By Pete Sigmund
CEG EDITORIAL CONSULTANT
In the following exclusive interview with Construction Equipment Guide, Donald Basham, chief of engineering construction of the U.S. Army Corps of Engineers in Washington, D.C., discussed important facets of protecting New Orleans against a Category 5 hurricane. Basham will direct this construction project, which is now in the study stage.
Q: Reports say that there are approximately 350 miles of levees around New Orleans and that their embankments are from 13 to 23 feet high. Is that correct?
A: Those numbers are probably a fairly decent range. The natural groundlines throughout that area govern the height of the levees. The protection depends on the level of the hurricane and the combination of surge level of water, plus the weight of water, plus wind. All that generates a top level. Depending on the ground level, you can have anywhere from 13 to 20 feet. There’s no standard height per se other than the top of the levee is expected to provide a certain level of protection.
Q: What about the width?
A: That will vary. Generally, the higher the levee, the larger the base, which is generally about 10 feet or 12 feet wide.
Q: The levees around New Orleans were built by the Corps of Engineers, right?
A: The Corps of Engineers built some of those levees, and the locals built some. It was all incorporated into one levee system. We are involved in getting federal funding for this system.
Q: The modern levee system as we know it was built about 1967, wasn’t it?
A: Yes, what we call the modern era started about then. Some of the system in the canal area was built in the late 1980s and early 1990s.
Of course, the levee system in New Orleans and up and down the Mississippi goes back to the 1800s and early 1900s. The system has grown over the years just as New Orleans itself has grown and changed.
I would add that, for part of the system, because of location and subsidence, which is inherent in that area of the country, we go back and validate the height of the levee every few years. We have built in a program that goes back and adds additional height to some of the levees as they subside.
Q: The system now is built to withstand a Category 3 hurricane?
A: Yes.
Q: How high would the surge be for a Category 5 hurricane compared with a Category 3 storm?
A: I couldn’t tell you that. We have a study ongoing now, which was started a couple of years ago. Congress asked us to look at what it would take to provide Category 5-level protection in the New Orleans area. We were just in the early stages of that study.
Q: That’s a five-year study, right?
A: Yes. A lot of factors go into it, like exactly where the hurricane could come through New Orleans and how it could impact Lake Pontchartrain and the Gulf Region area. You have to look at a number of different possible storm events, how they could combine and at least initially look at the maximum effect.
Q: Can you explain what would be involved in a study to construct levees that could withstand a Category 5 hurricane?
A: We will begin work to figure out just what level of protection would be required to withstand a Category 5. Obviously we have to do a lot of modeling and weather projections to look at different paths of hurricanes that would come up through the New Orleans area.
We have to take different surges and wave washes into account and predict, first, what the water levels would be in and around the whole area, from the Mississippi River side to the Lake Pontchartrain side to the Lake Borgne side to the Gulf side.
We have to look at all that because the entire area is exposed to those different combinations. From that information you can look at which different heights of levee you have to design to protect against that. Then you have to look at where they are going to be. You can have one solution for open areas and another for more-confined areas. Real estate is at a premium in the New Orleans proper so all of that gets to be a challenge.
Q: The study would continue for a couple of years?
A: Yes, my guess is that, if the Congress and the [Bush] Administration decides this is something they want to look at very hard and consider, I would anticipate there would be funding and we would expedite the effort.
Q: What’s the earliest date we could have a new levee system to withstand Category 5 storms?
A: That would be kind of hard to predict right now. It would take years, even if we started today, designing to that level, actually getting contracts out on the ground, buying real estate. Some of the old levee systems might have to be rebuilt. Building a new levee system on top of it.
In some cases you might be able to expand the existing levee system and work from that. It could be 10 or 15 or 20 years.
Q: Some reports say building a Category 5 levee system would cost $2.5 billion.
A: Yes, I’ve heard that figure. I wouldn’t venture a guess right now, but it wouldn’t surprise me if it cost at least that much or more.
Q: The surge on Hurricane Katrina was 25 or 28 feet, was it not?
A: It depended on the location and the trailing winds. There were areas as high as that right along the coast region, as I recall, though I don’t remember the specific numbers. That’s part of what the levees are designed for. You could have had areas as much as 28 feet in some areas and 16 feet in other areas. That [surge] will be one of the things that we will be looking at and analyzing very closely as we recover from this.
We will get with NOAH [the National Oceanic & Atmospheric Administration] and other weather organizations and try to replicate what exactly went on, so we can look at how our models predict all of that. It [the surge] varied up and down the coast and along the length of the storm. Eventually, we will be able to tell you all of that. We are in the process of launching some teams down there now to archive all that data so we can recreate exactly what happened.
Q: How can you tell how high the surge was?
