Meng Xia

Modeling of the Cape Fear River Estuary Plume

The Environmental Fluid Dynamic Code, an estuarine and coastal ocean circulation model, is used to simulate the distribution of the salinity plume in the vicinity of the mouth of the Cape Fear River Estuary, North Carolina. The individual and coupled effects of the astronomical tides, river discharge, and atmospheric winds on the spatial and temporal distributions of coastal water levels and the salinity plume were investigated. These modeled effects were compared with water level observations made by the National Oceanic and Atmospheric Administration and salinity surveys conducted by the Coastal Ocean Research and Monitoring Program. Model results and observations of salinity distributions and coastal water level showed good agreement. The simulations indicate that strong winds tend to reduce the surface plume size and distort the bulge shape near the estuary mouth due to enhanced wind-induced surface mixing. Under normal discharge conditions, tides, and light winds, the southward outwelling plume veers west. Relatively moderate winds can mechanically reverse the flow direction of the plume. Under conditions of weak to moderate winds the water column does not mix vertically to the bottom, while in strong wind cases the plume becomes vertically well mixed. Under conditions of high river discharge the plume increases in size and reaches the bottom. Vertical mixing induced by strong spring tides can also enable the plume to reach the bottom.

Numerical Simulation of Salinity and Dissolved Oxygen at Perdido Bay and Adjacent Coastal Ocean

Abstract Environmental fluid dynamic code (EFDC), a numerical estuarine and coastal ocean circulation hydrodynamic model, was used to simulate the distribution of the salinity, temperature, nutrients, and dissolved oxygen (DO) in Perdido Bay and adjacent Gulf of Mexico. External forcing factors included the coupled effects of the astronomical tides, river discharge, and atmospheric winds on the spatial and temporal distributions of salinity and DO. Modeled time series were in good agreement with field observations of water level, nutrients, temperature, salinity, and DO. Perdido Bay and adjacent northern Gulf of Mexico coasts can be divided into two areas according to salinity, water level, and DO concentrations. The first area was lower Perdido Bay and the associated Gulf of Mexico coasts, acting primarily under the influence of tidal forcing, which increases the vertical stratification. The second division was upper Perdido Bay, which was influenced by both tidal forcing and freshwater inflow. Simulations also indicated winds influenced the salinity and DO distributions, with an enhanced surface pressure gradient. Tidal effects were also important for conducting salinity and water quality simulations in Perdido Bay. Low amplitude tides induced relatively weak vertical mixing and favored the establishment of stratification at the bay, especially along deeper bathymetry. Flood tides influenced the distribution of salinity and DO more than ebb tides, specifically along shallow bathymetry.

Hydrodynamic Modeling of St. Louis Bay Estuary and Watershed Using EFDC and HSPF

Abstract St. Louis Bay estuary is a vital water body in the Mississippi Gulf Coast Region and greatly affects the water quality in the Mississippi Sound. As the first step of total maximum daily load (TMDL) study, a hydrodynamics model was developed by integrating Hydrological Simulation Program Fortran (HSPF) and Environmental Fluid Dynamics Code (EFDC). In this application the EFDC model was configured to simulate time-varying surface water elevation, velocity, salinity, and water temperature. The HSPF was applied to compute the fresh water discharge from the upstream watersheds. The model reasonably simulated the tidal range and phase. The simulated water temperature and salinity showed good and fairly good agreement with observations. The calculated correlation coefficients between computed and observed velocity were lower compared with those for water level, temperature, and salinity, but the magnitudes of simulated velocity were found to be in the range of observed data. The wind data was found to have strong impacts on velocity simulation by modeling verification tests. Near the study area, there is wind data available only at one station, which has been applied to the entire modeling domain. The lack of high-resolution wind data makes it very difficult to simulate the velocity distribution well. It is anticipated and recommended that the development of this model be continued to synthesize additional field data into the modeling process.

A Numerical Study of Storm Surge in the Cape Fear River Estuary and Adjacent Coast

Abstract The Cape Fear River Estuary (CFRE) region is a coastal domain that has experienced considerable threats and impacts from tropical cyclones. It is also an important nursery for juvenile fish, crabs, shrimp, and other biological species. Thus, predictions about the physical responses of the CFRE system to extreme weather events are important to the protection of life and property and to the economical well-being of local residents. In this study, the Princeton Ocean Model (POM) is used to simulate tropical cyclone storm–induced surge, inundation, and coastal circulation in the CFRE and the adjacent Long Bay using a three-level nesting approach. Hindcasts of the hydrodynamic responses of the CFRE system to historic events were performed for Hurricanes Fran, Floyd, Bertha, and Charley. Comparisons were also made for the modeling results and the observations.

Investigation of interbasin exchange and interannual variability in Lake Erie using an unstructured‐grid hydrodynamic model

Interbasin exchange and interannual variability in Lake Eries three basins are investigated with the help of a three-dimensional unstructured-grid-based Finite Volume Coastal Ocean Model (FVCOM). Experiments were carried out to investigate the influence of grid resolutions and different sources of wind forcing on the lake dynamics. Based on the calibrated model, we investigated the sensitivity of lake dynamics to major external forcing, and seasonal climatological circulation patterns are presented and compared with the observational data and existing model results. It was found that water exchange between the western basin (WB) and the central basin (CB) was mainly driven by hydraulic and density-driven flows, while density-driven flows dominate the interaction between the CB and the eastern basin (EB). River-induced hydraulic flows magnify the eastward water exchange and impede the westward one. Surface wind forcing shifts the pathway of hydraulic flows in the WB, determines the gyre pattern in the CB, contributes to thermal mixing, and magnifies interbasin water exchange during winter. Interannual variability is mainly driven by the differences in atmospheric forcing, and is most prominent in the CB.

