Salt Flux in a Partially Mixed Estuary with Varying River Discharge over a Spring-Neap Tide Cycle

2011 ◽  
Vol 137 ◽  
pp. 237-242 ◽  
Author(s):  
Li Rong Yuan ◽  
Rong Li Chen ◽  
Cheng Lu
Keyword(s):  
River Discharge ◽  
Neap Tide ◽  
Measurement Data ◽  
Salt Flux ◽  
Vertical Shear ◽  
Exchange Flow ◽  
Modaomen Estuary ◽  
Shear Dispersion ◽  
Order Of Magnitude ◽  
Estuarine Exchange

The mechanisms driving the salt flux in the Modaomen estuary were investigated using measurements from along-channel shipboard surveys over a spring-neap tide cycle. During the investigation, river discharge was controlled to increase sharply and then decrease gently. The measurement data were analyzed with composition method. The results show that exchange flow varied significantly over the spring–neap cycle, being weakest during middle tides and strongest during neap tides. It could be concluded that sub-tidal vertical shear dispersion, resulting from the estuarine exchange flow, is the dominant mechanism driving the salt flux since the salt flux resulting from vertical shear dispersion varies in amplitude by over an order of magnitude, which is close to two times as large as the variation of advective salt flux.

scholarly journals A Numerical Study of Salt Fluxes in Delaware Bay Estuary

2013 ◽  
Vol 43 (8) ◽  
pp. 1572-1588 ◽  
Author(s):  
María Aristizábal ◽  
Robert Chant
Keyword(s):  
River Discharge ◽  
Neap Tide ◽  
Numerical Study ◽  
Spring Tide ◽  
Delaware Bay ◽  
Salt Flux ◽  
Exchange Flow ◽  
Regional Ocean Modeling System ◽  
Salinity Field ◽  
Estuarine Exchange

Abstract The results of a numerical study of Delaware Bay using the Regional Ocean Modeling System (ROMS) are presented. The simulations are run over a range of steady river inputs and used M2 and S2 tidal components to capture the spring–neap variability. Results provide a description of the spatial and temporal structure of the estuarine exchange flow and the salinity field, as well the along-channel salt flux in the estuary. The along-channel salt flux is decomposed into an advective term associated with the river flow, a steady shear dispersion Fe associated with the estuarine exchange flow, and a tidal oscillatory salt flux Ft. Time series of Fe and Ft show that both are larger during neap tide than during spring. This time variability of Ft, which is contrary to existing scalings, is caused by the lateral flows that bring velocity and salinity out of quadrature and the stronger stratification during neap tide, which causes Ft to be enhanced relative to spring tide. A fit for the salt intrusion length L with river discharge Q for a number of isohalines is performed. The functional dependences of L with Q are significantly weaker than Q−1/3 scaling. It is concluded that the response of the salt field with river discharge is due to the dependence of Fe and Ft with Q and the relative importance of Ft to the total upstream salt flux: as river discharge increases, Fe becomes the dominant mechanism. Once Fe dominates, the salt field stiffens because of a reduction of the vertical eddy viscosity with increasing Q.

scholarly journals Mechanisms Driving the Time-Dependent Salt Flux in a Partially Stratified Estuary*

2006 ◽  
Vol 36 (12) ◽  
pp. 2296-2311 ◽  
Author(s):  
James A. Lerczak ◽  
W. Rockwell Geyer ◽  
Robert J. Chant
Keyword(s):  
Time Scale ◽  
Salt Content ◽  
River Estuary ◽  
Rate Of Change ◽  
Steady Shear ◽  
Longitudinal Dispersion ◽  
Salt Flux ◽  
Vertical Shear ◽  
Shear Dispersion ◽  
Order Of Magnitude

Abstract The subtidal salt balance and the mechanisms driving the downgradient salt flux in the Hudson River estuary are investigated using measurements from a cross-channel mooring array of current meters, temperature and conductivity sensors, and cross-channel and along-estuary shipboard surveys obtained during the spring of 2002. Steady (subtidal) vertical shear dispersion, resulting from the estuarine exchange flow, was the dominant mechanism driving the downgradient salt flux, and varied by over an order of magnitude over the spring–neap cycle, with maximum values during neap tides and minimum values during spring tides. Corresponding longitudinal dispersion rates were as big as 2500 m2 s−1 during neap tides. The salinity intrusion was not in a steady balance during the study period. During spring tides, the oceanward advective salt flux resulting from the net outflow balanced the time rate of change of salt content landward of the study site, and salt was flushed out of the estuary. During neap tides, the landward steady shear dispersion salt flux exceeded the oceanward advective salt flux, and salt entered the estuary. Factor-of-4 variations in the salt content occurred at the spring–neap time scale and at the time scale of variations in the net outflow. On average, the salt flux resulting from tidal correlations between currents and salinity (tidal oscillatory salt flux) was an order of magnitude smaller than that resulting from steady shear dispersion. During neap tides, this flux was minimal (or slightly countergradient) and was due to correlations between tidal currents and vertical excursions of the halocline. During spring tides, the tidal oscillatory salt flux was driven primarily by oscillatory shear dispersion, with an associated longitudinal dispersion rate of about 130 m2 s−1.

