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<br />Peak discharge-drainage area relationships for the Fryingpan and <br />Roaring FOrk Rivers are shown in Table l~ <br /> <br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of flooding from the <br />sources studied were carried out to provide estimates of the ele- <br />vations of floods of the selected recurrence intervals. <br /> <br />Water-surface elevations were computed using the COE HEC-2 <br />step-backwater computer program (Reference 10). <br /> <br />Cross section data for the backwater analysis of the as follows: <br />Roaring Fork River were obtained from topographic maps <br />(References 11 and 12). Cross sections for the Fryingpan River <br />and portions of the Roaring Fork River were digitized from aerial <br />photographs (References 13, 14, 15, and 16). Portions of the <br />Roaring Fork and Fryingpan Rivers were field surveyed to obtain <br />additional cross section data. All bridges and culverts were <br />field surveyed to obtain elevation data and structural geometry. <br /> <br />Locations of selected cross sections used in the hydraulic <br />analyses are shown on the Flood profiles (Exhibit 1). For stream <br />segments for which a floodway was computed (section 4.2), selected <br />cross section locations are also shown on the Flood Insurance Rate <br />Map (Exhibit 2). <br /> <br />Roughness factors (Manning's On") used in the hydraulic computa- <br />tions were chosen by engineering judgment and based on field <br />observations of the streams and flood plain areas. The ranges of <br />roughness values for the two detailed-study streams were as <br />follows: <br /> <br />Streams <br />Roaring Fork River <br />Fryingpan River <br /> <br />Channel <br />0.035-0.045 <br />0.035-0.080 <br /> <br />Overbank <br />0.045-0.150 <br />0.035-0.100 <br /> <br />The starting elevation <br />the slope-area method. <br />River was taken from a <br />(Reference 17). <br /> <br />for the Roaring Pork River was obtained by <br />The starting elevation for the Fryingpan <br />1978 FlOOd Plain Information report <br /> <br />The hydraulic analyses for this study were based on unobstructed <br />flow. The flood elevations shown on the profiles are thus con- <br />sidered valid only if hydraulic structures remain unobstructed, <br />operate properly, and do not fail. <br /> <br />All elevations are referenced to the National Geodetic Vertical <br />Datum of 1929 (NGVD). Elevation reference marks used in this <br />study are shown on the maps, the descriptions of the marks are <br />presented in Elevation,Reference Marks (Exhibit 3). <br /> <br />8 <br />