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<br />. <br /> <br />. <br /> <br />. <br /> <br />6 <br /> <br />in Table 1. The rainfall depths used for each storm event are given in Table <br />2. An aerial reduction factor of 9S percent was used for the storm over the <br />Cooper Slough basin and a factor of 76 percent was used for the combined <br />Boxelder Creek-Cooper Slough basin. Tables 3 and 4 contain the rainfall <br />hyetographs used for Boxelder Creek and Cooper Slough basins, respectively. <br /> <br />~ <br /> <br />2.2 Drainage Basin Parameters <br />The parameters required to determine runoff peaks and volumes from SWMM <br />are subcatchment area, width, percent impervious, infiltration rate, <br />depression and detention storage, overland roughness coefficient, channel <br />geometry, channel slope, channel roughness coefficient, and storage discharge <br />curves for ponded areas. The percentage of impervious area for developed con- <br />ditions was based on land use as defined by the Larimer County Urban Growth <br />Map (Figure 4). The percent of impervious area by land use is given in Table <br />S. The impervious area for existing basin conditions was based on U.S. <br />Geological Survey topographic maps and Larimer County topographic maps <br />(references 8 and 9). The hydrologic analysis of Boxelder Creek assumed deve- <br />loped basin conditions within the Urban Growth Area and existing conditions <br />outside the Urban Growth boundary. cooper Slough was analyzed for existing <br />and fully developed basin conditions. <br />Infiltration rates for pervious areas in the Cooper Slough basin were <br />based on soil types obtained from the ~arimer County Soil Survey" and <br />selected to be consistent with the Dry Creek report. Infiltration rates for <br />BoxelderCreek were based on soil types obtained from the "Larimer County Soil <br />Survey" with a maximum rate of one inch per hour. The maximum rate was recom- <br />mended by the Chief Hydrologist of the Omaha District of the U.S. Army Corps <br />in Omaha, Nebraska based on their experience on hydrologic analysis at the <br />front range. Depression and detention storage was selected to be consistent <br />with the Fossil Creek and Dry Creek studies. Subcatchment roughness charac- <br />teristics were based on field investigation and selected to be similar to the <br />previous studies. Channel unit lengths, slopes, and geometries were defined <br />from field surveys and from 1:1200 scale topographic maps from Larimer County, <br />Colorado, where available (reference 9) or U.S. Geological Survey 1:24000 <br />scale topographic maps (reference 8). Manning's roughness coefficients were <br />based on field investigation. Table 6 lists the parameters used and the range <br />