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I I I I I I I I I I I I I I I I I I I Ju(v 1973. The 1921 floodflow reached reach a flow of2.000 cf~ at Ouray, 2.400 cft beloll' Ouray and 4,080 cft at Colona. 3.0 ENGINEERING METHODS Surveying and Mapping A blueprint of the original mapping for the project was obtained from CWCB files for the Flood Plain Information Report (Reference I). The following description was provided in the report. Aerial photograph and topograhic mapping for this study were provided hy Ouray County in cooperation with the CWCB. The mapping was prepared hy Pafford and Associates of Los Angeles, California and A&S Consultants. The mapping is at a a scale of 1" = 200': the contour interval is 5 feet, with l' intervals dashed in the area in and around the Towns of Ridgway and Ouray. One hundred thirteen digitized cross sections on the Uncompahgre River were selected by A&S Consultants. These cross sections are shown on the mapping and are designated hy integer numhers. In addition, Mr. Larry Lang conducted a field investigation and provided guidance on the selection of Manning's n-values for the HEC-2 model. Hydrologic Analysis The Flood Insurance Study for the City of Ouray (Reference 5) provided the following description of the hydrologic analysis. Hydrologic ana(l'ses were carried out to establish the peak discharge.frequency relationships for each flooding source sflldied in detail affecting the community. Uncompahgre River floodflow ft'equency data were based on statistical ana(l'ses of streanifloll' records covering a 54-year period at gaging stations operated by the u.s. Geological Survey. These ana(l'ses followed the standard log-Pearson Type III method as omlined by the U.S. Water Resources Council. Peak discharge-drainage area relationships for the Uncompahgre River are show in Tahle 1. 3