Effects of bed sediment conditions on debris flow propagation from the two-phase flow modelling perspective.

• An improved depth-averaged two-phase continuum flow model is adopted to investigate the effects of pore-fluid pressure and bed sediment conditions on multi-size debris flow. • New mechanisms are proposed to numerically explain the phenomenon of "the larger sediment sizes of bed materials, the higher erosion rate" and front coarsen of debris flows on the erodible bed. • The non-dimension number and energetics of the debris flows are investigated to demonstrate it is necessary to incorporate water-sediment and particle-particle interactions. Debris flows are classical two-phase flows that are greatly affected by bed sediment conditions. However, the underlying mechanism remains uncertain from either a theoretical or numerical perspective. Here an existing depth-averaged two-phase continuum flow model is further improved by incorporating the effects of pore-fluid pressure and bed sediment conditions. By numerical simulation of the USGS experiments, we have numerically reproduced the mechanism that "the coarser the bed sediments and the larger the basal pore pressure, consequently the larger the erosion rates". Moreover, an additional mechanism for this phenomenon has been revealed. Specifically, debris flows on steep slopes are likely to fall into a high shear stress regime, under which conditions the sediment transport capacity always takes a maximum value and is independent of the sediment size. Therefore, the sediment settling velocity that is proportional to the sediment size affects the erosion rate directly. It is also observed that the added potential energy due to a net erosion has positive effects on the kinetic energy of the debris flow. Notably, coarse and fine particles exhibit distinct roles in energetics and the former is characterized by more intensive interactions with the water phase, which should be appropriately considered in the modeling. Finally, particle size coarsening is observed in the head region, which is attributed to the hiding/exposure mechanisms of nonuniform sediments. These findings can enhance the understanding of the mechanism between the bed conditions and debris flow. [ABSTRACT FROM AUTHOR]