By Lori Lovely
The Herbert Hoover Dike (HHD) around Florida’s Lake Okeechobee is undergoing extensive rehabilitation due to flooding, seepage and leakage issues. Considered one of the country’s six most at-risk of failing, the earthen dike system encircling the lake consists of 143 mi. (230 km) of levee with 19 culverts, hurricane gates and other water control structures to provide flood damage reduction, navigation, recreation and fresh water for the local community, water for agriculture, prevention of saltwater intrusion to coastal estuaries and enhancement of environmental resources.
Called “Big Water” by the Seminoles and stretching 730 sq. mi. — the second largest freshwater lake in the nation — Lake Okeechobee provides drinking water for communities around the lake and to more than six million people living along the lower east coast. Used as a source of irrigation for a $1.5 billion-a-year agricultural industry that produces sugar cane, winter vegetables, citrus and rice, it also serves as the gateway to the Everglades. Water flows into the lake from a watershed that includes the Kissimmee River. The lake features an average depth of 9 ft., but tropical storms and the rainy season can elevate water levels to dangerously high marks.
Big Water Trouble
The Herbert Hoover Dike, ranging from 32 to 46 ft. (9.7 t 14 m) above sea level, was built in 1915. Because the outlet canals are too small to accommodate rapid release to lower the level, it’s critical that the dike is capable of withstanding elevated lake levels for extended periods of time.
Over the years, however, the dike has not always performed, due primarily to construction methods. Geologically, the lake is surrounded by sediment containing sandy material and shells. Hydraulic dredge and dragline techniques were used, depositing concentrated amounts of pervious shell, rock and gravel within the dike. In addition, the foundation beneath the dike has pervious layers of limestone, sand, gravel and shell, which makes it prone to seepage.
The containment was breached by the storm surges from the Great Miami Hurricane in 1926 and the 1928 Okeechobee Hurricane. After these disasters, the Florida State Legislature created the Okeechobee Flood Control District. To alleviate future problems, the U.S. Army Corps of Engineers (USACE) drafted a new plan for the construction of floodway channels, control gates and major levees along Lake Okeechobee’s shores.
The USACE built a larger system of levees between 1932 and 1938 in accordance with the River and Harbor Act of 1930, which authorized construction of 67.8 mi. (109 km) of levee along the south shore and 15.7 mi. (25 km) along the north shore.
Even that wasn’t sufficient, however.
After a devastating hurricane in 1947 revealed the need for expanded flood and storm damage reduction work, Congress passed the Flood Control Act of 1948, authorizing the first phase of the Central and South Florida Project, a comprehensive plan to provide flood and storm damage reduction and other water control benefits. Completed in the late 1960s at a cost of $165 million, the new dike system was named after the president who personally inspected it after the devastating hurricanes of the 1920s.
Regrettably, the dike now is falling into disrepair, rendering it incapable of withstanding extreme conditions. As escalating development has shifted additional water storage to Lake Okeechobee, the dike, which historically stored water from a 4,000-sq.-mi. basin to the north, has undergone increased stress. The USACE, Jacksonville District, began compiling reports documenting areas of the dike susceptible to water seepage and piping (internal erosion) in 1984. Jacksonville District, established in 1884, is the second-largest civil works district in the country. Its area of responsibility encompasses Florida and the Caribbean.
On the Brink of a Breach
The population around the lake is about 40,000. Residents are vigilant during hurricane season (June 1 to Nov. 30) because strong winds, heavy rain and storm surges on the lake could potentially weaken the dike and lead to “over-topping” the dike.
Because there are towns and agricultural land close to the dike south and southwest of the lake, the rehab is a USACE dam safety top priority project. At a lake elevation of 18 ft. (5.5 m) or higher, flooding would be severe and warning time limited. There is less potential for dike failure when lake elevations are at 17 ft. (5 m) or lower.
To ease the pressure and prevent over-topping, the lake’s water level is lowered prior to hurricane season. Even so, during the unusually active hurricane season of 2004, the water level rose to more than 18 ft. when four hurricanes hit in quick succession. Lowering the water level through the St. Lucie canal and the Caloosahatche River damaged the coastal estuaries.
Other high water events also can stress the dike. Southern Florida’s subtropical climate means Lake Okeechobee receives between 55 and 60 in. of rain per year. Extended periods of rain cause the lake level to rise, placing stress on the dike. When the water level gets too high, water seeps through the dike, sometimes eroding underneath in the form of piping, which, over time, creates the potential for a breach that could result in disastrous flooding.
Piping can create large cavities in the dike.
“As the lake elevation changes,” explained Michael Rogalski, program manager/chief of HHD branch in charge of the rehabilitation, “water seeks a path through the embankment. That creates cavities that start collapsing. Over the years, that increases.”
