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Publication Abstract

Frequency Distributions and Hydroclimatic Characteristics of Heavy Rainstorms in Illinois Huff, Floyd A., and James R. Angel, 1989  Illinois State Water Survey, Champaign, IL,  ISWS B-70    Full Text Available

The type of rainstorm that most frequently produces flash floods in Illinois and the United States is very localized and produces a large amount of rainfall. According to Changnon and Vogel (1981), these storms usually last from 3 to 12 hours, significantly affect fewer than 400 square miles, and have l- to 4- hour rainfall totals in excess of 3 inches. Changnon and Vogel’s study indicates that approximately 40 of these storms will occur in an average year in Illinois, or about one for every 1,500 square miles of territory. These storms cause serious local flooding problems for farmers (crop damage) and urban areas, and interfere with small-reservoir operations.

Individually, the most damaging flood-producing storm experienced in Illinois is a larger version of the storm described above and occurs on the average of about once in two years within the state (Huff, 1986). These ‘blockbuster” storms generally last from 12 to 24 hours, produce extremely heavy rainfall over a 2,000- to 5,000-square-mile area, and typically create 10- to 12-inch amounts of rain at the storm center. Rainfall amounts in excess of the l00- year recurrence-interval value of point rainfall commonly encompass areas of several hundred square miles about the storm’s center.

A substantial portion of the maximum point rainfalls recorded in the 83-year sample used in the present study occurred in storms of this type. Although they are rather rare occurrences, they may occur in clusters. For example, two of the three storms that occurred in 1957 took place within two weeks of each other. On the other hand, there have been times when no blockbuster storm was observed for several consecutive years. ...

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