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Fieldwork

Tripods Deployed for Studies of Sediment and Pollutant Transport off Southern California, or CMG Scientists Loan More Gear to Neptune

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tripod deployment
Chris Sherwood (directly under tripod), Tim Elfers (to Chris' left), and Shad Baiz (Scripps marine technician, foreground) are about to deploy a Geoprobe at 60-m depth in the middle of Santa Monica Bay.
CMG's Los Angeles Shelf Project, led by Homa Lee, is developing sediment- and pollutant-transport models for Santa Monica Bay, just west of the city of Los Angeles. To gather data for that project, CMG deployed three tripods in the bay in January from the Scripps vessel R/V R.G. Sproul. Marlene Noble and Chris Sherwood served as co-chief scientists, and about a dozen additional CMG folks helped prepare and deploy the tripods.

Following December's successful deployment of tripods in the New York Bight (see related story in January issue of Sound Waves), Woods Hole technicians Marinna Martini and Rick Rendigs joined their Menlo Park counterparts -- Tim Elfers, Joanne Ferreira, and Dave Gonzales -- to toss the remaining gear into the West Coast deep. Actually, nothing was tossed (everything was gently lowered with precise seapersonship) and there may still be some instruments ashore, but the deployment on January 12 represents another major field effort this winter.

The tripods will stay in place until mid-May, recording currents, water properties, suspended sediments, and images of the bottom. The measurements will provide much-needed information about general circulation and pollution-transport patterns in the urban bay. They will also be used in models of sediment and contaminant transport.

multibeam bathymetric map of Santa Monica Bay
Multibeam bathymetric map of the floor of Santa Monica Bay off the City of Los Angeles. Three red stars show where tripods, or moorings, were deployed on January 12, 2000 (star in upper right is legend, not mooring site). The tripods will stay in place until mid-May, recording currents, water properties, suspended sediments, and images of the bottom. [larger version 59KB]
The team deployed gear at three strategically selected sites around Santa Monica Bay (see map). A "classic" Geoprobe tripod was deployed at 60-m depth in the middle of the bay, near the head of Santa Monica Canyon and just north of the Hyperion outfall, one of three huge LA sewer outfalls that, combined, serve 6 million people. This tripod is equipped with the time-tested instrumentation developed by Dave Cacchione, Dave Drake, George Tate, and Joanne Ferreira, combined with an upward-looking acoustic-doppler current profiler, a sediment trap, and a brand-new downward-looking acoustic backscatter suspended-sediment profiler. This new instrument was purchased from England with year-end money, cleared customs after Christmas, and made it to Marfac just as the truck left. It uses sound at three frequencies to record detailed profiles (1-cm resolution in the bottom 128 cm) of suspended sediment.

A "next-generation" Geoprobe was deployed at 50-m depth in the northern bay, just off Malibu. Instead of the traditional electromagnetic current meters, this tripod uses a downward-looking pulse-coherent acoustic doppler current profiler for boundary layer measurements in the bottom meter. Instead of measuring two components of velocity at four elevations, the new PC-ADP will measure three components at 13 elevations. With all that technology, is anyone surprised that the vendor delivered the instrument to the dock two days before the cruise?

Joanne Ferreira preps tripod
Joanne Ferreira surrounds optical transmissometers with foam donuts soaked with algacide, which helps prevent fouling. This is the last step before deployment.
We deployed one of Marinna's "minipods" at the southernmost site, located on a submerged outcrop that, according to the multibeam-mapping data of Jim Gardner and Pete Dartnell, has only a veneer of sediment. Is this region non-depositional because it is sediment-starved, or because wave focusing and/or current patterns elevate bottom stresses and frequently resuspend sediments? The minipod will record velocity, pressure, and optical estimates of suspended sediments that, along with data from the sediment trap, will help to answer this question.

After a sunny week at the Scripps San Diego marine facility preparing the tripods, the team (led by co-chief scientist Marlene and lead marine technician Marinna) deployed guard buoys, moorings, and tripods without so much as a bump or shout. There was some consternation when the subsurface float used to buoy the acoustic release for the third and final tripod came up and wedged itself between the Sproul's rudder and prop, but the very able crew of the Sproul sorted that out, and we were headed home within an hour. Huge thanks are due to the staff at the Scripps facility and the crew of the Sproul. In addition to the USGS crew aboard the Sproul, lots of credit goes to Marfac and Menlo Park staff who worked long hours to prepare instruments and logistics for the deployment. The staff includes Kevin O'Toole, Steve Wallace, Dave Hogg, Bill Robinson, Hank Chezar, Fred Payne, and George Tate (formerly from CMG, temporarily on contract).


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in this issue: Fieldwork Southern California

SF Bay Soil Liquifaction

Outreach Everglades Field Trip

Meetings cover story:
Interim Science Priorities Plan

Gulf of Mexico Integrated Science

CMG Webmasters

Carbonate Beaches 2000

Staff & Center News Colloquium to Honor Ernest Manheim

WHFC Arrivals

Robbins Keynote Address

Publications February Publications List


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