[StBernard] Leaky levee plan is a risk

[Westley Annis]

Leaky levee plan is a risk
Tidal flow shut off during storm threat 
Tuesday, January 02, 2007
By Matthew Brown
West Bank bureau 

As the state searches for pragmatic flood-protection solutions, experts
agree the region needs stronger levees and restored wetlands. But that
brings up a paradox that has bedeviled engineers and politicians for years:
Building more levees would only hasten the destruction of wetlands, by
choking off the freshwater sediment that nourishes the swamps. 

 
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."