Numerical modeling of suspended sediment transport during dam flushing: From reservoir dynamic to downstream propagation

Dam flushing is often performed to remove sediments of all sizes from reservoirs while minimizing downstream impacts. In this study, the reservoir flushing operation and the downstream transport of suspended sediments are simulated based on a numerical tool. The study site is the Arc River in the French Alps, focusing on two reaches: locally on the Saint Martin la Porte (SMLP) Reservoir and a few metres upstream (reach 1), and downstream the 120 km between the SMLP reservoir and the city of Grenoble (reach 2). The chosen numerical code is COURLIS, developed at EDFRD, and which could be coupled with codes of the open source Telemac-Mascaret system. The model was applied to the 2012 flushing event of the Arc River dams, which are managed by EDF (Electricity of France). The bed erosion and the suspended sediment transport dynamics in the most downstream reservoir (SMLP) were first computed. The initial state of the deposited fine sediment were defined using measured topographic data of the reservoir. The upstream boundary condition for the reach 2 model was set from estimated discharges and suspended sediment concentrations. The downstream study reach is approximately 120 km long, from SMLP dam to the city of Grenoble along the Arc and I sere rivers. Topographic data of the river bed were used to define the downstream river reach, taking into account the vegetated alternate bars. Water discharges and suspended sediment concentrations were also measured at several locations in the rivers downstream of the flushed reservoirs. A new formula is proposed to estimate two mean concentration values in the cross section: one for the main channel and the other for the overbank section. A good agreement with field data is obtained for both the reservoir and downstream reach dynamics. Moreover, the key role of the alternate bars in the fine sediment dynamics is highlighted, quantified and discussed.