Ocean surf 58

USGS Scientists Investigate Surf-Zone Hydrodynamics at San Francisco's Ocean Beach

Researchers from the U.S. Geological Survey (USGS)'s Pacific Science Center in Santa Cruz, Calif., recently conducted a study of surf-zone hydrodynamics at Ocean Beach in San Francisco, Calif. Ocean Beach is on the west side of San Francisco, within the Golden Gate National Recreation Area. The field effort is part of an ongoing study that began in April 2004 to document, analyze, and simulate the processes that control sand transport and sedimentation patterns along Ocean Beach and the mouth of San Francisco Bay. This study is part of the USGS project "Coastal Evolution: Process-Based, Multi-Scale Modeling."The recent surf-zone study was conducted over 5 days approaching spring tides in late January 2006 (maximum measured tide range was 2.2 m). Five current profilers—upward-looking Aquadopps from Nortek—were mounted on aluminum frames and placed on the sandy seabed at nine sites in the surf zone. The frames were manually deployed and retrieved at low tide by brave USGS scientists Patrick Barnard, Dan Hanes, Jodi Eshleman, Li Erikson, Peter Ruggerio, and Josh Logan, along with Andrew Schwartz of the Washington State Department of Ecology (DoE). To keep the instruments in place on the seabed within the high-energy surf zone, the frames were stabilized with two sand anchors on either side of the frame along the direction of breaking waves. In addition to sand anchors, tapered "feet" protruding from the bottom of each frame were buried in the sand. The Aquadopp current profilers collected time-series measurements of depths (pressure) and currents in the north-south and east-west directions at 10-cm intervals through the water column.Concurrent with the Aquadopp measurements, a video camera encased in a protective housing and mounted on the roof of the Cliff House restaurant was used to film the northern section of Ocean Beach (just south of the Cliff House; see Ocean Beach Webcam). The camera's field of view encompassed the locations of the northernmost Aquadopp instruments. Two variations of video images were generated (employing a system developed by Erdman Video Systems) and are currently being analyzed: (1) time-averaged images encompassing the camera's entire field of view and (2) "time stacks" along five cross-shore transects numbered T1 through T5.