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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 profilersupward-looking Aquadopps from Nortekwere 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.
Time-averaged images were created from consecutive video images averaged over 10-minute intervals. Because waves do not break consistently in the exact same place, a more easily discernible and stable image of the wave-breaking region is obtained with a suite of averaged images. The time-averaged images are analyzed for spatial determination of sand-bar dynamics and the presence of rip currents.
"Time stacks" are composite images created by extracting a line of pixels along each of the five transect lines in a video frame and pasting the lines of pixels side by side. The same set of pixels were extracted from consecutive video frames, taken at a rate of two frames per second, and stacked vertically to produce an image with time on the vertical axis and cross-shore distance (of the five transects) on the horizontal axis. Time-stack images are analyzed for maximum runup length (that is, maximum inshore distance of the leading edge of the waves), swash period, and cross-shore current velocities. Runup height (maximum elevation above sea level at the leading edge of the waves) is calculated by combining the data from time-stack images with high-resolution measurements of foreshore elevations (see below). A technique for obtaining alongshore current velocities from the cross-shore time stacks is being developed. Current-velocity measurements obtained with the Aquadopps are used to verify the cross-shore and alongshore velocities determined from time stacks.
In addition to data collected with the Aquadopp current profilers and the roof-top video camera, a suite of parameters related to surf-zone mechanics were also measured:
Field support for the various measurements was provided by the same people who maneuvered the Aquadopp current profilers (see above), along with Ann Gibbs, Gerry Hatcher, and Liron Friedman from the USGS Pacific Science Center; Jeff Hansen from San Francisco State University; and Lindsey Doermann from the DoE.
For more information about our work at Ocean Beach, visit the Ocean Beach Coastal Processes Study. For more information about the "Coastal Evolution: Process-Based Multi-Scale Modeling" project, of which the Ocean Beach study is a part, visit Coastal Evolution: Process-based Multi-scale Modeling.
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