floors calledĀ ground moraines.
  • The moraine in the centre of the glacial valley flanked by lateral moraines is calledĀ medial moraine.
  • They are imperfectly formed as compared to lateral moraines. Sometimes medial moraines are indistinguishable from ground moraines.


Factors triggering GLOFs

  • Glacier retreat, driven by anthropogenic factors, leads to rapid slope movement into the lakes and ice melting within the dams.
  • Human activities contributing to glacier retreat include mass tourism, developmental projects (roads and hydropower), and slash-and-burn farming.
  • Erratic and unpredictable monsoon rainfall patterns and increased climate variability have led to more frequent flood disasters.
  • Black carbon from incomplete combustion, reducing Earth’s albedo and accelerating glacier melting.
  • Other triggering factors include cascading processes, earthquakes, ice melting within dams, blocked subsurface outflow tunnels, and long-term dam degradation.


Impact of GLOFs

  • GLOFs can have catastrophic societal impacts, causing sudden and intense flooding that affects nearby communities.
  • They can influence ocean circulation patterns by reducing surface ocean salinity, impacting global climate.
  • GLOFs can significantly alter geomorphological features, including erosion-accumulation interactions, sediment dynamics, river channel erosion, meander shifts, and channel replacements.


Steps Taken

  • ISRO and other organizations monitor glacial lakes in the Himalayan region.
  • NDMA provides guidelines for GLOF management, including hazard and risk mapping, monitoring, awareness, preparedness, capacity development, disaster response, research, and action plans. India’s National Disaster Management Plan, based on the Sendai framework, focuses on understanding disaster risk, governance, risk reduction, and preparedness.
  • Sikkim installed a lake monitoring and information system (water level sensor) at South Lhonak lake and used HDPE pipes to siphon water from the glacial lake.
  • Early warning systems and timely information are crucial for minimizing flood impacts and improving response efficiency.
  • Mitigation measures include dam remediation, such as artificial dams, tunnels, open cuts, concrete outflows, and flood protection walls.
  • There’s a need to set up a disaster database, improve post-disaster assessments, and systematically capture climate change/variability impacts in disaster risk management and development sectors.
  • Central Water Commission (CWC) has meteorological observation stations in the Himalayan region, including telemetry-based stations.
  • Geological Survey of India (GSI) conducts various glacier studies and societal studies in the Himalayan region.
  • Way Forward
  • Nodal agency needed for inter-agency coordination in glacier management.
  • Expansion of high-altitude meteorological and discharge stations with modern technology.
  • Coordinated efforts to monitor the large number of glaciers in the Indian Himalayan Region.

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