Quasi-three-dimensional modelling of the morphology of longshore bars

A morphological quasi-three-dimensional (Q3D) area model for barred coasts has been developed. The model combines a two-dimensional depth integrated model for wave-driven currents with a model for undertow circulation currents. The combined model makes a simultaneous simulation of the bar-forming processes associated with the undertow and the horizontal wave-driven circulation currents, which may cause instabilities of the bar and the formation of rip channels. Situations with normal and oblique wave incidence are considered. Compared to the depth integrated approach the Q3D model produces less pronounced alongshore irregularities for obliquely incident waves. For normal incident waves the Q3D model produces a crescentic bar while the depth integrated model predicts almost straight sections of the bar interrupted by rip channels. The sensitivity to variation of wave angle and beach slope is further investigated.     

Potential climate-change impacts on the Chesapeake Bay

We review current understanding of the potential impact of climate change on the Chesapeake Bay. Scenarios for CO₂ emissions indicate that by the end of the 21^st century the Bay region will experience significant changes in climate forcings with respect to historical conditions, including increases in CO₂ concentrations, sea level, and water temperature of 50–160%, 0.7–1.6 m, and 2–6 °C, respectively. Also likely are increases in precipitation amount (very likely in the winter and spring), precipitation intensity, intensity of tropical and extratropical cyclones (though their frequency may decrease), and sea-level variability. The greatest uncertainty is associated with changes in annual streamflow, though it is likely that winter and spring flows will increase. Climate change alone will cause the Bay to function very differently in the future. Likely changes include: (1) an increase in coastal flooding and submergence of estuarine wetlands; (2) an increase in salinity variability on many time scales; (3) an increase in harmful algae; (4) an increase in hypoxia; (5) a reduction of eelgrass, the dominant submerged aquatic vegetation in the Bay; and (6) altered interactions among trophic levels, with subtropical fish and shellfish species ultimately being favored in the Bay. The magnitude of these changes is sensitive to the CO₂ emission trajectory, so that actions taken now to reduce CO₂ emissions will reduce climate impacts on the Bay. Research needs include improved precipitation and streamflow projections for the Bay watershed and whole-system monitoring, modeling, and process studies that can capture the likely non-linear responses of the Chesapeake Bay system to climate variability, climate change, and their interaction with other anthropogenic stressors.     

Geomorphology of a sand wave in lower Chesapeake Bay,Virginia, U.S.A.

Morphologic and sedimentologic studies of a single sand wave within a sand wave field in the lower Chesapeake Bay suggest that the bedform was originally formed by ebb currents, and is presently in static equilibrium with the circulation pattern. In this report, the concept of solitary sand wave is introduced to describe the state of a sand wave when further evolution of the sedimentary structure is mostly independent of adjacent bedforms. This concept can be applied to several bedforms in the area that are isolated from others by flats. A particular sand wave that is included in this category is discussed.Contribution number 79, Instituto Argentino de Oceanografia.     

Reconstructing pre-colonial oyster demographics in the Chesapeake Bay, USA

Recent estimates of growth and mortality rates in extant Chesapeake Bay, USA oyster (Crassostrea virginica) populations are used to quantify changes in both population abundance (dN/dT) and shell accretion (dS/dT) associated with modern population demographics. The demographics of oyster populations that would be required to maintain reef accretion rates commensurate with sea level rise over geological time frames are examined using estimates of oyster longevity in pre-colonial (pre -1600) times combined with parallel estimates of pre-disease endemic mortality. The analysis demonstrates that modern populations, with their disease related, age-truncated demographics, are generally not capable of maintaining and building biogenic reefs through accretion. Estimates of filtration rates associated with Chesapeake Bay oyster populations prior to 1600 considerably underestimate actual benthic-pelagic coupling during that period. Pristine oyster populations would have supported water column turnover rates on the order of minutes to hours. Thus, the spatial footprint of oyster reefs was limited by available productivity in the estuary. Accretion rate calculations for pristine (pre-1600) oyster reefs describe the intimate relationship between benthic-pelagic coupling and the presence or absence of oyster reefs and the associated communities.     

