Glacial Lake Outflow Hazard and Risk Probability in Sikkim

Glacial lakes are the main water source of Sikkim and its rivers, especially Teesta and Rangit without which economic activity in the state would have been next to impossible as agriculture and tourism are the main revenue sources. In this study, an attempt was made to analyse the glacial lake outflow risk probability in Sikkim along with a spatio-temporal change investigation of the hazardous glacial lakes over a period of thirty years (1990–2017) and also comparing them with the previous decades till 1974. The inventory map was used for change detection of the glacial lake. The hazardous lakes were determined using a site suitability model designed for the study area exclusively. The prediction of the hazard, which can be created by the hazardous glacial lakes, was done using the depth and volume determination of eight sample lakes with their probable water outflow. The susceptibility of villages was determined using network analysis of the flow rate of the glacial flood water. The lake area ranged from 0.005 to 200 Ha in the years 1974–2017 in Sikkim. A total of 282 glacial lakes (2017) were demarcated from the present work, and they are distributed throughout Sikkim mostly far from settlements and depending on the factors mentioned above—glacial lake connectivity, area, slope and distance from settlement and HEPs, growth of lakes, glacier connectivity—222 lakes were found to be potentially vulnerable. The hazardous lakes have increased from 138 out of 213 lakes in 1990 to 222 hazardous lakes out of 282 in 2017. Upon analysing the temporal changes and depth of the 8 sample lakes, it was found that there was tremendous increase in their size and volume increasing the vulnerability of the nearby villages and army camps of North Sikkim. Lachung and Thangu from North Sikkim are the most vulnerable villages, along with its nearby infrastructure (HEP), to GLOF hazard. An attempt has also been made to manage the risk of the impending disaster and to cope with its effects.