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Woods Hole Coastal and Marine Science Center > Coastal Model Applications and Field Measurements > Research Activities > OASIS

Coastal Model Applications and Field Measurements

Profiles of Suspended Particles in the Bottom Boundary Layer (OASIS Project)

We deployed a tripod with a new profiling arm at the Martha’s Vineyard Coastal Observatory (MVCO) in September 2011 as part of the Office of Naval Research (ONR) Optics Acoustics and Stress In Situ (OASIS) Project, supported jointly by ONR and the USGS Coastal and Marine Geology Program as part of the Coastal Model Applications and Field Measurements Project. The objective of the measurements was to relate optical and acoustic properties of suspended particles as wave- and current-induced stresses caused changes in particle size, concentration, and vertical distribution in the bottom boundary layer (bbl) near the sea floor. This information on the physics of particle resuspension and aggregation, and light penetration and water clarity, will help improve models of sediment transport, benthic primary productivity, and underwater visibility. There is a well-established technology for acoustic profiling, but optical profiles are more difficult to obtain because of the rapid attenuation of light in water. A specially modified tripod with a moving arm was designed to solve this problem by moving instruments vertically in the bbl, between the bottom and about 2 meters above the bottom. The profiling arm was designed, built, and tested during the spring and summer of 2011 by a team of scientists, engineers, and technicians from the USGS Woods Hole Coastal and Marine Science Center (WHCMSC). To accommodate power requirements and the large data files recorded by some of the optical instruments, the tripod was connected via underwater cable to the MVCO, operated by the Woods Hole Oceanographic Institution (WHOI). This connection afforded real-time internet communication with the embedded computers aboard the tripod. Instruments were mounted on the profiling arm, and additional instruments were mounted elsewhere on the tripod and nearby on the sea floor. The tripod (USGS Mooring 910) and a small mooring (911) for a profiling current meter were deployed on 17 September at the MVCO 12-m deep underwater node about 2 km south of Martha’s Vineyard, Massachusetts. Divers assisted in the deployment and cleaned the instrument surfaces on the tripod approximately once a week until the tripod and current meter were recovered on 23 October, 2011. There was a range of wave and current conditions during the 36-day deployment, including the distant passage of tropical storm Ophelia, several moderate wave events, and a significant local gale that generated wave heights greater than 4 m at the 12-m site and knocked over the tripod three days before it was recovered. All but one of the instruments functioned well and provided complete data sets. The details of these data and their locations were described in an open-file report (Sherwood et al., 2012).

Colored computer graphic of data

Perspective view of profiles from the LISST 100-X on the moving arm. Suspended particle volume concentration (z-axis, μL/L) as a function of elevation above the bottom (y-axis, meters), over 1.5 days in October (x-axis; hour:minutes), colored by the median particle size. The blue line on the back panel is measured current shear velocity u*c (cm/s).

Many of the results provide quantitative confirmation of expectations based on simple models. For example, estimates of particle settling velocity made for times when the mass balance is mostly between upward diffusion and downward settling (the Rouse equation) differ depending on the sensor properties and amount of turbulent resuspension. Acoustic backscatter intensity is proportional to total particle volume while optical sensors respond to total particle cross-sectional area. As a result, acoustic sensors are less sensitive to small particles. Our observations show that acoustic sensors predict stronger gradients in sediment concentration near the bed than optical sensors. This results in higher estimates of settling velocity when using the Rouse equation. Estimated settling velocities from both types of instruments increase during resuspension events, even as mean particle size measured by the LISST decreases, while total suspended mass is modulated by bottom shear stress. This indicates a shift in particle population from larger, lower-density flocs to smaller, higher-density particles.

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