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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />As a result, two different sets of rainfall depths were used in this study for the 24-hour storm, The <br />values for the 2-hour storm are the same for both areas, which is a result of the DCM equation used <br />to determine the 1-hour rainfall. Slightly higher elevations for the western basins tended to offset the <br />changes in rainfall depth used in the equation, The rainfall depths determined for all basin applications <br />are presented in Table 1.1, <br /> <br /> Table 1,1, <br /> Storm Duration <br /> (hours) <br />24 (General Colorado Springs/EI Paso 3,10 3,90 4,40 <br /> County) <br />24 (West Colorado Springs) 3,20 4,00 4,50 <br />1 (For deriving the 2-hour storm) 1.78 2,35 2,64 <br />2 (Derived from the 1-hour storm) 2.06 2,72 3,05 <br /> <br /> <br />The SCS Type IIA distribution for 24-hour duration is an intense rainfall occurrence whereby a majority <br />of the precipitation occurs between the fifth and sixth hour of the storm, Distribution for the 2-hour <br />storm is also intense, with a majority of the precipitation occurring between 20 and 35 minutes after <br />initiation of the rainfall. Both distributions, as presented in Figures 5-5a and 5-5b of the DCM, are <br />reproduced in Figure 1.2, <br /> <br />1.4 Resource Consultants & Engineers, Inc, <br /> <br /> <br />There is a single curve that applies to the 10-, 50-, and 100-year storm events under the 24-hour <br />Type IIA distribution, Separate curves for the to- and tOO-year events are required for the 1-hour <br />duration storm, No distribution is specified for the 50-year, 2-hour duration storm, Based on <br />engineering judgment, it was decided to evaluate the 50-year event with the same distribution as for <br />the tOO-year, 2-hour duration storm. <br /> <br />Antecedent moisture conditions (AMC) as defined by the SCS have been prescribed for use with <br />the two storm distributions by the DCM, AMC II reflects an average moisture condition in the soil <br />whereby some infiltration will occur before runoff starts, AMC III indicates saturated conditions with <br />little potential for infiltration, Although the DCM states "The use of AMC III is not recommended for <br />stormwater runoff analysis and drainage facility design within the City/County limits," several basin <br />planing studies have utilized the AMC III 2-hour storm and to be consistent, RCE utilized this <br />conditions for those basins, <br />