Glacial Lake Outburst Flood of South Lhonak Lake in Sikkim

Glacial Lake Outburst Flood in Sikkim

Context (TH | IE | IE | DTE): Due to the outburst of South Lhonak Lake, a glacial lake, flash floods have inundated Sikkim and breached the Chungthang Dam.

Glacial lakes form when a glacier erodes the land, and the depression is filled by glacial meltwater.
It typically forms at the foot of a glacier but may form on, in, or under it.

As glacial lakes grow larger, they become more dangerous because glacial lakes are mostly dammed by unstable ice or sediment.
In case the boundary around them breaks, huge amounts of water is released causing catastrophic floods downstream. This is called glacial lake outburst floods (GLOF).
GLOF can be triggered by several reasons, including earthquakes, heavy rains and ice avalanches.

South Lhonak Lake a located in Sikkim’s far northwestern region.
It is one of the fastest expanding lakes in the Sikkim Himalaya region, and was a potentially hazardous lakes susceptible to GLOFs.
The probable causes for the GLOF of South Lhonak Lake are:
- Cloudburst: Excess rainwater from cloudbrust has caused sudden surge in water level due to which the GLOF occurred.
- Nepal Earthquake: GLOF my have been triggered by 6.2 magnitude Nepal Earthquake that occurred a day before.
- Ice Dam Failure: It happens when glacial lake water volume exceeds the dam’s capacity, leading to potential weakening or rupturing of the dam, releasing a torrent of water downstream.

A cloudburst is a localised but intense rainfall activity.
However, not all instances of very heavy rainfall, are cloudbursts.
Rainfall of 10 cm or more in an hour over a roughly 10 km x 10 km area is classified as a cloudburst.
During a cloudburst event, a place receives about 10% of its annual rainfall within an hour.
While it can occur in plains, the phenomenon is most common in hilly regions.
In hilly regions, the local topology, wind systems, and temperature gradients between the lower and upper atmosphere facilitate the occurrence of such events.
Because of the terrain of hilly regions, cloudbrusts often trigger landslides and flash floods, causing extensive destruction downstream.

A flash flood is a sudden, intense flooding event triggered by heavy rain, storms, dam failures, or similar factors, leading to a rapid and overwhelming surge of water.
Flash floods can occur within minutes or hours of the triggering event.
They are characterised by their high intensity, short duration, and the potential for significant damage and danger to life and property.

The Chungthang Dam breached due to the sheer force and speed of the water from GLOF.
The huge amounts of water released from the dam further worsened the flooding of Sikkim.

Chungthang Dam is the largest hydropower project in Sikkim.
This dam is part of the 1,200-megawatt (MW) Teesta Stage III Hydro Electric Project, in which Sikkim government is the majority stakeholder.

The series of disasters that occurred in Sikkim after the triggering of GLOF of South Lhonak Lake again proves that cascading hazards are becoming frequent due to climate change and human activities.
It also reinforced that Himalayan regions are more susceptible to such disasters.
Addressing these disasters requires an “integrated disaster management approach” that considers the connection between hazards and specific incidents to formulate effective risk-mitigation plans.

Cascading hazards, also known as secondary hazards or chain reactions, refer to a sequence of hazards that occur due to an initial triggering hazard (primary hazard).