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<br />were used and the discharge-drainage area curves were deve~oped <br />for the ~O-, 50-, ~OO-, 500-year irecurrence interva~s. <br /> <br />Peak discharge-drainage area re~~tionships for the streams studied <br />in Pitkin County are shown in Ta~~e 2. On~y ~OO-year frequency <br />discharges are reported for the reaches to be studied by approximate <br />methods. <br /> <br />3.2 Hydrau~ic Ana~yses <br /> <br />Ana~yses of the hydrau~ic characteristics of f~ooding from the <br />sources studied were carried out:to provide estimates of the e~e- <br />vations of f~oods of the se~ecte9 recurrence interva~s. <br /> <br />Water-surface e~evations were computed using the COE HEC-2 step-back- <br />water computer program (Reference ~~). <br /> <br />Cross section data for the backwater ana~ysis of streams within <br />pitkin County were obtained from'topcgraphic maps as fo~~ows: Roaring <br />Fork River (References ~2 and ~3)i I Maroon Creek (Reference 3) I <br />Hunter Creek and Cast~e Creek (R~ference ~2)1 Brush Creek (Refer- <br />ence ~4)1 and Crysta~ River and qoa~ Creek (Reference ~8). Cross <br />sections for the Fryingpan River~ Snowmass Creek, and portions of <br />the Crysta~ and Roaring Fork Rivers were digitized from aeria~ <br />photographs (References ~7, ~a, 19, and 20). Parts of Brush Creek <br />and the Roaring Fork and Fryingpan Rivers were fie~d surveyed to <br />obtain additiona~ cross section data. A~~ bridges and culverts <br />were field surveyed to obtain e~~vation data and structura~ geometry. <br /> <br />Locations of se~ected cross sect~ons used in the hydrau~ic analyses <br />are shown on the Flood Profiles (Exhibit 1). For stream segments <br />for which a floodway was compute4 (Section 4.2), selected cross <br />section locations are also shown:on the Flood Insurance Rate Map <br />(Exhibit 2). <br /> <br />Roughness factors (Manning's On") used in the hydraulic computations <br />were chosen by engineering judgment and based on field observations <br />of the streams and flood plain a~eas. The ranges of roughness <br />va~ues for detailed-study stream~ were as follows: <br /> <br />Streams <br />Brush Creek <br />Castle Creek <br />Coal Creek <br />Crysta~ River <br />Hunter Creek <br />Maroon Creek <br />Roaring Fork Creek <br />Snowmass Creek <br />Fryingpan River <br /> <br />ci:hannel <br />0,040-0.050 <br />'0.045 <br />OJ040-0.050 <br />OJ035-0.055 <br />:0.040 <br />'0.045 <br />Ol035-0.045 <br />Ol040-0.045 <br />01035-0.080 <br /> <br />Overbank <br />0.060-0.070 <br />0.080-0.100 <br />0.060-0.080 <br />0.050-0.085 <br />0.080 <br />0.060-0.080 <br />0.045-0.150 <br />0.060-0.070 <br />0.035-0.100 <br /> <br />~4 <br />