Do Critical Stresses for Incipient Motion and Erosion Really Exist?

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Abstract
The concept of critical stress for the initial motion of noncohesive sediment beds under turbulent flow conditions is reviewed. Observational definitions of incipient motion are many and not entirely compatible. Some laboratory flume observations of sediment movement suggest that no true threshold exists. Current understanding of turbulent fluid motion at a sediment bed suggests that some particle movement must occur at all nonzero time‐mean velocities. A combined model for flume flow and sediment transport having no threshold is seen to explain features of data that have been previously used to support the threshold concept.