Observations of Turbulence in a Partially Stratified Estuary (original) (raw)
2005
Detailed field observations from the York River estuary, Virginia are used to examine the processes governing vertical density stratification and to evaluate the importance of spatial and temporal variations in turbulent mixing on estuarine dynamics and sediment transport. Contrary to previous findings that suggest wind stress acts predominantly as a source of energy to mix away stratification, this study demonstrate that the wind can play a more important role in “straining” the along-channel estuarine density gradient. As a result, down-estuary winds enhance the tidally-averaged vertical shear, which interacts with the along-channel density gradient to increase stratification. Conversely, up-estuary winds tend to reduce, or even reverse the vertical shear, reducing stratification. While wind straining can play a dominant role in governing the overall degree of turbulent mixing at sub-tidal time scales, tidal straining of the along-channel density gradient can result in asymmetries...
Quantifying vertical mixing in estuaries
Environmental Fluid Mechanics, 2008
Estuarine turbulence is notable in that both the dissipation rate and the buoyancy frequency extend to much higher values than in other natural environments. The high dissipation rates lead to a distinct inertial subrange in the velocity and scalar spectra, which can be exploited for quantifying the turbulence quantities. However, high buoyancy frequencies lead to small Ozmidov scales, which require high sampling rates and small spatial aperture to resolve the turbulent fluxes. A set of observations in a highly stratified estuary demonstrate the effectiveness of a vessel-mounted turbulence array for resolving turbulent processes, and for relating the turbulence to the forcing by the Reynolds-averaged flow. The observations focus on the ebb, when most of the buoyancy flux occurs. Three stages of mixing are observed:
Journal of Geophysical Research: Oceans, 2019
Time series of in situ measured velocity and suspended sediment concentration from Qiantang Estuary (China), and estimates of turbulence and sediment stratification parameters are presented. The data span a period of 9 days, and after phase averaged, they are used to explore spring‐neap tidal variations in flow, turbulence, and sediment stratification. A local balance between shear production, sediment‐induced buoyancy flux, and dissipation is found to hold during ebb for both neap and spring tides. During flood elevated turbulence dissipation rates are observed, attributed to nonlocal turbulence, most likely due to horizontal advection. Our results show that the effect of sediment stratification is successfully parameterized by adding the Monin‐Obukhov length scale to the classical logarithmic layer theory. Flood‐ebb asymmetry in both Rig and Rf is observed, with higher values attained during flood due to the higher sediment concentrations and the corresponding weaker velocity shea...
Modeling and understanding turbulent mixing in a macrotidal salt wedge estuary
Journal of Geophysical Research, 2011
1] A high-resolution three-dimensional numerical simulation is performed with the parallel, unstructured grid SUNTANS model to study the spatiotemporal dynamics of turbulent mixing in a shallow, macrotidal salt wedge estuary that experiences periodic mixing and strong stratification. Unresolved vertical mixing is parameterized with the k − kl closure scheme with the Canuto-A stability functions based on a careful comparison of multiple two-equation closure schemes and stability functions via the generic length scale approach. The predictions of velocity, salinity, Richardson number, and Reynolds stress are in good agreement with field observations, and the top and bottom salinity predictions achieve skill scores of 0.86 and 0.91, respectively. The model shows that the salt wedge starts to strengthen upstream at the beginning of weak ebb and gradually expands downstream during the weak tide. Mixing is most active along a density interface during the weak ebb, while it is most active in a bottom mixed layer during weak flood, consistent with the findings inferred from the observations. Stratification decays during the strong ebb in a mixing event along the horizontal extent of the salt wedge while it is also being advected offshore. Local mixing is shown to account for roughly half of the decay rate of the stratification in this process. Numerical experiments are performed to investigate the response of stratification and mixing to changes in the magnitude of the buoyancy. High sensitivity is shown under intermediate levels of stratification that occur in the real system, which becomes considerably weaker under more extreme conditions.
Estuaries, 2005
Observations from the York River Estuary, Virginia, demonstrate that the along-channel wind plays a dominant role in governing the estuarine exchange flow and the corresponding increase or decrease in vertical density stratification. Contrary to previous findings that suggest wind stress acts predominantly as a source of energy to mix away estuarine stratification, our results demonstrate that the wind can play a more important role in straining the alongchannel estuarine density gradient. Down-estuary winds enhance the tidally-averaged vertical shear, which interacts with the along-channel density gradient to increase vertical stratification. Up-estuary winds tend to reduce, or even reverse the vertical shear, reducing vertical stratification. In two experiments each lasting approximately a month, the estuarine exchange flow was highly correlated with the along-channel component of the wind. The changes in stratification caused by the exchange flow appear to control the amount of vertical mixing as parameterized by the vertical eddy viscosity. The degree of stratification induced by wind straining also appears to play an important role in controlling the effectiveness of wind and tidal mixing.
