Wind-wave effects on estuarine turbulence : a comparison of observations and second-moment closure predictions

Author Posting. © American Meteorological Society, 2018. 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 48 (2018): 905-923, doi:10.1175/JPO-D-17-0133.1.Observations of turbulent kinetic energy, dissipation, and turbulent stress were collected in the middle reaches of Chesapeake Bay and were used to assess second-moment closure predictions of turbulence generated beneath breaking waves. Dissipation scaling indicates that the turbulent flow structure observed during a 10-day wind event was dominated by a three-layer response that consisted of 1) a wave transport layer, 2) a surface log layer, and 3) a tidal, bottom boundary layer limited by stable stratification. Below the wave transport layer, turbulent mixing was limited by stable stratification. Within the wave transport layer, where dissipation was balanced by a divergence in the vertical turbulent kinetic energy flux, the eddy viscosity was significantly underestimated by second-moment turbulence closure models, suggesting that breaking waves homogenized the mixed surface layer to a greater extent than the simple model of TKE diffusing away from a source at the surface. While the turbulent transport of TKE occurred largely downgradient, the intermittent downward sweeps of momentum generated by breaking waves occurred largely independent of the mean shear. The underprediction of stress in the wave transport layer by second-moment closures was likely due to the inability of the eddy viscosity model to capture the nonlocal turbulent transport of the momentum flux beneath breaking waves. Finally, the authors hypothesize that large-scale coherent turbulent eddies played a significant role in transporting momentum generated near the surface to depth.This work was supported by National Science Foundation Grants OCE-1061609 and OCE-1339032.2018-10-1

Effects of hard clam (Mercenaria mercenaria) density and bottom shear stress on cohesive sediment erodibility and implications for benthic-pelagic coupling

The interacting effects of little neck hard clam (Mercenaria mercenaria) density and bottom shear stress on cohesive sediment erodibility were investigated. Short-term stepwise erosion experiments in 30 and 40 cm diameter Gust microcosms over a range of 0.0083 to 0.1932 Pa were performed using sequential 20-minute constant shear stress steps while sampling turbidity regularly. In addition, sediment erodibility was monitored in two one-month long ecosystem experiments with tidal resuspension and 0, 10, and 50 hard clams in 1 m3 shear turbulence resuspension mesocosms (STURM) with an initial stepwise erosion experiment (0.313 to 0.444 Pa). In short-term erosion experiments, a low density of hard clams did not significantly affect sediment erodibility, but a high density of hard clams destabilized muddy sediments through significantly decreased critical shear stresses and higher erosion rates, resulting in higher cumulative suspended mass (CSM). In long-term erosion experiments, the sediment stabilized over time between treatments and decreased to a CSM of approximately 60 g m–2 with different densities of hard clams. This was likely due to development of microphytobenthos, mediated by the filter-feeding clams, bottom shear stress and increased light. Bioturbation by a dense bed of hard clams in interaction with infrequent high bottom shear due to storms may increase CSM in the water column, with subsequent direct and indirect effects on the ecosystem. However, more controlled longer-term erosion studies to determine the interacting effects of long-term exposure to high bottom shear stress, benthos, and microphytobenthos on sediment erodibility and benthic-pelagic coupling are needed

Covariability of dissolved oxygen with physical processes in the summertime Chesapeake Bay

Long, rapidly sampled time series measurements of dissolved oxygen, temperature, salinity, currents, winds, tides, and insolation were collected during the summer of 1987 across the mesohaline Chesapeake Bay. Analyses of the data show that short term variability of dissolved oxygen was both large and spatially heterogeneous. Time scales of variability ranged from the longest period fluctuations resolved (several days) to the sampling interval (several minutes). The largest variability was associated with large amplitude, wind and tide forced lateral internal oscillations of the pycnocline in the mainstem of the Bay. These resulted in advection of saline, hypoxic water from below the pycnocline onto the flanks of the Bay and into the lower reaches of the Choptank River, an adjoining tributary estuary. Advective variability at higher frequencies was likely due to internal waves, internal mixing, and/or spatial patchiness. Dissolved oxygen also responded to the daily cycle of insolation, but lagged insolation by at least 90° (6 h). Advective variability of dissolved oxygen is implicated as an important characteristic of the majority of summertime benthic environments in the mesohaline Chesapeake Bay and lower reaches of adjoining tributaries

Surface wave effects on the translation of wind stress across the air–sea interface in a fetch-limited, coastal embayment

