Bed-Load Motion at High Shear Stress: Dune Washout and Plane-Bed Flow

Abstract

Experiments involving bed forms have usually been carried out in flumes, but in the present work a pressurized-conduit system was found to be appropriate for investigating sediment motion at high shear stress. The bed materials were three sands (1.1 mm, 0.6 mm, and 0.4 mm) and bakelite (1.0 mm and 0.7 mm). The grain-size Reynolds number was between 25.4 and 157.0. As the dimensionless shear stress (Shields parameter) was increased toward unity, the steepness of the bedforms diminished abruptly. This behavior indicates a sudden shift from the bedform regime to the upper-plane-bed (sheet flow) regime, rather than a gradual transition. The results for the upper-plane-bed regime show that this type of flow has a larger frictional resistance than that of a conventional rough boundary. The bedform results indicate that the frictional characteristics of the bedform regime cannot be expressed in terms of dimensionless shear stress alone. The observed behavior also suggests that, as a solid-transport mechanism, sand waves may be more efficient than upper-plane-bed flow.