Commentary: Magic flood math: A 10-year flood gets 100-year status | St. Louis Public Radio

Commentary: Magic flood math: A 10-year flood gets 100-year status

Aug 19, 2008

This article first appeared in the St. Louis Beacon: August 19, 2008 - The Great Flood of 1993 was a "200-year" flood at St. Louis and a "500-year" flood at many places upstream, according to the Army Corps of Engineers (2004) flow-frequency study. Incredibly, not quite 15 years later, the Great Flood of 2008 was another "200" or "500" year flood in northeast Missouri and set all-time record stages in southern Iowa.

Flooding was not as bad downstream, yet the disastrous water levels that overtopped levees at Winfield were the second highest ever, exceeding those for a "50 year" flood. These recurring, record and near-record events beg the question, why are we experiencing so many severe floods in recent time?

Well, flooding is clearly getting worse, for a combination of reasons that include climate change, watershed development and constriction of our rivers by levees and navigational structures. Unfortunately, government calculations on flood probabilities do not take these progressive changes into account, even though scientists pointed out this problem years ago. This has set the stage for repeated, costly disasters, while fostering a pattern of unwise regional decisions and risky floodplain development that continue today.

Consider the regulatory meaning of the "100-year" flood. This theoretical event has a 1 percent chance of occurring in any given year. The flood stage calculated for this event is used to set rates for federal flood insurance and define requirements for obtaining such insurance. It is also used, along with "500-year" levels, to describe levee protection.

The USGS explains that, because such floods are statistically defined, it is possible to have two "100-year" floods in consecutive years. Just because outcomes are unlikely does not mean that they are impossible, the geological service explains.

This dodge is not acceptable. The chance of having one "100-year" flood event occur during a 15-year interval is only 14 percent, less than the chance of rolling a dice and getting a six. The chance that two such events would occur in a 15 year period is less than 1 percent; you have half as good a chance of simultaneously rolling four dice, and getting four of a kind!

If you could live 170 years, you would have a 50-50 chance of seeing two "100-year" floods. Had you been born during the Crusades, you would have had only an even chance of witnessing two "500-year" floods, yet teenagers from Canton to Burlington just saw this happen.

The reality is that what are being called "100-year" and "500-year" floods are closer to "10-year" or "25-year" floods. The official calculations are not just flat wrong, they are dangerously misleading. Hopefully these calculations will be greatly revised in the near future, but in any case they will be discussed at the Flood Forum to be held on Nov. 11 at St. Louis University; details will be announced soon.

Sadly, our regional officials smugly persist in using these misleading terms to proclaim the integrity of our levees.

This language was used to convert the productive floodplain farms in Chesterfield Valley, inundated during 1993, to the "largest strip mall in the world."

This language is being used to promote the Premier 370 project in St. Peters, and to plan a new, much larger floodplain development project in Maryland Heights.

This language is even being used by EPA to justify the retention of illegally dumped radioactive waste at the West Lake landfill in Earth City.

Many local officials have turned a deaf ear to frustrated citizens who have tried to raise concerns about these issues. These "representatives" need to be more realistic or they simply need to go.

Robert Criss is a professor in the department of Earth and Planetary Sciences at Washington University. He is the coauthor of the 2003 book, "At the Confluence: Rivers, Floods, and Water Quality in the St. Louis Region."