Fine stock dynamics along an engineered Alpine river system

Understanding the dynamics of fine sediment stocks in the riverbed is important for river management. However, this dynamics is size dependent (the sand fraction against the silt-clay fraction). In this study, we investigate the dynamics of both silt-clay and sand fraction of fine stocks in an engineered Alpine river system characterized by alternate gravel bars (Arc-Isère system, France) using field data and 1D numerical modelling. The field investigation on gravel bars showed that the riverbed contains mostly sand. The sand content in the bed reaches 15%, while the silt-clay content is 3%. In addition, 1D simulation results showed that resuspension come mostly from the surface fine stocks since they are more easily mobilised than the subsurface stocks. This latter can only be re-entrained when the surface gravels are in movement. Such gravel erosion was only observed during high flows on the River Arc, while the bed shear stresses on the River Isère are always too low to induce significant gravel movement. Long-term simulations showed very different behaviours depending on the sediment class. For the silt-clay fraction, the system reaches an equilibrium with a balance between erosion and deposition induced by flood events. In contrast, sand tends to settle down quickly on the bed and its dynamics is sensitive to the upstream sediment supply. Finally, a very different dynamics was observed between the River Arc and the River Isère. Stocks in the River Arc tend to be in equilibrium while they increase continuously in the River Isère, especially due to sand deposition.