When Hurricane Katrina hit New Orleans on 29 August last year, US president George Bush initially claimed the city had 'dodged a bullet'. The storm's winds dropped from 282km/h, measured at sea, to 201km/h by the time it made landfall, and it appeared that, though it would sustain damage, the Big Easy had been spared the very worst that nature could unleash.
But Bush's optimism proved unfounded as huge storm surges rushed into the city's canals from Lake Pontchartrain to the north and the Gulf of Mexico to the south. Large sections of the city's floodwalls were overtopped and there were three major breaches, plus countless smaller instances where the levees buckled under the strain of the water. The levee breaches inundated large urban areas with up to 6.1m of water, destroying homes and businesses and leading to the death of 972 people.
In October, after the initial rescue efforts and plugging of the breaches by the US Army Corps of Engineers, US secretary of defence Donald Rumsfeld ordered a comprehensive investigation into why the levees failed.
Initial analysis of the breaches suggested that most were due to erosion. Principal levees consisted of earth embankments crowned by precast concrete panel walls. As water sluiced over the walls the leeward embankment slope was scoured away, exposing the embedded portion of and removing support from the wall elements. These were then either undermined by water, which burst through beneath them, or were toppled by the pressing weight of water.
But it is important for engineers to understand the failure mechanism in detail if defences are to be improved.
Rumsfeld's vestigation is being carried out by the Interagency Performance Evaluation Task Force (IPET), formed from over 40 organisations and led by the Corps. Using scientific and engineering methods, 10 teams are researching and recreating different aspects of the storm and local environment.
Leading one of these teams is Scott Steedman, a British geotechnical engineer. Steedman was the only non-American appointed to head an IPET research team and was tasked with the physical modelling of the levee and floodwall performance. Over the last 15 years he has worked regularly with the Corps, helping it develop one of the world's most powerful geotechnical research centrifuges at the Engineer Research & Development Centre (ERDC) in Vicksburg, Mississippi.
Steedman has recruited colleagues from the UK and from Dutch consultant Geodelft. 'We know from experience in the Netherlands that dyke failures keep happening by sliding mechanisms and by seepage, ' Steedman says.
'In [the Netherlands] there are various issues to do with constructing levees or dykes on soft clay or peat foundations. In New Orleans you have a mixture of foundations. Some are soft clay and peat and there are also large sand deposits in certain parts of the city.' New Orleans levees have generally been constructed without toeing into the underlying ground or with deeper cut-off walls.
Steedman and his team have set about constructing one fiftieth scale models of the levee sections that failed for testing in the ERDC's centrifuge.
Accelerating the centrifuge to create a gravitational force of 50G increases effective weight, forces and stresses in the model, making it behave like its full scale, real life counterpart.
'The point of the physical model is to have something independent of the numerical computer model [being generated by other parts of IPET]. Otherwise people may say 'that's just a computer model.
It looks pretty but how do I know it's telling me the truth-' The advantage of the physical centrifuge model is that we can have a parallel view and hopefully the two models will conform, and confirm the most likely mechanism of failure - explain the facts as we have them.' Teams are sharing information.
'All 10 [projects/teams] depend on a huge amount of information which is interrelated, ' says Steedman. The leaders have regular progress meetings.
Evidence gathered from witnesses, further soil samples and findings by the wave modelling team all contribute to Steedman's team constantly refining the physical models.
The bulk of the research has now been completed and Steedman is in the process of testing the physical models in the ERDC centrifuge. This will continue until April, when findings will be collated into IPET's report and presented to the US government on 1 June - just in time for the start of this year's hurricane season.
June is also the deadline for the Corps to complete repairs to New Orleans' flood protection system (News last week). Repairs are being carried out to original levee designs but as the IPET project advances, information that will improve levee design is being fed through.
'We are trying to incorporate the IPET recommendations and so far it has led to some changes in our design parameters, ' says Task Force Guardian project manager Brett Herr. Task Force Guardian is the name given to the race to restore the levees before the start of June.
'We are building to the proper design heights. In some cases the levees were below those heights when Katrina struck, ' he says. 'When you build a levee here it settles and a limitation in funding led to many of the levees being below the design grade height.' This is not the normal season for levee construction, Herr adds. Conditions on site are wet, making excavation and placement of clay a heavy, mucky business.
The Corps has also received funding to install flood control gates on the canals which drain water from the city into Lake Pontchartrain. When the levees failed, water rushed from the lake and along the canals into the city. The gates will in future be closed off when hurricanes and attendant tidal surges threaten.
Water will be discharged from the canals by 12, 1.5m diameter pumps at each outfall.
Task Force Guardian's work is budgeted at £440M, split across 59 contracts, 55 of which have been awarded so far and 10 of which are complete. With just three months to go, the work is only 30%-40% complete.
'There are probably at least 10 contracts we are concerned about getting done by June, ' admits Herr. 'These are the bigger ones with the most work to do.' Herr says that two of the projects behind schedule are big levee repair jobs, as well as a couple of the closure gates. He blames long design lead times for delaying construction.
Ultimately, the reconstruction effort may have been in vain, Steedman adds. 'What happens if the IPET team comes up with some piece of evidence which means that the work they [Task Force Guardian] have done in the last six months is redundant-' 'The answer is they will bite the bullet and redo it. If it comes to it they will have to immediately carry out additional works or replace what they have already done.' Rebuilding New Orleans' levees twice is something that Herr concedes is a real possibility.
'IPET is providing us with its findings and recommendations and hopefully they will be incorporated before June so we don't have that problem, ' he says. 'But if we do, we will have to go back and readdress the designs.'