Volumetric growth of debris flow on erodible bed by basal shear and collision: Theory and observations

Debris flows surge down slopes as fully shearing and agitated mixtures, which are frequent and destructive mass movement processes. The prime control on debris-flow hazard is flow volume, which can dramatically increase by the erosion and incorporation of bed sediment. However, an erosion theory for debris-flow volume growth regulated by basal shear and particle collision is currently absent. Here, we establish a mechanistic framework that incorporates basal shear stress, collisional stress, and attendant bed pore-fluid pressure (PP) based on the Coulomb criterion. We identify mechanical controls on bed erosion and validate the proposed model against laboratory experiments and field measurements at Illgraben catchment. The results show that shear and collision stresses collectively regulate the erosion process where debris flows interact with wet sediments with elevated bed PP. In contrast, collision stress is solely responsible for the erosion of relatively dry sediment with low PP because basal shear stress is not enough to overcome shear resistance. The bed PP facilitates progressive scour of the sediment through reducing bed resistance for shear traction and increasing penetration depth in the sediment for collision traction. Our theory can improve predictions of flow volume and hazardous impact for debris flows and offer implications for the erosion processes of other dense granular flows including pyroclastic flows and snow avalanches.