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Pacific Coastal & Marine Science Center

Coastal Processes

SF Bight Field Methods

3-D Beach Mapping

Monthly beach surveys are conducted using an all-terrain vehicle (ATV) with an onboard GPS receiver linked to a nearby GPS base station. This produces 3D topographic maps with centimeter accuracy of the beach surface. Sets of surveys can then easily be analyzed in a GIS or other mapping software to identify seasonal trends, storm effects, and areas of chronic erosion. Using this technique, we are able to gather over 20,000 survey points in a single 6-hour survey covering the entire 7 km stretch of Ocean Beach. This is far more efficient than traditional base station surveying, where only approximately 1% of the number of survey points could be gathered in the same time.  

 
Photo of ATV; see caption below
The ATV equipped with a GPS receiver on the back
Track lines on backscatter; see caption below
Track lines from the northern end of Ocean Beach during the July 6, 2004 ATV survey
Elevations; see caption below
Beach elevations calculated from the ATV survey using a standard kriging method
Elevation changes; see caption below
Beach elevation changes between the July and April surveys, showing a general trend of shoreface erosion and back beach accretion

Nearshore Mapping

The 3-D beach mapping is coupled with cross-shore surveys conducted using personal water craft (PWC) with onboard GPS and echo sounder equipment. A highly accurate GPS base station is set up onshore that allows us to measure depths with centimeter accuracy. The survey lines run from 1.8 km offshore through the surf zone to depths as low as 1 m. The survey lines are nested with 250-m spacing in the Sloat region at the southern end of Ocean Beach where the erosion hot spot exists. Periodic PWC profiles document the changes in beach and nearshore morphology caused by seasonal variations and storms.  

Photo of personal water craft
Getting the PWCs ready for deployment
Photo of personal water craft in action
PWC in action
Photo of personal water craft
Peter Ruggiero preps the computer
Photo of personal water craft in action
Using hand and arm signals to communicate with PWCs
Photo of personal water craft in action
Another long day in the water; the team gets a workout

Map with profiles from 4 surveys at 3 locations on Ocean Beach in 2004
Map shows location of the PWC survey lines at Ocean Beach. The arrows on this map point to the locations of the three profile plots on the right: Lines 3, 6, and 14. Three surveys are overlain on each plot, from May, June and November 2004. Profiles from Survey Line #6 illustrate the shoreward migration of a major sand bar.

Grain Size Analysis

Using a digital bed sediment camera (a.k.a "eyeball"), grain size surveys are conducted periodically at Ocean Beach as a proxy for tracking changes in the physical energy along the beach. The grain size parameters can be extracted from a digital image by an analysis of the variation and offset of the pixel intensities. This allows us to process over 300 sediment samples in less than a day, whereas traditional sieving could take several months of work. A standard digital camera is housed in a custom waterproof casing for use in the coastal environment.

Taking photo of sand grains
Student surveys the surface of Ocean Beach using the "eyeball" camera
Photo of sand grains
A digital image of the beach surface at Ocean Beach allows grain size to be determined in minutes. The image is 1 cm across.

Real-time Monitoring

A camera system, installed for a period of time at the top of the Cliff House Restaurant at the northern end of Ocean Beach, looking south, was used to monitor beach and nearshore morphology and processes in real-time. The system was comprised of an analog video camera and a digital still camera, housed in a single pan tilt unit, linked to a computer and DSL connection such that the camera was controlled remotely from our office in Santa Cruz. Monitoring Ocean Beach in real-time and archiving all images allowed us to track short-term shoreline changes, beach evolution, sand bar migration, wave direction, storm effects, beach response, and nearshore circulation. 

Photo of web cam on top of Cliff House
The web camera unit looks south from atop the roof of the Cliff House Restaurant at Ocean Beach

 

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URL: http://walrus.wr.usgs.gov/coastal_processes/sfbaycoastalsys/sfbight/methods.html
Questions to: Patrick Barnard
Maintained by: Laura Zink Torresan
Page Last Modified: 3 March 2014 (lzt)