Albert Y. Kuo

Hypoxia and salinity in Virginia estuaries

Hypoxia, periods of reduced dissolved oxygen concentrations, has been observed not only in the Chesapeake Bay but also in the deeper waters of the Virginia estuaries that are tributaries to the Chesapeake Bay. When water temperature exceeded 20°C, minimum oxygen concentrations were observed to be <50% of saturation concentrations in 75%, 50% and 2% of the surveys in the estuaries of the Rappahannock, York and James rivers, respectively. The observation that hypoxia rarely occurred in the James River is surprising, given the fact that it receives the greatest amount of wastewater. Analysis of the oxygen budgets in these estuaries indicates that the variations in the frequency, duration, and severity of hypoxia are related to the net movement of bottom waters. This relationship has significant implications for the management of water quality and marine fisheries.

A modeling study of a tidal intrusion front and its impact on larval dispersion in the James River estuary, Virginia

A tidally-induced frontal system regularly develops in a small area off Newport News Point in the lower James River, one of the tributaries of the Chesapeake Bay. In conjunction with the front, a strong counter-clockwise eddy develops on the shoals flanking the northern side of the channel as the result of tidal interaction with the local bathymetry and estuarine stratification. A three-dimensional hydrodynamic model was applied to simulate the eddy evolution and front development, and to investigate time-varying circulation and material transport over a spring-neap tidal cycle. The model results show that variation of tidal range, together with periodic stratification-destratification of the estuary, has a significant impact on the residual circulation of the lower James River. The net surface water circulation, which takes the form of a counterclockwise eddy on the Hampton Flats, is stronger during neap tide than during spring tide. Strong stratification and weak flood current during neap tide results in a dominant ebb flow at the surface, which delays flooding within the channel and advances the phase lead of flood tide on shoals adjacent to the channel, thus increasing both period and intensity of the eddy. Front development in the area off Newport News Point provides a linkage between shoal surface water and channel bottom water, producing a strong net upriver bottom transport. The existence of the vertical transport mechanism was independently demonstrated through tracer experiments. The impact of the dynamics on larval dispersion was investigated through a series of model simulations of the movement of shellfish larvae over multiple tidal cycles following their release at selected bottom sites. These results show that eddy-induced horizontal circulation and vertical transport associated with the frontal system are important mechanisms for the retention of larval organisms in the James River.

Spatial and temporal variabilities of hypoxia in the Rappahannock River, Virginia

Hypoxia/anoxia in bottom waters of the Rappahannock River, a tributary estuary of Chesapeake Bay, was observed to persist throughout the summer in the deep basin near the river mouth; periodic reoxygenation of bottom water occurred on the shallower sill at the river mouth. The reoxygenation events were closely related to spring tide mixing. The dissolved oxygen (DO) in surface waters was always near or at the saturation level, while that of bottom waters exhibited a characteristic spatial pattern. The bottom DO decreased upriver from river mouth, reaching a minimum upriver of the deepest point of the river and increasing as the water becaume shallower further upriver. A model was formulated to describe the longitudinal distribution of DO in bottom waters. The model is based on Lagrangian concept—following a water parcel as it travels upriver along the estuarine bottom. The model successfully describes the characteristic distribution of DO and also explains the shifting of the minimum DO location in response to spring-neap cycling. A diagnostic study with the model provided insight into relationships between the bottom DO and the competing factors that contribute to the DO budget of bottom waters. The study reveals that both oxygen demand, either benthic or water column demand, and vertical mixing have a promounced effect on the severity of hypoxia in bottom waters of an estary. However, it is the vertical mixing which controls the longitudinal location of the minimum DO. The strength of gravitational circulation is also shown to affect the occurrence of hypoxia. An estuary with stronger circulation tends to have less chance for hypoxia to occur. The initial DO deficit of bottom water entering an estuary has a strong effect on DO concentration near the river mouth, but its effect diminishes in the upriver direction.

A Tidal Prism Water Quality Model for Small Coastal Basins

Abstract A tidal prism water quality model (TPWQM) was developed to provide a tool for government agencies for water quality management of small coastal basins. It simulates physical transport using the concept of tidal flushing, includes one of the most sophisticated representations of eutrophication processes in water column and benthic sediment, and employs an innovative solution scheme that is simple, accurate, and computationally efficient. The predictive capability of the water column portion of TPWQM was demonstrated through successful calibration and validation of the model with extensive data sets collected from Lynnhaven Bay, Virginia. The models general applicability was examined for four other Virginia coastal basins. One value (0.3) of the returning ratio, the only calibration parameter for physical transport, is applicable to all five coastal basins and probably would be adequate for other Virginia coastal basins without further calibration. The values of kinetic parameters determined for Lynnhaven Bay are applicable to at least two of the other four coastal basins. The model underpredicts chlorophyll-a, total carbon, and total phosphorus in two of the tested basins, which is more likely the result of underpredicted nonpoint source loads than the inaccuracy of the kinetic coefficients. Therefore, the set of kinetic coefficients may be applicable to all the Virginia coastal basins with basin-specific refinement in the estimation of nonpoint source loads.

Vertical Transport Across an Estuary Front

An estuarine front was studied with a series of field measurements and theoretical analyses. The front is located off Newport News Point in the James River of Virginia, USA. The front develops at the early phase of flood tide, moves upriver at a speed of several meters per minute, then slows down over a region of steep bottom down slope, and eventually dissipates as the flood current wanes. Numerous tracer experiments demonstrate that surface waters are injected through the front to depths of 4 m or more in the vicinity of the depth transition.

