Crucially, the study found no evidence of a “triggering cloudburst event” in the vicinity of the lake. The Sikkim government had told the National Green Tribunal last year that a cloudburst had possibly preceded the glacial lake outburst flood (GLOF).
The flood had killed 55 people and destroyed the 1200-MW Teesta III hydel project, besides damaging the Teesta IV and V projects and Teesta Low Dams III and IV, as well as farms and buildings in Bangladesh.
The study says the South Lhonak lake remains “highly susceptible” to future GLOFs — a warning that comes just weeks after an Environment Ministry panel cleared a proposal to build a new 118.64-metre high concrete gravity dam in the place of the original 60-metre tall Teesta-III dam. This ministry approval came without a fresh public hearing and even as the design aspects of the dam are yet to be approved.
The investigation into the causes, dynamics and consequences of the GLOF was carried out by a team of 34 experts from nine countries and was led by Ashim Sattar, assistant professor, School of Earth, Ocean and Climate Sciences at the Indian Institute of Technology, Bhubaneswar.
According to the study, the collapse of the frozen glacial debris, called a moraine, created a 20-metre tsunami-like wave that breached the lake’s natural dam. It released 50 million cubic metres of water — equal to 20,000 Olympic-sized swimming pools — that took just about two hours to reach the hydropower dam, 67 km downstream.
Though the exact cause of its collapse is inconclusive, the moraine — formed by glaciers over several years — was found to be unstable for years preceding the event. It showed a maximum displacement of 15 metres per year between 2016 and 2023, the study revealed.
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The area around the glacial lake was also found to be unstable, owing to thawing of permafrost, or frozen ground, likely due to climate change.
The flood’s severity was also worsened by climate warming effects and heavy rainfall, which saturated the soil and increased its vulnerability to landslides, thus amplifying downstream impacts such as erosion, sediment transport and flood intensity, said a note issued by the authors of the study.
“The GLOF peak discharge vastly exceeds meteorological flood magnitudes, suggesting that it is a rare event in the historical context of this region, equivalent to a return period exceeding 200 years,” the note by authors stated.
“South Lhonak lake remains highly susceptible to future GLOF events. The northern lateral moraine still has a large and rapidly deforming zone despite the October 3, 2023 failure,” the note added, pointing to the continuing hazard posed by the glacial lake.
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“As we work to reduce the risks of GLOFs in the Himalayas, it is clear that we need a multi-faceted approach, including early warning systems, strengthened regulatory frameworks, a paradigm shift in the GLOF modelling approaches, and robust preparation programs and community education,” said lead author Sattar.
The team of experts used seismic signal data, satellite imagery and modelling techniques to back-calculate the GLOF’s movement and validated it using ground information available with the Indo-Tibetan Border Police (ITBP) and Sikkim State Disaster Management Authority.
The reconstruction showed that the moraine collapsed at 10.12 pm on October 3. At around 10.30 pm, the flood water reached an Indo-Tibetan Border Police camp, 7.12 km downstream, which was consistent with ITBP’s information. As the lake’s natural dam burst, its level dropped 28 meters.
The reconstruction showed that at 12:30 am, October 4, the flood waters reached the 1,200 MW Chungthang hydropower dam — this was in line with the reported time of 12.35 pm.
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The study also quantified the enormous amounts of sediment eroded by the GLOF — about 270 million cubic meters, enough to fill 1,08,000 Olympic-sized swimming pools. The sediment and boulders were deposited downstream causing immense devastation and damage. Satellite-imagery based mapping also identified 45 landslides caused by the GLOF along the Teesta valley.
Co-author Wolfgang Schwanghart of the Institute of Environmental Science and Geography, University of Potsdam, Potsdam-Golm, Germany said: “The South Lhonak outburst once again 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, we will likely see similar disasters in the future.”