Influence of physical forcing on bottom-water dissolved oxygen within Caloosahatchee River Estuary, Florida.

Environmental Fluid Dynamics Code, a numerical estuarine and coastal ocean circulation hydrodynamic and eutrophication model, was used to simulate the distributions of dissolved oxygen (DO), salinity, water temperature, and nutrients in the Caloosahatchee River Estuary. Modeled DO, salinity, and water temperature were in good agreement with field observational data from the Florida Department of Environmental Protection and South Florida Water Management District. Sensitivity analyses identified the effects of river discharge, atmospheric winds, and tidal forcing on the spatial and temporal distributions of DO. Simulation results indicated that vertical mixing due to wind forcing increased the bottom DO concentration. River discharge enhanced stratification in deep locations but propagated vertical mixing in the shallow upper estuary. Finally, tidal forcing heavily influenced bottom layer DO concentrations throughout the whole river estuary.

Dynamics of the Chesapeake Bay outflow plume: Realistic plume simulation and its seasonal and interannual variability

The three-dimensional unstructured-grid Finite Volume Coastal Ocean Model (FVCOM) was implemented for Chesapeake Bay and its adjacent coastal ocean to delineate the realistic Chesapeake Bay outflow plume (CBOP) as well as its seasonal and interannual variability. Applying the appropriate horizontal and vertical resolution, the model exhibited relatively high skill in matching the observational water level, temperature, and salinity from 2003 to 2012. The simulated surface plume structure was verified by comparing output to the HF radar current measurements, earlier field observations, and the MODIS and AVHRR satellite imagery. According to the orientation, shape, and size of the CBOP from both model snapshots and satellite images, five types of real-time plume behavior were detected, which implied strong regulation by wind and river discharge. In addition to the episodic plume modulation, horizontal and vertical structure of the CBOP exhibited variations on seasonal and interannual temporal scales. Seasonally, river discharge with a 1 month lag was primarily responsible for the surface plume area variation, while the plume thickness was mainly correlated to wind magnitude. On the interannual scale, river discharge was the predominant source of variability in both surface plume area and depth; however, the southerly winds also influenced the offshore plume depth. In addition, large-scale climate variability, such as the North Atlantic Oscillation, could potentially affect the plume signature in the long term by altering wind and upwelling dynamics, underlining the need to understand the impacts of climate change on buoyant plumes, such as the CBOP.

Winds and the orientation of a coastal plane estuary plume

[1] Based on a calibrated coastal plane estuary plume model, ideal model hindcasts of estuary plumes are used to describe the evolution of the plume pattern in response to river discharge and local wind forcing by selecting a typical partially mixed estuary (the Cape Fear River Estuary or CFRE). With the help of an existing calibrated plume model, as described by Xia et al. (2007), simulations were conducted using different parameters to evaluate the plume behavior type and its change associated with the variation of wind forcing and river discharge. The simulations indicate that relatively moderate winds can mechanically reverse the flow direction of the plume. Downwelling favorably wind will pin the plume to the coasts while the upwelling plume could induce plume from the left side to right side in the application to CFRE. It was found that six major types of plumes may occur in the estuary and in the corresponding coastal ocean. To better understand these plumes in the CFRE and other similar river estuary systems, we also investigated how the plumes transition from one type to another. Results showed that wind direction, wind speed, and sometimes river discharge contribute to plume transitions.

Water quality assessment of coastal Caloosahatchee River watershed, Florida

Caloosahatchee River watershed and estuary has experienced a general decline in the water quality over the last several decades due to agriculture practices, development, and other human activities. The objective of this study is to assess the water quality condition in coastal Caloosahatchee River watershed by analyzing the data collected by South Florida Water Management District and Lee County. Results indicated that during 1995 to 2006, averaged annually, Lake Okeechobee released 1124 million m3 of freshwater into the Caloosahatchee River, whereas the average annual freshwater discharge out of the Caloosahatchee River was approximately 2277 million m3. Lake Okeechobee might have more impacts on the water quality condition of Caloosahatchee River in dry season than wet season. The loads ratios of Lake Okeechobee to those out of Caloosahatchee River were much higher in dry season than wet season for flow (72% to 36%), total phosphorus (63% to 20%), total nitrogen (72% to 41%), organic nitrogen (85% to 47%), and NH3 (78% to 39%). In the coastal watershed area where the urban area is concentrated, of the total 5453 water samples, 74% of them have dissolved oxygen concentration less than 5 mg L− 1, the United States Environmental Protection Agency and Florida Department of Environmental Protection water quality standard. Only in January is the average monthly dissolved oxygen concentration higher than 5 mg L− 1.

Cape Fear River Estuary Plume Modeling: Model Configuration and Sensitivity Experiments

In this study, Environmental Fluid Dynamic Code (EFDC) is used to simulate the salinity plume distribution in the mouth of the Cape Fear River Estuary (CFRE). Effects of astronomical tide, river discharge and wind on the CFRE salinity plume were investigated. The spatial and temporal characteristics of the model simulated salinity plume are compared with observations measured by the Coastal Ocean Research and Monitoring Program (CORMP). The results indicate that model results and observations show a good agreement in water level and salinity. The simulations also indicate that strong winds tend to reduce the surface CFRE plume size and distorting the bulge region near the estuary mouth due to enhanced wind induced surface mixing. Even moderate wind speeds could fully reverse the buoyancy-driven plume structure in CFRE under normal river discharge conditions. Tide and the river. discharge also important factors ....