So the Army Corps of Engineers is designing a new type of structure intended to soften environmental impact.
The name sounds like an oxymoron: leaky levees. And in the wake of the levee breaches following Hurricane Katrina, corps officials are pushing a switch to the more technical "tidal interchange structures."
The idea is to build structures with gates and culverts that would let tidal currents flow somewhat naturally -- until a storm or flood threatens and the openings could be closed.
"Depending on who you listen to, it's either the first installment on the Great Wall of Louisiana, or an imaginative and effective protection program that allows wetlands to survive," said Donald Boesch, a University of Maryland professor and member of an independent group conducting peer reviews for the Louisiana Coastal Area Ecosystem Plan, the proposed 10-year start of the federal-state coastal restoration program.
Having seen unintended consequences from past levee projects, Boesch remains skeptical about whether the concept will work. So are many environmentalists and academics. They say the proposed structures threaten to cut the coast in two, in much the same way as traditional levees. One side would be choked off from freshwater sources that sustain marshes; the other from waters of higher salinity crucial to providing suitable nursery grounds for shrimp, crab and numerous fish species.
The first try
The first large-scale project that could serve as a test of the leaky levee concept is the $840 million "Morganza to the Gulf" levee. The 72-mile barrier would zigzag through the Terrebonne Basin between Larose and Houma, enclosing 550,990 acres, about half of it wetlands.
Thirty-four gates and culverts, or about one opening every two miles, would be installed to maintain water flow.
The environmental manager for the project, Nathan Dayan, acknowledged that while existing water channels would be maintained, "sheet flow" -- the natural movement of rain or tidal water over land or within marshes -- would be eliminated. For example, after a major tropical rainstorm it could take up to 14 days for waters inside the levee to drain through the culverts. With no levee, the water would drain in a day or two, Dayan said.
"Directly on either side of the levee you wouldn't get the interchange, so it does cause some channelization of water," Dayan said. "You can't have everything ... We need hurricane protection, and we're still getting the same volume of flow."
Flaps let water in, out
Another proposed levee, which would cut from Belle Chasse to LaRose, slicing the Barataria Basin in half, also is being designed as a leaky system, and the corps is considering other leaky levees across the Rigolets and Chef Menteur Pass, along the western shore of Lake Borgne, and south of the Harvey Canal.
Corps officials could name only one project in the New Orleans area where such a structure exists: a section of the eastern New Orleans levee enclosing about 13,000 acres of the Bayou Sauvage National Wildlife Refuge.
Constructed in the 1980s, the levee has four "flap gates" built into it to allow water movement. But refuge managers complain the gates are overly labor intensive and require frequent maintenance.
"They don't function like they were intended to function," said refuge lead biologist James Harris. "Most of the flap gates that control water are gone, they've rusted off. It makes management that much more difficult for us, because now we're constantly opening and closing the gates."
If the corps sticks with its plan for the Morganza-to-the-Gulf levee, Harris said, "I would think they would need to come up with a better system than what we have at Bayou Sauvage. I wouldn't recommend that system for widespread use."
Another bad idea?
Oliver Houck, a Tulane University environmental law professor and critic of the corps' levee proposals, said the concept reminds him of marsh management programs of the late 1980s. The idea then was to enclose marshes to protect them from too much vegetation-killing saltwater.
"They not only did not save the marshes, they turned out to hasten their destruction. All we did was drown the marshes," Houck said, because freshwater pooled in the marshes, rather than flowing naturally into and out with the tides.
He worried that leaky levees could have a different unintended consequence: restricting water flow to the point where the marshes die.
"A lot of the water that supplies marshes doesn't run over the surface of the land; it runs over two or three feet below the surface of the land as seepage ... The weight of the levee presses down and it acts like a tourniquet," Houck said. "You're breathing with half a lung. You're pumping the stuff out of your heart with one ventricle."
The state agrees with the corps that new levees are needed south of New Orleans and Houma, said Randy Hanchey, Louisiana's deputy secretary for natural resources. But to balance that need for protection with coastal restoration, he said the corps may have to do more than simply preserve existing bayous and other waterways.
Many of those waterways already have been altered over the decades, through the digging of canals to access oil wells or fishing and hunting camps. Rather than just maintain them in their present state, Hanchey said, drafting a new hurricane protection plan offers the chance to restore some areas to their historical conditions.
"The current conditions may not be desirable. If you're going to put these things in, we believe we've got an obligation to go beyond simply providing water flows between the existing channels," he said. "Once you build them, if you're wrong, if these water control structures really don't provide enough (water) exchange, it would be difficult to retrofit."
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