Tide-driven sediment variations on a large compound dune in the Jade tidal inlet channel, Southeastern North Sea

To understand the interplay of the local hydrodynamics, sediment parameters and geometry of subaqueous dunes, the spatial diversity and temporal change of grain size patterns of a large ebb directed compound dune (L ≈ 180m, H≈5m) in the Jade tidal inlet channel (German Bight, Southeastern North Sea) were studied. In the course of shifting tidal phases, repeated high resolution bathymetry and current velocity profiles were measured and 72 surface sediment samples were collected. Sediment parameters have been derived to describe grain size composition and statistics, in both space and time. Medium sand with an average mean grain size of ≈460 μm was the predominant fraction, covering most of the stoss and lee side of the dune. However, across the dune the mean grain size increased from medium sand of ≈350 μm at the lower lee and stoss sides to coarse sand of ≈700 μm on the crest. Regarding grain sorting, the bed material was better sorted towards the crest. Temporal sediment patterns were observed in the crest region, where the proportion of fine components decreased in the slack water phases and in the lee zone, where both fine and coarse components increased in proportion during the accelerated flood and ebb phases. The significant coarsening trend from trough to crest can be explained by an increase in flow velocity and transport capacity towards the crest region. So far interpretations of temporal trends are not feasible.