HR: 08:30h
AN: OS31A-01 INVITED     [Abstracts]
TI: Rippled Scour Depressions off Northern California - Processes and Patterns
AU: * Cacchione, D A
EM: dcacchione@charter.net
AF: CME (Coastal and Marine Environments), 2945 Granite Pointe Drive, Reno, NV 89511 United States
AU: * Cacchione, D A
EM: dcacchione@charter.net
AF: USGS (Emeritus, Coastal and Marine Geology), 345 Middlefield Road MS-999, Menlo Park, CA 94025 United States
AB: 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.
DE: 3022 Marine sediments--processes and transport
DE: 3045 Seafloor morphology and bottom photography
DE: 4219 Continental shelf processes
DE: 4558 Sediment transport
SC: Ocean Sciences [OS]
MN: 2005 Joint Assembly