Abstract ID: 3.11021 | Accepted as Poster | Talk/Oral | TBA | TBA

The Alps are a natural hazard-prone region with increasing losses from torrential events involving fluvial sediment transport, debris floods, and debris flows. Torrential hazards typically involve large volumes of sediment transported from a source to a depositional area. The volume of sediment stored in the catchment correlates with the hazard potential; more supply equals higher sediment concentration resulting in an increased hazard from the torrent. Hazard and risk dynamics are intensified by anthropogenic climate change. Increased sediment availability and shifting flooding patterns threaten new infrastructure and challenge previously installed mitigation measures. Past mitigation measures, while effective, mainly address processes upstream of the depositional fan apex and do not consider processes occurring in the confluence.

Confluences are critical sites in all river networks and are particularly vulnerable to increased sediment supply, as the sediment loads and hydraulic geometries from each channel must be accommodated to avoid overbank flooding and sedimentation into adjacent settlements. Hydrodynamic and morphological processes at lowland river confluences have been extensively studied, but there is a gap in our knowledge of mountain river confluences. To better understand the effects of increased sediment supply from Alpine catchments, a large-scale physical model, which is a generic representation of confluences typically found in the Alps, was constructed in the hydraulic engineering laboratory at the University of Innsbruck. In addition, a coupled and validated sediment transport model was used to further simulate sediment transport and morphological patterns in the confluence zone. Results from both physical and numerical approaches indicated that as the sediment concentration increased, the depositional extents increased causing severe backwater effects in the main channel and extensive deposition in the torrent channel. However, an extremal equilibrium morphology developed as a function of flood magnitude and was characterized by specific geomorphic units that were optimized to enhance sediment transport through the confluence. These results challenge the previous conclusions regarding the hydro-morphodynamics of confluences. Therefore, the flooding and sedimentation hazards that occur at mountain river confluences pose a unique hazard, and the application of methods and knowledge based on lowland regions may severely underrepresent the hydro-morphodynamic interactions and the resulting hazard potential.