Program: Fragile Environments
Project Title: Southern Inland and Coastal Systems (SICS) Model Development
Location of Study Area: South Florida/ Everglades Ecosystem
Project Start Date: June 1996
Project End Date: September 1999
Project Number: FL-61700
Project Chief: Eric Swain
Region/Division/Team/Section:
USGS/VRD Southeast Region, Miami Subdistrict
E-mail:edswain@usgs.gov
Phone: (305) 526-2895
Fax: (305) 526-2881
Mailing Address:
9100 NW 36 St. #107
Miami, FL 33178
Program Element(s)/Task(s)
Modeling and Support Studies for Southern Inland Coastal Systems of South Dade County (Including the Buttonwood Embankment) Task 1.6 - Sheet Flow Model of the Southern Marsh and Mangrove Fringe
Panel:
Collaborators, Clients:
The Army Corps of Engineers will be one of the major clients, as they require the surface water flows along the Florida Bay coast for their Inland and estuary models. The results of this project will yield important information for the development of model boundaries.

One of the most crucial of the data requirements for the project is the elevational data. The terrain in the study area Is not conducive to surveying. A full ground-based survey of the area with sufficient spatial resolution for a detailed numerical model would be an expensive and arduous task. The USGS Mapping Division is using helicopter based surveying to find point elevations on a 1/4 mile spacing. Areas with significant features, such as low points along the Embankment itself, are identified for additional surveying,

As part of another South Florida Fragile Environments (SFFE) project, flows in the major creeks that cut the Embankment are measured acoustically and conductivity measurements are made. As well as providing important flow and salinity information, these sites also provide access for ground-based surveying to validate and supplement the helicopter surveys.

Vegetative frictional resistance terms for the model are very important. A companion SFFE project is using lab and field studies to determine flow resistance terms for the types of vegetation found in this area. This information will be used, as well as data from the field site for this flow resistance study, which is at the north end of the Buttonwood model area.

Evapotranspiration (ET) in wetlands is an important hydrologic component, and difficult to ascertain. Another SHE project is determining ET rates for different wetland types in the area, and these rates will be used in the model. The numerical model requires some code modification to incorporate ET losses into the mass balance.

Boundary data comes from numerous sources, including other USGS projects, South Florida Water Management District, and Everglades National Park. The data from all these sources is being compiled into a database that can be accessed through a GIS system. This will be used to setup model grid data. As well as boundary data, data collected at sites Internal to the study area, such as water level, discharge, conductivity, wind, ET parameters, and frictional resistance parameters, can be referenced to location and interpolated for the model grid.

BACKGROUND NARRATIVES

Project Summary: In order to determine the effects of freshwater inflows and dynamic forcing mechanisms on flow patterns and salinity conditions in the subtidal embayments of northeast Florida Bay, a mathematical/numerical hydrodynamic/transport model of the Southern Inland and Coastal System (SICS) area is to be developed, implemented, calibrated, and verified with field-collected data. The model can also be used to study the significance of terrain relief, such as the Buttonwood embankment, and dynamic effects, such as wind and weather fronts, on flow patterns and salinity conditions and to provide boundary-condition information in the form of fluxes and gradients for Florida Bay model development.

Project Justification: One problem of particular concern and uncertainty to water management agencies for the South Florida Ecosystem is what effect future infrastructure and hydrologic changes to Taylor Slough and C-111 will have on the Everglades wetlands and the coastal mangrove ecotone of northeast Florida Bay. Specifically, hydroperiods and hydropatterns, which relate to the duration, timing, and extent of wetland inundation, in the southern part of the Everglades have been greatly distorted to the detriment of plant and animal life as evidenced by shifts in biologic and vegetative species. The quantity, timing, and location of freshwater flows to the subtidal embayments of northeast Florida Bay have been significantly altered by modification of inflows from the headwaters of the Taylor Slough and C- III drainage basins thereby contributing to aqueous stresses associated with the development of hypersaline conditions. Moreover, excess nutrients and contaminants are adding to the problems experienced by living organisms in both the wetlands and associated subtidal embayments of Florida Bay as well as the Bay itself. For more than a decade, the National Park Service, Army Corps of Engineers, and the South Florida Water Management District have been working jointly on design modifications to the Central and South Florida Project features to reestablish more natural surface flows through the Everglades National Park and into Florida Bay. Numerous process studies are underway and(or) planned to evaluate the effects of implemented redesigns, yet no project is focused on synthesizing and integrating process-study findings into a cohesive management tool to evaluate these plans prior to implementation or to assess the results of implemented restoration actions. A coupled hydrodynamic and constituent transport model is needed to integrate process-study findings in order to evaluate the variable forcing mechanisms that govern both the flow of water and concurrent transport of waterborne constituents in and through the southern Everglades wetlands that discharge into the subtidal embayments surrounding northeast Florida Bay. Once fully developed, implemented, and calibrated the hydrodynamic/transport model can be used to investigate wetland response to freshwater inflows and to compute resultant salinity patterns and concentrations in the subtidal embayments as functions of freshwater inflows and other dynamic forcing mechanisms in order to quantify, assess, and thereby systematically guide restoration efforts.

