Friedrichs Carl T.

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Carl T.

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  • Article
    Estimating hypoxic volume in the Chesapeake Bay using two continuously sampled oxygen profiles
    (John Wiley & Sons, 2018-09-12) Bever, Aaron J. ; Friedrichs, Marjorie A. M. ; Friedrichs, Carl T. ; Scully, Malcolm E.
    Low levels of dissolved oxygen (DO) occur in many embayments throughout the world and have numerous detrimental effects on biota. Although measurement of in situ DO is straightforward with modern instrumentation, quantifying the volume of water in a given embayment that is hypoxic (hypoxic volume (HV)) is a more difficult task; however, this information is critical for determining whether management efforts to increase DO are having an overall impact. This paper uses output from a three‐dimensional numerical model to demonstrate that HV in Chesapeake Bay can be estimated well with as few as two vertical profiles. In addition, the cumulative hypoxic volume (HVC; the total amount of hypoxia in a given year) can be calculated with relatively low uncertainty (<10%) if continuous DO data are available from two strategically positioned vertical profiles. This is because HV in the Chesapeake Bay is strongly constrained by the geometry of the embayment. A simple Geometric HV calculation method is presented and numerical model results are used to illustrate that for calculating HVC, the results using two daily‐averaged profiles are typically more accurate than those of the standard method that interpolates bimonthly cruise data. Bimonthly data produce less accurate estimates of HVC because high‐frequency changes in oxygen concentration, for example, due to regional‐weather‐ or storm‐induced changes in wind direction and magnitude, are not resolved. The advantages of supplementing cruise‐based sampling with continuous vertical profiles to estimate HVC should be applicable to other systems where hypoxic water is constrained to a specific area by bathymetry.
  • Article
    Tidal variation in cohesive sediment distribution and sensitivity to flocculation and bed consolidation in an idealized, partially mixed estuary
    (MDPI, 2019-09-25) Tarpley, Danielle R.N. ; Harris, Courtney K. ; Friedrichs, Carl T. ; Sherwood, Christopher R.
    Particle settling velocity and erodibility are key factors that govern the transport of sediment through coastal environments including estuaries. These are difficult to parameterize in models that represent mud, whose properties can change in response to many factors, including tidally varying suspended sediment concentration (SSC) and shear stress. Using the COAWST (Coupled Ocean-Atmosphere-Wave-Sediment Transport) model framework, we implemented bed consolidation, sediment-induced stratification, and flocculation formulations within an idealized two-dimensional domain that represented the longitudinal dimension of a micro-tidal, muddy, partially mixed estuary. Within the Estuarine Turbidity Maximum (ETM), SSC and median floc diameter varied by a factor of four over the tidal cycle. Downstream of the ETM, the median floc size and SSC were several times smaller and showed less tidal variation (~20% or less). The suspended floc distributions only reached an equilibrium size as a function of SSC and shear in the ETM at peak tidal flow. In general, flocculation increased particle size, which reduced SSC by half in the ETM through increased settling velocity. Consolidation also limited SSC by reduced resuspension, which then limited floc growth through reduced SSC by half outside of the ETM. Sediment-induced stratification had negligible effects in the parameter space examined. Efforts to lessen the computation cost of the flocculation routine by reducing the number of size classes proved difficult; floc size distribution and SSC were sensitive to specification of size classes by factors of 60% and 300%, respectively.
  • Technical Report
    Pressure/Temperature Logger (PTL) development and field deployment for the Great Bay, NH, tidal dynamics experiment
    (Woods Hole Oceanographic Institution, 1998-01) Friedrichs, Carl T. ; Spencer, Wayne D. ; Aubrey, David G.
    Durg 1992 and 1993 experiments were conducted in the shallow east side of Great Bay, New Hampshire. These experiments were conducted to better understand the morphodynamics and evolutionary tendencies of shallow tidal embayments and intertidal fiats. Hardware and software used in the collection of data are described. Discussed also are techniques used to collect data. Six pressure temperature loggers (PTL) and one current meter (TCSWG) were developed for the experiments. Both instruments are internally powered and internally recording. The instruments were developed because no company was found that manufactured a similar instrument within the price range of the project.
