Technique for estimating magnitude and frequency of floods in Illinois

A technique is presented for estimating flood magnitudes at recurrence intervals ranging from 2 to 500 years, for unregulated rural streams in Illinois, with drainage areas ranging from 0.02 to 10,000 square miles. Multiple regression analyses, using streamflow data from 241 sampling sites, were used to define the flood-frequency relationships. The independent variables drainage area, slope, rainfall intensity, and an areal factor are used in the estimating equations to determine flood peaks. Examples are given to demonstrate a step-by-step procedure in computing a 100-year flood for a site on an ungaged stream and a site on a gaged stream in Illinois. The report is oriented toward planners and designers of engineering projects such as highways, bridges, culverts, flood-control structures, and drainage systems, and toward planners responsible for planning flood-plain use and establishing flood-insurance rates.