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floodplain boundaries may lie above the be shown due to limitations of the detailed topographic data. flood elevations but cannot map scale and/or lack of Approximate lOa-year floodplain boundaries in some portions of the study area were taken directly from the Flood Insurance Rate Map for the Town of Deer Trail, Colorado (Reference 59). For the streams studied floodplain boundary is (Exhibit 2). by approximate methods, only the shown on the Flood Insurance lOa-year Rate Map 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 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 kept free of encroachment so that the lOa-year flood can be carried without substantial increases in flood height s. Minimum Federal standards limi t 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 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 2). The computed floodways are shown on the Flood Insurance Rate Map (Exhibit 2). 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 lOO-year flood by 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 8. 24