EFFECTS OF FEDERAL LEVEES AND RESERVOIRS ON 1993 FLOOD STAGES IN ST. LOUIS

A physical model of the Middle Mississippi River was used to perform quantitative analyses of the effects of federal levees on flood heights. Contrary to some opinions by the general public, these tests showed that today's stage-frequency relationship at the St. Louis gage is not greatly different than that existing before the onset of changes to the river by humans, beginning in the 1820s. Federal levees result in increased flood heights upstream of the protected area, but these increases have been fully offset by reductions resulting from federal flood control reservoirs for all floods in recent years. The occurrence of the Great Flood of 1993 further confirmed these findings and continued to demonstrate the value of the federal levees and reservoirs. Federal levees are estimated to have increased flood heights at St. Louis by 0.9 to 1.2 m (3 to 4 ft) during the crest of the 1993 flood ; however, federal reservoirs reduced flood levels at St. Louis by an equal amount, counterbalancing the levee increases. An estimated 19 billion dollars in flood damage was prevented by the federal levees and reservoirs throughout the Upper Mississippi and Missouri River Basins in 1993, a savings of more than the total construction cost of the projects. The model tests also found that the key historical flood discharges of the 1844 and 1903 floods, used in the design of the current levee system, may be overestimated by up to 33 percent.