In 1995 when the lake level rose to 18.6 ft. (5.6 m), an inspection team identified excessive seepage, piping and sinkhole formation on the dike crest. According to the USACE, cloudy water exiting the landside toe of the dike and accumulation of fine sands indicated internal erosion. Emergency repairs were made, but when the lake rose to 18.5 ft. (5.6 m) in 1998, conditions continued to deteriorate. The USACE reported additional boil formation, seepage and cumulative damage from successive high water events.
The USACE received congressional approval in 2000 to design a project to rehabilitate or reinforce the dike. Hurricane Katrina caused them to review the design, developing a new concept that includes a seepage berm for decreasing piping and a cutoff wall for sealing pre-existing piping pathways and added resilience.
Putting more emphasis on public safety, the altered new plan involves filling in the toe ditch on the landside and constructing a seepage berm and cutoff wall inside the dike foundation. In addition to meeting the new nationwide criteria for safety standards, key elements of the project plan include: redundancy (backups), resiliency (required for wall and features) and reliability, noted USACE’s Outreach and Communication Manager Nanciann Regalado. To ensure the three R’s, independent external reviews will evaluate progress.
Plan in Place
The HHD branch is managing and executing the rehabilitation plan in accordance with the USACE Project Management Business Process. According to Rogalski, the project consists of five parallel activities that are combined into one overall project:
• Construction of the cut-off wall and design analysis of the features.
• Extension of the seepage berm and landside features.
• Compliance with National Environmental Policy Act requirements associated with the rehabilitation both within and outside the Federal right-of way.
• Completion of supporting documentation that will recommend solutions, address risk assessment and systems-based solutions for future reaches of HHD rehabilitation through completion of major rehabilitation reports.
• Coordination with the state of Florida, local communities on project progress, possible land acquisition needs and solutions to existing structures that are currently located in HHD and are required to be brought into compliance with the USACE dam safety standards.
The South Florida Water Management District is responsible for all real estate acquisition that is necessary and the Corps is in charge of dike rehab and maintenance. When drawing up specifications for the work to be done, the heterogeneity of the soil was taken into consideration.
“When the dike was originally built,” Regalado detailed, “a lot of materials came from the lake” as they hydro-dredged. “There are many different layers of materials at different sites.” And there are changes throughout the area, she said: for example, in the south there’s a lot of peat. “We’ve done a lot of samples, tests and surveys. The solutions are simple in nature, the application is complicated — and has to be extrapolated over many miles.”
Extrapolating material amounts is a moving target. While they can specify the length and stationing, exact amounts of materials will vary per geography and whether they need to go all the way around the dike. The purpose of the project is to protect life and property and as work progresses, risks decrease, so once enough of the work is done, all of the planned work may not need to be completed.
Physical work began in 2005, Rogalski said, and the cutoff wall in 2007.
“We’re working first where the risk is highest.”
Because the land on the southern side of the lake is near sea level, the threat of breach is greatest in the southeast quadrant.
In the first phase of construction, Reach 1, Jacksonville District is currently rehabilitating the dike’s most vulnerable section, the 22-mi. (35 km) section along the southeast shore of the lake between Port Mayaca and Belle Glade. Rogalski anticipated work on this section to be completed in 2013, at which time work will move to the south portion.
The second phase includes Reaches 2 and 3, followed by Reach 7. The estimated completion date for the entire rehabilitation project is 2023-2024.
Funding comes from the federal dam safety program’s flood damage reduction business line through the Corps. The 2007 estimated cost is $856 million, which is almost three times the estimated cost of the 2000 design. Some of the increase can be attributed to rising costs of materials and labor, but the higher figure also is related to the redundant features — the cutoff wall and seepage berm — that provide additional defense against piping and improve reliability.
The Corps awarded multiple “task order” contracts in 2008 for cutoff wall construction to Bauer Foundation Corporation of Odessa, Fla., Hayward Baker Inc., based in Odenton, Md., and Treviicos South Inc., of Boston. To date, 15 mi. (24 km) of cutoff wall in the 22-mi. “Reach 1” section are now under construction or have been completed, totaling more than $145 million in cutoff wall contracts.
Bauer won a $40 million contract for a second task order in May — the fifth order under a single contract for placement of the partial cutoff wall. The Corps will award two more task orders for another 7 mi. (11 km) of cutoff wall in 2010.
Made of a slurry mix of concrete, the 27 to 30 in. (68 to 76 cm)-wide partially penetrating cutoff wall is being installed at the crest of the dike, Rogalski explained, and extends 50 to 80 ft. (15 to 24 m) deep through the foundation to a depth of 5 ft. (1.5 m) into the limestone. It serves as a barrier so water doesn’t seep.
