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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 lOO-year flood plain is divided into a floodway and a floodway fringe. The flood- way 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, pro- vided 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 presented in this study were computed on the basis of equal conveyance reduction from each side of the flood plain, The tributary streams to St. Vrain Creek are generally steep and severely encroached on by urban development. Further encroach- ment or conveyance-ability reduction would result in increased velocities above already-hazardous velocity conditions. Hazardous velocities are considered to be average overbank velocities in excess of 2 to 3 cfs. 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 flood- way boundary has been shown. Because the floodway for Dry Creek No. 1 was contained within its channel, no floodway is shown. The floodway boundaries for Spring Gulch are shown on the Flood Boundary and Floodway Map. The results of these floodway computa- tions are tabulated at selected cross sections for Spring Gulch (Table 2). No flooding is shown for Lefthand or St. Vrain Creeks in the City of Longmont as a result of the latest analyses of these drainages. Many factors affect the determination of the encroachment limits on each stream, including sand- and gravel-mining operations, over- topped bridges, development within the flood plain, and divided- flow conditions. TO determine the final floodway boundary at each cross section, characteristic velocities were considered, along with encroachment limits that cause no more than the maximum 1.0- foot rise in the flood height. Where appropriate, an equal degree of encroachment was permitted on each side of the main channel. The degree of conveyance reduction on each side of the channel was determined by hydraulic limitations. 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 between the floodway and the floodway fringe and their signif- icance to flood plain development are shown in Figure 10. 17