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Ecosystems, soil present challenges for levees

News | June 1, 2006 | By:

There is no shortage of challenges to building hurricane protection in Gulf Coast USA.

Soils are so soft that it’s difficult to get the land to support massive levees. The landscape itself continues to sink through natural and man-made processes. And a fragile ecosystem of coastal marshland likely will be affected by whatever is built.

Federal, state, and private engineers and designers have been meeting and tussling with such concerns, often through sessions such as workshops sponsored by the U.S. Army Corps of Engineers. Congress has mandated that the Corps come up with plans for a comprehensive system to protect against a Category 5 storm. One portion of this comprehensive plan will include the entire Louisiana coast. An initial report is to be issued to Congress this summer.

Corps officials said they want to come up with new ways to build strong, durable, stable levees. Peter Cali, an engineer with the Corps, said soft soils mean that earthen levees would need to be built much wider at the bottom to more evenly distribute weight. Such levees would sink more slowly than conventional ones, but would need to be very wide, he said.

A levee built to a 40-ft. elevation with a 10-ft.-wide crown would end up measuring 930 ft. wide at the base. If a levee was built by digging a “borrow pit” next to it—a standard practice—the entire “footprint” of the levee would be 3,500 feet across.

“You can see the magnitude of what you’d [need to] build in soft soil conditions,” Cali said.

Then there’s the issue of what materials to use to construct new levees, since digging a “borrow pit” often means using soft marsh soils. “One of our biggest challenges will be to find suitable building material,” Cali said.

Builders must consider many other factors. Among the issues: Every pipeline the system crosses will need to be relocated; the levees will affect how protected areas drain; and digging channels to build the levees will have an impact on coastal-restoration efforts.

Obviously, the construction itself comes with complications, Cali said. He described how levees are typically built by packing the dirt and adding additional layers after the structure sinks, often using geotextiles to help support some of the weight, plus metal sheet piles to stabilize the structures.

“This is what we do now, but we may need to do something different,” Cali said.

One option is to use concrete within earthen levees to strengthen weak soils. Another suggestion is using a soft-fill interior covered with tougher soil. A soft fill, which could be sand or even a type of geofoam, would allow the levee structure to be lighter than a conventional one. Another idea is using tubes filled with water or air—inflated on their own or on top of an earthen levee—to provide additional storm-surge protection, Cali said.

Testing would be required to see if those types of levees would be sturdy enough to withstand a hurricane’s storm surge, he said.

One complication in complying with the congressional mandate is determining exactly what “protect from a Category 5 storm” means. Category 5 describes a storm only in terms of wind speed, but that does not directly translate into how high or strong a levee needs to be.

“[There] could be a 10-ft. surge with a Category 2, or a 15-ft. storm surge with a Category 2,” said Nancy Powell, chief of the Corps’ hydrologic engineering section.

The strength of a surge depends on many factors, including the central intensity, the size and scope of a hurricane, wind speeds, what inland waters the storm crosses, the slope of the Continental Shelf in the area, and the temperature of waters in the Gulf of Mexico.

The Advocate of Baton Rouge, La., and Ron Bygness, editor of Geosynthetics (formerly GFR), contributed to this article.

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