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Fieldwork

Instruments Deployed near Martha’s Vineyard, Massachusetts, to Measure Bottom Shear Stress and Other Variables near the Seafloor



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Personnel from the U.S. Geological Survey (USGS) Woods Hole Coastal and Marine Science Center in Woods Hole, Massachusetts, placed instruments on the seafloor off Martha’s Vineyard in summer 2014 to measure the force created at the seabed by currents and waves, called bottom shear stress. In general, the higher the bottom shear stress, the greater the likelihood that bottom sediment will be picked up and moved by near-bottom currents. Information on the strength and variability of bottom shear stress, and the consequent erosion or deposition of sediment, has implications for seafloor geology, seafloor habitats, and the siting of structures such as seafloor cables or offshore wind turbines.

Researchers rigging a New Instrument for Making Bottom Boundary Layer Evaluations (NIMBBLE) for deployment
Above: Chris Sherwood (left) and Marinna Martini rigging a New Instrument for Making Bottom Boundary Layer Evaluations (NIMBBLE) for deployment from the research vessel Connecticut. Photograph by Elizama Pons-Montalvo, USGS summer student from City College of New York. [larger version]

USGS oceanographer Chris Sherwood, electronics engineer Marinna Martini, and summer students Elizama Pons-Montalvo (City College of New York) and Robert Forney (Rutgers University) deployed two New Instruments for Making Bottom Boundary Layer Evaluations (NIMBBLEs), designed to measure bottom shear stress. The NIMBBLEs have a pair of acoustic Doppler velocimeters (ADVs) that make rapid measurements of currents at two points that are about half a meter (a foot and a half) above the bottom and separated horizontally by about a meter and a half (five feet). Bottom shear stress is measured by examining turbulence near the seabed, and turbulence is estimated by examining the difference in currents measured by the two ADVs. This is a simple method based on the principle that most of the flow generated by waves, tides, and even big eddies is the same at these two nearby points, but turbulent fluctuations are different. The scientists also deployed two new large quadpods with multiple sensors for making sonar images of ripples on the seabed and for measuring bottom shear stress (using the same method as the NIMBBLEs), wave heights and periods, suspended-sediment concentrations, temperature, and salinity.

USGS electronics engineer rigs a quadpod for deployment in 10 meters (33 feet) water depth Scientists and crew watch as the quadpod is deployed from the research vessel Connecticut
Above Left: USGS electronics engineer Marinna Martini rigs a quadpod for deployment in 10 meters (33 feet) water depth off Martha’s Vineyard. Photograph by Elizama Pons-Montalvo, USGS summer student from City College of New York. [larger version]

Above Right: Scientists and crew watch as the quadpod is deployed from the research vessel Connecticut. Photograph by Elizama Pons-Montalvo, USGS summer student from City College of New York. [larger version]

Oceanographer Ellyn Montgomery and technician Chris Sabens helped prepare the USGS instruments, which were lowered onto the seafloor near the south shore of Martha’s Vineyard from the research vessel Connecticut on July 1, 2014. The NIMBBLEs were recovered, serviced, and redeployed in mid-summer using the Woods Hole Oceanographic Institution (WHOI) research vessel Tioga, and all of the instruments were recovered and serviced in September. They were redeployed in October and will remain in the water through December 2014, providing data for a range of fair-weather and storm conditions.

The same quadpod, after 84 days on the bottom
Above: The same quadpod, after 84 days on the bottom. Despite the fouling, good data were recovered from the acoustic sensors and the optical sensors that were equipped with mechanical wipers. USGS photograph by Chris Sherwood. [larger version]

The instruments were placed near the Martha’s Vineyard Coastal Observatory (MVCO), which continuously collects meteorological data (air temperature, wind velocity, relative humidity and so on) from an onshore station and oceanographic data (water temperature, salinity, current velocities, tides, and so on) from an offshore station, then posts the data in near-real time on the MVCO website. The USGS scientists chose the site of their experiment to take advantage of MVCO measurements and because they have detailed maps of bathymetry and backscatter from prior surveys (“Geophysical Data Collected off the South Shore of Martha's Vineyard, Massachusetts”). In the same area, a team of WHOI scientists is maintaining an array of moorings to measure water circulation and stratification, and also measuring surface currents from radar installations on Martha’s Vineyard.

Locations of bottom instrumentation south of Martha‘s Vineyard

Above: Locations of bottom instrumentation south of Martha‘s Vineyard. Permanent Martha‘s Vineyard Coastal Observatory (MVCO) sites (yellow triangles) include the Air-Sea Interaction Tower (ASIT) and the 12-meter undersea node. Summer 2014 NIMBBLE locations are shown as orange dots, quadpod locations as light-gray squares. In October 2014, NIMBBLEs were redeployed at sites marked by red triangles. Background is shaded with acoustic backscatter intensity from USGS geophysical surveys; lighter areas are higher backscatter, indicating coarse sand, and gray areas are lower backscatter, indicating fine sand and silt. [larger version]

Sherwood is co-principal investigator, along with Malcolm Scully and John Trowbridge (both with WHOI) of a project to study the effects of bottom roughness on bottom shear stress and current circulation on the inner continental shelf. The inner shelf is a critical link in the connection between coastal ecosystems and the open ocean. A better understanding of physical processes on the inner shelf will help scientists determine how they may affect such things as the movement of sediment and pollutants and the delivery of nutrients from deep waters to nearshore ecosystems. The research is supported by the National Science Foundation and the USGS Coastal and Marine Geology Program.

Learn more about students Elizama Pons-Montalvo and Robert Forney and the work they performed for the USGS last summer at “Summer Hires Assist Studies of Coastal Sediment Transport.” Learn more about bottom shear stress at “How Often Do Sediments on the Seafloor Move?


Related Sound Waves Stories
Summer Hires Assist Studies of Coastal Sediment Transport
May / June 2014
How Often Do Sediments on the Seafloor Move?
Mar. / Apr. 2012

Related Websites
MVCO website
WHOI
Geophysical Data Collected off the South Shore of Martha's Vineyard, Massachusetts
USGS

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in this issue:

Research
Natural Methane Seepage Is Widespread on the U.S. Atlantic Ocean Margin

Spotlight on Sandy
Field Investigations of Hurricane Sandy's Impacts on Fire Island, New York

Seafloor Mapping off the Delmarva Peninsula

#StrongAfterSandy—A Congressional Briefing Hosted by the USGS

This Woman ROCKS!

Fieldwork
Tripod Brings Data from the Deep Seafloor of the South China Sea

Instruments near Martha’s Vineyard Measure Seafloor Bottom Shear Stress

Coastal Streams in Central California Reflect the Region’s Drought

USGS Scientist Participates in National Geographic’s BioBlitz 2014

Outreach
Twenty Years of Ask-A-Geologist

Awards
Advancing Data Sharing Capabilities—2014 DeSouza Award

Staff & Center News
Postdocs Contributing to Climate-Change Studies

Feds Feed Families Food Drive

Publications
Sept. / Oct. Publications

Accessibility FOIA Privacy Policies and Notices

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