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Abstract

In the main channel of the typical T-shaped tidal area of Grådyb (mean tidal range ≈ 1.5 m), the maximum tidal currents are highest in both ebb (V1% ≈ 1.5 m/s; V50% ≈ 0.75 m/s) and flood (V1% ≈ 1.2 m/s; V50% ≈ 0.65 m/s) in the central part where the inlet splits up into two main tributaries behind the barriers. The ebb dominates over the flood in the outer parts and visa versa. The transport of fine-grained sediment in the area is extremely dependent on the weather conditions. In general, during windy periods the concentration level is above 40 mg/1 with mean values over the tidal period reaching a maximum of approximately 150 mg/1. In fair-weather periods the typical mean concentration in the main channel is between 15 mg/1 and 30 mg/1 and the in situ median grain size is surprisingly stable with a mean value of 26 µm. Recordings of the transport of suspended fine-grained sediment over 180 tidal periods, covering all seasons with typical weather conditions, showed that the important exchange of fine-grained material between the Wadden Sea and the open North Sea, is episodic. The investigated tidal area is exporting during stormy periods, concentrated in the winter term, where large amounts of fine-grained material are mobilized and apparently lost through the exchange of local turbid water with relatively clean water from the North Sea, The tidal area is importing during and after windy periods, following long periods of calm weather. This is speculated to be the outcome of fine-grained sediment settling on the shelf during long periods of calm weather, which increase the potential for high concentrations and large fall-velocities of suspended sediment. During subsequent windy periods those deposits are reworked and brought into the tidal area by the flood current. These conditions are most likely to appear in the summer.

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