Seasonal variations in the speciation of dissolved iodine in the Chesapeake Bay

The speciation of dissolved iodine and the distributions of the iodine species in the deep Chesapeake Bay underwent seasonal variations in response to changes in the prevailing redox condition. In the deep water, the ratios of iodate to iodide and iodate to inorganic iodine decreased progressively from the Winter through the Summer as the deep water became more poorly oxygenated before they rebounded in the Fall when the deep water became re-oxygenated again. The composition of the surface water followed the same trend. However, in this case, the higher biological activities in the Spring and the Summer could also have enhanced the biologically mediated reduction of iodate to iodide by phytoplankton and contributed to the lower ratios found during those seasons. Superimposed on this redox cycle was a cycle of input and removal of dissolved iodine probably as a result of the interactions between the water column and the underlying sediments. Iodine was added to the Bay during the Summer when the deep water was more reducing and removed from the Bay in the Fall when the deep water became re-oxygenated. A third cycle was the inter-conversion between inorganic iodine and ‘dissolved organic iodine’, or ‘‘DOI’’. The conversion of inorganic iodine to ‘DOI’ was more prevalent in the Spring. As a result of these biogeochemical reactions in the Bay, during exchanges between the Bay and the North Atlantic, iodate-rich and ‘DOI’-poor water was imported into the Bay while iodide- and ‘DOI’-rich water was exported to the Atlantic. The export of iodide from these geochemically reactive systems along the land margins contributes to the enrichment of iodide in the surface open oceans.     

The spectrum of subtidal variability in Chesapeake Bay circulation

The advent of long, continuous time series records of circulation in Chesapeake Bay has revealed the existence of large amplitude fluctuations within the subtidal range 0·03–0·6 cycles day⁻¹. These fluctuations represent direct and indirect response of the estuary to variations in wind stress, fresh water inflow, and coastal sea level. The fluctuations in circulation are accompanied by synchronous fluctuations in transportable properties such as salinity and temperature. A quantitative model is presented to explain this variability within the main stem of Chesapeake Bay in terms of a linear reponse to irregular, time-varying meteorological forcing. The model calculates transfer functions and energy spectra of laterally averaged transport, surface elevation, and salinity in two layers separated by a halocline, over the frequency band 0·03–0·6 cycles day⁻¹. Transfer functions between volume transport and wind stress obtained from one-month-long field experiments at three different cross sections in Chesapeake Bay are used to constrain model friction parameters. Using existing estimates for wind stress and coastal sea level energy spectra, energy spectra for volume transport and surface elevation are calculated as a function of longitudinal position. It is found that the observed volume transport spectrum at the mouth of the Bay can be explained quantitatively as the combined response to statistically indepentdent wind stress and sea level fluctuations. Variations in sea level account for 90% of the volume transport variance at the Bay mouth and dominate the volume transport spectrum below 0·375 cycles day⁻¹. In the upper Bay, longitudinal wind stress accounts for most of the variance. A maximum in the volume transport spectrum at 0·4 cycles day⁻¹, caused by a local maximum in the wind spectrum, is found at all upper Bay cross sections.     

