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<br />ranged from 0.035 to 0.050, while flood plain roughness values <br />ranged from 0.080 to 0.110 for all floods. <br /> <br />All the streams are supercritical. Therefore, critical depth was <br />used as the starting water-surface elevation. The only exception <br />is the upper reach of the Fraser River, which is controlled at the <br />downstream section by a box culvert. Starting water-surface ele- <br />vations at the upstream end of the culvert were calculated using a <br />rating curve developed from standard nomographs from the Federal <br />Highway Administration. <br /> <br />Floodflows along Leland Creek escape the channel on the left <br />overbank and cause shallow flooding along the west embankment of <br />U.S. Highway 40. This shallow flooding, with an average depth of <br />less than 1.0 foot, was determined from normal-depth computations. <br />Discharges downstream of U.S. Highway 40 on Leland Creek are limited <br />to 8 cfs for the 10-, 50-, 100-, and 500-year events. Since flooding <br />is contained within the channel downstream of U.S. Highway 40, no <br />profiles are presented for this portion of Leland Creek. <br /> <br />The hydraulic analyses for this' study were based on unobstructed <br />flow. The flood elevations sho~ 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 flooding along Jim Creek was determined from normal- <br />depth calculations. <br /> <br />All elevations are referenced bp the National Geodetic Vertical <br />Datum of 1929 (NGVD). Elevation reference marks used in this study <br />are shown on the maps. <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 (lOO-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 500-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 1:6,000 and 1:4,800, <br />with a contour interval of 10 and 2 feet (References 11 and 12). <br /> <br />16 <br />