The Army Corps of Engineers is about to launch a pilot program aimed at finding the best way to protect the crown and side of rebuilt earthen levees from erosion caused when they are overtopped by storm surges. The corps has determined that the best way to "armor" the levees from erosion is to install "high performance turf reinforcement mats" -- plastic fabric sheets that Bermuda grass will grow through. The sheets must be anchored to the earthen levee in a way that guarantees the mats will not be ripped up and damaged by mowing equipment and must be designed so they are not damaged by rutting from the wheels of mowing tractors or other equipment.
The armoring is part of a corps strategy to design the levees rebuilt after Hurricane Katrina to withstand the conditions that caused the old levees to fail in 2005. Each segment of the new levee system is high enough not to be overtopped by surges caused by a hurricane with a 1 percent chance of occurring in the Gulf of Mexico, also known as a 100-year storm.
But the corps also is designing each part of the levee system to be "resilient," defined as able to survive overtopping caused by a hurricane with a 0.2 percent chance of occurring, also known as a 500-year storm.
The new armoring mat must assure that even if the surges as high as 28 feet, as produced by Hurricane Katrina on the Mississippi Gulf Coast in 2005, were to overtop the New Orleans area levee system, the earthen parts of the system will remain in place until after the storm passes. And unlike during Katrina, it would mean that surge waters would stop entering the city when the water level drops below the top of the levees again.
Katrina's Mississippi surge is considered the equivalent of a 400-year hurricane event.
The combination of mat and grass for the earthen levees is only one of several methods being used to increase the resilience of the rebuilt levee system.
In St. Bernard Parish, for example, stronger inverted T-walls are built atop earthen levees, with sheet piling installed beneath their centers to much greater depths than before Katrina. H-shaped pilings are driven diagonally to even greater depths beneath the inverted T part of the wall to add to their stability.
Along most of the new and rebuilt floodwalls, contractors also have installed concrete splash guards that will reduce the eroding power of overtopping water.
In some locations, contractors mixed cement with soil, to increase the strength of the foundations of earthen levees, or built a complex series of wells and drainage systems, to redirect water that might seep beneath the levees.
Elsewhere, the corps has added concrete structures or walls to locations where earthen levees meet gates or other walls.
The corps has approved five companies to test their mat products on 1,000-foot segments of an east bank levee in St. Charles Parish, between Goodhope and Cross Bayou, and of a West Bank levee in Jefferson Parish along Louisiana 45. Each product also will undergo surge and wave tests at Colorado State University.
The mats would be installed and grass grown on the levees early this fall, with the tests completed in the spring.
If successful, workers will begin installing the selected armoring material next summer, corps officials said.
The finalist companies are ICS Nett of Vienna, Va.; Colbond, a Netherlands-based company with offices in Enka, N.C.; Propex Co. of Chattanooga, Tenn.; Tensar/North American Green of Poseyville, Ind.; and Western Excelsior Corp. of Mancos, Colo.
Regional levee authority officials remain concerned about the use of the mats and grass as the armoring method for the earthen levees, however. Bob Turner, executive director of the Southeast Louisiana Flood Protection Authority-East, said recently that he's not sure "how much resiliency will actually be there."
"It's important for everybody to understand ... that those 500-year overtopping rates are estimates at best," he said, pointing out that the actual levee design used storm-surge computer modeling based on 100-year storms, not the larger events.
Mark Schleifstein can be reached at firstname.lastname@example.org or 504.826.3327.