  • Article
    Model behavior and sensitivity in an application of the Cohesive Bed Component of the Community Sediment Transport Modeling System for the York River estuary, VA, USA
    (MDPI AG, 2014-05-19) Fall, Kelsey A. ; Harris, Courtney K. ; Friedrichs, Carl T. ; Rinehimer, J. Paul ; Sherwood, Christopher R.
    The Community Sediment Transport Modeling System (CSTMS) cohesive bed sub-model that accounts for erosion, deposition, consolidation, and swelling was implemented in a three-dimensional domain to represent the York River estuary, Virginia. The objectives of this paper are to (1) describe the application of the three-dimensional hydrodynamic York Cohesive Bed Model, (2) compare calculations to observations, and (3) investigate sensitivities of the cohesive bed sub-model to user-defined parameters. Model results for summer 2007 showed good agreement with tidal-phase averaged estimates of sediment concentration, bed stress, and current velocity derived from Acoustic Doppler Velocimeter (ADV) field measurements. An important step in implementing the cohesive bed model was specification of both the initial and equilibrium critical shear stress profiles, in addition to choosing other parameters like the consolidation and swelling timescales. This model promises to be a useful tool for investigating the fundamental controls on bed erodibility and settling velocity in the York River, a classical muddy estuary, provided that appropriate data exists to inform the choice of model parameters.
  • Technical Report
    Hydrodynamics and morphodynamics of shallow tidal channels and intertidal flats
    (Woods Hole Oceanographic Institution, 1993-02) Friedrichs, Carl T.
    In this thesis, mechanisms which control morphodynamics of shallow tidal embayments are investigated analytically. In the process of exploring these mechanisms (specifically asymmetries in bottom stress, τ), basis momentum and mass balances which govern flow in these systems are clarified. Temporal asymmetries in τ are investigated via a new perturbation scheme which quantifies nonlinear processes and combines geometric controls on asymmetry into a single non-dimensional parameter. Implications of spatial asymmetries in τ are investigated though stability criteria based on a uniform distribution of τ. Morphologic observations of both tidal channels and intertidal flats are consistent with a unifonn distribution of τ at equilibrium. Investigation of morphodynamic mechanisms leads to scalings of momentum and continuity which diverge from classical models. Scalings for prismatic channels with strong tidal asymmetries indicate friction often dominates acceleration in the momentum equation. The resulting "zero-inertia" balance gives a time-varing diffusion equation which requires along-channel amplitude to decay. Uniform τ justifies a new scaling of continuity for exponentially-shaped channels. In such channels, along-channel gradients in tidal velocity are small and are often dominated by gradients in cross-sectional area. The resulting first-order wave equation allows only constant amplitude, forward propagating waveforms which are independent of channel length.
  • Article
    Effects of density-driven flows on the long-term morphodynamic evolution of funnel-shaped estuaries
    (The Authors, 2018-10-13) Olabarrieta, Maitane ; Geyer, W. Rockwell ; Coco, Giovanni ; Friedrichs, Carl T. ; Cao, Zhendong
    Subtidal flows driven by density gradients affect the tide‐averaged sediment transport in estuaries and, therefore, can influence their long‐term morphodynamic evolution. The three‐dimensional Coupled Ocean‐Atmosphere‐Wave‐Sediment Transport modeling system is applied to numerically analyze the effects of baroclinicity and Earth's rotation on the long‐term morphodynamic evolution of idealized funnel‐shaped estuaries. The morphodynamic evolution in all the analyzed cases reproduced structures identified in many tide‐dominated estuaries: a meandering region in the fluvial‐tidal transition zone, a tidal maximum area close to the head, and a turbidity maxima region in the brackish zone. As the morphology of the estuaries evolved, the tidal propagation (including its asymmetry), the salinity gradient, and the strength of subtidal flows changed, which reflects the strong bathymetric control of these systems. The comparison with barotropic simulations showed that the three‐dimensional structure of the flow (induced by density gradients) has leading order effects on the morphodynamic evolution. Density gradient‐driven subtidal flows (1) promote near‐bed flood dominance and, consequently, the import of sediment into the estuary, (2) accelerate the morphodynamic evolution of the upper/middle estuary, (3) promote a more concave shape of the upper estuary and reduce the ebb‐tidal delta volume, and (4) produce an asymmetric bathymetry and inhibit the formation of alternate bars that would form under barotropic conditions. This latter effect is the consequence of the combined effect of Earth's rotation and baroclinicity.
