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<br />A. <br /> <br />, <br />; <br />, <br />.' .~ <br />i <br /> <br />;~, <br /> <br />:.iJ <br /> <br />Preliminary Steps <br /> <br />1. Clear Creek County - Chicago Creek above Idaho Springs Storage <br />Reservoir drainage area is approximately 5.01 sq. mile. <br /> <br />2. From Figure 20 <br />Probable maximum thunderstorm I-hour rainfall (point values in <br />inches) for area west of the 1050 meridian and above 8,000 feet <br />elevation. Page 53; Zone is III; <br /> <br />3. Hydrologic soil group "B": General soils map M7-E-22275-1O <br />by Colorado Land Use Commission (map at Division of Water <br />Resources Office). <br /> <br />From Table A-9, Design Thunderstorm Precipitation Direct Runoff <br />Curve Number. Page 544, Appendix A. <br /> <br />Cover Type: Brush, sage, grass, or combinations cover density at <br />ground surface - 50% or less, Zone III. Inflow design flood, probable <br />maximum thunderstorm rainfall, <br />Use CN = 75 <br /> <br />4. Determine T , Time of Concentration for the watershed. <br />FrOm map sfudy: L = 3.5 miles <br />Elevation difference between headwater and site: <br />13,600 - 10,617 = 2,983 feet. <br /> <br />Since this is the only data available, T is estimated by solution <br />c <br />of equation: <br /> <br /> 0.385 <br />T = [u: L J T = is in hrs. <br />c c <br /> L = length of longest <br /> 0.385 watercourse in miles <br /> = tU.9 x 3.53] <br /> (2983) H = elevation difference <br /> in feet <br /> = 0.51 hrs <br /> <br />- 7 - <br />