Author Posting. © American Meteorological Society, 2017. 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 47 (2017): 1921-1939, doi:10.1175/JPO-D-16-0146.1.The role of surface gravity waves in structuring the air–sea momentum flux is examined in the middle reaches of Chesapeake Bay. Observed wave spectra showed that wave direction in Chesapeake Bay is strongly correlated with basin geometry. Waves preferentially developed in the direction of maximum fetch, suggesting that dominant wave frequencies may be commonly and persistently misaligned with local wind forcing. Direct observations from an ultrasonic anemometer and vertical array of ADVs show that the magnitude and direction of stress changed across the air–sea interface, suggesting that a stress divergence occurred at or near the water surface. Using a numerical wave model in combination with direct flux measurements, the air–sea momentum flux was partitioned between the surface wave field and the mean flow. Results indicate that the surface wave field can store or release a significant fraction of the total momentum flux depending on the direction of the wind. When wind blew across dominant fetch axes, the generation of short gravity waves stored as much as 40% of the total wind stress. Accounting for the storage of momentum in the surface wave field closed the air–sea momentum budget. Agreement between the direction of Lagrangian shear and the direction of the stress vector in the mixed surface layer suggests that the observed directional difference was due to the combined effect of breaking waves producing downward sweeps of momentum in the direction of wave propagation and the straining of that vorticity field in a manner similar to Langmuir turbulence.This work was supported by National Science Foundation Grants OCE-1061609 and OCE-1339032.2018-01-1

Added Value of Combining Multiple Optical and Acoustic Instruments When Characterizing Fine-Grained Estuarine Suspensions

Various optical and acoustic instruments have specific advantages and limitations for characterizing suspensions, and when used together more information can be obtained than with one instrument alone. The LISST 100X, for example, is a powerful tool for estimating particle size distribution, but because of the inversion method used to determine the size distribution, it is difficult to distinguish two dominate populations that peak close to one another, especially among larger grain sizes. In the York River estuary, VA, additional information obtained through the deployment of a RIPScam camera system and an ADV along with the LISST 100X allowed differentiation between populations of resilient pellets and flocs in suspension close to the bed and how the populations varied over a tidal cycle. A second example of instrument pairing providing additional information was the use of a PICS video imaging system in the York River to verify the conditions under which use of the ADV Reynolds flux method was valid for estimating settling velocity of suspended particle populations

Characterization and modulation of Langmuir circulation in Chesapeake Bay

Author Posting. © American Meteorological Society, 2015. 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 45 (2015): 2621–2639, doi:10.1175/JPO-D-14-0239.1.Measurements made as part of a large-scale experiment to examine wind-driven circulation and mixing in Chesapeake Bay demonstrate that circulations consistent with Langmuir circulation play an important role in surface boundary layer dynamics. Under conditions when the turbulent Langmuir number Lat is low (<0.5), the surface mixed layer is characterized by 1) elevated vertical turbulent kinetic energy; 2) decreased anisotropy; 3) negative vertical velocity skewness indicative of strong/narrow downwelling and weak/broad upwelling; and 4) strong negative correlations between low-frequency vertical velocity and the velocity in the direction of wave propagation. These characteristics appear to be primarily the result of the vortex force associated with the surface wave field, but convection driven by a destabilizing heat flux is observed and appears to contribute significantly to the observed negative vertical velocity skewness. Conditions that favor convection usually also have strong Langmuir forcing, and these two processes probably both contribute to the surface mixed layer turbulence. Conditions in which traditional stress-driven turbulence is important are limited in this dataset. Unlike other shallow coastal systems where full water column Langmuir circulation has been observed, the salinity stratification in Chesapeake Bay is nearly always strong enough to prevent full-depth circulation from developing.The funding for this research was provided by the National Science Foundation Grants OCE-1339032 and OCE-1338518.2016-04-0

Modeling the impact of floating oyster (Crassostrea virginica) aquaculture on sediment−water nutrient and oxygen fluxes

