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

Laboratory simulation of wave shoaling over a medium sand bed (mean size = 250 mum) for a range of wave conditions (a natural spectrum, symmetrical an d asymmetrical wave groups) showed that a distinct horizontal fractionation of sediment by size occurred. Sediment became progressively coarser down-w ave (onshore) from the test section and progressively finer up-wave (offsho re) from the same source. This spatial sorting is explained by suspension t ransport alone and controlled by two factors: (a) a vertical profile of gra in-size in the suspension, in which there is a distinct increase in the pro portion of fines with elevation and a decrease in the mean size by approxim ately 18% between 0.04 and 0.24 m; and (b) a vertical profile of the mean m ass transport velocity, which revealed a down-wave (onshore) directed curre nt close to the bed and a current reversal at higher elevations. The averag e elevation for the reversal was approximate to 0.14 m under the range of w ave conditions simulated (H-8 = 0.22-0.78 m; T-pk = 2.25-3.78 s). 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, whic h was directed down-wave; at higher elevations as a response to a change in the phase coupling between concentration and velocity, the net transport w as 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 e levation 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 hig her elevations, an up-wave transport by the secondary wave was dominant, co mplementing the net transport due to the mean current. (C) 2001 Elsevier Sc ience B.V. All rights reserved.