Proposed mechanism for sediment entrainment by turbulent flows

Abstract

The means by which turbulent flows entrain sediment grains from alluvial stream beds were studied. Entrainment was considered to include both the initiation of sediment motion and the suspension of grains by the flow. Observations of grain motion induced by turbulent flows and by a pulsating jet led to the formulation of an entrainment hypothesis based on the concept of turbulent eddies disrupting the viscous sublayer and impinging directly onto the grain surface. It is suggested that grain entrainment results from the interaction between fluid elements within an eddy and the sediment grains. Experiments made in a laboratory flume with turbulent flows over sediment beds of different form are described. The resultant grain motion and the observed effects of sediment and fluid interaction are explained, in each case, in terms of the entrainment hypothesis.