SeaWiFS retrievals of chlorophyll in Chesapeake Bay and the mid-Atlantic bight

This paper presents three years (1998–2000) of chlorophyll a (chl a) data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) for Case 2 waters of Chesapeake Bay and the middle Atlantic bight (MAB) to describe phytoplankton dynamics on seasonal to interannual time scales. We used extensive data on inherent and apparent optical properties in conjunction with satellite retrievals to: (1) characterize the bio-optical properties of the study area relevant to processing and interpreting SeaWiFS data; (2) test the applicability of the SeaWiFS bio-optical algorithm (OC4v.4) for the estuarine and coastal waters; (3) evaluate the accuracy of the SeaWiFS remote sensing reflectance (R_RS) and chl a products on regional and seasonal bases using in situ observations. The characteristically strong absorption by chromophoric dissolved organic matter (a_cdom) and non-pigmented particulate matter (a_d) in estuarine and coastal waters contributed to overestimates of chl a using OC4v.4 applied to in situ radiances for the Bay (mean ratio 1.42±1.20) and the MAB (2.60±1.36). Values of R_RS from SeaWiFS in the blue region of the spectrum were low compared to in situ R_RS, suggesting that uncertainties remain in atmospheric correction. Direct comparisons of SeaWiFS retrievals of chl a with in situ chl a for the Bay showed larger biases and uncertainties (mean ratio 1.97±1.85) than for chl a estimated from OC4v.4 applied to in situ R_RS. The larger biases were attributed to errors in SeaWiFS radiances and the larger uncertainties to time-space “aliasing” of satellite observations and in situ measurements. To reduce the time differences between SeaWiFS and in situ data, we compared chl a obtained from continuous underway fluorometric measurements on selected ship tracks to SeaWiFS chl a and showed that SeaWiFS captured phytoplankton dynamics in much of the Bay. The agreement of SeaWiFS chl a with in situ chl a was strongest in the mid- (regions 3, 4) to lower Bay (regions 1, 2), and deteriorated toward the upper Bay (regions 5, 6), in part due to a reduction of sensitivity and an increase of noise for SeaWiFS products in the highly absorbing, low R_RS waters of the upper Bay. A three-year time-series of SeaWiFS and in situ data showed that SeaWiFS accurately and reliably captured seasonal and interannual variability of chl a associated with variations of freshwater flow. Significant short-term variability of chl a in summer that was unresolved with shipboard data was detected in the SeaWiFS time-series and the implications are discussed. The overall performance of SeaWiFS in the mid- to lower Bay and the MAB, combined with high spatial (∼1 km²) and temporal (∼100 clear scenes per year) resolution, indicate current SeaWiFS products are valuable for quantifying seasonal to interannual variability of chl a in estuarine and coastal waters.     

Multi-Model Validation of Currents in the Chesapeake Bay Region in June 2010

In this paper, we discuss the validation of water level and current predictions from three coastal hydrodynamic models and document the resource and operational requirements for each modeling system. The ADvanced CIRCulation Model (ADCIRC), the Navy Coastal Ocean Model (NCOM), and Delft3D have been configured and validated for the Chesapeake Bay region during a Navy exercise. Water level predictions are compared with a NOAA/NOS water level gauge at the Chesapeake Bay Bridge Tunnel location while current predictions are validated with Acoustic Doppler Profiler (ADP) measurement records at three locations in the lower Chesapeake Bay. Statistical metrics such as correlation coefficient and root mean square error (RMSE) are computed. Both the vertically-integrated currents and currents at varying water depths are compared as well. The model-data comparisons for surface elevation indicate all three models agreed well with water level gauge data. The two-dimensional version of ADCIRC, ADCIRC2D, and NCOM yield better statistics, in terms of correlation and RMSE, than Delft3D. For vertically-integrated currents, ADCIRC2D has the smallest RMSE at Thimble Shoal and Naval Station locations while NCOM has the smallest RMSE at Cape Henry. For the horizontal currents over the water column, the fully three-dimensional, baroclinic ADCIRC model, ADCIRC3D, and NCOM both showed better agreement with the ADP measurements.     

