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Reconstruction of Tamiami Trail provides avenue for improved water flow for Florida’s Everglades
Florida’s Everglades National Park is the largest designated wilderness in the southeast U.S. and the only subtropical preserve on the North American continent, comprising 1.5 million acres of a unique ecosystem that once dominated the entire southern tip of the state. With the arrival of 20th century development, however, the ecological balance of the Everglades has been seriously threatened.
Today, more than 1,700 miles of canals and levees crisscross this subtropical landscape, helping drain almost 1.7 million gal of water a day to both the Atlantic and Gulf coasts. The impact of water drainage and land development in the 20th century has been immense, reducing the overall size of the Everglades nearly by half. As a consequence, plant and wildlife populations have been affected dramatically.
Under the 1989 Everglades National Park Protection and Expansion Act, Congress authorized the U.S. Army Corps of Engineers to improve natural water flow to the park and restore the natural conditions of that flow to the greatest extent practicable. In 1992, the Corps issued a General Design Memorandum that mandated increased water flows to the Shark River Slough, a passageway that historically had been the location of a major channel in the Everglades responsible for critical flows to the area of the Everglades National Park.
11 miles of obstacle
Also in 1992, the Water Resources Act provided the Corps with authorization to begin a system-wide re-examination of the entire Everglades ecosystem. In partnership with the South Florida Water Management District (SFWMD), the Corps developed the Comprehensive Everglades Restoration Plan (CERP), a $7.8 billion strategy expected to take 30 years to complete that will entirely re-engineer the way the Corps manages flood protection, water supply and water flow for the entire Everglades ecosystem.
The main obstacle standing in the way of increasing flows in the Everglades is the Tamiami Trail, a two-lane east-west road that crosses the Everglades. Before the construction of I-75, the old Tampa-to-Miami highway was the primary route offering south Florida residents passageway from the Atlantic coast to the Gulf of Mexico.
An 11-mile length of the eastern end of the 80-mile highway that extends from the outskirts of metropolitan Miami to the northeast corner of the Everglades National Park has become the critical link because water-diversion projects will redirect flows from canals along the outer reaches of Miami to the Shark River Slough, a historic Everglades channel currently blocked by the Tamiami Trail.
Built in the late 1920s, the trail structure consists mainly of an embankment dug from an adjoining canal, a limerock base approximately 12 in. thick, and a 2- to 3-in.-thick asphalt covering. A series of timber bridges constructed during the 1940s were replaced in the 1950s by 19 sets of culverts, each approximately 60 ft long and located roughly 4 ft beneath the roadway. The maximum amount of water these culverts allow to flow beneath the Tamiami Trail is approximately 1,500 cu ft per second (cfs). The first round of hydrological studies for restoring flows to the Shark River Slough suggested that the existing culverts would be sufficient to handle the increased capacity.
Subsequent analysis, however, determined that flows increased to 4,000 cfs—the amount proposed under the Everglades Protection and Expansion Act—could back up against the Tamiami Trail embankment, potentially damaging the roadway’s substructure and flooding over low-lying sections of the road. The strategy envisioned under CERP would call for even greater water flows to the Northeast Shark River Slough, an amount estimated at 5,500 cfs.
The Corps, therefore, asked PBS&J to perform a study of alternatives that would protect the Tamiami Trail while allowing for this increased amount of water flow beneath it. Having worked previously with the Corps and the SFWMD on several Everglades-related projects, PBS&J is currently involved in the Everglades Partnership Joint Venture, providing ongoing support for CERP program management.
The results of the investigation became the basis for a further stage of analysis that produced a single recommendation from the Corps. Documentation of this process, with full commentary, is contained in a final report scheduled for public release this summer. In all, the options examined contributed to the formulation of eight basic alternative plans for the Tamiami Trail, seven of which were considered with the added option of water-quality treatment for roadway runoff and one of which was considered in two variations, resulting in a total of 16 viable alternatives.
Eight ways of solving
First to be considered were five basic road and bridge configurations:
• Maintain the existing alignment and profile of the highway while adding four new bridges;
• Maintain the existing alignment of the highway while raising the profile and adding four new bridges;
• Build a new roadway to the north of the existing alignment with eight new bridges;
• Build a new roadway to the south of the existing alignment with four new bridges;
• Construct an elevated roadway within the existing right-of-way for the entire length of the 11-mile section.
Subsequent analysis and opinions expressed both in public and agency forums resulted in the identification of three more alternatives required for a full consideration of options:
• Maintain the existing alignment of the highway while raising the profile and adding a single, 4-mile bridge;
• Maintain the existing alignment of the highway while raising the profile and adding a single 3,000-ft bridge; and
• Maintain the existing alignment of the highway while raising the profile and adding box culverts beneath the roadway.
The existing roadway, while subject to variable subsidence, is fundamentally sound and its construction conforms closely enough to current Florida DOT standards that it could be used as a base for improvements that would raise its profile well above flood levels. PBS&J then produced a set of engineering documents showing alignment, a typical roadway section, bridge layouts and estimated costs. For each of the formally identified alternatives, a range of options for bridge configurations, water-quality treatment and wildlife crossings also were examined. The options for bridge configurations primarily addressed problems associated with the staging of construction, including maintenance of traffic, impact on surrounding wetlands and overall cost related to span length and pier construction.
The Corps’ final report recommends the problem of increased flows to Everglades National Park be addressed by maintaining the current alignment of the Tamiami Trail, raising its profile with a 6-in. asphalt overlay to resist encroaching water levels and adding a 3,000-ft bridge that will contain two 12-ft travel lanes, two 8-ft shoulders and outside barriers. The construction cost for this alternative is estimated to be slightly more than $23 million, with a total life-cycle cost of approximately $31 million. As part of the final recommendation the Corps also requests that the federal government compensate the Florida DOT for all real estate rights necessary to complete the Tamiami Trail project, and in return acquire all water-flow rights, including conveyance and easement.
Water-quality treatment was not selected as an option due to the added cost, the minimal impact on the environment and the ability of the Everglades ecology to provide water-cleansing effects. Similarly, wildlife enhancement features were not selected by the Corps for funding, but may be provided by the SFWMD or the Department of the Interior. Funding considerations also proved a crucial factor in not selecting the alternative of an 11-mile causeway which, the Corps’ final report acknowledges, remains the option of choice for environmentalists and state environmental agencies.
After the Corps’ final report is approved, project design work and the execution of construction drawings can begin for this critical effort targeting the restoration of a vital ecosystem. The Tamiami Trail project will join an allied project already under way to install 56 sets of box culverts along 40 miles at the highway’s western end and another to be planned under CERP that will install more bridges on a 10-mile stretch of the highway directly to the west. Together, these efforts will help counteract decades of neglect and provide crucial water flows restoring a world-class resource in the immediate future and for many decades to come.
A draft of the final Tamiami Trail report and other information regarding the Congressionally authorized Modified Water Deliveries to Everglades National Park can be found on the website of the Jacksonville District of the U.S. Army Corps of Engineers (www.saj.usace.army.mil/dp/tamiami.htm).