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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 />1 <br />I <br /> <br />The new analyses were carried out for several additional sub- <br />basins utilizing the CUHPE!PC computer model. The drainage <br />basins and hydrologic design points are shown on Figure 1. <br />Since only the main channel of piney Creek was included in the <br />previous study, the design points defined were large and the <br />computed flows were published only for design points along the <br />main channel. <br /> <br />impervious retention were based on those shown in the USDCM for <br />the types of development that are expected to occur in each of <br />the basins. <br /> <br />Rainfall data used in modelling the tributary flows were <br />obtained from the Storm Drainage Design and Technical Criteria <br />manuals for Arapahoe and Douglas Counties. The rainfall depths <br />used are listed in Table 4. <br /> <br />The CUHPE!PC input data used for the modelling of the tributary <br />basins is shown in Table 5. A summary of the computed peak <br />discharges at selected hydrologic design points for the defined <br />drainage basins is presented in Table 6. The locations of <br />these hydrologic design points and the drainage basins are <br />shown on Figure 1. Discharge profiles and representative <br />hydrographs for all the study reaches are shown in Figures 2 <br />through 12. <br /> <br />Design Rainfall for <br />l-hr Rainfall <br />10-Year <br /> <br />Table 4 <br />Piney Creek Tributaries <br />Depth (inches) for Return period: <br />50-Year 100-Year <br /> <br />Douglas County <br /> <br />1. 65 <br />1.66 <br /> <br />2.32 <br />2.26 <br /> <br />2.67 <br />2.60 <br /> <br />Arapahoe County <br /> <br />The existing percent impervious area for each basin was estimated <br />from aerial photography flown in December of 1987, from USGS <br />quads, and from field observations carried out between June and <br />November of 1988. The percent impervious for fully developed <br />basin conditions was determined from land use maps, subdivision <br />plats, and discussions with planning and engineering staffs from <br />Arapahoe County, the City of Aurora, and Douglas County. <br /> <br />The basin areas, lengths, slopes, and lengths to centroid were <br />measured from USGS quads. Infiltration rates were based on the <br />soil types identified in the Soil Conservation Service Soil <br />Surveys. The infiltration rate for each pertinent soil group <br />was obtained from the recommended values in the Urban Drainage <br />and Flood Control District's, "Urban Storm Drainage Criteria <br />Manual" (USDCM) . The values used for pervious retention and <br /> <br />6 <br /> <br />