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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 />I <br />I <br /> <br />SECTION III <br />HYDROLOGIC ANALYSIS <br /> <br />A. INTRODUCTION <br />A hydrologic analysis was performed as part of the Big Dry Creek (ADCO) <br />North Area Outfall Planning Study which defined the storm runoff peak <br />flows and volumes al points of interest for the 100-year, 50-year and <br />lO-year storms. This hydrologic information was used as input to the <br />FHAD's hydraulic analyses to define floodplain areas and flood eleva- <br />tions. <br /> <br />B. RUNOFF MODEL <br />The Colorado Urban Hydrograph Procedure (CUHP) was used to compute <br />storm hydrographs for each subbasin. The subbasin input parameters for <br />CUHP include: identitler, area, characteristic flow length, length to <br />centroid of subbasin, characteristic subbasin slope, percent impervious <br />area, detention and retention storage losses, and soil infiltration <br />loss rates. Details of the input parameters are discussed in subse- <br />quent sections. <br /> <br />The UDFCD version of the Storm Water Management Model (SWMM) was used <br />to route the CUHP generated hydrographs. The routing includes the <br />translation of hydro graphs through the various channels, storm sewers, <br />and streets. The routing schematic shown in Figure III-I illustrates <br />the relationship between the subbasins of the CUHP model and the stream <br />elements of SWMM. The routing element parameters required for SWMM <br />are: element number, type (i.e., channel, pipe, etc.), length, slope, <br />and Manning's "n" value. <br /> <br />C. DESIGN RAINFALL <br />The design rainfall for the study area was derived using the procedure <br />outlined in the Urban Storm Drainage Criteria Manual (USDCM). A storm <br />duration of 2 hours was selected in accordance with the USDCM guide- <br />lines. <br /> <br />111-1 <br />