I found an article from 2004 that talks about worst-case scenarios for New Orleans in the case of a direct hit from a hurricane. At the time, Hurricane Ivan was thought to be on a path for The Big Easy.

The worst-case scenario here – a direct strike by a full-strength Hurricane Ivan – could submerge much of this historic city treetop-deep in a stew of sewage, industrial chemicals and fire ants, and the inundation could last for weeks, experts say.

If the storm were strong enough, Ivan could drive water over the tops of the levees that protect the city from the Mississippi River and vast Lake Pontchartrain. And with the city sitting in a saucer-shaped depression that dips as much as 9 feet below sea level, there would be nowhere for all that water to drain.

Even in the best of times, New Orleans depends on a network of canals and huge pumps to keep water from accumulating inside the basin.

“Those folks who remain, should the city flood, would be exposed to all kinds of nightmares, from buildings falling apart to floating in the water having nowhere to go,” Ivor van Heerden, director of Louisiana State University’s Hurricane Public Health Center, said Tuesday.

LSU’s hurricane experts have spent years developing computer models and taking surveys to predict what might happen.

The surveys predict that about 300,000 of the 1.6 million people living in the metropolitan area would risk staying.

The computer models show a hurricane with a wind speed of around 120 mph or more – hitting just west of New Orleans so its counterclockwise rotation could hurl the strongest surf and wind directly into the city – would push a storm surge from the Gulf of Mexico and Lake Pontchartrain over the city’s levees. Ivan had sustained wind of 140 mph Tuesday.

New Orleans would be under about 20 feet of water, higher than the roofs of many of the city’s homes. Besides collecting standard household and business garbage and chemicals, the flood would flow through chemical plants in the area, “so there’s the potential of pretty severe contamination,” van Heerden said.

Severe flooding in bayous also forces out wildlife, including poisonous snakes and stinging fire ants, which sometimes gather in floating balls carried by currents.

Much of the city would be under water for weeks. And even after the river and Lake Pontchartrain receded, the levees could trap water above sea level, meaning the Army Corps of Engineers would have to cut the levees to let the water out.

One note of optimism: the worst case scenario involves a hurricane hitting to the west of New Orleans. It currently appears that the storm will hit to the east. One note of caution: Katrina has much higher winds than Ivan, or the computer models.