Analysis of Complex Sand Waves in Raccoon Strait, San Francisco Bay

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Eaton, Mary
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College of Charleston BEAMS (Benthic Acoustic Mapping and Survey) Program students sailed aboard the eTrac, Inc. survey vessel S/V Pulse in December 2014 as part of a multibeam survey of Raccoon Strait, the channel separating Point Tiburon and Angel Island in San Francisco Bay. Multibeam data were processed using CARIS HIPS 9.0 software, revealing complex and dynamic bathymetry consisting of sand waves varying significantly in length, height, symmetry and orientation. Water depths within the strait range from 8 to 65 m and sand waves range in length from less than 5 m to more than 500 m, with one having a height exceeding 30 m. Wave symmetry, dimensions, and orientation were used to analyze the mechanisms influencing morphology including current direction, relative velocity, and channel width. Raccoon Strait is known to have some of the strongest tidal currents in the San Francisco Bay, due to both the narrow 1 km channel width and its proximity to the bay’s mouth. The strait’s southern margin sand waves are oriented northeastward towards the inside of the bay and northern margin sand waves are oriented southwestward towards the bay mouth, indicating forceful tidal currents in both flood and ebb directions, respectively. The distinctly different flow paths are the result of varying influences of both flood and ebb tidal currents acting within this large estuarine bay. This study shows how high resolution bathymetry can be used to study dynamic inshore sites. Repeated surveys of this area could be used to document migration of these large sand bodies.
sand waves, bedforms, multibeam data, bathymetry