DRAFT

Predictive Models for Florida Bay,
Florida Keys and Southwest Coast

Program Assessment and Status - February 1999

By

William K. Nuttle

Executive Officer,
Florida Bay and Adjacent Marine Systems Science Program

 

Interagency Science Center
98630 Overseas Highway
Key Largo, Florida 33037

I. Summary

An assessment of the modeling program reveals two areas of need that must be addressed if progress is to continue. First, the program needs clear direction from a set of objectives that are tied to the needs of management agencies in the region. Second, the function of operational management is lacking from the program. This function would address directly the issues related to coordination and integration of the component models, and it would direct the application of the predictive models in projects designed to meet the needs of management agencies.

Development of the Corps of Engineers’ hydrodynamic model has reached a point where simulations of salinity can be run for different scenarios of runoff from the Everglades. The development of FATHOM has reached a similar stage, with an analysis of the historical salinity database recently completed for the period 1965 through 1995.

II. Introduction

Predictive modeling of water quality in Florida Bay and adjacent systems relies on a suite of linked models. Each of these models is presently in development. When operational, these models will exchange information about water levels, water flow and water quality at their shared boundaries. For the most part, the area covered by each component model falls within one of the geographic sub-regions, Table 1. The exception is the SWIFT2D model, which spans the transition zone from coastal mangroves and into near-shore areas of northern Florida Bay. The modeling effort is similarly divided among the cooperating agencies of the Florida Bay Science Program. Therefore, the challenge of coordinating activities of separate agencies, which are each focused on separate geographic sub-regions, underlies the successful development and application of predictive water quality modeling in the region.

This report describes the elements of the program to support the development and application of predictive water quality models in Florida Bay. Two types of information are contained here. First, this report provides a brief overview and assessment of the predictive modeling program as it relates to the Science Program for Florida Bay and adjacent coastal systems. Second, this report reviews and summarizes the current status and immediate objectives for work on the component models. Information on the current status is a compilation of information provided by the modelers and/or their sponsoring agencies. Information on the mass-balance code, FATHOM, is included here. Although FATHOM is being developed principally as a data analysis tool; its development parallels, and to some degree complements, the development of the predictive models.

 

Table 1: Summary of models by agency and geographic sub-region

Everglades (including the Shark Slough drainage and south Dade county)

Inner Florida Bay

Outer Florida Bay (includes the inner Florida Shelf north to 10,000 Islands)

Florida Keys (including the Florida Keys National Marine Sanctuary)


III. Overview and Assessment of Modeling Program

This section provides an overview and assessment of the predictive modeling program in Florida Bay. The intent is to enhance coordination among cooperating investigators. The assessment of program needs draws on comments by members of the Program Management Committee (PMC) at a "brainstorming" session at their meeting in August 1998, recommendations by the Model Evaluation Group (MEG) from the meeting in May 1998, and to a lesser extent the recommendations of the Oversight Panel based on the Florida Bay Science conference in May 1998. Additional program elements, described at the end of this section, address the critical needs of the program revealed by this assessment. The assessment focuses exclusively on the development and application of the numerical models. Questions related to needs of the monitoring program and data management, which were raised by the MEG, are not considered here.

Objectives

The predictive modeling program is guided by two objectives. First, develop predictive models for coastal circulation and water quality with coverage of Florida Bay, the Florida Keys and the southwest Florida coast. The water quality model is understood to include the capability to simulate essential components of the benthic sea grass ecosystem and phytoplankton in the water column. Second, apply models to address the concerns of natural resource management, i.e. Everglades National Park, Florida Keys National Marine Sanctuary, State of Florida. Through these objectives, the modeling program serves the essential role within the Science Program of integrating the results of basic research on the dynamics of the bay’s ecosystems and providing a scientifically-based tool for resource managers with responsibility for these ecosystems.

Present capabilities

Development and application of the numerical, predictive models is integrated with and supported by other elements of the Science Program for Florida Bay and Adjacent Coastal Systems. The present status of the modeling program and supporting elements is summarized below:

    1. Progress has been made on the development of component circulation/water quality models for the regional subsystems. These component models, their current status and immediate objectives for work are described in the following section.

