Decadal scale stability of sorted bedforms, German Bight, southeastern North Sea

Complex, large-scale sorted bedforms have been investigated in the southeastern North Sea. We present a unique time series of sidescan sonar and multibeam backscatter data spanning 26 years (1977–2003). In the storm-dominated study area, sorted bedforms surrounded by fine sand plains, are widespread. Although, wave and current shear stresses indicate a frequent remobilisation of fine sediment and an episodic remobilisation of coarse sediment, it appears that existing patterns have remained essentially stable over a period of 26 years. Moreover, we observe the birth of new sorted bedforms. The observed patterns are interpreted as the product of a feedback-related sorting process consistent with a recent explanation of sorted bedform formation. served sorted bedforms tend to be oriented perpendicular to the ambient tidal currents, indicating the importance of tidal flows in their shaping. Moreover, they are dominantly symmetric in cross-section, i.e. the boundaries between coarse and fine sediment are sharp. This finding is consistent with a reversing tidal current of almost equal strength during ebb and flood. Similar to subaqueous dunes, sorted bedforms might therefore be subdivided into symmetric and asymmetric types, depending on hydrodynamic forcing. However, the newly emerging sorted bedforms are asymmetric, with an orientation independent of tidal current flow but perpendicular to the direction from which the highest storm waves approach the study area. We thus conclude that extreme storm events may play a major role in the generation of sorted bedforms, whereas the quasi-continuous tidal currents form and maintain their final shape.