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Fieldwork

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


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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."

Andrew Schwartz and Dan Hanes maneuver an Aquadopp current profiler and its frame in the surf.
Above: Andrew Schwartz (left) and Dan Hanes maneuver an Aquadopp current profiler and its frame in the surf. Photographs by Patrick Barnard. [larger version]

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.

Nortek Aquadopp current profiler mounted on an aluminum frame Video camera mounted on top of the Cliff House restaurant at the north end of Ocean Beach.
Above left: Nortek Aquadopp current profiler mounted on an aluminum frame (constructed by Kevin O'Toole at the USGS Marine Facility in Redwood City, Calif. Two handles facilitate moving the apparatus. Sand anchors on the handle ends and tapered "feet" protruding from the frame bottom and embedded in the sand help keep the frame and instrument steady in the waves. Photograph by Patrick Barnard. [larger version]

Above right: Video camera mounted on top of the Cliff House restaurant at the north end of Ocean Beach. DSL, digital subscriber line. Photograph by Ann Gibbs. [larger version]

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.

Locations of instruments used to measure wave heights and currents during the surf-zone study in January 2006, superimposed on two aerial photographs Aquadopp current-meter and acoustic-Doppler-profiler sampling locations near the north end of Ocean Beach. Also shown are the camera's field of view and cross-shore transects sampled for the time-stack analysis.
Above left: Locations of instruments used to measure wave heights and currents during the surf-zone study in January 2006, superimposed on two aerial photographs (upper half from California Spatial Information Library [CSIL]; lower half from San Francisco Bay Area Regional Database [BARD]. Aquadopps, Aquadopp current profilers; ADPs, acoustic Doppler profilers. [larger version]

Above right: Aquadopp current-meter (green dots) and acoustic-Doppler-profiler (ADP) (red square) sampling locations near the north end of Ocean Beach. Also shown are the camera's field of view and cross-shore transects sampled for the time-stack analysis (T1 through T5). [larger version]

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.

Typical time-averaged images showing sand bar and rip currents.
Above: Typical time-averaged images showing sand bar (broad, light band) and rip currents (narrow, slightly darker bands perpendicular to the shore). a, oblique view from the camera's actual position; b, overhead, or map, view produced by rectifying image a. [larger version]

"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:

  • currents and wave heights outside the surf zone were measured with two acoustic Doppler profilers (ADPs),
  • foreshore elevations were measured with a global positioning system (GPS) mounted on an all-terrain vehicle (ATV), and
  • sediment-grain sizes were measured with a camera system developed by Dave Rubin of the USGS (see Sound Waves article, "Patent Awarded to USGS Scientists for Underwater Microscope System").

Typical time-stack image obtained during the surf-zone study at Ocean Beach. Jeff Hansen with an all-terrain vehicle equipped with a geographic positioning system
Above left: Typical time-stack image obtained during the surf-zone study at Ocean Beach. Image was created by extracting a line of pixels along each of the five transect lines (T1 through T5) 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 (for each transect) on the horizontal axis. Shoreline is on the left in each transect. [larger version]

Above right: Jeff Hansen with an all-terrain vehicle (ATV) equipped with a geographic positioning system (GPS) used to measure topographic profiles of the foreshore. [larger version]

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.


Related Sound Waves Stories
Patent Awarded to USGS Scientists for Underwater Microscope System
March 2004

Related Web Sites
Ocean Beach Coastal Processes Study
U.S. Geological Survey (USGS)
Ocean Beach Webcam
U.S. Geological Survey (USGS)
Coastal Evolution: Process-based Multi-scale Modeling
U.S. Geological Survey (USGS)

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in this issue: Fieldwork
cover story:
Surf-Zone Hydrodynamics at Ocean Beach

Research Scientists Recreate Shaking from 1906 San Francisco Earthquake

Outreach Science Workshops for Girls

GIS Specialist Shares Expertise with Local Community

Spoonbill Bowl

Awards David Scholl Selected as AGU Fellow

Staff USGS Visits Deer Island Sewage Treatment Plant

Geomorphologist Joins the WCMG Team

Publications April 2006 Publications List


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