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<br />44 <br /> <br />TECHNICAL MANUAL <br /> <br />The primary consideration in developing an interim procedure for foothill <br />streams was to establ ish a transition zone between the rainfall flood zone <br />(lower region) and the snowmelt flood zone (Mountain Region). Because of the <br />lack of homogeneous flood-frequency data on small streams in the foothill <br />areas, an arbitrary division was required. The principal sources of informa- <br />tion utilized in establishing the transition zone were: 1~ Published reports <br />by Woolley (1946), Follansbee and Sawyer (1948), Matthai (1969), Fanner and <br />Fletcher (1971), Butler and Marsell (1972), and Snipes 'and others (I'J74); <br />2. Streamflow data collected by various State and Federal agencies; and <br />3. Field observations and measurements of stream channel characteristics. <br /> <br />Based on information from the above-l isted sources, several methods were <br />tested for technical soundness and simpl icity of appl ication. The method se- <br />lected for use is based on the assumption that areal extent and intensity of <br />floods resulting from thunderstorms decrease as elevation increases in the <br />foothill areas of Colorado. This assumption is supported by information con- <br />tained in the references and, to some degree, by field observations of channel <br />characteristics. This reduction is depicted in figure 11 as a reduction <br />factor versus elevation graph. The reduction factor is selected based on <br />elevation of the study site and is appl ied to the peak discharge regression <br />equations for the lower flood regions. Although highly subjective, the method <br />does provide a smooth transition between the lower and upper flood regions. <br />It is also easy to apply and provides seemingly realistic peak discharges <br />from rainfall at higher elevations. Strict interpretation of figure 11 in- <br />dicates that floods produced by thunderstorms do not occur in the Mountain <br />Region. The assumption used herein was that thunderstorms can occur at high <br />elevations but, owing to the small areal coverage and reduced intensities, <br />the resulting floods are smaller than snowmelt floods. This assumption <br />needs to be checked when studying small areas at high elevations. The interim <br />procedure is described in the section entitled "Appl icat'ion of Methods." <br />