Modelling Undular Bores: Rhone River Hydropowerplant Experimental Study

CNR is the first producer of 100% renewable energy in France, operating and managing 19 hydroelectric power plants on the Rhone River. When a sudden shut-down of turbines occurs (because of mechanical or electrical breakdown), the sudden disruption in discharge at the power plant generates waves that raise the headrace channel water level and propagate upstream over several kilometers. Theses waves are made of primary wave which amplitude is proportional to the incoming discharge and secondary waves, called Favre waves, which can be generated because of the vertical acceleration, that superimpose on the primary waves. These waves must be investigated with care since they may overflow and erode the dike crest, hence possibly damaging the dike core and even harming people. Traditional 1-D Shallow Water Equation numerical models based on the assumption of hydrostatic pressure distribution fail to accurately reproduce the complex patterns of the waves system. Improving the knowledge of this phenomenon and predicting it, is therefore something essential for CNR. In the frame of the project of the refurbishment of Châteauneuf-du-Rhône hydro-powerplant (1.6 billion kWh, nominal discharge 1850 m3 s−1, six Kaplan turbines), a dedicated study has been implemented to assess the consequence of increasing the nominal discharge on the waves amplitude and to anticipate any mitigation in case of adverse effects would be pointed out. Hybrid modeling of these phenomena has been initiated on the Montélimar run-of-river scheme, including a 1:35 undistorted physical model representing 1200 m of the upstream stretch of the powerplant and an overall numerical model of the 13 km headrace channel. In order to calibrate more accurately the large-scale numerical approach by 2D Basilisk code (Cierco et al. in New modelling paradigms for water issues? Paris, 2023 [1]), tests have been conducted on a physical model at CNR hydraulic laboratory. The results obtained thanks to an innovative way of modelling the turbines shutdown process in fully similitude (water hammer, initial conditions), completely match with the field observations and the measurements recorded during the onsite tests. This paper aims at exposing the numerous practical difficulties encountered during the carrying out of transient tests, including the generation of a wave in a pressure flow conduit as well as its propagation at the free surface of the physical model in complete similitude. The findings contribute to advance and extend the knowledge and the management of disjunction phenomena within the field of hydraulic engineering.