Entrainment‐Driven Transition From Avalanche to Debris Flow: Insights From the 2018 Sedongpu Event

Abstract Flow regime transitions from rock‐ice avalanches to debris flows remain insufficiently quantified, particularly in terms of the relative influence of internal meltwater production versus external water entrainment. On 17 October 2018, a large rock‐ice avalanche‐debris flow occurred in the Sedongpu gully, Southeastern Tibet, traveling over 10 km and impounding the Yarlung Tsangpo River to form a landslide‐dammed lake. Here we reconstruct the evolution of this catastrophic event using field investigations and a state‐of‐the‐art multiphase thermomechanical model, and assess how ice/snow melt and substrate entrainment influence the flow regime transition. Our results suggest that the highly saturated entrained moraine fosters the swift transition from rock‐ice avalanche to water‐rich debris flow, increasing its mobility and destructive potential. Water influx from the saturated substrate, which resulted from glacial meltwater and precipitation, was a more dominant driver of the flow regime transition than internal ice melt, with notable ice fragments remaining within the final deposits. Climate‐driven variations in substrate moisture and composition govern the flow regime as a rock‐ice granular avalanche or debris flow, highlighting the influence of seasonal and climatic changes on shaping flow behavior and ultimate runout. Recognizing this external control is crucial for anticipating rock‐ice flow behaviors in future warming scenarios.