    2. Broad, multi-agency research effort supports development and parameterization of the predictive models. Elements of this research include routine monitoring of physical, chemical and ecological components of the Florida Bay and adjacent systems, and basic research to develop an understanding of the fundamental mechanics of these systems.

    3. External peer review mechanisms have been established for research and technical review of predictive models, i.e. the Model Evaluation Group. These mechanisms provide objective guidance, independent of the cooperating agencies.

    4. Research teams have been set up to coordinate research in each of five strategic areas. These teams are one possible mechanism for researchers to provide input to the development and parameterization of predictive models.

    5. PMC provides central focus, coordination and common point of contact for the overall science program. The PMC does not have the ability to allocate resources directly. Therefore, PMC lacks the capacity to direct complex programs, such as that required for development and application of predictive water quality models.

    6. Executive officer of the PMC provides support through program review, integration and communication of results to a broad audience comprised of scientists and managers.

Critical needs

Two areas of critical need are indicated by comments by the PMC and recommendations by the MEG. First, develop a set of clear objectives/expectations for the models based on the concerns of management, preferably including the definition of modeling products as deliverables. The need for to provide guidance to the model development effort through a clear statement of objectives was recognized by both the PMC and the MEG. The connection to concerns of management is implied by the overall objectives of the Science Program. Second, coordinate model development by different agencies and provide operational management for the application of the models to provide the products/deliverables identified by resource managers. The absence of coordination only has only become obvious to the PMC recently. It appears that early expectations were that the Corps of Engineers would serve in this role. In fact, coordination of a regional, interagency modeling effort falls outside of the Corps’ mandate. The PMC itself is incapable of providing the direction needed, see point 5) above.

Proposed additional program elements

The identification of areas of critical need leads directly to a description of two additional elements for the modeling program. Therefore, it is proposed to expand the program in the following areas:

    1. Develop strategic goals, specific objectives and deliverables for the models by consulting regional resource managers.
    1. Florida Keys National Marine Sanctuary (Federal and State reps)
    2. Everglades National Park
    3. US Fish and Wildlife Service
    4. South Florida Water Management District
    5. Other State of Florida (?)
    6. Selected NGOs
    1. Establish within one of the cooperating agencies the function of operational management of the overall predictive modeling effort.
    1. Integrates component models by solving technical and data communications problem associated with managing the exchange of information between the linked models.
    2. Directs modeling exercises/simulations designed to meet specific objectives set by managers.
    3. Reports results to managers.

IV. Status of Component Models

The component models under development consist of a set of linked hydrologic, hydrodynamic and water-quality models, which covers the entire region, plus ancillary models. The set of linked models consists of 3DFEMWATER (Corps of Engineers), the Princeton Ocean Model (National Ocean Service), RMA10 (Corps of Engineers), and CE-QUAL-ICM (Corps of Engineers). The keystone of the linked models is RMA10, which provides a detailed solution for the hydrodynamics of Florida Bay and adjacent coastal areas. 3DFEMWATER and the Princeton Ocean Model provide the detailed hydrologic and hydrodynamic boundary conditions needed by RMA10. 3DFEMWATER provides the rates, distribution and timing of freshwater flows from the Everglades. The Princeton Ocean Model provides tidal water levels and currents at the domain boundaries along the reef tract in the Atlantic and over the Florida Shelf. In turn, RMA10 provides input to CE-QUAL-ICM, which will simulate water quality and ecological processes in Florida Bay and the adjacent coastal systems.

The development and application of the ancillary models complement and support the development of the set of linked models. The ancillary models provide frameworks for limited, detailed investigations independent of the need to implement the full suite of linked models. Indeed, these models permit progress on these detailed investigations while the set of linked models has not yet been fully implemented. The ancillary models of most interest with respect to Florida Bay are FATHOM (Everglades National Park) and SWIFT2D (U.S. Geological Survey). FATHOM is a mass-balance model capable, at present, of simulating averaged salinity in the ~40 basins defined by the system of anastomosing banks in Florida Bay. SWIFT2D is a surface water hydrodynamic code implemented for the area spanning the transition from the mangrove wetlands of Everglades National Park into the northern part of Florida Bay.

