Vertical Velocity Profiles in Natural Debris Flows: Insights into Different Flow Regimes

Debris flows are gravity-driven, channelized mass flows with a highly variable composition of solids and fluids. Due to the variability of grain size distribution and water content, flow resistance is expected to vary within single events as well as between different events. One approach to constrain the flow resistance of debris flows involves the measurement of vertical velocity distributions, i.e., average velocities and velocity fluctuations at different heights above the channel bed.

This study investigates vertical velocity distributions in natural debris flows observed at a monitoring station at the Gadria creek in South Tyrol, Italy. The first aim is to establish a robust methodology for estimating these distributions through a comprehensive parameter sensitivity analysis which forms the foundation of the present work. The second aim is to contrast velocity profiles during single debris-flow events and along different debris-flow events. For this we differentiate between relatively short, “quasi-steady” flow sections, characterized by no significant changes in bulk flow velocity, flow depth, or visually assessed composition of the passing debris, and unsteady flow periods, which are characterized by rapid and pronounced variations in velocity, flow depth, and mixture composition over short time scales, as typically occurring in debris-flow surges/waves or at granular debris-flow fronts.

In the current setting, we achieve a maximum temporal resolution for derivation of continuous vertical velocity profiles of 0.4 seconds. We observe substantial differences in the vertical velocity distributions of quasi-steady and unsteady flow regimes. Quasi-steady flow exhibits a constant velocity profile. For an initial analyzed quasi-steady section the profile follows a S-shape, which we interpret as indication of non-homogenous mixture composition along depth. For the unsteady flow section, represented by a sequence of waves/surges, we identify changing profile shapes, progressing from linear to S-shaped and finally to slightly concave.

In the future, we will analyze (quasi-)steady and unsteady flow sections of many debris-flow events and connect these with independent measurements of basal normal stresses and pore fluid pressure, as well as analyses of material samples and laboratory experiments. The outcomes of this study provide a basis for improved debris-flow model representation and validation.