Glacial lake outburst, excess rainfall led to Sikkim flash floods, says NDMA

The primary reason for the Sikkim flash floods appears to be a likely combination of excess rainfall and a GLOF (glacial lake outburst flood) event at the South Lhonak lake in north Sikkim, the National Disaster Management Authority (NDMA) said on Wednesday.

The apex disaster management authority added that nearly 25 glacial lakes in Sikkim are assessed to be at-risk.

At least 14 persons were killed and another 120 people, including 23 army personnel, went missing after the flash flood hit north Sikkim in the early hours of Wednesday. There was a sudden surge in water flow in the Teesta river, which washed away several bridges, parts of NH-10, the Chungthang Dam and has impacted several small villages, towns and infrastructure projects in the upper reaches of the river valley. The main districts affected are Mangan, Gangtok, Pakyong and Namchi. According to people familiar with the matter, the death toll is expected to rise sharply.

Also Read | Sikkim: 14 dead, 120 missing, bridges, dams, roads washed away in flash flood

“While scientists are investigating the exact cause of the flash flood, the primary reason for the sudden surge appears to be a likely combination of excess rainfall and a GLOF event at South Lhonak lake in North Sikkim,” NDMA said in a statement.

The lake is at a height of 5,200 metres, with a towering ice-capped feature at about 6,800 metres to the north of and in close proximity to the lake.

Satellite images received from the National Remote Sensing Centre or NRSC at 6am on Wednesday reveal “the draining out of more than half the lake, most likely as a result of an avalanche from the ice-capped feature”, NDMA added.

Also Read | Sikkim dam washed away in 10 minutes after flash flood: Official

The first surge of water was 19 metres above the maximum water level at Sangkalang at 1.30am and 4 metres above the maximum water level at Melli at 4am.

NDMA said that continued snowfall in upper reaches and rainfall and clouds in lower reaches is hampering deployment of helicopters and relief operations.

The Himalayan ranges, NDMA said, are host to many glacial lakes, estimated through remote sensing techniques at about 7,500.

“Of these, Sikkim has about 10%, of which nearly 25 are assessed to be at-risk,” NDMA added.

To mitigate the impact of a GLOF event in these lakes, an NDMA-led expedition in the first week of September 2023 had surveyed two at-risk lakes in order to eventually deploy early warning systems for real-time alerts.

Also Read | Sikkim flood victim says her house washed away within 30 minutes after warning

It has planned to install early warning systems for real-time alerts at most of 56 at-risk glacial lakes in India, NDMA said.

“Efforts to expand the mitigation programme are being expedited, while sustained investigations into the causes of this event are underway. Further, this region is known for highly localised heavy rainfall events. Therefore, efforts to improve the predictive ability for such events will also be intensified in collaboration with relevant agencies,” it said.

Meanwhile, Union cabinet secretary Rajiv Gauba chaired a meeting of National Crisis Management Committee (NCMC) to review the situation and asked officials to evacuate people stranded in tunnel of Chungthang dam and tourists in the area on “priority basis”.

He also asked for deployment of additional National Disaster Response Force (NDRF) teams and restoring the road and mobile connectivity at the earliest.