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, $k_s∕d$ . If the ratio of shear velocity $U*$ to particle fall velocity $V_f$ is sufficiently large, the shear layer may be destabilized, giving a rapid increase in turbulent suspension. New closed-conduit experiments with $0.11\mathrm{mm}$ sand show that for $U*∕V_f>\mathrm{6.5}$ the ratio $k_s∕d$ 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.