A: There are some sensors, normal water gauges to measure water elevation, out there [in the levees]. Some of those we believe failed when the waves, the surge, went high enough. We also have some eyewitness accounts from people who stayed until the last minute. Plus we will try to recreate some of that in models and try to match that with the information we got on the ground.
Q: Places like Amsterdam are using flood gates. It has been reported that the Corps of Engineers proposed this type of protection for New Orleans after Hurricane Betsy in 1965. They would be steel gates that could open or close between Lake Pontchartrain and the Gulf of Mexico. Is that one of the ideas you will be looking into?
A: Yes, that and a lot of others. We looked at the one that you just mentioned back in the early to mid-1960s as one alternative to try to reduce the amount of surge that would go into Lake Pontchartrain.
If you look at the eastern side of New Orleans proper, you have a northeast side where Lake Pontchartrain and Lake Borgne come together to go into the Gulf. That’s kind of the smallest area of landmass through which a certain direction of a hurricane could bring water into Lake Pontchartrain and then over the levee system proper. As I recall, a couple of gated structures were proposed through a few of the channels that go in through the lakes, in order to help control that water level.
Q: Could you open and close that floodgate system as needed?
A: I don’t have the specific details, but it would be something like that. They would be some type of gated structure that could open to let transit go in and out. If you had a storm, you could close them. They might even be used for some saltwater intrusion as well.
Q: Would they be steel?
A: I would think so, yes.
Q: Any other ideas? Would you consider increasing the height and width of the levees?
A: Yes, the gated structure I just mentioned would still only protect a certain portion of the New Orleans area. St. Bernard Parish and some of the other parishes a little further south of that would still need other levels of protection. Most likely, that other protection would be through some type of raising of a floodwall and levee system.
In areas where you have more ground to work with, you probably could just widen the levees to raise them up to a certain height. In other cases, you would still probably have a combination of earthen levee, with a steel or concrete wall on top of the levee.
Q: The levees are often grass-covered earthen embankments, right?
A: Most of them are earthen levees. We like to use the term “an engineered fill,” a material put in with a certain density and moisture content. We’ll generally have some type of core fill material, some type of impervious material. Depending on the height of the levee, and the subsurface material, you’ll have what we call a berm that can either be placed on the water side or the dry side to help control the seepage path.
When you have water up on the levee, you’ll have seepage of water trying to migrate through the underside of the levee. Sometimes, you’ll build another small blanket berm, which could be anywhere from two feet to four or five feet high along the toe of the levee to control that seepage.
We build these seepage berms according to models to be sure no uplift pressure from the water under the levee will lift up the ground material. Some earthen levees use a grass surface. In most cases those are turned over to the local sponsor once they’re completed in the community where they’re located and the community will operate and maintain them. This includes cutting the grass and keeping an eye on the gated structures, which let water out of the channels or canals.
In a major metropolitan area, you might go from an earthen levee system to a combination of concrete walls. Those walls depend on the height that you are trying to protect. They may be an I design wall or a T design in their structural shape. They are usually located in a more constrained area, like in a canal wall, where you don’t have the ability to go into a place perhaps 150 feet wide to start a levee system.
You’ll see concrete walls in many of the communities that the levee system runs through, particularly where it runs through a closure where a highway goes through a levee system. A T wall has a horizontal system at its base [in the ground] to resist overturning of the wall and a vertical system above ground that holds back the water.
Q: Any conclusions yet on what caused the breaches in the canals, flooding 80 percent of New Orleans?
A: Not yet. We’re trying to gather that data as quickly as we can. A lot of different combinations occurred there as the surge and wind came across Lake Pontchartrain and then went into those canals. They could have had an even-higher surge there than you would on the lake itself.
It’s too early to make conjectures on whether the walls were overtopped or whether additional forces, which weren’t anticipated, applied to the walls, or whether other types of erosion occurred. It’s just hard to tell right now, and it will be a challenge to get in there and try to recover as much data as you can because some of that has already been destroyed.
We’re trying to build access roads behind the one wall to close the breach. We’ve had a lot of folks trying to take as many pictures of existing conditions as best we can, but there again, as we were building that road, we already had the water there so it was difficult to view that area.
Q: Some reports said Lake Pontchartrain rose to 3.2 feet above sea level, compared with a normal average of 1.5 feet. Is there conclusive evidence on this yet?
A: There again, that will be something we will be gathering data on.
Q: New Orleans in general is about 10 feet below sea level isn’t it?
A: Maybe not quite that much. It might be six to eight feet below.
Q: Was this one of the worst natural disasters that the Corps of Engineers has had to combat?
A: Well, I think so. Not only for the Corps of Engineers, but for this country as a whole. CEG
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