Circulation Models & Tides
1996 Abstracts
Lisa C. Roig, U. S. Army Corps of Engineers, Waterways Experiment Station, Vicksburg, MS.
The Corps of Engineers, Jacksonville District (CESAJ) is considering a variety of freshwater release scenarios for operation of the C-111 canal system. Proposed operation plans will release freshwater into the Everglades and Florida Bay. The changes that these freshwater flows will have on circulation and salinity distributions in Florida Bay are at present unknown. The mixing rate of the freshwater under the influence of tides, storms, local evaporation and precipitation can be studied using a numerical model for hydrodynamics and salinity.
The USACE Waterways Experiment Station (WES) is developing a two dimensional (2D), vertically averaged numerical model for water surface, velocity, and salinity in Florida Bay. In conjunction with a groundwater flow model of southern Florida, the hydrodynamic/ salinity model will be used to estimate the effects of various C-111 operation plans on Florida Bay. A very large, detailed numerical mesh (approximately 35000 computational nodes and 13000 computational elements) has been developed. The mesh encompasses all of Florida Bay, part of the Atlantic Ocean south of the Florida Keys to the continental shelf, and part of the Gulf of Mexico has been developed. Because circulation in the Bay is strongly controlled by small scale bathymetric features, this detailed, unstructured grid will permit engineers to examine physical mixing process that are difficult to measure in the field. Once verified, the hydrodynamic model will accept freshwater flow information from the groundwater model and calculate the resultant salinity behavior of Florida Bay.
To model the effects of alternative freshwater releases on the circulation of Florida Bay, one must resolve the horizontal bathymetry of the basins, the partially submerged mudbanks, the mangrove swamps, and the coastal boundaries. Since the water column is generally well mixed, a two-dimensional, vertically averaged model of circulation in the Bay is adequate to simulate the majority of the hydrodynamic phenomena that have been observed. The equations that describe horizontal circulation and mixing in the Bay are the vertically averaged shallow water equations, the advection-dispersion equation for dissolved salts, and an equation of state relating salinity concentration to fluid density. The model to be used must be capable of resolving the complex horizontal distribution of mudflats, islands, and mangrove swamps that control circulation in the interior of the bay. The model must include an algorithm for describing the flooding and draining of the islands and mudbanks as water levels rise and fall. The modeling system that is being applied is the TABS-MD numerical modeling system that was developed and is maintained by WES.
At the heart of the TABS-MD system is the finite element model for two-dimensional, vertically averaged free surface flows known as RMA2-WES. This model was originally developed by Dr. Ian King at Resource Management Associates (RMA) under contract to WES. A sophisticated, user friendly, graphical user interface (GUI) known as the Surface Water Modeling System (SMS) has been developed at WES to facilitate the pre-processing of input files and the post-processing of model outputs. This GUI allows the user to visualize model results in a variety of ways, including contour maps, vector maps, and animated displays of time dependent solutions. A model of Florida Bay developed using these tools will be readily available to the local sponsors for in-house modeling studies.
The TABS-MD system uses an unstructured finite element grid to discretize the flow domain. The unstructured grid approach permits elements to be locally refined in the vicinity of small scale bathymetric features and locally graded to a coarser resolution in areas where the flow is more uniform. Simulation of flooding and draining over mudbanks requires a flexible grid system that accurately depicts shoreline movement. The TABS-MD system incorporates state-of-the-art algorithms for representing moving shorelines, while retaining the computational advantages of a fixed Eulerian grid.
WES is undertaking a field data collection effort to provide synoptic measurements of hydrodynamic fluxes through tidal inlets between the Keys that will be used to verify model behavior. Salinity maps and sediment bed maps provided by the USGS are being used to develop model initial conditions, boundary conditions, and verification data for the model. Open ocean boundary conditions and wind fields are being provided by NOAA for the model verification periods. The National Park Service, the South Florida Water Management District, the CESAJ, and the Florida Department of Environmental Protection are providing additional data for the model. Dr. Barbara Hayes of Rutgers University is assisting WES in the development of boundary condition data sets derived from these data sources. In a cooperative research effort, Dr. Ned Smith of the Harbor Branch Oceanographic Institute will provide analysis of the model time series to determine the quality of the simulations when compared to long term data records.
