Modeling glacial lake outburst flood process chains in Sikkim Himalaya: Hazard assessment of two potentially dangerous lakes

The presence of large and rapidly growing glacial lakes along the Himalayan Arc makes glacial lake outburst floods (GLOFs) a serious mountain hazard. While glacial lakes are mainly located in remote and unsettled mountain valleys, far-reaching GLOFs may claim lives and damage assets tens of kilometers downstream. Evaluating GLOF hazard is therefore of high importance, considering current and potential future climate-driven changes of glaciers and glacial lakes. A major concern in the Northeastern Indian Himalayan state of Sikkim is the damage potential these flood events can cause to hydropower plants and local vulnerable communities. This is particularly true for outburst floods potentially originating from the two lakes in Sikkim that are considered hazardous: the South Lhonak Lake and the Shako Cho Lake. Both lakes have been recognized in previous studies, and by local and state authorities, as being high priority sites for further monitoring and potential risk reduction measures. Recognizing the need for related risk reduction strategies to be based on robust scientific understanding, this study aims to combine remote sensing approaches with hydrodynamic flood modeling to identify key threats to lives and livelihoods.

This study also provides the first implementation of recently developed national guidelines on the management of GLOFs, where a detailed risk assessment including potential GLOF triggers, conditioning factors, and downstream impacts forms the scientific core. First results of only-water flow using HEC-RAS show that a high-potential scenario (dam breach depth = 40 m) produces flow depth and flow velocity up to 25 m and 9-12 m s^-1, respectively, at Chungthang, a town located close to a major hydropower station, 62 km downstream of the lake. The fact that GLOF flow rheology is often changing as it propagates downstream, further modeling has been undertaken with r.avaflow, which can simulate the entire process chain from initial avalanche triggering, to dam erosion, and downstream flow propagation with a multi-phase modeling approach. Hence, we can evaluate the potential downstream impact in the case of a GLOF transitioning into a debris flow process. Our results provide flow hydraulics including flow velocities, flow heights, and total downstream inundation. These parameters will provide important insights for risk reduction strategies, such as early warning systems and land-use planning under current and future glacial conditions.