Infiltration-driven momentum dissipation and bed strengthening constrain debris-flow entrainment on concave slopes

Bed entrainment increases the destructive potential of debris flows by amplifying their volume and momentum. Current entrainment models mostly assume uniform channel morphologies and saturated bed sediments, which are rarely observed in nature. The effects of complex channel geometries, like concave slopes, and their interaction with heterogeneous bed compositions on infiltration-driven erosion remain poorly quantified. Here, we experimentally investigate the coupled dynamics of debris flows, pore-pressure evolution and entrainment over unsaturated concave beds using a series of small-scale flume tests. The results demonstrate that downward fluid infiltration, coupled with an increasing bed transition angle, induces considerable momentum loss, as evidenced by velocity reductions. Pore-pressure measurements reveal that shear-induced excess pressures dissipate rapidly under all conditions, leaving the overburden load as the dominant mechanism of pore pressure generation. Bed-elevation analyses show net deposition in the middle reach and net entrainment at the upstream and downstream ends, with elevation changes consistently smaller than the true entrainment depths because of subsequent tail deposition. A quantitative comparison of entrainment rates calculated from single-phase and two-phase erosion models with the experimental data indicates that the single-phase approach substantially overestimates erosion for the coarse-grained bed, whereas the two-phase model reproduces the measurements across all sediment types. The superior performance of the two-phase model is attributed to its explicit treatment of solid–fluid interactions, which can account for the reduction in basal shear stress and the infiltration-induced seepage forces that are inferred to elevate the critical shear strength of the bed under the tested conditions. These findings deepen our understanding on debris-flow entrainment dynamics.