The addition of one generating unit at TIWAGs Kirchbichl Power Plant at the Inn river in Austria will result in the increase of the volumetric flow rate in the existing diversion channel by almost 100%. In order to ensure safe operation of the power plant it will be necessary to erect a new controlled spillway equipped with a controllable mechanical gate. Usually the performance for a certain hydraulic geometry of a controlled spillway is determined by using a model test in a hydraulic laboratory. However, for this assessment 3D-CFD simulations were performed to determine the hydraulic behaviour of the reverse radial gate. By means of multiphase simulations, not only the hydraulic characteristics could be investigated. A complex 3D model was generated capturing all relevant constructive details such as deflectors and also the existing gaps between steelworks and civil works. Hence, several relevant loads acting on the gate structure could be determined. These analyses were conducted for several positions for both the radial gate and the flap for various headwater levels. The results and findings of these simulations greatly facilitate the mechanical calculation of the relevant components. In addition, transient simulations were carried out to investigate possible vibration excitations on the mechanical structure triggered by the fluid flow. Therefore, a very fine numerical mesh had to be created to resolve the vortex shedding at the trailing edge of the flap. On the other hand, the tailrace flow field especially in the stilling basin as well as the aeration behaviour could be analysed. It is not possible to obtain this detailed information by means of a model test in a hydraulic laboratory.