  • Article
    Challenges associated with modeling low-oxygen waters in Chesapeake Bay : a multiple model comparison
    (Copernicus Publications on behalf of the European Geosciences Union, 2016-04-06) Irby, Isaac D. ; Friedrichs, Marjorie A. M. ; Friedrichs, Carl T. ; Bever, Aaron J. ; Hood, Raleigh R. ; Lanerolle, Lyon W. J. ; Li, Ming ; Linker, Lewis ; Scully, Malcolm E. ; Sellner, Kevin G. ; Shen, Jian ; Testa, Jeremy M. ; Wang, Hao ; Wang, Ping ; Xia, Meng
    As three-dimensional (3-D) aquatic ecosystem models are used more frequently for operational water quality forecasts and ecological management decisions, it is important to understand the relative strengths and limitations of existing 3-D models of varying spatial resolution and biogeochemical complexity. To this end, 2-year simulations of the Chesapeake Bay from eight hydrodynamic-oxygen models have been statistically compared to each other and to historical monitoring data. Results show that although models have difficulty resolving the variables typically thought to be the main drivers of dissolved oxygen variability (stratification, nutrients, and chlorophyll), all eight models have significant skill in reproducing the mean and seasonal variability of dissolved oxygen. In addition, models with constant net respiration rates independent of nutrient supply and temperature reproduced observed dissolved oxygen concentrations about as well as much more complex, nutrient-dependent biogeochemical models. This finding has significant ramifications for short-term hypoxia forecasts in the Chesapeake Bay, which may be possible with very simple oxygen parameterizations, in contrast to the more complex full biogeochemical models required for scenario-based forecasting. However, models have difficulty simulating correct density and oxygen mixed layer depths, which are important ecologically in terms of habitat compression. Observations indicate a much stronger correlation between the depths of the top of the pycnocline and oxycline than between their maximum vertical gradients, highlighting the importance of the mixing depth in defining the region of aerobic habitat in the Chesapeake Bay when low-oxygen bottom waters are present. Improvement in hypoxia simulations will thus depend more on the ability of models to reproduce the correct mean and variability of the depth of the physically driven surface mixed layer than the precise magnitude of the vertical density gradient.
  • Article
    Introduction to special section on The U.S. IOOS Coastal and Ocean Modeling Testbed
    (John Wiley & Sons, 2013-12-11) Luettich, Richard A. ; Wright, L. Donelson ; Signell, Richard P. ; Friedrichs, Carl T. ; Friedrichs, Marjorie A. M. ; Harding, John ; Fennel, Katja ; Howlett, Eoin ; Graves, Sara J. ; Smith, Elizabeth ; Crane, Gary ; Baltes, Rebecca
    Strong and strategic collaborations among experts from academia, federal operational centers, and industry have been forged to create a U.S. IOOS Coastal and Ocean Modeling Testbed (COMT). The COMT mission is to accelerate the transition of scientific and technical advances from the coastal and ocean modeling research community to improved operational ocean products and services. This is achieved via the evaluation of existing technology or the development of new technology depending on the status of technology within the research community. The initial phase of the COMT has addressed three coastal and ocean prediction challenges of great societal importance: estuarine hypoxia, shelf hypoxia, and coastal inundation. A fourth effort concentrated on providing and refining the cyberinfrastructure and cyber tools to support the modeling work and to advance interoperability and community access to the COMT archive. This paper presents an overview of the initiation of the COMT, the findings of each team and a discussion of the role of the COMT in research to operations and its interface with the coastal and ocean modeling community in general. Detailed technical results are presented in the accompanying series of 16 technical papers in this special issue.
