In south Florida's Everglades, water has been an issue in the spotlight for some time now. Concerns over impacts from agricultural runoff, as well as more fundamental concerns over the fundamental matter of getting adequate water in the first place, have drawn national attention.
To deal with those issues, a number of projects are in design or under construction. One of them is on the Seminole reservation in the Everglades, north of that far-south section of Interstate 75 known as Alligator Alley. When it's completed it will not only address water quality issues but also agricultural water supply issues as well.
At the heart of the project is concern over managing water used for agricultural purposes, notably irrigation of Seminole citrus groves. A decades-old system of canals brings water to the area, and the original irrigation system simply pumped water out of the canals for irrigation use, then returned the used water back to the irrigation canals. This tended to concentrate pollutants in the canal water — and when that "used" water was discharged from the canals (into the Everglades) it took the agricultural pollutants into the Everglades with it.
Clearly, what was needed was a way to remove the pollutants from the water in order to improve water quality and manage impacts on the surrounding ecosystems. And if such a system could also provide additional means of storing water for agricultural use, so much the better.
The solution came in the form of a plan that would allow irrigation water to be stored in a series of holding ponds and water reserve areas. The holding ponds and water reserve areas, defined by a network of about 17 miles of new earthen levees, are linked to create a multi-stage water storage system.
Once in operation, the system will manage water by pumping excess water from the main irrigation canal into a series of holding areas. After being used for agricultural purposes, the water will be returned to the holding areas and not to the feeder canals.
Here's how it will work. Initially, water will be pumped from the feeder canal and stored in large holding ponds. If conditions cause water levels to get too high in the holding pond areas, water will spill over into adjoining cypress-forest water reserve areas.
Should water levels continue to rise and become too high for the reserve areas, water will be transported to a second cypress forest reserve area. The only catch is that this second reserve area is located on the other side of the main feeder canal — a situation which requires construction of an enormous siphon system to carry that water under the canal and into that second cypress forest reserve area. Construction of this siphon proved to be one of the biggest challenges of the entire project.
In the event that all of these holding areas reach maximum capacity, the system is designed so that water will then flow through an outlet structure and back into the feeder canal.
The project, designed by engineers Burns & McDonnell, is a joint undertaking of the Army Corps of Engineers and the Seminole Tribe of Florida. Harry Pepper & Associates (HPA) is general contractor on the project, and Sid Timmons is serving as HPA's project superintendent.
HPA is building the pump stations, constructing the large siphon, handling mechanical construction, and building a portion of the levee system. The remaining portion of the levee system (about 90 percent of it) is being constructed by subcontractor WRS.
A key part of the overall project is the enormous siphon which will transport excess water under the feeder canal and into that second cypress reserve area. With a length of about 350 feet, the siphon consists of a pair of parallel 60-inch-diameter pipes. Siphons must be leak proof or they will loose the siphon action. Therefore, the pipe joints must be completely leak free and the pipe must be extremely corrosion resistant.
"The siphon is a very simple operation," Timmons says, "but it was very difficult to put in."
One challenge was the matter of completing the earthwork needed to install large pipe under a major canal.
Another challenge was to find suitable pipe for the siphon itself. The pipe used to construct the siphon was Weholite Profile Wall HDPE manufactured by KWH, a Canadian company.
Since the siphon had to be constructed under the canal, it was necessary to block off the canal at each end of the siphon construction area for the three weeks it took to complete the siphon installation. This was accomplished by constructing two large earth dikes — Timmons describes them as "two monstrous plugs in the canal" — to keep water out of the construction area, along with a diversion canal to carry water around the site during siphon construction.
The dikes and bypass canal were constructed in just three days, using a fleet of Case excavators and Case haul trucks. To put together a large part of the equipment fleet needed to complete this and other aspects of the overall project, Briggs Construction Equipment rental sales representative Keith Moody, West Palm Beach, worked with HPA's Timmons to specify a number of excavators, dozers and other machines.
The diked area was then pumped out, allowing excavation to begin. Crews excavated 10 feet below the bottom elevation of the canal, then placed 12 inches of #57 stone to provide a suitable bed for the pipe.
Meanwhile, crews were preparing the siphon pipe. The pipe arrived at the job site in 54-foot joints. The joints were welded together on-site. Angled joints where the inclined sections of the siphon merged with the horizontal run under the canal were given particular attention, and each joint was thoroughly tested.
The longest single run of pipe to be installed was the 162-foot-long horizontal run across the bottom of the cut, the run which would actually pass beneath the canal. Three carefully coordinated Case excavators lifted and placed this run of pipe in a single operation. Once set, the pipe cut was backfilled with stone (in 6-inch lifts) to the top elevation of the pipe.
"We chose to use stone to provide maximum stability for the pipe," Timmons says, adding that the pipe was also filled with water to keep it from floating.
With stone placement complete, HPA covered the top of the pipe with an additional 4 feet of compacted fill to bring the canal bottom back to its original level.
Throughout the siphon construction phase, control of groundwater was critical. HPA utilized a 2-foot-on-center wellpoint system rented from Service Pump & Compressor, plus four additional 12-inch pumps, to keep the construction area dry.
Another challenge facing the project team was construction of the 17 miles of new levees called for in the project plans. These levees define the holding areas.
Most of the levee construction was handled by subcontractor WRS, using excavators to dig out and then place levee material as construction of each dike moved ahead. Dozers were used to spread the material, and each lift of levee material was roller compacted before the next lift was placed.
The project also included construction of 10 pump stations. These pump stations, constructed by HPA, varied in size and complexity and combined precast concrete with some fairly elaborate on-site concrete work — and of course required careful attention to dewatering.
Most of the challenges on this project came in the areas of constructability and access. But there were non-construction challenges on the project as well, not the least of which were the occasional visits from alligators. The gators seemed to be fond of the construction sites, and it was not unusual for crews to have one or more of them drop in now and then.
"During siphon construction," Timmons recalls, "when we had the canal plugged off, there were a couple of gators that would come up and visit us fairly regularly."
Throughout the project, the construction team paid particular attention to protecting the Seminole orange groves. This included taking precautions to make sure that the equipment entering the site did not bring any diseases which might harm the citrus trees. To that end, HPA erected a spray station which washed down all machines and vehicles with a fine spray of an antimicrobial soap.
The project also included construction of about two miles of special fencing. This fencing was designed to keep wild animals (part of a local wildlife safari visitor attraction) separated from the canal system. The fencing was installed by Walkup Fence Co., based in Gainesville, Fla.
Currently, the overall project is scheduled for completion next year.
"Eventually," notes Timmons, "when construction is complete and the system is filled, it will be a balanced system."