A scaling law is proposed to describe the force experienced by a wall-like obstacle overflowed by a granular avalanche-flow when a stagnant zone is formed upstream from the wall and co-exists with an inertial zone above without granular jump. It relates the force on the wall relative to the force due to the kinetic energy of the undisturbed incident flow to (i) the Froude number, (ii) the wall height relative to the flow thickness, (iii) the slope angle and (iv) the various parameters associated with the properties of the flowing granular material. The scaling law is compared to small-scale discrete numerical simulations in two dimensions and data from granular laboratory tests. Finally, we discuss on the applicability of the new model to the full-scale granular flows.