Glacial lake outburst flood (GLOF)

Why in the News?

Incessant rains have caused flash floods in north Sikkim after the South Lhonak Lake burst, which has killed 14 people so far and 102, including at least 23 Army personnel, are missing. 

The South Lhonak Lake outburst:

1. 1. 1. It is a glacial lake situated in the state’s northwest at 17,000 ft.
      2. Heavy rains led to its burst leading to the release of water in downstream areas.
      3. This led to the rise of water levels in Teesta River with very high velocities near about 15m/sec, flooded at least four districts, including Mangan, Gangtok, Pakyong and Namchi.
      4. Numerous studies have highlighted in the past that
         1. South Lhonak Lake is rapidly growing size due to glacier melting 
         2. It is susceptible to glacial lake outburst flood (GLOF)

How did South Lhonak Lake become susceptible to GLOF?

1. Rising global temperatures have led to rapid melting of glaciers in Sikkim Himalayan, giving rise to many glacier lakes and expanding the already existing ones in the region. 
2. As per Sikkim State Disaster Management Authority, there are currently more than 300 glacial lakes in Sikkim Himalayan, of which 10 have been identified as vulnerable to outburst floods.
3. Lhonak has grown nearly 1.5 times and South Lhonak nearly 2.5 times of their initial size in 1989.

What is GLOF?

1. Glacial lakes are large bodies of water that sit in front of, on top of, or beneath a melting glacier.
2. They can grow larger due to glacier melting, which can pose danger as glacial lakes are mostly dammed by unstable ice or sediment composed of loose rock and debris.
3. If boundary around such dammed obstructions break, huge amounts of water will rush down the side of the mountains causing floods in the downstream areas. This is called as glacial lake outburst floods or GLOF.
4. It can be triggered by several reasons that include:
   1. Earthquakes
   2. extremely heavy rains
   3. ice avalanches
   4. landslides (as these lakes are often found in steep, mountainous regions)