Brief communication: In-situ measurements of basal sliding in natural debris flows

Abstract. The propagation of debris flows is expected to be controlled not only by the internal deformation of the material but also by basal sliding of sediment along the channel. In-situ measurements of basal slip velocities in field-scale debris flows are currently missing. This study introduces a novel monitoring setup that has been designed to directly measure basal slip velocities using paired conductivity sensors. Preliminary results of two events that were recorded at the Lattenbach catchment (Tyrol, Austria) in June 2025, are associated with a high degree of uncertainty, however, results indicate a tendential presence of basal sliding especially at the front of natural debris flows. Independently measured surface velocities were consistently larger than estimated basal slip. These preliminary findings support theoretical approaches that represent the granular nature of such flows. Future research will focus on refining the derivation methodology, the temporal resolution, an assessment of 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.