In many countries dams are found to have insufficient flood discharge capacity with respect to updated design floods. Upgradig of spillway discharge capacity has therefore become a significant issue for operators of hydropower plants. The Gloriettes concrete arch dam in the French Pyrenees, operated by EDF, shows a deficit of 80m3/s for the new design flood of 150m3/s. Therefore a complementary spillway on the right bank is to be implemented. The type of labyrinth weir known as the Piano Key Weir (PKW) was selected.