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<br />and increases flood hazards in areas beyond the encroachment <br />itself. One aspect of floodplain mliLnagement involves balancing the <br />economic gain from floodplain development against the resulting <br />increase in flood hazard. For purposes of the NFIP, a floodway is <br />used as a tool to assist local communities in this aspect of <br />floodplain management. Under thi s concept, the area of the 100- <br />year floodplain is divided into a floodway and a floodway fringe. <br />The floodway is the channel of a stream, plus any adjacent <br />floodplain areas, that must be kept free of encroachment so that <br />the lOO-year flood can be carried without substantial increases in <br />flood heights. Minimum Federal standards limit such increases to <br />1.0 foot, provided that hazardous velocities are not produced. The <br />f100dways in this study are presented to local agencies as minimum <br />standards that can be adopted directly or that can be used as a <br />basis for lildditiona1 floodway studies. <br />, <br /> <br />,. <br /> <br />The floodways presented in this study were computed for certain <br />stream segments on the basis of E!qua1 conveyance reduction from <br />each side of the floodplain. F100dwliLY widths were computed at cross <br />sections. Between cross sections, the f100dway boundaries were <br />interpolated. The results of the f100dway computations are <br />tabulated at selected cross sections (Table 2). In cases where the <br />floodway and 100-year floodplain boundaries are either close <br />together or collinear, only the f100dway boundary has been shown. <br /> <br />The area between the floodway and lOO-year floodplain boundaries is <br />termed the floodway fringe. The floodway fringe encompasses the <br />portion of the floodplain that c:ou1d be completely obstructed <br />without increasing the water-surfliLce elevation of the 100-year <br />flood more than 1.0 foot at any point. Typical relationships <br />between the floodway and the f100dwliLY fringe and their significance <br />to floodplain development are shown in Figure 2. <br /> <br />It( <br /> <br />100-YEAR FlOODPUlIIN <br /> <br />-I <br /> <br />t( <br /> <br />FlOODWA Y <br />FRINGE <br /> <br />.II( <br /> <br />FlOODWAY <br /> <br />lOODWAY <br />FRINGE <br /> <br />STREAM -+- <br />CHANNEL <br /> <br />FLOOD ELEVATION WHEN <br />CONFINED WITHIN FlOODWA Y <br /> <br /> <br />ENCROACHMENT <br /> <br />ENCROACHMENT <br /> <br />AREA OF FLOODPLAIN THAT COULD BE USED <br />FOR DEVELOPMENT BY RAISING GROUND <br /> <br />FLOOD ELEVATION BEFORE <br />ENCROACHMENT ON FLOODPLAIN <br /> <br />LINE AB IS THE FLOOD ELEVATION BEFORE ENCROACHMENT. <br />LINE CD IS THE FLOOD ELEVATION AFTER ENCROACHMENT. <br />'~)URCHARGE IS NOT TO EXCEED 1.0 FOOT (FIA REQUIREMENT) OR LESSER AMOUNT IF SPECIFIED BY ST ATE. <br /> <br />Figure 2. Floodway Schematic <br /> <br />II <br /> <br />/ ! <br />