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3.2 Hydraulic Analyses Analyses of the hydraulic charaC':eristics of streams were carried out to provide estimates of ~he elevations of floods of selected recurrence intervals along each stream studied in the conununity. Water-surface elevations of flood:;; of the selected J:ecurrence intervals were computE~d through 0.;;<2 of the U.S. AJ:my Corps of Engineers HEC-2 step-backwater computer program (Reference 6). Cross section data for the backwater analysis of the Roaring Fork River and Castle Creek were obtained from topographic maps at a scale of 1:1200 and 1:600, 'il,'.th a contour interv2,l of 2 feet (Reference 7). Cross sections on Maroon Creek were digitized from ael:ial photographs and obtained from a previous report (ReferencE~ 8). All brIdqes were field surveyed to obtain elevation data and structural geometry. Locations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibit 1). For stI'eam segments for which a floocwiOiY is computed (Section 4.2), selected cross section locations are also shown on the Flood Boundary and Floodway Map (Exhibit 21. The stream distances along the Roal:ing Fork River are mecsured from Emma Bridge near the pi tkinjEaq le County 1 ine. 'l'he st::eam distances for Castle and Maroon Creeks are measured from the confluence with the Roaring Fork liver. ,<oughness factol:S (Manning's "nU) used in the hydraul Ie computations were chosen by engineeJ:ing judgmE!nt and based on field observations of the str,c!ams and floodplain areas, Channel roughness values used for ~je Roaring Fork River ranged from 0.04 to 0.045 while the overbank areas were 0.08 to 0.1, The channel roughness valge for Castle Creek was 0.045 while '::he overbank values ranged from 0.08 to 0.,1. For Maroon Creek, the channel roughness was 0.045 and the overbank roughness was 0.08. Starting water-surface elevatiom; for the Roarinq Fcnk Ri vel: were taken from the adjoining pjtkin County Flood Insurance Study (Reference 9). Starting water-surface elevations for Castle and Maroon Creek were taken from the Roaring Fork River flood elevations. Flood prOfiles WerE! drawn elevations to an accuracy selected recurrence intervals show:i,:19 computed water,"su:::;:acE' of O,!'i foot: for, floods of the (Exhibit 1). 'l'he hydraulic analyses for ':,111:3 study were based 0'''' unobstructed flow. The flood elevations shown on the profiles are, thus, considered valid only If hydraulic structures remalD unobstructed, operate properly and do not fail. -5.-