SEDIMENT SUSPENSION UNDER WAVES AND CURRENTS - TIME SCALES AND VERTICAL STRUCTURE

Field measurements of the vertical structure Of near-bed suspended sed iment concentrations were obtained from arrays of fast response optica l backscatter suspended solids sensors to examine the time-dependent r esponse of sediment resuspension to waves and currents and the constra ints imposed by bedforms. Data were recorded from both a nonbarred, ma rine shoreface and a barred lacustrine shoreface, under both shoaling and breaking waves (significant heights of 0.25-1.50 m; peak periods o f 3 and 8 s) and in water depths of 0.5-5.0 m. Sediment concentrations are positively correlated with increasing elevation above the bed, bu t lagged in time. The time lag varies directly with separation distanc e between measurement locations and inversely with the horizontal comp onent of the near-bed oscillatory velocity. Both the presence of wave groups and the settling velocities of the sediment particules in suspe nsion influence the temporal changes in concentration at a given eleva tion. Sediment concentrations appear to respond more slowly to the inc ident wind wave forcing with distance away from the bed as a result of two factors: (i) the sequential increase in concentration induced by a succession of large waves in a group; and (ii) the relative increase in finer sediments with smaller settling velocities. Bedforms interac t with the near-bed horizontal currents to impose a distinct constrain t upon the timing of suspension events relative to the phase of the fl uid motion, and, therefore, the vertical structure of the suspended se diment concentration at a range of time scales. The near-bed concentra tions appear to be strongly dependent upon the vertical convection of sediment associated with the ejection from the wave boundary layer of separation vortices generated in the lee of ripple crests- Concentrati on gradients in the presence of vortex ripples are large, as are the c orrelation between concentrations measured at different elevations wit hin the fluid.