The physical modeling tests for the rehabilitation of St-Marc Dam with a PKW are presented. A particular focus is put on the energy dissipation downstream from the PKW. The adopted solution is a leaned “ski-jump gutter” placed at the contact line between the downstream face of the dam and the natural foundation rock. It consists of a cylindrical profile, developed around an inclined axis and closed by a horizontal reach at the end of the structure. The aim of this solution is to guide the flow issued from the PKW to the stilling basin of the left existing spillway. The experimental tests consider various operation conditions which required pressure measurements at different impact zones. Structural design of the PKW is impacted by the fact that the concrete of the dam is subjected to a noteworthy blowing reaction. For that reason, the new structure could not be anchored in the existing dam. Thus, the spillway behaves as a gravity structure.