Particle trapping in a stratified flood-dominated estuary

Observations in the Navesink River estuary in northern New Jersey demonstrate that buoyancy augments the particle trapping tendencies of flood-dominated systems because these estuaries heighten tidal period asymmetries in stratification. During the long and slow ebb which typifies flood-dominated systems, a positive feedback between tidal straining and weak vertical mixing stratifies the estuary. In contrast, during flood, turbulence generated by the stronger tidal currents augments overstraining of the density field and the water column becomes well mixed. The tidal period asymmetries in stratification have profound effects on the vertical structure and transport of suspended matter. During ebb, weak vertical mixing allows suspended material to settle downward. In contrast, strong turbulence during flood mixes suspended matter into the water column where it is transported up estuary. Furthermore, observations reveal that resuspension events are marked by multiple turbidity spikes, suggestive of multiple, limited layers of erodible material. The transport of the turbid waters is consistent with horizontal advection modified by horizontal dispersion. Periods of enhanced stratification are also marked by relatively low levels of turbidity during the ebb, consistent with more complete settling of suspended material following times of high river discharge. The interplay between buoyancy and tidal asymmetries are further elucidated with a onedimensional numerical model featuring a turbulent closure scheme and a passively settling tracer. Model results are generally consistent with the field observations, both emphasizing the robust particle trapping tendencies of a stratified flood-dominated estuary. We speculate that enhanced particle trapping following times of high river discharge may have important biological consequences

Tidal and spring-neap variations in horizontal dispersion in a partially mixed estuary

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 113 (2008): C07023, doi:10.1029/2007JC004644.A sequence of dye releases in the Hudson River estuary provide a quantitative assessment of horizontal dispersion in a partially mixed estuary. Dye was released in the bottom boundary layer on 4 separate occasions, with varying tidal phase and spring-neap conditions. The three-dimensional distribution of dye was monitored by two vessels with in situ, profiling fluorometers. The three-dimensional spreading of the dye was estimated by calculating the time derivative of the second moment of the dye in the along-estuary, cross-estuary and vertical directions. The average along-estuary dispersion rate was about 100 m2/s, but maximum rates up to 700 m2/s occurred during ebb tides, and minimum rates occurred during flood. Vertical shear dispersion was the principal mechanism during neap tides, but transverse shear dispersion became more important during springs. Suppression of mixing across the pycnocline limited the vertical extent of the patch in all but the maximum spring-tide conditions, with vertical diffusivities in the pycnocline estimated at 4 × 10−5 m2/s during neaps. The limited vertical extent of the dye patch limited the dispersion of the dye relative to the overall estuarine dispersion rate, which was an order of magnitude greater than that of the dye. This study indicates that the effective dispersion of waterborne material in an estuary depends sensitively on its vertical distribution as well as the phase of the spring-neap cycle.This research was supported by National Science Foundation Grant OCE04-52054 (W. Geyer), OCE00-99310 (R. Houghton), and OCE00-95913 (R. Chant)

Effects of locally generated wind waves on the momentum budget and subtidal exchange in a coastal plain estuary

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Oceans 124(2), (2019):1005-1028, doi:10.1029/2018JC014585.A numerical model with a vortex force formalism is used to study the role of wind waves in the momentum budget and subtidal exchange of a shallow coastal plain estuary, Delaware Bay. Wave height and age in the bay have a spatial distribution that is controlled by bathymetry and fetch, with implications for the surface drag coefficient in young, underdeveloped seas. Inclusion of waves in the model leads to increases in the surface drag coefficient by up to 30% with respect to parameterizations in which surface drag is only a function of wind speed, in agreement with recent observations of air‐sea fluxes in estuaries. The model was modified to prevent whitecapping wave dissipation from generating breaking forces since that contribution is integrally equivalent to the wind stress. The proposed adjustment is consistent with previous studies of wave‐induced nearshore currents and with additional parameterizations for breaking forces in the model. The mean momentum balance during a simulated wind event was mainly between the pressure gradient force and surface stress, with negligible contributions by vortex, wave breaking (i.e., depth‐induced), and Stokes‐Coriolis forces. Modeled scenarios with realistic Delaware bathymetry suggest that the subtidal bay‐ocean exchange at storm time scales is sensitive to wave‐induced surface drag coefficient, wind direction, and mass transport due to the Stokes drift. Results herein are applicable to shallow coastal systems where the typical wave field is young (i.e., wind seas) and modulated by bathymetry.This work was supported by National Science Foundation Coastal SEES grant 1325136. We acknowledge Christopher Sommerfield's Group, Jia‐Lin Chen, and Julia Levin who provided assistance with the model configuration. We also thank Nirnimesh Kumar, Greg Gerbi, Melissa Moulton, and the Rutgers Ocean Modeling group for constructive feedback. Insightful comments by two anonymous reviewers helped improve the manuscript. Model files are available in an open access repository (https://doi.org/10.5281/zenodo.1695900).2019-07-2

