Characteristics of velocity profiles along riffle-pool sequences and estimates of bed shear stress

Numerous velocity profiles were measured along a riffle-pool sequence for three different flow conditions. For each flow condition (ranging from baseflow to about 2/3 of bankfull depth), ten to fifteen velocity profiles were measured along the same transect. The objectives of the study were: (i) to differentiate between components of bed roughness and shear stress attributed to surface friction and large-scale bed undulations, and (ii) to investigate the effects of fluctuating discharges on the mean vertical flow structure in pools and riffles. No previous attempts have been made to investigate scales of bed roughness from velocity profiles along riffle-pool sequences and to determine how near-bed velocity gradients change with increasing flow discharge. At low flows (and as expected), riffles are characterized by higher near-bed velocity gradients than pools. Similarly, average bed shear stresses, as determined from measurements of flow depth and energy gradient, indicate a greater resistance and bed shear stress in riffle sections. Invariably, however, local shear stresses as determined from near-bed velocity gradients in pools and in riffles are much smaller than reach averages estimated from measurements of depth and slope. The shape of spatially averaged velocity profiles changes as discharges increases. The difference in terms of near-bed velocities between pools and riffles decreases as discharge increases which suggests that a velocity reversal may occur at or near bankfull. Further away from the bed surface, velocity differences between pools and riffles do remain significant. Finally, the curvature of the velocity profiles is more significant at high flows and surface roughness is maximized when flow conditions are intermediate, perhaps a result of increasing density of small-scale sedimentary structures.