Army officials recover a vehicle after the flood in Sikkim in October 2023 (File photo)
| Photo Credit: The Hindu/Ritu Raj Konwar
A glacial lake outburst flood in Sikkim in October 2023 led to the collapse of the Teesta III hydroelectric dam, killed more than 100 people, and affected thousands of others. And according to the World Meteorological Organisations ‘State of the Climate Asia 2023’ report, published in April, it was one of the worst climate-related disasters to have occurred on the continent last year.
The geohydrology of GLOFs
A body of water influenced by glaciers is called a glacial lake. As glaciers move, they erode the land underneath and the debris of rocks and soil scraped from the land from ridges called moraines. Glacial lakes are mostly on the margins of glaciers and evolve from ice-contact lakes — where the lake is in touch with the ice — into distal lakes as the glaciers retreat.
Moraines or ice blocks can form natural boundaries that hold back water and convert it into lakes. A glacial lake outburst flood (GLOF) happens when this holding structure becomes weaker, allowing the water to rapidly gush out., causing downstream flooding.
Glacial ice is sensitive to changes in regional temperature, precipitation, and surface radiation. When glacial ice melts, it affects sea level, regional water cycles, and creates hazards like GLOFs. The High-Mountain Asia region, centred on the Tibetan plateau, holds the largest volume of ice outside the polar regions.
The 2023 Sikkim disaster
Early on October 4, a breach in Sikkim’s South Lhonak resulted in a GLOF. The Teesta III dam – Sikkim’s biggest hydropower project – was located 60 km away at Chungthang. The GLOF destroyed the facility and rendered significant destruction downstream.
Satellite images in the aftermath from the National Remote Sensing Centre suggested around 1 sq. km of the glacier-fed lake had been drained, compared to its volume on September 28. These images didn’t reveal lake’s depth, however. In a press release published after the disaster, the National Disaster Management Authority (NDMA) cited the Central Water Commission’s monitoring stations to say the first surge of water was 19 m above the maximum water level at Sangkalang at 1.30 am and 4 metres above the maximum level at Melli at 4 am.
A few weeks later, GLOF expert and IIT Bhubaneswar assistant professor Ashim Sattar said a GLOF-triggered landslide had blocked the river channel 30 km downstream of the lake, resulting in a new lake. And although the new lake “drained partially through a channel beneath the landslide debris, [it] still exists and needs regular monitoring,” Dr. Sattar said in a post on X.
A ‘clear’ link to climate change
“The 2023 Sikkim event caused a lot of human fatalities and huge amount of damage to critical infrastructure such as the Teesta III HEP, roads, and bridges in Sikkim,” Miriam Jackson, a senior cryosphere specialist at the International Centre for Integrated Mountain Development (ICIMOD) told The Hindu.
According to Dr. Jackson, the South Lhonak glacier has been shrinking due to climate change. “As the glacier got smaller, a lake formed in front of it, dammed by the glacial moraine. There may have been some instability too because the ground was frozen, and such frozen ground has also been changing and thawing recently due to climate change,” she said. “Often there are also such moraine ridges at the sides of glacial lakes. In the case of south Lhonak, it seems there was failure at the side, and this triggered the GLOF.”
Rises in surface temperatures work differently in different areas. In colder, mountainous regions like the eastern Himalayas, even a small change can have drastic implications for local ecosystems because they melt glaciers and ice caps and thawing ice on the ground.
Recalling her experience of working on a glacial lake at Harbardsbreen in Norway, Dr. Jackson said although hydropower plants can exacerbate the damage, as they did in Sikkim, in many cases they include large reservoirs that can help to mitigate the flood’s damage. “At Harbardsbreen, we told the hydropower people to empty the reservoir as a GLOF would occur soon. They did, and a five million cubic metre flood did no damage.”
Max Van Wyk de Vries, assistant professor of natural hazards at the University of Cambridge, said on X that a landslide of waterlogged moraine triggered the GLOF, and the “destabilization of these lateral moraines and opening of the lake basin is closely tied to glacier frontal retreat”.
“Its link to climate warming is clearer here than in most cases,” he added.
Glacial lakes expanding
Satellite images from the Indian Space Research Organisation (ISRO) of the catchments of Indian Himalayan river basins from 1984 to 2023 reveal significant change in the prevalence of glacial lakes. Of 2,431 lakes larger than 10 ha identified in 2016-2017, 676 have notably expanded since 1984.
ISRO also said long-term change analyses of these lakes is essential to understand environmental impacts and develop risk-management and adaptation strategies in glacial environments. “One option is to not build critical infrastructure downstream from glacial lakes, but another is to build infrastructure that lessens the floods,” Dr. Jackson said.
“Adequate early warning systems and infrastructure that is built according to possible GLOFs are the most important actions to take. These events will continue to happen in the Hindu Kush Himalayas and probably become more frequent and bigger, but that doesn’t mean they need to cause more deaths and damage,” she added.