  • Preprint
    Susceptibility of salt marshes to nutrient enrichment and predator removal
    ( 2006-03-15) Deegan, Linda A. ; Bowen, Jennifer L. ; Drake, Deanne C. ; Fleeger, John W. ; Friedrichs, Carl T. ; Galvan, Kari A. ; Hobbie, John E. ; Hopkinson, Charles S. ; Johnson, J. Michael ; Johnson, David S. ; LeMay, Lynsey E. ; Miller, Erin ; Peterson, Bruce J. ; Picard, Christian ; Sheldon, Sallie ; Sutherland, Michael ; Vallino, Joseph J. ; Warren, R. Scott
    The sustainability of coastal ecosystems in the face of widespread environmental change is an issue of pressing concern throughout the world (Emeis et al. 2001). Coastal ecosystems form a dynamic interface between terrestrial and oceanic systems and are one of the most productive ecosystems in the world. Coastal systems probably serve more human uses than any other ecosystem and they have always been valued for their rich bounty of fish and shellfish. Coastal areas are also the sites of the nation’s and the world’s most intense commercial activity and population growth; worldwide, approximately 75% of the human population now lives in coastal regions (Emeis et al. 2001). Over the past three decades nutrient enrichment of coastal and estuarine waters has become the premier issue for both scientists and managers (National Research Council 2000). Our understanding of coastal eutrophication has been developed principally through monitoring of estuaries, with a focus on pelagic or subtidal habitats (National Research Council 2000, Cloern 2001). Because estuarine systems are usually nitrogen limited, NO3- is the most common nutrient responsible for cultural nutrient enrichment (Cloern 2001). Increased nitrogen delivery to pelagic habitats of estuaries produces the classic response of ecosystems to stress (altered primary producers and nutrient cycles and loss of secondary producer species and production; Nixon 1995, Rapport and Whitford 1999, Deegan et al. 2002). Salt marsh ecosystems have been thought of as not susceptible to nitrogen over-loading because early studies found added nitrogen increased marsh grass production (primarily Spartina spp., cordgrass) and concluded that salt marshes can adsorb excess nutrients in plants and salt marsh plant-derived organic matter as peat (Verhoeven et al. 2006). Detritus from Spartina is important in food webs (Deegan et al. 2000) and in creating peat that forms the physical structure of the marsh platform (Freidrichs and Perry 2001). However, the accumulation of peat and inputs of sediments and loss of peat through decomposition and sediment through erosion may be altered under high nutrient regimes and threaten the long-term stability of marsh systems. Nitrogen addition may lead to either net gain or loss of the marsh depending on the balance between increased marsh plant production and increased decomposition. Absolute change in marsh surface elevation is determined by marsh plant species composition, production and allocation to above- and belowground biomass, microbial decomposition, sedimentation, erosion and compaction (Friedrichs and Perry 2001). Levine et al. (1998) suggested that competitive dynamics among plants might be affected by nutrient enrichment, potentially altering relative abundance patterns favoring species with less belowground storage and thus lowering rates of peat formation. When combined with the observation that nutrient additions may also stimulate microbial respiration and decomposition (Morris and Bradley 1999), the net effect on the salt marsh under conditions of chronic nitrogen loading is a critical unknown. Although most research treats nutrient enrichment as a stand-alone stress, it never occurs in isolation from other perturbations. The effect of nutrient loading on species composition (both plants and animals) and the resultant structure and function of wetlands has been largely ignored when considering their ability to adsorb nutrients (Verhoeven et al. 2006). Recent studies suggest the response of estuaries to stress may depend on animal species composition (Silliman et al. 2005). Animal species composition may alter the balance between marsh gain and loss as animals may increase or decrease primary production, decomposition or N recycling (Pennings and Bertness 2001). Failure to understand interactions between nutrient loading and change in species composition may lead to underestimating the impacts of these stresses. The 'bottom up or top down' theory originated from the observation that nutrient availability (bottom up)sets the quantity of primary productivity, while other studies have shown that species composition (top down), particularly of top consumers, has a marked and cascading effect on ecosystems, including controlling species composition and nutrient cycling (Matson and Price 1992, Pace et al. 1999). Most examples of trophic cascades are in aquatic ecosystems with fairly simple, algal grazing pelagic food webs (Strong 1992). The rarity of trophic cascades in terrestrial systems has been attributed to the importance of detrital food webs (Polis 1999). Detritus-based aquatic ecosystems, such as salt marshes, bogs, and swamps, have classically been considered bottom-up or physically controlled ecosystems. Recent experiments, however, suggest that salt marshes may exhibit top-down control at several trophic levels (Silliman and Zeiman. 