Beyond their initial application on gravity dam crests as an element of an auxiliary spillway, Piano Key Weirs (PKWs) are more and more combined with weirs on rivers. Such applications comprise the level rise for river hydropower plants or to maintain a water level for inland navigation. PKWs are ungated structures, so that the “operational” risk is low. Furthermore, they have a hydraulic performant rating curve and a low risk of driftwood blocking, as compared to traditional weirs. Such a river application on a loose riverbed requests a laborious foundation, given that the structure is not directly built on rock or on dam concrete as for spillways. A key parameter for the stability of the PKW foundation is the ultimate scour-hole depth at the PKW toe. Further, measures to reduce the latter scour must be designed, as for instance a riprap protection of the riverbed downstream of the PKW. This paper reports of systematically conducted model experiments on PKW scour, and gives some indications on the parameters that are important to reduce the latter extend.