Heavy-mineral variability in bottom sediments of the lower Chesapeake Bay,Virginia

Heavy minerals in bottom-sediment samples of the lower Chesapeake Bay show distribution patterns and interrelationships that denote characteristic mineral suites associated with defined geographic provinces. The Baymouth province has a garnet—hornblende—pyroxene suite, which is largely attributed to the influx of littoral and shelf sediments; the Eastern Shore province has a similar suite, derived largely from coastal erosion of the Eastern Shore peninsula. The Northern and Combined River provinces have a zircon—tourmaline—staurolite assemblage, which reflects derivation from an Appalachian Piedmont—Atlantic Coastal Plain sourceland. The Western Shore province is associated with a zircon—epidote—staurolite assemblage, apparently derived jointly from tributary influx and coastal erosion of the western shore. Factor analysis identified two major factors that account for 63% of the total variation in the relative amounts of the seven most common heavy minerals. The dominant factor (44%) is based on a zircon—hornblende—staurolite—pyroxene relationship, which indicates that mineral stability, as influenced by sediment maturity, is a major contributing factor. The second factor (19%) based on a tourmaline—epidote—staurolite—garnet relationship indicates that provenance is another major cause of heavy-mineral variability within the lower bay.     

Methylmercury production in sediments of Chesapeake Bay and the mid-Atlantic continental margin

Methylmercury (MeHg) concentration and production rates were studied in bottom sediments along the mainstem of Chesapeake Bay and on the adjoining continental shelf and slope. Our objectives were to 1) observe spatial and temporal changes in total mercury (HgT) and MeHg concentrations in the mid-Atlantic coastal region, 2) investigate biogeochemical factors that affect MeHg production, and 3) examine the potential of these sediments as sources of MeHg to coastal and open waters. Estuarine, shelf and slope sediments contained on average 0.5 to 1.5% Hg as MeHg (% MeHg), which increased significantly with salinity across our study site, with weak seasonal trends. Methylation rate constants (k_meth), estimated using enriched stable mercury isotope spikes to intact cores, showed a similar, but weaker, salinity trend, but strong seasonality, and was highly correlated with % MeHg. Together, these patterns suggest that some fraction of MeHg is preserved thru seasons, as found by others [Orihel, D.M., Paterson, M.J., Blanchfield, P.J., Bodaly, R.A., Gilmour, C.C., Hintelmann, H., 2008. Temporal changes in the distribution, methylation, and bioaccumulation of newly deposited mercury in an aquatic ecosystem. Environmental Pollution 154, 77] Similar to other ecosystems, methylation was most favored in sediment depth horizons where sulfate was available, but sulfide concentrations were low (between 0.1 and 10 μM). MeHg production was maximal at the sediment surface in the organic sediments of the upper and mid Bay where oxygen penetration was small, but was found at increasingly deeper depths, and across a wider vertical range, as salinity increased, where oxygen penetration was deeper. Vertical trends in MeHg production mirrored the deeper, vertically expanded redox boundary layers in these offshore sediments. The organic content of the sediments had a strong impact on the sediment:water partitioning of Hg, and therefore, on methylation rates. However, the HgT distribution coefficient (K_D) normalized to organic matter varied by more than an order of magnitude across the study area, suggesting an important role of organic matter quality in Hg sequestration. We hypothesize that the lower sulfur content organic matter of shelf and slope sediments has a lower binding capacity for Hg resulting in higher MeHg production, relative to sediments in the estuary. Substantially higher MeHg concentrations in pore water relative to the water column indicate all sites are sources of MeHg to the water column throughout the seasons studied. Calculated diffusional fluxes for MeHg averaged  1 pmol m^− 2 day^− 1. It is likely that the total MeHg flux in sediments of the lower Bay and continental margin are significantly higher than their estimated diffusive fluxes due to enhanced MeHg mobilization by biological and/or physical processes. Our flux estimates across the full salinity gradient of Chesapeake Bay and its adjacent slope and shelf strongly suggest that the flux from coastal sediments is of the same order as other sources and contributes substantially to the coastal MeHg budget.     

