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i zation Reservoir Site Character Probable Maximum Flood A recent report (Harza, 1991) documents the PMF for this site as an inflow flood peak of 40,600 cfs. This report was approved by the Colorado State Engineer's Office (SEO) on July 8, 1991, and, based on recent communications with the SEO, is considered current. That report is therefore accepted at face value as the approved inflow design flood documentation without further evaluation. An earlier PMF report (CDOW, 1984) provides valuable reference material but has not specifically been used herein and has been superseded by the Harza report. These reports were prepared in response to a judgment made from the 1979 Phase I National Dam Safety Program inspection that the existing dam had inadequate spillway capacity to pass the PMF without overtopping. These three hydrologic analyses and several related reports (M- K, 1985a; 1985b, 1986) were prepared addressing both increased storage capacity for various purposes and increased spillway capacity to handle the PMF. While a design was even completed for one of the options, none were ever implemented. The issue of inadequate spillway capacity remains unresolved and integral to the enlargement study currently underway. of these two most recent PMF estimates Table 3-1 resents a tabulation summar . p y One-Hundred Year Flood r The PMF represents the upper limit of flood handling needs and is statistically indeterminate. As such, it is a valuable criteria for use on safety based issues but has little practical significance on the expected dam/reservoir operations during the project life. The one hundred year flood establishes an event of real time significance and one which serves as a basis for economic facility sizing and operational decisions which are directly related to actual reservoir expected performance. In particular, it serves as a frame of reference for sizing and positioning the service and emergency spillways. Since no documented 100-year flood exists for this site, and since both the 100-year peak flow rate and hydrograph were needed, these data were generated as a part of the study. The 100-year snowmelt flood was estimated at 2,000 cfs based upon regional analysis. The Soil Conservation Service design storm method was used to generate a 100-year rainfall flood (SCS, 1980). The 205 square mile drainage area illustrated on Figure 3-5 was divided into five sub- basins for evaluation. This permits more site specific characterization of basin parameters and loss rates. The hydrographs which were produced were then combined (three combinations) and routed (five routes) to the dam in a basin model. This model is superior in accuracy to a single basin evaluation as used in both previous models and allows the natural, site specific convolution of a hydrograph from actual physical parameters rather than the use of a standardized unit hydrograph as used in the 1991 Harza report. The basis for all three hydrologic studies is the SCS design storm method but each approach has unique distinguishing characteristics. The Harza report, for example, uses a rainfall to runoff abstraction which is not directly related to the SCS curve number procedure utilized herein. An inflow 100-year rainfall flood peak flow rate of 3,232 cfs was calculated by this method. It compares well with the regional analysis (CWCB, 1976) available for this area which produces a value of 2,800 cfs. There is no similar, convenient check for the resulting hydrograph volume and shape. 3-13 r