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<br /> <br />17 <br /> <br /> <br />2.0 <br /> <br />METEOROLOGICAL ANALYSIS <br /> <br />2.1 INTRODUCTION <br /> <br />The following is a summary of the procedure used to derive the ULDRS <br />(Upper Limit Design Rainstorm) for the drainage above Hoover Dam in <br />the Colorado River Basin. Details concerning this derivation are <br />fully addressed in a report prepared for Reclamation by MKE. That <br />report, titled "Hydrologic Design Data Acquisition - Determination <br />of an Upper Limit Design Rainstorm for the Colorado River above <br />Hoover Dam", was completed in March, 1989. The report developed two <br />separate ULDRS's - one critical for inflow to Glen Canyon Dam and <br />the other critical for inflow to Hoover Dam. Specific storm <br />analyses involved determination of the ULDRS magnitude, spatial and <br />temporal distributions, storm sequencing, and seasonal variation. <br />In addition to the development of these criteria for the two <br />individual ULDRS's determined from the contracted report, a third <br />ULDRS was analyzed for another location in the drainage above Glen <br />Canyon Dam. This ULDRS and associated data were developed by <br />hydrometeorologists in Reclamation's Flood Section. Development of <br />this third ULDRS was consistent with the agreed upon design <br />scenarios that would be used for the determination of the critical <br />design flood for Hoover Dam. <br /> <br />2.2 DATA COLLECTION <br /> <br />As with any study of this nature, it was first necessary to assemble <br />an exhaustive listing of all known major storms that have occurred <br />in or near the region surrounding the Colorado River Basin above <br />Hoover Dam. Due to the large area of the subject drainage and <br />the availability of extreme precipitation estimates from <br />Hydrometeorological Report No, 49 for areas less than <br />5000 square miles, the search for critical storm data concentrated <br />on finding severe rainfall events for areas greater than <br />5,000 square miles. A general literature search was performed. <br />The purpose was to find all pertinent investigations which could be <br />useful for the present analysis. This information, along with <br />severe storm data maintained in the files of the Hydrometeorological <br />Branch of the National Weather Service (NWS) and in the files of the <br />Flood Section of Reclamation's Denver Office, served as a basis for <br />identifying major storms of record. From a review of these data, <br />20 storms (table 2.1) were selected for detailed meteorological <br />analyses and used to determine the causes of large-area heavy <br />rainfall in the region. <br /> <br />In defining the ULDRS, determination of maximum average areal <br />rainfall for major storms of record occurring in the basin and major <br />storms of record considered transposable to the basin is required. <br />Of the twenty storms for which detailed meteorological <br />investigations were performed, thirteen storms (identified in <br />table 2.1 with an asterisk) were analyzed to provide the necessary <br />