Rapid Increase in Suspended Load at High Bed Shear

• Published in: Journal of hydraulic engineering (New York, N.Y.) N.Y.), 2005-01, Vol.131 (1), p.46-51
• Main Author: Wilson, Kenneth C
• Format: Article
• Language: English
• Subjects: Applied sciences; Buildings. Public works; Exact sciences and technology; Freshwater; Hydraulic constructions; TECHNICAL PAPERS
• ISSN: 0733-9429; 1943-7900
• DOI: 10.1061/(ASCE)0733-9429(2005)131:1(46)

When the dimensionless shear stress θ 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 θ , 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.