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August 30, 2005
The day after Hurricane Katrina smashed
into the Gulf Coast between New Orleans and Biloxi, leaving
a path of death and destruction from New Orleans to Biloxi,
the U.S. Army Corps of Engineers was scrambling to plug two
floodwall breaches in New Orleans to stop water from Lake
Ponchartrain from flowing into the city. Corps personnel from
New Orleans, Memphis and St. Louis convened Aug. 30 with hydraulics
engineers at Vicksburg District headquarters to confer with
state and local officials and contractors on the ground in
New Orleans to try to stem the inflow and close a 200-ft-long
breach along the 17th St. Canal. "Water was pouring in
from broken levees," says Adam Wine, a U.S. Coast Guard
spokesman, who flew over the city in a helicopter Monday.
"There are some areas where the water was 25-ft deep."
"We think the water overtopped the
floodwall last night and scoured out the foundation, leading
to a structural failure," says Al Naomi, senior manager
and head of the Lake Ponchartrain and hurricane protection
plan. The top of the concrete floodwall , which sits atop
an earthen levee, was 14-15 ft above sea level at the point
of failure, Naomi says. The city is 6 to 10 ft below sea level
at that point. Heavy rainfall from Katrina elevated Lake Ponchartrain's
flood stage to 5 ft above sea level, Naomi says.
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The New Orleans Sewerage and Water Board operates a vast
drainage system designed to collect water during storm events
and pump it from the city into the canal and on to the lake.
"There's the biggest drainage pumping station in the
world at the head of the canal, with many, many 1,000-cfs
(cubic feet per second) pumps," Naomi says, "but
because of the breach, it's shut down. So water is coming
in from the lake to the city. We have to devise a plan to
plug the breach and turn the pumps back on." By midday,
Tuesday, local accounts were estimating that 80% of the city
is flooded.
Downtown New Orleans was not submerged, Wine said, but most
of the eastern portion of the Big easy is inundated. In many
places, only overpasses break the surface. Elsewhere, he saw
burning buildings, out of reach of fire department equipment.
The lakefront airport was totally submerged, a ship was aground,
and cargo containers were scattered randomly. "I could
not believe the devastation," Wine said.
Corps engineers in Vicksburg were in contact with local officials
and emergency contractors trying to devise the best plan to
plug the breach and reverse the flow. "There are about
30 super-sized sandbags, 3 ft x 3 ft each, near the site,,"
says Naomi, "with more available nearby. It's very likely
that the contractors will fill them and try to put them in
place with heavy equipment to plug the hole. Then they would
come in and drive sheet piles."
Repairs are also needed at a second breach, at the Inner
Harbor Navigation Canal. Naomi says that the floodwall was
overtopped there as well. The storm surge was reported at
17-20 ft in St. Bernard Parish, on the Gulf of Mexico south
and east of the city, Naomi says. "It was 7 to 8 ft in
Lake Ponchartrain," He adds. "That's preliminary
data. Many of our gauges were washed away and we have to collate
information from the ones still in operation."
Another structure, the Superdome's roof, was not structurally
compromised by the loss of two roof panels blown off by the
hurricane's winds, says Larry Griffis, president of the Structures
Division of Walter P. Moore & Associates, Austin, Texas.
Griffis says that wind uplift with pressure of 80 to 100 lb
per sq ft appears to have torn off sheets of the single-ply
membrane roofing, leaving two 4 x 20-ft holes, but the pressures
and the breach did not affect the trusses making up the structure
of the dome. He says the extraordinary winds apparently caused
the failure of some weak welds. "The dome performed very
well, near design loads of 100-mile winds and 200-mile gusts,"
he says.
Walter P. Moore & Associates was the engineer of record
for the Houston Astrodome, whose roof was designed by Roof
Structures Inc., the same company that designed the Superdome's
roof. Griffis was well acquainted with Roof Structures' founder,
Louis Bass, and thus became familiar with the company's patented
lamella roof design. The Superdome's roof became a focus of
controversy after a freak hailstorm in 1980 apparently caused
it to leak. But Griffis says that roof's 25-mil poured-on
Hypalon coating was self-healing. The roof was replaced only
in 2000 with EPDM, which Griffis describes as an elastomeric
roof with single-ply membrane mechanically fastened to the
original metal deck.
As Katrina spills rain into the center of the country, the
worst may be yet to come, says Robert B. Flowers, CEO of HNTB
Federal Services Corp., Washington, D.C.-based engineering
security consultant. Flowers, former chief of the Corps, headed
the Mississippi River Commission, from 1995-97. The river
drains about 40% of the continental U.S., a vast area stretching
from the Rocky Mountains to the Appalachians, he points out.
"If we get big rainfall up through the Ohio Valley, Kentucky
and Tennessee, with water moving downriver for a week or so,
we could be looking at a real situation. The longer we have
water against the levees from one side and rising floodwaters
against them from the other, the more we're looking at erosion
and possible catastrophic failure."
Flowers says the loss of Louisiana wetlands over the past
200 years, as the Mississippi River levee system has been
put in place, heightened and reinforced, has weakened a natural
storm protection buffer zone. "Sediment is pushed out
into the Gulf, instead of building up wetlands," he says.
"It probably adds to the hypoxic or dead zone, and it
certainly doesn't help with hurricane protection." Coast
2050, a wetlands restoration plan, backed by former Louisiana
Sen. John Breaux, has helped somewhat, he says, by diverting
water and sediment from the river to augment wetland areas.
But much more needs to be done.
Naomi says that wetlands "didn't play a particular role
in this storm," but he agrees that the storm protection
system is overdue for an upgrade. The system is built to withstand
a Category 3 hurricane, he says. "We need to build a
system that can withstand a Category 5 storm. One preliminary
plan would feature a 30-mile flood engineered structural flood
control system, similar to those in the Netherlands. "It
would cost $2 billion to $2.5 billion, which is a drop in
the bucket, compared to what we're going to spend cleaning
up after Katrina."
Andrew
G. Wright and E.
Michael Powers with Thomas
F. Armistead
Courtesy Engineering News-Record
Related Links:
Katrina
Floods Gulf Coast, Killing 55 posted
Aug. 30, 2005
FEMA
Head: Katrina Was 'Catastrophic' posted
Aug.30, 2005
Louisiana
Officials Fear Major Oil and Gas Corridor May Be Under Water
posted Aug. 29, 2005
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