Variations in the boundary-drag coefficient in the tidal entrance to Chesapeake Bay,Virginia

Use of the quadratic shear-stress law for estimating boundary drag requires specific knowledge of the magnitude of a drag coefficient, C_D, and sectional mean velocity, $\text{u}\text{?}$. In previous attempts to adapt the relationship for use in studies of marine-sediment transport, the flow measurement has been standardized at a level 100 cm above the bed. The particularized value of the drag coefficient has been designated as C₁₀₀.In the entrance area to Chesapeake Bay, Virginia, C₁₀₀ has been found to range through unacceptably wide limits. Two-thirds of the values obtained are between 3.5 · 10^?3 and 5.4 · 10^?2. Mean C₁₀₀ for the area is 1.3 · 10^?2 as compared to 3 · 10^?3 for tidal channels within Puget Sound, Washington.Present data suggest that, given a moveable bed, a size hierarchy of mobile bed forms, time-varying flow, and a lack of equilibrium between flow and bed, C₁₀₀ changes continuously with boundary shear stress.Accurate evaluation of boundary shear stress in tidal entrances with high flow rates and mobile beds presently requires measurement of velocity profiles.     

Design and performance of headland bays in Chesapeake Bay,USA

The use of headland-breakwater systems along the shore of Chesapeake Bay began in the early 1980s. Properly designed and installed headland breakwaters with beach fill and wetlands plantings provide shore protection and create a “full” coastal profile of beach/backshore/dune which enhances habitat. They create a tertiary buffer for upland runoff and groundwater and provide access and recreation. The wetland grasses also create an erosion resistant turf. The coastal profile accommodates environmental permitting requirements of habitat enhancement for shore protection structures.     

Internal recycling of particle reactive organic chemicals in the Chesapeake Bay water column

Hydrophobic organic contaminants (HOCs) may be used as tracers of particle dynamics in aquatic systems. Internal cycling of polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) were studied in the mesohaline Chesapeake Bay to assess the role of resuspension in maintaining particle and contaminant inventories in the water column, and to compare settling and suspended particle characteristics. Direct measurements of sediment resuspension and settling conducted in conjunction with one of the sediment trap deployments indicate reasonable agreement between measurements of particle flux using the two different methods. Organic carbon and PCB concentrations in settling solids collected in near-surface sediment traps were remarkably lower than concentrations in suspended particles collected by filtration during the trap deployments, but higher PAH concentrations were found in the settling particles. The different behaviors of PAHs and PCBs in the settling particles are due to their different source types and association to different types of particles. Sediment trap collections in near bottom waters were dominated by resuspension. Resuspension fluxes of HOCs measured 2 m above the bay bottom were as high as 2.5 μg/m² day for total PCBs and 15 μg/m² day for fluoranthene, and were 25 and 10 times higher than their settling fluxes from surface waters, respectively. HOC concentrations in the near bottom traps varied much less between trap deployments than HOC concentrations in the surface traps, indicating that the chemical composition of the resuspended particles collected in the near bottom traps was more time-averaged by repeated resuspension than the surface particles.     

Chromophoric dissolved organic matter and dissolved organic carbon in Chesapeake Bay

