Twice since I began blogging in 2002 has New Orleans been threatened by a Category 5 hurricane, Lili in September 2002 and Isabel in September 2003. The city dodged the bullet both times. Lili moved away and made landfall in Texas and Isabel weakened before it went ashore, landing well to the east.

Now Katrina is a Cat 5 and on track to hit the city.

This is a live-linked image from the National Hurricane Center.

A satellite radar shot as of 7:23 a.m. today

According to a Corps of Engineer study done a few years ago, a Cat 5 hurricane would devastate New Orleans on a scale of near-biblical proportions. The French Quarter would be flooded 20 feet deep or more.

Basically, the part of New Orleans that most Americans—most people around the world—think is New Orleans, would disappear.

New Orleans has the levee equivalent of the Great Wall of China to protect flooding from the Mississippi River, which 100 years ago or so flooded the city and environs routinely. The Corps built 2,000 miles of levees to protect against such river flooding.

But only a few years ago the city and the Corps began understanding that in a very powerful hurricane, the worst flooding threat is from the sea and from Lake Ponchartrain, situated north of the city. As the huge lake overruns its banks from raina and wind-driven seawater, the river levees would trap and funnel the water to high levels, making the region between the lake and the city a giant bowl with no exit.

But that’s not all:

There’s always been a huge natural buffer that helps protect New Orleans from storms. There are miles of wetlands between here and the Gulf of Mexico: they slow hurricanes down as they blow in from the sea. But that buffer is disappearing. Every year, a chunk of wetlands the size of Manhattan crumbles and turns into open water.

Joe Suhayda explains, “So the hurricane can move closer to the city before it starts to decrease. So in effect, the city is moving closer to the Gulf as each year goes by.”

And he says, it’s partly because of those levees along the Mississippi River. When they stopped the river from flooding, they also prevented the wetlands from getting the regular doses of floodwater and mud that they need to survive. Studies show that if the wetlands keep vanishing over the next few decades, then you won’t need a giant storm to devastate New Orleans — a much weaker, more common kind of hurricane could destroy the city too.

Some Corps estmates of deaths reach to 100,000, with a low end of 20,000. But those estimates assume no one was evacuated beforehand. Fortunately, New Orleans’ mayor ordered a mandatory evacuation early this morning. The problem is that historically, only about 40 percent of New Orleans residents have heeded an evacuation order. Even if everyone heeded it, there remains the potential for terrible devastation of the city and its surrounding area.

Update: Informative links, courtesy Nashville is Talking: