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4.2 Floodways Encroachment on flood plains, 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 flood' plain management involves balancing the economic gain from flood plain 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 flood plain management. Under this concept, the area of the lOa-year flood plain is divided into a floodway and a floodway fringe. The floodway is the channel of a stream, plus any adjacent flood plain areas, that must be kept 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. The floodways for this study were divided into two categories based upon the location of the stream. The mountain area criteria were based upon the channel of the stream plus all flood plain areas where the depth of flooding was 18 inches or greater and the floodway fringe area was the area where flooding depths were 18 inches or less and velocities were 3 feet per second or less (Figure 7). The plains area criteria were based upon the criteria of equal- conveyance reduction from each side of the flood plain with a O.S-foot limitation on flood height, provided that hazardous velocities are not produced (Figure 8). The floodways for the Big Thompson River upstream of Lake Estes and Black Canyon Creek were based upon the criteria of equal conveyance reduction from each side of the flood plain with a 1.0-foot limitation of increased flood height, provided that hazardous velocities are not produced (Figure 8). The floodway for the Fall River was based upon the criteria of equal conveyance reduction from each side of the flood plain with a 1.O-foot limitation on increased energy grade line, It was decided for portions of the Dry.Creek floodway to permit encroachment to a point where the maximum rise in the lOa-year water-surface elevation would be no greater than 1.0 foot as computed in the Dry Creek report (Reference 1). The results of these computations are tabulated at selected cross sections for each stream segment for which a floodway is computed (Table 2). 22