The model development effort will produce a detailed, two-dimensional hydrodynamic and salinity model that can be used to test several different freshwater release scenarios for the C-111 canal system. The hydrodynamic/salinity model verification will be completed in calendar year 1997. The use of the model for evaluating alternative freshwater release scenarios will follow the release of the verified groundwater model which is scheduled for 1998. In advance of the groundwater model release the hydrodynamic model may be used to improve our understanding of circulation and mixing in Florida Bay, and to evaluate other possible scenarios, such as structural changes to widen the tidal inlets at selected locations in the Keys.
Brian E. Lapointe and William R. Matzie, Harbor Branch Oceanographic Institution, Inc., Big Pine Key, FL; and David Tomasko, Southwest Florida Water Management District, Venice, FL.
Anthropogenic increases in external nutrient loading to estuaries is the most frequently cited factor correlating with eutrophication, algal blooms, and decreased seagrass cover. In Florida, there are a number of locations where external nutrient loads have been manipulated by human activities. In Tampa Bay, seagrass losses during the 1960's and 1970's were attributed to increased nutrient loads that caused increased macroalgal biomass and water column chlorophyll-a. Recent management actions have decreased loads of the limiting nutrient - nitrogen - by some 35 %, which resulted in decreased macroalgal abundance and chlorophyll-a and an 11 % increase in seagrass coverage. In Sarasota Bay, studies have shown an inverse relationship between modeled nitrogen loads and measured seagrass biomass and productivity. Recently, modeled nitrogen loads to Sarasota Bay decreased by approximately 40 %, resulting in an increase in seagrass coverage of 7-11 %.
We tested the hypothesis that seagrass distribution and health in the western regions of Florida Bay may also be inversely related to external nitrogen loads. Following the diversion of stormwater runoff from Lake Okeechobee southwards towards the Everglades in the late 1970's, commercial fishermen observed increasing macroalgal biomass in the downstream waters of the eastern Gulf of Mexico and western regions of Florida Bay. To hindcast the possibility that these blooms may have been linked to increases in external N-loading, we performed physiological and biochemical studies during 1994 - 1995 of macroalgae at four stations in Florida Bay (Duck Key, eastern bay; Rankin Key, central bay; Murray Key, western bay; and Rabbit Key, southwestern bay); the studies included measurements of photosynthesis vs. irradiance curves, midday downwelling irradiance, alkaline phosphatase activity (a measure of P-limitation), and tissue analysis for carbon:nitrogen:phosphorus (C:N:P) ratios. These studies showed severe light attenuation/limitation of scant populations of macroalgae in the western bay due to the currently high concentrations of chlorophyll-a and suspended solids, which indicated a phase shift from historic submerged aquatic vegetation (SAV) to phytoplankton. Alkaline phosphatase activity of the red macroalga Laurencia intricata decreased from the eastern bay to the western bay, indicating a trend towards reduced P-limitation in western Florida Bay. The C:N ratio of Laurencia intricata also decreased from ~ 20:1 in the eastern bay to 10:1 in the western bay, also indicating N-enrichment in the western bay. Thus, it appears that the macroalgal blooms in the eastern Gulf of Mexico and western regions of Florida Bay in the late 1970's and early 1980's could have resulted, at least in part, from increased N loads from the Everglades and/or eastern Gulf of Mexico.
We also examined the possibility that the dramatic increases in chlorophyll-a and light attenuation in the western and central bay since 1991 could have been linked to increased flows and N-loads from the Everglades. Nutrient concentration and flow data from the South Florida Water Management District were used to determine nutrient gradients through the Everglades and to estimate potential external nutrient loadings to eastern Gulf waters and western Florida Bay for the 1980's compared to the period 1991-1995. These data indicated a trend of increasing N-loading to coastal waters begining in 1991, which correlated directly with the phase-shifts away from SAV towards increased phytoplankton biomass in the western and central bay since 1991. Thus, in Florida Bay, the health and distribution of seagrasses in western regions seems to be inversely related to external N loads, as is the case in Tampa Bay and Sarasota Bay. It would seem that unanticipated side effects of increased water deliveries to the Everglades might be associated with recent decreases in seagrass coverage and increased phytoplankton biomass in the downstream waters of the eastern Gulf of Mexico and Florida Bay.
