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<br />. <br /> <br /> <br />COLORADO STATEWIDE <br />DRAINAGE AND FLOODPLAIN MANAGEMENT CRITERIA MANUAL <br /> <br />Step 1: Determine the average 100-year, 6-hour rainfall depth and the <br />average 100-year, 24-hour rainfall depth in Basin A from Figures <br />CH5-F106 and CH5-F112, respectively (Basins that have highly <br />variable rainfall depths for a given frequency and duration may need <br />to be subdivided into areas of common rainfall depth. A weighted <br />average of the rainfall depth can then be calculated using the areas <br />and rainfall depths of the sub-basins). <br /> <br />D1OO,6 = 3.6 inches (Assumed for example purposes only) <br />D1oo,24 = 5.0 inches (Assumed for example purposes only) <br /> <br />The average 6-hr and 24-hr rainfall depths in Basin A for 2-, 5-, 10-, <br />25-, and 50-year storm events should also be estimated from Figures <br />CH5-F101 through CH5-F112. The average rainfall values of the six <br />recurrence frequencies for each storm duration (6- and 24-hr) should <br />be plotted on the retum-period diagram (Figure CH5-F113), and a <br />straight line of best fit should be drawn. If any recurrence frequency <br />rainfall value deviates substantially from the best-fit line, the value <br />read from the line should replace the original rainfall value from the <br />map. For the purpose of this example, it is assumed no adjustments <br />of the rainfall values are necessary. <br /> <br />Step 2: Calculate the average 100-year, 1-hour rainfall depth, Y100. <br /> <br />. <br /> <br />From Figure CH5-F114, determine the geographic Region of Basin A. <br />For the purpose of this example, Basin A is located in Region 1 and <br />the average basin elevation is 6,000 ft. <br /> <br />From Table CH5-T102, <br /> <br />Region 1, Y100 = 1.897 + 0.439[(3.6)(3.6/5.0)J - 0.008(60) = 2.56 <br />inches <br /> <br />Step 3: Determine the 100-year, 5 minute and the 100-year, 15-minute <br />rainfall values. The conversion ratios provided in Table CH5- T1 01 <br />are multiplied by the 1-hour rainfall depth. <br /> <br />RATI05 = 0.29 <br />RAT1015= 0.57 <br /> <br />D1OO,5 '" RATI05'D1OO,1 = 0.29'2.56 '" 0.74 inches <br />D100,15'" RATI015'D1OO,1 = 0.57*2.56 = 1.46 inches <br /> <br />Step 4: Compute the 100-year, 2-hour, 100-year, 3-hour, and the 100-year, <br />12-hour rainfall values using Equations CH5-100 and CH5-1 03, <br />respectively. <br /> <br />I <br />I <br /> <br />I. <br />II <br /> <br />D1OO,2 <br /> <br />= 0.342'D1OO,6 + 0.658'D1OO,1 <br />= 0.342'3.6+0.658*2.56 = 2.92 inches <br />'" 0.597'D1oo,6 + 0.403*D1oo,1 <br />'" 0.597*3.6+0.403*2.56 = 3.18 inches <br />'" 0.5 'D1OO,6 + 0.5 'D1OO.24 <br />'" 0.5'3.6+0.5*5.0 = 4.3 inches <br /> <br />CHAPTER 5 <br />HYDROLOGY <br /> <br />D1OO,3 <br /> <br />D1OO,12 <br /> <br />SECTION 1.0 <br />RAINFALL <br /> <br />AUGUST 2002 <br /> <br />RAINFALL <br /> <br />CH5-109 <br />