FLOOD10780 (3) - Laserfiche WebLink

Home Browse Search

cross section loc.at ions are also shown on the Flood Boundary and Floodway Map (Exhibit 2). Roughness factors (Manning's "n") used in the hydraulic computations for the study streams were chosen by engineering judgment and based on field observations of the flooding sources and floodplain areas. The roughness values used for the streams studied in this report are tabulated below. Rou2hness Coefficients Stream Main Channel Overbank Gunnison River 0.032 to 0.05 0.04 to 0.10 North Fork Gunnison River 0.04 to 0.07 0.04 to 0.11 Uncompahgre River 0.035 0.04 to 0.06 Minnesota, Creek 0.017 to 0.05 0.035 to 0.05 Surface Creek 0.017 to 0.04 0.035 to 0.07 Starting water-surface elevations for the Gunnison and Uncompahgre Rivers and Minnesota and Surface Creeks were determined by the slope-area method. For North Fork Gunnison River, the starting water-surface elevations were obtained from the HEC-2 analysis of Gunnison River. Flood profiles were drawn showing computed water-surface elevations to an accuracy of 0.5 foot for floods of the selected recurrence intervals (Exhibit 1). The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the flood profiles are thus considered valid only if the hydraulic structures assumed to stay in place remain unobstructed, operate properly, and do not fail. The approximate IOO-year flood elevations for Minnesota Creek and Cedar Run were determined by normal-depth calculations of typical sections based on field observation. All elevations are referenced to the National Geodetic Vertical Datum of 1929 (NGVD). Elevation reference marks, and descriptions used in the study are shown on the maps. 4.0 FLOODPLAIN MANAGEMENT APPLICATIONS The National Flood Insurance Program encourages State and local govern- ments to adopt sound. floodplain management programs. Therefore, each Flood Insurance Study includes a flood boundary map designed to assist communities in developing sound floodplain management measures. 4.1 Flood Boundaries In order to provide a national discrimination, the lOO-year flood standard without has been adopted regional by the 23