Atlas of Gulf of the Farallones Region, Central California

Mooring locations (74 Kb)

Current Patterns Over the Shelf and Slope Adjacent to the Gulf of the Farallones

"The increasing ecological, economical and societal interest in coastal waters has shown the need for scientific understanding of the coastal ocean in its complex totality."

--Drs. R. L. Smith and K. H. Brink, Research Oceanographers

Issue

The U. S. Environmental Protection Agency (EPA) was charged with choosing a site on the seafloor that is suitable for the disposal of material dredged from San Francisco Bay. It is important that currents do not carry dredged material from the site into the nearby Gulf of the Farallones or Monterey Bay National Marine Sanctuaries, either during dumping or after the material has settled on the sea floor.

Questions

• What are the circulation patterns over the seafloor west of San Francisco?
• How often and where will currents move dredged material dumped on the seafloor ?
• Do we already know enough to develop a model of the movement of this material?

Some answers

Almost no measurements of currents or sediment movement exist for the region. Hence, we conducted a year-long measurement program to collect the necessary information.

Our data suggest that currents flowing over the continental shelf (the gently-sloping seafloor from the shoreline to about 200 m depth) behave differently from currents flowing over the continental slope (a much steeper surface that slopes from the edge of the shelf down to the deep ocean floor):

• Strong, equatorward winds in spring and summer drive shelf currents to the southeast.
• Winds have less effect on slope currents, which tend to flow northwestward.

Significant variations of currents with geographic location and depth indicate that the fate of dredged materials will depend on the location of the disposal site. Materials that reach the seafloor will disperse less in quiet areas on the slope than in the more energetic regions on the shelf.

Partly on the basis of this data, the EPA chose a disposal site on the slope in 2,800 m. A predictive model for the dispersion and transport of dredged material dumped at this site used our data to show that dredged materials would not enter the two National Marine Sanctuaries

Summary

In February 1991, the USGS began a year-long field program to gather data on circulation patterns over the seafloor west of San Francisco. This program was undertaken to help the EPA choose among several candidate sites for the ocean-floor disposal of material dredged from San Francisco Bay.

Our program had two main elements

1. An array of six moorings took measurements every 30 minutes of current speed and direction, water temperature and salinity at various depths.
2. Surveys that show a snapshot of currents above 300 m over a large region and that measure the temperature, salinity, density and clarity of the water column were conducted every three months.

Measurements show that large currents, with speeds larger than 30 cm/s, exist for extended periods of time in water depths less than 800 m.

These currents can carry suspended materials, such as the finer particles from dredged materials, long distances.

The currents weaken below 800 m but may strengthen again near the seabed, potentially resuspending material already deposited on the seafloor.

The specific areas where strong, near-bed flows occur depend on local details of the topography. The EPA took these factors into account in selecting a disposal site.

For additional information on this study, contact Marlene Noble
Phone: (650)329-5486