Three years ago I visited New Orleans as part of a series looking at large-scale water and engineering projects. At the time, I was confidently told that nature had been conquered in the Mississippi delta: the huge system of levees, locks and floodways meant New Orleans would never flood again.
Over a hundred years of major civil engineering projects had seen to that. We all know now this wasn't true.
For the anniversary of the Katrina disaster, I returned to the city to look at what went wrong, where and why the levees were breached and to understand the political and engineering lessons that have been learnt from the hurricane.
There are many places in New Orleans where it looks like the flooding happened yesterday: whole neighbourhoods are deserted, flooded buildings still show the high watermarks, the mangled wreckage of smashed homes still present a health hazard.
Mountains of scrap metal, everything from refrigerators to buses, are piled up waiting to be taken away.
Low-level existence
Debbie Simon, a former disaster manager now working with the US Army Corps of Engineers in the city, has a phrase for it: "When people ask me what New Orleans is like, I say 'it's like landfill', thousands of square miles of landfill - but with people still living in it."
But why the city flooded is only now becoming clear. The hurricane did not pass directly over New Orleans. The city slowly filled up with salt water - water that remained for nearly three weeks.
There is no doubt that the flood defence system was inadequate: the levees were breached.
The US Army Corps of Engineers, which has responsibility for building and maintaining the levee system, has spent the last year looking into the minute detail of how and why this happened.
Their conclusions, shortly to be presented to the US Congress in the 7,000-page Interagency Performance Evaluation Taskforce (IPET) report, demonstrate the precarious nature of New Orleans' existence.
Most of the city is below sea level, 80% of it was flooded, and most of the water entered through two relatively small breaches of the levee system.
As we stood next to a brand new pumping station on the London Avenue Canal, Debbie Simon explained how New Orleans is subjected to tropical storms which if not immediately drained would flood the city every time it rains.
"When you have a tropical storm, it dumps 14 inches (35.5cm) in a couple of hours, and that water's gotta go somewhere; it goes into the canal."
'Like a bath'
This water flows from the streets to a series of drainage canals, which in turn take the water out to Lake Pontchartrain, a huge expanse of brackish water which is really an estuary to the Gulf of Mexico.
The lake borders the northern side of the city. As the lake is actually higher than most of the city, the water has to be pumped down the canals into the lake.
Hurricane Katrina caused huge waves of water to sweep across the lake. These created a storm surge, waves of water in the opposite direction to the rainwater flowing out of the canals into the lake.
"You had a storm surge coming down, rainwater going out. The only way was up. It was like putting the plug in and filling up a bathtub," said Simon. To try to prevent future storm surges from entering the canals, much needed flood gates and a large system of pumps are being installed at the mouth of both the 17th Street Canal and the London Avenue canal.
London Avenue was breached on both sides, flooding the 9th Ward, an area of predominantly wooden houses which were smashed by the floodwaters.
The Army Corps first proposed putting these gates into place nearly 10 years ago, but the idea did not get off the drawing board as it was opposed by local residents who thought it would spoil their view of the lake and environmental groups concerned about its effect on the ecology of the area.
A trade-off
Barry Fletcher from the Army Corps took me to the 17th Street Canal. This is where a large section of the levee gave way, flooding the neighbouring Lakeside area - the place where he used to live.
He is hoping to rebuild his home but cautions against placing too much faith in the new flood prevention methods.
"If the gates have to be closed due to a storm surge, that will reduce our ability to pump rain water out of the city and into the lake. This year, 2006, we will not have enough capacity to remove the rainwater from the city as fast as it can fall - that's just a reality.
"And as someone who lives here, I would rather have a few litres of rainwater in the streets than 3m of salt water for two weeks." About 200 miles north of New Orleans at Vicksburg is the Army Corps' main research station - the Engineer Research and Development Center (ERDC).
To assess what happened during Katrina, they have built a 1/50th scale model of the New Orleans basin. Using a wave generator, they were able to recreate the hurricane conditions in their model.
This showed exactly where the failures occurred and the forces needed to produce them.
Here, they tested scale models of the 17th Street Canal, in particular the section of canal wall that failed.
"We built the model, and subjected it to a water level that existed before Katrina, and it held up fine," explained Wapawi Ellis, the ERDC's assistant director.
"Then we increased the water level to flood height and within minutes saw it fail. The wall itself didn't fail; it was the soil beneath it. This couldn't hold the wall tight."
New Orleans is built on river delta. There is no bedrock for hundreds of metres down. The soil is very absorbent and unstable. But to people whose homes were flooded the distinction between soil and wall failure is immaterial - many in New Orleans are angry at the Corps. The city is plastered in posters saying, "Hold the Corps responsible".
It is an issue acknowledged by Wayne Stroupe from ERDC: "The failures on 17th Street Canal, London Avenue Canal - they failed before water got up to the top of the flood wall. It's hurt the Corps; we helped contribute to the flooding of New Orleans."
Nature as ally
For now, the repairs are only being undertaken to bring levees up to pre-Katrina levels. The current thinking on flood defence for the city is that it is not enough just to build higher levees.
The city's population has to get more involved, houses will have to be rebuilt higher and evacuation plans followed more rigidly.
The Corps is also taking a fresh look at what help the natural environment might provide.
After years of tightly controlling the amount of water flowing down the Mississippi, they are beginning to realise that the river might actually be a useful aid to buffering hurricanes.
"Now, we've got to shift to allow the river to do what it did naturally: to deposit sediments and nutrients, to utilise the river as a sustaining and building block for the estuaries," explained Gregory Miller from the Corps' Coastal Restoration Office.
By allowing the delta to reform, building up barrier islands and marshes with flows of nutrients from further upstream, the restored wetlands should act as a barrier to incoming storms and hurricanes, providing a first line of defence for human settlements.
Julian Siddle's journey around the flood defences of New Orleans can be heard on Discovery: Katrina: A Year On, to be broadcast on the BBC World Service on Wednesday, 30 August, at 1130 GMT. The programme will then be archived on the Discovery website.
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