A Model Study of Flushing Characteristics of the Elizabeth River, Virginia

VIMS HEM-3D was applied to study flushing characteristics of the Elizabeth River, a tributary of the James River Estuary in the lower Chesapeake Bay. The model grid was designed to cover the entire tidal portion of the James River but have higher resolution in the Elizabeth River. The model was verified with a spatial-temporal distribution of water levels and salinity structures. For a flushing characterization study, the model was forced with 3 tidal constituents (M 2 , N 2 , and S 2 ) to encompass perigean-spring to apogean-neap tides. Fluxes temporally and spatially integrated over a cross section indicate the primary flushing is dominated by tidal transport but non-tidal transport is not negligible and becomes more significant at upper reach of the estuary. Data from a dye study and particle tracking supplement the characterization. Flushing for the overall system is not considered to be poor but locally varying.

Assessment of Long-term Water Quality Impacts of the Craney Island Eastward Expansion, Elizabeth River, Virginia

The Craney Island Dredged Material Management Area (CIDMMA) is a 2400acre, federally-owned and operated facility located in Hampton Roads, Virginia adjacent to the city of Portsmouth. A 520-acre designed expansion of the CIDMMA to the east was shown in a previous study to have the least impact to the hydrodynamic circulation of all evaluated land expansions. The 3D baroclinic hydrodynamic model HEM-3D coupled with the water quality model was applied over a domain spanning the James and Elizabeth Rivers. The model was calibrated, validated, and used to assess the long-term environmental impact of the terminal expansion, as well as the impact resulting from an intermediate construction phase. The study showed that a slight reduction in tidal prism was compensated for by an increase in the non-tidal residual flow during flood and ebb tide phases, thereby enhancing flushing. Dissolved oxygen (DO) levels due to the eastward expansion were mitigated by the enhanced exchange of Elizabeth River water with that from the oxygen-rich Lower James River. As a result, the increase of advective DO flux from the James River and local vertical mixing overcome the increase of low DO volumes. Overall, the impacts to DO levels due to both the south cell and the full expansion are minimal and are well within the range of detection limits.

Eutrophication Model Calibration as a Coupled Inverse Problem

An inverse algorithm was integrated into a real time, vertical, two-dimensional eutrophication model to assist in calibrating of the model kinetic parameters. The problem of the eutrophication model calibration was posed as a coupled inverse problem in a framework of multiobjective optimization. The solution was found by minimizing a global objective function, which was a weighted least-square of residuals between modeled water quality state variables and their corresponding observed values. Both conjugate gradient and modified Gauss-Newton methods were used to update unknown parameter values. The gradient vectors of the objective function, with respect to the model parameters, and the sensitivity coefficient matrix used to estimate the Hessian matrix were obtained by using the variational method and the influence coefficient method, respectively. In comparison to the variational method, using the influence coefficient method to calculate the sensitivity matrix provides an alternate way to estimate the Hessian matrix and Gauss-Newton direction. It requires much less effort in coding and is very efficient for estimating limited parameters. Because the sensitivity matrix is calculated during the iteration process, the convergence speed of the inverse model is improved. The quick convergence compensates for the time consumed in computing the sensitivity matrix. A series of model experiments with a real time eutrophication model of the tidal Rappahannock River were conducted. The results of the numerical experiments demonstrate the efficiency and accuracy of the inverse model. Thirteen unknown kinetic parameters were calibrated with hypothetical data sets generated by the forward model. Both data sets, with and without random errors, were used to test the inverse model. The results of model calibration demonstrate that the inverse model is capable of conducting model calibration. With the use of the inverse model, the unknown parameters can be estimated satisfactorily.

Three Dimensional Hydrodynamic Modeling Study Craney Island Eastward Expansion, Lower James River and Elizabeth River, Virginia

Recommended Citation Wang, H. V., Kim, S. C., Boon, J. D., Kuo, A. Y., Sisson, G. M., Brubaker, J. M., & Maa, J. P. (2001) Three Dimensional Hydrodynamic Modeling Study, Craney Island eastward expansion, lower James River and Elizabeth River, Virginia. Special report in applied marine science and ocean engineering ; no. 372.. Virginia Institute of Marine Science, College of William and Mary. https://doi.org/10.21220/V5372G

Water Quality in a Small Tidal Creek: Parker Creek, Virginia

Parker Creek is a branched tidal cn~ek located on the Eastern Shore of Virginia. In its southern branch, the creek receives waste inputs from a poultry processing plant. A study has been conducted to determine the effects of these inputs and to formulate a mathe~matical model of the creek system suitable for water quality planning. The model and field studies show the creek may be divided into two zones, an upstream zone dominated by freshwater flows and waste inputs, and a downstream zone dominated by conditions in adjacent Metomkin Bay. In the upstream zone of the waste-receiving branch, conditions of elevated nutrient and depressed dissolved oxygen concentrations exist. In the downstream zone, conditions are close to natural. For purposes of comparison, surveys were conducted in three similar non-impacted tidal creeks and in Metornkin Bay. From a plann.ing standpoint, the most significant result of these surveys is that violations of minimum dissolved oxygen standards may occur as a natural condition in tidal creeks.