The Biggest Threat
I was depressed and saddened from footage on NBC tonight so I did a quick google search ("why below sea level new orleans built") about New Orleans and how it was built below sea level. I didn't find my answer yet, but the second ranked article was this one. The title is Vulnerable Cities: New Orleans (also articles for Long Island, Tampa, Miami Galveston and Wilmington).
A hurricane approaching the city from the east, virtually at the mouth of the Mississippi River, "would drive the lake water southward into the city. So under the right circumstances, the flooding may be more severe coming from the lake than that coming from the Gulf (of Mexico)," said Jay Grimes, Louisiana State Climatologist.
The levees that protect the city from flooding are also a flood threat themselves. "The biggest threat that the city has is that of a slow moving Category 3, 4 or 5 hurricane, which would create a surge of water that could be up to 30 feet high. Now if this (high) water comes into the city, it will top the levees. It will go over the top of the levees and actually fill up the city," said Hijuelos.Sadly, now this article has been proven correct.