SIZE FRACTIONATION BY SUSPENSION TRANSPORT: A LARGE SCALE FLUME EXPERIMENT WITH SHOALING WAVES

Abstract

Laboratory simulation of wave shoaling over a medium sand bed (mean size=250μm) for a range of wave conditions (a natural spectrum, symmetrical and asymmetrical wave groups) showed that a distinct horizontal fractionation of sediment by size occurred. Sediment became progressively coarser down-wave (onshore) from the test section and progressively finer up-wave (offshore) from the same source. This spatial sorting is explained by suspension transport alone and controlled by two factors: (a) a vertical profile of grain-size in the suspension, in which there is a distinct increase in the proportion of fines with elevation and a decrease in the mean size by approximately 18% between 0.04 and 0.24m; and (b) a vertical profile of the mean mass transport velocity, which revealed a down-wave (onshore) directed current close to the bed and a current reversal at higher elevations. The average elevation for the reversal was ~0.14m under the range of wave conditions simulated (Hs=0.22-0.78m; Tpk=2.25-3.78s). These two factors were enhanced by a frequency-dependent transport in the wave field. The primary waves produced a maximum net transport close to the bed, which was directed down-wave; at higher elevations as a response to a change in the phase coupling between concentration and velocity, the net transport was reversed. In contrast, the net transport associated with the group-bound long wave was directed up-wave at all elevations and the decay rate with elevation was significantly less than that associated with the primary waves. Thus, near the bed, transport by primary and secondary waves was roughly equal and opposite (resulting in little wave-induced net transport); at higher elevations, an up-wave transport by the secondary wave was dominant, complementing the net transport due to the mean current.