By Adam Blenford
For generations, many who live and work in the Big Easy have feared the worst.
Once the water broke into New Orleans it had no way of flowing out
New Orleans lies in a wide, shallow bowl on delicate marshlands well below sea level, with the Mississippi River running through it.
It perches under the lip of a vast lake more than twice its size, while to the south and east lies the Gulf of Mexico, one of the world's most fertile hurricane zones.
To protect the city from disaster, city planners designed and built a complex system of flood defences after the Mississippi burst its banks in devastating fashion in 1927.
Nevertheless, officials had little option but to order a mass evacuation of New Orleans when they saw that Hurricane Katrina was headed for a direct hit.
New Orleans' extensive levee system was built to withstand a strong category three storm. When Katrina appeared on the weather maps it was a category five - the strongest Atlantic hurricane in a generation.
As water levels rose in Lake Pontchartrain to the city's north, and the Mississippi to the south, two levees - semi-permanent dams built to hold back high waters - buckled under the strain.
The Mississippi and Missouri rivers form a giant drainage system
Swollen by the storm, the lake spilled inexorably into the heart of the city.
Experts agree that a "catastrophic" failure of the levee system has been avoided - narrowly.
The US Federal Emergency Management Agency counts a direct hurricane strike on New Orleans as one of the biggest threats the nation faces, ranked alongside terror attacks and earthquakes.
A frontal assault by a category four or five hurricane would break or destroy many more of New Orleans' famous levees. The city would drown.
Instead a last-minute curve away from its plotted course saved New Orleans from a direct hit.
A changing environment
Yet if the levees saved the city from utter ruin, they may have contributed to its partial destruction.
Years of flood control engineering, inspired by the need for a major city and port in the oil and gas-rich Mississippi delta, have altered the natural landscape of the region beyond recognition.
The levee system meant that the Mississippi, a vast river that drains the whole of the eastern US, was tamed by man.
New Orleans and the Mississippi are crucial oil and gas hubs
Without regular river floods to feed the swampy delta with precious silt and nutrients, vast swathes of Louisiana's coastal wetlands have disappeared in the past 75 years.
Sprawling coastal wetlands can bear the brunt of a hurricane better than the concrete passageways of a modern city.
The US Geological Survey calls the wetlands a "natural buffer" in a high-risk area. Plans to stop further erosion have run aground in Congress.
Erosion meant that instead of falling on the delta, Katrina's rains swelled the Mississippi and filled Lake Pontchartrain.
In the oldest part of New Orleans, the French quarter, the river runs along the crest of a ridge, a testament to man's engineering but a snub to nature.
When the pressure was too much to take, two of the levees broke, pouring water into the city.
Two days after the storm, water levels had fallen in the lake and the river, but the city remained a muddy swamp.
The broken levees allowed the water in, but the dozens of barriers that remained intact stopped the floods receding.
Getting New Orleans back on its feet will take three to four months, the city's mayor has said.
Restoring confidence in its flood defences may take a little while longer.