South Lhonak Lake in Sikkim was Drained after a Cloudburst

According to satellite image of the Indian Space Research Organisation (ISRO) South Lhonak Lake in Sikkim was drained out after a cloudburst that triggered flash floods on 4th October.

• On October 4th, there was a significant flooding event at Teesta River in the town of Chungthang, located in Sikkim. 
• According to the Sikkim State Disaster Management Authority, this flooding was caused by the outburst of South Lhonak Lake, resulting in a Glacial Lake Outburst Flood (GLOF) in portions of Lhonak Lake, Mangan District. 
• The rapid rise in water levels with high velocity downstream along the Teesta River Basin caused severe damage in Mangan, Gangtok, Pakyong, and Namchi Districts, particularly in the early hours of October 4th, 2023.

What is GLOF?

South Lhonak Lake is an example of a glacial lake, a large body of water located near a melting glacier. 

• As these lakes increase in size, they become more hazardous because they are often dammed by unstable ice or sediment made up of loose rock and debris.
• Large amount of water rush down the mountainside as boundaries around these lakes break. This can lead to flooding in downstream areas, as seen in Sikkim.
• This phenomenon is called glacial lake outburst floods or GLOF.
• There could be several reasons behind GLOF, like earthquakes, extremely heavy rains and ice avalanches.

In 2013, one such event occurred in Uttarakhand’s Kedarnath when the region witnessed flash floods and a GLOF caused by the Chorabari Tal glacial lake, killing thousands of people.

Reasons for South Lhonak Lake becoming susceptible to GLOF

Due to increasing global temperatures, glaciers in the Sikkim Himalayan region are melting rapidly. This has led to the formation of many new glacier lakes and the expansion of existing ones. 

• As per a report by Mongabay in 2020, there are currently over 300 glacial lakes in Sikkim Himalayan, out of which 10 are considered vulnerable to outburst floods. One of these lakes is the South Lhonak Lake, which has been under observation by government authorities for several years. 
• A report by the Sikkim Forest and Environment Department has revealed that the lake's area has significantly increased over the past five decades.

Action to tackle the expanding South Lhonak Lake

In 2016, a group including the Sikkim State Disaster Management Authority and the Department of Science and Technology and Climate Change in Sikkim, along with others, worked together to remove water from South Lhonak Lake. 

• Innovator Sonam Wangchuk supervised the process, which involved installing three 130-140 meter long, eight-inch wide High Density Polyethylene pipes in the lake to extract the water.