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Instruments Deployed near Martha’s Vineyard, Massachusetts, to Measure Bottom Shear Stress and Other Variables near the Seafloor
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.
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.
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 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.
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?”
in this issue:
Instruments near Martha’s Vineyard Measure Seaﬂoor Bottom Shear Stress
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