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<br />(Section 4.2), selected cross section locations are also shown on the Flood Insurance Rate <br />Map (Exhibit 2). <br /> <br />The hydraulic analyses for this study were, based on unobstructed flow. The flood <br />elevations shown on the profiles are thus considered valid only if hydraulic structures <br />remain unobstructed, operate properly, and do not fail. <br /> <br />All elevations are referenced to the National Geodetic Vertical Datum of 1929 (NGVD). <br />Elevation reference marks (ERMs) and their descriptions are shown on the maps. ERMs <br />shown on the FIRM represent those used during the preparation of this and previous Flood <br />Insurance Studies. The elevations associated with each ERM were obtained andlor <br />developed during FIS production to establish vertical control for determination of flood <br />elevations and floodplain boundaries shown on the FIRM. Users should be aware that <br />these ERM elevations may have changed since the publication of this PIS. To obtain up- <br />to-date elevation information on National Geodetic Survey (NGS) ERMs shown on this <br />map, please contact the Information Services Branch of the NGS at (301) 713-3242, or <br />visit their website at www.ngs.noaa.gov. Map users should seek verification of non-NGS <br />ERM monument elevations when using these elevations for construction or floodplain <br />management purposes. <br /> <br />4.0 FLOODPLAIN MANAGEMENT APPLICATIONS <br /> <br />The NPIP encourages State and local governments to adopt sound floodplain management <br />programs. To assist in this endeavor, each PIS pro'{ides loo-year floodplain data. which may <br />include a combination of the following: 10-, 50-,100-, and 50o-year flood elevations; delineations <br />of the loo-year and 500-year floodplains; and 100-year floodway. This information is presented <br />on the PIRM and in many components of the PIS, including Flood Profiles, Floodway Data tables <br />and Summary of Stillwater Elevation tables, Users should reference the data presented in the FIS <br />as well as additional information that may be available at the local community map repository <br />before making flood elevation andlor floodplain boundary determinations. <br /> <br />4.1 Floodplain Boundaries <br /> <br />To provide a national standard without regional discrimination, the I-percent annual <br />chance (IOO-year) flood has been adopted by FEMA as the base flood for floodplain <br />management purposes. The 0.2-percent annual chance (500-year) flood is employed to <br />indicate additional areas of flood risk in the community. For each stream studied by <br />detailed methods, the 100- and 5oo-year floodplain boundaries have been delineated using <br />the flood elevations determined at each cross section. Between cross sections, the <br />boundaries were interpolated using topographic maps at a scale of I" = 1 00 feet, with a <br />contour interval of 2 feet (Reference 6). <br /> <br />The 100- and 5oo-year floodplain boundaries are shown on the Flood Insurance Rate Map <br />(Exhibit 2). On this map, the lOO-year floodplain boundary corresponds to the boundary <br />of the areas of special flood hazards (Zones A, AE, and AH), and the 5oo-year floodplain <br />boundary corresponds to the boundary of areas of moderate flood hazards. In cases where <br />the 100- and 500-year floodplain boundaries are close together, only the loo-year <br />floodplain boundary has been shown. Small areas within the floodplain boundaries may <br />lie above the flood elevations but cannot be shown due to limitations of the map scale <br />andlor lack of detailed topographic data. <br /> <br />6 <br />