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<br />4,2 Floodways <br /> <br />Encroachment on flood plains, such as artificial fill, reduces the <br />flood-carrying capacity and increases flood heights, thus increasing <br />flood hazards in areas beyond the encroachment itsel f. One aspect of <br />flood plain management involves balancing the economic gain from flood <br />plain development against the resulting increase in flood hazard. For <br />purposes of the National Flood Insurance Program, the concept of a <br />floodway is used as a tool to assist local communites in this aspect <br />of flood plain management. Under this concept, the area of the <br />lOO-year flood is divided into a floodway and a floodway fringe. The <br />floodway is the channel of a stream, plus any adjacent flood plain <br />areas, that must be kept free of encroachment so that the lOO-year <br />flood may be carried without substantial increase in flood heights. <br />Minimum standards of the Federal Emergency Management Agency 1 imit <br />such increases in flood heights to 1.0 foot, provided that hazardous <br />velocities are not produced. The floodways are presented to local <br />agencies as minimum standards that can be adopted or that can be used <br />as a basis for additional studies. <br /> <br />The area between the fl oodway and the boundary of the lOO-year fl ood <br />is tenned the floodway fringe. The floodway fringe thus encompasses <br />the portion of the flood plain that could be completely obstructed <br />without increasing the water-surface elevation of the lOO-year flood <br />more than 1.0 feet at any point. Typical relationships between the <br />floodway and the floodway fringe and their significance to flood plain <br />development are shown in Figure 2. <br /> <br />I- <br /> <br />tOO.YEAR FLOOD PLAIN <br /> <br />-I <br /> <br />FLOQDWA Y <br />FRINGE <br /> <br /> <br />FLOODWAY <br /> <br />FLOOOWAV <br />FRINGE <br /> <br />STREAM <br />CHANNEL <br /> <br />FLOOD ELEVATION WHEN <br />CONFINED WITHIN FLOOQWAY <br /> <br /> <br />AREA OF FLOOD PLAIN THAT COULD <br />BE USED FOR DEVELOPMENT BY <br />RAISING GROUND <br /> <br />FLOOD ELEVATION <br />BEFORE ENCROACHMENT <br />ON FLOOD PLAIN <br /> <br />LINE AS IS THE FLOOD ELEVATION BEFORE ENCROACHMENT. <br />LINE CD IS THE FLOOD ELEVATION AFTER ENCROACHMENT. <br />.SURCHARGE IS NOTTO ExceeD 1.0 FOOT (FEMA REQUIREMENT) OR LESSER AMOUNT IF SPECIFIED BY STATE. <br /> <br />Floodway Schematic Figure 2. <br /> <br />8 <br />