Breakwaters influence coastal wave climate and circulation by blocking and dissipating wave energ... more Breakwaters influence coastal wave climate and circulation by blocking and dissipating wave energy. Accurate representation of these effects is essential to the determination of coastal circulation and wave processes. MIKE21SW and SWAN are two third-generation spectral wave models which are used widely in coastal research and engineering applications. Recent improved versions of the models are able to consider the influence of breakwater structures. In this study, we used available observations to evaluate the accuracy of model simulations of waves in New Haven Harbor, Connecticut, USA, an estuary with three detached breakwaters near the mouth. We then compare the accuracy and computational efficiency of MIKE21SW and SWAN. Both models were executed on the same unstructured triangular grid. The boundary conditions were derived from a bottom mounted ADCP on the offshore side of the breakwaters. Wind forcing was applied using data from the Central Long Island Sound buoy. We find that b...
Proceedings of the IEEE Fifth Working Conference on Current Measurement, 1995
A broadband ADCP and CTD, mounted aboard a towed platform (TOAD), were used to characterize the f... more A broadband ADCP and CTD, mounted aboard a towed platform (TOAD), were used to characterize the front of the Connecticut River plume in Long Island Sound on April 5, 1994. The CTD was at ~0.75 m and the ADCP estimated velocities in 0.25 m intervals from 1.1 m to the bottom, ~10 m. Data were processed in 5-s intervals yielding
Nonlinear mechanisms give rise to overtide frequencies that are integer multiples of the principl... more Nonlinear mechanisms give rise to overtide frequencies that are integer multiples of the principle tidal frequency. The M6 is an overtide with a frequency three times that of the primary M2 semidiurnal tide. Overtides are of interest because they modulate or distort the shape and timing of the fundamental tide. Because their generation and propagation is sensitive to a combination of physical factors, they can also serve as a means by which to evaluate hydrodynamic model performance. Observations in Long Island Sound (LIS) indicate the M6 overtide shows a notable increase in amplitude in the Western Sound compared to the East. M6 generation, however, should be greatest in the East, where the primary M2 current is strongest. We examine the generation and propagation of the M6 in simple channels using numeric and analytic methods and show that, despite its higher frequency, attenuation at the M6 frequency is less than that at the M2. The observed spatial distribution of M6 amplitudes ...
Numerical modeling has emerged over the last several decades as a widely accepted tool for invest... more Numerical modeling has emerged over the last several decades as a widely accepted tool for investigations in environmental sciences. In estuarine research, hydrodynamic and ecological models have moved along parallel tracks with regard to complexity, refinement, computational power, and incorporation of uncertainty. Coupled hydrodynamicecological models have been used to assess ecosystem processes and interactions, simulate future scenarios, and evaluate remedial actions in response to eutrophication, habitat loss, and freshwater diversion. The need to couple hydrodynamic and ecological models to address research and management questions is clear because dynamic feedbacks between biotic and physical processes are critical interactions within ecosystems. In this review, we present historical and modern perspectives on estuarine hydrodynamic and ecological modeling, consider model limitations, and address aspects of model linkage, skill assessment, and complexity. We discuss the balance between spatial and temporal resolution and present examples using different spatiotemporal scales. Finally, we recommend future lines of inquiry, approaches to balance complexity and uncertainty, and model transparency and utility. It is idealistic to think we can pursue a Btheory of everything^for estuarine models, but recent advances suggest that models for both scientific investigations and management applications will continue to improve in terms of realism, precision, and accuracy.
