Rapid Increase in Suspended Load at High Bed Shear

When the dimensionless shear stress (theta) exceeds 0.8 a plane shear layer develops on the top of a granular bed. This layer is stabilized by the mixture density gradient within it. Its thickness typically increases linearly with (theta), and so does the ratio of the effective roughness size to particle diameter, ks/d. If the ratio of shear velocity U* to particle fall velocity Vf is sufficiently large, the shear layer may be destabilized, giving a rapid increase in turbulent suspension. New closed-conduit experiments with 0.11  mm sand show that for U*/Vf > 6.5 the ratio ks/d has a pronounced increase above the typical shear-layer trend, and so does the suspended-load concentration (measured at the mid-height between the stationary bed and the top of the flow). These observations differ significantly from the sediment pick-up function in common use, indicating that the latter must be reconsidered. The present experiments also show a tie-in with hyperconcentrated flow as observed in the rivers of Northern China.