A systematic analysis of eight decades of incipient motion studies, with special reference to gravel‐bedded rivers

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Abstract
Data compiled from eight decades of incipient motion studies were used to calculate dimensionless critical shear stress values of the median grain size, . Calculated . values were stratified by initial motion definition, median grain size type (surface, subsurface, or laboratory mixture), relative roughness, and flow regime. A traditional Shields plot constructed from data that represent initial motion of the bed surface material reveals systematic methodological biases of incipient motion definition; . values determined from reference bed load transport rates and from visual observation of grain motion define subparallel Shields curves, with the latter generally underlying the former; values derived from competence functions define a separate but poorly developed field, while theoretical values predict a wide range of generally higher stresses that likely represent instantaneous, rather than time‐averaged, critical shear stresses. The available data indicate that for high critical boundary Reynolds numbers and low relative roughnesses typical of gravel‐bedded rivers, reference‐based and visually based studies have ranges of 0.052–0.086 and 0.030–0.073, respectively. The apparent lack of a universal for gravel‐bedded rivers warrants great care in choosing défendable values for particular applications.