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4.2 Floodways Encroachment on floodplains, such as structures and fill, reduces flood-carrying capacity, increases flood heights and velocities, and increases flood hazards in areas beyond the encroachment itself. One aspect of floodl'lain management involves balancing the economic gain from floodplain development against the resulting 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 floodway is the channel of a stream, plus any adjacent floodplain areas, that must be kept free of encroachment so that the lOO-year flood can be carried without substantial increases in flood heights. Minimum Federal standards limit such increases to I 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 flood way studies. The floodways presented in this study were computed for cer;ain stream segments on the basis 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 for selected cross sections (see Table 2, "Floodway Data"). In cases where the floodway and 100-year floodplain boundaries are either close together or collinear. only the flood way boundary is shown. Downstream of Interstate 70 at Silver Plwne, no floodway encroachments were made because of high flow velocities. The area between the floodway and lOO-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 100-year flood more than I foot at any point. Typical relationships between the floodway and the floodway fringe and their significance to floodplain development are shown in Figure 1. I' l00.YEARFlOOOPlAlN fLOODWAY 1- flOOOWA.Y fRINGE S1lI:E.&..M CHANNEL LOODWAY H~IN(jr ROQOHEVATION WIiEN CONFINED WITHIN FLOOOWAY FLQODELEVATION 8EFOFlE ENCROACHMENT OPII FLOODPLAIN UNE A8 IS THE flOOD ELEVATION SHORE ENC!\OACHMENT. LINE COIS THE fLOOD ELEvATION AFTER ENCROACHMENT. -'iUIKHARGE IS NOT TO ExceeD 1.0 fOOT (FIA REQUIREMENT) OR LtSS~R AMOUNT IF SPECIFIED liv STAn: Figure I. Floodway Schematic 7