By mid-March, only about 3,000 ft of a hardened, 123-mile-long storm-surge defensive ring around New Orleans remains to be closed. Contractors have sheet pile and HESCO baskets on hand to defend those last points in case of storm emergency.
The city's storm surge risk reduction system is undergoing a $14.6-billion upgrade under the direction of the U.S. Army Corps of Engineers following Hurricane Katrina's devastation in August 2005. The new system's design is based on computer modeling of 152 storms with multiple criteria and approach paths to identify discrete vulnerabilities under a host of scenarios.
New Orleans has better protection than ever, says Col. Edward Fleming, commander of the Corps’ New Orleans District, but he adds a warning: “There is always going to be a bigger storm. There will always be residual risk.”
Evacuations, resiliency, foreshore protection and coastal restoration are other risk-reduction tools that should also be used, Flemming says.
Some of the last, big chunks of protective infrastructure were put in place March 15—17 as Alberici Constructors, St. Louis, installed two, 230-ton leaves of a 95-ft-wide sector gate at the $165-million Seabrook Floodgate Complex on the city's northeast side. A dense fog delayed the schedule and turned a planned one-day operation into a three-day marathon.
“This is a good day for the City of New Orleans,” said Fleming as he watched the first gate leaf swing into place. “This is a critical component of the overall, active system that works with interior drainage, pumping stations, levees and floodwalls to provide protection for New Orleans East and the metro area.”
All that remains now of heavy construction in the system is the replacement of three temporary pumping stations with permanent structures to seal off the ends of the city's big, stormwater drainage outfall canals. The temporary stations were built quickly after Katrina, but a previously awarded contract for their permanent replacements is embroiled in a contract-award dispute.
The Seabrook complex, with its sector gate and two vertical lift gates and walls, forms an operable barricade at the north end of the city's Inner Harbor Navigation Canal. The industrial canal links Lake Pontchartrain on the north, to the Gulf Intracoastal Waterway on the east and the Mississippi River on the south. The Seabrook gates will work in tandem with gates in the IHNC-Lake Borgne Surge Barrier to block storm surge up to a height that has a 1% chance of occurring in any given year.
“When Hurricane Katrina hit, there was no gate here to close and block storm surge,” Fleming says of the Seabrook installation. Circular cofferdams installed by Alberici for construction of the complex have effectively blocked the canal's Lake Pontchartrain end since June 2011, however.
After Alberici installed two vertical-lift gates in January, the contractor sealed off the sector gate cofferdam with needle girders to keep it in the dry while the rest of the site was flooded, partly in preparation for staging the sector gate installation.
Coffer cells immediately south of the sector gate were removed first to allow Bisso Marine, Houston, to position its 600-ton-capacity derrick crane barge, the Lili Bisso, so it could lift and lower the leaves into position.
Derrick cranes have no side-to-side movement like other cranes, so Bisso had to shift the barge as it lifted the 34-ft-tall leaves over obstructions and lowered them into place.
“Within the old cofferdam limits, the sector gate cofferdam is the only area on site that is dry,” says Dave Calcaterra, Alberici project manager and vice president. “They have to lift off the barge, up and over the bulkhead and into the sector gate area.” If water is at 0.0 ft elevation, that means Bisso had to lift over the +10 ft bulkhead and lower over the +5 ft needle girders to an elevation of -18 ft.
“Each leaf of the gate is below 60% capacity of the crane, so we are well within safe capacity,” Calcaterra adds. He says the biggest concerns came from working off two floating platforms—the derrick and the delivery barge with the leaves—while maneuvering the 230-ton elements into place.
But working in dynamic conditions is the norm with floating cranes, says Marcel Garsaud, Bisso’s heavy lift manager. “We do this all the time. We install oil rigs, pipelines, remove oil rigs and pipelines, and we are also worldwide salvors.”
Challenges at Seabrook were in using multiple cranes—operating from the spud barge, derrick barge and material barge within very tight tolerances, Garsaud says. “The weight is actually a bit below average for our typical lift, but most of our lifts are from material barge to shore, or from shore to ship,” he says. “We typically don’t work in such tight quarters. Some sheet pile of the coffer cells is still in place, and we had to maneuver inside that in order to set the gate structure.”
Alberici planned to install both leaves on the same day, reducing the cost of the crane and operator rental, but heavy fog cloaked the city for several days, pushing back installation of the first gate on March 15 from the early a.m. to mid-afternoon.
Alberici had planned to trim some beam stub-outs after lowering the leaf into the dam prior to final positioning, but with the weather delay, the contractor was forced to set the leaf on cribbing and perform trimming and final positioning March 16. That pushed installation of the second gate back to March 17, which started out with another fog delay.
For final positioning, Alberici is using six, 4-ft-square air skates, which work by feeding compressed air through aluminum plates sandwiched together and riddled with small holes. “When you put air pressure to it, it allows the sandwich plate to ride on a bed of air,” says Mike Arlt, an Alberici superintendent. “We use them quite a bit for moving heavy machinery. With an air skate, an average person can push a 5,000-pound of load with one arm.”
Alberici used a unified, eight-point jacking system to raise the gates and lower them onto the air skates which were then used to guide the lower chord truss members to the pintle base. Alberici used laser scanning on “all critical dimensions for anchor bolts at king pin base and upper hinge assembly to verify for accuracy prior to setting the gates,” Calcaterra says.
Fleming says contractors have done 95% of the $14.6 billion worth of work that has gone into building the storm surge risk reduction system, and the civilian architectural and engineering community has performed at least 75% of the design.
Until Seabrook's mechanical hydraulics are installed, the Corps will manually open and close the gates for navigation traffic. The gate will be mechanically operational by June 19. Hurricane season begins June 1.