Land-Surface Topography, Surface-Water Slope, and Water-Level Fluctuations

WCA-2A

The present slope of the land surface in WCA-2A is similar to pre-drainage conditions, 3.3 x 10^-5, towards the south-southeast. The water surface in WCA-2A generally slopes toward the southwest on a grade similar to the average land-surface slope. Sometimes, the water surface slopes toward the southeast, depending on where the water releases through structures are occurring (Romanowicz and Richardson, 2000). During the wet season, particularly when water is released from WCA-1 to WCA-2A, the water-surface slope can increase by as much as a factor of 2.

Long-term data collected by USGS and simulations by SFWMD suggest that water management imparts the dominant control on water-level fluctuations in WCA-2A. The largest and most rapid fluctuations in surface-water level in WCA-2A are caused by water releases from WCA-1, and not by precipitation and evapotranspiration. Comparisons between the Natural Systems Model and South Florida Water Management Model showed that the amplitude of water-level fluctuations for the pre-drainage Everglades in the vicinity of WCA-2A typically were 25 to 50 percent of present-day fluctuations.

A constructed berm on the extreme south side of the Hillsboro canal/levee complex (fig. 3), adjacent to WCA-2A, has an important effect on surface-water releases from WCA-1, and on water flow in WCA-2A. The purpose of the berm is to direct flow in the canal toward the east side of WCA-2A before the water enters the wetland and flows southward. When the water level rises rapidly in the tailwater canal because of a water release from WCA-1, overtopping of the berm eventually will result (at approximately 13.5 ft NGVD). After overtopping occurs, sheet flow across the berm supplies surface water directly to the wetland at all points along the Hillsboro levee.

ENR

The slope of the land surface in ENR is much steeper than in WCA-2A, because of the engineering involved. Because of subsidence, the land surface slopes in the direction opposite (northwest) of the estimated pre-drainage topographic gradient (southeast). The area of steepest slope (0.001) is immediately to the west of the L-7 levee and continues across the eastern third of ENR. Continuing to the west the ground slope is 2 x 10^-4 across the western two-thirds of ENR. Those slopes are approximately 30 times and 7 times greater, respectively, than the average land slope in WCA-2A. The ENR was designed so that surface water would flow toward the southwest. The engineered slope in that direction also is relatively steep (1 x 10^-4). Even though the land-surface slope is much greater than WCA-2A, the average water-surface slopes are similar (approximately 3 x 10^-5).

Average differences in water level across the WCA-1 levee are similar at ENR and WCA-2A, 2.4 and 2.8 ft, respectively. Variability of those water-level differences was greater at WCA-2A. Expressed as a coefficient of variation, the temporal variability of water-level differences across levees was 50 percent at WCA-2A compared with 18 percent at ENR. At ENR, surface-water levels are more widely monitored and tightly controlled by water-resources managers in an attempt to maintain relatively stable water levels in ENR. Greater variability at WCA-2A results because of water releases that can substantially decrease WCA-1 levels and increase WCA-2A levels in a matter of hours.