The ''equilibrium'' distribution of grain size fractions and its implications for cross-shore sediment transport: A conceptual model

The relative distribution of individual grain size fractions across the cross-shore coastal profile of Terschelling (North Sea) displays a constant shape when ''average'' conditions are considered. The shape of the distribution curve depends on the hydrodynamic processes acting in the littoral zone and, if the curve is contemplated in a normalized, relative form, it can be considered as a permanent feature of the nearshore zone. The normalized curve is an ''equilibrium distribution curve'' and it can be used in the prediction of the cross-shore redistribution of sediments when the textural distribution of the supplied sediment is known. Sediment may be imported into the active profile by either natural or man-induced processes such as e.g. nourishments. The application of the conceptual model in the prediction of the sediment redistribution from the shoreface nourishment carried out in front of the Island of Terschelling indicates a predominant sediment transport from the nourished area towards the shoreline (58%) and a subordinate cross-shore transport in an offshore direction (17%). About 25% of the supplied sediment remains in the nourished area. Comparison of these results with the volumetric estimations of the sediment transport based on the morphological evolution shows a good agreement and suggests that the method gives reliable results. The method may become a simple, additional and useful tool in predicting the behaviour and dispersion of naturally or artificially imported volumes of sediment. The conceptual model is a typical black-box model and it cannot replace the existing and future generations of generic coastal morphodynamic models or process-related models such as 2DV coastal profile models. In addition the application of this model is definitely limited to coastal environments that satisfy certain hydrodynamic and geological conditions that are related to e.g. a longshore uniformity in processes and the lack of influence of sedimentary outcrops.