When it comes to building on flood-prone land, no place does it like St. Louis.

"The St. Louis area is the epicenter of floodplain encroachment nationwide," says Nicholas Pinter, an SIUC geologist whose article on the subject appeared in an April 2005 issue of the journal Science.

"Because of the extent of human development on the floodplain, our research suggests that the next [flood] may well be bigger than the last," he says.

"More than any other place, the greater St. Louis metropolitan area is allowing its floodplain to turn into new strip mall development. It has $2.2 billion in new development on land that was under water in 1993--18,000 acres either in construction behind levees or in the planning stages. It's unprecedented."

The Federal Emergency Management Agency spent $56 million to buy out floodplain properties in Missouri and Illinois after the great Midwest flood of 1993. But subsequent floodplain development, which has been concentrated in the St. Louis metropolitan area, is "counterbalancing" the good that FEMA did, Pinter asserts.

FEMA guidelines specify that no floodplain construction should cause more than a 1-foot rise in flood level. Many states have established a stricter threshold than FEMA's. Missouri, however, passed legislation prohibiting its counties from setting stricter limits.

Pinter has studied flooding on the Mississippi and Missouri Rivers and on major rivers in Europe. Analyzing such factors as water height, flow, and volume; channel width and depth; land-use changes; and engineering structures reveals how and why river behavior changes over time. A fixed amount of rainfall can cause dramatically different flood levels, depending on these factors.

To get a fix on floods along the middle Mississippi, from just north of St. Louis to Cairo, Ill., Pinter and his students analyzed the detailed measurement record from St. Louis (which goes back to 1861) and other locations. That work was described in the Fall 2001 issue of Perspectives; see High-Water Mark. Among other things, the team confirmed that levees have made floods higher and more frequent along this stretch. The phenomenon is called "human forcing" of floods.

With funding from a National Science Foundation grant, Pinter and his students are now working to provide better tools for planners to assess how new engineering structures in the Mississippi River system, such as new or raised levees, would affect flood levels.

"Levees make floods higher when they come through because areas that would convey and store the flood flows are blocked," Pinter explains.

"Before 1900, there were very few levees in most areas (along the middle Mississippi). For example, along a portion of the Mississippi floodplain in St. Charles County (Mo.), there was only one low levee as recently as 1930, but by 2000, they're all over the place, and they're enormously higher--in some places as much as 10 times higher." Developers in the St. Louis region also have been granted wetland fill permits to raise parts of the Missouri and Mississippi floodplains, says Pinter.

"When you look at what was there before (in terms of enclosed or raised floodplain), what's been added since 1993, and what's being proposed, it's a huge amount," he says.

But since levees force floodwaters higher than they otherwise would be, the newly enclosed or raised floodplains are not necessarily safe from major floods--and there are many more structures on them now.

Taxpayers will be the one to pay the price, says Pinter. Records show that the St. Louis area's 10 highest floods in the historical record all occurred in the last 65 years. The next severe flood, he says, is "just a matter of time."

--by K. C. Jaehnig, Media & Communication Resources, and Marilyn Davis, ed.

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