Author(s): Theo St Pierre Ostrander; Maria Gorfer; Johannes Holzner; Andrea Andreoli; Francesco Comiti; Bernhard Gems
Keywords: River Confluence; Fluvial Hazard; Tributary Torrent; Bedload; Physical Scale Model
Abstract: Large volumes of transported sediment in mountain torrents pose a substantial hazard to human life, property, and infrastructure. Intense sediment transport in tributary torrents, typically associated with extreme hydrological events, can induce flooding at the torrent-receiving river confluence. Massive sediment supplies from torrent catchments and insufficient transport capacities of the receiving channels facilitate deposition in the confluence leading to overbanking on the torrential fan. While being geomorphologically active, torrential fans are home to communities in mountain areas. Exposure to torrential hazards has increased, fueled by developmental pressures, tourism, and increases in Alpine recreation. Extensive research has been carried out investigating hydrodynamic and morphological processes at lowland river confluences where it has been determined that the confluence angle and discharge ratio have the greatest influence on channel morphology and hydraulics. However, there is a lack of information concerning confluences with low width and discharge ratios, high sediment concentrations and gradients, typically found in mountain rivers and torrents. In this study, results from large-scale laboratory experiments with a standardized river confluence geometry are presented. Thereby, transport capacities, shape and volume of the deposition zone in the confluence, bedload dispersion characteristics, morphological dynamics, and boundaries were analysed for various hazard impact conditions, characterized by the discharge ratio and the sediment concentration. The receiving channel features a gradient of 0.5 % and a moveable bed allowing for 0.2 m of erosion. The torrent channel bed is rigid with a gradient of 10 %, channel bed roughness was identified with a series of clear water experiments. The confluence angle is 90°. With this configuration, a set of experiments based on steady-state hydraulic conditions was accomplished. The receiving channel discharge was exclusively clear water, sediment was supplied to the torrent channel in concentrations from 5 % to 15 %. The discharge ratio varied between 0.1 and 0.3. Model validation was achieved through field surveys, analyses of historic events, and GIS-based studies of the geometries at relevant confluences found in Tyrol (Austria) and South Tyrol (Italy). Referring to these natural conditions, transferring model results yields a range of Froude-based scale factors from 20 to 40. Results indicate that in addition to the confluence angle and the discharge ratio, the stream power of the torrent channel has a significant impact on confluence morphology, size and location of the distinct morphological zones, and depositional processes in the torrent channel. High sediment concentrations create feedbacks between confluence hydraulics and morphology occurring during torrential events. Bedload deposition affects hydraulics which influences transport processes in both the confluence and tributary channel. Throughout certain experiments, regressive aggradation caused the tributary channel to gradually fill with sediment reducing gradient and discharge capacity leading to potential overbanking of the sediment-laden flow.