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<br />MANUAL FOR ESTIMATING FLOOD CHARACTERISTICS <br /> <br />43 <br /> <br />Mixed-Population Flood Areas <br /> <br />Many streams in Colorado originate in high mountain areas and flmv out of <br />the mountains to much flatter plain or plateau areas. Floods on these streams <br />at higher elevations are caused by snowmelt ,'unoff but at lower elevations, <br />floods can result from snowmelt, rainfall, or a combination of rain on snow. <br />Reaches of streams that receive annual maximum floods from both phenomena are <br />herein referred to as foothill streams and the part of a drainage basin affec- <br />ted is called a mixed-population flood area. The existence of the problem of <br />mixed-population floods in Colorado was impl ied by Jenkins (1960, p. 6). <br /> <br />During the early phases of this study it was noted that the flood records <br />for foothill streams did not adequately fit the log-Peal'son Type III statisti- <br />cal distribution. In general, flood-frequency relations for gaging stations <br />on foothill streams exhibit large positive skewness; that is, the relations <br />are concave upward in shape. Another characteristic feature of these rela- <br />tions is that several of the highest floods, which usually occur during summer <br />months, plot extremely high in spite of the large positive skewness of the re- <br />lations. This lack of a satisfactory fit of the higher floods results in low <br />estimates for the larger recurrence-Interval floods. These features were <br />noted for flood records of gaging st~tions throughout Colorado but \'/ere more <br />pronounced for streams along the eastern foothills of the Rocky Mountains. <br />This report contains the assumption that mixed-population flood areas form <br />the transition zone between the Mountain Region and each of the other three <br />flood regions, except for that part of the Southern Plateau Region south of <br />the Gunnison River. Flood records for this part of the State did not provide <br />sufficient evidence to define separate flood regions for snowmelt and rainfall <br />floods. <br /> <br />.. <br /> <br />The gaging-station records for foothill streams were separated from other <br />records and an Investigation was started to separate the flood-frequency rela- <br />tions into snowmelt and rainfall components. Flood records for several gaging <br />stations along the Front Range provided useful and interesting information but <br />a satisfactory procedure for computing flood discharges of foothill streams <br />could not be derived in the limited time available for this study. An interim <br />procedure discussed in the following paragraphs was developed based on prel im- <br />inary results of the foothill streams investigation and a review of literature <br />relating to rainfall-produced floods in Colorado and Utah. <br /> <br />, <br />