The effect of East River on the barotropic motions in Long Island Sound

A linearized, frequency-dependent analytical model is developed to examine the effect of East River on the barotropic motions in Long Island Sound. At tidal frequencies, East River creates a slightly imperfect reflecting wall at the western end of Long Island Sound, resulting in moderate reduction in the resonance of the M2 tide. At subtidal frequencies the presence of East River permits a significant amount of volume exchange through the western end of Long Island Sound, causing large scale adjustments in both the amplitude and the phase of the barotropic flow well into the interior of the Sound. It appears that the impact of East River has to be considered for a proper assessment of low frequency motion in Long Island Sound

On the relative importance of the remote and local wind effects on the subtidal exchange at the entrance to the Chesapeake Bay

Water velocity data from acoustic Doppler current profilers and electromagnetic current meters deployed at six separate locations across the entrance of the Chesapeake Bay from mid-April to early July of 1999 and from early September to mid-November of 1999 were used in conjunction with wind velocity and sea level records to describe the characteristics of the wind-induced subtidal volume exchange between the bay and the adjacent continental shelf. The current measurements were used to estimate volume fluxes associated with the local and remote wind-induced bay-shelf exchange over time scales of 2–3 days. The results show that at these relatively short subtidal time scales (1) the net flux integrated over the entrance to the estuary adequately describes the unidirectional (either inflow or outflow over the entire cross-section) barotropic volume flux associated with the coastally forced remote wind effect, (2) during the first deployment there is always a bi-directional exchange pattern (inflow and outflow existing simultaneously over different parts of the cross-section) superimposed on the sectionally integrated unidirectional exchange, (3) the magnitude of the bi-directional transport associated with the local wind effect may be a significant fraction of the unidirectional transport associated with the remote wind effect, and (4) the relative importance of the local wind effect in producing estuary-shelf exchange changes appreciably with season, depending on the characteristic frequency of the wind events and the degree of stratification in the estuary

The variability of currents and sea level in the upper Delaware estuary

The variability of currents and sea levels in the upper Delaware estuary are examined based on measurements from bottom mounted acoustic Doppler current profilers (ADCP) deployed at two sites (New Castle and Tinicum) from 18 March to 10 June 2003. New Castle is located 104 km from the mouth, and Tinicum is located another 32 km up-estuary. Supplemental data, including sea level at the mouth of the estuary, river discharge, and wind speed and direction, were also obtained from various federal agencies. The instantaneous current represents a superposition of variability driven by the tide, wind, and river discharge. Over the short (\u3c36 hr) time scale, the tide is the dominant forcing mechanism, with M2 being the principal tidal constituent. The amplitude of the M2 tide increases from the mouth to the upper estuary and gives rise to a vigorous M2 current of the order 80 cm s–1. On time scales of 36 to 120 hr, the effect of wind drives a weak subtidal current with a standard deviation of 2 cm s–1 in the upper estuary. At time scales longer than 120 hr, the subtidal current variability, with a standard deviation of 6 cm s–1, is dominated by the barotropic response of the upper estuary to variations in the river discharge. The upper estuary exhibits a strong down-estuary mean current of the order—15 cm s–1. At Tinicum, river discharge accounts for more than half of the mean current, which is characterized by down-estuary flow throughout the water column. The magnitude of the river discharge-induced mean current is reduced at New Castle, in direct response to the down-estuary increase in the cross-sectional area. Tidally rectified current accounts for the remainder of the overall mean flow at Tinicum, and the effect of tidal rectification may be more important than river discharge in producing the mean flow at New Castle. There is no evidence of a baroclinic gravitational circulation, as the salt intrusion generally does not extend into the upper estuary

The tidal and subtidal variations in the transverse salinity and current distributions across a coastal plain estuary

