Sediment suspension under waves and currents: time scales and vertical structure

Abstract

Field measurements of the vertical structure of near‐bed suspended sediment concentrations were obtained from arrays of fast response optical backscatter suspended solids sensors to examine the time‐dependent response of sediment resuspension to waves and currents and the constraints imposed by bedforms. Data were recorded from both a nonbarred, marine shoreface and a barred lacustrine shoreface, under both shoaling and breaking waves (significant heights of 0·25–1·50m; peak periods of 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, but lagged in time. The time lag varies directly with separation distance between measurement locations and inversely with the horizontal component of the near‐bed oscillatory velocity. Both the presence of wave groups and the settling velocities of the sediment particules in suspension influence the temporal changes in concentration at a given elevation. Sediment concentrations appear to respond more slowly to the incident wind‐wave forcing with distance away from the bed as a result of two factors: (1) 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 interact with the near‐bed horizontal currents to impose a distinct constraint upon the timing of suspension events relative to the phase of the fluid motion, and, therefore, the vertical structure of the suspended sediment concentration at a range of time scales. The near‐bed concentrations 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. Concentration gradients in the presence of vortex ripples are large, as are the correlation between concentrations measured at different elevations within the fluid.

Frequency dependent cross-shore suspended sediment transport. 2. A barred shoreface

“…8b and c) as spilling breakers propagate from the bar crest across the trough. The low-frequency oscillation associated with the shoaling wave field reflects clearly the presence of a group-bound forced long wave (see also Greenwood and Osborne, 1992;Osborne and Greenwood, 1992a). In contrast, lowfrequency oscillations in the trough most probably represent energy transferred from the forced long wave to long waves constrained by the presence of the bar crest and the time-varying position of the breaker line (e.g.…”

“…These transitions may be explained by changes in the time-velocity relationships under increasingly skewed and asymmetric waves and by the phase lags which exist between velocity and concentration (Osborne and Greenwood, 1992a). Under this hypothesis the lowest frequencies in the wind-wave band would always tend to induce an onshore transport of sediment; during the onshore phase of the motion, the larger speeds would induce larger concentrations, but the length of the onshore half-wave cycle would still be long enough to allow material to rise above the bed (z ~ 0.04 m) and be advected shoreward during the same half-wave cycle.…”

“…A basic understanding of both sediment and bar dynamics will therefore be achieved by examining the suspended sediment transport response to changes in the wave he'~ght-to-water depth ratio through space and time (as in OG92). The constraints imposed by bedforms on suspended sediment transport rates will also be considered (as in Osborne and Greenwood, 1992a).…”

Temporal and spatial variations of surf-zone currents and suspended sediment concentration

“…In order to examine the influences of the complicated temporal variations and phase coupling of the surf-zone current and sediment concentration for the determination of sediment flux, u x,y and c are often partitioned as (e.g., Osborne and Greenwood, 1993;Grasmeijer and Van Rijn, 1999;Thornton et al, 1996) u x;y ¼ū x;y þũ x;y À low þũ x;y À high ð3Þ c ¼c þc low þc high ð4Þ…”

Size fractionation by suspension transport: a large scale flume experiment with shoaling waves

“…More recently, Wang et al (1998) used streamer traps (Kraus, 1987) to demonstrate a strong homogeneity in size and size-distribution with respect to distance vertically away from the bed, at least between z 0.05 and 0.50 m. Over a rippled bed and under non-breaking waves, Nielsen (1983) showed a decrease in size between the suspended and bed material, but was unable to detect any signi®-cant difference in the mean grain size within the suspended load with elevation. Under similar conditions, Osborne and Greenwood (1993) noted a small (18%) decrease in mean particle diameter between 0.04 and 0.50 m above the bed, with the most signi®cant change being a marked increase in the relative proportion of the ®nest fractions. In such cases, onshore transport close to the bed should distribute more of the coarser sediment towards the shoreline and offshore transport at higher elevations should distribute more of the ®ner sediment towards deeper water.…”