Particle Overpassing on Flat Granular Boundaries

by Craig H. Everts, Oceanographer; Coastal Engrg. Res. Ctr., Fort Belvoir, VA,

Serial Information: Journal of the Waterways, Harbors and Coastal Engineering Division, 1973, Vol. 99, Issue 4, Pg. 425-438

Document Type: Journal Paper


Sand-sized sediment under certain boundary shear conditions may overpass, i.e., may move as bed load, across a loose, flat boundary of unlike size and density without being deposited, and without appreciably disturbing the boundary stability. To define the boundary shear stress necessary for overpassing, and shear stress at incipient motion for bed particles, a series of flat bed, uniform flow tests was conducted in a flume using uniform sand sizes in the range 0.13 to 3.57 mm and density 2.65 to 4.70 g/cc. Flow depths were varied from 1.0 to 8.8 cm and mean flow velocities ranged from 13.1 to 50.3 cm/s producing critical boundary shear stresses between 0.8 and 16.5 dynes/cm². With a bed grain diameter smaller than 0.9 mm, and a grain density of 2.65 g/cc, the size range of overpassing particles was greatest for grains larger than the boundary. Minimum overpassing boundary shear appears to be a function of boundary roughness and of the thickness of the laminar sublayer with respect to the diameter of the overpassing grain.

Subject Headings: Boundary shear | Shear stress | Grain (material) | Bed materials | Shear flow | Particles | Overpasses and underpasses | Granular materials

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