Suspended sediment transport in a river network: testing signal propagation and modelling approaches

The AlpRhineS2S project, a collaboration between ETH Zrich and the University of Bern, researches the interplay of geological, geomorphological and hydrological processes within the sedimentary system of the Alpine Rhine in the canton of Grisons, Switzerland. Distributed river network hydrology-sediment models are being used in Alpine basins for the prediction of source activation and transport rates, for both fine and course sediment. Fine sediment input in such models may be generated by hillslope mass movements in hotspots of erosion (Demmel et al., 2024) which can be tracked, facilitating the development of sediment budgets (Garipova et al., 2024). However, despite the utility of hydrology-sediment models, the propagation of the sediment signal along channels is rarely tested against exact solutions and observations. In this contribution, we investigate the propagation of suspended sediment signals along channels and compare modelling simplifications with observations and theory. Averaged over long timescales, suspended sediment load represents the erosion rates of the catchment. At shorter timescales, from seasonal to hourly, sediment fluxes can describe the spatial distribution and activation of sediment sources and sinks across the basin. Active sediment sources and sinks constitute points of discontinuity in the basin, which create turbidity signals along the river network. Here we ask the questions: To what extent can channel flood wave propagation describe the sediment dynamics? Do current modelling approximations capture the richness of turbidity signals carried across the river network? The observation data used here are retrieved from flushing events and environmental flow releases across selected Alpine rivers. The turbidity signal properties of the different events are compared in non-dimensional terms, and synthetic common properties across the samples are determined. Modelling is compared through a successive approximation approach starting with a 1D solution for unsteady flow with the model 1D BASEMENT (Vanzo et al., 2021) for a range of channel geometries and slopes. Then the sediment propagation is analysed with the steady flow assumption of the parabolic and kinematic flood wave, in analytical form and in the TOPKAPI-ETH model, which we plan to use in the AlpRhineS2S Project for sediment fluxes and sediment source identification (Battista et al., 2020). Results highlight the extent to which numerical models can represent the channel sediment dynamics and what is consecutively missing from the introduced approximations. Findings also show that the suspended sediment propagation, even during controlled release events, cannot be described as a boundary condition problem: the interplay of deposition and resuspension along with local morphology and vegetation also play a fundamental role in the signal description. References Battista, G., Schlunegger, F., Burlando, P., Molnar, P. (2020): Modelling localized sources of sediment in mountain catchments for provenance studies, https://doi.org/10.1002/esp.4979. Demmel, S., Agostini, L., Garipova, S., Leonarduzzi, E., Schlunegger, F., Molnar, P. (2024): Climatic triggering of landslide sediment supply in the Alpine Rhine, EGU24. Garipova, S., Mair, D., Demmel, S., Agostini, L., Akar, N., Molnar, P., Schlunegger, F. (2024): Source-to-Sink Sediment Tracing in the Glogn River Catchment, EGU24. Vanzo, Davide, et al. "BASEMENT v3: A modular freeware for river process modelling over multiple computational backends." (2021)