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<br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of flooding from the <br />riverine sources studied were carried out to provide estimates of <br />the elevations of floods of the selected recurrence intervals. <br />Users should be aware that flood elevations shown on the FIRM <br />represent rounded whole-foot elevations and may not exactly reflect <br />the elevations shown on the Flood Profiles or in the Floodway Data <br />tables in the FIS report. Flood elevations shown on the FIRM are <br />primarily intended for flood insurance rating purposes. For <br />construction and/or floodplain management purposes, users are <br />cautioned to use the flood elevation data presented in this FIS in <br />conjunction with the data shown on the FIRM. <br /> <br />Analyses of the hydraulic characteristics of the Arkansas River <br />study reach were carried out by Camp Dresser & McKee, while Great <br />Divide Engineering & Surveying completed the analyses of the <br />hydraulic characteristics of the NE Canon Drainage Area, to provide <br />estimates of the water-surface elevations for floods of the <br />selected recurrence intervals. Water-surface profiles were <br />calculated using the COE HEC-2 step-backwater surface profile <br />computer program. Water-surface profiles were computed for the <br />10-, 50-, 100-, and 500-year floods and are plotted for the <br />Arkansas River in the vicinity of the City of Florence and City of <br />Canon City areas, while water-surface profiles were computed for <br />the lOO-year flood and are plotted for the NE Canon Drainage Area. <br /> <br />Cross-sections for the backwater analyses of the various streams <br />were field surveyed at close intervals above and below bridges and <br />cuI verts in order to compute the significant backwater effects of <br />these structures. Additional digitized cross sections were secured <br />when the topographic maps were compiled. <br /> <br />For the Arkansas River and Chandler, Coal, and Oak Creek, divided <br />flow analysis was used on the railroad bridges and other bridges to <br />proportion the flow between the channel and the overbanks of the <br />streams. Additional cross sections were obtained and computations <br />made to define the separated flow characteristics of the overbank <br />flow paths. Backwater computations, using HEC-2, were made for <br />each independent flow path using a random range of discharges. At <br />the point of flow separation, rating curves were constructed using <br />backwater results and added to obtain a composite rating. Flows <br />for each frequency were proportioned for each flow path by <br />reference to the rating curves. From the computations, the <br />probable flow direction was determined and equal potential <br />water-surface contours were projected into adjacent ponding or low <br />veloci ty areas. The constrictions that produce the divided flow <br />also limit the quantities of flow in the channels downstream. The <br />flow regulation imposed by these structures limit the discharge in <br />the downstream channel to one maximum value for all frequency <br />floods, as reflected in the profiles for these areas (Reference 1). <br /> <br />Locations of selected cross sections in the hydraulic analyses are <br />shown on the Flood Profiles (Exhibit 1). <br /> <br />Channel roughness coefficients (Manning's "nil) for these <br />computations were assigned on the basis of field inspection of the <br /> <br />7 <br />