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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 100-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 100-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 floodway for Brotherton Gulch was computed on the basis of equal-conveyance reduction from each side of the flood plain. For Fortification Creek, the floodway presented in this study was developed by confining it to channel areas and smoothing it through sharp meanders. The results of these computations are tabulated at selected cross sections for each stream segment for which a floodway is computed (Table 2). No floodways were computed for Cedar Mountain Gulch through the study limits and for Pine Ridge Gulch downstream of State Highway 13 due to 100-year sheet flow diversions from Cedar Mountain Gulch and Pine Ridge Gulch, and due to the complex backwater conditions between Pine Ridge and Cedar Mountain Gulches created by the triple box culvert under State Highway 13 (County Road 7). The complete filling of these sheet flow areas will increase floodflows downstream thereby increasing the 100-year flood hazard. For any channel modification or fill in the sheetflow areas, a detailed review of the improvement should be undertaken to determine the effects downstream. As shown on the Flood Boundary and Floodway Map (Exhibit 2), the floodway boundaries were computed at cross sections. Between cross sections, the boundaries were interpolated. In cases where the floodway and 100-year flood plain boundaries are either close together or collinear, only the floodway boundary has been shown. The area between the floodway and 100-year flood plain boundaries is termed the floodway fringe. The floodway fringe encompasses the portion of the flood plain that could be completely obstructed without increasing the water-surface elevation of the 100-year flood by more than 1.0 foot at any point. Typical relationships 13