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.