Near-bottom shear stresses in a small, highly stratified estuary
Journal of Geophysical Research: Oceans, 2005
The shear stresses obtained from acoustic Doppler current profilers (ADCPs) operating in the recently developed mode 11 at a high spatial resolution are examined in detail. Measurement noise of the ADCP is estimated using autocorrelation analyses and is found to be small but correlated with velocity. Measurement noise in estimates of Reynolds stress is found to be somewhat smaller than that obtained in earlier studies for the ADCP operating in mode 4. Internal waves may have produced observed correlated oscillatory motions. The effect of these motions on Reynolds stress estimates is quantified and shown to be at least an order of magnitude less than the total measured stress and probably much less. The high-spatialresolution profiles from the ADCP operating in mode 11 allow excellent measurements of near-bed vertical velocity gradients. Log profile estimates of stress are poorly related to Reynolds estimates, even when stability measures based on near-bed density gradients are taken into account. We show that upper layer pycnoclines likely inhibit turbulence and bias log profile estimates even at a half meter above the bed, where the water column is well mixed.
Internal hydraulics and mixing in a highly stratified estuary
Journal of Geophysical Research: Oceans, 2000
Shipboard acoustic Doppler current profiler and conductivity-temperaturedepth data obtained during highly stratified conditions in the Hudson River estuary along a section of variable width and breadth are presented. The observations emphasize tidal period asymmetries in the vertical structure of current and salinity. However, these asymmetries exhibit significant along-channel structure which is determined by channel morphology. During the ebb the flow is linearly sheared, and steep halocline slopes in the vicinity of channel contractions are maintained by momentum advection. A minimum in vertical shear across the pycnocline occurs in channel contractions. During food the pycnocline sharpens and flattens with a middepth velocity maximum embedded in the pycnocline which separates a stratified surface layer from a bottom mixed layer. The along-channel structure in vertical shear is consistent with a lateral vorticity equation. Estimates of Richardson numbers suggest that vertical mixing across the pycnocline is enhanced downstream of channel contractions.
Shear and turbulence production across subtidal channels
Journal of Marine Research, 2006
In intertidal regions with subtidal channels, effects of bathymetry on overlying flow vary greatly with tidal stage. Around low water when mudflats and marsh are exposed, flow is constrained to channels, but when water depths are greater, tidal forcing may not necessarily be aligned with meandering channel axes. Flow across the channel can generate strong shear and turbulence at the elevation of the channel banks and can significantly increase turbulent energy in the middle of the water column. Field observations in a mudflat channel of San Francisco Bay indicate that cross-channel shear regularly occurs there early in ebb tides. With increased freshwater flow, baroclinic forcing can enhance shear by decoupling flow between dense water flooding in the channel and fresher water ebbing above the channel banks. A water column numerical model with k-ε turbulence closure is modified to represent the cross-channel shear production. Numerical results with uniform density indicate that turbulence production increases with the angle between the barotropic tidal forcing and the channel axis. When a longitudinal salinity gradient is imposed, cross-channel shear production contributes to breakdown of periodic stratification. Turbulence produced at the channel banks locally exceeds dissipation, and the excess energy is either lost to buoyancy or diffuses vertically to lower energy regions near the surface and near the bed. The balance among shear production, buoyancy production, and diffusion of turbulence depends on the flow angle and the strength of the longitudinal salinity gradient.
Estuarine, Coastal and Shelf Science, 2004
The overall goal of this study was to strengthen understanding of the hydrographic structure in shallow estuaries as influenced by seasonal and depth-dependent variability, and by variability from extreme meteorological events. The mesohaline Neuse Estuary, North Carolina, U.S.A., which was the focus, receives surface inputs from upriver and tributary freshwater sources and bottom inputs from downriver high-salinity sound water sources, resulting in varying degrees of stratification. To assess depth-dependent, estuary-wide changes in salinity, a multiple time series was created using data from four discrete depths (surface and 1, 2, and 3 m G 0:25 m). The database was developed from weekly to biweekly sampling of the entire water column, and included sidechannel as well as mid-channel data. We characterized seasonal differences in halocline depth affecting the hydrographic structure of the mesohaline estuary and site-specific variation in nutrient concentrations, based on a comprehensive eight-year physical/chemical database. The first two years of the record showed an expected seasonal signal and included events that impacted the surface layer from freshwater inputs. Remaining years had greater variability over seasons and depths, with freshening events that affected all depths. Halocline depth was compared at specific locations, and a ''snapshot'' view was provided of the relative depth of these water masses within the estuary by season. We also examined flow patterns at the same cross-estuary sites over a three-year period, using a boat-mounted acoustic Doppler current profiler (ADCP) with bottom-tracking capability. Composite visualizations constructed with single-transect ADCP data revealed a classical estuarine circulation pattern of outflow at the surface/southern shore and inflow at the bottom/northern shore. Although this pattern deviated under extreme climatological events and was sometimes variable, the estuary generally exhibited a high probability of direction of flow. Wind fields, hurricanes, and small-scale, high-precipitation events represented significant forcing variables.