Excess rainfall, glacial lake outburst likely cause of Sikkim flood: NDMA

The National Disaster Management Authority (NDMA) on Wednesday said the possible cause of the flash flood in Sikkim could be a combination of excess rainfall and a Glacial Lake Outburst Flood (GLOF) event at South Lhonak lake in North Sikkim.

In a statement, the NDMA also said the Himalayan ranges are host to many glacial lakes, estimated through remote sensing techniques at about 7,500 and of these, Sikkim has around 10 per cent of which nearly 25 are assessed to be at-risk.

Giving details of the incident, the NDMA said 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 affected districts are Mangan, Gangtok, Pakyong and Namchi.

"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 (Glacial Lake Outburst Flood) event at South Lhonak lake in North Sikkim," it said in the 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 ISRO at 0600 hours 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.

The Central Water Commission's monitoring stations revealed that the first surge of water was 19 metres above the maximum water level at Sangkalang at 0130 hours and 4 metres above the maximum water level at Melli at 0400 hours.

As soon as news was received, the NDMA said, alerts were issued to downstream habitations and relief and rescue operations were initiated by the NDRF, SDRF, ITBP, Army, and Sikkim civil administration.

Several persons, including 23 Army men are missing, it said.

Continued snowfall in upper reaches and rainfall and clouds in lower reaches is hampering deployment of helicopters and relief operations.

However, the NDMA said consistent efforts are underway to assist the state government in all manner possible

Relief camps have been set up at several locations in the affected areas.

Recently, in order 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.

At the next stage, the NDMA has planned to instal early warning systems for real-time alerts at most of 56 at-risk glacial lakes in India.

Efforts to expand the mitigation programme are being expedited, while sustained investigations into the causes of this event are underway, the statement said.

Further, the North Sikkim 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, the NDMA said.