Verification of a 2D Hydrodynamic Model of Florida Bay

Topical Area: Water Circulation and Currents

Keu W. Kim, Robert McAdory and Thad Pratt, US Army Engineer Waterways Experiment Station (WES), Vicksburg, MS

Objectives: The objectives of this study are: 1) to develop a two-dimensional (2D) hydrodynamic and salinity model of Florida Bay; 2) to use the model to study the effects of alternative freshwater releases on circulation and salinity distribution in the bay; and 3) to provide flow fields to a water quality model of the bay.

Technical Approach: The TABS-Multi Dimensional (MD) numerical modeling system is being used in this study. The TABS-MD system is an existing suite of hydrodynamic, salinity, and sedimentation numerical models which are used in conjunction with the graphical user environment within SMS (Surfacewater Modeling System) which has seen widespread application to estuarine and riverine water bodies. Gulf of Mexico area estuarine applications include Laguna Madre and Galveston Bay, TX, and Atchafalaya Bay and Biloxi Marshes, LA. Florida Bay is a complex system, wherein circulation and mixing are controlled by small-scale geomorphic features such as cuts, inlets, and breaches between islands and mud banks that connect the many interior basins. The TABS-MD system model RMA10-WES was used for simulation of circulation and mixing in Florida Bay because it incorporates features such as an unstructured grid that allows resolution to be used where needed to define system geomorphic features, marsh porosity wetting and drying to allow for emergent areas such as mud flats during tidal cycles or wind events, evaporation and rainfall, coupled salinity gradient forcings, and the capability to model sediment transport or extend calculations to three-dimensions as needed. The modeled area extends for about 120 km from Barnes Sound in the north to Big Pine Key in the south and includes the Shark River region on the southwest corner of the Florida peninsula. Boundary conditions include tides and ocean salinity around the southern and western perimeters of the bay in, respectively, the Florida Straits and the Gulf of Mexico, winds and evaporation/rainfall over the modeled area, and freshwater inflows from the mainland. Model verification primarily involved reproducing tides, velocities, salinities, and fluxes through cuts and inlets.

Field Data Collection Program: To aid in model verification and to provide boundary conditions, a field data collection effort was conducted during March 1996 through April 1997. Long term water surface elevations, salinity, water velocity, and meteorological data were collected at several locations in the bay. Two short term intensive data collection efforts were conducted in September 1996 and February 1997, including ADCP data collected over 20 ranges to attempt to define the water flux through the cuts and inlets at or near the eastern and southern fringes of the bay. During the long term data collection, five stations were maintained near the southern part of the bay near Florida Key inlets to provide water surface elevation and salinity data, three meteorological stations were maintained along an east west line through the bay, and ADP velocity data were also collected from three stations along the 81° 05' meridian on the western edge of the bay proper. These data were supplemented with data from the Everglades National Park (NPS), the USGS, and the Harbor Branch Oceanographic Institute (HBOI). Offshore water level boundary conditions were obtained from the results of a large-scale numerical modeling of the Atlantic Ocean, the Gulf of Mexico, and the Caribbean Sea.

Summary of Results to Date: The numerical mesh generated to represent the modeled system consists of 33,824 surface nodes in 12,473 elements. The modeled area was divided into 48 subregions based on the bottom materials (e.g., mud banks, dense sea grass) of the region. Model verification was initially carried out for the two intensive data collection periods, along with NPS and HBOI data. Comparisons were then made to the longer data records. Water surface elevations, velocities, and inlet fluxes were reproduced well. Due to the small amount of extant data for freshwater inflow, initial salinity comparisons were made to model calculations using estimates for inflows. Initial salinity comparisons were, therefore, based on qualitative behavior and trends.

Outlook for Remaining Work: The USGS has recently begun providing freshwater inflows for several stations in and near Florida Bay. As flow data become available, we are incorporating them into our calculations and reassessing the salinity verification. Additionally, sensitivity experiments are under way to determine the influence of freshwater inflows on salinity in the interior of the bay. Later in 1998, we will initiate production experiments for inflow scenarios derived from a companion study of the groundwater and hydrology of south Florida, and calculations in support of the companion water quality model will be continued.

Permission was granted by the Chief of Engineers to publish this information.