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Although the PMP is the theoretical upper limit for rainfall amounts, there are <br />currently no theoretical approaches for computing the PMP. The PMP estimates are <br />based on detailed analyses of the most extreme historic storms, evaluating as many <br />aspects of the storm dynamics and rainfall patterns as possible. By selecting only the <br />most extreme storms in this historical record, it is assumed that these storm events were <br />as efficient as possible in their production of rainfall. The storm rainfall is analyzed for <br />its spatial coverage, temporal distribution, and rainfall totals. Procedures are then used to <br />increase the observed rainfall to a level that is considered to be the greatest depth <br />physically possible for the location of the basin being studied. Many storms are <br />evaluated in a site - specific study, and the storms with the largest rainfall values are <br />selected for detailed consideration. These rainfall amounts are enveloped in both time <br />and space to provide a consistent analysis for various rainfall durations and various types <br />of storms that could occur over the watershed location. Each of these procedures tends to <br />increase the rainfall values while attempting to maintain consistent physical assumptions <br />within the analysis (Tomlinson 2001). <br />The results from both the generalized and site - specific PMP studies provide large <br />rainfall amounts compared to the observed rainfall from most storm events. Often, these <br />values appear to be unrealistically large. Caution must be used to insure that overly <br />conservative and/or inconsistent assumptions are not made (Tomlinson 2001). However, <br />PMP values should be extreme since extreme rainfall events can and do occur in the <br />regions of western Colorado, Wyoming, Utah, and northern Arizona. Examples are the <br />Crown King, Arizona, event of 1951, the Bug Point, Utah, storm of 1970, and the Opal, <br />Wyoming, storm of 1990. The purpose of site - specific PMP studies is to provide detailed <br />analyses to determine the theoretically greatest rainfall amounts associated with the most <br />extreme storms. Each site - specific study needs to address all of the factors associated <br />with extreme rainfall over the basin using updated analyses of historic storm events and <br />state -of -the- science meteorological analysis techniques (Tomlinson 2001). <br />1.2 Site - Specific PMP Studies <br />HMRs provide generalized estimates of PMP values for large, climatologically <br />diverse areas. Site - specific PMP studies for specific drainage basins can incorporate <br />more site - specific considerations. These include using topographical, climatologically, <br />and geographical characteristics of the basin. Site - specific studies also provide an <br />opportunity to incorporate new advances in the meteorological community's <br />understanding of extreme rainfall events as well as provide updated analyses of extreme <br />storms that have occurred since the appropriate HMR was published (Tomlinson 2001). <br />Both federal and state regulators have accepted results from site - specific PMP studies <br />because of their incorporation of state -of -the- sciences techniques, updated extreme storm <br />analyses, and detail topographic consideration. <br />For the study of Climax Mine tailings ponds, HMR 49 is the appropriate NWS hydro - <br />meteorological report since it covers the geographic region of the Colorado River and <br />Great Basin drainages. Because HMR 49 was published in 1977, twenty -two years of <br />additional storm data were considered for this study. <br />12 <br />