This section provides information on the current status and immediate objectives for work on each of the component models. This information has been gathered in consultation with the researchers involved in model development or, in the case of the Corps of Engineers models, from the Jacksonville District Office, which directs the work of the Waterways Experiment Station. A summary of this information is presented first, followed by more extensive information on each model.

 

3DFEMWATER
Sponsor: Corps of Engineers

Function: 3DFEMWATER is a combined surface and groundwater hydrology and salinity model with explicit representation of control structures. For Florida Bay, 3DFEMWATER is used to provide the amounts, timing and spatial distribution of freshwater flows from the Everglades into Florida Bay and along the west coast.

Status: Two separate implementations are being developed for the region south of Tamiami Trail. One covers Everglades National Park and the other covers south Dade county and the east coast. Some simulation runs have been done using the Everglades National Park model.

Objectives: Development work is scheduled to continue until summer 1999.

 

RMA10
Sponsor: Corps of Engineers

Function: RMA10 is a 3-D hydrodynamic model that has been implemented for Florida Bay in 2-D format, the near-shore region of the Florida Shelf north to the 10,000 Islands, and through the Keys onto the reef tract down to the lower Keys. RMA10 provides detailed information on tides and currents in response to tides on the boundaries, wind, rainfall, runoff, and evaporation. RMA10 also has the capability of simulating salinity.

Status: Initial development and implementation of RMA10 is complete. The draft final report provides results of salinity simulations for three runoff scenarios.

Objectives: The PMC is requesting simulated salinity output from a series of management scenarios for discussion at the next Florida Bay Science Conference.

 

CE-QUAL-ICM
Sponsor: Corps of Engineers

Function: CE-QUAL-ICM is a detailed water quality model that incorporates simulation of the seagrass ecosystem. The model domain for CE-QUAL-ICM is coincident with the domain of the RMA10 model (above). CE-QUAL-ICM relies on output from a hydrodynamic model – in this case RMA10.

Status: Implementation of CE-QUAL-ICM with the RMA10 model is new, and this required development of additional code to translate the hydrodynamic solution of RMA10 into input that CE-QUAL-ICM could use. This task has been completed.

Objectives: CE-QUAL-ICM is in development until fall 1999. Additional development work will implement and test modules for sediment resuspension and for simulating the seagrass ecosystem.

 

Princeton Ocean Model
Sponsor: National Ocean Service (NOAA)

Function: Output from the Princeton Ocean Model, implemented for the Florida Shelf region, provides water level and current boundary conditions for the RMA10 hydrodynamic model.

Status: Results of an 18-month simulation (Sep 95 through Feb 97) have been provided for input for the RMA10 scenarios. The 18-month simulation is based on a barotropic version of the Princeton Ocean Model calibrated for tides and with wind added.

Objectives: Results of the 18-month simulation are being compared with available oceanographic data in the region for verification of the wind-plus-tide results. Efforts are being made to implement the model with the most recently available data on winds and water levels from operating monitoring stations.

 

SWIFT2D
Sponsor: U.S. Geological Survey

Function: SWIFT2D simulates 2-dimensional surface flow and transport through the mangrove wetlands in the lower eastern region of Everglades National Park and into Florida Bay.

Status: Initial calibration and testing has been performed on the period July and August 1997. A simulation has also been performed for the period August 1996 through February 1997 to coincide with the results of the Corps of Engineers modeling. Results obtained with SWIFT2D suggest that wind exerts a large influence on water movement in the mangrove wetlands an on magnitude and direction of exchange flows with Florida Bay.

Objectives: SWIFT2D is being expanded to refine model processes and include adjacent areas.

 

FATHOM
Sponsor: Everglades National Park

Function: FATHOM is primarily a mass-balance, i.e. box, model that is currently capable of simulating the spatial and temporal variation in salinity in inner Florida Bay.

