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.
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.
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.
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.