Abstract. Mountain debris cones in the Alpine region often provide space for dense population and cultivation. Hence, a great number of buildings are exposed to torrential hazards. In order to protect the settlement areas against flooding and overbank sedimentation, torrent defence structures are implemented at various locations within catchments. Directly at the debris cones, these protection measures often include a deposition basin at the fan apex and/or a confined channel that passes through the settlement.

The work presented within this paper deals with the effect of specific outlet structure layouts, situated at the lower end of a selected deposition basin, on bed-load transport processes and flood protection. A case study analysis was accomplished comprising a 3-D numerical model (FLOW-3D) and a physical scale model test (1 : 30). The subject of investigation was the deposition basin of the Larsennbach torrent in the Austrian Northern Limestone Alps. The basin is situated on a large debris cone and opens out into a paved channel. Since the basin is undersized and the accumulation of sediment in the outlet section reduces the available cross section during floods, adjoining settlements are considerably endangered of lateral overtopping of both clear water and sediment. Aiming for an upgrade in flood protection, certain layouts for a "closing-off structure" at the outlet were tested within this project. For the most efficient design layout, its effect on flood protection, a continuous bed-load output from the basin and the best possible use of the retention volume are pointed out. The simple design of the structure and the key aspects that have to be taken into consideration for implementation are highlighted.