Morphodynamic Behavior of Sorted Bed Forms on the Continental Shelf II
Presiding: A Trembanis, Woods Hole Oceanographic Institution; E R Thieler, U.S. Geological Survey
OS31A-01 INVITED 08:30h
Rippled Scour Depressions off Northern California - Processes and Patterns
Striking seafloor features called "rippled scour depressions" were discovered on the inner continental shelf off northern California in the early 1980's. These features are similar to erosional patterns found off other coasts and are reexamined in light of additional data and new analyses. The fine-to-medium sand surface of the inner shelf is disrupted by elongate channel-like depressions of negative low relief (< 1 m) that generally extend shore-normal to slightly shore-oblique from the coast into water depths of 70 m. The widths of the depressions vary widely in the range of 50-500 m, and generally thin seaward. The sidewalls of the depressions are usually quite distinct, with one wall marking a sharp contact between the fine to medium sand of the adjacent elevated inner shelf surface and the coarser sand and gravel in the depressions. The other wall is usually less distinct and irregular, often showing interfingering of the contrasting sediment types. The coarse sand and gravel in the depressions form large symmetric ripples with crests aligned normal or slightly oblique to the sidewalls (ripple heights and wavelengths are 10-15 cm and 1.0 to 1.8 m, respectively). Based on previous measurements obtained with bottom instrumented tripods, the large ripples are generated by wave-induced bottom stresses during storms. Deposition during low energy periods produces a covering of finer sediment atop the large ripples. This surficial cover is swept clean during subsequent large wave events, and the ripples are reactivated. Low, broad crescentic dunes comprised of fine to medium sand were observed within the channel-like depressions in side-scan sonar images. Vibracores indicated that these dunes were riding on the coarse sand and gravel. The orientation of the crescentic dunes indicates that their migration pattern changes from offshore in the shallowest depths to nearly alongshore in deeper water. The migration rate and direction of the dunes appear to be controlled by the combined bottom stresses of waves and currents. Additional side scan sonar data shows that these features occur off many of the rocky coastal headlands along the northern California coast. Their pattern and distribution confirms earlier predictions that increased wave bottom stresses around bathymetric highs and enhanced downwelling flows off the rocky promontories generate and maintain the rippled scour depressions.
OS31A-02 08:55h
The Birth and Maintenance of Sorted Bedforms: Insights From Field Data, German Bight, South-Eastern North Sea
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.
OS31A-03 09:10h
Stratigraphic Evidence of Sorted Bedform Evolution
An examination of inner shelf sediments off Tairua on the north east coast of New Zealand provides stratigraphic evidence of the manner in which `sorted bedforms' or `rippled scour depressions' evolve and the oceanographic or geological conditions control their growth. Side-scan sonar, seismics and 15 vibracores (6 m long), interpreted in the context of wider field studies including swath mapping, seabed sampling and tripod deployments, provide a picture of the surface features and internal architecture of a Holocene sand body, along with detail of the alternating bands of coarse and fine sedimentary units interpreted as sorted bedforms. The coarse fraction is typically shell hash with some rock fragments. The coarse units are thin and typically the thickness of a bedform or two, while the associated fine sand units can be several meters thick. The boundaries between the units are typically distinct, but can be transitional. The coarse units are massive, show upward fining, or are just abruptly coarser at the base, reflecting differing modes of deposition such as settling from suspension, partial reworking of the coarse layer of lateral movements of large ripple over the bed, and the lateral back-and-forth oscillations of the boundaries of the units that we mapped at the surface. The stratigraphy reveals a picture of continual and progressive sorting and the difference between the coarse sand units represents various stages in bedform evolution frozen in time by burial by fine sand. In contrast the fine sands are massive, or exhibit weak horizontal laminae of shell fragments about 1 sand grain thick. While the coarse units are generally well developed and contain little fine sand, we see some that have much higher sand content and may represent coarse units that failed to develop fully before burial. The occurrence of the coarse units is unrelated to the thickness of the unconsolidated Holocene, and there is no obvious layer of coarse source sediments lying at shallow depth below the coarse and fine sedimentary units. In fact the very old C14 dates returned from shell material in the coarse units of surficial sediments suggest that the coarse layers take a long time to accumulate and shell is reworked from older deposits to do this. We present evidence suggesting that the coarse units develop best in areas of low sediment supply. Numerical modeling may help explain these processes. The Foundation for Research, Science and Technology (contract C01X0401) and the Andrew W. Mellon Foundation supported this work.
OS31A-04 09:25h
Development of Sorted Bedforms in the Nearshore: Geology or Hydrodynamics?
Recent investigations of sorted bedforms and ripple scour depressions have focused largely on measuring or modeling hydrodynamic mechanisms that may explain the development of these features. In contrast, this study questions the role geology may play in the development of sorted bedforms in the nearshore, and specifically tests if certain pre-existing characteristics of the underlying, framework geology may be more conducive for their formation. Results from bathymetric and sub-bottom surveys from the nearshore of the North Carolina Outer Banks document: 1) clusters of shore-oblique bars with adjacent gravel-filled troughs that span the nearshore (2-12 m depths), 2) persistent or recurring bar and trough morphology through storms and fair weather, and 3) the gravel exposed in the shore-oblique troughs are surface outcrops of a much larger, underlying stratum. A total of five large clusters of shore-oblique bars and troughs were identified in the 56-km long field site. In every case, evidence of a buried paleo-fluvial channel was found underneath the groups of shore-oblique bars and troughs. The morphology and surface sediment distribution of much of the nearshore (approximately 85% of the field site), nevertheless, did not show any evidence of sorted bedforms. This site-specific occurrence differs from the inner shelf surveys conducted by the US Geological Survey, which reveal a nearly ubiquitous presence of sorted bedforms covering very large regions of the US East Coast. The similar morphology of sorted bedforms found on both the inner shelf and nearshore suggest the same physical processes may be responsible for shaping and maintaining them but it remains unclear what parameters- geology, hydrodynamics, or a combination thereof - dictate the location and initial development of these features in the nearshore. The US Army Research Office (Grant &35;DAAD19-02-1-0307) and the US Geological Survey North Carolina Cooperative (&35;02ERAG0034) supported this work.
OS31A-05 09:40h
Process Model of Sorted Bedforms
Complex sorted bedforms, also known as rippled scour depressions, have been modeled previously using rules-based approaches developed from non-linear complexity theory. The models have had some success in explaining the origin and maintenance of sorted bedforms in a heuristic manner, but fall short of demonstrating a detailed understanding of the mechanics involved. Here we describe efforts to apply a deterministic model that includes detailed processes for hydrodynamics and sediment dynamics. Our objective was to determine whether a high-resolution application of a fairly standard collection of process models was sufficient to explain the formation and/or maintenance of these ubiquitous features. The three-dimensional physical oceanographic model is the hydrostatic Regional Oceanographic Modeling System (ROMS; http://marine.rutgers.edu/po/index.php?model=roms) and includes formulations for non-cohesive sediment resuspension and armoring, ripple geometry, and bottom stresses under wave and currents. A two-equation turbulence submodel was used to parameterize eddy viscosity and sediment diffusivity. Waves were modeled with the community model SWAN. We modeled a simple test case that represents the inner shelf of an enclosed basin, forced by winds and waves. Separate simulations were designed to 1) test the potential for formation of sorted bedform features from mixed sediment assemblages and 2) test the persistence of imposed features to the dispersive processes of sediment transport. We will present the results of these simulations and compare them with the heuristic model results.