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<br />For the streams studied by approximate methods, only the lOO-year floodplain boundary is <br />shown on the Flood Insurance Rate Map (Exhibit 2). <br /> <br />Approximate lOO-year floodplain boundaries in some portions of the study area were taken <br />directly from the Flood Insurance Rate Map for the unincorporated areas of Weld County, <br />Colorado, dated September 28, 1982 (Reference 9). <br /> <br />4.2 Floodways <br /> <br />Encroachment on floodplains, such as structures and fill, reduces flood'carrying capacity, <br />increases flood heights and velocities, and increases flood hazards in areas beyond the <br />encroachment itself. One aspect of floodplain management involves balancing the economic <br />gain from floodplain development against the resulting increase in flood hazard. For purposes <br />of the NFIP, a floodway is used as a tool to assist local communities in this aspect of <br />floodplain management. Under this concept, the area of the 1000year floodplain is divided <br />into a floodway and a floodway fringe. The floodway is the channel of a stream, plus any <br />adjacent floodplain areas, that must be kept free of encroachment so that the 1000year flood <br />can be carried without substantial increases in flood heights. Minimum Federal standards <br />limit such increases to I foot, provided that hazardous velocities are not produced. The <br />floodways in this study are presented to local agencies as minimum standards that can be <br />adopted directly or that can be used as a basis for additional floodway studies. <br /> <br />1l1e floodways presented in this study were computed for certain stream segments on the basis <br />of equal-conveyance reduction from each side of the floodplain. Floodway widths were <br />computed at cross sections. Between cross sections, the floodway boundaries were <br />interpolated. The results of the floodway computations are tabulated for selected cross <br />sections (see Table 2, "Floodway Data"). In cases where the floodway and IOO'year <br />floodplain boundaries are either close together or collinear, only the floodway boundary is <br />shown, <br /> <br />The area between the floodway and 1000year floodplain boundaries is termed the floodway <br />fringe. The floodway fringe encompasses the portion of the floodplain that could be <br />completely obstructed without increasing the water,surface elevation of the lOO'year flood <br />more than I foot at any point. Typical relationships between the floodway and the floodway <br />fringe and their significance to floodplain development are shown in Figure I. <br /> <br />5.0 INSURANCE APPUCATION <br /> <br />For flood insurance rating purposes, flood insurance zone designations are assigned to a community <br />based on the results of the engineering analyses. These zones are as follows: <br /> <br />Zone A <br /> <br />Zone A is the flood insurance rate zone that corresponds to the 1000year floodplains that are <br />determined in the Flood Insurance Study by approximate methods. Because detailed hydraulic <br />analyses are not performed for such areas, no base (IOO-year) flood elevations (BFEs) or depths are <br />shown within this zone. <br /> <br />6 <br />