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and increases flood hazards in areas beyond the encroachment itself. One aspect of floodplain management involves balancing the economic gain from floodplain developmen't against the resul t ing increase in flood hazard. For purposes of the NFIP, 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 floodplain is divided into a floodway and a floodway fringe. The floodo/ay is the channel of a stream, plus any adjacent floodplain areas, that must be kept free of encroachment so that the lOa-year flood can be carried without substantial increases in flood heights. Minimum Federal standards limit such increases to 1.0 foot, provided that hazardous velocities are not produced. The floodways in this study are presented to local agencies as minimum standards that can be adopted directly or that can be used as a basis for additional floodway studies. The floodways presented in this study were computed for certain stream segments on the basi.s of equal conveyance reduction from each side of the floodplain. Floodway widths were computed at cross sections. Between cross sections, the floodway boundaries were interpolated. The results of the floodway computations are tabulated at selected cross sections (Table 2). In cases where the floodway and lOa-year floodplain boundaries are either close together or collinear, only the floodway boundary has been shown. The area between the floodway and lOa-year floodplain boundaries is termed the floodway fringe. The floodway fringe encompasses the portion of the floodplain that could be completely obstructed without increasing the water-surface elevation of the lOa-year flood more than 1.0 foot at any point. Typical relationships between the floodway and the floodway fringe and their significance to floodplain development are shown in Figure 2. I~ 100. YEAR FLOODPLAIN .1 .. FLOODWAY FRINGE . FLOODWAY FLOODWAY FRINGE STREAM _ CHANNEL FLOOD ELEVA nON WHEN CONFINED WITHIN FLOODWAY ENCROACHMENT FLOOD ELEVATION BEFORE ENCROACHMENT ON FLOODPLAIN C ,. ............... .................. . .~:.:.::.::::::.::.:::.::::.:::::::.:.::::.. -- ~.~..':":"'.~..~.~..~.~..~:. - - - .................. ........... ................ .......... ........... ........... AREA OF FLOODPLAIN THAT COULD BE USED FOR DEVELOPMENT BY RAISING GROUND LINE AB IS THE FLOOD ELEVATION BEFORE ENCROACHMENT. LINE CD IS THE FLOOD ELEVATION AFTER ENCROACHMENT. .SURCHARGE IS NOT TO EXCEED 1.0 FOOT (FIA REOUIREMENT) OR LESSER AMOUNT IF SPECIFIED BY STATE. Figure 2. Floodway Schematic 11