2001, Silliman and Bertness 2002, Quiñones-Rivera and Fleeger 2005). One abundant, ubiquitous predator, a small (<10 cm total length) killifish (Fundulus heteroclitus, mummichog) has been suggested to control benthic algal through a trophic cascade because they prey on the invertebrates that graze on the benthic algae (Kneib 1997, Sarda et al. 1998). In late summer, killifish are capable of consuming 3-10 times the creek meiofauna production and meiofauna in the absence of predators appear capable of grazing over 60% of the microalgal community per day (Carman et al. 1997). Strong top-down control by grazers is considered a moderating influence on the negative effects of elevated nutrients on algae (Worm et al. 2000). Small-scale nutrient additions and predator community exclusion experiments have demonstrated bottom-up and top-down control of macroinfauna in mudflats associated with salt marsh creeks (Posey et al. 1999, Posey et al. 2002). Together, these observations suggest mummichogs are at the top of a trophic cascade that controls benthic algae (Sarda et al. 1998). Mummichogs are also omnivorous and ingest algae, bulk detritus and the attached microbial community (D’Avanzo and Valiela 1990). As a result, marsh decomposition rates may be limited by top-down controls through trophic pathways or by release from competition with algae for nutrients. Whole-ecosystem experiments have shown that responses to stress are often not predictable from studies of the individual components (Schindler 1998). Developing the information needed to predict the interacting impacts of nutrient loading and species composition change requires experiments with realistic alterations carried out at scales of space and time that include the complexities of real ecosystems. Whole ecosystem manipulation experiments have been used effectively in other ecosystems (Bormann and Likens 1979, Carpenter et al. 1995), but they are rare in coastal research. Experiments in salt marshes have traditionally been less than a few m2. Our understanding of the response of salt marsh plants to nutrient enrichment is from small (<10 m2), plot-level additions where uniform levels of dry inorganic fertilizer (20 to > 1000 g N m-2 y-1) are sprinkled on the marsh surface at low tide. Dry fertilizer additions were usually made every two weeks or monthly and the duration of elevated nutrient levels after these additions was usually not determined. Tidal water is the primary vector for N delivery to coastal marshes, suggesting that dry fertilizer addition to the marsh surface may not be the best basis for determining if Spartina production responds to nutrient enrichment of tidal waters. Similarly, our understanding of top-down controls in salt marshes also relies on small (1 - 4 m2) exclusion experiments that use cages to isolate communities from top consumers. While the design of these cage experiments has improved, there are some remaining drawbacks. For example, it is impossible to selectively exclude single species using cages, and recruitment or size-selective movement into or out of the cages may obscure interpretations. In addition, while these small-scale experiments provide insight into controls on isolated ecosystem processes, they do not allow for interaction among different parts of the ecosystem which may buffer or alter the impacts and are not appropriate for determining the effects of populations of larger more motile animals on whole-ecosystems or the effects of ecosystem changes on populations. For example, interactions may be caused when a motile species alters its distribution among the habitats available to it because of an experimental treatment. Small-scale experiments generally do not allow such events to happen. Complex feedbacks among physical and biological processes can alter accumulation rates and affect marsh elevation relative to sea level rise making extrapolation of small plot level experiments to whole marsh ecosystems problematic. We are conducting an ecosystem-scale, multi-year field experiment including both nutrient and biotic manipulations to coastal salt marsh ecosystems. We are testing, for the first time at the ecosystem level, the hypothesis that nutrient enrichment and species composition change have interactive effects across multiple levels of biological organization and a range of biogeochemical processes. We altered whole salt marsh creek watersheds (~60,000 m2 of saltmarsh) by addition of nutrients (15x ambient) in flooding waters and by a 60% reduction of a key fish species, the mummichog. Small marsh creek watersheds provide an ideal experimental setting because they have the spatial complexity, species composition and processes characteristic of the larger salt marsh ecosystem, which are often hundreds of thousands of m2. Manipulating entire salt marsh creeksheds allowed us to examine effects on large motile animals and the interactive effects of motile species changes on ecosystem processes without cage artifacts. Because our manipulations were done on whole-marsh