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I I I I I I I I I I I I I I I I I I I 8 Infiltration rates were determined by overlaying the soils map (Figure 2) on the subcatchment del ineation maps (Figure 4 and 5) and proportioning the amounts of soil type and infiltration rates. Table 3 gives a summary of input data for CUHP. C. Channel Routing Individual subcatchment hydrographs were routed dovm the drainage system using the UDSI,tlH2-PC computer model. The drainage system was modeled using a system of channels and direct flow elements. The channel input includes length, slope, cross section, and roughness coefficients. Direct flow ele- ments do not route upstream elements but instead directly add upstream hydrographs to give a direct translation of incoming flows. The routing system is depicted schematically in Figures 6, 7 and 8. The SWMH channel system is shown on Figures g and 10. Existing channel elements are modeled as a low-flow channel with an overflow section. Slopes were measured froM USGS maps. Lengths were measured from USGS maps neglecting small bends in the low-flow channel. Typical cross sections for the low-flow channel and overbanks were developed based on USGS maps and field inspection. Roughness coefficients were estimated based on Equation 2 in the UDSWM2-PC manual. Future channel element cross section, slope and roughness were modified from existing conditions. Future roughness values were set at 0.035 and future channel slopes were set at 0.5 percent with side slopes of 4 horizontal to 1 vertical (4H:1V). The main stems of Hayesmount Gulch and Box Elder Creek were assumed to remain in their natural condition in the future, and no adjustments were made to the channel section. SWMM and CUHP input listings are given in the Appendix.