Construction recently began in Florida on a key component of the Central Everglades Planning Project, an ambitious joint effort by the U.S. Army Corps of Engineers and the South Florida Water Management District (SFWMD) to increase flows of water headed south through the Everglades by an average of approximately 370,000 acre-ft/yr. On April 30, Florida governor Ron DeSantis, the SFWMD, and the Florida Department of Environmental Protection announced the start of initial site work on a new 6,500-acre stormwater treatment area (STA) known as the A-2 STA. Located in Palm Beach County, the treatment wetland ultimately will receive water from the planned 240,000 acre-ft Everglades Agricultural Area (EAA) Reservoir. All told, the design and construction of the A-2 STA and the EAA Reservoir are expected to cost an estimated $1.8 billion.
Authorized by Congress in 2016, the Central Everglades Planning Project aims to restore more natural flows of water within and through the central and southern Everglades. A key aspect of this approach entails storing, treating, and conveying water south of Lake Okeechobee, the 730 sq mi water body that historically released vast quantities of sheet flow southward, sustaining the Everglades and Florida Bay at the southern tip of the state. During the 20th century, development greatly disrupted these flows to the south, contributing to the ecological decline of the Everglades and Florida Bay.
As part of its efforts to regulate water levels within Lake Okeechobee, the Corps discharges flows to the Caloosahatchee River to the west and the St. Lucie Canal to the east. In recent years, large freshwater releases to these waterways have contributed to harmful algal blooms and disrupted ecological conditions within their estuaries. By reducing the extent to which the Corps will need to discharge water from Lake Okeechobee to the state’s western and eastern coasts, the Central Everglades Planning Project will help improve the health of the Caloosa-hatchee and St. Lucie estuaries while also rejuvenating the Everglades.
In this way, the Central Everglades Planning Project offers a “double benefit,” says Matthew Schultz, P.E., a vice president in the West Palm Beach, Florida, office of Brown and Caldwell. In early 2019, the SFWMD selected Brown and Caldwell to be the design consultant and engineer of record for the A-2 STA. “The project reduces the discharges of water to the northern estuaries, and it treats the water and sends it to where it should go, which is the Everglades,” Schultz says.
To be completed by the SFWMD in 2023, the STA is designed to treat water that has been released from Lake Okeechobee and is flowing south by means of either the Miami Canal to the west of the project or the North New River Canal to the east. To reach the STA, flows within the Miami and North New River Canals will be diverted into an approximately 10 mi long east–west oriented inflow canal that is currently under construction. After undergoing biological treatment in the STA, the clean water will be discharged to the Miami Canal, eventually flowing south through Everglades National Park and into Florida Bay. After the Corps completes the adjacent 10,000-acre EAA Reservoir in 2028, the STA will receive flows from the reservoir itself rather than from the inflow canal.
Once in the treatment area, the water will enter distribution canals that regulate the flow of water across the three treatment cells.
Until then, a temporary pump station will lift water from the inflow canal and convey it to the STA. Once in the treatment area, the water will enter distribution canals that regulate the flow of water across the three treatment cells, which are designed to channel water from east to west. “The distribution canals smooth out that water across the whole six thousand five hundred acres, so that you have a nice, even pattern of flow that’s not short-circuiting through the stormwater treatment area,” Schultz says. More than 25 mi of perimeter and internal levees will be constructed to form the boundaries of the STA and its three treatment cells.
The STA will be designed to treat up to 650 cfs of water. Within the treatment area, subaqueous vegetation as well as grasses and cattails that grow above the water surface will help remove phosphorus from the water column. As the water flows slowly through the treatment area, the plants will remove nutrients from the water. At the western end of the STA, collection canals will gather the treated water and convey it to an outflow canal. In turn, the outflow canal will return the water to the Miami Canal, where it will resume its southward journey.
Currently, the land that will become the A-2 STA consists of sugar cane fields that include many canals and access roads. During design of the project, care will be taken to ensure that such features do not hasten the westward flow of water through the treatment area. “We can’t have anything that short-circuits water going east–west because basically that’s going to short-circuit the treatment process,” Schultz says. Therefore, any existing canals that are situated in an east–west orientation need to be filled in. In certain locations, existing roads will have to be lowered to facilitate the desired flow patterns.
additional modeling was conducted to determine the likely seepage volumes and if the inflow canal would be capable of controlling them sufficiently.
A key challenge that the design team encountered early on involved the decision to add a seepage canal to prevent flows of water between the STA and the agricultural land that borders it to the north. In its original form, the project did not call for a seepage canal. “The concept for the project was to manage the elevation of the inflow canal to control seepage at the same time,” Schultz says. With a bottom width of 55 ft, the inflow canal is a “big canal,” he says—one that will be capable of conveying up to 3,000 cfs. “Its primary purpose isn’t to control seepage,” Schultz notes. As a result, using the canal to try to control seepage was bound to prove “operationally challenging,” he explains.
With this concern in mind, the design team sought to determine whether it made sense to add a seepage canal. To this end, additional modeling was conducted to determine the likely seepage volumes and if the inflow canal would be capable of controlling them sufficiently. “We had to get the SFWMD, the Corps, and our own experts all on board with the need to add this as a necessary component to the project,” Schultz says. Ultimately, the decision was made to add a small seepage canal and a pump station capable of pumping up to 150 cfs. The seepage canal will have a bottom width of approximately 10 ft and the same depth as the inflow canal. “With that smaller cross section, you’re moving a lot less water in and out in order to control the elevation of the canal,” he says. “The smaller canal is much more manageable in being able to dial in operations so that they can really control seepage.”
Although site work has begun on the A-2 STA, the design for the entire project is scheduled for completion this fall. Besides Brown and Caldwell, the design team includes two firms providing geotechnical support: Terracon and Radise International. Two companies—Pickett and Associates and the Engenuity Group Inc.—are handling surveying. “There’s a tremendous amount of surveying associated with this project,” Schultz says. Hydrologic and hydraulic modeling are being conducted by Collective Water Resources LLC and A.D.A. Engineering Inc. Mock-Roos Consulting Engineers is providing civil engineering support, and Hillers Electrical Engineering Inc. is the electrical
subconsultant.
This article first appeared in the September 2020 issue of Civil Engineering.
