Distributed Seismic Sensing of Debris Flow Dynamics at Illgraben, Switzerland

Recent years have shown the destructive nature associated with debris flows in alpine regions, including densely inhabited regions in Central Europe. Surge fronts within debris flows increase peak discharge and the dynamical complexity, which contributes much to the hazard potential. In recent years, numerical models helped to gain insights into the surging behavior of debris flows, in particular into the formation of surging waves including roll waves and erosion-deposition waves (Edwards & Gray, 2014). In order to capture the dynamic processes involved in the formation and propagation of flow surges, it is necessary to obtain distributed observations in the spatio-temporal domain. However, demanding field installations have confined studies to theoretical or laboratory settings, and results have yet to be validated under large-scale, real-world conditions. In this study, we close this gap by utilizing distributed, near-torrent seismic measurements at the Illgraben debris flow observatory maintained by the Swiss Federal Institute of Forest, Snow and Landscape Research WSL.In 2024, a chain of 33 seismic nodes was deployed along a 2-kilometer section of the Illgraben torrent, with a spacing of 70 m between each node. In total, 10 debris flows with front velocities varying between 0.2 and 6 m/s and maximum flow heights varying between 1 - 3 m were recorded. The nodal array detected debris flow signals up to 2 km away, at a stage when the flows were still mobilizing in Illgraben’s upper catchment. The seismic record is characterized by high-frequency signals commonly attributed to particle-ground impacts within the debris flow. Additionally, it is found that steps in torrent geometry (check dams) produce a strong, low-frequency (1 – 10 Hz) background signal that is detectable kilometers away from the torrent.Our measurements provide novel data of the spatio-temporal evolution of debris flows: Bifurcations of surge fronts and spawning of erosion-deposition waves can be observed and traced along the torrent. The data furthermore reveal the interaction between surging waves and the debris flow front. Our dense seismic recordings thus show how and where surging waves develop and how they modify maximum discharge and thus allow inferring the debris flows destructive potential.The distributed seismic measurements at Illgraben offer new perspectives on measuring flow instabilities such as surge fronts and roll waves, allowing us to track them along extended torrent sections. They furthermore enabled us to refine our understanding of the seismogenesis of torrential processes, which is often only investigated with single stations or sparse networks. In a next step we plan to use these findings to better represent pulsing behavior in numerical debris flow models.  Edwards & Gray, 2014, J. Fluid Mech., doi:10.1017/jfm.2014.643