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W. Rockwell Geyer

Abstract

A set of field experiments was conducted to determine the water-following characteristics of mixed-layer drifters with “holey-sock” drogues. Through the use of a drifting current meter array, direct estimates of slip velocity (or the difference between the velocity of the drifter and that of the water surrounding the drogue) were obtained with precision of better than ± 1 cm s−1. The range of slip velocities was 1 to 4 cm s−1, with the orientation of the slip principally in the downwind direction. The results are consistent with a simple model for slip induced by a wind-drift current, assuming a static balance of drag forces.

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Tao Wang and W. Rockwell Geyer

Abstract

Salinity variance dissipation is related to exchange flow through the salinity variance balance equation, and meanwhile its magnitude is also proportional to the turbulence production and stratification inside the estuary. As river flow increases, estuarine volume-integrated salinity variance dissipation increases owing to more variance input from the open boundaries driven by exchange flow and river flow. This corresponds to the increased efficient conversion of turbulence production to salinity variance dissipation due to the intensified stratification with higher river flow. Through the spring–neap cycle, the temporal variation of salinity variance dissipation is more dependent on stratification than turbulence production, so it reaches its maximum during the transition from neap to spring tides. During most of the transition time from spring to neap tides, the advective input of salinity variance from the open boundaries is larger than dissipation, resulting in the net increase of variance, which is mainly expressed as vertical variance, that is, stratification. The intensified stratification in turn increases salinity variance dissipation. During neap tides, a large amount of enhanced salinity variance dissipation is induced by the internal shear stress near the halocline. During most of the transition time from neap to spring tides, dissipation becomes larger than the advective input, so salinity variance decreases and the stratification is destroyed.

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Robert D. Hetland and W. Rockwell Geyer

Abstract

Classic models of estuarine circulation are reexamined using a three-dimensional, primitive equation numerical ocean model. The model is configured using an idealized estuary/shelf domain with rectangular cross section, constant vertical mixing, and steady riverine discharge. Tidal dispersion is neglected, so the analysis does apply to well-mixed estuaries and lagoons. Estuarine scales for the length of steady-state salt intrusion, vertical stratification, and estuarine exchange flow estimated from steady-state model results are found to have the same functional relationships to vertical mixing and riverine discharge as the classic analytic solutions. For example, for steady-state conditions, the stratification is found to be virtually independent of the strength of vertical mixing. The estuarine structure was controlled by the interior estuarine circulation, and not by limited exchange at the mouth. Thus, the numerical solutions were not “overmixed,” although the solutions showed a dependence on freshwater flux functionally similar to the overmixed solution. Estuarine adjustment time scales are also estimated from the simulations, and they are related to the steady-state estuarine scales. Two classes of nonsteady solutions are examined: the response to a step change in riverine discharge and estuarine response to changes in vertical mixing. Spring/neap tidal variations are examined by modulating the (spatially constant) vertical mixing with a fortnightly period. Unlike the steady solutions, there is a clear dependence of stratification on mixing rate in the time-dependent solutions. The simulations involving changes in riverine discharge show asymmetries between response to increasing and decreasing river flow that are attributed to quadratic bottom drag.

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Daniel G. MacDonald and W. Rockwell Geyer

Abstract

Observations at the mouth of the Fraser River (British Columbia, Canada) indicate an abrupt frontal transition between unstratified river outflow and a highly stratified river plume with differences in salinity greater than 25 psu across a few meters in the vertical direction and several hundred meters in the horizontal direction. The front roughly follows a natural break in the bathymetry, crossing the channel at an angle of approximately 45°, and is essentially stationary for a period of approximately 3.5 h centered on the low tide following the larger of two daily ebbs. The location of the front is coincident with observations of significantly supercritical internal Froude numbers at the front, based on velocities in the along-flow direction. This observation contradicts the one-dimensional theory, which indicates that the Froude number should be 1. However, because the front is oriented obliquely to the outflow, a coordinate system can be selected that is normal to the front and for which a critical Froude number of 1 is obtained. This indicates that a Froude angle, similar in application to a Mach angle for transonic flows, can be used to determine critical conditions when the front is oblique to the principal flow direction.

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Malcolm E. Scully and W. Rockwell Geyer

Abstract

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 when direct turbulent mixing through the pycnocline is prohibited by strong stratification.

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Derek A. Fong and W. Rockwell Geyer

Abstract

The alongshore transport of a surface-trapped river plume is studied using a three-dimensional model. Model simulations exhibit the previously observed rightward veering (in the Northern Hemisphere) of the freshwater and establishment of a downstream geostrophically balanced coastal current. In the absence of any ambient current, the plume does not reach a steady state. The downstream coastal current only carries a fraction of the discharged freshwater; the remaining fraction recirculates in a continually growing “bulge” of freshwater in the vicinity of the river mouth.

The river mouth conditions influence the amount of freshwater transported in the coastal current relative to the growing bulge. For high Rossby number [O(1)] discharge conditions, the bulge shape is circular and the coastal current transport is smaller than for the model runs of low Rossby number discharges. For all model runs conducted without an ambient current, the freshwater transport in the coastal current is less than the freshwater discharged at the river mouth.

