Massive collapse of moraine into South Lhonak lake in October 2023 sparked a 20-metre-high ‘tsunami-like’ wave that sent 20,000 Olympic swimming pools’ worth of water downstream – making it one of the most catastrophic to hit the Himalayas in recent decades.
Key findings:
- Moraine instability: An unstable section of the lateral moraine had been moving at velocity exceeding 15 meters per year between 2016 and 2023. The collapse of this section likely triggered the disaster.
- Permafrost Thaw: The presence of permafrost (frozen ground) in the collapsed area suggests that thawing may have contributed to the instability.
- Sediment Erosion: The GLOF eroded approximately 270 million cubic meters sediment along its path, depositing massive volume of sediment downstream which caused severe destruction in settlements.
- Landslides: The GLOF triggered 45 landslides, seven of which directly damaged buildings and National Highway-10.
- Weather Impact: A low-pressure system from the Bay of Bengal brought heavy rainfall along the Teesta valley, exacerbating the floods impact in West Bengal and Bangladesh.
- Future Risks: South Lhonak Lake remains highly susceptible to future GLOF due to altered slope geometry, with small-scale mass movements already visible on the failure slope.
Kathmandu – A newly published scientific study provides the first comprehensive analysis of October 2023’s devasting Glacial Lake outburst flood (GLOF) for South Lhonak Lake in Sikkim, India.
The disaster resulted in 55 casualties, with 70 people still missing and caused extensive damage to critical infrastructure including hydropower facilities, bridges, highways, and buildings in the Teesta Valley. This event ranks among the most catastrophic GLOFs to hit the Himalaya in recent decades.
The research, led by a team of international scientists from nine countries, examines the triggers, dynamics, and cascading impacts of the flood, offering critical insight for Himalayan communities vulnerable to climate-induced disasters.
Using high-resolution satellite imagery and modelling techniques, the study reveals that the GLOF was triggered by a collapse of up to 14.7 million cubic meters of frozen moraine material into the South Lhonak Lake. This collapse generated a tsunami-like wave approximately 20 meters high, which eroded the lake’s frontal moraine dam, releasing about 50 million cubic meters of water – equivalent to 20,000 Olympic sized swimming pools.
“The South Lhonak GLOF serves as a grim reminder for similar high-risk lakes across the region,” said Ashim Sattar, Assistant Professor at IIT Bhubaneshwar and lead author of the study. “We urgently need a multi-faceted approach, including early warning systems, strengthened regulatory frameworks, and community education, to mitigate these risks.”
The flood severely impacted settlements and infrastructure along the Teesta River, breaching the Teesta III dam in Chunthang district. The study highlights the role of cascading hazards, where initial flooding triggered secondary landslides and debris flows, compounding the destruction.
Key recommendations from the study underscores:
- Enhanced Early Warning Systems: Urgent implementation of improved hazard detection and response systems across the Himalaya.
- Advanced GLOF Modelling: Current models often underestimate risks by failing to account for sediment entrainment and riverbank collapses. Improved techniques are critical for effective risk management.
- Adaptative Risk Management: The recent disaster shows the need for urgent, adaptive plans, particular for basin-scale early warning systems.
- Resilient Infrastructure: Strengthening regulations and risk assessments for hydropower projects near glacier lakes to safeguard communities and investments.
“This research paper exemplifies global collaboration among Earth scientists, using cutting-edge satellite imagery and modelling techniques to unravel the complexities of cryospheric hazards.” said Mohd. Farooq Azam, Intervention Manager, Cryosphere, ICIMOD, and a co-author of the study.
GLOFs occur when lakes fed by glacier melt release large volumes of water due to dam failures triggered by glacier calving, heavy rainfall, avalanche. These events pose increasing threats to mountain communities as climate change accelerates glacial retreat and destabilises moraine dams.
“This study reveals how climate change is amplifying glacial hazards, with the 2023 Sikkim GLOF triggering an unprecedented cascade of destruction. It underscores the urgent need for better early warning systems, resilient infrastructure, and stronger regional cooperation to mitigate future risks.” said Arun Shrestha, Senior Climate Change Specialist, ICIMOD.
Published in the journal science, this study is a significant contribution to the growing body of research on climate change-driven cryosphere hazards. It provides crucial insights for policymakers, disaster response agencies and researchers to safeguard vulnerable mountain communities.
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