Status: Implementation and testing of the mass-balance solution code is complete. Results of an investigation into the influence of variation in net freshwater supply on salinity are reported in a manuscript submitted for publication. Results of sensitivity studies with FATHOM are reported in the Year-1 report submitted to Everglades National Park.

Objectives: Work for year-2 will improve the calibration of the model, with attention to the period 1989 through 1995, and perform simulations to estimate residence times in each of the basins in Florida Bay. The PMC is requesting simulated salinity output from a series of management scenarios for discussion at the next Florida Bay Science Conference.

 

V. Information on the Models

Corps of Engineers:

3DFEMWATER (IMG meeting, 14 Dec 98)

RMA10 (WES web site and project report, "Field and Model Studies in Support of the Evaluation of Impacts of the C-111 Canal on Regional Water Resources, South Florida", dated October 1998)

 The two-dimensional model includes dynamic coupling between the salinity and hydrodynamic fields. Model development and verification were accomplished using extensive data sets taken by WES and data supplied by other agencies and organizations, such as the National Park Service, USGS, Harbor Branch Oceanographic Institute, NOAA, and the Water Management District.

Comparison of the model and prototype (field) data shows that the model reproduces tides and currents well in and around Florida Bay. The salinity modeling shows that the model responds to freshwater flows as expected and calculates a reasonable salinity field for the bay, given large uncertainties in initial conditions and in freshwater inflow. Verification period is that period for which WES data were taken March 1996 to April 1997. Model initialized with the August 1995 salinity distribution from the USGS surveys. Freshwater inflows from USGS gauged flows into northeast portion of the bay and measured net (rainfall - evaporation) atmospheric fluxes from the three WES meteorological stations. Verification (calibration) of the model was attained by adjusting bottom roughness in the model to provide the best match to tides, currents and salinity variation (in that order) over a relatively short period of time (20 days in September 1996 and February 1997).

The first model calculations examined the salinity of the Bay under varying wet season freshwater inflows. Three flow scenarios were considered. The Base flow is representative of the flow in a wet year. The NSM (Natural System Model) flow represents a conjectured flow through the everglades before human intervention. Alternative D represents a proposal for future water usage. The calculations for Florida Bay based on these three flow scenarios used tides, net rainfall, and winds for the year 1996. Calculations were made for the three-month period, September through November. Results of the three inflow calculations show that diverting more freshwater into the northeastern part of the Bay through the Taylor Slough system has these effects:

•Lowers the salinity of the northeast corner of the Bay in approximate inverse proportion to the increase in freshwater flows
•Pushes these salinity changes out to mid Bay
•Creates a sharper gradient of salinity in the mid Bay area.

The model provides flow fields for a WES water quality study of Florida Bay. To enable the model hydrodynamics to be used in conjunction with a water quality model with different spatial resolution, a unique projection technique was developed through a partnership between WES researchers and the University of Texas at Austin. This projection tool enables two-dimensional hydrodynamic results to be applied to water quality modeling without the two efforts having the same computational mesh, so that the ideal resolution for each portion of the modeling effort can be used.

CE-QUAL-ICM (progress report for period ending September 1998, approximately half way through the project term)

The objective of this two-year study is to develop a calibrated water quality and sea grass model of coarse spatial scale for Florida Bay and provide results for a management scenario of freshwater diversions. This model relies on the RMA-10 hydrodynamic code for input. (Progress on that model is reported above.) In addition, work on the water quality model requires construction of new code to manage input of velocity field to the water quality model from the RMA-10, verification of components unique to the water quality model, and compilation of nutrient flux data for Florida Bay.

An overlay mesh of approximately 1000 elements and the associated map files exist for WQM representation of Florida Bay. The WQM mesh coincides to the HM mesh lines, but with fewer elements. We now have a projection program that has acceptable run times and can be used as a practical tool for HM-WQM linkage for large, complex applications, such as the Florida Bay model mesh. All of the post-processing for HM-WQM linkage can now be executed in about a day for a month of HM output

The WQM team received two 24-hour hydrodynamic data sets, one with no precipitation/evaporation and one with precipitation/evaporation. The first set was linked to the water-quality model and conservation tests were conducted. Initial and boundary conditions for all water quality variables were set at 1.0 mg/L. All kinetics parameters were set to zero. The water quality model was run for the duration of the hydrodynamic data set and all constituents remained at their initial values. This indicates that we can run the hydrodynamic model, project output on to a reduced grid, run the water quality model on the reduced grid, and still conserve mass.

