Sudden starting and stopping of turbines and pumps lead to highly unsteady flow in tailrace channels of hydropower plants (HPP). Submergence of Pelton turbines and air entrainment in pumps due to surge waves must be avoided by technical measures. The existing 240 MW Veytaux pumped-storage plant in Switzerland will be enlarged by a new powerhouse adding another 240 MW, using the same tailrace channel and intake. Therefore, surge waves, induced by the two combined HPPs’ operation, were investigated using a physical model. For its validation, the flow behavior of the present scheme was tested and compared to prototype measurements. The enhanced scheme was investigated to analyze critical scenarios, to optimize the plant operation rules, and to define its limits. The effect of the layout on wave reflection in pump mode is highlighted and compared to theoretical approaches.