Vertical and Lateral Variability of Suspended Sediment Transport in the Rhine River

Suspended sediment is one of the major contributors to the total sediment load transported by rivers. Suspended sediment transport is highly variable both in time and space, driven by complex interactions between tectonics, climate change, and anthropogenic activity. Large waterways often underlie strong anthropogenic impact, for example, to ensure navigability and pursue economic objectives. However, stability and ecological integrity of the river system are equally important. For both, economic and ecologic objectives, processes related to the transport, deposition, and resuspension of fine sediment must be understood and quantified. Starting in the 1960s the Waterways and Shipping Administration (WSV) and the Federal Institute of Hydrology (BfG) started an extensive monitoring program to quantify suspended sediment transport in German navigable waterways. To cope with the large spatiotemporal variability of suspended sediment transport the WSV combined work-daily single point measurements and infrequent multi-point measurements. The aim of our study is to quantify the vertical and lateral variability of suspended sediment transport along the largest waterway in Germany, the Rhine River, and investigate drivers of cross-sectional variability. We link results from multi-point measurements with single-point measurements to assess the representativity of surface samples compared to cross-sectional means of suspended sediment concentration. The comparison of these two monitoring programs reveals that surface samples strongly underestimate suspended sediment loads. Main drivers that could be quantified are vertical gradients of suspended sediment concentration by means of Rouse profiles and lateral variability which is partially explained by mean channel curvature and related to the underestimation of suspended sediment transport relying on surface samples. Further, we observe that the magnitude of lateral variability is comparable to vertical variability, but often neglected in suspended sediment monitoring. Our study contributes to the refinement of existing monitoring schemes and shows how empirical data verifies and falsifies transport dynamics and processes.