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<br /> <br />19 <br /> <br /> <br />which would enhance the rainfall potential of the storm. This <br />Hypothetical-Upper Limit Design Rainstorm (HYPO-ULDRS) would occur <br />sometime between late June and well into the fall season. The <br />second HYPO-ULDRS scenario occurs from an optimum extra-tropical <br />cyclone having abundant moisture inflow from the tropical Pacific. <br />This type of major rainfall event could occur in either Mayor June <br />and from mid September through October. <br /> <br />2.4 MAGNITUDE OF ULDRS <br /> <br />Since the study basin is located in a region of complex topography <br />which produces a significant effect on total storm rainfall, it was <br />necessary to estimate likely storm centerings and associated <br />"generic" isohyetal patterns prior to development of the ULDRS. An <br />important consideration in the development of likely storm <br />centerings was the specific location of Glen Canyon Dam in relation <br />to Hoover Dam. The objective was to provide the necessary design <br />storms that would affect not only the design of Hoover, but also the <br />design of Glen Canyon, or the two dams operating in combination. <br />Examination of the isohyetal and isopercental (analysis of storm <br />rainfall percentages of some rainfall index) patterns of rainfall <br />associated with major storms occurring in the drainage were <br />particularly useful in identifying three storm centerings at 37045' <br />N, 107045' W (San Juan Mountains): 3800' N, 111040' W (Boulder <br />Mountains): and 37015' N, 113010' W (Pine and Cedar Mountains) and <br />their related generic isohyetal patterns. Figure 2.1 indicates the <br />location of the three storm centerings. <br /> <br />The ULDRS magnitude for each of the three storm centerings was <br />evaluated by two separate approaches. Traditional techniques of <br />design storm development were employed in developing each approach. <br />However, both methodologies incorporated the use of a restriction to <br />inflow moisture. This restriction to moisture availability was <br />developed because the large areal extent of the drainage and <br />corresponding design storms required evaluation. Differing from <br />typical design storm studies, the full storm moisture maximization <br />technique was restricted to the storm rainfall region of primary <br />moisture inflow. Rainfall occurring beyond this region was adjusted <br />by a reduced moisture maximization factor. . <br /> <br />In developing the ULDRS magnitude, the first approach examined made <br />use of techniques provided in NWS Hydrometeorological Report No. <br />S5A. This approach is commonly referred to as the "storm <br />separation" method whereby observed areal storm precipitation is <br />separated into components (convergence and orographic). Each <br />precipitation component is treated and evaluated separately, and <br />later recombined, to provide total design storm precipitation. The <br />procedure is based on: (1) estimating the convergence <br />(nonorographic) portion of individual storm rainfall for key area <br />sizes and durations, (2) moisture maximization and transposition of <br />the convergence rainfall, (3) evaluating the adjusted convergence <br />rainfall for orographic contributions, and (4) determining ULDRS <br />values for a complete array of storm area sizes and durations. <br /> <br />The second approach made use of the traditional method of storm <br />moisture maximization and transposition. In this procedure, <br />