New unsaturated erosion model for landslide: Effects of flow particle size and debunking the importance of frictional stress

Abstract

Flow-type landslides, such as debris flows and rock avalanches, erode soil bed material as they flow downslope. The eroded material increases the volume of a landslide, and thus, its destructive potential. Field reports of debris flow show that collisional stresses generally occur at the flow front while frictional stresses generally occur at the base of the flow body. However, it remains unclear which of these competing effects is more dominant in terms of soil bed erosion. Moreover, existing theories do not consider the flow basal slip condition on the erosion dynamics at the flow-bed interface, nor do they consider the effects of volumetric response on the shear strength of unsaturated soil beds. In this study, an erosion model that considers the effects of basal slip condition and the volumetric response of unsaturated soil beds is proposed and evaluated with a unique set of experiments. Boundary collisional stresses drive soil bed erosion, while boundary frictional shear stresses are counteracted by the soil strength mobilized by overburden induced by volume contraction. The flow particle size and Froude number are shown to be adequate indicators of the boundary collisional stresses that govern the soil bed erosion rate. Findings from this study hint at the conjecture that existing erosion models may not be equipped with the necessary physics to provide realistic hazard assessments of flow-type landslides eroding soil beds.