Sikkim flash flood: A disaster that waited to strike

South Lhonak Lake is at an altitude of 5,200 metres above sea level

As devastation continues due to flash floods in Sikkim, at least 14 people have been killed and 104 are missing.

A cloudburst over the South Lhonak Lake in northwest Sikkim on October 4 caused a sudden gush of water in the Lhonak River. This in turn triggered a massive flood in the Teesta valley. Around 105 hectares (1.05 sq km) of South Lhonak Lake in Sikkim was drained out after a cloudburst, the Indian Space Research Organisation (ISRO)  observed with satellite imagery.

According to various reports, multiple government agencies and scientific expeditions red-flagged the vulnerability of Lhonak Lake in North Sikkim to a glacial lake outburst flood (GLOF) for at least a decade.

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Lohnak Lake has two depressions- north and south. A Down To Earth report stated that the lake is one of the fast-growing lakes out of 14 potentially dangerous lakes vulnerable to Glacial Lake Outburst Floods (GLOFs) in the Sikkim Himalayan region.

Trigger factors

South Lhonak Lake is at an altitude of 5,200 metres above sea level, on the windward side of the Eastern Himalayas. According to National Disaster Management Authority (NDMA), the primary reason for the sudden surge in water level appears to be a likely combination of excess rainfall and a Glacial Lake Outburst Flood (GLOF) event at South Lhonak lake in North Sikkim.

Sikkim Chief Secretary VB Pathak told NDTV that a cloudburst over Lhonak lake in northwest Sikkim raised water levels. The lake overflowed and water gushed towards the Teesta river, which flows through Sikkim and West Bengal before entering Bangladesh.

Satellite images released by the Hyderabad-based National Remote Sensing Centre showed that the area of south Lhonak lake was reduced by over 100 hectares compared to its size on September 17. This suggested that the lake outburst caused a flash flood in the Teesta River basin in north Sikkim. CNBC-TV18 quoted a study published in the journal Science Direct in 2021. According to the study, since these lakes are mainly situated in remote and unsettled mountain valleys, GLOFs could claim lives and damage assets tens of kilometers downstream. GLOFs occur when lakes formed by melting glaciers suddenly burst open.

Also  read: NHPC shares falls as it shuts Sikkim plant

Impact of Nepal earthquake

Scientists are exploring whether the Nepal earthquake might be a cause of the flash floods in Sikkim. The lake was already vulnerable and covered an area of 168 hectares. Its area has now reduced to 60 hectares, indicating that about 100 hectares of water volume has breached the level, a senior official at the Central Water Commission (CWC) told PTI.

According to the official, while it's difficult to determine right now, "a cloud burst does not cause such results”.

Some of the largest GLOF events have occurred in the Himalayas- the 2013 Chorabari lake outburst, known as the Kedarnath disaster in Uttarakhand. The swelling of the Jhelum river that led to Kashmir floods in 2014, and Parechu river flash floods that occurred in Himachal Pradesh in 2005 are also examples of GLOF events.