Boundary shear stress of granular flows

Abstract

The shear stress exerted at the bed and walls of granular flows is an important quantity for modeling and predicting runout, bulking up, channel erosion, and entrainment. Although there are some measurements of boundary shear stress for granular flows in the eld and laboratory, we lack systematic measurement of shear stress for a range of flow properties such as particle size, particle shape, and fluid content, especially with natural sediment or for long durations where the flow may evolve. We used two vertically rotating drums to study the boundary shear stress of granular flows, a smaller drum that was 56 cm diameter and 15 cm wide, and a larger drum that was 4 m diameter and 80 cm wide. The materials we used included combinations of different sized glass marbles, sand, gravel, fine sediment, and water. We compared two ways of estimating the boundary shear stress using the force balance of the particle flow in the drum. We varied particle size, particle shape, and fluid content. Larger and more angular particles increased the total boundary shear stress, as did a decrease in fluid content. This study illustrates some of the mechanisms and particle dynamics that control the boundary shear stress in natural geological flows, which has implications for debris flow modeling and hazard mitigation.