Swash Bar Morphodynamics in the Danish Wadden Sea: Sand Bed Oscillations and Suspended Sediment Flux during an Accretionary Phase of the Foreshore Cycle

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Abstract
The landward migration of an intertidal swash bar at Skallingen (Danish Wadden Sea), together with the associated hydrodynamics and suspended sediment transport, were monitored over several spring tidal cycles during an accretionary phase of the foreshore cycle. The landward migration was not a simple linear displacement of the bar form; instead, regular intra-tidal sand bed oscillations (average height ∼ 0.12 m; average spacing ∼ 24 m; average steepness ∼ 0.005) moved sand landward across the seaward slope. The estimated migration rate of the bed oscillations was ∼ 0.032 m min−1. These values compare favourably with other citations in the literature. The sediment flux associated with these bed oscillations was the suspended sediment transport dominated by a net landward transport primarily by gravity wave frequencies. Infragravity waves were important at certain times in the tidal cycle but were variable in frequency and the associated transport was highly variable spatially. The suspended sediment transport by mean currents was smaller than that induced by the waves, and in almost all cases was directed offshore. The sand bed oscillations appear to be driven primarily by the incident gravity waves, which were also the primary forcing for the landward migration of the swash bar as a whole.