scholarly journals Influence of Advective Accelerations on Estuarine Exchange at a Chesapeake Bay Tributary

2012 ◽  
Vol 42 (10) ◽  
pp. 1617-1634 ◽  
Author(s):  
Nuvit B. Basdurak ◽  
Arnoldo Valle-Levinson
Keyword(s):  
Spatial Distribution ◽  
Chesapeake Bay ◽  
Momentum Balance ◽  
Exchange Flow ◽  
Hampton Roads ◽  
Relative Contribution ◽  
Advection Term ◽  
Nonlinear Advection ◽  
Order Of Magnitude ◽  
Estuarine Exchange

Abstract The influence of nonlinear advection on estuarine exchange flow was investigated with observations at the transition between the James River and Chesapeake Bay, Hampton Roads, Virginia. Data were collected under different tidal forcing, wind forcing, and river discharge in 2004 and 2005. The relative contribution of nonlinear advective terms to the along-channel momentum balance had the same order of magnitude as pressure gradient and friction, verifying recent analytical and numerical model results. Both the magnitude and the spatial distribution of nonlinear advection showed fortnightly variability. Nonlinear advection was more influential on along-channel flow at spring tides than at neap tides because of increased tidal velocities, in a cross-sectionally averaged sense. The flow structures induced by each nonlinear advective process were investigated for the first time with observations. The lateral advection term υuy was found to enhance laterally sheared exchange acting along with Coriolis forcing at spring tides and opposing it at neap tides. Vertical advection wuz showed similar spatial distribution as υuy at spring tides but was vertically sheared (landward at middepth and seaward in the rest of the water column) at neaps. Longitudinal advection uux augmented landward flow in the channel.

scholarly journals Estuarine Exchange Flow Variability in a Seasonal, Segmented Estuary

2020 ◽  
Vol 50 (3) ◽  
pp. 595-613 ◽  
Author(s):  
Ted Conroy ◽  
David A. Sutherland ◽  
David K. Ralston
Keyword(s):  
Salinity Gradient ◽  
Low Flow ◽  
Exchange Flow ◽  
Strongly Correlated ◽  
Wet Season ◽  
Event Time ◽  
Shear Dispersion ◽  
Salinity Variability ◽  
Tidal Velocity ◽  
Estuarine Exchange

AbstractSmall estuaries in Mediterranean climates display pronounced salinity variability at seasonal and event time scales. Here, we use a hydrodynamic model of the Coos Estuary, Oregon, to examine the seasonal variability of the salinity dynamics and estuarine exchange flow. The exchange flow is primarily driven by tidal processes, varying with the spring–neap cycle rather than discharge or the salinity gradient. The salinity distribution is rarely in equilibrium with discharge conditions because during the wet season the response time scale is longer than discharge events, while during low flow it is longer than the entire dry season. Consequently, the salt field is rarely fully adjusted to the forcing and common power-law relations between the salinity intrusion and discharge do not apply. Further complicating the salinity dynamics is the estuarine geometry that consists of multiple branching channel segments with distinct freshwater sources. These channel segments act as subestuaries that import both higher- and lower-salinity water and export intermediate salinities. Throughout the estuary, tidal dispersion scales with tidal velocity squared, and likely includes jet–sink flow at the mouth, lateral shear dispersion, and tidal trapping in branching channel segments inside the estuary. While the estuarine inflow is strongly correlated with tidal amplitude, the outflow, stratification, and total mixing in the estuary are dependent on the seasonal variation in river discharge, which is similar to estuaries that are dominated by subtidal exchange flow.

scholarly journals Circulation and salt balance in the São Francisco river Estuary (NE/Brazil)

