FLUME EXPERIMENTS AIMED AT UNDERSTANDING ENTRAINMENT PROCESSES IN STEEP, SHALLOW FLOWS

The potential for debris flows to form from runoff on steep slopes covered with non-cohesive sediment has been cited often, but the mechanism behind the formation of these rapidly entraining flows is poorly understood. We have performed a variety of experiments in a laboratory flume with a plane, uniform, cohesionless bed in order to study the conditions in which the primary mode of particle transport transitions from bedload to granular sheet flow, the latter which is often described as an immature debris flow in the literature. We define the bedload regime to occur when only a portion of the particles from the surface layer of the bed are entrained in the flow. By contrast, granular sheet flow is associated with higher sediment transport rates and involves entire layers, often several grain diameters deep.

Theoretical and empirical models state that the occurrence and thickness of the granular sheet flow layer is simply a function of increasing basal shear stress. Observations suggest that granular sheet flows occur in shallow flows at steeper slopes, as opposed to deeper flows and shallower slopes. In our experiments we vary the particle size and shape (glass spheres versus natural grains), the bed slope, and the water flow depth to determine the depth of the mobilized layer and the conditions at which the onset of granular sheet flow occurs. Using high speed images of the sediment motion and velocity profile measurements in the fluid layer, we can estimate momentum transfer from the liquid layer to the bed surface and between the particle layers in the bed. We observed entire layers moving together and mobilizing lower layers. This work gives us a better insight into the interaction between the fluid and solid phases at high particle concentrations, which is necessary to predict entrainment rates and/or volumes by runoff on non-cohesive beds, and to define under what conditions sheet flows transition to fully developed debris flows.