Dynamic analysis of the multi-staged ice-rock debris avalanche in the Langtang valley triggered by the 2015 Gorkha earthquake, Nepal

On April 25, 2015 a disastrous rock avalanche was triggered by the Gorkha earthquake in the Langtang valley of central Nepal, that buried a touristic village and killed >350 people. This was a dual-phase avalanche comprising of glacier snow-ice and rock debris mixture, which interacted in multiple paths from different source areas. In this study, the runout and deposition process of the Langtang avalanche were analyzed using a two-phase mass flow model (Pudasaini, 2012) with the r.avaflow software to recreate the flow heights, velocity, and deposition structure and sequence. The simulation results indicated that the duration of the total failure process was 140 s, and the avalanche flowed with an average velocity of 36 m/s. The interaction of the avalanche material from two different stages deflected its path, directing it toward the downstream end of the valley and generating a splashing air blast, which caused significant damage in that direction. Parameter calibration, model performance, and verification were performed numerically and based on field observations. This study gave insight into the failure mechanisms of the dual phase and multi-staged avalanche occurring in the snowline regions and explained the path deflections due to the stage interactions. The results provide a reference for hazard zonation in such mountainous regions.