Dynamic Flow Resistance in Debris Flows: High-Resolution Insights from Remote Sensing

Debris flows are fast-moving, destructive mass transport processes that frequently occur in mountainous regions, posing severe threats to infrastructure and communities. Despite extensive research on debris flows, high-resolution velocity and flow depth data from full-scale natural events to test and parameterize empirical equations remain scarce. This study utilizes pulse-Doppler (PD) radar to continuously track debris-flow velocities at Illgraben, Switzerland, during the 2022 season. We analysed three debris flows and one debris flood, initially assessing four empirical mean velocity estimation equations: Newtonian Laminar Flow, Dilatant Grain Shearing, Manning-Strickler, and Chézy.

Flow resistance coefficients were back-calculated for each equation to evaluate their applicability and define their plausible value ranges. Based on these findings, we identified optimal Manning-Strickler (n = 0.16; n = 0.09) and Dilatant Grain Shearing (ξ = 25.5; ξ = 51.2) coefficients for the three debris flows and the debris flood, respectively, to estimate discharge and volume, highlighting both the strengths and limitations of these approaches. However, substantial variability in M-S and DGS coefficients—both within individual events and across different flows—challenges the conventional assumption of constant friction coefficients in debris-flow modeling.

Additionally, analysis of the relationship between flow height and velocity revealed a progressive decrease in yield stress across successive surges in two events, indicating a transition toward more fluidized flow behaviour. These findings contribute critical data for refining debris-flow models and improving predictive capabilities.