Objectives of this work are to experimentally analyse the dynamic impact of dry avalanches on structures and to examine the possibility of extending the results to debris flows; to identify structure efficiency in terms of extension and shape of the deposition area; to numerically validate the adequacy of shallow-water equations in reproducing the impact process and the maximum force on the obstacle; finally to propose an analytical expression for estimating the design force. Experiments were carried out on two dry sandy mixtures flowing down a steep chute and impacting on obstacles of different shape that reproduce check-dams, filter-dams, “houses” and flow-breakers. Effects of mixture composition, obstacle shape and orientation— with respect to mean flow direction—on force values and deposit extension were examined, showing that only check-dams can significantly reduce the kinetic energy and retain the flowing material. Numerical simulations were performed with a 1D code, based on shallow-water equations and the weighted average flux method, to represent forces on an infinite high obstacle (i.e., a wall) in time. Promising results allowed the use of a simple expression directly derived from momentum balance to compute the force exerted by the flow on the structure for given upstream flow depth and velocity. Limitations of the experiments in reproducing impact on defence works in real events are highlighted and suggestions for improving structural design are drawn.