The Gloriettes concrete arch dam in the French Pyrenees showed a deficit of 80 m3/s for the new design flood of 150 m3/s. To address the problem, a Piano Key Weir (PKW) as an additional spillway was designed. The existence of a geotechnically unstable zone downstream of the PKW site, as well as requirements for environmental integration, did not allow a direct trajectory of the tailrace channel, but imposed an abrupt change in direction halfway. A stepped channel including an intermediate stilling basin was conceived for this purpose. The energy dissipation over the total length of the restitution channel for this condition was found to be about 90 per cent. Since the construction works could only be carried out in summer, the works were split into two stages: summer 2009 (excavation of the stepped reaches and the stilling basin) and summer 2010 (construction of the PKW and completion).