The South Lhonak Lake in Sikkim experienced a devastating burst that resulted in the loss of lives and destruction of infrastructure. Scientists had previously warned about the high probability of a sudden outburst and the potential for flash floods downstream back in 2013. Even earlier, in 2001, the Sikkim Human Development Report highlighted the catastrophic impact of glacial lake outburst floods (GLOF) and the retreat of Onglokthang, Rathong Chu, and Zemu glaciers.
A GLOF occurs when lakes formed by melting glaciers burst open, releasing a large volume of water all at once and causing flash floods. The Department of Science and Technology of the Sikkim government issued a warning in 2016, and a study published in 2021 flagged the accelerated growth of glacial lakes in the state, particularly the South Lhonak glacier.
According to the studies, the South Lhonak glacier has been receding due to climate change and global warming. This melting has increased the water level in the lake. Satellite images released by ISRO showed that the lake held nearly 170 hectares of water before the cloudburst occurred, but the water level drastically decreased afterward.
The studies predicted a peak discharge of water in the event of a breach, with the 2021 study estimating a peak discharge seven times worse than the previous prediction. The growth of glacial lakes in Sikkim has raised concerns about the potential hazards for heavily populated downstream areas.
The flash flood caused significant damage, including the destruction of the Chungthang power station, loss of lives, and destruction of infrastructure. The Sikkim government had taken some preventive measures, including partially draining the excess water from the South Lhonak Lake in 2016. Plans were also underway to install early warning systems at the lake. However, the cloudburst occurred before these measures could be implemented.
Sikkim has numerous glacial lakes, and at least 10 of them are considered vulnerable to GLOF events. The installation and functionality of early warning systems at these lakes remain a question.