ORLEANS PARISH, LA – Following Hurricane Katrina, Congress approved more than $14 billion worth of upgrades to the federal levee system surrounding the New Orleans area, including one of the largest storm surge barriers in the world. The costly venture also necessitated research on levee preservation. Dr. Jeffery Beasley rose to the forefront.
“I am working with the Army Corps of Engineers (USACE) in the New Orleans district helping them to write vegetation specifications for vegetating earthen levees,” said Beasley, Associate Professor in Louisiana State University’s School of Plants. “They recognize that vegetation is a key component in stopping levee erosion. And, you’re talking about miles and miles of levees.”
According to Beasley, levee projects are most probably the largest civil engineering projects in the United States.
A levee is an elongated naturally occurring ridge or artificially constructed fill or wall, which regulates water levels. It is usually earthen and often parallel to the course of a river in its floodplain or along low-lying coastlines.
USACE’s Interagency Task Force Reports on levee breeches have included foundation induced failures to a combination of overtopping and scouring (erosion of an earthen levee due to wave and water friction). To address the issue of scouring, the USACE decided to use scour protection in the form of armoring. The most common form, on earthen levees, is grass.
“There are two primary types of grass that are recommended – Bermuda and Bahia,” said Beasley. “We like those because they are aggressive and they are pretty tolerant of a lot of different environments.”
Continue Beasley, “It’s a concerted effort to make a system better. It’s not just one thing that makes it better. It’s everybody doing their part. That’s a part of the problem; however, it’s not the only part.”
According to Beasley, the specifications for vegetation on levees and some tests for materials used for levee construction have been completed and are being implemented.
The part of the project that’s still under consideration is the research on the use of High Performance Turf Reinforcement Mats (HPTRM) for erosion control. These are being considered in areas where there’s potential for overtopping of levees.
An HPTRM is composed of a three-dimensional matrix of yarns that are designed in a uniform configuration to lock soil in place. It exhibits high tensile strength and interlock/reinforcement capacity with both soil and root system. It holds seed and soil in place while vegetation grows and provides permanent reinforcement to enhance vegetation’s natural ability to filter soil particles and prevent soil loss during storm events.
“The reality is, it wasn’t the species of vegetation chosen but more importantly how contractors were establishing it,” said Beasley. “As soon as you finish building an earthen levee, the soil starts to shift. We get a lot of rain in Louisiana. Every time it rains and you don’t have the vegetation, that’s literally money washing away. Soil is washing off. So the name of the game is – How quickly can I establish the vegetation to prevent erosion and also strengthen the levee.”
HPTRM’s must be anchored to the earthen levee in such a way that the mats will not be ripped up or damaged by mowing equipment. The mats must assure that even if the surge is as high as 28ft. the soil will remain intact.
The combination of HPTRM’s and grass for earthen levees is only one of several methods being used to increase the resilience of the rebuilt levee system.
“Earthen levees are the most implemented ones, probably the easiest and most cost effective. Plus we have soil that’s readily available throughout Louisiana,” said Beasley. “Not only are earthen levees more economical, they are also very effective. When you get a pot hole in concrete, you have you have to wait until somebody comes and fills it. But grass – if it gets worn down, it can grow back.
Beasley added, “Unfortunately, research is a slow process. We still have a long way to go; however, I can see that New Orleans residents have benefitted from the improved levee system. When Hurricane Isaac came, the levees held.”