The transverse structure in the current and salinity distributions across the mouth of Delaware Bay are examined over the tidal and subtidal time scales. Results show that the across-estuary variation in bathymetry, in the form of a channel-shoal configuration, has a very significant impact on the characteristics of transverse variability. The mean current over the channel is characterized by a strong outflow of low salinity water in the upper layer and a strong inflow of high salinity water in the lower layer, consistent with the density-induced gravitational circulation. The mean flow pattern in the shallow waters over the shoals is marked by transverse rather than vertical variation. The subtidal current and salinity fluctuations are primarily driven by the effect of local atmospheric forcing. The subtidal current fluctuations in the upper layer of the channel are frictionally coupled to the local wind, resulting in downwind currents. The subtidal current fluctuations in the lower layer of the channel, however, flow in the direction of local setup and against the wind. With a wind blowing down the estuary, the wind-induced current tends to reinforce the two-layer structure of the gravitational circulation and substantially enhance the vertical shear and surface to bottom salinity difference. The reverse occurs with a wind in the up-bay direction. The subtidal currents in the shallow areas to the right of the channel exhibit largely depth-independent response to the effect of local wind, with downwind currents at both the surface and the bottom. At tidal frequencies the currents show only a modest variation across the bay mouth. Tidal currents are highly deterministic, but the characteristics of the tidal variability in salinity exhibit significant changes over long time scales. These long-term changes in the intratidal salinity variability are caused by the nonlinear interactions between the tidal and subtidal motions. The residual salt flux through the bay mouth shows significant subtidal fluctuations. The leading factor responsible for producing such subtidal fluctuations is the advection of salt by the wind-induced subtidal currents, but the effect of tidal pumping also contributes significantly to the overall residual salt flux into the estuary

Observations of the wind-induced exchange at the entrance to Chesapeake Bay

Water density and velocity data from two ~75-day deployments across the entrance to the Chesapeake Bay were used in conjunction with wind velocity and sea level records to describe the transverse structure of wind-induced subtidal exchange. Acoustic Doppler current profilers, electromagnetic current meters, and conductivity-temperature-depth recorders were deployed at the entrance to the bay from mid-April to early July of 1999 and from early September to mid-November of 1999. Three main scenarios of wind-induced exchange were identified: (1) Northeasterly (NE) winds consistently drove water from the coast toward the lower Chesapeake Bay as well as water from the upper bay to the lower bay, which was indicated by the surface elevation slopes across the lower bay and along the bay. This resulted in water piling up against the southwestern corner of the bay. The subtidal flow over the southern portion of the bay entrance was directed to the left of the wind direction, likely the result of the influence of Coriolis and centripetal accelerations on the adjustment of the sea level gradients. Over the northern shallow half of the entrance, the subtidal flows were nearly depth-independent and in the same direction as the wind. (2) Southwesterly (SW) winds caused opposite sea level gradients (relative to NE winds), which translated into near-surface outflows throughout the entrance and near-bottom inflows restricted to the channels. This wind-induced circulation enhanced the two-way exchange between the estuary and the adjacent ocean. (3) Northwesterly winds produced the same exchange pattern as NE winds. Water piled up against the southwestern corner of the bay causing net outflow in the deep, southern area and downwind flow over the shallow areas. Northwesterly winds greater than 12 m/s caused the most efficient flushing of the bay, driving water out over the entire mouth of the estuary

Fortnightly Variability in the Transverse Dynamics of a Coastal Plain Estuary

Current velocity and water density profiles were obtained along two crossestuary transects with the purpose of determining the fortnightly variability of the transverse dynamics in a partially stratified coastal plain estuary. The profiles were measured with a towed acoustic Doppler current profiler and a conductivity-temperaturedepth recorder in the James River estuary, Virginia. The cross-estuary transects were sampled during the spring tides of October 26-27, 1996, and the ensuing neap tides of November 2-3, 1996. The transects were-4 km long, featured a bathymetry that consisted of a channel flanked by shoals, and were sampled repeatedly during two semidiurnal tidal cycles (25 hours) in order to separate semidiurnal, diurnal, and subtidal signals from the observations. This work concentrates on the subtidal transverse dynamics. The transverse baroclinic pressure gradients were larger during neap tides than during spring tides. During spring tides the advective accelerations were predominantly greater than the Coriolis accelerations, most markedly over the edges of the channel. Both effects combined with frictional influences to balance the pressure gradient in the transverse direction. During neap tides, advective accelerations were not as dominant over Coriolis accelerations as during spring tides. Also, during neap tides, Coriolis played a more relevant role, compared to spring tides, in combining with friction to balance the pressure gradient. This behavior was indicative of the momentum balance approaching gravitational circulation modified by the Earth\u27s rotation, weak friction, and nonlinear advection during neap tides. The balance became more influenced by nonlinear advection and friction and less influenced by the Earth\u27s rotation during spring tides. These results showed that transverse dynamics of a partially stratified estuary are far from being in geostrophic balanc