Chromophoric dissolved organic matter (CDOM) is the light absorbing fraction of dissolved organic carbon (DOC). The optical properties of CDOM potentially permit remote sensing of DOC and CDOM, and correction for CDOM absorption is essential for remote sensing of chlorophyll a (chl a) in coastal and estuarine waters. To provide data for this purpose, we report the distributions of CDOM, DOC, and chl a from seven cruises in Chesapeake Bay in 1994–1997. We observed non-conservative distributions of chl a and DOC in half of the cruises, indicating net accumulations within the estuary; however, there were no net accumulations or losses of CDOM, measured as absorption at 355 nm or as fluorescence. Freshwater end member CDOM absorption varied from 2.2 to 4.1 m⁻¹. Coastal end member CDOM absorption was considerably lower, ranging over 0.4–1.1 m⁻¹. The fluorescence/absorption ratio was similar to those reported elsewhere for estuarine and coastal waters; however, in the lower salinity/high CDOM region of the Bay, the relationship was not constant, suggestive of the mixing of two or more CDOM sources. Chl a was not correlated with the absorption for most of the cruises nor for the data set as a whole; however, CDOM and DOC were significantly correlated, with two groups evident in the data. The first group had high CDOM concentrations per unit DOC and corresponded to the conservative DOC values observed in the transects. The second group had lower CDOM concentrations per unit DOC and corresponded to the non-conservative DOC values associated with net DOC accumulation near the chl a maximum on the salinity gradient. This indicates the production of non-chromophoric DOC in the region of the chl a maximum of Chesapeake Bay. In terms of remote sensing, these data show that (1) the retrieval of the absorption coefficient of CDOM from fluorescence measurements in the Bay must consider the variability of the fluorescence/absorption relationship, and (2) estimates of DOC acquired from CDOM absorption will underestimate DOC in regions with recent, net accumulations of DOC.     

Accretion rates in brackish marshes of a Chesapeake Bay estuarine tributary

Vertical accretion rates were determined for brackish marshes in an estuarine tributary on the Eastern Shore of Chesapeake Bay. These rates determined on the basis of the peak phase of early European settlement as recorded in pollen spectra of the marsh sediments range between 0. 18 to 0.74 cm yr⁻¹ over approximately the last 194 years and generally decrease down the estuary. More recent accretion rates (estimated from changes in pine pollen concentration) appear to have accelerated, exceeding the present local rate of sea level rise. The implications for using estuarine marsh accretion rates as surrogates for changes in sea level are discussed.     

Determining the age of water and long-term transport timescale of the Chesapeake Bay

The concept of age of water (AW) is applied to the Chesapeake Bay to investigate the long-term transport properties for dissolved substances. A real-time calibrated hydrodynamic Chesapeake Bay model in 3 Dimensions (CH3D), employing a boundary-fitted curvilinear grid, is used for the study. The long-term transport properties, represented by AW, are investigated under the conditions of low river inflow of 1995 and high river inflow of 1996, as well as for constant mean inflows. The influences of freshwater, density-induced circulation, and wind-induced transport on age distribution have been investigated. Model results show that river inflows, wind stress, and density-induced circulation play important roles in controlling the long-term transport in the Bay. The model results shows that it requires 120–300 days for a marked change in the characteristics of the pollutant source discharged into the Bay from the Susquehanna River to affect significantly the conditions near the mouth under different hydrodynamic conditions. An increase of river discharge results in increases of downstream residual current and gravitational circulation, and thus reduces AW. The density-induced circulation contributes to the transport substantially. The dissolved substances discharged into the Bay are transported out of the Bay more rapidly when the estuary becomes more stratified. Southeasterly and southwesterly winds have strong impacts on the transport compared to the northeasterly and northwesterly winds. The former increases lateral and vertical mixing significantly. Consequently, the gravitational circulation is reduced and the transport time is increased by 50%. The model results provide useful information for understanding the long-term transport processes in the Bay.     

Intensification of Subtidal Surface Currents Over a Deep Channel in the Upper Chesapeake Bay

A three-dimensional hydrodynamic model of the Upper Chesapeake Bay was used to examine the nature and cause of an intensification of subtidal, southward surface current in the middle reaches of the basin. The deep navigation channel along the eastern boundary was found to be ultimately responsible. The deep channel allows the density and tidally-induced subtidal currents to intensify over it, producing the eastern intensification. Both mechanisms operate in the non-rotating limit and consequently do not diminish with vanishing effect of the earth's rotation. Density-induced forcing is predominantly baroclinic, generating a northward undercurrent in the deep channel and a southward current aloft which attenuates westward. Tidal forcing is mostly barotropic, producing southward mean current in the deep channel and return flow to the west. Historic data lend support to the model results.