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Last modified
11/23/2009 12:50:35 PM
Creation date
5/15/2007 10:43:00 AM
Metadata
Fields
Template:
Floodplain Documents
County
Statewide
Stream Name
Colorado River
Title
Colorado River Basin Probable Maximum Floods, Hoover and Glen Canyon Dams
Date
9/1/1990
Prepared By
US Department of the Interior Bureau of Reclaimation
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br /> <br />21 <br /> <br /> <br />individual storms were moisture maximized in place. A transposition <br />index was selected to account for differences in storm precipitation <br />due to orographic and distance from moisture source effects when <br />individual sto~ms were transposed from in situ centerings to <br />selected design storm centerings. Transposed storm rainfall data <br />were enveloped to complete an array of design storm areal and <br />durational depths. <br /> <br />After evaluation of the assumptions and uncertainties involved in <br />application of each approach, the results of each method were <br />averaged to produce the final array of ULDRS depths. Table 2.2 <br />provides the final averaged ULDRS precipitation values for each of <br />the selected storm centerings. <br /> <br />2.5 SPATIAL AND TEMPORAL DISTRIBUTIONS OF ULDRS <br /> <br />Due to the large basin and storm areas involved, it was necessary to <br />describe the spatial distribution of average areal ULDRS <br />precipitation provided in table 2.2. This would permit the <br />determination of average subbasin precipitation for any group of <br />subbasins delineated within the total study region. To avoid the <br />necessity of determining the spatial distribution of ULDRS <br />precipitation for each of the area sizes indicated in table 2.2, <br />hydrologic trials were conducted by Reclamation hydrologists using <br />preliminary average areal precipitation. These results implied <br />that, for an ULDRS type event, a storm area of 40,000 square miles <br />was critical for development of the maximum inflow flood for both <br />Hoover and Glen Canyon Dams. The ULDRS results for this area <br />(table 2.2) were used in conjunction with DAD relations from major <br />storms located in and surrounding the subject basin to develop ULDRS <br />DAD curves. The ULDRS DAD relations and the 100-year, 24-hour <br />precipitation-frequency maps that apportion regional rainfall were <br />used to develop total storm and incremental isohyetal patterns for <br />the three storm centerings. The total storm (72 hours) isohyetal <br />maps for each selected centering are provided on figures 2.2 to 2.4. <br /> <br />Major storms in the region were examined to determine characteristic <br />time distributions. Results from these investigations indicated a <br />different temporal distribution would be appropriate for the storm <br />centering located in the San Juan Mountains, compared to that found <br />at the Pine/Cedar Mountain location. The same temporal distribution <br />indicated for the San Juan Mountain centering was adopted for the <br />Boulder Mountain centering. Figure 2.5 provides the temporal <br />distributions for the ULDRS centerings used in this study. <br /> <br />2.6 <br /> <br />STORM SEQUENCES <br /> <br />It was hypothesized that critical inflow to the dams could result <br />from a series of storms occurring in sequence. Investigations were <br />undertaken to define the relation between storm magnitude and dry- <br />period interval separating sequenced storms. Due to differences in <br />storm types controlling the ULDRS on a seasonal basis, relations <br />developed were separated into distinct spring and late summer/early <br />fall categories. Pairs of storms that provided major rainfalls in <br />
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