Deciphering Debris-Flow Bank Erosion: Insights from the Illgraben Torrent, Switzerland

Debris flows increase in size by channel bed and bank erosion, enhancing their hazardous potential. While bed erosion by debris flows has been studied extensively through field measurements, laboratory experiments, and numerical modeling, our understanding of bank erosion remains limited. Therefore, we have no information on the spatial and temporal dynamics of debris flow bank erosion. Due to the infrequent occurrence of debris-flow events and the difficulty in accessing debris-flow channels, there have been no torrents for which there are (1) detailed measurements of debris-flow properties, (2) high-resolution topographic measurements of bank erosion before and after debris flow events, and (3) detailed measurements of bank composition and strength. We need the combination of these three processes to be able to physically explain the conditions that lead to high bank erosion rates. We use a comprehensive, long-term dataset from the Illgraben torrent, one of the most active debris-flow channels in the European Alps, to investigate bank erosion processes. This unique record includes field measurements of debris-flow characteristics and 51 high-resolution DEMs, spanning over 70 debris flow events between 2020 and 2025. By generating DEMs of Difference (DoD) to quantify bank erosion and integrating these with flow parameters derived from RAMMS modeling and field measurements, we investigate the controls on debris-flow bank erosion. Our preliminary results indicate that bank erosion often lags behind major debris-flow events. Large erosion episodes commonly occur after a high-magnitude flow. Smaller flows can gradually erode the bank toe during successive events, creating progressive undercutting that reduces stability until a sudden, larger bank failure occurs. A better understanding of debris-flow bank erosion processes and controls provides insights into the timing and magnitude of volume amplification, improving the accuracy of debris-flow models and fostering the development of strategies to reduce debris-flow erosion and mitigate its hazards.