Almost five years to the day since Hurricane Katrina ripped across Louisiana, engineers are bracing themselves for the final phase of works to reduce the risk of storm damage to the city of New Orleans.
A massive programme has to be completed by 1 June 2011 ahead of the hurricane season that begins in late summer. And as Sunday marked five years since Katrina caused mass flooding that left nearly 2,000 dead and destroyed countless structures, the deadline is looming.
Arcadis global director of water resources Pieter Dircke says the New Orleans Hurricane and Storm Damage Risk Reduction System (HSDRRS) is comparable with the Dutch Delta plan in scale. “That’s a 40 to 50 year plan – and that was considered fast,” he says.
In five years, New Orleans has seen improvements made to, or new construction of, levees and flood walls stretching 218km. Work has also taken place on 73 pumping stations, three canal closure structures and four gated outlets.
The US Army Corps of Engineers has responsibility for ensuring the flood defences are built while consultant Arcadis has had a major design role on many of the key structures - its two largest contracts total $200M (£130M) of the almost £10bn total scheme costs.
It’s a huge sum, but around £65bn worth of infrastructure is at risk from storm damage in New Orleans. It is a factor that contributed to the tight deadline, which has in turn created a novel approach where accepted practices have been flexed or abandoned.
“There was a commitment made to build it all in five years,” says Arcadis Corps Projects vice president, program manager Dan Hitchings. Hitchings previously worked as an engineer with the US Army Corps until early 2007 when he moved to Arcadis. “We said ‘we can’t do this the way we normally do things’.”
“It got a little loose with the contracts. It was a case of saying: ‘You got a [framework] contract? Let’s use that and go get something done’.”
“It got a little loose with the contracts. It was a case of saying: ‘You got a contract? Let’s use that and go get something done’.”
For some works, the US Army Corps turned to early contractor involvement for the first time in its civils works history to create a faster turnaround – getting contractors on board to start work while design continued.
Most of the work is being completed at least three times faster than normal, while some structures are speeding ahead up to 15 times faster than would be expected. However, engineers reject the suggestion that working at such speed exposes the project to undue risk. “Yeah, there’s the potential that some mistakes could be made,” says Hitchings. “But the level of checks is extraordinary and everything’s been so overdesigned that there’s a good level of redundancy too.”
With less than a year to go, there is much physical evidence of the scheme being built.
One of the key structures – the 3km long Inner Harbor Navigation Canal (IHNC) surge barrier – is already in place, if not entirely complete. This is positioned across a marsh west of Lake Borgne and will reduce the risk of the storm surge that contributed to the worst flooding in the city’s most vulnerable areas - including the Ninth Ward and St Bernard Parish.
“It’s an active navigation channel,” says US Army Corps resident engineer Vic Zilmer. “But we needed a strategic change in how we protect the city. There are 150 gates for businesses alone in the canal area and they need about three days’ warning for their closure.
“It was challenging but we didn’t want to deal with that hassle so we took the fight [and the barrier] 13km away from town.”
Contractor Shaw Environmental & Infrastructure won the contract from the US Army Corps in April 2008 and work started on the barrier in May last year. In a little over a year, the predominantly piled structure is mostly in place.
Ground conditions are extremely poor for foundation work in the city. “There’s no bedrock anywhere so all the piles are working in friction,” says Zilmer.
Shaw installed nearly 1,300, 1.7m diameter, precast concrete soldier piles at depths of nearly 44m to form the main structure. Each of these 94t piles were driven into place before steel reinforcing bars were concreted in the centre of each. There are two closure piles between each of these and the remaining 150mm gap is filled with concrete pours.
For extra robustness, 645 steel tubular batter piles with a diameter of 910mm prop the wall on the protected side at an angle of 57˚ from the vertical, down to 60m.
Because it is a navigable channel, the barrier incorporates a 45m wide sector gate with a 45m wide concrete swinging barge gate supplemented further south by a smaller vertical lifting gate for commercial fishing vehicles.
Time is of the essence
“It would take around 15 years to complete a barrier of that size,” says Zilmer. “It’s taking only one year and that’s never been done before.”
Despite costing less than a tenth of the total project cost at around £780M, Zilmer says this is the “most important” 3km of the entire HSDRRS. It stands almost 8m above the water line. The size is significant, but at its most extreme, the water levels during Katrina reached over 9m.
The site has clocked up 1.8M man hours and to date work is three quarters complete. “The walls are on schedule, adds Zilmer. “The gates are more of a challenge. But I will meet my deadline.”
“The walls are on schedule. The gates are more of a challenge. But I will meet my deadline.”
The sector gate is the most complex of the three being built into the surge barrier. Two wedge-shaped, steel gates are swung on a spindle, their curved edges meeting when closed. It is a similar design replicated in two other parts of the HSDRRS.
One of the other two forms part of the wider IHNC system. This is the 29m wide Seabrook Floodgate, which along with vertical sliding gates flanking it, reduce the risk of a storm surge swamping the city via Lake Pontchartrain to the north.
The third is part of one of the other dominant systems within the New Orleans plan - the Gulf Intracoastal Waterway West (GIWW)Closure Complex situated to the south.
The plan here is to create flood defences along the west bank of the Mississippi River, with mass dredging of the Algiers and Harvey Canals to inrease stormwater drainage capacity.
However, the GIWW complex has a vast, 150m long closure system that boasts the largest pumping station and sector gates ever built in the US, according to the US Army Corps. The wedged sector gate is almost 69m wide and when closed for a storm the pump station will have to work at a rate of over 540m3/s to evacuate water from the two canals.
Connecting these two key projects across much of the city are new and improved levees and flood walls. In many places, walls and levees restored immediately after Katrina are being bolstered by up to four rows of new steel H-beam foundations on which concrete walls stand 4.5m high.
Future coastal protection
Beyond June 2011, engineers expect the State of Louisiana to shift its focus onto coastal restoration to defend the region.
Historical creation of the levees has eroded the amount of coastal land in the area as sediment has been prevented from settling. Consultant Arcadis is now working on a plan to restore some of this lost land.
“We know that to create a sustainable Louisiana you either need to create freshwater diversions or carry out dedicated dredging to get that sediment back,” says Arcadis vice president, technical expert Falcolm Hull.
The firm is studying lessons from other countries about how to reduce associated costs. Hull says that these are likely to account for a huge portion of the $2.1bn (£1.4bn) Louisiana has set aside for coastal restoration in the next three years.
Operation and maintenance
Debate continues over who will maintain and operate the flood defences.
With State funds low in the current economic downturn, Louisiana is resisting taking full responsibility.
“As a result, we’ve made most of the wall as low maintenance as possible,” says Zilmer. For example, steel batter piles are protected from saltwater corrosion by encasing them in high density plastic.