The behaviour of intertidal sandwaves during neap‐spring tide cycles and the relevance for palaeoflow reconstructions

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Abstract
Variations in migration distances and shape characteristics of sandwaves in relation to flow conditions were studied on the Ossenisse intertidal shoal in the Westerschelde estuary. The purpose was to analyse bedform behaviour, to establish the threshold and the time lags involved, to find differences in two‐ and three‐dimensional sandwaves and to determine the implications for palaeoflow reconstructions.Sandwave migration is well correlated with the peak depth‐averaged flow velocity of the dominant tide. Thus the latter parameter may be estimated from the thickness of the tidal bundles. Other flow parameters such as shear velocity, Chezy C or roughness length do not show a correlation with the migration and cannot be used in palaeoflow analysis.Flow depth does not correlate with sandwave height or with length. Consequently, neither sandwave height and length nor set height and length can be used for palaeoflow depth determination.Sandwaves start moving when the peak dominant flow velocity exceeds 0.5–0.6 m s−1, and appreciable changes in shape occur at 0.75–0.8 m s −1. Complete reversal of sandwaves is accomplished if both the dominant and subordinate peak depth‐averaged current velocities exceed 0.85 m s−1.Two‐ and 3‐D sandwaves appeared to have different stability fields in the velocity‐depth diagram and in the diagram of the Froude number versus the depth‐grain‐size ratio. In addition the distinction between 2‐D and 3‐D sandwaves appeared to be related to a variability in current direction during periods of appreciable sand transport. There are also differences in sedimentary structures between the two types of sandwaves.