Size fractionation by suspension transport: a large scale flume experiment with shoaling waves
✍ Scribed by Brian Greenwood; Zhiming Xu
- Book ID
- 104156979
- Publisher
- Elsevier Science
- Year
- 2001
- Tongue
- English
- Weight
- 298 KB
- Volume
- 176
- Category
- Article
- ISSN
- 0025-3227
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✦ Synopsis
Laboratory simulation of wave shoaling over a medium sand bed (mean size 250 mm) 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 ®ner up-wave (offshore) from the same source. This spatial sorting is explained by suspension transport alone and controlled by two factors: (a) a vertical pro®le of grain-size in the suspension, in which there is a distinct increase in the proportion of ®nes with elevation and a decrease in the mean size by approximately 18% between 0.04 and 0.24 m; and (b) a vertical pro®le 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 < 0.14 m under the range of wave conditions simulated (H s 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 ®eld. 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 signi®cantly 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.