Pore pressure evolution and entrainment mechanism of convex unsaturated colluvial beds with divergent compositions under debris flow overriding

A debris flow descending through an erodible convex colluvial bed, originating from a landslide dam and its upstream deposits, can entrain massive amounts of sediment, dramatically increasing the debris flow volume. Most existing erosion models assume that bed sediments are fully saturated, although this condition is rarely observed in nature. Therefore, a thorough understanding of debris flow overtopping erosion on a convex unsaturated bed is crucial for quantifying disaster risk. In this study, we experimentally investigated the effects of sediment composition, specifically coarse-grain size distribution and fine particle content, on the pore pressure evolution and entrainment of debris flows overriding a convex unsaturated colluvial bed. The average entrainment rate at convex sites for continuously graded bed sediment was higher than its discontinuous counterpart. The measured pore pressures within the unsaturated bed sediments were primarily generated by the passing debris flows. Furthermore, it was found that these pressures decreased as the fine particle content increased and the coarse-grain size of the erodible substrates decreased. When the coarse-grain size of the debris flow was smaller than that of the bed sediment, only a portion of the eroded material was entrained by the moving debris flow. In contrast, when the coarse-grain size of the debris flow was equal to or greater than that of the bed sediment, nearly all of the eroded material was entrained. The findings of this study could contribute to the assessment of hazard amplification and inform the design of mitigation and prevention strategies.