Vertical Structure of Dissipation in the Nearshore

Abstract The vertical structure of the dissipation of turbulence kinetic energy was observed in the nearshore region (3.2-m mean water depth) with a tripod of three acoustic Doppler current meters off a sandy ocean beach. Surface and bottom boundary layer dissipation scaling concepts overlap in this region. No depth-limited wave breaking occurred at the tripod, but wind-induced whitecapping wave breaking did occur. Dissipation is maximum near the surface and minimum at middepth, with a secondary maximum near the bed. The observed dissipation does not follow a surfzone scaling, nor does it follow a “log layer” surface or bottom boundary layer scaling. At the upper two current meters, dissipation follows a modified deep-water breaking-wave scaling. Vertical shear in the mean currents is negligible and shear production magnitude is much less than dissipation, implying that the vertical diffusion of turbulence is important. The increased near-bed secondary dissipation maximum results from a decrease in the turbulent length scale.

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Estuarine Boundary Layer Mixing Processes: Insights from Dye Experiments*

Journal of Physical Oceanography ◽

10.1175/jpo3088.1 ◽

2007 ◽

Vol 37 (7) ◽

pp. 1859-1877 ◽

Cited By ~ 34

Author(s):

Robert J. Chant ◽

Wayne R. Geyer ◽

Robert Houghton ◽

Elias Hunter ◽

James Lerczak

Keyword(s):

Boundary Layer ◽

Richardson Number ◽

Flood Tide ◽

Buoyancy Flux ◽

Bottom Boundary Layer ◽

Buoyancy Frequency ◽

Entrainment Rate ◽

Diapycnal Mixing ◽

Bottom Boundary ◽

Shear Production

Abstract A series of dye releases in the Hudson River estuary elucidated diapycnal mixing rates and temporal variability over tidal and fortnightly time scales. Dye was injected in the bottom boundary layer for each of four releases during different phases of the tide and of the spring–neap cycle. Diapycnal mixing occurs primarily through entrainment that is driven by shear production in the bottom boundary layer. On flood the dye extended vertically through the bottom mixed layer, and its concentration decreased abruptly near the base of the pycnocline, usually at a height corresponding to a velocity maximum. Boundary layer growth is consistent with a one-dimensional, stress-driven entrainment model. A model was developed for the vertical structure of the vertical eddy viscosity in the flood tide boundary layer that is proportional to u2*/N∞, where u* and N∞ are the bottom friction velocity and buoyancy frequency above the boundary layer. The model also predicts that the buoyancy flux averaged over the bottom boundary layer is equal to 0.06N∞u2* or, based on the structure of the boundary layer equal to 0.1NBLu2*, where NBL is the buoyancy frequency across the flood-tide boundary layer. Estimates of shear production and buoyancy flux indicate that the flux Richardson number in the flood-tide boundary layer is 0.1–0.18, consistent with the model indicating that the flux Richardson number is between 0.1 and 0.14. During ebb, the boundary layer was more stratified, and its vertical extent was not as sharply delineated as in the flood. During neap tide the rate of mixing during ebb was significantly weaker than on flood, owing to reduced bottom stress and stabilization by stratification. As tidal amplitude increased ebb mixing increased and more closely resembled the boundary layer entrainment process observed during the flood. Tidal straining modestly increased the entrainment rate during the flood, and it restratified the boundary layer and inhibited mixing during the ebb.

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A numerical investigation into the bottom boundary layer flow and vertical structure of internal waves on a continental slope

Continental Shelf Research ◽

10.1016/s0278-4343(97)00067-8 ◽

1998 ◽

Vol 18 (1) ◽

pp. 31-65 ◽

Cited By ~ 15

Author(s):

Peter E. Holloway ◽

Belinda Barnes

Keyword(s):

Boundary Layer ◽

Numerical Investigation ◽

Continental Slope ◽

Internal Waves ◽

Boundary Layer Flow ◽

Vertical Structure ◽

Bottom Boundary Layer ◽

Bottom Boundary ◽

Layer Flow

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Acceleration of a Stratified Current over a Sloping Bottom, Driven by an Alongshelf Pressure Gradient*

Journal of Physical Oceanography ◽

10.1175/jpo2744.1 ◽

2005 ◽

Vol 35 (8) ◽

pp. 1305-1317 ◽

Cited By ~ 14

Author(s):

David C. Chapman ◽

Steven J. Lentz

Keyword(s):

Boundary Layer ◽

Pressure Gradient ◽

Vertical Shear ◽

Bottom Boundary Layer ◽

Buoyancy Frequency ◽

Coriolis Parameter ◽

Model Calculations ◽

Bottom Boundary ◽

Bottom Stress ◽

Sloping Bottom

Abstract An idealized theoretical model is developed for the acceleration of a two-dimensional, stratified current over a uniformly sloping bottom, driven by an imposed alongshelf pressure gradient and taking into account the effects of buoyancy advection in the bottom boundary layer. Both downwelling and upwelling pressure gradients are considered. For a specified pressure gradient, the model response depends primarily on the Burger number S = Nα/f, where N is the initial buoyancy frequency, α is the bottom slope, and f is the Coriolis parameter. Without stratification (S = 0), buoyancy advection is absent, and the alongshelf flow accelerates until bottom stress balances the imposed pressure gradient. The e-folding time scale to reach this steady state is the friction time, h/r, where h is the water depth and r is a linear bottom friction coefficient. With stratification (S ≠ 0), buoyancy advection in the bottom boundary layer produces vertical shear, which prevents the bottom stress from becoming large enough to balance the imposed pressure gradient for many friction time scales. Thus, the alongshelf flow continues to accelerate, potentially producing large velocities. The acceleration increases rapidly with increasing S, such that even relatively weak stratification (S > 0.2) has a major impact. These results are supported by numerical model calculations.

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