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For the streams studied by approximate methods, only the lOO-year floodplain boundary is shown on the Flood Insurance Rate Map (Exhibit 2), Approximate lOO-year floodplain boundaries in some portions of the study area were taken directly from the Flood Insurance Rate Map for the unincorporated areas of Weld County, Colorado, dated September 28, 1982 (Reference 9). 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 1000year 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 1000year 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 floodway studies. 1l1e 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 (see Table 2, "Floodway Data"). In cases where the floodway and )()()-year floodplain boundaries are either close together or collinear, only the floodway boundary is shown. The area between the floodway and 1000year 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 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 I, 5.0 INSURANCE APPUCATION For flood insurance rating purposes, flood insurance zone designations are assigned to a community based on the results of the engineering analyses. These zones are as follows: Zone A Zone A is the flood insurance rate zone that corresponds to the 1000year floodplains that are determined in the Flood Insurance Study by approximate methods, Because detailed hydraulic analyses are not performed for such areas, no base (IOO-year) flood elevations (BFEs) or depths are shown within this zone. 6