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I I I I I I I I I I I I I I I I I I 1.2. Modeling Procedure The U,S, Soil Conservation Service (SCS) hydrology model TR-20 was used for all of the hydrologic analysis conducted, This model takes into account the contributing basin rainfall-runoff characteristics of area, slope, soil type and land use, together with specific storm events to generate flow discharges, Effects of channel flood routing, detention storage routing and stream diversions can also be represented, Standard SCS basin parameters for model input include soil and land use Curve Number (CN), T c' and runoff area, These were derived using guidelines prescribed in Procedures for Determinino Peak Flows in Colorado (Ref 1,1), Reach flow characteristics are represented by x and m parameters defined by TR-20 manual (Ref 1.2) for use in determining channel flood routing, Basin soil characteristics were obtained from the SCE Soil Survev of EI Paso County Area, Colorado (Ref 1,3), As specified in the FEMA guidelines, determination of peak flood discharges associated with the 10-, 50-, 100-, and 500-year rainfall events. are required, For basins where limited detailed studies were conducted, only the 1 DO-year discharge was determined, For this study, the 10-, 50-, and 100- year discharge values were generated directly from the computer modeling of TR-20, The 500-year peak discharge was obtained from extrapolation of a probability curve constructed from the 10-, 50-, and 1 DO-year results, 1.3. Storm Criteria Rainfall depth, duration and distribution for the 10-, 50-, and 100-year storm events were obtained from specifications set forth under the City of Colorado Sorinos and EI Paso County Drainaoe Criteria Manual (DCM) (Ref, 1.4), Two possible distributions for the critical design storm are presented with the final choice determined by definition as, "the storm distribution producing the greatest level of protection for flood conveyance and storage facilities," One storm is for a 24-hour duration of rainfall according to the SCS Type IIA distribution, The other is a 2-hour duration based on the 1-hour rainfall depth with distribution similar to the Colorado Urban Hydrograph Procedure 2-hour storm distribution, Rainfall in most of the Colorado Springs area does not vary much from basin to basin, An exception to this occurs in the foothill basins located in the western part of the City, These areas tend to get slightly higher rainfall amounts as determined from the NOAA Atlas II (Ref 1,5) isopluvial maps, 1.3 Resource Consultants & Engineers, Inc,