The Gage II concrete arch dam shows some limiting constraints in its structural behavior when submitted to particular hydrostatic and thermal load conditions. Its sensitivity to low temperature has led to determine two different maximum water levels during winter and summer, respectively. Therefore, a new complementary spillway, composed by a PKWeir and a flap gate weir has been designed by EDF and optimized using a physical scaled model at LCH. The experimental tests have been carried out for different configurations of the spillway. The output flow capacities of the PKWeir and the flap gate weir including their interaction through the restitution channel have been also investigated. The geometric optimization process has been conducted on several spillway parameters: number of the PKWeir units, its position relative to the flap gate weir, and the height, width and bottom slope of the restitution channel. The tested configurations have been compared based on their flow output capacity.