San Francisco Bight Coastal Processes Study

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 ATV equipped with a GPS receiver on the back	Track lines from the northern end of Ocean Beach during the July 6, 2004 ATV survey
Beach elevations calculated from the ATV survey using a standard kriging method	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.
 

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

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.