Thu. Jan 2nd, 2025 2:43:53 PM

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Satellite images released by the Indian Space Research Organisation (ISRO) have revealed how a possible avalanche led to the outburst of the Lhonak Lake in north Sikkim which led to a flash flood in the Himalayan state, killing scores and washing away roads, bridges and dams

A flood-affected locality at Singtam in Gangtok district on Wednesday. (PTI)
A flood-affected locality at Singtam in Gangtok district on Wednesday. (PTI)

The death toll in the Sikkim flash flood in the Teesta river has risen to 14 while more than 100 people are missing, the state government confirmed Thursday; at least 22 army personnel are also among the 102 missing people while at least 26 injured are seeking treatment in local hospitals.

Senior officials of the India Meteorological Department (IMD) in Gangtok said the lake had rapidly increased in size in just 11 days between September 17 and September 28. It grew by more than three percent of its size during that period.

Experts said that only 37% of the lake now remains there while the rest of the water has been drained out.

“It is observed that lake is burst and about 105 hectares area has been drained out (28 September 2023 image versus 04 October 2023) which might have created a flash flood downstream,” said an ISRO statement.

Satellite images of South Lhonak Lake.
Satellite images of South Lhonak Lake.

Experts said that while the total area of the proglacial lake, situated high in mountains at altitude of 5245 metres, was around 162.7 hectares on September 17, it shrunk to around 60.3 hectares on October 4 morning.

“From the satellite images it is clear that it was a Glacial Lake Outburst Flood (GLOF). More than 60% of the lake’s water has been drained out and only around 37% of the lake now remains there,” said Ashim Sattar, a scientist with the Indian Institute of Science ( IISc) in Bengaluru, who has been studying the lake and the glacier for several years.

Also Read: Sikkim flash floods: 14 dead, over 100 missing; 3,000 tourists stranded

Lhonak lake is one of the largest and fastest growing proglacial lakes in Sikkim and is among the 11 large glacial lakes monitored by the Central Water Commission.

While the Lhonak glacier has been retreating, the lake has been growing making it vulnerable to GLOF. During the past 29 years, the glacier’s length has reduced from 6.4 km to 5.1 km.

The Lhonak lake, which measured just 0.17 square kilometres in 1977, grew to 0.78 square kilometres in 2002 and 1.35 square kilometres in 2019

“The lake has been growing in size for many years. But between September 17 and September 28, its size shot up by around 5 hectares, which is around three percent of its size. It was probably due to melting (of snow). The reasons are yet to be ascertained,” said GN Raha, director of IMD’s regional office in Gangtok.

A report compiled by the Sikkim State Disaster Management Authority in 2019 notes that in 2013, scientists from the National Remote Sensing Centre in Hyderabad reported that the Lhonak Lake was “highly vulnerable” to a GLOF event, which had the ability to cause extensive damage to life and property in downstream areas.

It warned of the possibility of “flash floods” with important infrastructure such as dams and power houses susceptible to damage. “Townships like Chungthang, Dikchu, Singtam, Rangpo are vulnerable. Loss of life and property likely,” it said.

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