RBRH ◽  
2017 ◽  
Vol 22 (0) ◽  
Author(s):  
Geórgenes Cavalcante ◽  
Luiz Bruner de Miranda ◽  
Paulo Ricardo Petter Medeiros
Keyword(s):  
River Discharge ◽  
Neap Tide ◽  
Spring Tide ◽  
River Estuary ◽  
Salt Transport ◽  
Salt Flux ◽  
Sao Francisco River ◽  
Tidal Cycles ◽  
São Francisco River ◽  
And Diffusion

ABSTRACT In order to evaluate how river discharge and tidal currents forcing influence the circulation and salt transport within the São Francisco River-Estuary, a two 25 hour surveys campaigns and along-channel profiles were performed in the estuary main channel (18/19 and 25/26 February 2014). The current intensity showed higher asymmetry between flood (v<0) and ebb (v>0) currents in spring tide (-0.6 and 1.1 m/s), as compared to neap tide (-0.4 and 1.0 m/s), which was attributed to the intense buoyancy energy enhanced by the stronger discharge in spring. Overall, salinity presented high stratified conditions and varied between 0.0<S<36.6, and 0.5<S<36.1 from surface to bottom, during spring and neap, respectively. Although river discharge reduced from spring to neap tide partially mixed-highly stratified conditions (type 2b) remained in both periods, with advection and diffusion contributing with 34% and 66% to the net upstream salt flux in spring, respectively, and 56% and 44% in neap tide, respectively, which was attributed to the balance of tidal and baroclinic forcing and river discharge. The dominant downstream advective salt transport in both spring (12.5 kg.m-1s-1) and neap (15.2 kg.m-1s-1) tidal cycles suggests that there is little salt accumulation inside the São Francisco estuary.

scholarly journals Calculating Estuarine Exchange Flow Using Isohaline Coordinates*

2011 ◽  
Vol 41 (6) ◽  
pp. 1116-1124 ◽  
Author(s):  
Parker MacCready
Keyword(s):  
Residence Time ◽  
River Estuary ◽  
Columbia River ◽  
Time Dependent ◽  
Salt Flux ◽  
Exchange Flow ◽  
Exact Formulation ◽  
Volume Flux ◽  
Columbia River Estuary ◽  
Estuarine Exchange

Abstract A method for calculating subtidal estuarine exchange flow using an isohaline framework is described, and the results are compared with those of the more commonly used Eulerian method of salt flux decomposition. Concepts are explored using a realistic numerical simulation of the Columbia River estuary. The isohaline method is found to be advantageous because it intrinsically highlights the salinity classes in which subtidal volume flux occurs. The resulting expressions give rise to an exact formulation of the time-dependent Knudsen relation and may be used in calculation of the saltwater residence time. The volume flux of the landward transport, which can be calculated precisely using the isohaline framework, is of particular importance for problems in which the saltwater residence time is critical.

scholarly journals Subtidal Exchange in Eastern Long Island Sound

2016 ◽  
Vol 46 (8) ◽  
pp. 2351-2371 ◽  
Author(s):  
Michael M. Whitney ◽  
David S. Ullman ◽  
Daniel L. Codiga
Keyword(s):  
River Discharge ◽  
Long Island ◽  
Long Island Sound ◽  
Salt Flux ◽  
Freshwater Flux ◽  
Spatial And Temporal Variability ◽  
Connecticut River ◽  
The North ◽  
Shear Dispersion ◽  
Island Sound

AbstractLong Island Sound (LIS) is a large and wide macrotidal estuary with distributed river inputs, including the Connecticut River (the largest freshwater source) that flows into the eastern LIS near the mouth. In 2010, shipboard surveys of salinity, temperature, and currents were collected along an across-estuary transect in eastern LIS. Numerical model results are compared to these observations and used to study the spatial and temporal variability of salinity, velocity, and freshwater and salt fluxes over a 4-yr period. For all low wind conditions, observations and model results indicate an outward-flowing, low-salinity wedge on the south side with an inward-flowing, higher-salinity area underneath and to the north. Observations and model results during the low wind surveys indicate that stratification substantially decreases with increased tidal amplitude and decreased river discharge; the velocity field is less variable among surveys. Model analysis indicates strong sensitivities to both tides and river discharge; with discharge response strongest for salinity and freshwater flux and tidal response larger for velocities, volume flux, and salt flux. The long-term average net freshwater and salt fluxes are outward and inward, respectively. For both flux types, subtidal shear dispersion is twice tidal oscillatory diffusion, and both contributions are in the same direction as the net flux. The uniform flow contribution is small for freshwater flux, yet it is the largest single term for salt flux and partially counters the inward contributions.

scholarly journals The Tidally Averaged Momentum Balance in a Partially and Periodically Stratified Estuary

