Atlas of Gulf of the Farallones Region, Central California

Gulf of the Farallones National Marine Sanctuary (162 Kb)

Disposal of San Francisco Bay Dredged Material on the Farallon Slope

"....Conducting side scan sonar imaging and physical oceanographic characterization studies offshore of the Gulf of the Farallones is an excellent example of fostering partnerships between agencies to gather valuable scientific information and also to assist environmental management."

--Janet Y. Hashimoto; Chief, Environmental Protection Agency, Marine Protection Section. September, 1993

Issue

Harbors and other navigable waterways in San Francisco Bay require maintenance dredging to sustain economic growth and strategic utilization of ports. An economy of $5.4 billion and 35,000 jobs is directly dependent on navigable waterways.

Roughly 400 million cubic yards of sediment will be dredged from San Francisco Bay waterways over the next 50 years. The dredged material must be disposed of in a manner that is environmentally safe and acceptable to a wide variety of competing interests.

Three ocean sites considered for disposal of dredged material lie on the Farallon slope, a part of the seafloor west of San Francisco.

Questions

Is it environmentally safe and acceptable to dispose of dredged materials on the Farallon slope?

To provide policy makers with sufficient scientific information to help answer this question the following questions, among others, must be addressed:

• What is the natural sediment at the three disposal sites, how thick is it, and is it likely to slide downslope?
• Are the potential disposal sites characterized by sediment accumulation or erosion?

Some Answers

Preliminary mapping shows that the three sites are characterized by areas of slow accumulation and erosion of sediment.

Sediment consists largely of silts and clays, with rock cropping out in places.

Analyses indicate that sediment is stable under normal conditions and marginally stable when subjected to extreme earthquakes.

Partly on the basis of this study, a disposal site was designated, referred to as the San Francisco Deep Ocean Disposal Site (SF-DODS), at about 2,800 m depth on the Farallon slope.

Our results can serve as a baseline for subsequent studies to monitor the fate of the dredged sediment and its effect on the natural environment.

Summary

The U.S. Geological Survey (USGS), in cooperation with the U.S. Environmental Protection Agency (EPA), the U.S. Army Corps of Engineers, the U.S. Navy, and the National Oceanic and Atmospheric Administration/Gulf of the Farallones National Marine Sanctuary, undertook an interdisciplinary investigation of the Farallon slope to establish a scientific baseline.

We collected sidescan-sonar, bathymetric, geophysical, sediment- property, sediment-stability, and underwater-photography data over a 3,500-sq-km study area.

The acoustic data allowed us to map the geology and shape of the seafloor and to identify areas of sediment accumulation and erosion over the entire region. Analysis of sediment samples yielded information about the character of the sediment and its stability. This information combined with underwater photographs verified our interpretations of the acoustic data.

The results of the investigation allowed the USGS to provide credible scientific information to the EPA and the Army Corps of Engineers. Policy makers at these agencies were subsequently able to designate a specific disposal site on the Farallon slope on the basis of scientific data.

For additional information on this study, contact John Chin
Phone: (650) 329-5270