Causes for Record High Flood Losses in the Central United States

Abstract

Flood losses in the north central United States are the nation's highest since 1980. Losses in Iowa rank first nationally and those in two adjacent states rank fourth (Missouri) and sixth (Illinois). Explanations were sought for this three-state regional maximum of flood losses, including geophysical, societal, and climatological conditions. Comparisons of conditions with those of surrounding areas revealed the three states had more land exposure to the major flood-prone rivers (Mississippi, Missouri, and Ohio) than any other state or group of states. The region also has a higher population density than surrounding areas and contains three of the five largest metropolitan areas of the Midwest. Farm land values rank just behind those of the three states to the immediate west. Further, the region is the center of the nation's complex surface and riverine transportation systems that are highly vulnerable to flooding. The assessment of regional hydroclimatic factors on enhanced flooding in the three-state region was based on a case study of historic floods on two adjacent basins located near the center of the region. Historic flood data showed systematic, long-term increases in both flood incidence and magnitude. Certain precipitation conditions on the basins also showed systematic temporal increases since the 1920s, including annual precipitation, number of seven-day heavy rain events, and the number of days with precipitation. This climatic shift to more multi-day periods of heavy rain appears to be the major reason that hydrologic flooding in the three states has increased since the 1920s. Analysis of the individual major hydrologic floods revealed five different types of precipitation conditions, including snow-melt situations that produced major floods. The primary cause of major floods was prolonged periods of four- to 13-day durations with intermittent moderate to heavy rains, and these events often included heavy, 15-centimeter rainstorms capable of producing flash floods. Climatic data reveal that the three states experience more heavy rain-producing synoptic weather conditions, cyclonic activity, and intense rainfalls than do most surrounding states. Collectively, these geophysical, hydroclimatic, and societal factors combine to cause the high flood losses in the nation's central region