Bivalve aquaculture relies on naturally occurring phytoplankton, zooplankton, and detritus as food sources, thereby avoiding external nutrient inputs that are commonly associated with finfish aquaculture. High filtration rates and concentrated bivalve biomass within aquacul- ture operations, however, result in intense biodeposition of particulate organic matter (POM) on surrounding sediments, with potential adverse environmental impacts. Estimating the net deposi- tional flux is difficult in shallow waters due to methodological constraints and dynamic processes such as resuspension and advection. In this study, we combined sediment trap deployments with simulations from a mechanistic sediment flux model to estimate seasonal POM deposition, resus- pension, and processing within sediments in the vicinity of an eastern oyster Crassostrea virginica farm in the Choptank River, Maryland, USA. The model is the stand-alone version of a 2-layer sediment flux model currently implemented within larger models for understanding ecosystem responses to nutrient management. Modeled sediment−water fluxes were compared to observed denitrification rates and nitrite + nitrate (NO2 −+NO3 −), phosphate (PO4 3−) and dissolved O2 fluxes. Model-derived estimates of POM deposition, which represent POM incorporated and processed within the sediment, comprised a small fraction of the material collected in sediment traps. These results highlight the roles of biodeposit resuspension and transport in effectively removing oyster biodeposits away from this particular farm, resulting in a highly diminished local environmental impact. This study highlights the value of sediment models as a practical tool for computing inte- grated measures of nitrogen cycling as a function of seasonal dynamics in the vicinity of aquaculture operations

Lateral circulation and sediment transport driven by axial winds in an idealized, partially mixed estuary

Author Posting. © American Geophysical Union, 2009. 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 114 (2009): C12006, doi:10.1029/2008JC005014.A 3D hydrodynamic model (ROMS) is used to investigate lateral circulation in a partially mixed estuary driven by axial wind events and to explore the associated transport of sediments. The channel is straight with a triangular cross section. The model results suggest that driving mechanisms for lateral circulation during axial wind events are different between stratified and unstratified conditions. When the water column is largely unstratified, rotational effects do not drive significant lateral circulation. Instead, differential advection of the axial salinity gradient by wind-driven axial flow is responsible for regulating the lateral salinity gradients that in turn drive bottom-divergent/convergent lateral circulation during down/up-estuary winds. From the subtidal lateral salt balance, it is found that the development of lateral salinity gradient by wind-induced differential advection is largely counterbalanced by the advection of salt by lateral circulation itself. When the water column is stratified, the lateral flow and salinity structures below the halocline closely resemble those driven by boundary mixing, and rotational effects are important. Lateral sediment flux and the event-integrated sediment transport are from channel to shoals during down-estuary winds but reversed for up-estuary winds. Potential impacts of wind-generated waves on lateral sediment transport are evaluated with two cases representing event conditions typical of upper Chesapeake Bay. Accounting for wind wave effects results in an order of magnitude increase in lateral sediment fluxes because of greater bottom stresses and sediment resuspension.Financial support from ONR through the Community Sediment Transport Modeling (CSTM) project

Surgical versus nonsurgical treatment for lumbar degenerative spondylolisthesis.

BACKGROUND: Management of degenerative spondylolisthesis with spinal stenosis is controversial. Surgery is widely used, but its effectiveness in comparison with that of nonsurgical treatment has not been demonstrated in controlled trials. METHODS: Surgical candidates from 13 centers in 11 U.S. states who had at least 12 weeks of symptoms and image-confirmed degenerative spondylolisthesis were offered enrollment in a randomized cohort or an observational cohort. Treatment was standard decompressive laminectomy (with or without fusion) or usual nonsurgical care. The primary outcome measures were the Medical Outcomes Study 36-Item Short-Form General Health Survey (SF-36) bodily pain and physical function scores (100-point scales, with higher scores indicating less severe symptoms) and the modified Oswestry Disability Index (100-point scale, with lower scores indicating less severe symptoms) at 6 weeks, 3 months, 6 months, 1 year, and 2 years. RESULTS: We enrolled 304 patients in the randomized cohort and 303 in the observational cohort. The baseline characteristics of the two cohorts were similar. The one-year crossover rates were high in the randomized cohort (approximately 40% in each direction) but moderate in the observational cohort (17% crossover to surgery and 3% crossover to nonsurgical care). The intention-to-treat analysis for the randomized cohort showed no statistically significant effects for the primary outcomes. The as-treated analysis for both cohorts combined showed a significant advantage for surgery at 3 months that increased at 1 year and diminished only slightly at 2 years. The treatment effects at 2 years were 18.1 for bodily pain (95% confidence interval [CI], 14.5 to 21.7), 18.3 for physical function (95% CI, 14.6 to 21.9), and -16.7 for the Oswestry Disability Index (95% CI, -19.5 to -13.9). There was little evidence of harm from either treatment. CONCLUSIONS: In nonrandomized as-treated comparisons with careful control for potentially confounding baseline factors, patients with degenerative spondylolisthesis and spinal stenosis treated surgically showed substantially greater improvement in pain and function during a period of 2 years than patients treated nonsurgically. (ClinicalTrials.gov number, NCT00000409 [ClinicalTrials.gov].)