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

College of Charleston BEAMS (BEnthic Acoustic Mapping and Survey) students sailed aboard the eTrac, Inc. 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 and SIPS 9.0, revealing complex and dynamic bathymetry of Raccoon Strait, consisting of sand waves varying significantly in length, height, 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 several having heights exceeding 300 m. These sand waves were classified by their geomorphology, and further characterized using backscatter intensity. Wave symmetry, dimensions, and orientation were used to compare the relative ebb and flood tidal energy flowing through the channel. 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 eastward towards the inside of the bay and northern margin sand waves are oriented westward towards the bay mouth indicating forceful tidal currents in both ebb and flood directions. The distinctly different flow paths may be the result of Coriolis influence within this large estuarine bay. This study shows how high resolution bathymetry and backscatter can be used to study dynamic inshore sites. Repeated surveys of this area could be used to document migration of these large sand bodies.