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I I I I I I I I I I I I I I I I I I I 6 Ill. HYDROLOGIC ANALYSIS A. Introduction A hydrologic analysis was conducted to determine the 2-, 5-, 10-, and IOO-year peak flows for two development conditions (existing, and future with new airport). The Colorado Urban Hydrograph Procedure (CUHP) (computer model version CUHPE-PC) was used to develop runoff hydrographs from individual sub- catchments. The individual hydrographs were routed down the drainage system us i ng the Storm Water r'lanagement ~~ode 1 (SW/.1~1) (computer model vers i on UDSW~12-PC) . B. Colorado Urban Hydrograph Procedure (CUHP) The input data for CUHP include rainfall, imperviousness, basin area, basin length, shape factor, soil infiltration, and surface storage. Input data were prepared using guidelines and values recommended in the Urban Storm Drainage Criteria Hanual (USDC~I) and the CUHPEPC users manual. Basin area, length, length to centroid, amount of impervious area, proportion of soil types and weighted basin slope were measured and input files prepared using AutoCAD with an in-house developed input data preparation program (CUHPCAD). Discussions of specific input parameters follow. 1. Design Rainfall The design rainfall for the study area was determined using procedures outlined in the USDCM. One hour point rainfall values for each basin vlere interpolated from the rainfall Depth-Duration-Frequency maps found in' the USDCH. The I-hour point rainfall depths were adjusted to a 2-hour rainfall distribution according to Table 3-1 of the USDCt-l. The I-hour point rainfall for the 2-, 5-, 10-, and IOO-year recurrence intervals are given in Table 1. Adjustments to the 2-hour rainfall distribution are necessary for the Hayesmount and Box Elder Basins. Both basins are greater than five square miles and less than ten square miles. Table 4-2 of the USDCM was used to develop a 2-hour rainfall pattern for both Hayesmount Gulch and Box Elder Creek. The 2-hour rainfall distributions are shown in the CUHP input file (Appendix). Individual hydrographs for the Hayesmount Tributary and Unnamed Basins were developed using unadjusted rainfall values.