An initial version of the resuspension module has been developed and installed in the WQM. The module contains an efficient wind/wave shear stress formulation. The module was successfully tested in a stand-alone mode against an independent data set. An investigation into seagrass frictional effects for inclusion in the resuspension module was also conducted. Some additional testing and refinements remain to be completed.

A large data base for development and application of the seagrass model to Florida Bay has been assembled.

Dr. W. W. Walker, through a contract with Everglades National Park (ENP), developed 10 (1988-1997) year loading data for the WQM. Dr. Walker has completed Phase I of his contract (completed during the third quarter of FY98). Phase II will be conducted during FY99 and will be devoted to refining these loadings. The results of this work can be reviewed on a web site, http://www2.shore.net/~wwwalker/flabay. Also, ocean boundary condition data have been assembled.

NOS: Princeton Ocean Model (Status Report, January 1999. Frank Aikman III, William P. O’Connor, and George L. Mellor)

GOALS: Our long-range goal is to determine how open ocean forcing can be incorporated into a model of Florida Bay (FB) and to address the Bay's interaction and exchange with the adjacent shelf and Gulf of Mexico. This is a key component of the NOAA focus on the large-scale regional oceanographic, atmospheric, ecological, and fisheries context within which FB restoration will proceed.

OBJECTIVE: To develop, test, evaluate, and implement a Florida Shelf (FS) domain model that will provide accurate information on the boundary conditions for hydrodynamic models of Florida Bay.

RATIONALE: Any numerical model of the unique and complex FB hydrodynamics, such as the barotropic circulation model (RMA-10) being applied by the U.S. Army Corps of Engineers (COE), requires adequate open ocean boundary condition information. In fact, one of the most difficult aspects of limited-area bay modeling is the adequacy of the lateral boundary condition information available. For FB, the barotropic forcing from wind and tides is probably of primary importance, with the baroclinic forcing from, for example, meandering of the Loop Current or the influence of Loop Current Eddies most likely being of secondary importance.

FB regional observations have not had sufficient spatial or temporal coverage to provide adequate boundary condition information, and very little is actually known about the relative importance of the boundary forcing from the east and south (Florida Keys) versus the west (West Florida Shelf) of FB. However, recently obtained and on-going measurements of currents, salinity, temperature, and bottom pressure (Tom Lee [RSMAS]; Lisa Roig [COE]; Ned Smith [HBOI]) will prove useful for both defining what the conditions at the boundary are and for the evaluation of the boundary condition information derived from the National Ocean Service (NOS) model simulations.

METHODOLOGY: NOS is applying the Princeton Ocean Model (Blumberg and Mellor, 1987) for the FS domain to simulate the barotropic boundary conditions for FB. Using tidal boundary conditions modified from Schwiderski (1980) and wind forcing from the 29-km Eta atmospheric forecast model (Black, 1994), the FS model has been calibrated for tides and is now being evaluated for wind-plus-tide-driven simulations. Output water level and barotropic current fields are provided to the COE to serve as boundary conditions for their barotropic model (RMA-10) of FB. Analyzed (Eta 48-km) wind fields have been tested and evaluated using the FS model to see if this offers an improvement over the Eta forecast winds, and the assimilation of water level gauge data into the barotropic model will be tested for the FS domain.

STATUS and ACCOMPLISHMENTS:

1. An 18-month (1 September 1995 to 28 February 1997) barotropic FS model simulation of wind-plus-tide-forced water level and currents has been completed and we are in the process of evaluating the results using NOS water level gauge data, observations of bottom pressure and both moored and drifting buoy current meter data from Tom Lee (RSMAS) and Ned Smith (HBOI). At the request of the COE (Keu Kim, personal communication), the original 14-month FS simulation was extended four months to the end of February 1997, driven by analyzed Eta 48-km winds, and the output was interpolated to the RMA-10 model grid and delivered to the COE. This will allow the COE to complete a 7-month (August 1996 - February 1997) simulation for salinity evaluation of their RMA-10 model in FB.