Plans for the Florida Bay Water Quality Model
Mark S. Dortch, U.S. Army Corps of Engineers, Waterways Experiment Station, Vicksburg, MS.
A work plan is being prepared to define the procedures, tasks, and time and costs associated with the development and application of a water quality model of Florida Bay. The Florida Bay Program Management Committee (PMC) and various federal and state partners require this model to address questions related to the future environmental health of the Bay. The Jacksonville District, U.S. Army Corps of Engineers (CESAJ), tasked the Waterways Experiment Station (WES) to develop the work plan. The PMC, along with the CESAJ, WES, and the South Florida Water Management District (SFWMD), has planned a workshop during October 22-24, 1996, to scope the model design and specifications. Workshop recommendations will be used to develop the workplan.
Questions to be addressed by the model include the following.
Other model uses and benefits are: to gain an improved understanding of the Bay; to investigate fate of nutrients and their export through the keys towards the reefs; to evaluate the effects of various freshwater flow alternatives and other management options; to investigate the relative importance of key processes; to foster synergism with other Bay studies; and to help focus data needs and monitoring design.
It has been suggested that a dual track modeling strategy be pursued, where a box model of limited spatial resolution is initiated immediately while a more sophisticated model with much greater spatial resolution is being prepared. The simpler model will be used to quickly gain insights about the Bay and to help guide the detailed model. The simpler model can be an existing version of the detailed water quality model. Gross-scale circulation needed to drive the box model can be extracted from the Corps RMA2 hydrodynamic model presently being applied to the Bay to evaluate salinity regimes associated with changes in freshwater flows.
The conceptual model of Florida Bay states that nutrients affect phytoplankton and epiphytes, which affect light attenuation and, thus, seagrass growth. Additionally, suspended sediments affect light and seagrass. Changes in seagrass coverage affect currents and waves. Changes in current and wave conditions can influence sediment resuspension. Therefore, there is potentially a need for rather complex model interactions and feedback among hydrodynamics, sediment transport, water quality, and seagrass. A major challenge of this project will be to devise a methodology to reduce the complexity of model linkages and feedback to obtain practical solutions while retaining sufficient realism to provide reliable model predictions.
The detailed Florida Bay water quality model will consist of several individual model components. Components of the model package will include a hydrodynamic model (HM), wind-wave model, sediment transport model (STM), water quality model (WQM), benthic nutrient diagenesis model, and a seagrass model. The HM is required to drive the transport terms of the STM and WQM. The wind-wave model is needed to compute sediment erosion due to short-period waves. Information on suspended sediment from the STM will be used in the WQM to attenuate light and allow for phosphorus partitioning to solids. The WQM, benthic diagenesis model, and seagrass model have been developed within the CE-QUAL- ICM to work together interactively. The CE-QUAL-ICM model was originally developed during a study of Chesapeake Bay and has since been applied to a number of other systems. Revisions to the WQM and its submodels are envisioned to better represent Florida Bay.
It is recommended that the model be calibrated for several annual periods and confirmed for the past decade. The model must be operated for at least decade-long periods to forecast the effects of management options on water quality and seagrass. Therefore, it will be necessary to demonstrate that the model can capture changes in water quality and seagrass coverage observed during the past 10 to 15 years.
Several technical obstacles and data gaps that impede this model development have already been identified. Successful completion of this project will require significant partnering and collaboration among the Florida Bay scientific community and the modelers.
This presentation will summarize the conclusions and recommendations of the October workshop and the overall plans for model development and application as being detailed in the work plan, which is scheduled for completion during December 1996.
Last updated: 2/26/98
by: Monika Gurnée
gurnee@aoml.noaa.gov