Background

In Sikkim Himalaya many glacial lakes are in the process of formation. There are high chances of bursting of these lakes due to their formation in weak moraine structures. It may cause huge devastation in the downstream in the case of bursting. The hazard of Glacial Lake Outburst Flood (GLOF) may get aggravated in case of cloud burst situation in the area.

South Lhonak glacial lake, located in the extreme North-western parts of Sikkim, is one of the fastest growinglakes in Sikkim Himalaya. The lake is fast increasing in its size in an extremely abnormal way due to the melting of the South Lhonak glacier attached with the lake and addition melt water from adjoining North Lhonak glacier and main Lhonak glacier. The abnormal growth of the volume of the lake is a great concern in terms of glacial lake outburst floods. The lake is purely a glacial moraine dammed lake which is vulnerable in terms of GLOFs.

Objectives

• To assess the probability of Glacial Lake Outburst Flood (GLOF) in South Lhonak lake
• To determine the health of moraine damming south Lhonak lake with the help of Electrical resistivity survey
• To conduct Bathymetry for determining depth and volume of lake.

Institutions Involvement

The State Climate Change Cell under Sikkim State Council is actively engaged in the study of South Lhonak lake in support of Department of Science and Technology, Government of India, Snow & Avalanche Study Estt. Chandigarh, Geological Survey of India, Centre for Development of Advanced Computing, Pune and Wadia Institute of Himalayan Geology, Dehradun etc.

Outcomes of Research Under GLOF

• The study of past satellite data of Lhonak glacier lake area revealed that the area of the lake has increased from 18 ha or 180,000 m2 in 1976 to 126 ha or 1,260,000 m2 in 2013.
  
  The increase in the area has been rapid at an approximate rate of 25,000 m2 to 35,000 m2 per year from 1976 to 2011. With this figure, the lake became one of the longest and largest lake within the territory of Sikkim.
• The lake has formed on the loose glacial moraines debris brought down by the South Lhonak glacier.
  
  This enormous growth on loose moraines within the short period of time makes it one of the vulnerable lakes in Sikkim in terms of glacial hazard in the form of glacial lake outburst flood.
• In order to mitigate the threat, preliminary investigation has been carried out. So far geophysical survey of the moraines along the outlet area of the lake has been conducted in order to know the resistive characteristics of the moraines. The study shows the presence of dead ice in the depth of the moraines. In long run, the slow melting of this ice may weaken the moraines structures.
• In another studies, the bathymetry survey of the lake has been conducted to know the volume of the lake. This is the first systematic bathymetric work in Sikkim carried out at an altitude of 5200m above mean sea level, at an extreme windy and glaciated terrain.
  

   

  

   

  Bathymetric map of South Lhonak Glacial Lake (A) and 3D Terrain model generated through DEM acquired through bathymetric survey of South Lhonak Lake (B)
  • The bathymetric analysis revealed that the storage volume of the lake is calculated to be around 53 million m3. The maximum depth of the lake is recorded more than 260 feet in the interior areas of the lake. This figure will help in analysing the risk level in case of lake burst. Based on these studies further mitigation measures are being recommended.
  • Disaster risks were analyzed by numerical simulation of South Lhonak Glacial Lake Outburst using the ANUGA and SMS TUFLOW models (as done by Working Group from field bathymetric data). The simulation of peak discharge from the lake has revealed that the water flow would take 3-4 hours to reach the first habitat in the region, in case of GLOF event.