(1a) Estimates of the cross-FB sea level slope have been compared between observations and the model simulation. The results indicate that the barotropic model water levels are in close agreement with the observations around FB (rms difference ~ 7-8 cm; correlation coefficient ~0.9), that the model cross-FB sea level slope is in qualitatively good agreement with the observations, and that the observed cross-FB sea level slope is primarily a barotropic feature.

(1b) At a number of locations, where Ned Smith (HBOI) has processed moored current meter and bottom pressure records in the western part of FB, the model currents and water levels were subjected to harmonic (tidal) analysis and compared to the identical analysis applied to the observed data. This analysis was carried out by Ned Smith (personal communication). In general, and for all tidal constituents, the model tidal water levels and east-west velocity components are in good agreement with the observed tides while the amplitude of the model north-south tidal velocity components is about twice that of the observed. This may be explained, in part, by the poor resolution of the FL Keys in our FS model (5' resolution).

2. For the seven months of August 1996 - February 1997 the FS model has also been driven with Eta 48 km analyzed winds to compare the water level results over the same period of the model driven with Eta 29 km forecast winds. The results indicate that there is not a significant difference, although the correlation coefficients (observed vs. model water levels) are slightly higher and the rms differences are slightly lower with the model results driven by analyzed winds.

3. The hourly water level and barotropic current fields of the 18-month simulation have been interpolated to the RMA-10 model grid and delivered to the COE (Keu Kim, WES, Vicksburg, MS). The COE is testing these as boundary conditions for their barotropic model simulations of FB.

4. Evaluation results and plans for further 2-D simulations and 3-D model development and simulations have been presented at the following meetings:

(4a) 5th Estuarine and Coastal Modeling Conference, American Society of Civil Engineers, Alexandria, VA, October 22-24, 1997 (O’Connor et al., 1998a);
(4b) 2nd Conference on Coastal Atmospheric and Oceanographic Prediction and Processes, American Meteorological Society, Phoenix, AZ, January 11-16, 1998, (O’Connor et al., 1998b);
(4c) Florida Bay Science Conference, Miami, FL, May 12-14, 1998; and
(4d) Marine Technology Society Ocean Community Conference, November 16-19, 1998, Baltimore, MD (Aikman and O’Connor, 1998).

FUTURE PLANS:

1. Analyzed wind fields will soon be available from the RUC-2 (Rapid Update Cycle) model, which will have the advantage of an hourly data assimilation cycle (versus 3 hours for the Eta model) and will be of 40 km resolution. We plan to test these wind fields with the FS model.

2. NOS is also examining the regional availability of real-time meteorological observations, especially on the West Florida Shelf (Mark Luther, USF, personal communication) and will consider developing our own analyzed wind fields for the region.

3. NOS will develop and test the assimilation of water level gauge data into the barotropic FS model using a number of different nudging techniques. Depending on the results, such techniques could be used for operational nowcasting using the real-time availability of NOS water level gauge data in the region.

4. A fully 3-dimensional version of the Princeton Ocean Model is being tested at Princeton University, which will encompass the extension of the Coastal Ocean Forecast System (Aikman et al., 1996) into the Gulf of Mexico. This model will be used to determine the importance of baroclinicity in the coastal circulation on the West Florida Shelf and to evaluate the baroclinic effects on FB currents and water levels. If required, 3-dimensional boundary condition information (including estimates of the 3-D salinity, temperature and density) could be provided to the COE for FB, as well as for other regional studies associated with FB, the West Florida Shelf, and the Florida Keys National Marine Sanctuary.

REFERENCES:

Aikman, F. III and W.P. O’Connor. 1998. Model-based regional boundary conditions for Florida Bay. In: Proceedings of the Marine Technology Society Ocean Community Conference, November 16-19, 1998, Baltimore, MD.

