The present study focuses at improving our basic understanding of size-graded and uniform sand transport mechanisms in oscillatory sheet-flows. Sheet-flow is the dominant transport mode during storm conditions, when coastal and seabed erosion and deposition processes are generally very strong. Moreover, this study aims at improving the performance of model concepts for the description of both the rate and size-composition of the transported sand. Improved modelling tools are developed through a strong interaction between experimental research (scale 1:1) and of mathematical transport modelling. Experiments were performed using fairly uniform sand sizes and non-uniform sands under 2nd-order Stokes waves. Graded sand experiments showed that, size-gradation has almost no effect on the net total transport rates, provided that the grain sizes of the sand mixture are in the range of 0.21 < D < 0.97 mm. If very fine grains (D = 0.13 mm) are present in the mixture net transport rates of graded sand may be different from those of uniform sand (with the same D_(50)). The net transport of each size-fraction in a mixture is strongly influenced by the presence of other fractions. Fine particles in sand mixtures are less transported than in a uniform sand bed (hiding effect), while the opposite occurs for coarse fractions in a mixture (increased exposure effect). In this study an assessment is made of measured net total transport rates and transport rates per size fraction and computed results from existing transport models (Dibajnia & Watanabe 1996 and Ribberink/Dohmen-Janssen 1999). The comparison is based on applying a size-fraction approach in order to improve the model results and to predict the size composition of the transported sand as well. Finally, on the basis of the new results model improvementsare proposed.
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