Hazard, and risk modelling of glacial lakes in the Sikkim Himalaya: Integrating qualitative and quantitative approaches for hazard assessment

Glacial lakes are prone to outburst due to breaching or overtopping of natural dams, leading to Glacial Lake Outburst Floods (GLOFs). These floods represent a substantial danger to communities downstream and essential infrastructure. To address this issue, it is crucial to investigate certain key factors, including gaining insight into the dynamics of glacial lakes, identifying dangerous ones, and assessing the associated level of risk. In this study, we scrutinized the temporal changes of glacial lakes in the Sikkim Himalaya for the years 1990, 2000, 2010, and 2020 using Landsat satellite imageries. Furthermore, we performed a hazard assessment of GLOFs in terms of both qualitatively, and quantitatively assessment. Moreover, qualitative evaluation involves seven external triggers and quantitative assessment employed by utilizing the Fuzzy Analytical Hierarchy Process (FAHP) method, considering six causative factors. Finally, we modelled the degree of risk associated with glacial lakes by quantifying downstream impacts with the help of GIS-based Stochastic Inundation Monte-Carlo Least Cost Path model (MC-LCP). The findings revealed that the number of glacial lakes increased from 309 to 440, with a total area expansion from 22.83 km² to 30.71 km² over the 30-year period. Also, among the 51 glacial lakes susceptible to GLOFs, seven lakes were classified as highly hazardous and another six were deemed to pose high risk. Overall this study contributes to planning field investigations and developing effective mitigation strategies to minimize the significant socio-economic losses caused by GLOFs in the Sikkim Himalaya.