Assessing the risks from a potential Glacial Lake Outburst Flood in the Alaknanda Basin, Central Himalaya

The Himalayan region is experiencing a higher rise in temperature than the global mean, leading to glacier retreat. This retreat is contributing to the rapid formation and expansion of numerous moraine-dammed lakes. Simultaneously, the Himalayan region is witnessing a surge in development activities, including road and tunnel construction, hydropower projects, rapid urbanization, and a booming tourism industry. These changes increase the region's susceptibility to glacial lake outburst floods (GLOFs) by altering the natural ecosystem. However, infrastructure development activities can require significant time, during which retreating glaciers may create new lakes, creating new challenges for risk management. Consequently, regions that appear safe today may become vulnerable in the future. In this context, our study focuses on the Alaknanda basin, an area with a high concentration of glaciers and extensive developmental activity.

In this study, we have identified potential lake sites in the Alaknanda basin and assessed their impact on the infrastructure. We used the Himalayan Glacier Thickness Mapper (HIGTHIM) tool to estimate future glacial lakes. It uses the laminar flow method, which estimates ice thickness by balancing factors such as ice surface slope, glacier geometry, and basal shear stress. This method identifies potential subglacial depressions that may become exposed during glacier retreat, thus predicting the area and volume of future glacial lakes.

We identified 28 potential lake sites with a total area of 471.81 ha and a potential to store 113.4 million m³ of water. A highly susceptible lake in the Vishnuganga sub-basin, with an area of 38 ha and a volume of 14.5 million m³, is assessed to understand the impact of GLOF on the downstream region. This study evaluates a possible GLOF caused by a moraine-dam failure using the hydrodynamic model HEC-RAS. This model uses SRTM DEM to understand the channel geometry and downstream topography. The simulation predicts an average flood depth of 10 m and a flood velocity of 5 ms^-1 within the settlement, located 10 km from the lake. This settlement experiences flooding within 53 minutes with a peak discharge of 6200 m³s^-1 in a high-risk scenario.

Considering the uncertainties in future moraine formations, a sensitivity analysis was conducted by varying the moraine breaching parameters to model different risk scenarios. In addition, the study incorporates hazard, vulnerability, and risk mapping of the downstream flood-affected area to assess potential GLOF impacts comprehensively. These findings highlight the critical role of GLOF prediction in safeguarding downstream communities and guiding future planning efforts. Considering the ongoing and future development activities in this region, it is essential to integrate GLOF risk assessments into development strategies to minimize the potential impact on vulnerable communities and infrastructure.