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<br />dc>wnstream edge of the pits, as indicated by the topographic mapping. This adjustment <br />prevented the excavated area in the overbanks from being included as excessive eff(~ctive flow <br />area in the hydraulic computations. <br />5.3 Roughness Values <br />T~e roughness value (Manning's "n") used in the hydraulic computations fClr the Cache <br />La Poudre River chmmel was set at 0.035 based on the channcl roughness values used in the <br />previous hydraulic studies of the Cache La Poudre Riv'~r in Weld County. Roughncss values for <br />the flood plain overbanks wef(~ :Jascd on field observa1::.ons and aerial and surface photography. <br />The overbank "n" values ranged frem 0.045 for open pasture or cropland to 0.085 for urban <br /> <br />areas. <br /> <br />No high wate:r marks from r~cent large floods with overbank flows were available. Water <br />surface measurements were made ill June] 997 and used 'wit~ flow data from the stream gage <br />located a1 Fern Avenue to check th, selected roughncs~: value using the HEC-2 bydraulic models. <br />The water surface profile computed with the recorded flow followed the measured elevations so <br />the chanr:.el roughness value of 0.0: 5 was not adjusted. <br />5.4 Starting Water Surface EI[ev~tiolJ\s <br />The starting water surface e. evation on the Cache la Poudre River near the Platte Rivcr <br />confluence was developed usin[. th, slope-area method of HEC2. The starting slope of 0.005 <br />was based on the South Platte Rive) slope in the confluence area. Because the hydraulic model <br />for the study area was continuous, ;tarting elevations wen~ only needed at the South Platte River <br />confluence. <br />S.S Brid ges. <br />Flow through bridges was cnmputed using the normal bridge or special bridge options of <br />HEC-2. All hydraulic computation; were based on unobstructed flow. <br />5.6 Modifications to the Reach 2 Hydraulic Model <br />In order to make the 1997 h:rdraulic model for Reach 2 tbrough Grceley consistent with <br />the hydralllic models developed for this study for Reach] and Reach 3, several modifications <br />were mac.e. The cross sections;n the 1997 study for Rea.cl12 were oriented from left-to-right <br />looking upstream. For this study, the cross section orientation 'wa.s reversed using a computer <br /> <br />4 <br />