ecosystems, we are able to evaluate the integrated and interactive effects on all habitats (e.g., water column, tidal creeks and marsh) and on populations. These experiments are similar in many respects to the small watershed experiments carried out in forested catchments. Our nutrient enrichment is novel compared to past studies in two important ways. We added nutrients (N and P) directly to the flooding tidal creek waters to mimic the way in which anthropogenic nutrients reach marsh ecosystems. All previous experimental salt marsh nutrient enrichment studies used a dose-response design with spatially uniform dry fertilizer loading on small plots (<10 m2). Nutrients carried in water will interact and reach parts of the ecosystem differently than dry fertilizer. Our enrichment method also creates a spatial gradient of nutrient loading across the landscape that is proportional to the frequency and depth of inundation in the marsh. Spatial gradients in loading within an ecosystem are typical in real world situations in many terrestrial and aquatic ecosystems. Because of our enrichment method, at any location in the ecosystem, nutrient load will be a function of the nutrient concentration in the water, the frequency and depth of tidal flooding and the reduction of nutrients from the flooding waters by other parts of the ecosystem. Uniform loading misses important aspects of the spatial complexity of ecosystem exposure and response. This work is organized around two questions that are central to understanding the long-term fate of coastal marshes: 1. Does chronic nutrient enrichment via flooding water increase primary production more than it stimulates microbial decomposition? 2. Do top-down controls change the response of the salt marsh ecosystem to nutrient enrichment? Here we present findings on the first 2 years of these experiments including 1) water chemistry, 2) standing stocks and species composition of benthic microalgae, 3) microbial production, 4) species composition and ecophysiology of macrophytes, 5) invertebrates, and 6) nekton. Because even highly eutrophic waters result in nutrient loading that is an order of magnitude less than most plot level experiments, we expected little stimulation of salt marsh vascular plant growth. However, moderate levels of nutrient enrichment in the water column were expected to increase benthic algal biomass and to stimulate bacterial activity and detrital decomposition throughout the ecosystem because of direct uptake of nitrogen from the water column and availability of more high quality organic matter from increased algal production. We predicted nutrient enrichment would increase invertebrate production because of an increase of high quality microalgal and microbial production at the base of the food web. Finally, we predicted that fish reduction would reduce predation on benthic invertebrates resulting in increased abundance of benthic invertebrates that would graze down the benthic algae.
  • Article
    Predicting seabed burial of cylinders by wave-induced scour : application to the sandy inner shelf off Florida and Massachusetts
    (IEEE, 2007-01) Trembanis, Arthur C. ; Friedrichs, Carl T. ; Richardson, Michael D. ; Traykovski, Peter A. ; Howd, Peter A. ; Elmore, Paul A. ; Wever, Thomas F.
    A simple parameterized model for wave-induced burial of mine-like cylinders as a function of grain-size, time-varying, wave orbital velocity and mine diameter was implemented and assessed against results from inert instrumented mines placed off the Indian Rocks Beach (IRB, FL), and off the Martha’s Vineyard Coastal Observatory (MVCO, Edgartown, MA). The steady flow scour parameters provided by Whitehouse (1998) for self-settling cylinders worked well for predicting burial by depth below the ambient seabed for Ο (0.5 m) diameter mines in fine sand at both sites. By including or excluding scour pit infilling, a range of percent burial by surface area was predicted that was also consistent with observations. Rapid scour pit infilling was often seen at MVCO but never at IRB, suggesting that the environmental presence of fine sediment plays a key role in promoting infilling. Overprediction of mine scour in coarse sand was corrected by assuming a mine within a field of large ripples buries only until it generates no more turbulence than that produced by surrounding bedforms. The feasibility of using a regional wave model to predict mine burial in both hindcast and real-time forecast mode was tested using the National Oceanic and Atmospheric Administration (NOAA, Washington, DC) WaveWatch 3 (WW3) model. Hindcast waves were adequate for useful operational forcing of mine burial predictions, but five-day wave forecasts introduced large errors. This investigation was part of a larger effort to develop simple yet reliable predictions of mine burial suitable for addressing the operational needs of the U.S. Navy.
  • Article
    Combining observations and numerical model results to improve estimates of hypoxic volume within the Chesapeake Bay, USA
    (John Wiley & Sons, 2013-10-03) Lanerolle, Aaron J. ; Friedrichs, Marjorie A. M. ; Friedrichs, Carl T. ; Scully, Malcolm E. ; Lanerolle, Lyon W. J.
