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I.' I I I I I I I I I I I I I I I I I I . . ~4- Phase I Task 1 - Because the Sheriff Dam and Reservoir are classified as high hazard, the SEO requires that the reservoir have a spillway capable of safely passing the probable maximum flood (PMF) resulting from the probable maximum precipitation (PMP) in the catchment above the dam. The engineering analy- sis methods used to determine the PMF for a given catchment are sensitive to assumptions made concerning the basin charac- teristics. Hence, spillway modification requirements for Sheriff Reservoir will be sensitive to basin characteristics used in the hydrologic evaluations. The previously completed COE Phase I Inspection Report i~cluded hydrologic evaluations which were based on analysis methods outlined in the U.S. Bureau of Reclamation's "Design of Small Dams" (DSD). Based on our preliminary review of the Phase I Inspection Report, it is our opinion that the study did not consider several impor- tant factors related to the catchment above the reservoir, including: 1. Heavy basin vegetation, 2. Geologic and other site conditions affecting runoff, and 3. Actual records of basin runoff. We will consider these factors in our analyses. Because these factors were not considered in the early study of the COE, we believe that the work completed under this task could result in a reduction in the required pMF magnitude. These factors are discussed in greater detail in a subsequent section of this proposal. Conversely, it should be noted that a recently released state-of-the-art rainfall document for Colorado, HMR-49, published by the National Weather Service, was not available at the time of the 1980 study. Based on DSD and HMR-49 docu- ments, the rainfall intensities and temporal storm distribu- tions may be different. The information contained in HMR-49 will likely outline higher total PMP rainfall values for use in determining PMF flows. It will be important to include the new HMR-49 information in our evaluations, in order to satisfy current SEO mandates. Task 2 - The reservoir is located on property managed by the White River National Forest. The Forest Service property extends approximately 6 miles downstream and privately owned properties do not begin until downstream of that point. Design regulations promulgated by the SEO allow owners of dams to perform "Incremental Damage Assessments" to evaluate incre- mental downstream effects of the failure of a dam. Provided