The simulation of runoff in Alpine catchment areas is essential for the optimal operation of high-head storage hydropower plants (HPP) under normal flow conditions, but also in case of flood events. A semi-distributed conceptual numerical approach has been developed, combining hydrological modelling and operation of hydraulic works. Routing System allowed runoff generation, simulation of the operating mode of complex HPP and the impact on the downstream river network for different scenarios. An overview on the hydrological modelling approach and its application for the upper Aare River catchment in Switzerland, where about half of the area is operated by a complex hydropower scheme, is given. The retention effect of the reservoirs and their management, including preventive turbine operations, on flood routing in the Aare River upstream of Lake Brienz is presented for the 2005 historical flood event.