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Tidal Inlets, Deltas and Flats
Time-Series Study of Mesoscale Tidal Bedforms, Martens Plate, Wadden Sea, Germany
Continuous bathymetric profiles, side-scan sonar surveys, and current velocity measurements were taken over a complete lunar tidal cycle through a tidal channel system in the German Wadden Sea near the islands of Spiekeroog and Wangeroog. Data were collected at low, mid-flood, high and mid-ebb stages under neap, mean and spring tidal conditions. A broad spectrum of medium- and large-scale bedforms (dunes) displays a range in size, shape and orientation that is related to specific tidal conditions. The nature of bottom configuration, time-velocity relationships and tidal stage show that although there are patterns displayed at individual locations, there is great geographic and temporal variation.
Irregularities in the channel floor caused by various scales of bedforms provide shielding from tidal currents during given phases of the tidal cycle. This shielding inhibits deterioration and may prevent reorientation of bedforms during tidal reversals. Locations only hundreds of metres apart may display both ebb- and flood-dominated bedforms. Height of bedforms ranges from tens of centimetres to greater than two metres at a single location between neap and spring tide conditions.
Velocity profiles show a log-normal distribution, with the accelerating phase displaying a steeper slope than the decelerating phase. Greatest shear velocities occur on the decelerating phase of the tidal cycle and bed roughness values peak near the end of the decelerating tidal currents.
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