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Virginia Institute of Marine Science

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[1] A benthic boundary layer tripod supporting six current meters and three profiling acoustic backscatter sensors (ABS) documented storm and swell conditions during the fall of 1996 at a depth of 13 m on the inner shelf off Duck, North Carolina. Sediment concentration was higher in the wave boundary layer (WBL) during storm conditions but higher similar to40 cm above the bed (cm ab) during swell conditions. To test the applicability of a diffusive balance during storm versus swell, ABS data were used to invert the vertical diffusion equation and solve for eddy diffusivity from 1 to 50 cm ab. During the storm period, diffusivity derived from the ABS up to similar to40 cm ab agreed well with viscosity derived above the WBL from observed current profiles and from the Grant-Madsen-Glenn (GMG) model. During the swell period, diffusivity derived from the ABS up to similar to40 cm ab did not agree with observed mean current shear above this level nor with the GMG model. Diffusivity did agree with viscosity derived from shear stress due to waves within the WBL extrapolated to a height greater than the modeled WBL. We speculate that during swell conditions, shedding vortices enhanced mass and momentum exchange, extending the eddy viscosity associated with waves above the predicted WBL; during storm conditions, strong currents prevented vortices from penetrating beyond the predicted WBL. Rouse diffusion models with two- and three-layered eddy diffusivity and combined diffusion-advection models with one and three-layer were applied to the observational data set. During the storm the two- and three-layered Rouse models including multiple grain sizes and bed armoring reproduced the observed concentration well. During swell (weak current conditions) all the models considered underpredicted the observed concentration if applied with a standard WBL thickness. To correct this, enhanced vertical exchange was represented by a thickened WBL whenever mean currents were weak relative to the estimated jet velocity associated with wave-induced vortex shedding. The two- layer Rouse model then reproduced the concentrations observed during swell remarkably well. This implies that mean sediment suspension dominated by wave-induced advection may still be approximated by a diffusion-like process under some circumstances.




sediment; suspension; diffusion; advection; turbulence; Duck; North Carolina