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Large ice/rock avalanches have been known to generate extremely powerful air blasts capable of causing fatalities and destruction far beyond the runout of the movement mass. Thus, an appraisal of their possible destructive air blast should be considered during the risk assessments. However, much less is known about how avalanche-induced air blasts occur and how destructive they are. Here an existing DEM-CFD coupled method was innovatively employed to quantitatively analyze the initiation and propagation mechanism of the air blast caused by the largest ice-rock avalanche during Nepal's 2015 Gorkha earthquake, and compared the results with some recorded events. Our results highlighted the contribution of valley morphology to air blast generation. Large high-velocity ice/rock avalanches that have an airborne trajectory or a travel path with turning points can suddenly release tremendous energy and more probably cause destructive air blasts. Additionally, the Langtang air blast presented a propagation of 1 km from the avalanche impact point and equivalent destruction to an EF2–EF5 tornado, causing mass casualties. The large impact region and destructive force of avalanche-induced air blasts suggests their disastrous consequences that should provoke assessment of such type of cascading effect during the landslide risk assessment, especially for the long-term sustainability of planned infrastructure in high-altitude regions. [ABSTRACT FROM AUTHOR]

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. • Langtang avalanche is a dual phase and multi-staged process in a snowline region. • Runout analysis is conducted with r.avaflow to track debris deposition sequence. • Optimum flow parameters for such avalanche is back-calculated. • Simulation results are verified using displaced rock block and other field data. [ABSTRACT FROM AUTHOR]