BEHAVIOUS OF MOBILE BEDS AT HIGH SHEAR STRESS

Kenneth C. Wilson, Fidelia N. Nnadi

Abstract


Beds of granular material show various types of behaviour as the dimensionless shear stress or Shields ordinate, Y, is increased. This quantity is defined as r/pg(S-l)d, where r is boundary shear stress, p is fluid density, g is gravitational acceleration, S is the ratio of solids density to fluid density and d is particle diameter. No movement occurs until Y exceeds a critical value. As Y is increased beyond the critical sand waves form, first increasing and then decreasing in steepness with successive increases in Y. Finally, in the highshear- stress region, say Y > 0.8, the bed becomes plane (or exhibits antidunes in cases of critical or supercritical free-surface flow). This high-stress condition, sometimes called the upper plane-bed region, may be encountered for rivers in flood, large flows in estuaries, and closures or breaches of cofferdams or dykes. Because of the very high rates of sediment transport this type of flow has a disproportionate effect on both natural topographic features and engineering works. it is not easy to investigate this behaviour by traditional flume experiments, but the use of enclosed pressurised conducts can eliminate many of the experimental difficulties (Wilson, 1966).

Keywords


shear stress; mobile bed; bed behavior

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