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<br />Flood profiles were drawn showing computed water-surface elevations <br />to an accuracy of 0.5 foot for the floods of the selected rec~rrence <br />intervals (Exhibit 1). <br /> <br />Starting water-surface elevations were based on critical depth. <br /> <br />The hydraulic analyses for this study were based on unobstructed <br />flow. The flood elevations shown on the profiles are thus con- <br />sidered valid only if hydraulic structures remain unobstructed, <br />operate properly, and do not faiL <br /> <br />Approximate elevations were determined by field reconnaissance, <br />historical accounts of flooding provided by local residents, and <br />engineering jUdgment. <br /> <br />All elevations are referenced to the National Geodetic Vertical <br />Datum of 1929 (NGVD). <br /> <br />4.0 FLOOD PLAIN MANAGEMENT APPLICATIONS <br /> <br />The NFIP encourages State and local governments to adopt sound flood <br />plain management programs. Therefore, each Flood Insurance Study produces <br />maps designed to assist communities in developing flood plain management <br />measures. <br /> <br />4.1 Flood Boundaries <br /> <br />To provide a national standard without regional discrimination, <br />the 1 percent annual chance (lOa-year) flood has been adopted by <br />FEMA as the base flood for flood plain management purposes. The <br />0.2 percent annual chance (500-year) flood is employed to indicate <br />additional areas of flood risk in the community. For each stream <br />studied in detail, the 100- and 50 a-year flood plain boundaries <br />have been delineated using the flood elevations determined at each <br />cross section. Between cross sections, the boundaries were inter- <br />polated using topographic maps at a scale of 1:2,400, with a contour <br />interval of 5 feet (Reference 7). <br /> <br />Due to the scope of work for the original report, no 500-year flood <br />boundaries were delineated in this study. <br /> <br />Topographic mapping at a scale of 1:2,400 (Reference 7) was used <br />for the delineation of approximate flood boundaries for Dallas, <br />Pleasant Valley, and Cascade Creeks. <br /> <br />Additional approximate flood boundaries for this study were taken <br />from USGS Flood Prone Area mapping (Reference 8). <br /> <br />The lOa-year flood plain boundaries are shown on the Flood Boundary <br />and Floodway Map (Exhibit 2). In cases where the 100- and 500- <br /> <br />10 <br />