VARIATION IN EBB-TIDAL DELTA-GEOMETRY ALONG THE COAST OF THE NETHERLANDS AND THE GERMAN BIGHT

The morphology of the ebb-tidal deltas along the Dutch and German Nort h Sea coast is examined in relation to wave, offshore and inshore tida l regimes. Remarkable differences exist in the shape of shoals and tid al channels in the ebb-tidal deltas along the coast from the SW Nether lands to the German Bight. The large ebb-tidal deltas along the wester n Wadden Islands have a seaward protruding shape strongly oriented to the left with respect to the offshore direction (or updrift with respe ct to the direction of the general regional longshore drift). Ebb chan nels also turn to the left in their most seaward part. On the other ha nd the ebb-tidal deltas along the southwestern coast of the Netherland s as present before the tidal inlet closures of Delta Project showed a sharp-edged triangular shape pointing landward. Their ebb channels bi furcated seaward, orienting updrift and downdrift on both sides of the ebb-tidal deltas. It is shown that these variations are due to differ ences in interaction of offshore tidal currents and inshore tidal curr ents between the two areas. In the original ebb deltas of the SW Nethe rlands, the ebb channels bifurcating to the left are more pronounced t han the channels bifurcating to the right. This is probably related to a significant decrease in tidal range along the coast to the north. I n the Wadden Sea the small tidal deltas, in contrast to the large ebb tidal deltas, protrude downdrift. This is due to the large longshore c omponent of wave energy flux relative to the small tidal prisms throug h the inlets. A descriptive model of ebb-tidal delta geometry is propo sed, which considers the relative influence of waves as compared to ti dal currents, wave direction and the interaction of offshore and insho re tidal currents.