2010 ◽  
Vol 40 (11) ◽  
pp. 2418-2434 ◽  
Author(s):  
Mark T. Stacey ◽  
Matthew L. Brennan ◽  
Jon R. Burau ◽  
Stephen G. Monismith
Keyword(s):  
Water Column ◽  
Time Scale ◽  
Eddy Viscosity ◽  
Momentum Equation ◽  
Momentum Balance ◽  
Exchange Flow ◽  
Bed Stress ◽  
Turbulent Momentum ◽  
Stratified Estuary ◽  
Estuarine Exchange

Abstract Observations of turbulent stresses and mean velocities over an entire spring–neap cycle are used to evaluate the dynamics of tidally averaged flows in a partially stratified estuarine channel. In a depth-averaged sense, the net flow in this channel is up estuary due to interaction of tidal forcing with the geometry of the larger basin. The depth-variable tidally averaged flow has the form of an estuarine exchange flow (downstream at the surface, upstream at depth) and varies in response to the neap–spring transition. The weakening of the tidally averaged exchange during the spring tides appears to be a result of decreased stratification on the tidal time scale rather than changes in bed stress. The dynamics of the estuarine exchange flow are defined by a balance between the vertical divergence of the tidally averaged turbulent stress and the tidally averaged pressure gradient in the lower water column. In the upper water column, tidal stresses are important contributors, particularly during the neap tides. The usefulness of an effective eddy viscosity in the tidally averaged momentum equation is explored, and it is seen that the effective eddy viscosity on the subtidal time scale would need to be negative to close the momentum balance. This is due to the dominant contribution of tidally varying turbulent momentum fluxes, which have no specific relation to the subtidal circulation. Using a water column model, the validity of an effective eddy viscosity is explored; for periodically stratified water columns, a negative effective viscosity is required.

Buoyancy effects in vertical shear dispersion

1992 ◽  
Vol 242 ◽  
pp. 371-386 ◽  
Author(s):  
Ronald Smith
Keyword(s):  
Vertical Plane ◽  
Solvable Model ◽  
Solution Procedure ◽  
Vertical Shear ◽  
Shear Dispersion ◽  
Exactly Solvable ◽  
Parallel Shear Flows ◽  
Miscible Fluid ◽  
Explicit Formulae ◽  
Nonlinear Shear

Density gradients modify the flow and hence the shear dispersion of one miscible fluid in another. A solution procedure is given for calculating the effects of weak buoyancy for vertical laminar parallel shear flows. A particular extrapolation to large buoyancy gives an exactly solvable nonlinear diffusion equation. For the particular case of vertical plane Poiseuille flow explicit formulae are derived for the flow, for the nonlinear shear dispersion coefficient and for the onset of instability. The exactly solvable model gives reasonably accurate results for the buoyancy-modified shear dispersion over a range from half to one-and-a-half times the non-buoyant value.

scholarly journals Impact of evaporation and precipitation on estuarine mixing

2021 ◽  
Author(s):  
Marvin Lorenz ◽  
Knut Klingbeil ◽  
Hans Burchard
Keyword(s):  
River Runoff ◽  
Freshwater Flux ◽  
Exchange Flow ◽  
Surface Salinity ◽  
Estuarine Mixing ◽  
Freshwater Transport ◽  
Freshwater Forcing ◽  
Order Of Magnitude ◽  
Inverse Estuary ◽  
The Persian Gulf

AbstractRecent studies could link the quantities of estuarine exchange flows to the volume-integrated mixing inside an estuary, where mixing is defined as the destruction of salinity variance. The existing mixing relations quantify mixing inside an estuary by the net boundary fluxes of volume, salinity, and salinity variance which are quantified as Knudsen or Total Exchange Flow bulk values. So far, river runoff is the only freshwater flux included and the freshwater exchange due to precipitation and evaporation is neglected. Yet, the latter is the driving force of inverse estuaries, which could not be described by the existing relations. To close this gap, this study considers evaporation and precipitation to complete the existing mixing relations by including cross-surface salinity variance transport. This allows decomposing the mixing into a riverine and a surface transport contribution. The improved relations are tested against idealized two-dimensional numerical simulations of different combinations of freshwater forcing. The mixing diagnosed from the model results agrees exactly with the derived mixing relation. An annual hind-cast simulation of the Persian Gulf is then used to test the mixing relations, both exact and approximated, e.g., long-term averaged, for a realistic inverse estuary. The results show that the annual mean mixing contributions of river discharge and evaporation are almost equal, although the freshwater transport due to evaporation is about one order of magnitude larger than the river runoff.

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