Key New Orleans Storm Surge Barrier Gate Set Ahead of Schedule
Six weeks ahead of schedule, contractors for the U.S. Army Corps of Engineers placed a 140-ton, steel vertical lift gate near the south end of an 1.8-mile-long storm surge barrier nearing completion in New Orleans.
The placement marks the first closure of three openings on the surge barrier. It represents a significant milestone toward closing the barrier and having hurricane protections in place by the 2011 hurricane season.
Manson Gulf LLC, Houma, La., used a 500-ton ringer crane for the March 24 installation of the gate where the Lake Borgne Inner Harbor Navigation Canal surge barrier crosses the Bayou Bienvenue channel. The surge barrier lies at the confluence of the recently-de-authorized Mississippi River Gulf Outlet, the Gulf Intracoastal Waterway and the IHNC. It stands 12 miles east of downtown New Orleans and is designed to reduce the risk of storm surge on the eastern part of the city and St. Bernard Parish.
In addition to almost two miles of +26-ft elevation concrete and steel wall and the gate just placed, the barrier also has two 150-ft-wide gates near the north end where it crosses the GIWW to allow passage for deep draft barges and ships. “We finished the wall about two months ahead of schedule,” says Col. Robert Sinkler, commander of the Corps’ Hurricane Protection Office in New Orleans. “The barge gate was floated in location about two months ahead, and we are placing the Bayou Bienvenue gate in its slot six weeks ahead of schedule. Fabrication of the sector gate is ahead of schedule, and concrete work is on schedule.”
The 34-ft-tall, 62-ft-wide vertical lift panel at Bayou Bienvenue will create an opening 56 ft wide to allow passage for recreational boats.
Prime contractor Shaw Environmental, Baton Rouge, La. fabricated the gate at Shaw Global in Delcambre, La. The design is by Waldemar S. Nelson and Co. Inc., New Orleans, which has designed many offshore platforms and dock structures. The gate is engineered to resist a maximum lateral force from the surge of a storm that has a 1% chance of occurring in a given year, as per Corps specifications for the entire risk reduction system. "It's designed for water to go all the way to the top [+26 ft], with the water two ft below [sea level--a differential of 28 ft] on the back side," says Stephen Johns, vice president of Nelson. "One of the design challenges was designing the structure for the force/weight of water coming over the top and cascading down." The gate will resist a lateral load of 3,000 kips (3 million lb), Johns says.
For the initial gate design Nelson consulted with Richard Daniel, a Dutch engineer who designed the 98-meter-long Hartel Canal Storm Surge Barrier gate near Rotterdam. The gates in that structre are larger than the Bayou Bienvenue gate and have an arch design, while the Bayou Bienvenue gate is a truss or space frame design, Johns says. However, for the BB gate, designers borrowed some Hartel details, most notably the use of ultra high molecular weight polyethylene bearing pads on the edge of the gate and the configuration of the slots in the receiving, concrete monoliths. The 6-ft-thick gate also features rounded corners and tubular members to minimize sharp corners, where corrosion begins, Johns says.
The 16-in.-wide, 4-in.-thick UHMW pads reduce friction and provide some give as the gate is placed in the slots, says�Dale Miller, regional vice president for TetraTech INCA, Metairie, La., which performed design on the concrete structure and steel towers that support the steel lift gate.�The UHMW pads also transfer horizontal water pressures along the vertical concrete slots during storm surges. �In addition to providing a low coefficient of friction, the UHMW is “not susceptible to marine life or deterioration in salt water,” Miller says.
“The entire structure [with monoliths and gate], with the gate lowered in place, resists overturning forces from a hurricane surge across 138 linear ft of barrier wall,” says Sergio Gaitan, TetraTech INCA’s senior project manager. Approximately 46,000 kips (46 million lb) of concrete and steel were used for the structure, from the foundation piles to the top of the concrete walls at +26 ft. “The 8 ft thick base slab was placed in three different pours inside of the sheet pile cofferdam from -16 ft to -8 ft,” Gaitan says. “All these walls and abutments were accomplished in seven different pours, each placed in a single, 32-ft lift, lasting eight to nine hours.”
The gate will be lifted and lowered by motors and winches bolted to 54-ft-tall tubular steel towers bolted to the concrete slab atop the barrier wall, for a total height of +80 ft. “Cables running through sheaves on the towers will hold the gate in the raised position such that vessels can have a clearance of about 36 ft,” Gaitan says.
A lift bridge, to allow vehicular passage along the floodwall at Bayou Bienvenue, will also be lifted from a platform supported by the towers. The towers and bridge have yet to be installed.
The gate will remain in the down position until the rest of the structure is operational, says Jon Kennedy, Shaw’s construction manager. Once placed, the gate was lowered to check the seal, and raised and lowered by crane to verify that it will slide normally, and then blocked at the bottom with wooden cribbing to maintain a 1 ft tidal passage until it is operational, Kennedy says.
A vertical lift gate could be used because it is for recreational passage only, says Col. (ret.) Vic Zilmer, Corps project manager. “If we used this at the wider, navigable crossings, they would be like huge, steel sails, adding wind load to the structure.” The added wind load is a concern during thunderstorms. “The Dutch have big vertical lift gates, 100 m wide, but not as tall,” Zilmer says. “This is small enough so you don’t get wind torque.”
An advantage to using the lift gate is that the steel normally is above the water in the raised position and less exposed to corrosion.
“We went to great lengths to reduce O&M [operations and maintenance] costs,” Sinkler says. “The batter piles in neoprene sleeves on the back side of the barrier are a great example.” Other elements in Corps’ specifications throughout the system that are designed to reduce O&M costs include concrete guide walls at the surge barrier sector gate instead of timber and deep soil mixing and wick drains in earthen levees to reduce settling and the need for future levee lifts. Construction of the Bayou Bienvenue vertical lift gate structure will continue into 2012.