An integrated nutrient observatory is being developed within the Northeastern Regional Associatio... more An integrated nutrient observatory is being developed within the Northeastern Regional Association of Coastal Ocean Observing Systems (NERACOOS), capable of monitoring nutrient dynamics year-round at temporal and spatial scales necessary to address critical needs of stakeholders throughout the Northeast region. Nutrient levels and fluxes drive total biological productivity throughout the region, from phytoplankton to commercially exploited fish stocks. Nitrate sensors (Satlantic SUNAs) are being installed on existing mooring assets in western Long Island Sound, Narragansett Bay (Prudence Island), Great Bay in New Hampshire, Massachusetts Bay, three sites along the coastal shelf of the Gulf of Maine (GOM), at five depths in Jordan Basin in the interior GOM, and at two depths in the GOM Northeast Channel. Phosphate and ammonium sensors (WET Labs Cycle-PO 4 and Cycle-NH 4) are also being deployed at the three nearshore sites. The measurements from these sensors will extend the current sparse, long-term records of nutrients from discretely collected samples in the Northeast region and will dramatically improve temporal resolution and continuity of the data for use in studying potential impacts of climate change. Nearshore measurements will be used by NERACOOS stakeholders to help assess, regulate, and mitigate the adverse impacts on water quality associated with excessive pollutant loadings. Measurements throughout the GOM will be used to assess basinwide nutrient variability and to initialize harmful algal bloom (Alexandrium fundyense) forecast models.
The Seventh ACM International Conference on Underwater Networks and Systems, 2012
ABSTRACT This paper presents collective efforts from four institutions, University of Connecticut... more ABSTRACT This paper presents collective efforts from four institutions, University of Connecticut, University of Washington, University of California, Los Angeles, and Texas A&M University, on a community Ocean Testbed for Underwater Networks Experiments (Ocean-TUNE). This proposed community testbed will enable a wide range of research in the areas of underwater communications, networking, engineering, and marine science, and hence will promote unprecedented progress towards practical solutions in diverse aquatic applications.
With an analytic model, this paper describes the subtidal circulation in tidally dominated channe... more With an analytic model, this paper describes the subtidal circulation in tidally dominated channels of different lengths, with arbitrary lateral depth variations. The focus is on an important parameter associated with the reversal of the exchange flows. This parameter (δ) is defined as the ratio between the channel length and one-quarter of the tidal wavelength, which is determined by water depth and tidal frequency. In this study, a standard bottom drag coefficient, CD = 0.0025, is used. For a channel with δ smaller than 0.6–0.7 (short channels), the exchange flow at the open end has an inward transport in deep water and an outward transport in shallow water. This situation is just the opposite of channels with a δ value larger than 0.6–0.7 (long channels). For a channel with a δ value of about 0.35–0.5, the exchange flow at the open end reaches the maximum of a short channel. For a channel with a δ value of about 0.85–1.0, the exchange flow at the open end reaches the maximum of a...
Measurements of turbulence were performed in four frontal locations near the mouths of Block Isla... more Measurements of turbulence were performed in four frontal locations near the mouths of Block Island Sound (BIS) and Long Island Sound (LIS). These measurements extend from the offshore front associated with BIS and Mid-Atlantic Bight Shelf water, to the onshore fronts near the Montauk Point (MK) headland, and the Connecticut River plume front. The latter feature is closely associated with the major fresh water input to LIS. Turbulent kinetic energy (TKE) dissipation rate, ε, was obtained using shear probes mounted on an autonomous underwater vehicle. Offshore, the BIS estuarine outflow front showed, during spring season and ebb tide, maximum TKE dissipation rate, ε, estimates of order 10 -5 W/kg, with background values of order 10 -6 to 10 -9 W/kg. Edwards et al. [Edwards, C.A., Fake, T.A., and Bogden, P.S., 2004a. Spring-summer frontogenesis at the mouth of Block Island Sound: 1. A numerical investigation into tidal and buoyancy-forced motion. Journal of Geophysical Research 109 (C12021), doi:10.1029/2003JC002132.] model this front as the boundary of a tidally driven, baroclinically adjusted BIS flow around the MK headland eddy. At the entrance to BIS, near MK, two additional fronts are observed, one of which was over sand waves. For the headland site front east of MK, without sand waves, during ebb tide, ε estimates of 10 -5 to 10 -6 W/kg were observed. The model shows that this front is at the northern end of an anti-cyclonic headland eddy, and within a region of strong tidal mixing. For the headland site front further northeast over sand waves, maximum ε estimates were of order 10 -4 W/kg within a background of order 10 -7 -10 -6 W/kg. From the model, this front is at the northeastern edge of the anti-cyclonic headland eddy and within the tidal mixing zone. For the Connecticut River plume front, a surface trapped plume, during ebb tide, maximum ε estimates of 10 -5 W/kg were obtained, within a background of 10 -6 to 10 -8 W/kg. Of all four fronts, the river plume front has the largest finescale mean-square shear, S 2 ~0.15 s -2 . All of the frontal locations had local values of the buoyancy Reynolds number indicating strong isotropic turbulence at the dissipation scales. Local values of the Froude number indicated shear instability in all of the fronts.
