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<br />engineering judgment and based on field observations of the <br />flooding sources and floodplain areas. <br /> <br />Starting water-surface elevations were either based on coincident <br />condition elevations or the slope-area method. <br /> <br />Flood profiles were drawn showing the computed water-surface <br />elevations for floods of the selected recurrence intervals. <br /> <br />The hydraulic analyses for this study were based on unobstructed <br />flow. The flood elevations shown on the profiles are thus <br />considered valid only if hydraulic structures remain unobstructed, <br />operate properly, and do not fail. <br /> <br />All elevations are referenced to the National Geodetic Vertical <br />Datum (NGVD) of 1929. Elevation reference marks used in this study <br />are shown on the maps; the descriptions of the marks are presented <br />in Elevation Reference Marks (Exhibit 3). <br /> <br />4.0 FLOODPLAIN MANAGEMENT APPLICATION <br /> <br />The NFIP encourages State and local governments to adopt sound <br />floodplain management programs. Therefore, each FIS provides 100-year <br />flood elevations and delineations of the 100- and SOD-year floodplain <br />boundaries and 100-year floodway to assist communities in developing <br />floodplain management measures. <br /> <br />4.1 Floodplain Boundaries <br /> <br />To provide a national standard without regional discrimination, the <br />I-percent annual chance ODD-year) flood has been adopted by FEMA <br />as the base flood for floodplain management purposes. The <br />0.2-percent annual chance (SOD-year) flood is employed to indicate <br />additional areas of flood risk in the community. For each stream <br />studied in detail, the boundaries of the 100- and SOD-year <br />floodplains have been delineated using the flood elevations <br />determined at each cross section. <br /> <br />Between cross sections, the <br />topographic maps at a scale <br />interval of 2 feet (References <br /> <br />boundaries were interpolated using <br />of 1:24,000 feet, with a contour <br />1, 2, 3, S, and 6). <br /> <br />The 100- and SOD-year floodplain boundaries are shown on the Flood <br />Insurance Rate Map. On this map, the 100-year floodplain boundary <br />corresponds to the boundary of the areas of special flood hazards <br />(Zones A, AE, AH, and AO), and the SOD-year floodplain boundary <br />corresponds to the boundary of areas of moderate flood hazards <br />(shaded Zone X). In cases where the 100- and SOD-year floodplain <br />boundaries are close together, only the 100-year floodplain <br />boundary has been shown. Small areas within the floodplain <br />boundaries may 1 ie above the flood elevations but cannot be shown <br />due to limitations of the map scale and/or lack of detailed <br /> <br />9 <br />