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use of the U.S. Army Corps of Engineers River Analysis System (HEC-RAS) for <br />the City of Cortez studies, the Dolores and West Dolores Rivers, and for the <br />detailed portion of the Mancos River studied both upstream and downstream of <br />the Town of Mancos. <br />For the detailed studies of Chicken Creek and the 1Vlancos River through the <br />Town of Mancos, basic cross section input was derived from both <br />photogrammetric and field survey methods. In addition, a11 bridges and other <br />significant structures that might divert or restrict flood-flows were surveyed. It is <br />noted that because the digitized cross sections show the water surface elevation in <br />each stream on the date of photography, the cross sections were adjusted by field <br />measurements to reflect the streambed elevations and an area beneath the mapped <br />water surface level for a discharge of 300 cfs in the stream at the time of <br />photography (Muller Engineering, March 1982). For the detailed study of the <br />Mancos River both upstream and downstream of the Town of Mancos, cross <br />sec~ions were cut using GeoRAS based on 2-foot contour mapping, breakline <br />information and a ground survey of Highway 160, generated as part of the 2006 <br />Mancos River Hydraulic Study (Anderson Consulting Engineers, Inc., August <br />2006). For the detailed studies in the City of Cortez, cross sections were cut <br />based on 2-foot contour mapping generated for the City of Cortez (Goff <br />Engineering, October 2005). For the Dolores and West Dolores Rivers, cross <br />sections were cut using GeoRAS based on a triangulated irregular network (TIN), <br />which was generated from an aerial survey of the subject area in spring of 2003 <br />(U.S. Army Corps of Engineers, Sacramento District, August 2005). Cross <br />sections for Lost Canyon Creek w~ere derived from survey information furriished <br />by the Colorado Water Conservation Board. <br />Starting water surface elevations for the Cortez Studies were either based on the <br />slope-area method or known water surface elevations if the water course <br />discharged into a lake (Goff Engineering, October 2005). The downstream <br />boundary condition for the Dolores and Mancos Rivers were based upon a bed <br />slope of the river at the downstream study limit (LJ.S. Army Corps of Engineers, <br />Sacramento District, August 2005 and Anderson Consulting Engineers, Inc., <br />August 2006 respectively). <br />Floodplain roughness factors (Manning's n) used in the hydraulic computations <br />were selected based on field inspection and flood photos of the actual floodplain <br />conditions. In the case of the Mancos River study both upstream and downstream <br />of the Town of Mancos Manning's n values were determined through a <br />combination of field reconnaissance efforts and the application of Cowan's <br />Method (Ven Te Chow, 1959). Roughness factors for all streams studied by <br />detailed methods are shown in Table 2. <br />The hydraulic analyses for this Flo~d Insurance Study are based on unobstructed <br />flow. The flood elevations shown ~n the profiles are thus considered valid only if <br />hydraulic structures remain unobstr~acted, operate properly, and do not fail. <br />13 <br />