Determination of Sediment Transport Paths in Macrotidal Shoreface Environments: A Comparison of Grain-Size Trend Analysis with Near-Bed Current Measurements

摘要

The applicability of grain-size trend analysis for determining sediment transport pathways was investigated on two macrotidal shorefaces of the southern North Sea and Dover Strait where sediment transport is strongly controlled by well-defined, shore-parallel, tidal currents. Sediment transport directions were defined following the Gao and Collins (Sedimentary Geology, 80, 47–60) method, using surficial sediment samples collected from 0 to 8 m water depths. Grain-size trend analysis produced results in relatively good agreement with the directions of near-bottom currents measured in the vicinity of the sampling sites in some cases, but also yielded more confused patterns of calculated transport vectors in others. The best results were obtained when using discrete sets of samples collected during a distinct half tide–cycle. Comparison of threshold shear velocity for sediment motion with estimates of bed shear velocity obtained from near-bottom current data recorded simultaneously with sediment sampling indicates that sediment transport was most likely occurring when sediment samples were collected, suggesting that the grain-size trends observed in the surface sediments probably reflect the last transport event. Our results suggest that grain-size trend analysis can be efficient for determining the mean transport direction of the last sediment transport event when applied to small areas and when sampling is limited to the thickness of the active layer of sediment remobilisation. This method may be appropriate, however, for defining time-averaged residual transport directions when applied to larger areas over which progressive variations in surface sediment distribution reflect a longer term adjustment to hydraulic regime.