Levees must be armored, corps says
Fabric, rock layers would add strength, slow erosion
Monday, January 30, 2006
By Bob Marshall
During nearly 300 years of living in one of nation's most flood-prone
spots, New Orleanians have spent a lot of time talking about levees --
How high? How strong? How safe? -- while hardly ever mentioning the word
Hurricane Katrina changed that.
As state and federal agencies race to fix almost 170 miles of local
levees damaged by the storm before the next hurricane season begins June
1, armoring -- protecting the surface of levees with concrete, rocks or
synthetic fabric to prevent erosion and scouring -- has become a
constant in discussions about the effort.
The subject will be back in the headlines this week when the Army Corps
of Engineers expects to forward a request to the White House for an
appropriation of up to $600 million for levee armoring in the New
Orleans area. Corps officials said the proposal will carry an "urgent"
tag the agency hopes will speed its journey to approval in a Congress
facing tighter budgets and a lengthy to-do list.
"If we can get this approved by the end of April, we can have the
armoring in place in another three to four months," said Dan Hitchings,
civilian director of the corps' Task Force Hope. "We think we can get
this done in time."
The reason for the urgency is simple: The corps and its critics agree
armoring could have dramatically reduced the destruction and death
Katrina spread across the area.
Engineers say armoring is to levees what air bags are to seat belts: a
little extra protection that can greatly improve survivability.
"You can build a Category 3 levee but get Category 4 or 5 survivability
when you armor a levee," said David Rogers, a University Missouri-Rolla
professor who is one of the nation's leading experts on levee and dam
failures and a member of a National Science Foundation team
investigating levee failures in the New Orleans area. "You may get
overtopped by a larger storm than you designed for, but your chances of
holding up are tremendously improved.
"And if your levee stands up when it's overtopped, you greatly increase
Help with overtopping
By placing a layer of water-impervious material on the outside of a
levee, engineers involved in the repairs said, they can almost eliminate
the chances it will collapse due to erosion caused by a hurricane's two
most serious threats: the pounding from large, fast-moving waves on the
lakeside of a levee that can tear vegetation from its surface, exposing
its soils to erosion; and overtopping by surge or waves which, as the
water rushes down the land-side levee slope, can dislodge vegetation,
resulting in rapid scouring and eventual collapse.
Katrina proved overtopping is the far more serious threat to the metro
area, investigators said. Evidence clearly showed overtopping caused the
Industrial Canal I-wall collapse that flooded much of the 9th Ward and
parts of St. Bernard Parish. Overtopping also was the main cause of the
wholesale disintegration of the Mississippi River-Gulf Outlet levee,
which allowed Lake Borgne to rush over much of northern St. Bernard
Parish, engineers said.
Had either system stayed intact while being overtopped, the flooding
would have been much reduced, because the storm surge receded below the
tops of the levees and flood walls in little more than two hours,
records released by the corps show. But when the levees collapsed, the
water continued to pour through the breaches for hours until the lake
receded to sea level or the hole was filled by the corps.
Armoring would likely have prevented both failures, the investigators
"There's no question the people in St. Bernard and the 9th Ward would
have had a much easier time of it," said Hassan Mashriqui, a storm-surge
researcher at the Louisiana State University Hurricane Center. "There
would have been flooding, but it would not have been catastrophic.
"They absolutely must armor these levees now, because there is a chance
this area probably will see more storms like Katrina."
Alternatives looked at
The corps agrees, and has been researching an armoring effort for weeks,
Tentative plans call for armoring the land side of the Industrial Canal
floodwall; all locations where levees attach to floodgates because such
spots saw almost universal scouring during the storm; the front sides of
many sections of levees that are routinely pounded by waves from large
boat traffic, and both sides of all levees that are exposed directly to
storm surge, such as the stretch of the MR-GO levee south of the Bayou
Options for armoring flood walls such as those on the Industrial Canal
typically include a simple concrete spill shelf, much like a sidewalk,
on the land side of the levee crown, or rock and concrete riprap in the
same location. Simply by diminishing the force of the water flowing over
the wall, such shelves can prevent scouring and wall failures, corps
Armoring levees requires various strategies. One is the use of rigid
barriers such as reinforced concrete, the type of protection that can be
seen on Mississippi River levees in the metro area. But in marshes,
where subsidence is almost certain, such rigid concrete slabs are a poor
selection because they cannot remain in contact with the soil as the
So the corps is looking at two other alternatives, agency officials
One option is the use of synthetic fabrics that prevent erosion by
anchoring grass against the force of moving water. These come in a
variety of types, from single-sheet materials to spongelike mats of
woven threads. The major advantages of these synthetics include ease of
installation, flexibility to adapt to changing levee profiles due to
soil subsidence, and the region's subtropical climate in which grass
would quickly grow through the fabric to help seal the levee. Further,
these mats could be easily and quickly removed when levees have to be
rebuilt due to sinking.
However, timing could be a serious drawback for the use of fabric on
this project, engineers said. Ideally, the fabric is laid down as each
section of levee is completed, allowing quick growth of grasses and
reducing the amount of erosion that might take place before the armoring
is set. But if Congress doesn't approve the appropriation until the
summer, many levee repairs will already be finished, and the fabric
would have to be retrofitted, a process that would increase the expense
and produce mixed results.
The second option is the use of "gabion mattresses," 6-foot-by-12-foot
frames of flexible wire filled with rocks that are laid on top of the
levees after construction. The thickness of the mattress usually varies
between 6 and 18 inches, depending on the degree of protection required,
and the strength of the levees to hold the extra weight of the rocks.
Grass and other vegetation would quickly fill the spaces between the
rocks, forming a tight seal against erosion if overtopped.
Industry suppliers said the advantage of gabions is that they are
durable and will last for years, manufacturers could supply enough to
cover what could be as much as 100 miles of levee, and while
installation is not as quick as fabric, it could proceed fast enough to
complete the job this summer. Finally, gabions can be added after a
levee has been built without any loss of performance.
The drawbacks for gabions, however, are serious. The levee design did
not include the possibility of the extra weight, which could be as much
as 400 pounds per square yard, industry sources said. Engineers
disagreed on the impact of that extra weight. Some said it might reduce
the factor of safety, meaning it could increase its risk of failure from
causes other than scouring. And gabions are more expensive.
Gabions also would be difficult to remove if a levee had to be rebuilt
due to subsidence.
Hitchings said the corps, while currently favoring gabions, has made no
"Much of this is still very speculative," Hitching said, noting that the
budgeting process is involved. "For example, we know we're probably
going to send a request for around $600 million to the administration,
but we don't know the shape of the request they send to Congress. And
we're hopeful Congress will act very quickly. But that's not certain
either. So our plans could change."
But one thing is no longer speculative when designing and building
hurricane protection levees in this area, Hitchings said.
"We have to armor these things," he said. "That's a given."
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Bob Marshall can be reached at firstname.lastname@example.org or (504)