Separating Baroclinic Flow From Tidally Induced Flow in Estuaries

A simple method is used to separate the tidally induced and density-driven subtidal flows in a coastal plain estuary. This method is applicable to weak wind conditions and to systems with appreciable fortnightly variation of tidal amplitude. The baroclinic density-driven motion is assumed to depend on the river discharge, which generates a horizontal density gradient, and is weakened by vertical mixing, which in turn depends on tidal forcing. The barotropic tidally induced motion is assumed to be a function of the tidal amplitude. By Taylor series expansions, two equations are obtained. These equations show the dependence of the tidally induced how component on the tidal amplitude and the dependence of the density-driven flow component on the ratio between river discharge and tidal amplitude, respectively. The method is applied to water velocity data obtained in the James River, Virginia, in October-November 1996. The data cover two spring tidal cycles and two neap tidal cycles. The vertical structures, as well as the depth mean, of both tidally induced and density-driven components of the subtidal flow are obtained. Results show that the tidally induced component has a predominant seaward how in the channel and a landward flow over the shoals. The density-driven exchange how is seaward over the shoals and landward in the channel. These results are consistent with theoretical model results which show that the tidally induced component and density-driven component compete against each other. The increased tidal mixing and tidally induced exchange flow during spring tides reduce density-driven motion, which results in a weak net subtidal flow. In contrast, during neap tides, both the tidally induced flow component-of the subtidal how and tidal mixing are weak, and the tidally induced flow is overwhelmed by the density-driven flow component, which results in a stronger subtidal how. By extending the proposed method, we suggest that future studies use a least squares fitting technique to obtain an optimal estimate for the tidally induced and density-driven subtidal flow components

Convergence of Lateral Flow Along a Coastal Plain Estuary

A set of velocity profiles obtained in the James River estuary with an acoustic Doppler current profiler was used in combination with the results of an analytic tidal model to depict the appearance of surface lateral flow convergences (δv/δy) during both flood and ebb stages of the tidal cycle. The bathymetry of the estuary was characterized by a main channel and a secondary channel separated by relatively narrow shoals. Lateral surface flow convergences appeared over the edges of the channels and were produced by the phase lag of the flow in the channel relative to the shoals. Flood convergences developed in the late tidal stages and ebb convergences appeared soon after maximum currents. Most of these convergences caused fronts in the density field and flotsam lines that also appeared over the edges of the channel and that lasted \u3c2 hours. The transverse flows associated with the convergences were mostly in the same direction throughout the water column. In fact, the vertically averaged flow produced the same convergence patterns as those near the surface. The analytic tidal model reproduced well the timing and location of the convergences as observed in the James River. Model results with different bathymetry emulated the results in other estuaries, e.g., axial convergence in an estuary with a channel in the middle. This work showed that the strength of lateral convergences along the estuary was proportional to the tidal amplitude and the channel steepness. It also suggested that the convergences were produced mainly by the tidal flow interacting with the channel-shoal bathymetry, i.e., that they did not require the presence of density gradients. However, the analytic model underestimated the magnitude of the convergences and did not account for vertical circulations associated with fronts. The formation of fronts resulted from the interaction of the tidal flow with the bathymetry and the density field

Estimation of Drag Coefficient in James River Estuary Using Tidal Velocity Data from a Vessel-Towed ADCP

[1] A phase-matching method is introduced to calculate the bottom drag coefficient in tidal channels with significant lateral variation of depth. The method is based on the fact that the bottom friction in a tidal channel causes tidal velocity to have a phase difference across the channel. The calculation involves a few steps. First, the observed horizontal velocity components are analyzed to obtain the amplitude and phase of the velocity at the major tidal frequency. The phase of the longitudinal velocity is then fitted to a relationship derived from the linearized momentum balance. The drag coefficient is then calculated. This method is applicable to narrow (approximately a few kilometers wide) tidal channels without strong stratification and where the cross-channel variation of surface elevation is negligible compared to tidal amplitude. This analytic approach is easy to implement when appropriate observational data are available. It allows a spatial variation of the drag coefficient, and the resolution is only limited by that of the observations. The method is validated by identical twin experiments and applied to tidal velocity data, obtained in the James River Estuary, using an acoustic Doppler current profiler during spring tides and neap tides in October-November 1996. The obtained bottom drag coefficient ranged from 1.2 x 10(-3) to 6.9 x 10(-3) at different positions along two cross-channel transects each 4 km long and 2 to 14 m deep. The maximum drag coefficient is found in the shallowest water near the banks of the estuary, while the minimum values occur between 9 and 12 m in the center of the channel. The friction of the lateral boundary may have contributed to the apparent increase of the bottom friction on the banks. The transverse mean values of the drag coefficient ranges between 2.2 and 2.3 x 10(-3) for the spring and neap tides, respectively