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Public Radio's Environmental News Magazine (follow us on Google News)

Wing Dams

Air Date: Week of

The Great Lakes Radio Consortium’s Lester Graham reports on a study that raises questions about the Army Corps of Engineers’ use of so-called wing dams to control rising waters during floods.


CURWOOD: The flood control projects of the U.S. Army Corps of Engineers have changed some of the biggest rivers in America and prevented a lot of flood damage. But on April 30th the Army Corps announced that it was suspending work on as many as 150 water-related projects to assess their economic and environmental value. A recent study by the National Academy of Sciences found that some Corps projects are destructive to certain river ecosystems. And two researchers at Washington University in St. Louis say that some Corps projects may serve to make major floods worse.

They say that wing dams—structures that jut out into rivers—could cause waters to rise higher during floods than they otherwise would. Lester Graham, of the Great Lakes Radio Consortium, reports.


GRAHAM: The Mississippi and the Missouri Rivers are two of the major arteries for barge transportation in America. Millions of tons of grain and raw materials are ported up and down the rivers each year. It’s the Army Corps of Engineers’ job to keep the rivers open to barge traffic. The Corps has been doing their job for the past 150 years. But since the 1930’s that effort has taken on immense proportions. Huge dams hold back the rivers, keeping the water high enough for the barges to travel up and downstream. Big earthen dikes called "levies" wall in the rivers, keeping them from flooding farms and towns, but also keeping the water from reaching the natural flood plains.

Robert Criss and Everett Shock study flood levels and the effects of the Corps of Engineers’ projects. Criss says those dams and levies alone might be enough to disrupt the flow of the river and cause flood stages to be higher.

CRISS: The other component is these structures called "wing dams," which are jetties of rock that project out perpendicularly into the channel. For high flow conditions these act something like scale in a pipe: they impede the flow, restricting the channel. That slows the velocity of the water down and that also makes the flood stages higher.

GRAHAM: The purpose of wing dams is to force the current to the middle of the river to scour out the navigation channel to keep it open for barges. Researcher Everett Shock.

SHOCK: So they do the job they’re intended to do. It seems that there’s a, perhaps, unintended consequence of all these constructions along the river that shows up when we have a big flood and makes it—on the basis of our study—makes these big floods worse.

GRAHAM: Criss and Shock say their study finds that since these flood control projects have been erected there have been more big floods, such as the one in 1993 that flooded the Mississippi and some of its tributaries for most of the summer. Robert Criss:

CRISS: The fact is, before World War II a flood stage of 38 feet is very rare and now it happens every five years.

GRAHAM: But not everyone agrees with the methodology used by the researchers. The Corps of Engineers dismisses the researcher’s study, saying they used flawed data. Corps officials point to a study at the University of Missouri-Rolla. That study compared the 19th century method of measuring the river’s flow by timing how fast floats moved in the current to the methods used today. Dave Busse is a scientist with the Army Corps of Engineers. He says the original stream flow measurements, the ones Criss and Shock used, were inaccurate:

BUSSE: The flows were over-estimated by 30 percent using these float measurements, rather than the measurements that we use today.

GRAHAM: Criss and Shock are skeptical of the new numbers the Corps prefers, saying it seems awfully convenient for the Corps because changing the numbers makes the historic floods look smaller and, therefore, makes the 1993 flood look unprecedented. Criss and Shock say based on the original record there was as much water in past floods as in the 1993 flood, but lower water levels. Criss and Shock say the difference between then and now is that the Corps’ dams, levies and wing dams constrict the river’s flow and make floods higher.

The Corps, however, has other criticism of the Criss and Shock study. Dave Busse says the researchers ignored the role of the Corps’ reservoirs in the rivers’ watersheds. Busse says reservoirs hold back water that would otherwise be part of a flood. And Busse says another flaw is the researchers’ conclusions about wing dams. The Corps says the wing dams force the water to deepen the channel and that increases the flow of the river.

BUSSE: It’s a reshaped river, but its carrying capacity is actually higher now. We can actually carry more water at the same stage. After we put wing dams in, the river got deeper. Therefore, this conclusion that they’ve made is wrong.

GRAHAM: The Corps says there’s more to managing the river than the researchers have considered. Criss and Shock, meanwhile, say their study is not the first to be dismissed by the Corps of Engineers. They say other studies have found similar results, but the Corps dismissed them, as well. Environmentalists have been arguing for decades that levies and dams keep flood waters from spreading out on the natural flood plains and, as a result, cause higher flood levels.

The Criss and Shock study adds to their arsenal of arguments to change the way the rivers are managed. But most environmentalists concede that we’ve become somewhat dependent on the Corps’ flood control projects. Chad Smith, with the environmental group American Rivers:

SMITH: In most ways, both of these camps are right. The Corps is right that putting some of this structure in has helped to reduce the kind of annual flood events that always happen on a big river like this. But what they unfortunately have done is to exacerbate what happens when you have bigger floods and the wing dams and the levies and the dams themselves all are a part of that.

GRAHAM: The Army Corps of Engineers says it’s reviewing its way of managing rivers in the light of the 1993 flood, but it also notes that while flood stages might be higher more often than they were in the nineteenth century, most of the time those flood waters remain behind the flood walls and levies, protecting communities from high water. And the Corps says in the end that’s the only fact that really matters. For Living on Earth, this is Lester Graham.

[MUSIC: Rachel’s, "Good Bye," SELENOGRAPHY (Quarter Stick – 1999)]



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