The present paper deals with the hydrodynamic force from granular streams overflowing a wall-like obstacle. An analytical continuum model is proposed to derive this force both for steady and unsteady flow conditions. The force derived from the continuum model is successfully compared to the time-averaged force from discrete numerical simulations of free-surface granular flows down an incline and overflowing a wall. A good agreement is found in a large range of slope inclinations from a slow dense regime to a rapid dilute one. The continuum model gives the various contributions of the total force. Application to avalanche dam design is discussed.