Multi-scale experimental and numerical approaches are combined to investigate the dynamics of rapid mass movements such as snow avalanches and granular debris flows, and the destructive forces these flows are able to exert on structures. At grain scale, laboratory experiments and discrete numerical simulations on steady free-surface gravity-driven granular flows are performed. The main flow characteristics (depth, velocity and density profiles) are measured, which allows us to derive the constitutive law governing flow propagation. In agreement with prior studies, this constitutive law is expressed in the form of an effective coefficient of friction depending on the inertial number I, which measures the ratio between a typical time scale for grain rearrangement and a macroscopic time scale linked to the average deformation. At the continuum scale, this constitutive relationship is implemented in a numerical model based on smoothed particle hydrodynamics (SPH) method. This model allows us to simulate complex free-surface gravity-driven flows such as finite volume granular avalanches propagating around obstacles. We consider here a rather simple obstacle geometry consisting of a wall normal to the flow bottom. SPH results are directly compared to results from discrete numerical simulations and small-scale experiments with similar flow and geometrical conditions. We analyse the hydrodynamic effects of the wall on the incoming granular flow (dead zones, shocks, jets), and derive the force exerted on the wall. Lastly, at full-scale, we are interested in understanding the behaviour of rapid granular-like mass movements around protection structures. We show results of numerical simulations aiming at predicting the propagation of a natural snow avalanche and its interaction with a complex protection system located at Taconnaz (France). The model is based on shallow-water equations, including an erosion-deposition model. The flow depth and the depth-averaged velocity derived from these full-scale simulations can be combined with the outcome of the intermediate scale study, in order to estimate the expected force exerted on the structures. At the end, the physical vulnerability of structures submitted to snow avalanches can be assessed.