Debris flows and snow avalanches are among the most catastrophic hazards in mountainous settings. Surface erosion, slope failure or dam breaching are among the initiation mechanisms of debris flow, which can be magnified during the subsequent flowing process, with disastrous consequences for urbanized settings. Understanding whether an initial rainfall forcing evolves into a hazardous debris flow requires a modelling framework capable of representing the different stages of the process and their interactions. To this end, analytical approaches and numerical modelling provide fundamental insights into the interplay of mechanical equilibrium, soil saturation and flow propagation. These components can be combined into predictive applications for the design of early warning systems. We invite contributions addressing the modelling of debris flows at different levels of complexity and scales, from watershed to the interaction with hydraulic works, including rainfall-induced slope failure and surface erosion, transformation of landslides into debris flows, flow initiation and acceleration, entrainment of bed material, flow-topography interactions, rheological behaviour and deposition. Similar modelling challenges arise in snow avalanches, where the thermal forcing often causes the transition from snowpack instability to rapid flow development involving strongly nonlinear interactions between snow properties, terrain and flow dynamics. Contributions combining different modelling approaches or coupling different physical processes are particularly encouraged, as are studies investigating how information generated at one stage of the process can be transferred to the next. The session aims to foster discussion on how process-based numerical modelling can move from the simulation of individual phenomena towards integrated predictive frameworks, contributing to bridge the gap between fundamental process understanding and operational applications for hazard mitigation and early warning.