Sediment dynamic processes and products on the shoreface of the East Frisian barrier-island system, southern North Sea

Abstract

The investigations on the Otzum ebb-delta were, amongst others, carried out to test the sediment bypassing model postulated in the 1980's and before. To achieve this, sediment distribution patterns, bedform migration (swash bars and dunes) as well as sediment transport directions were determined. The main body of the ebb-tidal delta is composed of fine sands, whereas superimposed swash bars on the inlet shoals consist of medium-coarse sands. Furthermore, the dip-directions of cross-bedded sands revealed in box-cores mainly trend towards the south or southeast, i.e. towards the inlet and not towards the supposed down-drift bypass corridor. As a consequence, sediment transport follows a semi-circular recycling pattern due to the interaction between tidal currents and high waves generated by strong northwesterly winds and storms. The inlet sediment bypassing model postulated in earlier publications is therefore refuted. On the other hand, the lower shoreface off Spiekeroog island is characterized by extensive shoreface-connected sand ridges striking in a NW-SE direction. Thus, the grain-size pattern shows trough-ward coarsening due to the presence of exposed palimpsest lag deposits such as gravels in the troughs. By comparing the spatial sediment distribution patterns of repeated surveys between 1989 and 2005, the most remarkable feature is that the coarse sediments (coarse sands to gravels) normally occupying the outer trough (around 15 m water depth) expanded onto the lower ridge flanks. This phenomenon is hard to be explained with tidal current action alone since the tidal current speeds in this environment are insufficient to transport such coarse material. Accordingly, wave action or wave-amplified tidal currents are the only mechanism capable of explaining this coarse sediment expansion. In order to illustrate this, the oscillatory threshold velocities of the 'effective' wave bases for different wave periods and heights characteristic for the local seasonal wave climates were calculated.