SUSPENSION OF FINE-GRAINED SEDIMENT

The suspension, transport, settling, and resuspension of fine sediment on shallow lakes and river margins subject to wind and tides can lead to problems of shoaling and poor water quality. Incipient motion of cohesive sediment involves physico-chemical bonding forces among bed particles. These forces must be overcome by the shear stress of the flow at the bed surface. Experimental results are used to understand the combined effects of unidirectional and oscillatory flow. It was found that 1) incipient motion occurred within a range of values rather than at a fixed shear stress or velocity; 2) compaction over time did not increase the shear stress needed lo cause incipient motion. 3) incipient motion of the sediment bed for oscillatory flow occurred at lower shear stress and velocity than those for unidirectional flow; and 4) oscillatory flow tended to remain laminar at velocities which, for unidirectional flow, would correspond to turbulent flow.