We present an approach that allows the estimation of vertical eddy diffusivity coefficients from ... more We present an approach that allows the estimation of vertical eddy diffusivity coefficients from buoy measurements made at two or more depths. By measuring the attenuation and phase lag of a scalar signal generated periodically at the surface as it propagates downwards, the vertical eddy diffusivity coefficients can be calculated as K v = ωΔz 2 /2ln 2 (α 2 /α 1), where α 2 /α 1 is the ratio of the real amplitudes at frequency ω at the two depths separated by Δz = z 2 − z 1 ; as K V = ωΔz 2 /2φ 2 , where φ is the phase lag at the frequency ω; or as K v = iωΔz 2 /ln 2 (U 2 /U 1), where U 2 /U 1 is the ratio of the complex signal amplitudes at the two depths. The method requires that horizontal fluxes be small at the ω frequency and that the signal-to-noise ratios at the two depths allow the determination of the amplitude and phase of ω. Application of this method to summertime 2004 western Long Island Sound oxygen and temperature buoy measurements at two depths provides a time-series of two-day average vertical eddy diffusivity estimates. Using these eddy diffusivities in conjunction with measured vertical concentration gradients, we obtain a time-series of vertical transport rates for oxygen and heat and estimate mean downward fluxes for June and July as 150-260 mMol m − 2 day − 1 and 100-400 W m − 2 respectively. These estimates are of a similar magnitude to subpycnocline O 2 and heat demands of 240 ± 200 mMol m − 2 day − 1 and 180 ± 60 W m − 2 that we infer from simple budgets, implying that vertical transport is significant to both budgets. The eddy coefficients obtained from the independent O 2 and temperature measurements have a 68% correlation, and the O 2 flux estimates show a correlation of 41% to measured rates of change in bottom dissolved oxygen levels. Our results indicate that extended time-series of eddy diffusivity coefficients can be obtained from in situ buoy measurements and the method shows promise as a way to constrain the vertical transport variability in budgets of dissolved materials in estuaries.
... 1. Department of Marine Sciences, University of Connecticut, Groton, Connecticut, 06340, USA ... more ... 1. Department of Marine Sciences, University of Connecticut, Groton, Connecticut, 06340, USA 2. Corresponding author. email: diane.bennett@uconn.edu 1 Page 2. 2 Journal of Marine Research [68, 1 Figure 1. Bathymetric map of Long Island Sound. ...
An important aspect of particle trajectory modeling in the ocean is the assessment of the uncerta... more An important aspect of particle trajectory modeling in the ocean is the assessment of the uncertainty in the final particle position. Monte Carlo particle trajectory simulations using surface currents derived from standard-range and long-range CODAR HF radar systems were performed using random-walk and random-flight models of the unresolved velocities. Velocity statistics for these models were derived from the covariance functions of differences between CODAR and drifter estimates of surface currents. Comparison of predicted trajectories and drifter tracks demonstrate that these predictions are superior to assuming the drifters stay at their initial position. Vertical shear between the effective depth of long-range CODAR measurements ($2.4 m) and that of drifters (0.65 m) causes the drifters to move more rapidly downwind than predicted. This bias is absent when standard-range CODAR currents (effective depth $0.5 m) are used, implying that drifter leeway is not the cause of the bias. Particle trajectories were computed using CODAR data and the random-flight model for 24-hour intervals using a Monte Carlo approach to determine the 95% confidence interval of position predictions. Between 80% and 90% of real drifters were located within the predicted confidence interval, in reasonable agreement with the expected 95% success rate. In contrast, predictions using the random-walk approach proved inconsistent with observations unless the diffusion coefficient was increased to approximately the random-flight value. The consistency of the random-flight uncertainty estimates and drifter data supports the use of our methodology for estimating model parameters from drifter-CODAR velocity differences.
