Interaction of lateral baroclinic forcing and turbulence in an estuary (original) (raw)
Related papers
The impact of tidal straining on the cycle of turbulence in a partially stratified estuary
Continental Shelf Research, 2005
Fast sampling ADCPs (Acoustic Doppler Current Profilers) have been used to observe the cycle of turbulent properties in a partially stratified estuary over a springs-neaps cycle in parallel with measurements of water column T=S from an anchored vessel. Using the variance method, estimates of the Reynolds stress t; the rate of turbulent kinetic energy production P and the eddy viscosity N z are derived from measurements of the along beam velocities with a vertical resolution of 0.5 m. Combined with vertical profile measurements of temperature and salinity, the results reveal the important influence of tidal straining in controlling the structure and intensity of turbulent stresses and TKE production on both ebb and flood phases of the tide. Over the M 2 cycle at neaps, t and P both showed a significant response to water column structure with higher values restricted to the lower part of the water column at times when the water column was stratified by tidal straining. Conversely at the end of the flood, when stratification was minimal due to the de-stabilising effect of straining in supplementing tidal mixing, significant stresses and relatively high production levels extended upwards to near the surface. At springs, strain-induced stratification is relatively weaker but the influence of straining is still apparent in the development of some stability at the end of the ebb and in relatively high values of N z on the flood due to the action of tidal straining in inducing a negative buoyancy flux. This asymmetry between mixing conditions on the ebb and the flood is apparent in plots of N z versus the local gradient Richardson number Ri: N z generally increases rapidly for Rio1 but there is considerable scatter and only limited conformity to functional relations like that of Munk and Anderson.
The Role of Advection, Straining, and Mixing on the Tidal Variability of Estuarine Stratification
Journal of Physical Oceanography, 2012
Data from the Hudson River estuary demonstrate that the tidal variations in vertical salinity stratification are not consistent with the patterns associated with along-channel tidal straining. These observations result from three additional processes not accounted for in the traditional tidal straining model: 1) along-channel and 2) lateral advection of horizontal gradients in the vertical salinity gradient and 3) tidal asymmetries in the strength of vertical mixing. As a result, cross-sectionally averaged values of the vertical salinity gradient are shown to increase during the flood tide and decrease during the ebb. Only over a limited portion of the cross section does the observed stratification increase during the ebb and decrease during the flood. These observations highlight the three-dimensional nature of estuarine flows and demonstrate that lateral circulation provides an alternate mechanism that allows for the exchange of materials between surface and bottom waters, even wh...
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...
Observations of Turbulence in a Partially Stratified Estuary
Journal of Physical Oceanography, 1999
The authors present a field study of estuarine turbulence in which profiles of Reynolds stresses were directly measured using an ADCP throughout a 25-h tidal day. The dataset that is discussed quantifies turbulent mixing for a water column in northern San Francisco Bay that experiences a sequence of states that includes a weak ebb and flood that are stratified, followed
Turbulent mixing in a strongly forced salt wedge estuary
2010
1] Turbulent mixing of salt is examined in a shallow salt wedge estuary with strong fluvial and tidal forcing. A numerical model of the Merrimack River estuary is used to quantify turbulent stress, shear production, and buoyancy flux. Little mixing occurs during flood tides despite strong velocities because bottom boundary layer turbulence is dislocated from stratification elevated in the water column. During ebbs, bottom salinity fronts form at a series of bathymetric transitions. At the fronts, near-bottom velocity and shear stress are low, but shear, stress, and buoyancy flux are elevated at the pycnocline. Internal shear layers provide the dominant source of mixing during the early ebb. Later in the ebb, the pycnocline broadens and moves down such that boundary layer turbulence dominates mixing. Mixing occurs primarily during ebbs, with internal shear mixing accounting for about 50% of the total buoyancy flux. Both the relative contribution of internal shear mixing and the mixing efficiency increase with discharge, with bulk mixing efficiencies between 0.02 and 0.07. Buoyancy fluxes in the estuary increase with discharge up to about 400 m 3 s −1 above which a majority of the mixing occurs offshore. Observed buoyancy fluxes were more consistent with the k-" turbulence closure than the Mellor-Yamada closure, and more total mixing occurred in the estuary with k-". Calculated buoyancy fluxes were sensitive to horizontal grid resolution, as a lower resolution grid yielded less integrated buoyancy flux in the estuary and exported lower salinity water but likely had greater numerical mixing.
How winds and river discharge affect circulation in a mesotidal estuary, San Francisco Bay, USA
2022
Previous studies suggest importance of wind forcing on salt intrusion length and salt flux in river-dominated microtidal estuaries (with tidal range < 2 m). In this study, we investigate the role of wind forcing on salt intrusion in a mesotidal estuary, San Francisco Bay (SFB), with tidal ranges between 2 m and 4 m, through an open-source model of high transferability, the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM). Meanwhile, we investigate circulation and salinity variation of San Francisco Bay. The model’s performance in hydrodynamics at tidal, spring/neap and seasonal time scales is validated through model-observation comparisons. Through realistically forced and process-oriented experiments, we demonstrate that spring/neap tides can cause fortnightly variations in salinity and currents by modulating vertical mixing and stratification; and seasonal variability of circulation in North Bay is determined by change of river discharge and modified by w...
