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.. .. ~.~.-- The computational methods and techniques used are accepted procedures for hydrologic analyses, and produced results considered reasonable for the Rangely area. Unexpected findings were not encountered in carrying out the hydrologic analyses for this Flood Insurance Study. The Frequency-Discharge, Drainage Area Curves for the White River and the other streamways under study are shown in Figure 2. 3.2 Hydraulic Analyses Analyses of the hydraulic characteristics of streams in the community are carried ou.t to provide estimates of the elevations of floods of the selected recurrence intervals along each stream studied in detail. Flood elevations were comput:ed through use of the U. S. Army Corps of Engineers HEC-2 step-backwater computer program (Reference 4). Cross section data for backwater analysis were obtained from topo- graphic maps at a scale of 1:2400 feet, with a contour interval of 2 feet (Reference 5). The State Highway 64 bridge over the White River and the culverts in Dragon Wash at White Avenue and near the community ball park were field surveyed to obtain elevation data and structural geometry. Roughness coefficients (Manning's "n") were determined by field observa1:ion. The coefficients used were 0.040 for the main channel and 0.060 for overbank areas on the White River, and 0.040 for both the main streamways and overbank areas of Dragon and St. Timothy Washes and High School Ravine. Flood pl~ofiles were drawn showing computed water-surface elevations to an accuracy of 0.5 foot for floods of the selected recurrence intervals (Exhibit 1). Stage data for the White River were based on 51 years of flow data and available historical flood data. Stage data for Dragon Wash were based on analyses of drainage basins having similar hydrological characteristics. .A. normal. depth computa- tion was used t.O determine starting water-surface elevations for all flood events. Hydraulic analyses of ice jam data for the White River indicate that ice jam events would result in water-surface elevations approximating the lOa-year flood. Computed elevations in areas studied by approximate methods were determined using normal depth analysis. All elevations are referenced to the National Geodetic vertical Datum of 1929. Elevation reference marks used in the study are shown on the maps. The hydraulic analyses in this study reflect unobstructed flow. Flood e~evations, as shown on the profiles, are thus considered valid only if the State Highway 64 bridge and the culverts on Dragon Wash remain unobstructed. 9 ...