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<br />I <br /> <br />25 <br /> <br />I <br />. <br /> <br />2. The rainfall is uniformly distributed throughout the whole area <br />or the drainage basin. The small basins in South Boulder make <br />this a valid assumption. <br /> <br />3. The base or time duration of the design runoff due to an effective <br />rainfall of unit duration is constant. <br /> <br />. <br />. <br />I <br />I <br />. <br /> <br />4. The ordinates of the design runoff with a common base time are <br />directly proportional to the total amount of direct runoff repre- <br />sented by each hydrograph. <br /> <br />5. The effects of all physical characteristics of a given drainage <br />basin, including shape, slope, detention, infi ltration, drainage <br />pattern, channel storage, etc., are reflected in the shape of the <br />unit hydrograph for that basin. <br /> <br />Equations <br /> <br />I <br />I <br />I <br /> <br />There are two basic equations used in defining the limits of the synthetic <br />hydrograph. The first equation defines the lag time of the basin in terms <br />of time to peak, t , which, for the Colorado Urban Hydrograph Procedure, is <br />defined as the ti~ from the center of the unit storm duration to the peak <br />of the unit hydrograph. This study used a ten-minute unit storm duration. <br /> <br />tp= Ct (L Lc) <br /> <br />.3 <br /> <br />whe re t <br />p <br /> <br />= time to peak of hydrograph from midpoint of unit <br />rainfall in hours <br /> <br />L = length along stream from study point to upstream <br />limits of the basin in miles <br /> <br />I <br /> <br />L <br />ca <br /> <br />= distance from study point along stream to the centroid <br />of the basin in miles <br /> <br />I <br /> <br />Ct = a coefficient related to time to peak <br /> <br />The second equation defines the unit peak of the unIt hydrograph <br /> <br />I <br />. <br />I <br /> <br />q = <br />p <br /> <br />C 640 <br />p <br />t <br />p <br /> <br />whe re <br /> <br />qp = peak rate of runoff in cfs per square mi Ie <br /> <br /> <br />C = a coefficient related to the peak of the unit hydrograph. <br />p <br /> <br />I <br />,. <br />I <br /> <br />The C and Ct values are empirical and should be based on similar basins <br />that Rave been instrumented to obtain the actual rainfall-runoff relation- <br />ships. The C and Ct values used In thIs study are based on values recom- <br />mended in Tab~e 4-1 of the Urban Storm Drainage Criteria Manual. When <br />selecting the Cp and Ct values for the various basins, the area, shape, and <br />