Effects of Grain-Size Composition on Flow Resistance of Debris Flows: Behavior of Fine Sediment

Investigations are presented on the effects of grain-size composition on the flow resistance of debris flows via laboratory experiments with bidisperse granular materials. Two models focusing on whether the smaller particles behave as a completely solid or completely fluid phase are considered to describe the experimental flow resistance. To examine their performance, a blending factor is introduced that represents the relative contributions to the experimental flow resistance between these two models; results show that some cases are described by neither of the two models, which indicates that the smaller particles behave neither as a completely solid nor a completely fluid phase. As the blending factors are positively correlated with the suspension conditions of the smaller particles and are described by the ratio of the shear velocity or turbulent velocity to the settling velocity, the smaller particles behave in an intermediate way, with some as a solid phase and the rest behaving as a fluid phase, depending on the suspension conditions. © 2019 American Society of Civil Engineers.