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

Abstract

Osborne, P.D. and Greenwood, B., 1992. Frequency dependent cross-shore suspended sediment transport. 2. A barred shoreface. Mar. Geol., Field measurements of the local time-varying suspended sediment flux across a barred shoreface demonstrate that sediment transport is a response to high frequency wind wave oscillatory currents, low frequency waves and mean flows. The relative importance of the various transport components varies spatially and temporally in association with variability in the incident wave energy. In contrast to the non-barred shoreface, where variations were strongly depth-dependent, sediment transport across the barred shoreface is constrained by position with respect to the bars.On the lakeward slope of the bar, wind-wave oscillatory currents induce large rates of onshore transport near the bed; low frequency oscillatory currents (< 0.05 Hz) in this region are driven by the group-forced, bound long wave and produce an offshore transport, often equal in magnitude to that induced by the wind waves. The sediment transport attributable to mean currents near the bed is always offshore, resulting in a net offshore sediment flux across the slope. On the bar crest, net sediment transport is seen to be close to zero, owing to a balance between the offshore mean transport and the onshore net oscillatory transport (resulting from interaction between both high and low frequency waves).Landward of the bar crest and in the trough, sediment transport by wind waves decreases owing to dissipation of energy during wave breaking. In contrast, the contribution to suspended sediment transport by low frequency waves increases relatively, and furthermore this transport is now predominantly landward. This transport is probably associated with the release of the group-bound long wave as a free wave and the associated landward sediment flux approximates the offshore transport induced by the mean currents.Local variability in the frequency dependent transport, both in terms of magnitude and direction is closely related to the bedforms present and their variation in response to increasing and decreasing wave energy. Specifically, the oscillatory transport attributable to wind-wave frequencies is predominantly onshore in the presence of steep vortex ripples, but is directed offshore as the flow regime shifts to lower amplitude post-vortex ripples.

Beach-Profile Evolution under Spilling and Plunging Breakers

“…To simplify the complex temporal variations of the surf-zone sediment motion, current velocity and sediment concentration were often partitioned as ͑e.g., Osborne and Greenwood 1992;Thornton et al 1996͒ uϭū ϩũ low ϩũ high (5) cϭc ϩc low ϩc high (6) where ū and c ϭtime-averaged velocity and sediment concentration, respectively. ũ and c are oscillatory components of velocity and concentration.…”

Spectral analysis of turbulent flow and suspended sediment transport over fixed dunes

“…Osborne and Greenwood [ 1991a, 1991b have used spectral and cospectral analyses to determine the link between sediment transport and near-bed wave velocity. Their results show that the vertical convection of sediment can be related to ejections of fluid in the wave boundary layer over bed forms.…”

Transport of Fine Sands by Currents and Waves

“…Detailed field data are scarce because of the difficulties in measuring both the fluid velocities and sediment concentrations under wave conditions. Most of the available existing data are related to the longshore sediment transport processes in the surf zone (Jaffe et al 1984;Osborne and Greenwood 1992). Flume studies of sand transport by waves and currents were mostly related to bed-load transport (Inman and Bowen 1963;Harms 1969;Quick 1983).…”