Time bomb: Sikkim glacial lake’s size trebled in 3 decades before bursting | Kolkata News

North Sikkim’s South Lhonak glacial lake, the epicentre of the massive flash flood that ravaged the region on Wednesday, had been literally bursting at the seams after more than tripling in size from 0.42 sq km to 1.35 sq km in less than three decades. As Lhonak grew between 1990 and 2019 into a giant reservoir holding 65 million cubic feet of water, a 2021 study warned it was just a matter of time before a glacial lake outburst flood, or GLOF, caused massive devastation downstream.
As one of Sikkim’s largest and fastest-growing glacial lakes, South Lhonak was always a potential risk for ‘GLOF’, a sudden release of water from a lake formed by glacial melt.“This was a known vulnerability,” said Ashim Sattar, the study’s lead author, of the flash flood that claimed at least 38 lives and wreaked havoc.

Sikkim has 14 other glacial lakes that are just as large as the one at South Lhonak that burst and caused massive devastation on Wednesday, said Ashim Sattar, a researcher at the Bengaluru-based Indian Institute of Science’s Divecha Centre for Climate Change, who had written a paper in 2021 warning about the growing danger from the South Lhonak lake.

One study projects a three-fold increase in the risk of GLOF hazards across high-mountain Asia, with the eastern Himalayas a hotspot.
When a glacier melts, water collects into a lake. The build-up of water can cause the lake to suddenly breach its natural barriers. Avalanches can also cause such a breach, with a recent study by Sattar pointing out that 70% of glacial lakes in the Hindu Kush Himalayas were vulnerable to landslide and avalanche-type events. An avalanche may have triggered the South Lho nak lake outburst, Sattar said.

According to the study, South Lhonak was susceptible to a GLOF for several reasons, including its expansion towards steep slopes that heightened the risk of potential landslides and avalanches. The lake lying in a seismic zone is another risk factor.
Sattar said the lake had grown large enough to cause a lot of damage if it burst. Luckily, as evidenced by satellite images, all of the water in the lake didn’t drain out when the dam burst. “It could have been worse,” he said.
Sattar’s study says “many settlements and assets along the river channel at Chungthang are potentially exposed to GLOFs, indicating the need to conduct a full environmental impact assessment and potentially undertake GLOF risk mitigation measures”.
Wednesday’s catastrophe also highlights the risk to hydropower and other development projects in the Himalayas. “When planning a project, it’s important to investigate what is happening upstream carefully,” said Farooq Azim, a glaciologist at IIT Indore. “The lake didn’t appear overnight.”
A study published last year warns of risks to more than 650 hydropower projects being planned or constructed close to glaciers, or in the path of potential lake bursts. Many of these new projects are upslope of existing ones, and in locations close to glaciers and glacial lakes, it points out.
Azam cites the disaster as an example of a “cascading hazard”. In this case, a cloudburst or avalanche combined with a GLOF, eventually leading to a dam being breached and causing even more water to flood the region. “This is a case of one hazard inducing another hazard,” he said.
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