Brief communication: First field observations of basal slip velocities in natural debris flows

Abstract. The internal dynamics of field-scale debris flows are challenging to observe. Constitutive equations for debris flows, often base on simplified shear models that assume a no-slip condition at the bed. However, laboratory experiments suggest that basal slip can occur under fixed bed conditions, influenced by the variable material composition of debris flows, which includes grains and boulders interacting with the basal boundary. This study presents results from a novel monitoring system that has been designed to directly measure basal slip velocities in natural debris flows using paired conductivity sensors. The system, which was installed in the Lattenbach catchment (Tyrol, Austria), captures high-resolution temporal data from debris-flow events. A detailed examination of two events that occurred in June 2025 has revealed the presence of persistent slip velocities, particularly during surge phases and granular flow fronts. Basal sliding was found to be consistently lower in comparison to surface velocities. These findings call into question the no-slip assumption of traditional rheological models and underscore the significance of basal slip in the dynamics of debris flows. Future research will focus on refining the detection depth of the sensors, analysing additional events, and conducting comparative studies across different catchments to further understand the role of basal slip in debris flows.