Evaluating 3D Hydraulic Conditions to Favour Sediment Transport and Erosion Through a Reservoir: Hybrid Modelling of Champagneux Run-of-River Dam on the Rhône River, France

Abstract

By decreasing the velocity of the flow and turbulence, reservoirs controlled by dams are likely to force inflowing sediments to settle. This process can be more or less temporal and intensive, depending on particle size and the reservoir characteristics. To reduce the reservoir sedimentation and prevent a disruption of sediment continuity, one possible option for dam operators is to recover favourable flow conditions either for routing inflowing sediments or to remobilize previous deposits. In the case of cohesive sediments, one of the main challenges to deal with is that deposition and erosion thresholds are often radically different as a result of deposit consolidation. For the last 30 years, the Champagneux run-of-river dam (Rhône River, France) has experienced significant deposition processes affecting mainly fine sediment fractions. As a dam operator, CNR has wanted to determine hydrodynamic conditions to favour transport and erosion of fine fractions of sediment. In fact, this simple question covers many points of complexity, and thorough analyses on shear stress evaluation have been conducted by CNR. The evaluation of bottom shear stress throughout the reservoir and for different hydrological and operating conditions relies on a hybrid approach. Even if three components of hybrid model have been initially deployed in the frame of a more general project, this experience has shown that such a comprehensive approach is required to obtain relevant values of the shear stress and flow velocity close to the river bottom. The final goal is to achieve a correlation of numerical results with critical thresholds coming from core sampling lab tests. For the dam operator, such a comprehensive survey provides useful information to enhance the management of the reservoir, and feedback obtained will contribute to an optimization of the methodology for other similar cases.