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UDFCD (Reference 53). The boundary portions of Hidden Lake Drainageway was for that drainage system (Reference 46). of the lOO-year flood for taken from the UDFCD study 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 lOO- 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 l.O 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 City of Brighton, the City of Thornton, and Adams County have ordinances which limit the increase in flood heights to 0.5 foot above the pre-floodway elevation; therefore, floodways having no more than a 0.5-foot surcharge have been delineated for these communities. Floodways were not computed along through the City of Federal Heights the scope of this study. Tributary M of Niver because they were not Creek within The lOO-year floodflows on Brantner Gulch are basically confined to the channel; therefore, the floodway corresponds to the lOO-year flood boundary. 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 lOO-year floodplain boundaries are either close together or collinear, only the floodway boundary is shown. 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 lOO-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 17. 34