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year flood plain boundaries are close together, only the 100-year flood plain boundary has been shown. Small areas within the flood plain boundaries may lie above the flood elevations but cannot be shown due to limitations of the map scale and/or lack of detailed topographic data. For the streams studied by approximate methods, only the lOa-year flood plain boundary is shown. 4.2 r'.JOdways 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. No floodway was computed for the Uncompahgre River as part of this study. Work submitted by the study contractor did not meet FEMA requirements for establishing a floodway. Regulatory elevations computed at selected cross sections used in the hydraulic analyses (Section 3.2) are shown in Table 2. 5.0 INSURANCE APPLICATION TO establish actuarial insurance rates, dat~ from the engineering study must be transformed into flood insurance criteria. This process includes the determination of reaches, Flood Hazard Factors, and flood insurance zone designations for each flooding source studied in detail affecting the unincorporated areas of OUray County, Colorado. 5.1 Reach Determinations Reaches are defined as sections of flood plain that have relatively the same flood hazard, based on the average weighted difference in water-surface elevations between the 10- and lOa-year floods. This difference may not have a variation greater than that indicated in the following table for more than 20 percent of the reach: Average Difference Between 10- and laO-Year Floods Variation Less than 2 fee t 2 to 7 feet 7.1 to 12 feet More than 12 feet 0.5 foot 1. 0 foot 2.0 feet 3.0 feet The locations of the reaches determined for the flooding sources of Ouray County are shown on the Flood Profiles (Exhibit 1) and summarized in Table 3. 11