Laboratory Experiments on Sediment Replenishment in Gravel-bed Rivers

Worldwide, many rivers are affected by various river engineering measures and exhibit ecological deficits as a result. Sediment input from upstream is often limited by the presence of hydropower plants, sediment retention basins, and other river training measures, resulting in a lack of bed-load and morphological degradation. Sediment deficit can undermine structures, negatively influence groundwater formation, and decrease habitat quality for aquatic flora and fauna. Sediment replenishment by means of artificial gravel deposits or enhanced by bank erosion are two options when attempting to compensate for sediment deficits in rivers and to improve their ecological conditions. The ability to predict bank erosion rates and their impact on the river reach are important when attempting to implement restoration projects successfully. In the first part of this study, experiments are conducted on the erosion pattern of artificial gravel deposits to (i) investigate the influence of the governing parameters on the erosion process of gravel deposits, (ii) improve understanding of fluvial bank erosion in case of non-cohesive, granular bank material, and (iii) identify the fundamentals to implement and improve bank erosion in numerical models. In a second test series, the ability of non-submerged structures to enhance bank erosion is evaluated. Erodible and non-erodible structures are tested to obtain bank erosion volumes that are favourable from river engineering and ecological perspectives. In a third test series, the long-term evolution of the bed of a typical Swiss river as a result of changing sediment supply is investigated. Predicting equations intended to describe the erosion pattern of artificial gravel deposits and to estimate the backwater rise that occurs due to gravel deposits are proposed. This study demonstrates that an excess shear stress relation can describe the temporal evolution of the mean erosion rate of an artificial gravel deposit. Non-erodible structures perpendicular to a river’s bank and groins on the opposite bank are efficient to enhance bank erosion for discharges where no or negligible bank erosion occurs without a structure being present. The introduction of large wood debris jams is a promising option for enhancing bank erosion, as this approach combines low maintenance requirements with the immediate creation of complex habitats. In the third test series, channel responses to a long-term change in sediment supply took the form of a combination of (i) an adjustment on the grain scale by a smoothening of the bed; (ii) a change of the cross-sectional shape by sediment deposition, primarily in the thalweg; and (iii) and a reduction of the longitudinal bed slope. This study’s findings allow for improved predictions of the rate and magnitude of bank erosion and their subsequent impact on the downstream river reach; thus, they can consequently aid in determining design frequency and deposit volumes for planned restoration projects