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# 9.1: Introduction to Threshold of Movement

One of the classic problems in sediment transport is to predict the flow strength at which sediment movement first begins. This condition for incipient movement is usually expressed in terms of a critical shear stress or threshold shear stress, which I will denote by $$\tau_{\text{o}c}$$. The problem can be viewed either as the minimum shear stress needed to move a given particle, or as the largest grain size that can be moved by a given shear stress. The latter is termed competence by geologists.

Incipient movement should be one of the simplest problems in sediment transport, because at that point the flow has not yet become a two-phase flow, and all the principles and techniques of sediment-free flow—what is called rigid-boundary hydraulics—should still apply. Even in this simplest of problems, however, understanding is far from complete, which should put you on your guard about the great many approaches and formulas in the literature that are supposed to deal with established sediment movement.