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referenced to the National Geodetic Vertical Datum of 1929 (NGVD). Elevation reference marks used in the study are shown on the maps. 4.0 FLOODPLAIN MANAGEMENT APPLICATIONS A prime purpose of the National Flood encourage state and local governments to management programs. Each Flood Insurance a flood boundary map designed to assist sound floodplain management measures. Insurance Program is to adopt sound floodplain Study, therefore, includes communities in developing 4.1 Flood Boundaries In order to provide a national standard without regional discrimination, the 100-year flood has been adopted by the Federal Insurance Administration as the base flood for purposes . ------of--Hfloodplain---management meas.ures.~---- _TheSOO--ye=-~:fLoo.d -. is employed to indicate additional areas of flood risk in the community. For each stream studied in detail, the boundaries of the 100-year, and the 500-year floods have been delineated using the flood elevations determined at each cross section; between cross sections, the boundaries were interpolated by photogrammetric methods using topographic maps from aerial photos at a scale of 1:6,000, with a contour interval of 2 feet (References 3 and 6). Areas of shallow sheet flow outside the aerial photomapping limits were delineated using a topographic map at a scale of 1:24,000 with a contour interval of 10 feet (Reference 7). Flood boundaries are indicated on the Flood Insurance Rate Map (Panel 0005). In this map, the 100-year boundary corresponds to the boundary of the areas of special flood hazards (Zones A6, A7, A9, AlO, A12, and A13); and the 500-year boundary corresponds to the boundary of areas of moderate flood hazards (Zone B). In cases where the 100-year and 500-year flood boundaries are close together, only the 100-year boundary has been shown. Small areas within the flood boundaries may lie above the flood elevations, and therefore, not be subject to flooding; owing to limitations of the map scale, such areas are not shown. 4.2 Floodways Encroachment on floodplains, such as artificial fill, reduces the flood-carrying capacity and increases flood heights, thus increasing flood hazards in areas beyond the encroachment itself. One aspect of floodplain management involves balancing the economic gain from floodplain development against the resulting increase in flood hazard. For purposes of the Flood Insurance Program, the concept of a floodway is used as a tool to assist local commtlnitics in this aspect of floodplain manag t 1S concept, t.e area 0 e -year 00 1S v floodway and a floodway fringe. The floodway is the channel of a 12