Project Objectives: Based on site parameter information from other Fragile Environments projects, the USGS Surface-Water Integrated Flow and Transport two- dimensional hydrodynamic/transport model (SWIFT2D) is to be particularized to this mixed sheetflow, canal, and tidal regime. 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. Once fully developed, implemented, and calibrated the model will be available to investigate hydroperiods and hydropatterns representing current conditions and to test and evaluate hypotheses pertaining to recommended restoration scenarios. All relevant forcing functions having a potential impact on restoration decisions will thus be concurrently and conjunctively treated in a consistent and rigorous mathematical/numerical model framework. The findings of all restoration initiatives and process studies will thereby be synthesized for ease of management evaluation of restoration plans resulting in improved decision making.

Overall Strategy, Study Design, and Planned Major Products: This project will integrate the results of ongoing process studies to verify the mathematical representation of critical forcing mechanisms and(or) to empirically define relations to particularize the generic SWIFT21) model, as appropriate, to represent the hydrologic, hydrodynamic, and transport properties of the SICS study area. The model will be implemented to the study area using land-surface elevations, topographic features, vegetation characteristics, soil conditions, bottom-material classifications, and bathymetric data collected within ongoing and proposed mapping efforts. Once fully developed and implemented to characterize the study area, the model will be calibrated and verified using concurrent sets of measured data defining mass fluxes and salinity concentrations at strategic transcct locations within the wetlands. Flow discharges and specific conductance values for determination of salt concentrations being measured at key outflow points along the mangrove fringe wilt be used for model calibration and verification along the wetland-bay interface. Sensitivity experiments will be conducted with critical model parameters both to establish error bands for simulation results and to identify critical factors controlling flow dynamics and transport properties throughout the SICS study area. Boundary-condition influences will be evaluated, demonstrated, and documented by numerical experiments. Design and setup of numerical simulations representing past and current flow and transport properties will be developed using available data, or hypothesized values as necessary, to establish baseline conditions for evaluating and contrasting the effects of future changes to the ecosystem using the calibrated model. A final report will cover the model development and calibration from the initial studies to the final model predictions of the hydrologic regime.

WORK PLAN

Overall:
The model requirements can be summarized as such:
1) Representing two-dimensional overland flow with dynamic terms, wind effects, and drying and rewetting of grid cells.
2) Represent dynamic transport of salinity with coupled effects on density and flow.
3) Represent one-dimensional flow through creeks on smaller spatial scale than the overland flow grid.
4) Represent ground-water leakage as boundary conditions to the overland flow.

The SWIFT2D model in its standard form satisfies the first two requirements. Model grid spacing is 1000 feet, based on how much interpolation is reasonable with the field data, and to create a manageable size of the model (98 by 148 cells). Figure 1 shows the study area and model grid for the initial inodel without the extension to Shark River that is proposed for FY98. The field conductivity data is used to calibrate the salinity transport component of the model.

The third requirement, representing the creeks with a one-dimensional model, is necessary because the creeks that cut the Embankment are less than 100 feet in width and it is not feasible to represent the creeks at the scale of SWIFT21) model cells. The one-dimensional dynamic surface water digital model BRANCH is recoded to be incorporated with SWIFT21). The upstream boundary of a creek represented in BRANCH is set to the water level in the corresponding SWIFT213 cell, and the computed BRANCH flow is removed from the SWIFT21) cell.

Insight into the development of ground-water leakage boundaries, for requirement four, can be obtained from the mass balance of the model at the crucial areas around C-111 and the Buttonwood Embankment. Local scale ground-water leakage will manifest itself as unexplained mass transfer through a levee or the Embankment. This leakage may also be a significant transport mechanism for salinity. The local scale leakage is represented as a head dependent flow boundary.

Hypothetical scenarios can be simulated to predict flow and transport under various hydrologic conditions. Geologic investigative techniques are being applied to determine the elevations of the Embankment in the past, and predict future elevations. These different conditions can easily be simulated with the SICS model. Different boundary inflows at Taylor Slough Bridge (at the headwater of Taylor Slough) and along C-111, based on a variety of Everglades Restoration scenarios, can be simulated in the model. Model results will show the timings and distributions of flows in ENP and through the embankment to Florida Bay, salinity exchange with the Bay, frequency and extent of inundation, and changes in the system that Restoration may induce.

    1. Initial rough model calibration                            September 1997 
    2. Final comollation of all input parameters 
       from external sources                                      December 1997
    3. Incorporation of salinity transport component              April 1998
    4. Completion of all field velocity measurements              May 1998
    5. First area calibrated model results                        July 1998
    6. Initial model of second area                               Dec 1998
    7. First draft of report                                      Feb 1999 
    8. Final calibrated model results                             June 1999 
    9. Final report approved                                      July 1999
    10. Report published and distributed                          Sept 1999