We present a simple advection-dispersion model for the subtidal salt distribution in estuaries wi... more We present a simple advection-dispersion model for the subtidal salt distribution in estuaries with linearly varying cross-sectional area and a nonzero net salt flux. A novel analytic solution allows investigation of the dependence of the curvature and gradient of the longitudinal salinity distribution on runoff, dispersion coefficient, and channel contraction or expansion. The model predicts that in estuarine segments that contract toward the fresher boundary, the salinity gradient is stronger than in a prismatic channel. When the dispersion coefficient is large compared to the salinity intrusion lengthscale, $\frac{LR}{A0 (the product of segment length and net volume flux divided by cross-sectional area at the ocean boundary), the curvature of the salt concentration may be negative, a characteristic not possible in uniform channel models. The main effect of up-estuary salt flux is to strengthen the salinity gradient. The model can be extended to multiple segments in order to simulate geometrically complicated estuaries. The model is employed to estimate an effective dispersion coefficient and to describe the salinity variation in the western 53 km of Long Island Sound where the cross section of the basin varies linearly. Using 8 years of monthly observations at seven stations we find that, since the curvature of the vertically averaged salinity is negative, the model and data are consistent only if the net volume flux and salt flux are toward the fresher boundary, the East River. Combining prior estimates of the magnitudes of the fluxes and their uncertainties with the model and salinity observations using a least squares approach, we estimate the dispersion coefficient for the Western Sound as 580 m2/s.
Breakwaters influence coastal wave climate and circulation by blocking and dissipating wave energ... more Breakwaters influence coastal wave climate and circulation by blocking and dissipating wave energy. Accurate representation of these effects is essential to the determination of coastal circulation and wave processes. MIKE21SW and SWAN are two third-generation spectral wave models which are used widely in coastal research and engineering applications. Recent improved versions of the models are able to consider the influence of breakwater structures. In this study, we used available observations to evaluate the accuracy of model simulations of waves in New Haven Harbor, Connecticut, USA, an estuary with three detached breakwaters near the mouth. We then compare the accuracy and computational efficiency of MIKE21SW and SWAN. Both models were executed on the same unstructured triangular grid. The boundary conditions were derived from a bottom mounted ADCP on the offshore side of the breakwaters. Wind forcing was applied using data from the Central Long Island Sound buoy. We find that b...
Proceedings of the IEEE Fifth Working Conference on Current Measurement, 1995
A broadband ADCP and CTD, mounted aboard a towed platform (TOAD), were used to characterize the f... more A broadband ADCP and CTD, mounted aboard a towed platform (TOAD), were used to characterize the front of the Connecticut River plume in Long Island Sound on April 5, 1994. The CTD was at ~0.75 m and the ADCP estimated velocities in 0.25 m intervals from 1.1 m to the bottom, ~10 m. Data were processed in 5-s intervals yielding
Nonlinear mechanisms give rise to overtide frequencies that are integer multiples of the principl... more Nonlinear mechanisms give rise to overtide frequencies that are integer multiples of the principle tidal frequency. The M6 is an overtide with a frequency three times that of the primary M2 semidiurnal tide. Overtides are of interest because they modulate or distort the shape and timing of the fundamental tide. Because their generation and propagation is sensitive to a combination of physical factors, they can also serve as a means by which to evaluate hydrodynamic model performance. Observations in Long Island Sound (LIS) indicate the M6 overtide shows a notable increase in amplitude in the Western Sound compared to the East. M6 generation, however, should be greatest in the East, where the primary M2 current is strongest. We examine the generation and propagation of the M6 in simple channels using numeric and analytic methods and show that, despite its higher frequency, attenuation at the M6 frequency is less than that at the M2. The observed spatial distribution of M6 amplitudes ...
