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<br />2. From fig~re 4: <br /> <br />Q50 = 77,000 cubic feet per second <br /> <br />QlOO - 87,000 cubic feet per second <br /> <br />. <br /> <br />The examples illustrate the various ways of using and interpreting <br />flood-frequency data. For a better undertanding of probability concepts <br />for design, the reader is referred to Chow (l964), and Linsley and Franzini <br />(1964). <br /> <br />SlOOIARY <br /> <br />The report describes a method for estimating the magnitude and <br />frequency of floods on unregulated streams in Iowa. Five hydrologic <br />regions are defined on the basis of physiographic characteristics. <br />Flood-frequency equations were developed by least-squares regression <br />analysis using data for 25l gaged stations. Regression analysis of <br />regionalized data relates flood discharage to the drainage-basin area and <br />can be used to estimate flood discharage at ungaged sites for 2-, 5-. 10-. <br />25-, 50-, and 100--year recurrence intervals. Drainage-basin size was the <br />most significant independent variable and it accounted for 71 to 95 percent <br />of the variance in all cases. Discharge of selected mainstem streams where <br />peak discharge is affected by factors of basin geometry was determined <br />graphically. The method described in this report is only applicable to <br />sites on ungaged streams that are not affected significantly by regulation <br />upstream from the sites and that the areas upstream from the sites are not <br />mostly in urban areas. <br /> <br />. <br /> <br />19 <br />