Shear-stress analyses provide easily determined estimates of the competence of rivers to transport coarse bedload material. Relevant data from diverse geological and engineering reports are integrated, and a good correlation between competent particle size and shear stress results when data points are derived in a uniform way.
Because flow mechanics differ in contrasting fluvial environments, the shear stresses needed to entrain large sediment may diverge considerably from values based on theoretical grounds. In shallow rivers, entrainment may occur at lower shear stresses than predicted by the Shields theory because hydrodynamic lift and bank caving provide additional transporting force. In very deep flows, the shear stresses needed to initiate particle movement are greater than theoretical values. Potential use of the empirical relationship between particle size and shear stress is limited by the following considerations: (1) random nature of turbulent life forces, (2) problems in interpreting the significance of the sedimentary deposit, (3) sediment finer than 5 cm in diameter is not considered, and (4) the analysis ignores special considerations of sediment packing, shape, and grain-size distributions.