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

Data compiled from eight decades of incipient motion studies were used to calculate dimensionless critical shear stress values of the median grain size, tau*(c50). Calculated tau*(c50) values were stratified by initial motion definition, median gram 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; tau*(c50) 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 tau*(c50) ranges of 0.052-0.086 and 0.030-0.073, respectively. The apparent lack of a universal tau*(c50) for gravel-bedded rivers warrants great care in choosing defendable tau*(c50) values for particular applications.