Bathymetric influences on tidal currents at the entrance to a highly stratified, shallow estuary
Continental Shelf Research, 2013
Bathymetric effects on tidal currents are investigated at Main Pass, which is the primary inlet of Mobile Bay, Alabama. A 12-h ship-mounted ADCP survey, which covered nearly one-half of the diurnal tide during flood conditions, included 24 repetitions. The resulting velocity data demonstrate significant tidal variability in the horizontal and vertical current structure between the ship channel and the shoals. The diurnal tidal flows, the dominant tidal forcing, are 721 (4.8 h) ahead of the water level throughout shallower areas of Main Pass, indicating near-standing wave conditions. Moving across the mouth, a phase lag (5.371 or 20 min) develops with the deep channel tidal currents lagging the shoal region. The vertical tidal structure is also modified across the mouth where near-bottom flows change their direction first in the ship channel, while near-surface flows change their direction first over the western shoal. This may be related to the seaward pressure gradient associated with the relatively large (~1715 m 3 /s) freshwater discharge or the discharge interaction with a nearby opening, Pass-aux-Herons. Current magnitudes over the shoals and in the ship channel vary by as much as 1 m/s. Flows at the east side of Main Pass, close to Mobile Point, behave oppositely to those in the rest of the transect during the survey. This inconsistent flow pattern is caused by an anticyclonic eddy that is triggered by flow separation at Mobile Point.
Journal Of Geophysical Research: Oceans, 2020
The Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) modeling system was used to examine axial wind effects on vertical stratification and sediment transport in a convergent estuary. The model demonstrated that stratification dynamics in the upper estuary (Kelvin number <1; Ke ¼ fB ffiffiffiffiffi ffi g′hs p) are dominated by longitudinal wind straining, whereas the dominant mechanism governing estuarine stratification in the lower estuary (Kelvin number~1) is lateral wind straining. Barotropic advection contributes to seaward sediment transport and peaks during spring tides, whereas estuarine circulation causes landward sediment transport with a maximum during neap tides. Down-estuary winds impose no obvious effects on longitudinal sediment flux, whereas up-estuary winds contribute to enhanced seaward sediment flux by increasing the tidal oscillatory flux. The model also demonstrates that bottom friction is significantly influenced by vertical stratification over channel regions, which is indirectly affected by axial winds. Plain Language Summary Winds have significant impacts on estuarine density stratification and sediment dynamics, which may affect water quality, siltation of navigation channels, and the overall health of estuarine ecosystems. Despite this great influence, the mechanisms controlling stratification and sediment transport by axial winds in a convergent estuary have not received adequate attention. Here we use a coupled ocean model system to examine axial wind effects on vertical stratification and sediment transport in a convergent estuary. The ocean model reproduced the observed water elevation, velocity, salinity, and sediment concentration well and shows that the density stratification in the upper bay is mainly controlled by longitudinal wind straining, whereas that in the lower bay is dominated by lateral wind straining. Up-estuary wind enhances seaward sediment transport by increasing the tidal oscillatory flux, whereas down-estuary wind has a limited effect. Through this study we advanced our understanding on stratification and sediment transport in a convergent estuary and noted that winds have great influence on tidal oscillatory sediment transport, which may change the sedimentary budget in an estuary.
Geo-Marine Letters, 2020
The vertical density gradients of salinity and suspended sediment concentration (SSC) cause stratification in estuaries, which play a vital role in the turbulence structure, water mixing, and sediment transport. To investigate the effect of stratification, especially SSC-induced stratification, on maintaining the estuarine turbidity maximum (ETM), we conducted in situ measurements on sediment dynamics at the upper and central ETM sites in the South Passage of Changjiang Estuary in July 2018. The gradient Richardson number was estimated as a proxy for the stratification that is attributable to salinity or/and SSC. We found that salinity-induced stratification was observed mainly on the surface and in the middle layers, whereas SSC-induced stratification occurred mainly in the near-bottom layers. Furthermore, at the central ETM, the baroclinic effect was enhanced during the neap tide when the salinity-induced stratification was stronger than that during the spring tide. In the early phase of floods with minimum velocity during the neap tide, salinity-induced stratification suppressed the turbulence and vertical diffusion of sediments. Moreover, the flocculation enhanced the settling process within the water column. Consequently, high concentrations of fine-grained sediments formed near the bottom and promoted SSC-induced stratification, thereby leading to the continuous accumulation and trapping of sediments. In conclusion, the interactions among the "salinityand SSC-induced stratification" processes served as crucial constraints of the temporal and spatial variations of the ETM in the Changjiang Estuary.
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.