Aikman, F. III, G. L. Mellor, T. Ezer, D. Sheinin, P. Chen, L. Breaker, K. Bosley, and D. B. Rao. 1996. Towards an operational nowcast/forecast system for the U.S. East Coast. In: Modern Approaches to Data Assimilation in Ocean Modeling. P. Malanotte-Rizzoli (Editor). Elsevier Oceanography Series, 61, 347-376.

Black, T. L. 1994. The new NMC mesoscale Eta model: Description and Forecast Examples. Weather and Forecasting, 9, 265-278.

Blumberg, A.F., and G.L. Mellor. 1987. A description of a three-dimensional coastal ocean circulation model. Three-Dimensional Coastal Ocean Models, 4, edited by N. Heaps. American Geophysical Union, 208p.

O’Connor, W.P., F. Aikman III, E.J. Wei, and P.H. Richardson. 1998a. Comparison of observed and forecasted sea levels along the West Florida coast. In: Estuarine and Coastal Modeling, Proceedings of the 5th International Conference, American Society of Civil Engineers, M.L. Spaulding and A.F. Blumberg (editors), October 22-24, 1997, Alexandria, VA, 601-612.

O’Connor, W.P., F. Aikman III, E.J. Wei, and P.H. Richardson. 1998b. Comparison of observed and forecasted sea levels for the Texas coast near Galveston Bay. Preprints of the 2nd Conference on Coastal Atmospheric and Oceanographic Prediction and Processes, American Meteorological Society, January 11-16, 1998, Phoenix, AZ, 23-29.

Schwiderski, E. W. 1980. On Charting Global Ocean Tides. Reviews of Geophysics and Space Physics, 18(1), 243-268.

USGS: SWIFT2D (TWO-DIMENSIONAL SIMULATION OF FLOW AND TRANSPORT TO FLORIDA BAY THROUGH THE SOUTHERN INLAND AND COASTAL SYSTEMS. By Eric D. Swain)

The large number of process studies in the southern Everglades area have been integrated into a two-dimensional numerical model of flow through the wetlands and into Florida Bay. The model chosen for this task is the Surface Water Integrated Flow and Transport in Two Dimensions (SWIFT2D) code. SWIFT2D numerically solves finite-difference forms of the vertically integrated equations of mass and momentum conservation in conjunction with transport equations for heat, salt, and constituent fluxes. An equation of state for salt balance is included in the model to account for pressure-gradient effects, thus the hydrodynamic and transport computations are directly coupled. Information on ground-surface elevations, rainfall, evapotranspiration, frictional resistance terms, boundary water levels and flows, wetland flow velocities, ground-water inflows, and wind frictional terms are all derived from the process studies and incorporated into the model. This serves to synthesize the results of the process studies into an integrated representation of the hydrologic regime.

Modifications to the model code allow more sophisticated representations of parameters developed from the process studies. Evapotranspiration is computed for each model cell based on field derived parameters, and frictional resistance terms are based on vegetation types. Rainfall in distributed between stations, and ground-water inflows are accounted for.

For initial calibration and testing a simulation period of July-August 1997 was used. This period coincided with field measurements of flow velocities in the wetlands and boundary flows. The field data that are most significant for model comparison are the continuous discharge measurements made at five coastal creeks. The topologic high along the coast, referred to as the Buttonwood Embankment, acts as a major hydraulic control, and the flow at the creeks is a good indication of the behavior of the system. Since field research into most of the controlling processes yields most of the input data, the calibration parameters are kept to a minimum. This allows for a more confident simulation results.

The comparison with creek flows highlighted the importance of wind forcing on water movement. Frequent flow reversals (inland direction) measured in the creeks were not simulated whatsoever without incorporating wind into the model. Using only field measured wind speed and direction in the model, these reversals were well simulated. Testing shows that the water level fluctuations in the Bay due to wind are not the primary driving force in flow reversals. Simulating the measured water-level fluctuations in the Bay without wind forcing does not represent the flow reversals. Inland effects of wind are important. With the exception of the deeper portions of Taylor Slough, flow in the wetlands may be induced in any direction by the wind.

