Construction features of the Central and Southern Florida Project are suspected of contributing to the development of hypersaline conditions in nearshore embayments of central and northeastern Florida Bay as a result of diminished freshwater deliveries from the Taylor Slough and Canal C-111 drainage basins. Recent modifications in the C-111 project of the U.S. Army Corps of Engineers Central and Southern Florida Comprehensive Project Review Study (RESTUDY) include removal of 54 residual spoil mounds along the southwest bank of the canal to enhance sheet flow through the Everglades National Park (ENP) wetland and into Florida Bay. The wetland due south of C-111 in the "panhandle" area of ENP is of particular concern to current environmental restoration efforts because it constitutes a primary pathway for freshwater to reach nearshore embayments. Strategic operation of two hydraulic control structures, S-18C and S-197, at the endpoints of this 10.5-km segment of the C-111 canal can generate and(or) moderate canal to wetland flow exchanges and thereby regulate the magnitude and duration of sheet flow to Florida Bay. The objectives of this project, which is focused on the interconnected C-111 canal and wetland system, are to design, develop, and test data-collection and analytical methods to quantify the flow exchanges between the canal and the adjacent wetlands. The purposes of the research are to devise and demonstrate new methods by which to assess the effectiveness of implemented restoration actions and to develop improved models to simulate and evaluate new operational management strategies.

In September 1997, near the conclusion of the approximately 18-month-long spoil-removal effort, extensive flow measurements were made along the C-111 canal bank, the degraded spoil area, and the adjacent wetland. These data are being used to quantify and analyze canal and wetland flow exchanges, to evaluate the magnitude and extent of enhanced sheet flow into the wetlands, to develop an improved technique and approach to simulate such flow interactions, and to investigate the effect of recent construction modifications on flow-pattern and hydroperiod changes. Flow measurements were made in the 8.5-km segment of the C-111 overbank area that begins 1.5 km downstream of S-18C and ends 0.5 km upstream of S-197. Data were collected along nine transect lines perpendicular to the canal, spaced approximately 1 km apart, beginning at the southwest bank and extending about 1.5 km into the adjacent wetlands. Flow-velocity measurements were made at the canal bank, at the edge of the degraded spoil area, and at variably spaced intervals into the wetlands along the transect lines. Flow velocities were measured using portable acoustic Doppler velocity meters that have a resolution of 0.1 mm/s. The meters were retrofitted with internal electronic compasses and tilt sensors to provide flow directions that are geodetically referenced to Earth coordinates. Velocities were measured for 2 minutes, using a sampling rate of 10 Hz, at each of three points in the water column (0.2, 0.5, and 0.8 depths) for depths greater than 15 cm. For shallower depths, a 2-minute velocity measurement was made only at middepth.

Inspection of these multiple sets of velocity data revealed a south to south-southwest sheet flow primarily toward the eastern segment of Joe Bay and the western segment of Long Sound in the northeastern corner of Florida Bay. Measured flow velocities were typically on the order of 1 cm/s or less in the wetlands, and flow depths averaged about 30 cm. Velocities were generally greater in deeper areas having less dense vegetation, which verifies that flows typically follow paths of least resistance. Further analyses of these data are currently underway to determine the extent and patterns of sheet flow and to evaluate their correlation with concurrent flows through the control structures. Depth-integrated unit-flow discharges are being computed at the velocity-measurement sites from which mass fluxes through transect lines in the wetlands can be calculated. These computed fluxes in the wetland will be compared with flow discharges measured at three fixed acoustic velocity-meter installations in the canal to determine and contrast the quantity of water conveyed in the canal with that transferred to the wetland.

In a separate and related data-collection effort, flow velocities were measured along three transect lines beginning near the midpoint of the 8.5-km canal segment and extending approximately 2.3 km into the wetland. Flow measurements were made in the center and about 150 m on both sides of an established airboat trail that traverses the wetland southward from the canal. Vegetation within this and other established airboat trails in the area is substantially compressed and presents less biomass throughout the water column than does vegetation in the surrounding undisturbed wetland. Inspection of these data indicated that flow velocities in the airboat trail were consistently equal to or greater than those in the adjacent wetland. Although velocity differences were small, these findings serve to substantiate the role that vegetation plays in exerting resistance to flow. Additional research and focused data-collection efforts are needed to evaluate the significance and to quantify the effects of established airboat trails on flows and drainage within the wetland south of C-111. A number of such established trails are clearly shown in aerial photographs, including a major trail that parallels the east-west boundary of ENP and presents a conduit to potentially channel sheet flow toward the east and away from nearshore embayments in central Florida Bay. These preliminary analyses indicate that in hydraulically sensitive environments, such as the Everglades, even the most subtle alterations to the natural system can have lasting residual effects on flow patterns and resulting hydroperiods.

Investigation and simulation of canal and wetland flow interactions require basic data of finer resolution and greater accuracy than that required for evaluation of regional-scale processes. In this regard, continuing project efforts are focused on collection of additional sets of refined data that are needed to quantify canal/wetland flow exchanges and to test new numerical algorithms and model improvements. These data-collection efforts include measuring a wider range of canal and wetland-flow conditions, surveying the degraded spoil area and adjoining wetland-surface elevations at a finer resolution and to a greater accuracy, and improving characterization of the wetland vegetation for better definition and treatment of spatially variable flow-resistance effects. Collection and interpretation of these basic data, and subsequent findings derived from models built on them, support the development of improved techniques to formulate plans and to evaluate restoration actions for preservation of the ecosystem, lead to the development of better tools for managers to assess changes in the ecosystem in response to imposed alterations, and to enhance understanding of the consequences of human interventions on the ecosystem.

(This abstract was taken from the Proceedings of the South Florida Restoration Science Forum Open File Report)