GWEN IFILL: It is now nearly impossible to tell where Lake Pontchartrain ends and the city of New Orleans begins. The crescent city, wedged at the mouth of the Mississippi River below sea level, has finally lost its century-long battle against the waters which surround it.
Cascades are rushing in through two large breaches in the wall of levees designed to keep combined waters of the river, the gulf and the lake at bay.
CHOPPER PILOT: There is no delineation between the river and this neighborhood. It looks like the neighborhood was built in the Mississippi River.
GWEN IFILL: Mayor Ray Nagin told ABC this morning the situation is dire.
MAYOR RAY NAGIN: So what's happening right now is the bowl effect that everybody talked about, that's happening as we speak, and the water will rise to try and equal the water level of the lake, which is three feet above sea level.
That's significant because on St. Charles Avenue, one of our most famous avenues, is six feet below sea level in elevation. So there will be nine feet in that area and probably twenty feet in other areas.
GWEN IFILL: The levees are a principal component in a line of defenses erected over the years to protect New Orleans. They act like the walls of a fortress. On the city's southern edge, they keep the Mississippi River penned in. And on the city's northern border, they keep the 600 square miles of Lake Pontchartrain at bay.
In the east, a levee has breached along the Industrial Canal which connects the lake to the river; a large swath of east New Orleans is under deep water as a result.
On the north side of town, a 150-yard section of the 17th Street Canal has been decimated. The results: Simply disastrous.
As this cross section of the city shows, nearly the entire city is below sea, river, or lake level. There is nowhere for the water to go once it invades.
Ordinarily, a vast network of giant pumps moves water through and out of the city, but the rising tide has knocked out many of those pumps. The Army Corps of Engineers built or fortified much of the present system.
This afternoon, Louisiana Emergency Management officials outlined efforts to plug the levees with giant sandbags.
JOHNNY BRADBERRY: There are 3,000 pound sandbags. Some have gone in,okay. We've got another 100 that are ready to go into the hole as soon as the slings arrive on location. We have 20,000-pound sandbags that are going to be filled and ready for drop into the hole sometime later today and tonight.
GWEN IFILL: Once the levees are repaired, it could take up to a month to pump out the contaminated waters now inundating New Orleans.
GWEN IFILL: For more now on the design of the levees and the difficulties ahead, I'm joined by John Rennie, the editor of Scientific American Magazine.
John Rennie, many of us are hearing about what these levees are and what they do for the first time. In the case of New Orleans, a very unique case, what is it that this set of levees were supposed to do and how were they breached?
JOHN RENNIE: Well, really the levees were basically constructed to make it possible for there to be a city of New Orleans. New Orleans is built basically in a bowl beside Lake Pontchartrain. If the levees weren't there, there wouldn't be enough dry land for you to be able to build the city on.
In this case, unfortunately, what happened was that the surge of water associated with Hurricane Katrina moved through the lake, struck the levees and opened up these holes in a few places allowing the waters from the lake to then start to flow down into the city.
GWEN IFILL: These levees were built to withstand a hurricane of this magnitude, obviously not, but what was the idea about what they would be able to withstand?
JOHN RENNIE: New Orleans has been hit by a lot of hurricanes over the years and the levees really are constructed so that they can withstand a lot of the sorts of pressures and strains associated with typical hurricanes.
The fact is that even if Katrina had really hit New Orleans dead on and if we'd seen the kind of 25-foot surge that was associated with other parts in the worst part of the storm, if that had hit New Orleans, even if the levees had held up, an enormous amount of water would have still spilled over them and flooded the city.
So really nothing is built to withstand something the strength of Katrina. But the fact is that in practice, New Orleans didn't have to experience that extraordinary level of force. It was still enough though to break open these areas of the levees.
GWEN IFILL: There are also canals and there's a pump system. Does the pump system kick into gear at a time when the levees fail?
JOHN RENNIE: That's right. Automatically, these pumps are supposed to start pumping water back over into the areas in the lake to help keep the city dry. They're normally designed, I think, to be able to handle, for example, in heavy rains something on the order of an inch of water an hour arriving. However, they do break down and a lot more water was coming in a lot faster than that this time.
GWEN IFILL: Was there anything avoidable in this circumstance or was this a catastrophe waiting to happen?
JOHN RENNIE: Unfortunately, this really was a catastrophe that was waiting to happen by almost every measure. Back in 2001, Scientific American published an article on this subject.
Because the levees themselves needed a lot of work, it was obvious that eventually some sort of disaster could occur. But also the lands involving -- the marshy lands surrounding New Orleans have also degraded so fast that they're a kind of natural buffer that would usually help to protect the city against them, and in fact have done that in the past. But they're disappearing very, very fast and, unfortunately, with every year, they leave city more and more exposed to the raw power of the ocean and hurricanes like this.
GWEN IFILL: I read in that Scientific American article that you mentioned that the city was actually losing an acre every 24 minutes, an acre of wetland was going away.
JOHN RENNIE: That's right.
GWEN IFILL: So what is it that the city, the state, the government could have been doing to shore up these levees and the systems, these irrigation systems as it were, against this sort of catastrophe?
JOHN RENNIE: Well, it is a tricky problem. In fact, since 1995, the Army Corps of Engineers has been involved in a project of trying to fortify the levees and modernize the pumping systems. Unfortunately, in the last few years, some of the spending on that actually had fallen off what was recommended. I'm sure that's going to be controversial as people are looking who to blame.
Beyond that though, really the biggest problem is that it calls for a lot commitment to look at these areas outside of the city proper and start to try to allow those wetlands to be reconstructed. And that's an expensive project. It's something that goes against natural development interests and, you know, unfortunately it would have been expensive, but it's -- it would have been a lot less expensive than what we are looking at having to do in New Orleans now.
GWEN IFILL: Assuming that they find a way to plug these holes in the dike, as it were, how long would it take for the city, this bowl, to drain?
JOHN RENNIE: That's a very good question. I'm seeing different estimates of that. One estimate I saw said that once they can close that up, assuming that the levels of water don't rise a lot higher, maybe in a month, maybe more they would be able to pump that back out, which is actually an astonishing feat. But at this point, I think people are still coming to grips with just how bad the problem is and no one really knows how hard it's going to be to fix.
GWEN IFILL: And how do they plug it up? I mean, plug it up makes it sound simpler than it probably is.
JOHN RENNIE: It is, although fundamentally that's basically the problem. You have this rupture in the levee. It's a big hole in the dam in effect and the water is spilling through. They've already tried to do things like drop big sandbags down into that to fill it up. And that hasn't worked so far.
This is kind of an engineering nightmare because it's very hard to -- for the engineers to get access to these points where they have to try to build these patches. And the problem is that even when they start to build these patches, the more of the hole they fill, the faster the water that's coming through to the remaining open part. So it keeps tearing away at the surface. It's very, very hard.
They're not even sure how big the ruptures in some parts of the levees are. I've seen estimates of 300 yards, but I've also seen estimates of 500 yards and maybe growing.
GWEN IFILL: So, now that the water has stopped rising, which they say has happened, now what? Do they just wait for it to go down some more or do they just keep dropping sandbags until you hope it stops?
JOHN RENNIE: Well, you have to keep on throwing down those sandbags and you have to start the pumping and it's going to be a very, very difficult.
Unfortunately, you can't wait for gravity to take the water away because, again, New Orleans itself -- it is built under the sea level. So naturally water wants to cover New Orleans to a depth of, you know, in some places, 20 feet.
GWEN IFILL: John Rennie, a tough problem. Thanks a lot for helping us with it.
JOHN RENNIE: Thank you.