Numerical modeling has emerged over the last several decades as a widely accepted tool for invest... more Numerical modeling has emerged over the last several decades as a widely accepted tool for investigations in environmental sciences. In estuarine research, hydrodynamic and ecological models have moved along parallel tracks with regard to complexity, refinement, computational power, and incorporation of uncertainty. Coupled hydrodynamicecological models have been used to assess ecosystem processes and interactions, simulate future scenarios, and evaluate remedial actions in response to eutrophication, habitat loss, and freshwater diversion. The need to couple hydrodynamic and ecological models to address research and management questions is clear because dynamic feedbacks between biotic and physical processes are critical interactions within ecosystems. In this review, we present historical and modern perspectives on estuarine hydrodynamic and ecological modeling, consider model limitations, and address aspects of model linkage, skill assessment, and complexity. We discuss the balance between spatial and temporal resolution and present examples using different spatiotemporal scales. Finally, we recommend future lines of inquiry, approaches to balance complexity and uncertainty, and model transparency and utility. It is idealistic to think we can pursue a Btheory of everything^for estuarine models, but recent advances suggest that models for both scientific investigations and management applications will continue to improve in terms of realism, precision, and accuracy.
An integrated nutrient observatory is being developed within the Northeastern Regional Associatio... more An integrated nutrient observatory is being developed within the Northeastern Regional Association of Coastal Ocean Observing Systems (NERACOOS), capable of monitoring nutrient dynamics year-round at temporal and spatial scales necessary to address critical needs of stakeholders throughout the Northeast region. Nutrient levels and fluxes drive total biological productivity throughout the region, from phytoplankton to commercially exploited fish stocks. Nitrate sensors (Satlantic SUNAs) are being installed on existing mooring assets in western Long Island Sound, Narragansett Bay (Prudence Island), Great Bay in New Hampshire, Massachusetts Bay, three sites along the coastal shelf of the Gulf of Maine (GOM), at five depths in Jordan Basin in the interior GOM, and at two depths in the GOM Northeast Channel. Phosphate and ammonium sensors (WET Labs Cycle-PO 4 and Cycle-NH 4) are also being deployed at the three nearshore sites. The measurements from these sensors will extend the current sparse, long-term records of nutrients from discretely collected samples in the Northeast region and will dramatically improve temporal resolution and continuity of the data for use in studying potential impacts of climate change. Nearshore measurements will be used by NERACOOS stakeholders to help assess, regulate, and mitigate the adverse impacts on water quality associated with excessive pollutant loadings. Measurements throughout the GOM will be used to assess basinwide nutrient variability and to initialize harmful algal bloom (Alexandrium fundyense) forecast models.
The Seventh ACM International Conference on Underwater Networks and Systems, 2012
ABSTRACT This paper presents collective efforts from four institutions, University of Connecticut... more ABSTRACT This paper presents collective efforts from four institutions, University of Connecticut, University of Washington, University of California, Los Angeles, and Texas A&M University, on a community Ocean Testbed for Underwater Networks Experiments (Ocean-TUNE). This proposed community testbed will enable a wide range of research in the areas of underwater communications, networking, engineering, and marine science, and hence will promote unprecedented progress towards practical solutions in diverse aquatic applications.
With an analytic model, this paper describes the subtidal circulation in tidally dominated channe... more With an analytic model, this paper describes the subtidal circulation in tidally dominated channels of different lengths, with arbitrary lateral depth variations. The focus is on an important parameter associated with the reversal of the exchange flows. This parameter (δ) is defined as the ratio between the channel length and one-quarter of the tidal wavelength, which is determined by water depth and tidal frequency. In this study, a standard bottom drag coefficient, CD = 0.0025, is used. For a channel with δ smaller than 0.6–0.7 (short channels), the exchange flow at the open end has an inward transport in deep water and an outward transport in shallow water. This situation is just the opposite of channels with a δ value larger than 0.6–0.7 (long channels). For a channel with a δ value of about 0.35–0.5, the exchange flow at the open end reaches the maximum of a short channel. For a channel with a δ value of about 0.85–1.0, the exchange flow at the open end reaches the maximum of a...
