The 5th International Symposium on Slope Risk and Resilience

The partitioning of sliding mass at bifurcations is mainly controlled by a combination of inertia, basal friction, and deflecting barriers, yet the transient dissipation process governing branch dominance is not well understood. In this study, we have experimentally and numerically analysed granular flows bifurcating at a splitter, with a variation in slope angle, basal roughness, and deflector orientation. We proposed a flow phase-dependent, branch-specific quantification of kinetic energy dissipation, based on decay times obtained from the post impact velocity results. By analysing the post-peak decay, the results indicate that dissipation between smooth and rough branches is not universal, but changes with slope. At 30°slope angle, rough branches retain kinetic energy longer than smooth branches, whereas at high slopes (60°) rough branches dissipate KE much more quickly. Whereas the intermediate slope angle (45°) is highly sensitive to deflector orientation, highlighting a tipping point where flow momentum and deflector drag compete. These results reveal that, contrary to common assumptions, rough surfaces do not always cause greater energy loss in gravitational flows. Furthermore, a brief delay between the splash zone and the onset of branch-specific dissipation was observed across all the cases (Figure 1). This delay shows an inertia-controlled transient phase, followed by a roughness-controlled decay phase, highlighting a disparity of timescales in bifurcating flows. The results provide a detailed overview of how basal friction and deflecting barriers jointly control flow partitioning in debris flows and avalanches.