Sikkim’s South Lhonak lake, which had disgorged water matching 20,000 Olympic swimming pools into the Teesta river valley 16 months ago claiming 55 lives, remains highly susceptible to another glacial lake outburst flood, a study has suggested.
The study, by an international team of scientists, underlines the need for both risk mitigation strategies and early warning systems against GLOFs, the sudden release of water from glacial lakes caused by the failure of natural rock and sediments to hold the water back.
The researchers have combined satellite imagery, seismic and weather data, field observations, and computer simulations to reconstruct the cascade of events and generate the most comprehensive analysis yet of the disaster that unfolded on October 3, 2023, night.
“Our reconstruction has implications for future preparedness,” said Ashim Sattar, a glaciologist at IIT Bhubaneswar and lead author of the study, published on Thursday in the US research journal Science.
“Both the triggers and the impacts of GLOFs are complex — we need to take into account myriad factors, both upstream that could trigger a GLOF and downstream where the impacts will unfold,” he said.
The analysis by Sattar and his colleagues has shown that intense episodes of rainfall on October 3, 2023, had primed the landscape for the landslide that dumped 14.7 million cubic metres of sediments — enough to fill 14 Empire State Buildings — into the lake.
The collapse generated a 20-metre-tall, tsunami-like wave that breached a natural-rock-and-sediment barrier and released roughly half the lake’s water — 50 million cubic metres, or enough to fill 20,000 Olympic pools — downstream into the Teesta river valley.
The gushing water chipped away at the valley’s flanks, triggering smaller landslides and eroding an estimated 270 million cubic metres of sediments along the valley up to 70km downstream.
The study comes three weeks after a Union environment ministry panel had, on January 10, approved a proposal to rebuild a dam at Chungthang where the GLOF had washed away Sikkim’s largest hydroelectric unit, the 1,200MW Teesta III project.
Public opposition to what is being viewed as “hasty” approval for the new dam is brewing in the Sikkim-Darjeeling region.
The (South Lhonak) lake is exposed to unstable rocky ridges and rock-and-soil buildup flanking its sides, making it “susceptible to potential GLOFs”, the study has suggested.
The October 2023 GLOF’s impacts reached up to 385km from its source, its floodwaters leaving 55 people dead and 74 missing, and triggering over 40 smaller landslides along the valley.
The flood debris impacted other hydro projects, including Teesta V and Teesta VI in Sikkim and Teesta Low Dams III and IV in Bengal, apart from the Chungthang hydroelectric unit that was destroyed.
The study has warned that current GLOF models could at times underestimate flood risks.
“Current GLOF models fail to adequately consider sediment entrainment and riverbank collapses, leading to potential underestimation of flood risks,” the study says.
It adds that climate-warming effects and preceding rainfall intensified the destruction. It says the left flank of the lake had been unstable and moving towards the lake at about 10 metres a year between 2016 and 2023.
“To reduce the risks of GLOFs in the Himalayas, we will need a multi-faceted approach, including early warning systems, strengthened regulatory frameworks, a paradigm shift in GLOF modelling approaches, and robust preparation programmes and community education,” Sattar said.