Measurements of turbulence were performed in four frontal locations near the mouths of Block Isla... more Measurements of turbulence were performed in four frontal locations near the mouths of Block Island Sound (BIS) and Long Island Sound (LIS). These measurements extend from the offshore front associated with BIS and Mid-Atlantic Bight Shelf water, to the onshore fronts near the Montauk Point (MK) headland, and the Connecticut River plume front. The latter feature is closely associated with the major fresh water input to LIS. Turbulent kinetic energy (TKE) dissipation rate, ε, was obtained using shear probes mounted on an autonomous underwater vehicle. Offshore, the BIS estuarine outflow front showed, during spring season and ebb tide, maximum TKE dissipation rate, ε, estimates of order 10 -5 W/kg, with background values of order 10 -6 to 10 -9 W/kg. Edwards et al. [Edwards, C.A., Fake, T.A., and Bogden, P.S., 2004a. Spring-summer frontogenesis at the mouth of Block Island Sound: 1. A numerical investigation into tidal and buoyancy-forced motion. Journal of Geophysical Research 109 (C12021), doi:10.1029/2003JC002132.] model this front as the boundary of a tidally driven, baroclinically adjusted BIS flow around the MK headland eddy. At the entrance to BIS, near MK, two additional fronts are observed, one of which was over sand waves. For the headland site front east of MK, without sand waves, during ebb tide, ε estimates of 10 -5 to 10 -6 W/kg were observed. The model shows that this front is at the northern end of an anti-cyclonic headland eddy, and within a region of strong tidal mixing. For the headland site front further northeast over sand waves, maximum ε estimates were of order 10 -4 W/kg within a background of order 10 -7 -10 -6 W/kg. From the model, this front is at the northeastern edge of the anti-cyclonic headland eddy and within the tidal mixing zone. For the Connecticut River plume front, a surface trapped plume, during ebb tide, maximum ε estimates of 10 -5 W/kg were obtained, within a background of 10 -6 to 10 -8 W/kg. Of all four fronts, the river plume front has the largest finescale mean-square shear, S 2 ~0.15 s -2 . All of the frontal locations had local values of the buoyancy Reynolds number indicating strong isotropic turbulence at the dissipation scales. Local values of the Froude number indicated shear instability in all of the fronts.
We present an approach that allows the estimation of vertical eddy diffusivity coefficients from ... more We present an approach that allows the estimation of vertical eddy diffusivity coefficients from buoy measurements made at two or more depths. By measuring the attenuation and phase lag of a scalar signal generated periodically at the surface as it propagates downwards, the vertical eddy diffusivity coefficients can be calculated as K v = ωΔz 2 /2ln 2 (α 2 /α 1), where α 2 /α 1 is the ratio of the real amplitudes at frequency ω at the two depths separated by Δz = z 2 − z 1 ; as K V = ωΔz 2 /2φ 2 , where φ is the phase lag at the frequency ω; or as K v = iωΔz 2 /ln 2 (U 2 /U 1), where U 2 /U 1 is the ratio of the complex signal amplitudes at the two depths. The method requires that horizontal fluxes be small at the ω frequency and that the signal-to-noise ratios at the two depths allow the determination of the amplitude and phase of ω. Application of this method to summertime 2004 western Long Island Sound oxygen and temperature buoy measurements at two depths provides a time-series of two-day average vertical eddy diffusivity estimates. Using these eddy diffusivities in conjunction with measured vertical concentration gradients, we obtain a time-series of vertical transport rates for oxygen and heat and estimate mean downward fluxes for June and July as 150-260 mMol m − 2 day − 1 and 100-400 W m − 2 respectively. These estimates are of a similar magnitude to subpycnocline O 2 and heat demands of 240 ± 200 mMol m − 2 day − 1 and 180 ± 60 W m − 2 that we infer from simple budgets, implying that vertical transport is significant to both budgets. The eddy coefficients obtained from the independent O 2 and temperature measurements have a 68% correlation, and the O 2 flux estimates show a correlation of 41% to measured rates of change in bottom dissolved oxygen levels. Our results indicate that extended time-series of eddy diffusivity coefficients can be obtained from in situ buoy measurements and the method shows promise as a way to constrain the vertical transport variability in budgets of dissolved materials in estuaries.
