Death toll rises to 40 in Sikkim: What is glacial lake outburst flood that caused massive damage

A Glacial Lake Outburst Flood is a catastrophic flood which occurs as a result of the failure of a dam containing a glacial lake and releasing a massive amount of water downstream. This catastrophic flood occurs due to the rapid melting of glaciers or any build-up of water in the lake due to heavy precipitation or the inflow of glacier-melted water. These floods are usually triggered by several possible factors which include changes in the volume of the glacier, changes in the water level of the lake, and earthquakes as well.

According to the National Disaster Management Authority (NDMA), the glacial retreat has become frequent due to climate change occurring in most parts of the Hindu Kush Himalayas and has also contributed to the formation of new glacial lakes.

The Lhonak Lake, which caused the massive flood, is a glacial lake and it has been surrounded by many glaciers.

Cloudburst is a geo-hydrological hazard. The huge scale and destruction of rainfall are hazardous and in India, the cloudburst occurs mostly during the South-West monsoon season. Cloudbursts are caused when the saturated clouds are unable to produce rain because of the upward movement of warm air currents. Instead of dropping the rain down, it gets bigger in size and gets pushed up by the air current. Eventually, they become too heavy to hold and drop rapidly down, leading to unusually heavy compared to the usual volume.

A flood is associated with heavy or excessive rainfall in a short duration of time. Flash floods are usually characterised by raging torrents after heavy rains which rip through river beds, urban streets, or mountains sweeping most of the surrounding area. They can occur within minutes or even a few hours of excessive rainfall. Besides, flash floods can also occur without heavy rain in cases of the collapse of boundaries of glacial lakes or rising water levels due to the melting of glaciers.