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4.2 Floodways Encroachment on floodplains, such as structures and fi 11, reduces flood-carrying capacity, increases flood heights and velocities, and increases flood hazards in areas beyond the encroachment itself. One aspect of floodplain 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 k.ept 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. For Indian Wash, floodways were not computed for areas of 100-year sheetflow diversions. However, the complete filling of these sheetflow areas may increase flood flows downstream, thereby increasing the lOa-year flood hazard. For any channel modification or fill in the sheetflow areas, a detailed review of the improvement should be undertak.en to determine the effects downstream. The floodways presented in this study were computed for certain 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 (Table 3). In cases where the floodway and lOa-year floodplain boundaries are either close together or collinear, only the floodway boundary is 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 7. For the streams studied floodplain boundary is (Exhibit 2). by approximate methods, only the lOa-year shown on the Flood Insurance Rate Map 20