December 2001

From Washington University in St. Louis

Floods ain't what they used to be; Study shows wing dams have made them worse

Like so many other things, floods just aren't what they used to be.

In the Midwest, they are worse than ever, according to Robert Criss, Ph.D., and Everett Shock, Ph.D., both professors of earth and planetary sciences in Arts & Sciences at Washington University in St. Louis. And they say that recent flood magnitudes and frequencies cannot be blamed on global warming or climate change, the popular notions. They point to human engineering of the rivers to try to control them for navigation.

In their paper, "Flood Enhancement through Flood Control," published in the October, 2001 issue of Geology, Criss and Shock lay the prime blame for increased flood levels on the Missouri and Mississippi rivers over the past century on the placement of wing dams on the nation's largest rivers.

Wing dams are jetties of rock placed nearly perpendicular along river banks, and are intended to stabilize channels and to keep water levels high in mid-river for barge traffic. In the reaches of both rivers in Missouri there are literally thousands of wing dams, many visible through your car's windshield. Most of them were built in the 1930s and '40s by the U.S. Army Corps of Engineers. During low flow, the wing dams keep the channel deeper for barge traffic, and increase water velocity in the center for a stable, self-scouring channel. But under flood conditions, Criss says, the structures act like scale in a pipe. They actually slow water velocity and constrict the channel, impeding the flow of water, forcing flood levels to rise.

"The main problem with wing dams is that they make flood waters deeper," said Criss. "For floods of a given magnitude, the depth of water is much greater when you have them compared with places without them. In many areas of Missouri with wing dams, flood water can be ten feet higher than it was before they were built."

Criss and Shock compared flood stage levels of the middle-Mississippi River (from the confluence of the Missouri River down to the Ohio River), and the lower Missouri River , both heavily lined with wing dams, to the Meramec River in Missouri, which is one of the few free-flowing rivers in the United States, and the Ohio River at Cincinnati. The Ohio there is free of wing dams but does have levees and navigational locks and dams, which show little effect on water depth over 140 years of data. Both the Meramec and Ohio rivers show a horizontal line on graphs Criss and Shock drew up; the Mississippi and Missouri rivers, laden with wing dams, show distinctly rising lines throughout the past century.

Criss said his study is not the first to place the prime blame on wing dams (he says levees also contribute to increased flood stages, but have less effect than wing dams). Scientists in the mid-'70s also made this claim, but he said their research largely was bitterly criticized and subsequently ignored. What makes this study different is the comparison of rivers without wing dams or other structures, such as the Meramec River, to rivers that have been profoundly modified by a variety of different methods, using data gathered since the 1860s. This comparison clarifies the consequences of different engineering practices over time.

"Where none of this kind of engineering occurred, the records today look just like the records of 100 years ago," said Criss. "Such is not the case on the heavily engineered Mississippi River at St. Louis. Before World War II, floods that reached 38 feet or higher at St. Louis were very rare, occurring only about every 50 years, but now flood stages of this magnitude occur every five years or so."

"Severe flooding is commonplace now. If you look at our table and graphs, you see the trends are going up. The government is misleading the public by saying the Great Flood of '93 was a once in two-hundred-year event, or even a 70-year-event. Our data show it won't take a century for a flood like that to reoccur. I would not even be surprised if it happened in the next 15 years."

Criss said that over nearly two centuries the U.S. Army Corps of Engineers has done" a lot of marvelous good for the Midwest in making rivers navigable and stable. I don't know if we can ever go back to the rivers being the way they were in the days of Lewis and Clark, or if we'd even want to. But I do think we should look at the consequences of what we build. There are some downsides to certain practices that have to be incorporated into our thinking. It's essential that we recognize what these effects are."

This article comes from Science Blog. Copyright 2004

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