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The two-hour rainfall distributions for each return period are presented on Table 3.2. Table 3,2 10-Year 50-Year 1 DO-Year SOD-Year Time Rainfall Rainfall Rainfall Rainfall Min (Inches) (Inches) (Inches) (Inches) 5 0.03 0.03 0.03 0.03 10 0.06 0.08 0.08 0.10 15 0.14 0.11 0.12 0.15 20 0.25 0.18 0.21 0.26 25 0.41 0.34 0.36 0.45 30 0.20 0.56 0.65 0.80 35 0,09 0.27 0.36 0.45 40 0.07 0.18 0.21 0.26 45 0.06 0.11 0.16 0.20 50 0.05 0.11 0.13 0.16 55 0.05 0.07 0.10 0.13 60 0.05 0.07 0.10 0.13 65 0.05 0.07 0.10 0.13 70 0.05 0.05 0.05 0.06 75 0.05 0.05 0.05 0.06 80 0.04 0.04 0,03 0.04 85 0.03 0.04 0.03 0.04 90 0.03 0.03 0.03 0.04 95 0.03 0.03 0.03 0.04 100 0.03 0.03 0.03 0.04 105 0.03 0.03 0.03 0.04 110 0.03 0.03 0.03 0.04 115 0.03 0.03 0.03 0.04 120 0.02 0.03 0.03 0.04 Total 1.88 2.57 2.98 3.73 3,6 CUHPFiPC Modeling 3.6. I Delineation of Subwatersheds: Subwatersheds were delineated on the USGS Kassler, Colorado quadrangle map enlarged to a scale of 1"=1,000'. The boundaries were determined based on the topography, existing roadways, bridges, and existing culverts and existing development. The subwatershed areas, lengths, and centroid lengths were estimated from the enlarged 1"=1,000' scale USGS quadrangle maps. 3.6.2 Imperviousness: The imperviousness of each subwatershed was determined by superimposing the land use information on the USGS map. The future development conditions imperviousness was based on full development of the land use within each watershed, The following percentage of imperviousness was assigned to each of the land uses within the major watersheds: . 1%-2% . 20% . 5% . 30% . 45% . 85% Undeveloped or Agricultural Areas Future Development Conditions in Agricultural Areas Rural Residential Residential West of Hogback Residential East of Hogback General Industrial and Business Based on the assumed impervious values for each land use area, a weighted impervious percentage for each subwatershed was calculated. 3.6.3 ~: The slopes for each subwatershed were estimated according to the weighted slope method documented in Volume I of the Urban Storm Drainage Criteria Manual dated 1969 (USDCM, Volume 1). The length of each subwatershed was broken up into segments of approximately equal slope. The slope of each of the segments was computed and corrected according to Figure 4-1 of the Runoff chapter of the USDCM. The segment lengths and corrected slopes were entered into Equation 4-5 of the Runoff chapter of the USDCM to determine the weighted subwatershed slope. Weighted subwatershed slopes varied from 1.7 percent to 6.0 percent in the study area. Six percent is the maximum slope for use in CUHPF/PC models. The natural slope exceeded 10% in many of the upper reaches. CUHPF/PC models for future development conditions were completed for the combined watersheds of Willow Creek, East Willow Creek, and Little Willow Creek. The 10- 50-,100-, and 500-year storm events were modeled using CUI-IPF/PC. The subwatershed characteristics required for CUI-IPFIPC include the identification number, area, length, centroid length, slope, impervious percentage retention storage losses, and infiltration losses. The methods for estimating the subwatershed characteristics are described below and a summary table of the subwatershed characteristics is presented in Table 3.3 (located in the Appendix) for future development conditions respectively. A summary of the peak discharges computed by CUI-IPF/PC for each of the storm events is included in the Appendix to this report (Table 3.4). 13