Impact of Offshore Winds on a Buoyant River Plume System

Idealized numerical simulations utilizing the Regional Ocean Modeling System (ROMS) are carried out to examine the response of buoyant river plume systems to offshore-directed wind stresses. It is found that after a few inertial periods of wind forcing the plume becomes detached from the coast and reaches a steady state in terms of the plume’s offshore position, width, and plume-averaged depth, salinity, and velocity. The steady-state offshore position of the plume is a balance between the cross-shore advection driven by the estuarine outflow and the alongshore advection driven by the Ekman velocities, and is described using the ratio of the outflow Froude number and the plume Froude number. The steady-state salinity structure is maintained by a balance between the cross-shore advection of salt creating stratification, the turbulent vertical mixing, and the downstream transport of freshwater continually resetting the system. Plume mixing is also analyzed using a salinity coordinate system to track the changes in freshwater volume in salinity space and time. A dynamical plume region classification is developed with use of a Richardson number–based critical mixing salinity criterion in salinity space. This salinity class–based classification agrees well with a classification based on an alongshore analysis of the salt flux equation. For this classification the near field is dominated by large cross-shore fluxes and the midfield by a diminishing cross-shore salt flux, and in the far field there is a balance between the alongshore salt flux and turbulent mixing

Wave generation, dissipation, and disequilibrium in an embayment with complex bathymetry

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Wave generation, dissipation, and disequilibrium in an embayment with complex bathymetry. Journal of Geophysical Research-Oceans, 123(11), (2018): 7856-7876, doi:10.1029/2018JC014381.Heterogeneous, sharply varying bathymetry is common in estuaries and embayments, and complex interactions between the bathymetry and wave processes fundamentally alter the distribution of wave energy. The mechanisms that control the generation and dissipation of wind waves in an embayment with heterogeneous, sharply varying bathymetry are evaluated with an observational and numerical study of the Delaware Estuary. Waves in the lower bay depend on both local wind forcing and remote wave forcing from offshore, but elsewhere in the estuary waves are controlled by the local winds and the response of the wavefield to bathymetric variability. Differences in the wavefield with wind direction highlight the impacts of heterogeneous bathymetry and limited fetch. Under the typical winter northwest wind conditions waves are fetch‐limited in the middle estuary and reach equilibrium with local water depth only in the lower bay. During southerly wind conditions typical of storms, wave energy is near equilibrium in the lower bay, and midestuary waves are attenuated by the combination of whitecapping and bottom friction, particularly over the steep, longitudinal shoals. Although the energy dissipation due to bottom friction is generally small relative to whitecapping, it becomes significant where the waves shoal abruptly due to steep bottom topography. In contrast, directional spreading keeps wave heights in the main channel significantly less than local equilibrium. The wave disequilibrium in the deep navigational channel explains why the marked increase in depth by dredging of the modern channel has had little impact on wave conditions.Funding was provided by National Science Foundation Coastal SEES: Toward Sustainable Urban Estuaries in the Anthropocene (OCE 1325136) and Ministry of Science and Technology (MOST 107‐2611‐M‐006‐004). We thank James Kirby, Fengyan Shi, and the two anonymous reviewers for their careful reading of our manuscript and their insightful comments. We thank Tracy Quirk for providing wave measurements in Bombay Hook, DE and Stow Creek, NJ. We thank Katie Pijanowski for compiling historical and modern bathymetric data for the estuary. Data supporting this study are posted to Zenodo (http://doi.org/10.5281/zenodo.1433055).2019-04-0

Turbulent mixing in a far‐field plume during the transition to upwelling conditions : microstructure observations from an AUV

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 45 (2018): 9765-9773, doi:10.1029/2018GL078543.A REMUS 600 autonomous underwater vehicle was used to measure turbulent mixing within the far‐field Chesapeake Bay plume during the transition to upwelling. Prior to the onset of upwelling, the plume was mixed by a combination of energetic downwelling winds and bottom‐generated shear resulting in a two‐layer plume structure. Estimates of turbulent dissipation and buoyancy flux from a nose‐mounted microstructure system indicate that scalar exchange within the plume was patchy and transient, with direct wind mixing constrained to the near surface by stratification within the plume. Changing wind and tide conditions contributed to temporal variability. Following the separation of the upper plume from the coast, alongshore shear became a significant driver of mixing on the shoreward edge of the plume.NSF Grant Numbers: OCE‐1334231, OCE‐1745258, OCE‐13343982019-03-2

