Geyer
W. Rockwell
Geyer
W. Rockwell
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ArticleTidal and spring-neap variations in horizontal dispersion in a partially mixed estuary(American Geophysical Union, 2008-07-18) Geyer, W. Rockwell ; Chant, Robert J. ; Houghton, R.A sequence of dye releases in the Hudson River estuary provide a quantitative assessment of horizontal dispersion in a partially mixed estuary. Dye was released in the bottom boundary layer on 4 separate occasions, with varying tidal phase and spring-neap conditions. The three-dimensional distribution of dye was monitored by two vessels with in situ, profiling fluorometers. The three-dimensional spreading of the dye was estimated by calculating the time derivative of the second moment of the dye in the along-estuary, cross-estuary and vertical directions. The average along-estuary dispersion rate was about 100 m2/s, but maximum rates up to 700 m2/s occurred during ebb tides, and minimum rates occurred during flood. Vertical shear dispersion was the principal mechanism during neap tides, but transverse shear dispersion became more important during springs. Suppression of mixing across the pycnocline limited the vertical extent of the patch in all but the maximum spring-tide conditions, with vertical diffusivities in the pycnocline estimated at 4 × 10−5 m2/s during neaps. The limited vertical extent of the dye patch limited the dispersion of the dye relative to the overall estuarine dispersion rate, which was an order of magnitude greater than that of the dye. This study indicates that the effective dispersion of waterborne material in an estuary depends sensitively on its vertical distribution as well as the phase of the spring-neap cycle.
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ArticleAsymmetric tidal mixing due to the horizontal density gradient(American Meteorological Society, 2008-02) Li, Ming ; Trowbridge, John H. ; Geyer, W. RockwellStratification and turbulent mixing exhibit a flood–ebb tidal asymmetry in estuaries and continental shelf regions affected by horizontal density gradients. The authors use a large-eddy simulation (LES) model to investigate the penetration of a tidally driven bottom boundary layer into stratified water in the presence of a horizontal density gradient. Turbulence in the bottom boundary layer is driven by bottom stress during flood tides, with low-gradient (Ri) and flux (Rf) Richardson numbers, but by localized shear during ebb tides, with Ri = ¼ and Rf = 0.2 in the upper half of the boundary layer. If the water column is unstratified initially, the LES model reproduces periodic stratification associated with tidal straining. The model results show that the energetics criterion based on the competition between tidal straining and tidal stirring provides a good prediction for the onset of periodic stratification, but the tidally averaged horizontal Richardson number Rix has a threshold value of about 0.2, which is lower than the 3 suggested in a recent study. Although the tidal straining leads to negative buoyancy flux on flood tides, the authors find that for typical values of the horizontal density gradient and tidal currents in estuaries and shelf regions, buoyancy production is much smaller than shear production in generating turbulent kinetic energy.
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ArticleEstuarine boundary layer mixing processes : insights from dye experiments(American Meteorological Society, 2007-07) Chant, Robert J. ; Geyer, W. Rockwell ; Houghton, Robert ; Hunter, Elias J. ; Lerczak, James A.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.