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

Abstract

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.