Andrew, my understanding from basically no research is that New Orleans didn’t start below sea level. As the city grew, they needed more land, so they built levies and pumped water out of the basin. This meant that the city started to sink further below sea level. Additionally, another problem is that the city is cited on a major oil deposit, which they’re pumping oil out of. To get the oil out, they dug canals, with removed the natural barriers which protect from storms. The city sinks about an inch a year. So, the problem is the world’s greed for oil, not poor city planning.
This isn’t a well researched piece. Please feel free to leave a comment if I need educating.