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All elevations are referenced to ~he National Geodetic Vertical Datum of 1929 (NGVD). ~leva~ion ~eference marks used in the study are ~hown on the maps. 1,0 FLOODPLAIN MANAGEMENT A,PPLICA~'IONS A prime purpose of the National Flood Insurance Program is to encourage State and local governments ~o adopt sound floodplain management programs. Each Flood In:>llrance Study, therefore, includes a flood boundary map designed to assist communities in developing sound floodplain management measure:>. 4.1 Flood Boundaries In order to prqvide a nationa:! standar-d w:il:hout I:egional discrimination, the lOO-year flooB has been pdopted by the Federal Emergency Management Agency as the base flood for purposes of floodplain management measures. The SOD-year flood is employed to indicate additional areas of flood risk in the communi ty. For each st.ream stude! ed in detai 1" the boundar i es of the 100- and SOO-year floods have been delineated using the flood elevations determined at each cross section; between cross sections, the boundaries were interpolated using topographic maps at a scale of 1:~200 and 1:600, with a contour interval of 2 feet (Reference 7). In cases where the 100- and SOD-year flood boundaries are close together, only the 100-year flood boundary has been shown. Flood boundaries 'for the 100- and :;Oa-year floods are shown on the Plood Boundary and Floodway Map (Exhibit 2). Small areas within the flood boundacles 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, ~lUch as artificial fill, reduces the flood-carrying capacity and increases flood heights, thus increasing flood hazards in area~; beyond the encroachment itself. One aspect of floodplain management involves balancing the economic gain from floodplain development against the resul ting increase in flood hazard. For purposes of the National Flood Insurance Program, loh'e concept of a floodway is used as a tool to assist local communities in this aspect of floodplain management. Under this concept, the area of the 100-year flood is divided into a floodway ~nd a floodway fringe. The floodway is the channel of a stream, plus any adjacent floodplain areas,- that must be kept free of encroachment in order that the 100-year flood be carried without substantial increases in flood heights. As minimum standards, the Federal Emergency ~'[anagement Agency 1 imi ts such increases in flood heights to 1.0 ~oot, provided that hazardous velocities are not produced. -6-,