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, i i 4.1 Flood Boundaries I I I In order'to provide a national standard without regional discrimina- tion, the lOO-year flood has been adopted by the Federal Insurance Administration as the base flood for purposes of flood plain ;;; managemeI[1t measures. The SOO-year flood is employed to indicate additional areas of flood risk in the community. For each stream studied ~n detail, the boundaries of the 100- and SOO-year floods have beem delineated using the flood elevations determined at each cross se~tioni between cross sections, the boundaries were interpo- lated using topographic maps at scales of 1:1,200 and 1:24,000 i (enlarged to a scale of 1:6,000), with contour intervals of 2 and i 40 feet (References 8, 9, and 10). i In cases Iwhere the 100- and SOD-year flood boundaries are close together, only the 100-year flood boundary has been shown. I Shallow ~looding boundaries were delineated on topographic maps at a scale df 1:1200 (References 8 and 9). ., For the stream studied by approximate methods, the boundary of the 100-year IfloOd was taken from the Flood Hazard Boundary Map for the Town of ~risco, produced in October 1976 (Reference 11). The stream cliannel has been relocated south of Belford Street near Eighth A~enue. The new stream location was obtained from a school site pla~ dated July 1978 (Reference 12). The Flood Hazard Boundary Map floog boundary was revised accordingly. I I I Flood borlndaries for the 100- and SOD-year floods are shown on the Flood BO~ndary and Floodway Map (Exhibit 2). I Small areias wi thin the flood boundaries may lie above the flood i elevations and, therefore, not be subject to flooding; owing to I limitatiops of the map scale, such areas are not shown. 4.2 F 100dwaysI i Encroachm~nt on flood plains, such as artificial fill, reduces the i flood-car~ying capacity and increases flood heights, thus increasing flood haz~rds in'areas beyond the encroachment itself. One aspect of flood plain management involves balancing the economic gain from floo~ plain development against the resulting increase in flood hazard. For purposes of the National Flood Insurance Program, the concept of a floodway is used as a tool to assist local communities in this akpect of flood plain management. Under this concept, the I area of the lOa-year flood is divided into a floodway and a floodway fringe. rhe floodway is the channel of a stream, plus any adjacent flood pla~n areas, that must be kept free of encroachment in order that the ~OO-year flood be carried without substantial increases in flood heights. As minimum standards, the Federal Insurance Administr1tion limits such increases in flood heights to 1.0 foot, provided that hazardous velocities are not produced. .'": 7