Variation in ebb-delta morphologies along the West and East Frisian Islands, The Netherlands and Germany

Abstract

Ebb-delta morphologies in the West and East Frisian Islands differ mainly in terms of the direction and the degree of asymmetry of the ebb-deltas. In the East Frisian Islands, the ebb-tidal deltas and their main ebb channels are asymmetrically directed downdrift (to the east). In the West Frisian Islands, they are asymmetrically directed updrift (to the west), except for a few of the small ebb-tidal deltas which are asymmetrically directed downdrift. Earlier theories for the East Frisian Islands considered wind-driven waves coming dominantly from the west to be the main cause of the downdrift asymmetry of the ebb-tidal deltas. Theories for the West Frisian Islands related the updrift-directed ebb-tidal deltas to tidal currents. None of these theories can explain the morphological variation in the ebb-tidal deltas for both the West and East Frisian Islands. Observation of the ebb-delta morphologies of the West and East Frisian Islands in relation to tidal and wave regimes shows that the above-mentioned difference in the ebb-delta morphologies is due to the varying importance of the tidal prism and the wave-induced longshore drift for the different inlets. The difference is especially pronounced between the large and the small ebb-tidal deltas because their tidal prisms are different. The inlets with a large tidal prism have large ebb-tidal deltas and they are dominated by tidal currents. Interaction of marine shore-parallel tidal currents with onshore-offshore inlet tidal currents, which is enhanced by the ebb-delta shoals, causes the large ebb-tidal deltas and their main ebb channels to be asymmetrically directed updrift (to the west). The inlets with a small tidal prism have small ebb-tidal deltas and they are dominated by waves. Eastward-moving deep-water wave and wave-induced longshore drift drive the small ebb-tidal deltas asymmetrically downdrift (to the east). The general model proposed not only explains the varying morphologies of the ebb-tidal deltas of the West and East Frisian Islands, but is also applicable to other ebb-tidal deltas.