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only if hydraulic structures properly, and do not fail. remain unobstructed, operate The City of Florence identified Oak Creek as having potential flood hazards during storm events. Based on discussions with City representatives, a hydraulic baseline was drawn along the centerline of Oak Creek. The width of the cross sections varies based on the extent of the flood plains. The lower limit of this revision is approximately 330 feet upstream of the confluence with the Arkansas River (3+30) and the upper limit is approximately 70 feet upstream of the abandoned Atchison, Topeka and Santa Fe Railroad (98+89). Roughness coefficients (Manning's "n") used in the hydraulic analysis were determined using engineering judgement and based on field observations, orthophoto topographic mapping, and ground level photography of the channel and flood plain areas. Factors affecting the determination of the Manning I s "n" include channel material, channel irregularity, obstructions, vegetation, and the degree of channel meandering. The roughness value used for the main channel was 0.045, and for the flood plain areas the roughness values range from 0.035 to 0.15. The selected roughness coefficients are referenced in tables presented in the handbook Cpen Channel Hydraulics (Reference 19). The recorrunended values for the contraction and expansion coefficients for gradual transitions are 0.1 and 0.3, respectively. When the change in effective cross-sectional area is abrupt and increased losses occur, such as at bridges or cuI verts, the recorrunended contraction and expansion coefficients are 0.3 and 0.5, respectively. The computer program HEC-RAS (Reference 18) was used to develop the 10.0%-, 2.0%-, 1.0%-, and 0.2%-chance water surface elevations for Oak Creek. The computer analysis uses the mixed flow regime method which computes both supercri tical and subcritical solutions. For the supercritical flow regime, the model starts with critical at the upstream cross section. The subcritical flow regime uses the downstream cross section and the slope-area method for computing the water surface elevations for the 10.0%-, 2.0%-, 1.0%-, and 0.2%-chance floods. Although Oak Creek is supercritical, the bridge or culvert crossings are subcritical and the backwater affects several cross sections upstream of the structures. Users should be aware that flood elevations shown on the FIRM represent rounded whole-foot elevations and may not exactly reflect the elevations shown on the Flood Profiles or in the Floodway Data tables in the FIS report. Flood elevations shown on the FIRM are primarily intended for flood insurance rating purposes. For construction and/or floodplain management purposes, users are cautioned to use the flood elevation data presented in this FIS in conj unction with the data shown on the FIRM. 14