Estuarine boundary layer mixing processes : insights from dye experiments

Author Posting. © American Meteorological Society, 2007. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 37 (2007): 1859-1877, doi:10.1175/jpo3088.1.A series of dye releases in the Hudson River estuary elucidated diapycnal mixing rates and temporal variability over tidal and fortnightly time scales. Dye was injected in the bottom boundary layer for each of four releases during different phases of the tide and of the spring–neap cycle. Diapycnal mixing occurs primarily through entrainment that is driven by shear production in the bottom boundary layer. On flood the dye extended vertically through the bottom mixed layer, and its concentration decreased abruptly near the base of the pycnocline, usually at a height corresponding to a velocity maximum. Boundary layer growth is consistent with a one-dimensional, stress-driven entrainment model. A model was developed for the vertical structure of the vertical eddy viscosity in the flood tide boundary layer that is proportional to u2*/N∞, where u* and N∞ are the bottom friction velocity and buoyancy frequency above the boundary layer. The model also predicts that the buoyancy flux averaged over the bottom boundary layer is equal to 0.06N∞u2* or, based on the structure of the boundary layer equal to 0.1NBLu2*, where NBL is the buoyancy frequency across the flood-tide boundary layer. Estimates of shear production and buoyancy flux indicate that the flux Richardson number in the flood-tide boundary layer is 0.1–0.18, consistent with the model indicating that the flux Richardson number is between 0.1 and 0.14. During ebb, the boundary layer was more stratified, and its vertical extent was not as sharply delineated as in the flood. During neap tide the rate of mixing during ebb was significantly weaker than on flood, owing to reduced bottom stress and stabilization by stratification. As tidal amplitude increased ebb mixing increased and more closely resembled the boundary layer entrainment process observed during the flood. Tidal straining modestly increased the entrainment rate during the flood, and it restratified the boundary layer and inhibited mixing during the ebb.The work was supported by the National Science Foundation Grant OCE00-95972 (W. Geyer, J. Lerczak), OCE00-99310 (R. Houghton), and OCE00-95913 (R. Chant, E. Hunter)

Vertical distribution of bivalve larvae along a cross-shelf transect during summer upwelling and downwelling

Abstract Previous time-series studies of meroplankton abundances in the LEO-15 research area off Tuckerton, New Jersey, USA (39°28¢N, 74°15¢W) indicated shortlived (6-12 h) pulses in larval surfclam (Spisula solidissima Dillwyn) concentration often associated with the initiation of downwelling. To examine possible larval surfclam (and other bivalve) concentrating mechanisms during upwelling and downwelling, six sets of adaptive mobile zooplankton pump samples were taken in July 1998 at different depths at five to six stations along a 25-km transect perpendicular to the coastline and crossing Beach Haven Ridge at LEO-15. Sampling was guided by near real-time, satellite imagery of sea surface temperature overlain by sea surface currents from a shore-based ocean surface current radar (OSCR) unit. A Seabird CTD on the mobile pump frame near the intake provided information on thermocline depth, and sampling depths were adjusted according to the temperature profiles. Near shore, the thermocline was tilted down during downwelling, and up during upwelling. The highest concentrations of surfclam larvae occurred near the bottom at a station near Beach Haven Ridge during downwelling, and just above the thermocline 3 km further offshore during well-developed upwelling. For other bivalve taxa, the larvae were concentrated near the thermocline (Anomia simplex Orbigny and Pholadidae spp.) or concentrated upslope near the bottom (Mytilidae spp.) during upwelling, and the larvae were concentrated near the bottom or were moved downslope during downwelling. Donax fossor Say larvae were found near the surface or above the thermocline during upwelling and downwelling. The general patterns of larval bivalve distribution appear to be influenced by water mass movement during upwelling and downwelling. The larval concentration patterns of individual species are likely a consequence of advection due to upwelling and downwelling circulation, vertical shear in the front region, species-specific larval behaviors, and larval sources

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

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

situ particle size distributions and volume concentrations from a LISST-100 laser particle sizer and a digital floc

Abstract A LISST-100 in situ laser particle sizer was deployed together with a digital floc camera during field work in the Newark Bay area (USA) and along the Apennine margin (the Adriatic Sea, Italy). The purpose of these simultaneous deployments was to investigate how well in situ particle (floc) sizes and volume concentrations from the two different instruments compared. In the Adriatic Sea the two instruments displayed the same temporal variation, but the LISST provided lower estimates of floc size by a factor of 2-3, compared to the DFC. In the Newark Bay area, the LISST provided higher values of floc size by up to a factor of 2. When floc size was computed using only the overlapping size bins from the two instruments the discrepancy disappeared. The reason for the discrepancy in size was found to be related to several issues: First, the LISST measured particles in the 2.5-500 mm range, whereas the camera measured particles in the 135-9900 mm range, so generally the LISST should provide lower estimates of floc size, as it measures the smaller particles. Second, in the Newark Bay area scattering from particles 4500 mm generally caused the LISST to overestimate the volume of particles in its largest size bin, thereby increasing apparent floc size. Relative to the camera, the LISST generally provided estimates of total floc volume that were lower by a factor of 3. Factors that could explain this discrepancy are errors arising from the accuracy of the LISST volume conversion coefficient and image processing. Regardless of these discrepancies, the shapes of the size spectra from the instruments were similar in the regions of overlap and could be matched by multiplying with an appropriate correction coefficient. This facilitated merging of the size spectra from the LISST and the DFC, yielding size spectra in the 2.5-9900 mm range. The merged size spectra generally had one or more peaks in the coarse end of the spectrum, presumably due to the presence of flocs. The fine end (o100 mm) of the spectrum displayed a flat tail with equal concentration of particles in all size classes. Size spectra with this shape indicate that the classical Junge model for description of in situ particle size spectra is reasonable for particles smaller than 100 mm but not for larger particles. Floc fraction was computed for the merged spectra by using a diameter-to-mass conversion and found to vary between 0.34 and 0.95, within the range reported by other authors.