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Debris flows can grow dramatically in volume and mobility as they override bed sediment due to the reduction in bed-friction resistance caused by high pore-fluid pressure (PP). However, the mechanisms that control PP evolution in overridden bed sediment are still unclear. Here, a new mathematical model clarifies how diverse styles and magnitudes of PP evolution can result from regulation of the flow diffusion and shear contraction of bed sediment. Normalization of the model equations shows that the propensity for PP generation depends on timescales of PP diffusion and deformation of the bed grains. The PP of saturated bed sediment under immobile conditions is equal to the fluid pressure at the bottom of the flow due to a non-flux basal boundary. However, the PP of unsaturated bed sediment is lower than that of the overlying flow. PP diffusion from a debris flow into an unsaturated bed increases with the bed's permeability and water content. The shear deformation behavior changes from undrained to drained with increasing permeability or decreasing shear velocity of the saturated bed sediment, leading to a reduced magnitude of pore pressure. By contrast, the shear deformation transitions from drained to undrained behavior with increasing permeability and water content of unsaturated bed sediment. The entrainment rate and erosion pattern of bed sediment are closely related to the PP evolution and liquefaction ratio of the bed sediment due to Coulomb-friction shear tractions. Our models can be used to interpret the feedback of PP on the flow momentum during debris-flow entrainment.

Landslide-debris flow is a sudden geological hazard in mountain areas, which is characterized with large scale, fast speed and wide impact range, and often causes disastrous accidents. In this study, an indoor sliding chute test was used to study the movement process of the landslide-debris flow and its accumulation pattern in the valley, taking into account the initiated gradient and particle size distribution. Besides, the model test was reproduced by PFC and the numerical models were constructed to fit the actual situation of landslide-debris flow. The results show that the collision of particles occurs during the movement of landslide-debris flow, and obvious sorting phenomena occur in the final deposit. Coarse particles distribute in the front and surface of the deposit while fine particles distribute in the back and bottom. The initiated angle has a certain effect on the morphology of the deposit: larger initiated angle makes the deposit closer to the opposite bank of the valley. Particle gradation has a significant impact on the form and distribution of deposit as well, with the increase of the proportion of coarse particles, the deposit of fine particles shrinks to the center of the rear edge, the profile of the deposit is more flat and uneven, the deposit is closer to the opposite bank of the valley, and the angle of the deposit profile increases significantly.

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