Abstract

The morphologies of the ebb-tidal deltas of Texel, Terschelling and Ameland inlets and the resulting sand transport patterns are greatly influenced by the interaction of shore-parallel tidal currents with tidal currents through the inlets. These three inlets and their ebb-tidal deltas are similar in many aspects, and together they differ from North American Atlantic coast examples and from those of the East Frisian Islands. Sedimentary facies and sequences, and sand body geometry of these three inlets/ebb-tidal deltas are strongly controlled by this interaction of offshore tidal currents, tidal currents through the inlets, and waves. The interaction and relative strengths of both tidal current systems determine the flow patterns and orientations of the main ebb channels in the ebb-tidal deltas. Different orientations of the (asymmetrical) ebb-tidal deltas as compared to those along the East Frisian Islands and the U.S. east coast, as well as a change of the general orientation of the main ebb channel of the ebb-tidal delta of Texel Inlet since the 16th century, are related to differences and changes in the relative importance of tidal currents and waves. Waves and longshore drift tend to force the ebb-tidal deltas into a downdrift asymmetry, whereas the shore-parallel tidal currents tend to force the ebb-tidal delta into an updrift asymmetry. Under the present regime of waves and longshore drift in this area, the critical boundary between updrift and downdrift asymmetry appears to be at a tidal prism on the order of 500 × 10⁶ m³. The volume of ebb delta sand bodies is approximately 1 km³ (Texel), a considerable part of which can be preserved in the fossil record. Holocene deposits off the West Frisian Islands indeed show that preservation of ebb-tidal delta deposits during a rise of relative sea level is possible under certain conditons. The strength of tidal influence is of great importance for determining both the basal boundary and the seaward extent of the ebb delta systems. Thus, it is inferred that preservation potential of ebb-tidal delta deposits can be positively correlated with the relative dominance of ebb-tides during their formation.