Composition of freshly deposited fine sediments during the 2023 summer flood event in the Rhine River basin: implications for sediment transfer and provenance

Assessing sediment transfer and provenance in large river basins is complex due to the variety of processes involved and the variability of their controlling factors. In this study, we attempt to quantify the provenance and transfer of fine sediment in the Rhine basin by adopting a synoptic sampling approach. Following a minor flood event during the end of August and beginning of September 2023, which originated in the alpine part of the Rhine basin, we collected samples of freshly deposited fine sediments along the banks of the main branch of the Rhine River and its four major tributaries (Aare, Neckar, Main, Mosel). These samples were mostly collected from hard surfaces (e.g., bank reinforcements, ferry landings) just above the water line. The samples were analysed for elemental composition using ICP-MS. A principal component analysis was performed on the element concentrations. The first principal component was interpreted as the main factor reflecting the  geogenic variation of the sediment composition. Next, a sediment transfer model that accounts for sediment supply to and sediment retention within the river network was set up. The model inputs include a digital elevation model of the river basin, the interpolated scores of the first  principal component based on element concentrations from the FOREGS geochemical atlas, and RUSLE-based estimates of sediment production. The model was calibrated using the ‘observed’ scores of the first principal component in the High Rhine and impounded section Upper Rhine (section between the Rhine-Aare confluence and Iffezheim).

The model results reveal that spatial variation in sediment supply to the river network is primarily controlled by area-specific event runoff and, to a lesser extent, by long-term sediment production. Furthermore, the model results demonstrate the relative importance of nearby sediment sources over sources further upstream: on average the relative importance of the source declines by 1.1% per kilometre downstream transport. It is likely that both retention of fine sediments in the channel network during transport and entrainment of fine sediments due to bank erosion or channel bed incision are at play and explain this decline. The patterns of deviations of the model predictions from measured sediment composition in the free-flowing section of the Upper Rhine and in the upper part of the Lower Rhine suggests that about 50% of the fine sediments reaching the Rhine delta may be derived from sediment nourishments to mitigate channel bed incision.