Slip happens: Field evidence of basal sliding in natural debris flows

Basal sliding along the channel bed may play a significant role in debris flow propagation, however a lack of field measurements has limited our ability to understand the conditions that may occur in in-situ debris flows. Laboratory experiments have demonstrated that such sliding can occur under both fixed-bed and erodible conditions, driven by interactions between the heterogeneous debris flow material and the basal boundary. We introduce a novel monitoring setup designed to directly quantify basal slip velocities using paired conductivity sensors and report preliminary results from two natural debris-flow events recorded in the Lattenbach catchment (Tyrol, Austria) in June 2025.

The preliminary analysis indicates that basal slip was present in both events and consistently lower than independently measured surface velocities. Sixty-second binned median slip velocities were mostly below 2 m s⁻¹; fronts exhibited the highest values, followed by stabilization around 0.5–1 m s⁻¹. Event-scale ratios of  daveraged approximately 0.2 for both events, with instantaneous values ranging from 0.1 to 0.5 for the 15 June event and from 0 to 1 for the 30 June event. The latter comprised three surge-like phases, including a middle surge that briefly matched surface velocity. We note that the effective detection depth of the sensor pairs remains uncertain and likely varies with mixture conductivity and fluid content; if substantial, measured velocities may reflect the motion of lowermost flow layers rather than true bed slip.

These observations suggest that no-slip boundary conditions on non-erodible channel sections may not adequately represent debris-flow mechanics. Future work will improve temporal resolution, constrain detection depth, analyse additional events, and conduct cross-catchment comparisons.