“It stops internal erosion,” Rogalski explained.
The height of the wall will be enough to provide protection to a lake elevation of 26 ft. (7.9 m).
Cutoff walls have been done in other parts of the world, but Rogalski believed this is the first time one has been constructed in the United States. The unique project is not without challenges, due to the magnitude of the project and the requirement that regular maintenance, such as mowing, continues during the rehabilitation.
“The dike needs to operate,” Regalado stated.
Because it’s such an unusual project, the HHD is drawing a lot of attention.
“So many people want to come see it and tour,” Regalado marveled, adding that a significant number of interested visitors have come from the Netherlands. Access is limited for safety reasons, but an on-site office is helping coordinate public interest and outreach.
The wall offers no support; instead, it acts as a barrier or seal to cut off internal erosion, working with the relief trench and wells to uplift and stop pressure on the earthen structure.
“The berm and wells extend the flow pattern of seepage, relieve pressure and bring water to the surface and let it out,” Rogalski explained.
However, crews working on the cutoff wall will have no interaction with water; all work is performed within the embankment. The work, divided between three contractors, is being conducted via three methods — for flexibility, Rogalski contended. With a limited number of companies qualified to do this kind of work — and each proven through projects in other parts of the world — he said multiple companies and varied techniques improve production and offer an economy of scale. Each method must meet specifications, he noted.
Construction is staged to stop where there are culverts or penetrations, so there’s no overlap of contractors. The separate wall sections don’t meet — yet. That’s a separate contract to be let later, Rogalski added.
One contractor mixes soil with the slurry. Using some equipment imported from Japan and Europe, such as the TRD (which works like a vertical chain saw) for trench cutting, they mix soil with the slurry mix and pump it in. Although it’s difficult to perform rehab work on the structure in place, Rogalski said a benefit of this method is the elimination of the need to remove and haul material offsite. In addition, he said work is progressing at a rate of 100 ft. (30.5 m) per day, whereas the others advance 50 to70 ft. (15 to 21 m)/day.
Another contractor installs the wall in panels. This panel method involves installing the wall in sections by pouring solid concrete into the earth. The third contractor excavates a trench and fills in with sections of the wall. This hydromill method entails cutting into the bedrock and filling the wall.
The seepage berm has been proposed as the best long-term solution to control piping. It will serve two functions: strengthening the dike by providing additional mass to resist water pressure from the lake and serving as a filter to prevent piping of material from the landside of the dike.
Rogalski reported that they’re still working on the design to provide a cost-effective solution for slope stability and erosion reduction. He anticipated variance in the size of the berm, depending on the need and available space. The geology of some areas may eliminate the need for a berm. Other areas may not have enough space. In those instances, other solutions are being considered, such as construction of pressure relief walls and replacement of the peat layer with filler material. While the HHD branch hopes to minimize impact on nearby homes and structures, “safety is our priority” Rogalski reiterated. “Our goal is to protect life and property.”
Working Through the Bumps
Such an extensive and uncommon project gives rise to many unexpected obstacles. Rogalski said they try to anticipate and resolve challenges as they arise. Some are easier than others.
A batch plant has been set up on-site and nearby in staging areas to save time and expense. Rogalski listed equipment at the dike as “dump trucks, cranes, excavators, concrete trucks — all the basic earthmoving equipment.”
In addition to moving dirt for wall installation, Regalado said they’re taking the opportunity to fill a quarry that exacerbated seepage.
Crews also removed an eagle’s nest, but Regalado said that didn’t slow the project. Other species have been spotted in the area as well. A project biologist and representatives from U.S. Fish & Wildlife continually monitor for potential nests because, as she explained, a lot of species nest along the lake bank at certain times of the year.
While it’s difficult to predict the weather, plans are in place for events like a hurricane. Wet and dry seasons were factored into the schedule because the water level goes up from 12.5 to 15.5 ft. (3.8 to 4.7 m) in the wet season, Regalado said and that affects installation. The lower level is an elevation below ground level and is maintained to reduce the risk of dike breach during hurricane season. The plan was implemented prior to the 2006 hurricane season as a result of flooding after Hurricane Katrina.
Despite on-again-off-again drought, this year has been rainier than usual, resulting in lake levels 5 in. (12.7 cm) above normal even before the summer rainy season began. In April and May, the Sun Sentinel reported Lake Okeechobee water levels were on the rise, renewing concerns that led to the USACE “dumping” 22 billion gallons of lake water out to sea in order to ease strain on the 75-year-old dike.
The South Florida Water Management District relies on a system of pumps, culverts and 2,600 mi. of canals and levees to protect farms and towns against flooding. Once the rehabilitation project is complete, they will be able to count on the Herbert Hoover Dike for increased protection. CEG
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