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Geosynthetic materials provide hurricane protection through erosion control for Louisiana levees, and a Florida canal
By Andy Burran
Introduction
Hurricanes and other extreme storm events seem to be intensifying, putting the U.S. infrastructure at increasing risk of failure. As the nation assisted the Northeast in rebuilding after Hurricane Sandy last fall, we were reminded of the importance of our infrastructure and how much we take it for granted.
The American Society of Civil Engineers has continually given the nation’s infrastructure poor grades for the last 15 years. Specifically, the levee and inland waterway systems have consistently been ranked as some of the worst infrastructure systems in the country.
When these systems were analyzed to determine the principal mode of failure, it was determined that in levees, canals, and other water conveying structures, the main source of structural failure was erosion. When the levees that failed during Hurricane Katrina were examined, the U.S. Army Corps of Engineers (USACE) determined that it was the water that cascaded down the “dry” side of the levee just before the levees were overtopped that caused breaches, followed by collapses and failures.
After Katrina tore through coastal Louisiana and Mississippi with devastating winds and destructive waves, state and local leaders searched to find permanent protections. They wanted to believe the newly constructed levees would hold back the rising waters of the Gulf of Mexico along with hundreds of lakes, rivers, and bayous that provide the commercial and residential foundation for unique cultures.
State officials and environmental scientists, along with the USACE, have developed and built pumping capacities and levee systems at an astounding rate to convince locals that Katrina was a one-time occurrence. But the seeds of doubt sown in 2005 have made people think twice when confronted with the thought that it could happen again with similar results.
Armoring the levees
When Hurricane Katrina made landfall in late August 2005, it caused devastating flood damage to large portions of southeastern Louisiana—but it was not the only hurricane to hit the area that year. Hurricane Rita made landfall in late September, causing catastrophic tidal inundation to the community of Lafitte, south of New Orleans.
One of the many levees that needed rehabilitation was the Pen Levee in Lafitte. The Louisiana Department of Transportation and Development and the Lafitte Area Independent Levee District made the decision to install an anchored reinforced vegetation system into the Pen Levee project design.
The Decamp Street Levee which intersects the Pen Levee was also armored, providing protection from erosion and scour forces caused by wave overtopping and storm surge similar to what happened during Katrina. When wave overtopping occurs, the levee soil surface is subject to severe erosion and scour that can cause breaches leading to catastrophic failure. The engineered armoring solution provides permanent erosion protection to the levee, resisting soil movement and uplift.
The design for the 1.8-mile (2.90-km) Pen Levee included the armor system across the levee crest (top) and down the backside slope to the Borrow Canal. The project was installed in two phases. Phase I installation started 5ft (1.5m) from the sheet pile wall to 18ft (5.5m) across the levee crest.
Phase II installation started where Phase I ended, 25ft (7.6m) down the back side of the slope. Prior to installation, the levee was prepared by “scalping” the vegetation with a mower and then back-dragging it to fill in any depressions and remove any obstructions.
The high-performance turf reinforcement mat (HPTRM) component was then installed and secured with Type 2 earth percussion anchors in a checkerboard grid pattern at 5ft (1.5m) centers. The earth percussion anchors act as a tie-down mechanism with 500lbs (227kgs) of maximum pull-out strength for greater factors of safety.
The installation of each phase was completed in about six weeks each, compared to typical rock riprap that might have taken several months. The Decamp Street Levee was an additional 1,300ft (396m), installed from the front toe of the levee over the crest and down to the backside toe.
Gustav, Ike, and Isaac
Three years after Katrina, in September 2008, the Pen Levee was subjected to heavy rain from Hurricane Gustav and a 12ft (3.66m) storm surge from Hurricane Ike. The system held its ground during the superstorm conditions and levee overtopping, resisting erosion and scour.
Video footage taken at the time shows that Ike generated 6ft swells and wave action resulted in overtopping causing water on the backside of the levee to rise. After three hours of constant inundation, the levee continued to hold its ground as the storm surge continued to cascade down the landside of the levee.
During the evening of Sept. 13, 2008, the levees took the full force of Ike’s waves. The water level rose to 8.5ft, submerging the levee to within 6in. of the top of the sheet pile wall. The system continued to perform for an additional three days after the storm surge reached its peak.
Erosion control
for Canal C-41A
In 2012, Tropical Storm Isaac crossed Florida dumping approximately 16in. of rain on Highlands County in the south-central part of the state. One of the many canals that the South Florida Water Management District (SFWMD) has to maintain, Canal C-41A, was in the path of Isaac when it crossed Florida on its way to the Gulf of Mexico.
Approximately one year prior to Isaac, Section 1 of C-41A was under construction because of excessive erosion under the waterline. The mode of failure on this canal was due to the water undercutting the canal banks and causing severe erosion at and even below the waterline. SFWMD contracted with Underwater Engineering Services Inc. (UESI) to provide rehabilitation to the canal’s eroding banks. UESI elected to rehabilitate the canal’s banks with a combination of an anchored/reinforced vegetation system and nonwoven geotextile.
The design called for this system along the canal banks that would extend approximately 10ft below the waterline to protect the entire area in danger of erosion from the flowing water. Because the geotextiles that were used in this project were made of polypropylene, which has a specific gravity less than 1, UESI had to find a solution that would allow erosion control protection under the waterline while ensuring that the products would stay in position during the service life of the project.
The solution that UESI developed included sewing a panel of nonwoven geotextile to the end of each TRM panel. The nonwoven geotextile was sewn into a pocket, which was the filled with rock ballast.
The rock pocket was folded over itself creating a “burrito wrap” that served as the mechanism to keep the reinforcement under water and secure while the earth anchors were driven into the canal bottom by professional divers. An assembly-line system was devised on-site where the individual panels were fabricated and folded at one station then transported to the canal bank where workers on the shoreline and the divers in the water received them.
An excavator was used to hoist the panels into a position where workers on the shore and in the water could then place the fabricated panels for final installation. Just ahead of the installation, a long-reach excavator was used to perform the canal bank regrading work. The installation was fast and, through the use of geosynthetics, saved SFWMD a significant amount of money.
Tropical Storm Isaac crossed over Florida on Aug. 27, 2012, and dumped massive quantities of rain on south Florida as it continued into the Gulf of Mexico. During this storm event, the water level in Canal C-41A almost reached record-setting levels.
However, the canal erosion was successfully mitigated and, after the water receded, there was no evidence of damage to either the armoring system or the underlying soils beneath it. This system had successfully controlled erosion during an extreme weather event, saving SFWMD an untold of amount of money in canal repair work in the aftermath of Tropical Storm Isaac.
Conclusion
Whether it’s a hurricane, tropical storm, or another potential disaster, geosynthetic flexible lining systems are proven to save time and money during installation, maintenance, and beyond. Whether the project is a major levee, canal, slope, or other civil structure, erosion will always occur unless the project is designed to actively prevent it—providing proactive erosion control and preventing erosion before it happens.