Page - 1066 - in Book of Full Papers - Symposium Hydro Engineering

Results from two-dimensional modeling (2D) in the pre-study concluded that the reservoir levels needed to be increased to successfully evacuate the new design flood, since too high velocities would occur otherwise in the channel between the existing spillway and the new one. In order to achieve acceptable velocities at FRRL, an extensive excavation of the mentioned channel is needed, and it is not likely to get a permit for this with respect to environmental aspects. Figure 6 shows the results from the 2D- modeling: a comparison between FRRL and FRRL+0.75. Figure 6: Reservoir flow patterns at FRRL and FRRL+0.75 m The new spillway is designed with a stilling basin to reduce the risk of erosion in the river bed downstream the spillway and dams. Several configurations of the stilling basin were examined in terms of its floor elevation, end-sill elevation and basin length. Its placement was mainly governed by the hydraulic requirements to produce a stable hydraulic jump. It is also restrained by the geological conditions as it mainly will be excavated from existing bedrock. Figure 7 shows the final layout of the basin, with a diverging shape downstream and a floor length of 40 m. The length of the sloping chute is about 80 m. The floor is placed at el. +255 m; the end sill is situated 6 m higher. The basin is 30 m wide at its downstream end. FRRL FRRL +0.75 m Qout= 2000 m3/s Qout= 1700 m3/s Qout= 800 m3/s Qout= 500 m3/s 1066