Evolution of the debris flow channel from the Illgraben catchment in the Swiss Alps

Abstract

The mechanism of erosion in debris flows is inherently complex due to their dynamic nature. Here, the spatio-temporal evolution of the outflow channel from the Illgraben catchment resulting from 26 debris flows between May of 2020 and July of 2023 is analyzed. The presence of check dams heavily influences the spatial distribution of erosion and deposition resulting in a memory effect. This effect emerges from the interplay between the geometry of the channel and whether the previous flow was more depositional or erosional. Correlation analysis showed that flow properties, such as peak discharge, flow volume and cumulative shear stress had the strongest linear correlation to the amount of erosion for the whole 3.6 km study reach. A secondary analysis, incorporating data from 2019, was done for the lowest 800 meters of the channel where peak flow velocity and antecedent rainfall was strongly correlated with erosion along with the peak discharge, flow volume and cumulative shear stress. These flow characteristics represent the collective forces acting on the channel bed. These results demonstrate the importance of check dams in stabilizing the channel bed and the influences the flow and bed characteristics have on the spatio-temporal change. Emphasizing the importance of integrating these factors in hazard assessments.