Formation, propagation, and annihilation of surge waves in debris flows

In recent years, the destructive impact of debris flows in alpine regions has become increasingly evident. Surge waves within debris flows increase peak discharge and magnify the hazard potential. Hence, understanding the dynamic complexity of debris flows is crucial to mitigate their risk. In order to capture the dynamic processes involved in the formation and interaction of surge waves, it is necessary to obtain distributed observations in the spatiotemporal domain. In this study, we present near-torrent distributed seismic measurements to monitor the Illgraben channel located in the Swiss Alps. With 33 nodal sensors, we detected and tracked surge waves along a 2-kilometer torrent section across the Illgraben fan. This provided valuable new insights into the spatial scales relevant for surge wave formation and the long-distance propagation characteristics from surge wave formation to their annihilation. We observed erosion-deposition waves that emerged out of the muddy flow tail and propagated with constant velocity along the torrent, reaching meter-scale flow heights within only 100s of meters of flow distance. We can differentiate flow regimes based on their seismic signature and track them along the torrent, thus mapping the debris-flow evolution in time and space. The observations elucidate large-scale debris flow dynamics and improve our understanding of hazard potential and the effectiveness of structural countermeasures. • Debris flows were recorded using a densely spaced linear nodal array. • Distributed seismic measurements enable the tracking of surge waves. • Formation and growth of surge waves were observed in the flow tail of debris flows. • Observed surge waves affect debris flow hazard by increasing peak flow height.