Holocene tidal conditions and tide-induced sand transport in the southern North Sea

A numerical model of Holocene tides and sand transport in the North Sea was used to establish potential tidal sand transport patterns in the Southern Eight and sand budgets for the Dutch and Belgian coast. The sand transport formulation yields magnitude and direction in addition to erosion/deposition values, in contrast to previous studies. All palaeobathymetries were obtained using the present bathymetry and sea level change results from an isostatic rebound model. The sea first invaded the Southern Eight through the Strait of Dover, and the microtidal range increased rapidly to mesotidal. The amphidromic system developed when the land bridge between Britain and the continent flooded and the northern tidal influence increased. Simultaneously, the current ellipses rotated from shore normal to shore parallel, while the flow velocities increased. The net tidal sand transport direction changed from toward the Dutch and Belgian shore to northward alongshore in the north and southward in the south. Until 7000 B.P. erosion dominated at sea, and deposition dominated near shore. Subsequently, erosion occurred in the south, and sedimentation occurred in the north. To the north of the Southern Eight, transport rates were low. Since 7500 B.P., sand has been transported mainly in suspension. Dominant transport by tidal asymmetry changed to transport by tidal residual currents and back in large areas. Comparison with previous studies illustrates that isostatic rebound is important for constructing palaeobathymetries. Most sediment was supplied to the Dutch and Belgian coast before 6000 EP. Substantial tidally induced mean sea level changes occurred in the early Holocene.