The presence of an ambient current (in the same direction as the geostrophic coastal current) augments the transport in the plume such that its downstream freshwater transport matches the freshwater source, and the plume evolves to a steady-state width. The steady-state transport accounted for by the ambient current is independent of the strength of the ambient current. The amplitude of the ambient current only determines the time required to reach a steady-state plume width. A key result of this study is that an external forcing agent (e.g., wind or ambient current) is required in order for the entire freshwater volume discharged by a river to be transported downstream.

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W. Rockwell Geyer and David M. Farmer

Abstract

The time variations of the salt wedge intrusion in the Fraser estuary, British Columbia, were monitored during a variety of tidal and run-off conditions using instruments and sampling methods that provided high resolution of the velocity and water properties in space and time. The salt wedge was found to vary considerably in position and vertical structure through the tidal cycle due to the interaction of the tidal flow with the density-driven motion of the salt wedge. During the flood, the salt wedge progressed up the estuary as a gravity current, while during the ebb the salinity structure was eroded by shear instability. The difference in character of the flow between flood and ebb is attributed to the transition between a subcritical and supercritical internal Froude number. During flood tide, the internal state is subcritical, and the estuarine shear flow is maintained with a small amount of vertical mixing, while in the supercritical ebb flow, vertical shears become so large that the pycnocline becomes unstable, and the salt wedge structure breaks down.

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W. Rockwell Geyer and J. Dungan Smith

Abstract

Shear instability is found to be the principal mechanism of vertical exchange within the pycnocline of a salt wedge estuary. A field program involving high-resolution velocity and density measurements, as well as high-frequency acoustic imagery, allowed direct comparison of instantaneous Richardson number distributions to the occurrence of shear instability. The theoretical stability threshold of 0.25 is consistent with the measurements, based on estimates of gradients that contain the mean as well as fluctuations due to internal waves. An effective stability threshold based on mean gradients is found to be approximately one-third, reflecting a significant contribution of internal wave shear. The integral effect of the mixing process is to homogenize the gradients of velocity and density, producing linear profiles of these quantities across the pycnocline. A turbulent Prandtl number of unity is suggested by the vertical distributions of velocity and density during periods of active vertical mixing. Based on these observations, a simple model for mixing in stratified shear flows is proposed, which is applicable to estuaries and other environments with a dominant mean shear.

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Tao Wang, W. Rockwell Geyer, and Parker MacCready

Abstract

The linkage among total exchange flow, entrainment, and diffusive salt flux in estuaries is derived analytically using salinity coordinates, revealing the simple but important relationship between total exchange flow and mixing. Mixing is defined and quantified in this paper as the dissipation of salinity variance. The method uses the conservation of volume and salt to quantify and distinguish the diahaline transport of volume (i.e., entrainment) and diahaline diffusive salt flux. A numerical model of the Hudson estuary is used as an example of the application of the method in a realistic estuary with a persistent but temporally variable exchange flow. A notable finding of this analysis is that the total exchange flow and diahaline salt flux are out of phase with respect to the spring–neap cycle. Total exchange flow reaches its maximum near minimum neap tide, but diahaline salt transport reaches its maximum during the maximum spring tide. This phase shift explains the strong temporal variation of stratification and estuarine salt content through the spring–neap cycle. In addition to quantifying temporal variation, the method reveals the spatial variation of total exchange flow, entrainment, and diffusive salt flux through the estuary. For instance, the analysis of the Hudson estuary indicates that diffusive salt flux is intensified in the wider cross sections. The method also provides a simple means of quantifying numerical mixing in ocean models because it provides an estimate of the total dissipation of salinity variance, which is the sum of mixing due to the turbulence closure and numerical mixing.

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James A. Lerczak and W. Rockwell Geyer

Abstract

The dynamics of lateral circulation in an idealized, straight estuary under varying stratification conditions is investigated using a three-dimensional, hydrostatic, primitive equation model in order to determine the importance of lateral circulation to the momentum budget within the estuary. For all model runs, lateral circulation is about 4 times as strong during flood tides as during ebbs. This flood–ebb asymmetry is due to a feedback between the lateral circulation and the along-channel tidal currents, as well as to time-varying stratification over a tidal cycle. As the stratification is increased, the lateral circulation is significantly reduced because of the adverse pressure gradient set up by isopycnals being tilted by the lateral flow itself. When rotation is included, a time-dependent, cross-channel Ekman circulation is driven, and the tidally averaged, bottom lateral circulation is enhanced toward the right bank (when looking toward the ocean in the Northern Hemisphere). This asymmetry in the tidally averaged bottom lateral circulation may lead to asymmetric sediment transport, leading to asymmetric channel profiles in straight estuaries. For the weakly stratified model run, advection due to lateral currents is a dominant term in both the along-channel and cross-channel momentum equations over a tidal cycle and for the tidally averaged momentum equations. In the tidally averaged, along-channel momentum equation, lateral advection acts as a driving term for the estuarine exchange flow and can be larger than the along-channel pressure gradient. Therefore, it should not be ignored when estimating momentum budgets in estuaries.

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