Guest Column                                                        

A flood of misinformation flowed from the Great Flood of 1993 By MISGANAW DEMISSIE

This disaster ranks as the greatest flood in North America's modern history. But what have we learned from it? As scientists and engineers collect and analyze mounds of data, from flood elevations to satellite images, the general public is force-fed by the news media and selected "experts." But too often, media information is wrong, incomplete or misleading. Unfortunately, it will have significant impact on government response to the flood. As the debate heats up on who or what to blame, it becomes clear that the scientific and engineering community has not adequately educated the public and policymakers on how our natural environment and our flood control structures operate. Sadly, people seem more knowledgeable about the odds of a lottery win (let's say one in a million) than about the chances of a 100-year flood (one in 100) or the chances of a flood control levee designed for a 50-year flood being overtopped in any year (two in 100).

Athletic records are routinely broken, which the public accepts. But when flood records fall, so too does the public's trust in experts. It's a trust that's hard to relinquish. We want to believe in experts. We want to believe we're knowledgeable

Most people thought they would never see a 100-year flood in their lifetimes, simply because we don't live that long. Or the problem could be that TV "experts" don't understand engineering concepts and deliver the wrong messages, albeit wrapped in confidence and elegance: "A 100-year flood occurs only once in 100 years," or "We'll never see this again." Such misinformation has led to a disregard for the laws of nature when it comes to flooding and flood control.

Often heard along the flood route has been, "After the '73 flood, they said this wouldn't happen again for 100 to 500 years." And when told that the 1993 flood is the greatest on record, "We heard the same thing in '65 and '73." This could have been misinformation or misinterpretation. Either way, it was ineffective communication.

Athletic records are routinely broken, which the public accepts. But when flood records fall, so too does the public's trust in experts. It's a trust that's hard to relinquish. We want to believe in experts. We want to believe we're knowledgeable. We want to believe in flood control and feel secure about flood control structures.

We as scientists must admit that we do not totally understand how our natural environment behaves, and we haven't done an adequate job in communicating that uncertainty. Now is the time to begin promulgating clear and concise explanations of the major topics of the day; how flood frequency works; how often rivers can be expected to top their banks and reach their floodplains; what a design flood is; the functions and limitations of levees; how bridges, flood walls and levees affect flood elevations; and how locks and dams control a river and impact flooding. We must respond to questions and concerns such as: Why are we surprised when a major flood occurs? Why are many "experts" saying that a flood of this magnitude will not recur for a long time? Why are we surprised that levees designed for a 50-year flood fail in a flood that surpasses the 100-year level? Why do people risk their lives sandbagging atop levees that were designed to fail weeks earlier? Why do we spend money and emotion protecting cornfields that are expected to be flooded an average of once every 50 years? Why do many feel that the levees caused this flood?

The answers touch on social, political, economic, scientific, environmental and basic survival issues. As scientists, it is our responsibility to communicate effectively — to provide answers, or at least to explain when there are no answers. The key is that we must respond — accurately, directly and honestly. Some basic concepts about the flood and its aftermath are outlined below.

Flood Frequency. This statistical term expresses the chance that a flood of a particular magnitude could occur in any year, or how frequently certain floods are expected to occur on a long-term basis. For example, a 100-year flood has a 1 percent chance of occurring in any year, a 500-year flood has a 0.2 percent chance, and so on. The occurrence of the major flood this year does not reduce the chances of another major flood next year. When we say a 100-year flood occurs on the average of once in 100 years, we don't mean that it occurs only once every 100 years. We mean that on a long-term basis, it will average out as once in 100 years. Perhaps the terminology is to blame for the confusion, and "chance of occurrence" might be a more appropriate term.

Flood of Record. The 1993 event is

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now the flood of record for most of the Mississippi, superseding the 1973 flood, which was identified then as the flood of record for some segments of the river. This does not mean that the 1993 flood will not be exceeded in the future. Records are made to be broken, and the record goes back only about 100 years.

Floodplains. The floodplain is the area adjoining a river or stream that is inundated by floodwaters whenever the capacity of the stream is exceeded and water overflows the streambanks. This is a natural process. The extent of the flood-plain is defined to correspond to the magnitude of floods: The 100-year floodplain is inundated by a 100-year flood, and so on. It is important to realize that most of the damage from the current flood is confined within the Mississippi's floodplain.

We must learn to accept the risk that extreme floods will overcome levees designed to withstand lesser floods of greater frequency. It is economically infeasible, if not technically impossible, to design levees that will never fail

Levees. Levees and flood walls channelize a river, confining the floodway and increasing the water's elevation. But it is very unlikely that levees change the discharge in a river as large as the Mississippi. Most of the agricultural levees along the river were designed only for a 50-year flood, and this level was surpassed a long time ago. Thus, they performed beyond, what they were designed to do. Have we "oversold" the levees? Probably. Have we encouraged an unwarranted sense of security among those who rely on them? Very likely. We must learn to accept the risk that extreme floods will overcome levees designed to withstand lesser floods of greater frequency. It is economically infeasible, if not technically impossible, to design levees that will never fail. *

Misganaw Demissie is director of the office of sediment and wetland studies for the Illinois State Water Survey. This column was reprinted from the fall 1993 issue of Currents.

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