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I I I I I I I I I I I I I I I I I I I All bridges were field checked 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 stream segments for which a floodway is computed (Section 4.2), selected cross section locations are also shown on the Flood Boundary and Floodway Map (Exhibit 2). Channel roughness factors (Manning's "n") used in the hydraulic computations were chosen by engineering judgment and based on field observations of the streams and flood plain areas. A roughness value of 0.040 was selected for the Blue River, Straight Creek, and Willow Creek channels, while flood plain roughness values were in the range of 0.100 for all floods. Starting water-surface elevations for the Blue River, Straight Creek, and Willow Creek were calculated using the slope-area method at the downstream end of the each river or tributary River. Flood profiles were drawn showing computed water-surface elevations to an accuracy of .::to. 5 foot for floods of the selected recurrence intervals (Exhibit I). The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the profiles are, thus, considered valid only if hydraulic structures remain unobstructed, operated properly, and do not fail. All elevations are referenced to the National Geodetic Vertical Datum of 1988 (NGVD). Elevation reference marks used in the study are shown on the maps. 4.0 FLOOD PLAIN MANAGEMENT APPLICATIONS A prime purpose of the National Flood Insurance Program is to encourage State and local governments to adopt sound flood plain management programs. Each Flood Insurance Study, therefore, includes a flood boundary map designed to assist communities in developing sound flood plain management measures. 4.1 Flood Boundaries In order to provide a national standard without regional discrimination, the 100-year flood has been adopted by the Federal Insurance Administration as the base flood for purposes of flood plain management measures. The 500-year flood is employed to indicate additional areas of flood risk in the community. For each stream studied in detail, the boundaries of the 100- and 500-year floods, have been delineated using the flood elevations detennined at each cross section; between cross sections, the boundaries were interpolated using topographic maps at a scale of 1:2400, with a contour interval of 2 feet. In cases where the 100- and 500-year flood boundaries are close together, only the 100-year flood boundary has been shown. Flood boundaries for the 100- and 500-year floods are shown on the Flood Boundary and Floodway Map (Exhibit 2).