    The overall size of the “dead zone” within the main stem of the Chesapeake Bay and its tidal tributaries is quantified by the hypoxic volume (HV), the volume of water with dissolved oxygen (DO) less than 2 mg/L. To improve estimates of HV, DO was subsampled from the output of 3-D model hindcasts at times/locations matching the set of 2004–2005 stations monitored by the Chesapeake Bay Program. The resulting station profiles were interpolated to produce bay-wide estimates of HV in a manner consistent with nonsynoptic, cruise-based estimates. Interpolations of the same stations sampled synoptically, as well as multiple other combinations of station profiles, were examined in order to quantify uncertainties associated with interpolating HV from observed profiles. The potential uncertainty in summer HV estimates resulting from profiles being collected over 2 weeks rather than synoptically averaged ∼5 km3. This is larger than that due to sampling at discrete stations and interpolating/extrapolating to the entire Chesapeake Bay (2.4 km3). As a result, sampling fewer, selected stations over a shorter time period is likely to reduce uncertainties associated with interpolating HV from observed profiles. A function was derived that when applied to a subset of 13 stations, significantly improved estimates of HV. Finally, multiple metrics for quantifying bay-wide hypoxia were examined, and cumulative hypoxic volume was determined to be particularly useful, as a result of its insensitivity to temporal errors and climate change. A final product of this analysis is a nearly three-decade time series of improved estimates of HV for Chesapeake Bay.
  • Article
    Sediment pumping by tidal asymmetry in a partially mixed estuary
    (American Geophysical Union, 2007-07-28) Scully, Malcolm E. ; Friedrichs, Carl T.
    Observations collected at two laterally adjacent locations are used to examine the processes driving sediment transport in the partially mixed York River Estuary. Estimates of sediment flux are decomposed into advective and pumping components, to evaluate the importance of tidal asymmetries in turbulent mixing. At the instrumented location in the estuarine channel, a strong asymmetry in internal mixing due to tidal straining is documented, with higher values of eddy viscosity occurring during the less-stratified flood tide. As a result of this asymmetry, more sediment is resuspended during the flood phase of the tide resulting in up-estuary pumping of sediment despite a net down-estuary advective flux. At the instrumented location on the adjacent shoal, where no pronounced tidal asymmetry in internal mixing was found, both the pumping flux and advective flux were directed down-estuary. The down-estuary pumping of sediment on the shoal appears to be driven by asymmetries in bed stress. The impact of tidal asymmetries in bed stress at the channel location was negated because the amount of sediment available for resuspension was limited. As a result, the pumping flux was dominated by the overlying asymmetries in internal mixing. The asymmetries in stratification appear to exert an important control on the vertical distribution of sediment by both impacting the eddy diffusivity as well as the fall velocity. During the more turbulent flood tide, the fall velocities are smaller suggesting the Kolmogorov microscale is setting the upper bound on floc diameter.
  • Article
    The importance of tidal and lateral asymmetries in stratification to residual circulation in partially mixed estuaries
    (American Meteorological Society, 2007-06) Scully, Malcolm E. ; Friedrichs, Carl T.
    Measurements collected in the York River estuary, Virginia, demonstrate the important impact that tidal and lateral asymmetries in turbulent mixing have on the tidally averaged residual circulation. A reduction in turbulent mixing during the ebb phase of the tide caused by tidal straining of the axial density gradient results in increased vertical velocity shear throughout the water column during the ebb tide. In the absence of significant lateral differences in turbulent mixing, the enhanced ebb-directed transport caused by tidal straining is balanced by a reduction in the net seaward-directed barotropic pressure gradient, resulting in laterally uniform two-layer residual flow. However, the channel–shoal morphology of many drowned river valley estuaries often leads to lateral gradients in turbulent mixing. Tidal straining may then lead to tidal asymmetries in turbulent mixing near the deeper channel while the neighboring shoals remain relatively well mixed. As a result, the largest lateral asymmetries in turbulent mixing occur at the end of the ebb tide when the channel is significantly more stratified than the shoals. The reduced friction at the end of ebb delays the onset of the flood tide, increasing the duration of ebb in the channel. Conversely, over the shoal regions where stratification is more inhibited by tidal mixing, there is greater friction and the transition from ebb to flood occurs more rapidly. The resulting residual circulation is seaward over the channel and landward over the shoal. The shoal–channel segregation of this barotropically induced estuarine residual flow is opposite to that typically associated with baroclinic estuarine circulation over channel–shoal bathymetry.