HR: 08:55h
AN: OS31A-02    [Abstracts]
TI: The Birth and Maintenance of Sorted Bedforms: Insights From Field Data, German Bight, South-Eastern North Sea
AU: * Diesing, M
EM: md@gpi.uni-kiel.de
AF: Institute of Geosciences, Christian-Albrechts-University, Olshausenstr. 40-60, 24098 Kiel, Germany
AU: Kubicki, A
EM: akubicki@gpi.uni-kiel.de
AF: Institute of Geosciences, Christian-Albrechts-University, Olshausenstr. 40-60, 24098 Kiel, Germany
AU: Winter, C
EM: acwinter@uni-bremen.de
AF: Research Center Ocean Margins, University of Bremen, Klagenfurter Str., 28359 Bremen, Germany
AU: Schwarzer, K
EM: kls@gpi.uni-kiel.de
AF: Institute of Geosciences, Christian-Albrechts-University, Olshausenstr. 40-60, 24098 Kiel, Germany
AB: Complex, large-scale sorted bedforms have been investigated in the south-eastern North Sea. We present a unique time series of side-scan sonar and multibeam backscatter data spanning 26 years (1977 - 2003). In the tide- and storm-dominated study area sorted bedforms (SBs), surrounded by fine sand plains, are widespread. Although wave and current shear stresses indicate a frequent mobilisation of both the fine and the coarse sediment, it appears that existing patterns have remained essentially stable over a period of 26 years. Moreover, we observe the birth of new SBs. Sediment cores, taken before the formation of these SBs has started, indicate a relatively thick top layer of fine sand on the order of up to a few meters. It is therefore unlikely that these newly emerging SBs have formed by the excavation of underlying coarse-grained strata. The observed patterns are interpreted as the product of a feedback-related sorting process in line with recent theory of SB formation.
The observed SBs tend to be oriented perpendicular to the ambient tidal currents, which indicates 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, which is in line with a reversing tidal current of almost equal strength during ebb and flood. Similar to subaqueous dunes, SBs might therefore be subdivided into symmetric and asymmetric types, depending on hydrodynamic forcing. However, the newly emerging SBs 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 SBs, whereas the continuous tidal currents form and maintain their final shape.
DE: 3022 Marine sediments--processes and transport
DE: 3384 Waves and tides
DE: 4219 Continental shelf processes
DE: 4558 Sediment transport
SC: Ocean Sciences [OS]
MN: 2005 Joint Assembly