BED-LOAD MOTION AT HIGH-SHEAR STRESS - DUNE WASHOUT AND PLANE-BED FLOW

Experiments involving bed forms have usually been carried out in flume s, but in the present work a pressurized-conduit system was found to b e 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 ba kelite (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 regi me to the upper-plane-bed (sheet flow) regime, rather than a gradual t ransition. The results for the upper-plane-bed regime show that this t ype of flow has a larger frictional resistance than that of a conventi onal 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.