Physical and numerical modelling of a bedload deposition area for an Alpine torrent

Abstract. Floods including intensive bedload transport represent a severe hazard to the often densely populated alluvial fans of small Alpine watersheds. In order to minimize the risk of future inundation, an existing bedload deposition area on the fan upstream of the village Vorderberg in southern Austria is planned for reconstruction. The suggested concept for protection measures includes dividing the area into three similar sections of reduced slope. The three sections are to be separated by a block ramp. To test this concept and to optimize the sedimentation process, an analysis was performed by using both a physical scale model (1:30) and a numerical simulation tool (SETRAC). Four configurations for the section-outlet were tested based on three flood scenarios. The results support the general protection concept and suggest a minimum construction configuration, including a woody debris filter. Employing a physical scale model for analysing small watershed processes is rarely found in literature. This contribution represents an applied study and provides quantitative information on bedload deposition and outflow from a deposition area. We test a novel simulation tool for bedload transport on the steep slopes against the measurements in the laboratory and show that the combination of physical and numerical modelling is a valuable tool to evaluate the efficiency of planned measures for torrent hazard mitigation.