Insights on Debris Flow Growth: Collisions and Contractile Skins

Soil bed erosion is perhaps the most important momentum exchange process that governs the growth and destructive potential of landslides, such as debris flow. Existing erosion models only consider shear stresses induced by the basal friction of the flow as the driving mechanism and adopt saturated soil mechanics to describe the failure of the soil bed. However, field observations have hinted at the importance of collisional stresses as a major driving mechanism of erosion and soil beds in nature are rarely saturated when landslides occur. In this plenary paper, unsaturated soil mechanics is used to characterize soil bed erosion by flows dominated by collisional stresses. Experiments were conducted to model the erosion of unsaturated sandy beds with a wide range of initial matric suction, which is a measure of the capillary stresses, by gravel flows. Key findings and their implications to the delineation of debris flow hazards are discussed.