Efficiency and sustainability of gravel augmentation to restore large regulated rivers: Insights from three experiments on the Rhine River (France/Germany)

Over the last two centuries, rivers have been strongly regulated, inducing notable sediment starvation and flow reduction. This regulation has altered aquatic habitats, biocenosis and ecosystem services. Gravel augmentation (GA) is increasingly promoted to restore bedload transport, rejuvenate bed sediments and diversify aquatic and riverine habitats. However, practical questions remain in terms of efficiency and sustainability. The objective of this study is to provide unprecedented feedback from three GA experiments conducted along the Rhine River downstream of the Kembs dam (France/Germany). The restored sites were monitored over periods of 1 to 7 years and the stockpile deposits (SDs) and channel responses were compared to highlight the strengths and limitations of such experiments. The monitoring was based on topo-bathymetry, bedload tracking and grain size surveys. Two-dimensional hydraulic modelling was also performed to link the SD erosion patterns to the critical flow conditions. We show that (i) SD erosion is dependent on the grain size of the introduced sediments and the local flow velocities, (ii) sediment dispersion occurred mostly along the thalweg and increased over time, with a progressive decrease in the virtual velocities, (iii) bathymetric simplification and bed grain size fining were observed when downstream sediment diffusion began, (iv) in the mid-term (~5 years), local channel diversification occurred due to the fragmentation of sediment waves, with deposition occurring preferentially on riffles, and (v) new sediment starvation conditions appeared when the sediment waves travelled downstream in the absence of an upstream sediment supply. These results allow us to propose operational recommendations and management scenarios to improve both the efficiency and the sustainability of GAs conducted in large regulated rivers.