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The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the profiles are thus con- sidered valid only if hydraulip structures remain unobstructed, operate properly; and do not f~il. All elevations are referenced :to the National Geodetic vertical Datum of 1929 (NGVD). Elevation reference marks used in this study are shown on the maps. 4,0 FLOOD PLAIN MANAGEMENT APPLICATIONS The NFIP encourages State and local sovernments to adopt sound flood plain management programs. Therefor'e, each Flood Insurance Study pro- duces maps designed to assist communities in developing flood plain man- agement measures, 4.1 Flood Plain Boundaries To provide a national standarq without regional discrimination, the 1 percent annual chance (~OO-year) flood has been adopted by FEMA as the base flood for flqod plain management purposes. The 0,2 percent annual chance (500-year) flood is employed to indicate additional areas of flood ris~ in the community. For each stream studied in detail, the 100- arid 500-year flood plain boundaries have been delineated using the flood elevations determined at each cross section. Between cross isections, the boundaries were inter- polated using topographic map~ at scales of 1:4,800 and 1:6,000, with contour intervals of 2 a~d 10 feet, respectively ~References 6 and 21, respectively). The 100- and 500-year flood p~ain boundaries are shown on the Flood Boundary and Floodway Map (Exhibit 2), In cases where the 100- and 500-year flood plain boundaries are close together, only the 100-year flood plain boundary 'has been shown. Small areas within the flood plain boundaries may lie above the flood elevations but cannot be shown due to limitations of the map scale and/or lack of detailed topographic data. Floodplain boundary delineations have been revised to present the lastest information available; as shown on topographic work maps included with flood plain information reports for Lefthand Creek, Dry Creek NO.1, and st, Vrain Creek (References 1-4), 4,2 Floodways Encroachment on flood plains, :such as structures and fill, reduces flood-carrying capacity, incr~ases flood heights and velocities, and increases flood hazards i~ areas beyond the encroachment itself, One aspect of flood plain man4gement involves balancing the economic gain from flood plain develop~ent against the resulting increase 16