Factors regulating rock-ice avalanche mobility: insights from the 2020 Zelunglung event

On 10 September, 2020, a large rock-ice avalanche occurred in the Zelunglung Ravine, Southeastern Tibet. The avalanche travelled over 9.78 km, dammed the Zhibai Bridge, and partially blocked the Yarlung Tsangbo. This study reconstructs the event using field investigations and numerical modelling, examining the effects of air temperature, initial ratio of the volume fractions of ice and water (i.e., ice/water fraction), and bed wetness on flow mobility. The results suggest that avalanche mobility is heavily governed by the material composition of the flow system, particularly the proportion and distribution of ice and water. Warm conditions promote meltwater production, increase flow depth, and enhance runout distance. The internal ice and water composition controls how these lubricated materials are incorporated into the mixture, producing a nonlinear response with a critical ratio yielding minimum resistance. In addition, bed wetness strongly affects the entrainment process, leading to rapid increases in water content and further enhancing mobility in a wet scenario. These findings underscore that ambient air temperature, internal material composition, and substrate properties jointly control rock-ice avalanche mobility, providing valuable implications for hazard assessment in high-altitude regions.