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<br />MANUAL FOR ESTIMATING FLOOD CHARACTERISTICS <br /> <br />5 <br /> <br />FLOOD INFORMATION AT lINGAGEO SITES <br /> <br />This method consists of a series of regression equations which <br />flood characteristics to measurable basin and cl imatic parameters. <br />resulting equations are of the form: <br /> <br />re I a te <br />The <br /> <br />I <br />. <br />1 <br /> <br />b) b <br />Y t=ax) x2 2 <br /> <br />where <br /> <br />Yt <br /> <br />= a flood characteristic, either peak discharge or peak flood depth, <br />for recurrence interval t; <br />basin and cl imatic parameters as defined in a later section; <br />regression constant; and <br />regression coefficients. <br /> <br />Xl' x2= <br />a, = <br />b),b2= <br /> <br />The basin and cl imatic parameters used in this study were selected based <br />on the results of trial regressions using 9aging stations with 25 or more <br />years of record. The basin and cl imatic parameters are defined below with a <br />brief explanation of computation procedures, <br /> <br />Drainage area, A,--The total area of a basin contributing to flood dis- <br />charges, In square miles, as measured from U,S. Geological Survey topographic <br />maps, usually by planimetering. Simpler but less reI iable indexes of drain- <br />age-area size might be obtained from smaller scale maps or by overlaying a <br />grid of known square sizes on the outlined catchment and counting the number <br />of squares. <br /> <br />In mixed-population flood areas, the basin needs to be outl ined and <br />divided according to the elevation criteria given on page 9, and an area <br />computed for each subbasin. <br /> <br />, <br /> <br />Mean annual precipitation, P.--The 1931-60 mean annual precipitation for <br />a basin, in inches, as computed from a U.S, Weather Bureau map (1967). The <br />value is computed by a grid-overlay method" The basin is outlined on the maps <br />and the grid is selected so that at least 20 points fall inside the basin, A <br />precipitation value for each grid point is read and recorded. The values are <br />then summed and an average precipitation value for the basin is computed. <br />Copies of the maps can be obtained from the Colorado '''!ater Conservation Board. <br />