Avalanches of granular geomaterials (rock, soil, snow, ice, etc.) are frequent and pose varying degrees of risk to land use, infrastructure, and personal safety in mountainous areas of the world.One of the main goals of avalanche research is to forecast the flow-obstacle interaction in order to (i) design civil engineering structures able to withstand the impact forces and (ii) assess the physical vulnerability of existing structures in avalanche prone-areas. The granular nature of sliding geomaterials is considered the crux of flow-obstacle interaction. At present, the granular mechanisms involved in the flow-obstacle interaction and the resulting forces remain poorly understood.In order to provide efficient tools to calculate impact forces stemming from full-scale granular avalanches and to contribute to hazard mapping and risk management, we propose to investigate the mechanical behavior of granular flows around obstacles, accounting for grain-size dynamics.Key objectives of the study are: (1) to improve our fundamental understanding of the dynamics of avalanche-flows around obstacles and the induced forces with attention paid to the role of grain-size dynamics with the help of innovative discrete numerical simulations, laboratory tests and field-based observations coupled with continuum mechanics theory, (2) to develop universally recognized reliable methods for the design of efficient protection structures built to brake, divert and stop the avalanche-flow in run-out zones, and of safe civil engineering structures, (3) to introduce overall final project deliverables to wider European and international communities (researchers, practitioners, decision makers, general public) through publications, presentations and outreach activities.The proposed methodology is likely to be further developed to explore other important questions such as the effect of inter-particle cohesion (aggregation and breakage) and of particle shape on flow-obstacle interaction.