If you drive into New Orleans from the west, you travel on elevated expressways built on concrete pilings that tower above the swamps and bayous below.

You have entered the land that drainage forgot.

New Orleans is sandwiched between Lake Pontchartrain to the north and the Mississippi River, about 160 kilometres north of where the river joins the Gulf of Mexico, one of the world's most fertile hurricane zones.

French fur traders first chose the city's location in part because it was the only patch of relatively high ground along that part of the river, and because of the narrow portage, favoured by the native Indians of the area, between the river and the lake.

New Orleans is home to 485,000 people -- about 160,000 less than Winnipeg, a city that also has some familiarity with flooding -- although the area population is 1.3 million.

While living in Winnipeg is like living on a tabletop, living in New Orleans is more like being in a sunken soup bowl.

The majority of the city is an average of 1.9 metres below sea level -- with the lowest point six metres below sea level and the highest ground still 0.3 metres lower than the sea.

Almost half of New Orleans' 907-square-kilometre surface area is comprised of water, not land.

Since the area is naturally flood-prone, engineers have worked to build an intricate system of canals, pumps and elevated embankments called levees, which form the bowl, to protect the city.

As little as 2.5 centimetres of rain can trigger some degree of local flooding. With its semi-tropical climate, the city gets an average of 90 cm per year.

That reality helps explain one colourful aspect of New Orleans and southern Louisiana -- people are buried in aboveground crypts, not underground graves.

A history of near misses

Levees exist up and down the Mississippi River valley. Breaches, and the damage and heartbreak that follows, are ingrained in southern folklore.

For example, "When the Levee Breaks" is the name of a 1929 blues tune by Memphis Minnie, made famous by Led Zeppelin.

"Now, cryin' won't help you, prayin' won't do you no good, When the levee breaks, mama, you got to move," the lyrics warn.

The earliest New Orleans levees were erected soon after the city's founding in 1718. The system has been expanded and strengthened ever since.

In 1965, when Hurricane Betsy struck the coast near New Orleans, the levees encircling the city and outlying parishes were raised to heights ranging up to about seven metres.

Since then, the Big Easy has had nothing but near misses as hurricane after hurricane wreaked havoc elsewhere along the Gulf Coast.

In 1998, Hurricane Georges headed straight for New Orleans, then veered at the last minute to strike Mississippi and Alabama. Hurricane Lili blew herself out at the mouth of the Mississippi in 2002. And last year's Hurricane Ivan swerved to the east as it came ashore, barely grazing the city.

Nevertheless, experts repeatedly cautioned that the levee system was unlikely to protect the city against a Category 4 or 5 storm.

"We're talking about an incredible environmental disaster," Marine scientist Ivor van Heerden of Louisiana State University, who has developed flooding models for New Orleans, said before the storm arrived.

He predicted that floodwaters would overcome the levee system, fill the low-lying areas of the city and remain trapped there.

When meteorologists forecasted that Hurricane Katrina was bearing down on the Big Easy, city officials were grim.

"We are facing a storm that most of us have long feared," New Orleans' Mayor Ray Nagin said Sunday as he issued a mandatory evacuation order. "The storm surge will most likely topple our levee system."

He was right.

When the levee breaks

The 560-kilometre-long hurricane levee system, mostly along Lake Pontchartrain to the north and the Mississippi River on the south -- was designed to withstand a fast-moving Category 3 hurricane, which carries with it a storm surge of up to 5.5 metres.

As it crossed the Gulf of Mexico after pummeling south Florida, Katrina was rated a rare Category 5 hurricane, with winds of 280 kilometres per hour and a predicted storm surge of 8.5 metres. Meteorologists thought the storm would bring also 38 centimetres of rain.

When Katrina hit early on Aug. 29, it had been downgraded to a still-dangerous Category 4 storm. As well, it veered to the east when it came ashore, sparing New Orleans a direct hit.

But peak winds still hit 160 km/h as Katrina lashed New Orleans for eight hours.

That night, residents believed they had escaped major damage. They were wrong.

As water levels in Lake Pontchartrain rose in the city's north, the levee system buckled under the strain.

There was one levee break reported at the east end of town -- the Industrial Canal breach -- but only localized flooding resulted.

However, there was a failure of a large section of the vital 17th Street Canal levee where it connects to the Old Hammond Highway Bridge. The London Avenue Canal breach was another blow.

The gap -- first reported to be about 60 metres wide, but now about 150 metres -- allowed millions of litres of water from Lake Pontchartrain to flood New Orleans, turning it into an urban swamp.

According to The New York Times, this breach was at a spot that had received more attention than other areas in the region.

Shea Penland, director of the Pontchartrain Institute for Environmental Studies at the University of New Orleans, said that breach was particularly surprising because it occurred "along a section that was just upgraded."

"It did not have an earthen levee," Penland told the newspaper. "It had a vertical concrete wall several feel thick."

By late afternoon Tuesday, Mayor Nagin reported that 80 per cent of his city was now underwater.

Engineers believe high winds pushed water over the levees' top and eroded them from behind. Other experts studying flood prevention speculated that any dip in the retaining levee system might have allowed water to slosh over, triggering the collapse.

Plugging the breaches

The U.S. Army Corps of Engineers had hoped to plug the breaches by dropping 1,360-kilogram sand bags from twin-rotored CH-53 helicopters. Another plan was to use shipping containers filled with gravel.

But neither option initially went well.

"The challenge is an engineering nightmare," Louisiana Gov. Kathleen Blanco of Louisiana told ABC's Good Morning America on Aug. 31. "The National Guard has been dropping sandbags into it, but it's like dropping it into a black hole."

In addition, there was a report of a pump failure in the evening of Aug. 30.

On the morning of Aug. 31, Blanco called for the complete evacuation of New Orleans.

Later that day, experts from the Army Corps of Engineers arrived on the scene, assessing ways to repair the breaches.

"We're attempting to contract for materials, such as rock, super sand bags, cranes, and also for modes of transportation like barges and helicopters, to close the gap and stop the flow of water," said Walter Baumy, the Corps' manager for the project.

According to reports on Sept. 1, the Army Corps of Engineers said it was having trouble getting the sandbags to the affected site because the waterways were blocked by boats, debris, and loose barges.

By Sunday, authorities reported progress in fixing the levees. Military helicopters continued to drop huge sandbags. But because Lake Ponchartrain's level has started to drop, some of the water is flowing out of the city and back over the levee into the lake.

On Monday, authorities said they had repaired the 17th Street breach and had almost closed off the London Avenue one. In addition, some of the pumps were being started up.

Even after the breaches are plugged, electricity will still have to be restored and the remaining dried otu pumps repaired before the long process of pumping the water out of New Orleans can begin in earnest. In a worst-case scenario, that might not be done until late November.

Once the water is removed, people will still be unable to come home immediately. That's because the floodwaters have turned New Orleans into a toxic soup bowl, with chemicals, human waste and rotting animal carcasses and human corpses all mixing together to make the trapped water highly polluted. Submerged homes will have to be cleaned out and surveyed to see if they are still liveable.

 "It's hard to say how many homes may be structurally salvageable," Blanco said.

As a result, hundreds of thousands of people will be stuck in temporary housing for months and months.

As the song laments: "When the levee breaks, I'll have no place to stay. Mean old levee, taught me to weep and moan."