|Home||Archived February 20, 2019||(i)|
A field of giant underwater sand waves was mapped at high resolution for the first time just west of the Golden Gate Bridge in a cooperative effort by the U.S. Geological Survey (USGS) and California State University, Monterey Bay (CSUMB). These features are among the largest anywhere in the world, rivaling sand waves at such well-known sites as the Bay of Fundy in Nova Scotia and Cook Inlet in Alaska. Striking images and a description of these features were published in the July 18, 2006, issue of Eos (Transactions of the American Geophysical Union).
More than 40 large (greater than 50-m wavelength) sand waves were mapped, with crest-to-crest lengths of as much as 220 m (722 ft) and heights of as much as 10 m (33 ft). The scale of these massive features is unusual because of the modest tidal range in the region (max 2.65 m [8.7 ft] between low and high tide), as opposed to other sites where large sand waves are present (for example, the typical tidal range in the Bay of Fundy is 17 m [56 ft]). But these features persist because tides force an enormous flow through the relatively narrow Golden Gate straita total volume of 2 billion m3 (528 billion gallons) every 6 hoursresulting in tidal currents that typically exceed 2.5 m/s (5.6 mph). These strong flows effectively sweep all mobile sediment through the narrowest part of the channel. However, the large sediment-transport capacity of these flows diminishes as they emerge from the Golden Gate, spread out, and slow down, dropping much of their sediment to form one of the largest sand-wave fields in the world.
Most of the sand and gravel that supplies this sand-wave field presumably originates from the Sierra Nevada mountain range and is carried into San Francisco Bay by the San Joaquin and Sacramento Rivers; local erosion of the Marin Headlands (to the north) may also contribute sediment.
The bathymetric survey was completed during 44 days in fall 2004 and fall 2005, using a multibeam sonar system. Patrick Barnard, Dan Hanes, and Dave Rubin of the USGS commissioned and analyzed the survey data, which was collected by the Seafloor Mapping Laboratory at CSUMB, headed by Rikk Kvitek. In addition to the USGS, the U.S. Army Corps of Engineers and the National Oceanic and Atmospheric Administration also provided funding for this survey.
The San Francisco Chronicle ran a front-page article about the sand waves on July 20, 2006. Articles appeared in additional media outlets, such as CNETNews.com (view article) and the San Jose Mercury News (view article). The full citation of the scientific report is Barnard, P.L., Hanes, D.M., Rubin, D.M., and Kvitek, Rikk, Giant sand waves at the mouth of San Francisco Bay: Eos (American Geophysical Union Transactions), v. 87, no. 29, p. 285, 289.
The sand waves also appear on the recently released USGS Scientific Investigations Map 2917, entitled "Under the Golden Gate BridgeViews of the Sea Floor Near the Entrance to San Francisco Bay, California," and they will be featured in a USGS Scientific Investigations Map now in preparation, with the working title "Bedforms at the Mouth of San Francisco Bay."
The existence of these sand waves was first documented by USGS scientists David Rubin and David McCulloch during a sidescan-sonar survey focused primarily inside the bay in the late 1970s in the following paper: Rubin, D.M., and McCulloch, D.S., 1979, The movement and equilibrium of bedforms in central San Francisco Bay, in Conomos, T.J., ed., San Francisco Bay; the urbanized estuary: San Francisco, Calif., American Association for the Advancement of Science, Pacific Division, p. 97-114 [URL http://www.estuaryarchive.org/archive/conomos_1979/8/].
in this issue:
Underwater Sand Waves Seaward of Golden Gate Bridge
|Home||Archived February 20, 2019|