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Woods Hole Coastal and Marine Science Center > Coastal Model Applications and Field Measurements > Research Activities > Bottom-Stress Measurements on the Martha’s Vineyard Inner Shelf

Coastal Model Applications and Field Measurements

Bottom-Stress Measurements on the Martha’s Vineyard Inner Shelf

Bottom stress plays a critical role in shaping both mean flow and large-scale horizontal eddies in coastal circulation, is the dominant parameter controlling sediment transport, and is an important determinant for benthic habitat. Bottom stress depends on flow velocity and how the associated momentum is transferred to the sea floor, a turbulence process that is often characterized in terms of hydrodynamic bottom roughness. Long-standing models postulate that bottom stress in the coastal ocean depends on currents, waves, sediment type, and bottom geometry, and is affected by stratification of heat, salinity, and suspended sediment. When wave-orbital motions are present, the hydrodynamic roughness is enhanced by wave-current interactions in the thin (a few cm thick) wave boundary layer. Measurements of bottom stress are important for determining the hydrodynamic roughness and evaluating these models, but stress measurements are complicated by wave motions. One approach that seems to provide a robust solution to the problem is the direct covariance method proposed by our colleague John Trowbridge at WHOI (Trowbridge, 1998), which builds on the argument that momentum-carrying turbulent eddies scale as the distance from the sea floor The differences between perfect velocity measurements by multiple instruments exposed to the same flow regime, but separated by several turbulence length scales, can be attributed to turbulence. The method can provide accurate estimates of mean Reynolds stress, even with imperfect measurements. Our goal was to design a platform that implements this approach and can be easily relocated, in order to study the spatial variability of bottom stress resulting from bottom types and forcing conditions.

Photo of instruments from ship

Deployment of an instrumented quadrapod off Martha’s Vineyard, November 2014.

           Drawing of instruments on sled

Illustration of the the NIMBBLE (New instrument for making bottom boundary layer evaluations). The NIMBBLE is a low-profile platform with two acoustic Doppler velocimeters and an upward-looking acoustic Doppler profilers.

We have designed the New Instrument for Making Bottom Boundary Layer Evaluations (NIMBBLE) for measuring bottom stress in the presence of waves and currents (See instruments section) The NIMBBLEs were initially deployed for a five weeks on the inner shelf south of Martha’s Vineyard beginning July 1, 2014. This region has sorted grain-size features that trend offshore in alternating bands of fine sand and coarse sand, providing contrasting bottom types. The fine sand usually has small (height ~1 cm, wavelength ~10 cm) sub-orbital wave ripples, whereas the coarse sand has larger (height ~10 cm, wavelength 60 – 80 cm) orbital ripples. In the first experiment, two NIMBBLEs were placed on a band of coarse sand at different depths (10 m and 14 m). In the second 45-day experiment, the 14-m site was reoccupied, and the second NIMBBLE was located about 800 meters east, at the same depth, but on coarse sand. Two large quadpods (see figure) with a complete array of bottom-boundary layer and sediment-transport instrumentation were deployed for the entire period to provide comparative information on time-varying processes at fixed locations. One was located offshore (20-m depth) on fine sand, and the second 1200 to 2000 m east of the NIMBBLEs on coarse sand. We are processing the time series from the ADVs to estimate bottom stress using the Trowbridge method and friction velocity and bottom roughness from logarithmic profiles. Preliminary analyses of the data show periods of elevated turbulence that may represent enhanced roughness caused by wave-current interaction over the large ripples, but further analyses are required to ensure that there were no artifacts associated with instrument performance or flow over the sleds.

 

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