Guwahati: The catastrophic South Lhonak Glacial Lake Outburst Flood (GLOF) that devastated parts of Sikkim in October 2023 was not triggered by a cloudburst or an earthquake alone, but by a dangerous chain reaction involving glacier collapse, landslides and moraine failure that unfolded over several months, according to a new scientific study.
The study, published in Scientific Reports, was authored by researchers Litan Kumar Mohanty of the Wadia Institute of Himalayan Geology and the Central Ground Water Board, Prateek Gantayat of the Institute of Science and Technology Austria, Ankur Dixit of Cornell University, Manik Das Adhikari of Gangneung-Wonju National University, Rahul Biswas of the CSIR-National Geophysical Research Institute, and Vivek Kumar Singh of the Singapore-MIT Alliance for Research and Technology.
The researchers reconstructed the sequence of events behind the disaster, which killed at least 24 people, left more than 70 missing, destroyed 13 bridges and swept away the Chungthang hydropower dam, affecting over 60,000 people downstream in the Teesta basin.
The analysis found that nearly 38 million cubic metres of debris from a landslide crashed into South Lhonak Lake shortly before the outburst, while another 7 million cubic metres of glacier ice broke off into the lake through rapid calving.
Together, the impacts displaced around 45 million cubic metres of water, generating massive flood waves that breached the moraine dam holding back the lake.
According to the study, the disaster was the culmination of years of instability in one of the Himalaya’s fastest-expanding glacial lakes.
“The rapid glacier retreat completely destabilised the left lateral moraine,” the researchers noted, pointing to accelerated melting and repeated calving events prior to the flood.
Satellite analysis showed that the South Lhonak Glacier had retreated by more than 100 metres between February and September 2023 alone, weakening the surrounding moraine structure and leaving behind “dead ice” beneath the slopes.
The researchers also detected long-term land subsidence around the lake. Using Sentinel-1 satellite data, they found that sections of the lateral moraine had been sinking by an average of around 22 mm per year between 2017 and 2021, indicating deep structural instability beneath the surface.
Contrary to early speculation, the study ruled out both cloudburst and earthquake activity as the primary triggers behind the disaster.
While southern Sikkim experienced heavy rainfall in early October 2023, the researchers found that North Sikkim — where South Lhonak Lake is located — remained relatively dry during the critical period. Weather simulations and satellite precipitation data showed no evidence of an extreme cloudburst directly over the lake.
Similarly, seismic activity was deemed too weak to have caused the breach. The study concluded that even the nearby Goalpara earthquake generated only “weak shaking” in the lake region, insufficient to trigger a GLOF.
Instead, the researchers identified what they described as a “compound hazard” scenario: glacier retreat weakened the moraine, moderate rainfall increased liquefaction and seepage, meltwater streams eroded unstable slopes, and finally a landslide triggered further glacier collapse into the lake.
The paper also warned that such disasters could become more common across the Himalaya as glacial lakes continue to expand under rising temperatures.
According to the study, the area of South Lhonak Lake increased nearly eightfold between 1975 and 2023, with particularly rapid growth in recent years due to repeated calving events.
The researchers have called for continuous satellite monitoring of Himalayan glacial lakes, especially to track lake expansion, ground subsidence and glacier calving activity, saying these may provide critical early warning signs before future GLOFs occur.
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