MOTION OF MOBILE BEDS AT HIGH SHEAR STRESS

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 T/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. Following the zone of no particle movement at low Y, there is a range at which sand waves are found, and finally, in the high shear-stress region where Y exceeds 0.8, the bed tends to become plane. This high-stress or upper-plane-bed condition may be encountered in rivers in flood, large flows in estuaries, and closures or breaches of cofferdams or dykes. Because of the very high rates of sediment transport associated with this type of flow, it has a disproportionate effect on both natural topographic features and engineering works. The investigation of such behaviour by traditional flume experiments is not easy, but testing in enclosed pressurised conduits can eliminate many of the difficulties (Wilson, 1966; Nnadi and Wilson, 1992).