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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 /> <br />6 <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />IV. SUMMARY OF HYDROLOGY <br />A hydrologic analysis was conducted to determine the 10-, <br />50-, 100-, and 500-year peak flows for future watershed <br />development condition. It is estimated that the current <br />watershed lS approximately 85 percent developed. The Urban <br />Drainage Storm Water Management Model (UDSWM2) was used to <br />develop runoff hydrographs from individual subcatchments and to <br />route the hydrographs through the entire drainage system. <br />UDSWM2 is the name assigned by the UDFCD to the runoff block <br />of the Environmental Protection Agency's (EPA) Storm Water <br />Management Model (SWMM, version 2) as modified by the Missouri <br />River Division (MRD) of the U.S. Corps of Engineers (CaE). <br />UDSWM2 offers channel, pipe and reservoir routing capabilities. <br />The input data for UDSWM2 include rainfall, depth <br />imperviousness, basin area and slope, basin tributary width, <br />roughness coefficient, soil infiltration rate, surface retention <br />depth, channel/pipe length and slope, channel/pipe configuration <br />and roughness coeff icient, detention pond storage and outf low <br />capaci ty, etc. Input data were prepared using guidel ines and <br />values recommended in the Urban Storm Drainage Criteria Manual <br />(USDCM) and the UDSWM2 users manual. Discussions of specific <br />input parameters follow. <br /> <br />2. Drainage Basin Parameters <br />The drainage basin parameters used In this study are listed <br />In Table 2. A total of 29 subcatchments were delineated, and <br />shown on Figure 2. <br />Subcatchments were delineated uSlng USGS quadrangle maps <br />with a scale of 1" = 2000' and 10-foot contour interval and other <br />available mapplng obtained from the City. Subcatchment <br />boundaries were established based on physical drainage <br />boundaries, expected future development and logical design <br />points. <br />Subcatchment areas were measured and divided into idealized <br />rectangular areas to determine an appropriate tributary basin <br />width. The basin width can be adjusted for the purpose of <br />calibration. <br /> <br />1. Design Rainfall <br />The design rainfall for the study area was determined using <br />procedures outlined in the users manual. One hour point rainfall <br />values for each basin were interpolated from the rainfall Depth- <br />Duration-Frequency maps found In the USDCM. The I-hour point <br />rainfall depths were adjusted to a 2-hour rainfall distribution <br />according to Table 3-1 of the USDCM. The 2-hour storm for the <br />10-, 50-, and 100-year recurrence intervals used in the UDSWM2 <br />model are given in Table 1. <br /> <br />Subcatchment slopes were calculated for the <br /> <br /> <br />rectangular areas by using the average basin slope. <br /> <br />assumed to be uniform through the subcatchment. <br /> <br /> <br />Subcatchment impervious areas were obtained by overlaying <br /> <br /> <br />the current land use maps provided by the City on the <br /> <br /> <br />subcatchment delineation and proportioning the appropriate <br /> <br /> <br />impervious percentages. Table 3 presents the relationships <br /> <br /> <br />between the land use and percent of imperviousness. <br /> <br />The surface resistance factor (Manning's 'n') and surface <br /> <br /> <br />retention storage (depth) for both impervious and pervious areas <br /> <br />were determined by following the ODSWM2 users manual and the <br /> <br />idealized <br />Slopes were <br /> <br />USDCM. <br />Inf i 1 tration rates were determined by over laying the soils <br />map on the subcatchment del i neation maps and proportioning the <br />amounts of soil type and infiltration rates. Infiltration rates <br />were taken from the USDCM. Soil types were obtained from the <br />Adams and Arapahoe County Soil Surveys. <br /> <br />3. Channel Routing <br />Individual subcatchment hydrographs were routed down the <br />drainage system using UDSWM2. The drainage system was modeled <br />using a system of channels and direct flow elements (Figtirc 2). <br />