... 1. Department of Marine Sciences, University of Connecticut, Groton, Connecticut, 06340, USA ... more ... 1. Department of Marine Sciences, University of Connecticut, Groton, Connecticut, 06340, USA 2. Corresponding author. email: diane.bennett@uconn.edu 1 Page 2. 2 Journal of Marine Research [68, 1 Figure 1. Bathymetric map of Long Island Sound. ...
An important aspect of particle trajectory modeling in the ocean is the assessment of the uncerta... more An important aspect of particle trajectory modeling in the ocean is the assessment of the uncertainty in the final particle position. Monte Carlo particle trajectory simulations using surface currents derived from standard-range and long-range CODAR HF radar systems were performed using random-walk and random-flight models of the unresolved velocities. Velocity statistics for these models were derived from the covariance functions of differences between CODAR and drifter estimates of surface currents. Comparison of predicted trajectories and drifter tracks demonstrate that these predictions are superior to assuming the drifters stay at their initial position. Vertical shear between the effective depth of long-range CODAR measurements ($2.4 m) and that of drifters (0.65 m) causes the drifters to move more rapidly downwind than predicted. This bias is absent when standard-range CODAR currents (effective depth $0.5 m) are used, implying that drifter leeway is not the cause of the bias. Particle trajectories were computed using CODAR data and the random-flight model for 24-hour intervals using a Monte Carlo approach to determine the 95% confidence interval of position predictions. Between 80% and 90% of real drifters were located within the predicted confidence interval, in reasonable agreement with the expected 95% success rate. In contrast, predictions using the random-walk approach proved inconsistent with observations unless the diffusion coefficient was increased to approximately the random-flight value. The consistency of the random-flight uncertainty estimates and drifter data supports the use of our methodology for estimating model parameters from drifter-CODAR velocity differences.
We present a simple advection-dispersion model for the subtidal salt distribution in estuaries wi... more We present a simple advection-dispersion model for the subtidal salt distribution in estuaries with linearly varying cross-sectional area and a nonzero net salt flux. A novel analytic solution allows investigation of the dependence of the curvature and gradient of the longitudinal salinity distribution on runoff, dispersion coefficient, and channel contraction or expansion. The model predicts that in estuarine segments that contract toward the fresher boundary, the salinity gradient is stronger than in a prismatic channel. When the dispersion coefficient is large compared to the salinity intrusion lengthscale, $\frac{LR}{A0 (the product of segment length and net volume flux divided by cross-sectional area at the ocean boundary), the curvature of the salt concentration may be negative, a characteristic not possible in uniform channel models. The main effect of up-estuary salt flux is to strengthen the salinity gradient. The model can be extended to multiple segments in order to simulate geometrically complicated estuaries. The model is employed to estimate an effective dispersion coefficient and to describe the salinity variation in the western 53 km of Long Island Sound where the cross section of the basin varies linearly. Using 8 years of monthly observations at seven stations we find that, since the curvature of the vertically averaged salinity is negative, the model and data are consistent only if the net volume flux and salt flux are toward the fresher boundary, the East River. Combining prior estimates of the magnitudes of the fluxes and their uncertainties with the model and salinity observations using a least squares approach, we estimate the dispersion coefficient for the Western Sound as 580 m2/s.
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