Behaviour of a Scour Protection Overlay With Randomly Distributed Concrete Prisms in Plunge Pools Downstream of Mobile Barrages for Exceptional Operation Conditions

Uncontrolled scour is affecting multiple mobile dams. The plunge pool of the Chancy-Pougny barrage, located on the Rhone River bordering France and Switzerland, has recently been reported to develop in a potentially unfavourable direction, after almost 100 years of acceptable scour development. In order to estimate the future scour potential of this asymmetrical plunge pool and validate and optimize possible solutions to limit its development, a physical model was built at LCH-EPFL. First-stage diagnosis tests showed the presence of recirculation surface currents interacting with the spillway outflow jets, producing downward vorticity responsible for scour progression. In partnership with the responsible engineering company different solutions were proposed and tested, starting with the installation of a vertical guidewall to prevent the formation of the return currents. A solution with randomly-arranged concrete prisms was finally retained as the most favourable compromise between the effective protection of the riverbed and site implementation. Herein the robustness of this solution is analysed for two different, but plausible, operation scenarios. Both the closure of a spillway gate and the lowering of the water level proved the effectiveness of the chosen solution, without endangering the stability of the prism-arrangement. The technical solution presented herein enlarges the range of alternative solutions for uncontrolled scour control not only at the Chancy-Pougny barrage but also for a large number of structures with similar layout