A SYSTEMATIC ANALYSIS OF 8 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 tradi tional Shields plot constructed from data that represent initial motio n of the bed surface material reveals systematic methodological biases of incipient motion definition; tau(c50) values determined from refe rence bed load transport rates and from visual observation of grain mo tion define subparallel Shields curves, with the latter generally unde rlying the former; values derived from competence functions define a s eparate but poorly developed field, while theoretical values predict a wide range of generally higher stresses that likely represent instant aneous, rather than time-averaged, critical shear stresses. The availa ble data indicate that for high critical boundary Reynolds numbers and low relative roughnesses typical of gravel-bedded rivers, reference-b ased and visually based studies have tau(c50) ranges of 0.052-0.086 a nd 0.030-0.073, respectively. The apparent lack of a universal tau(c5 0) for gravel-bedded rivers warrants great care in choosing defendable tau(c50) values for particular applications.