Figure 1. Southeastern Florida showing water-conservation areas, primary canals, and location of study site. Click for larger image.

In 1948, the United States Congress authorized the Central and Southern Florida Flood Control Project (currently managed by the South Florida Water Management District) in response to catastrophic floods that had occurred in south Florida. This enormous undertaking resulted in the construction of levees, canal networks, pumping stations, and water-conservation areas (fig. 1) to provide adequate control of water levels and surface-water routing. The initial effort established an interconnected network of levees and adjacent canals from central Palm Beach County to southern Miami-Dade County. This 80-mile long network of levees and borrow canals constitutes the eastern limit of the water-conservation areas and prevents Everglades overland sheetflow from reaching the developed areas to the east. This network includes the 14-mile long Levee 30 and adjacent canal in central Miami-Dade County. Completed in 1954, Levee 30 is part of the eastern boundary of Water Conservation Area 3B.

Determining the volume of water seeping from the water-conservation areas to the underlying aquifers is important in managing water levels in the conservation areas and freshwater deliveries to Everglades National Park. From Water Conservation Area 3B, water seeps into the Biscayne aquifer and flows relatively fast (due to high permeability of the aquifer) toward the urban and agricultural areas to the east. Ground water is also discharged to the canal along the eastern part of Levee 30. The stage in the canal, which affects the rate of discharge, is controlled by structures at the northern and southern ends of the canal. This seepage to the aquifer and canal discharge of water are critical for water-supply wells to the east and for preventing saltwater intrusion. However, lowered ground-water levels to the east have resulted in higher ground-water seepage and canal discharge, reducing surface-water flows to the south in the water-conservation area. The altering of historical flow directions and water-level durations has adversely affected parts of the Everglades ecosystem. Water managers want to restore predevelopment flow conditions and keep this ecosystem viable, while also providing for urban and agricultural needs.

A two-dimensional, cross-sectional, finite-difference, ground-water flow model and a simple application of Darcy's law were used to quantify ground-water flow from the wetland beneath Levee 30. Geologic and geophysical data, vertical seepage data from the wetland, canal discharge data, ground-water-level data, and surface-water stage data collected during 1995 and 1996 were used to develop boundary conditions and to calibrate the ground-water flow model. These data were also used as input for the application of Darcy's law.

Vertical seepage data (fig. 2) indicated that water from the wetland infiltrated the subsurface near Levee 30 at rates ranging from approximately 0.03 to 0.25 foot per day, with the gates at the structures in Levee 30 canal closed. During the same period, stage

Figure 2. Relations between head difference and change in discharge rate and between head difference and seepage rate. Click for larger image.

differences between the wetland (Water Conservation Area 3B) and Levee 30 canal ranged from approximately 0.10 to 1.25 feet (fig. 3). A layer of low-permeability limestone, located 7 to 10 feet below land surface, restricts vertical flow between the surface water in the wetland and the ground water. Based on measured water-level data, ground-water flow appears to be generally horizontal, except in the immediate vicinity of the canal. The increase in canal flow rate along a 2-mile reach of the Levee 30 canal ranged from approximately 9 to 23 cubic feet per second per mile (fig. 2) and can be primarily attributed to ground-water inflow. Flow rates in Levee 30 canal were greatest when the gates at the control structures were open.

The ground-water flow model data were compared with the measured ground-water heads and vertical seepage from the wetland. Estimating the horizontal ground-water flow rate beneath Levee 30 was difficult, owing to the uncertainty in the horizontal hydraulic conductivity of the main flow zone of the Biscayne aquifer. Measurements of ground-water flows into Levee 30 canal, a significant component of the water budget, were also uncertain, which lessened the ability to validate the model results. Because of vertical ground-water flows near Levee 30 canal and a very low hydraulic gradient east of the canal, a simplified Darcian approach does not accurately estimate the horizontal ground-water flow rate. Horizontal ground-water flow rates simulated with the ground-water flow model (for a 60-foot deep by 1-foot wide section of the Biscayne aquifer) ranged from approximately 150 to 450 cubic feet per day west of Levee 30 and from approximately 15 to 170 cubic feet per day east of Levee 30 canal. Vertical seepage from the wetlands within 500 feet of Levee 30 generally accounted for 10 to 15 percent of the total horizontal flow beneath the levee. Horizontal ground-water flow was highest during the wet-season simulations and when the gates at the control structures were open.

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report (PDF, 8.7 MB))

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