Deceptive Numbers Used to Described Floods

Of all the deceptive numbers in circulation, the most ridiculous, expensive, and tragic are the numbers the U. S. Army Corp of Engineers use to describe the levees that they build. I saw an interview with General Michael J. Walsh on the NBC news this past week. In it he referred to the rainfall flooding Cedar Rapids as a “500-year storm.” The levees were not designed for such a rare event. According to General Walsh: “A lot of levees have over topped. We don’t consider that a failure.”

He is consistent with other officials I’ve seen interviewed, who refer to levees as having “100-year” or “500-year” designs. The terminology means that according to the statistical models used to predict floods, water should top a 100-year levee about once every 100 years and top a 500-year levee once every 500 years.

You wonder how engineers and government officials can quote these numbers with a straight face. Does the U. S. Army Corp of engineers understand anything about what they are doing along the Mississippi. Clearly once levees are constructed along the river all the flood statistics become meaningless.

Timothy Kusky, director of the Center for Environmental Sciences at St. Louis University, described in an interview on NPR how before levees were built, the Mississippi River was 4000 ft wide at St. Louis. Today the river is 1500 ft wide. As Kusky stated: “It is a simple concept; confine the river to a narrower channel and there is nowhere for the water to go but up.” The result according to Kusky is that we’ve had 15 hundred-year floods in the past hundred years, and many more 500-year floods in the past 150 years. Building structures changes all the statistics.

The other problem is that the flooding statistics are based on past events. Climate change models predict a 20% increase in rainfall in the coming decades for the Midwest. With that rainfall will come a 50% rise in the height of the rivers.

But, communities, developers, and home owners continue to invest large sums of money on the basis of the bogus statistical claims made by the Army Corp of Engineers. Chesterfield, Missouri now has the largest strip mall in the United States—3 miles long. The mall, along with 30,000 new homes, is built on land that was completely under 10 ft of water during the 1993 flood. The developer erected a “500-year” levee for protection and declared the area safe.

Robert E. Criss, Ph.D., professor of earth and planetary sciences at Washington University in St. Louis has strongly criticized such development as “ignoring geological reality.” On a Washington University news website, Criss describes as an "absurd exaggeration” the claim that a levee will withstand floods for 500 years. “If some private company were making claims that they'll sell you a car that will run for 500 years, they'd be in jail. Somehow, the government feels justified making absurd claims that have no basis."

It appears that little of substance was learned from the 1993 floods. The conclusion that higher levees need to be built ignores the reality that all of the water must have some place to go. A higher levee in one location will force water over the top in another location. The statistics that are the basis for the levee building and development become meaningless. But government officials and developers still rely on these absurd numbers and the result is heartbreaking for the people affected.

Joseph Ganem is a physicist and author of The Two Headed Quarter: How to See Through Deceptive Numbers and Save Money on Everything You Buy