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Ongoing Midwest Flooding Threatens Mississippi River Levees

June 17, 2008 at 6:10 PM EDT
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As the Mississippi River continues to rise through Iowa, Illinois and Missouri, federal officials fear many of the existing levees will fail, stressing the region's flood prevention system. A civil engineer discusses the science of levees.
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GWEN IFILL: We begin with the latest on the floods in the Midwest, where there are new fears tonight about high water heading south.

The Mississippi River is rising, and fast, as floodwaters from swollen tributaries in the Upper Midwest dump into the big river.

And now, as the waters continue to surge, there is an added concern: a system of levees that holds back the Mississippi in Iowa, Illinois and Missouri is in danger of being overtopped.

Millions of sandbags are being placed on the levees in hopes of repelling the river. But the Army Corps of Engineers has said there is no way to predict whether the levees will hold.

The Illinois levee, which broke today, forced the closure of the Great River Bridge, which spans the Mississippi between Gulfport, Illinois, and Burlington, Iowa. The river will crest there today at 26 feet.

Missouri is now bracing for record flooding as the deluge flows down river. Sandbagging crews were out in force. Hannibal, Missouri, saw its first flooding today. The worst is not expected there until later this week.

In parts of Iowa, the flooding continues. Some residents can’t beat back the tide.

FLOOD VICTIM: You’ve got to let it go, because we can’t keep fighting it up.

FLOOD VICTIM: It’s up. We gave up. Mother Nature wins.

GWEN IFILL: And the floodwaters coursing throughout the stricken region carry a toxic stew of garbage, chemicals, fertilizers, manure and fuel. An official in Iowa City warned citizens to beware.

MAYOR REGENIA BAILEY, Iowa City, Iowa: The floodwaters are full of hazards themselves. The water, people should take precautions about being in floodwater. That’s very important. If they have open cut or open skin, they should make sure that they have a current tetanus booster. That’s very important, as well, if they’re in floodwaters.

GWEN IFILL: There is also enormous damage, both to property and to the region’s agriculture industry. Corn and soybean farmers are just now beginning to take stock of their fields and determine whether their crops are salvageable.

Levees are center of attention

Erik Loehr
University of Missouri
If they get overtopped, levees aren't really designed to be able to withstand overtopping. So once the water tends to go over the levee, they'll tend to erode away and create a much larger breach.

GWEN IFILL: We get more now on the levees that are causing so much concern from Erik Loehr, professor of civil engineering at the University of Missouri.

Welcome, Professor Loehr.

ERIK LOEHR, University of Missouri: Hello.

GWEN IFILL: We have heard so much about levees in the wake of the Katrina floods in 2005. How are the levees that we see along the Mississippi River different from what we saw down in New Orleans?

ERIK LOEHR: Well, they're different in a couple of ways. One, they're loaded a little bit differently. In Katrina or in coastal levees, they tend to be loaded by wave-type loading and high tides and such, whereas along the rivers they tend to be subjected to currents that sort runs along the levee, rather than into the levee.

But they're also relatively similar. Sometimes there are different kinds of levees, but the levees that we have along the rivers aren't terribly different from what we see down on the coastal areas.

GWEN IFILL: The levees that we see along the Mississippi River are made of what, for instance?

ERIK LOEHR: For the most part, they're earthen levees, so they're compacted earth to form a slope, an embankment to hold back the water when it raises to high levels. But in other instances, there may be floodwalls on top of those levees or just floodwalls alone.

GWEN IFILL: And when we hear about a levee failing or a levee being breached or overtopped, how does a levee fail?

ERIK LOEHR: Well, they can fail in a number of ways. If they get overtopped, levees aren't really designed to be able to withstand overtopping. So once the water tends to go over the levee, they'll tend to erode away and create a much larger breach.

And so that's the one that typically gets seen. If the flood elevation, the water elevation gets higher than the levee, it comes over the top and kind of erodes the levee away.

They can fail in other ways, through what we call underseepage, which is where seepage actually occurs through the levee or, more commonly, down into foundation. If that seepage gets severe enough, it can cause internal erosion, erosion down in the foundation, and that can then compromise the levee and it can fail and wash away, as well.

First test on levees since 1993

Erik Loehr
University of Missouri
Levees are kind of challenging structures in the sense that they don't get loaded very often. It's only when we have a severe flood, like is going on in portions of the Midwest now, that they really get tested.

GWEN IFILL: The last time we saw such tremendous damage from flooding in the Midwest was 1993. Have there been improvements made in the levee system that were designed to avoid what we're seeing now in the interim, in the time since?

ERIK LOEHR: Certainly, there's been work to improve the levees. In this particular area, I know the Corps of Engineers has deconstructed some levees, actually moved them further back from the river channel proper, to both provide more volume space, in essence, for the water to flow, as well to sort of rebuild them to make them more confident.

That hasn't happened everywhere, but that's certainly -- in some instances, that's happened.

GWEN IFILL: Does it take a flood, the worst-case scenario, in order to know whether a levee might fail? Is there some way to test it, to monitor it, to know whether this might happen?

ERIK LOEHR: That's the challenge, I think. Levees are kind of challenging structures in the sense that they don't get loaded very often. It's only when we have a severe flood, like is going on in portions of the Midwest now, that they really get tested.

And like an earth dam, levees are essentially just smaller dams, but there's miles and miles and miles of them. But earth dams get tested pretty frequently, and we can kind monitor them and see how they're performing, and then try to take some action.

With a levee, they're sort of just sitting there most of the time. And it's only when they get tested like now that we can really do things.

Now, having said that, there are ways that we can try to evaluate their condition, through inspections, through certain tests and things that we can do, but it's a challenge to do that.

Can sandbags save towns?

Erik Loehr
University of Missouri
It's a matter of getting the financing to be able to support that construction and kind of balancing the risks and the cost that -- the risks that we have and the cost that it will take to sort of reduce those risks.

GWEN IFILL: You mentioned the miles and miles and miles of levees there are along the Mississippi River. We have seen entire towns sandbagging and trying to hold the water at bay. What is it about -- is sandbagging actually the best solution in a situation like this, or is it just holding back the inevitable?

ERIK LOEHR: Well, sandbagging is, in essence, an attempt to raise the level of the levee slightly, by a few feet, in order to try to prevent overtopping, which can erode the levee away. And so sandbagging certainly can be effective in certain cases.

You can't raise the levee by much more than three or four feet with sandbags. But you can, you know, in a temporary basis, use the sandbagging to raise the levee a little bit.

There are other things that are kind of similar that can be used at times, but they require generally more mechanized equipment and such to get those installed.

GWEN IFILL: When President Bush says today he's going to provide some help to the flood region, is there a government policy or is there a way for government to best spend its money in order to help shore up these levees?

ERIK LOEHR: Well, that's the problem. I mean, as far as engineers go, there are times when maybe we don't understand things as well as we'd like, but for the most part we know how to design levees to withstand the floods.

It's a matter of getting the financing to be able to support that construction and kind of balancing the risks and the cost that -- the risks that we have and the cost that it will take to sort of reduce those risks.

And so the problem is you can't just shore up one section of the levee. Then the next section becomes the most critical link. And so it's a complicated public policy kind of decision in order to -- like much of our other infrastructure, to balance the cost it's going to take to maintain that infrastructure, maybe even enhance that infrastructure, without challenging the other things.

GWEN IFILL: Professor Erik Loehr of the University of Missouri, thank you very much for helping us out.

ERIK LOEHR: Thank you.