Influence of sand on hydraulics and gravel transport in a braided gravel-bed river

Measurements in a braided reach of the proglacial White River, Washington, are used to compare the bed load capacity and competence of flow over the predominant gravel bed and over a narrow ribbon of mobile sand which extended alongside the talweg. Analysis of 144 velocity profiles and 222 bed load samples shows that shear stress and roughness height were significantly lower over sand than gravel, but maximum bed load diameter and total gravel transport rate significantly higher, as were transport rates of given size fractions at similar shear stresses. These differences in competence and capacity are explained by relative size effects on the threshold shear stress for entrainment. The influence on gravel transport of local variation in bed material size may explain the self-sorting of “bed load sheets” described by other workers. However, in channels like that described here the continuous longitudinal ribbon of sand allows the flow to develop a lower shear stress over the smoother bed, which partly offsets the relative size effects on competence and capacity.