The wind effects are also apparent in comparisons with measured wetland flows. Flow direction varies so rapidly that model-produced daily averages cannot be compared to field measurements made over a period of several minutes. However, simultaneous model and field values for unit discharge (average velocity times depth) coincide well. Both the model and field measurements indicated times when southward flow in the deeper parts of Taylor Slough occurred simultaneously with wind driven flow to the north in adjacent shallow wetlands.

The flat, slow flowing wetlands have very low gradients; the largest water-level gradients and the major flow control occur at the coast. Water-level gradients tend to be on the order of 0.15 ft/mile in the inland wetlands, but model results show gradients of 0.5 ft/mile at times along the coast. This finding is supported by specific water level measurements made along one of the coastal creek. The topologic high along most of the coast is the most important flow control in the system, and usually confines flow to the coastal creeks.

During wetter periods, negligible salinity exists in Joe Bay and the other sub-embayments, and a relatively sharp salinity gradient exists offshore. Despite periodic wind-induced flow reversals, salinity is generally flushed from inland areas in wet periods. During dry periods, significant salinity levels are possible in the Seven Palms Lake area, and some in Joe Bay.

A longer period of August 1996 - February 1997 is simulated to represent coastal boundaries for hydrologic models developed by the U. S. Army Corps of Engineers (ACOE). Two ACOE models are under development; one has the Florida Bay coast as a southern boundary, the other has the coast as a northern boundary. Thus, a reliable predictor of coastal flows is needed, as supplied by this longer period simulation. This simulation used many of the same input parameters as the shorter simulation. One difference is the replacement of the northern flow boundary at Taylor Slough Bridge with a water level boundary. This is due to a lack of flow data for the seven month period. This seven-month simulation allows a quantification of the long-term effects of wind on total flow at the coast.

Although the short-term effects can be dramatic, the question of the net effect on cumulative volumes required separate analysis. At the largest outflow point, Trout Creek, computed volumes are 18 percent lower with wind effects. Measured and computed volumes are within 9 percent of each other, so the wind differences can be considered significant. At smaller outflow points, such as Taylor Creek, wind may not have such a larger effect; differences were less than 2 percent. Trout Creek has a deeper water area upstream, Joe Bay, within which seiching probably accounts for the higher sensitivity to wind.

Both sets of simulations indicate that the Upper portion of Taylor Slough flows southwesterly, but the water generally proceeds southeasterly out of the lower Slough towards the Joe Bay area. The West Lake/Seven Palms Lake area, which appears inline with the southwesterly orientation of Taylor Slough, actually receives less water from the Slough than Joe Bay. This is due to the relatively high topography north of the Lakes area and the higher hydraulic gradient between Taylor Slough and Joe Bay. The West Lake/Seven Palms Lake area can be almost completely cut off from Taylor Slough during dryer periods. These results are backed up by the flow measurements at the coastal creeks.

The SICS model is a useful tool in understanding the effects of controlling processes on the coastal hydrology of Florida Bay and defining boundaries for other models. It is currently under expansion to include adjacent areas and further refinement of the model processes.



ENP: FATHOM

FATHOM simulates variation in salinity in Florida Bay based on hydrologic inputs for the period 1965 through 1995. Refinement and verification of the solution routine were completed in FY98. Preliminary simulation results and comparison to Robblee’s historical salinity database were delivered to ENP in September. The annual report for FY98 includes results of a model sensitivity study. A manuscript describing these results is in preparation for publication, tentatively planned for the Journal of Coastal Research.

Work planned for FY99 will investigate the bay-wide response of salinity to different hydrologic scenarios, which will reflect the anticipated changes in Everglades hydrology due to restoration. Further work also will include refinements in the bathymetry and salinity input data. The bathymetry GIS data have been analyzed to provide an objective measure of bank widths. This information is parameterizes the hydraulic solution for tidal mixing in the model. For work completed in FY98, this information was input manually. Also, FATHOM will be modified to accept a time-varying salinity boundary condition along the Keys and the western boundary with the Gulf of Mexico. Results of work in FY98 suggest that this is a significant source of variation in salinity in Florida Bay.