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| spillway_type = uncontrolled overflow | res_name = | res_capacity_total = | res_catchment = | res_max_depth = | res_max_length = | lat_d = 34| lat_m = 32| lat_s = 49 | lat_NS = N | long_d = 118| long_m = 30| long_s = 45| long_EW = W | coordinates_type = region:US-CA_type:landmark | extra = }} The St. Francis Dam was a curved concrete gravity dam, built to create a large regulating and storage reservoir for the City of Los Angeles, California. The reservoir was an integral part of the city's Los Angeles Aqueduct water supply infrastructure. It was located in San Francisquito Canyon of the Sierra Pelona Mountains, about northwest of Downtown Los Angeles, and approximately north of the present day city of Santa Clarita. The dam was designed and built between 1924 and 1926 by the Los Angeles Department of Water and Power, then named the Bureau of Water Works and Supply. The department was under the direction of its General Manager and Chief Engineer, William Mulholland. At 11:57PM on March 12, 1928, the dam catastrophically failed, and the resulting flood took the lives of as many as 431 people. The collapse of the St. Francis Dam is considered to be one of the worst American civil engineering disasters of the 20th century and remains the second-greatest loss of life in California's history, after the 1906 San Francisco earthquake and fire. The disaster marked the end of Mulholland's career.〔 ==Planning and design== In the early years of Los Angeles, the city's water supply was obtained from the Los Angeles River. This was accomplished by diverting water from the river through a series of ditches called ''zanjas''. At that time a private water company, the Los Angeles City Water Company, leased the city's waterworks and provided water to the city. Hired in 1878 as a ''zanjero'' (ditch tender), William Mulholland proved to be a brilliant employee who after doing his day's work would study textbooks on mathematics, hydraulics and geology, and taught himself engineering and geology. Mulholland quickly moved up the ranks of the Water Company and was promoted to Superintendent in 1886.〔 In 1902, the City of Los Angeles ended its lease with the private water company and took control over the city's water supply. The city council established the Water Department with Mulholland as its Superintendent and when the city charter was amended in 1911, the Water Department was renamed the Bureau of Water Works and Supply. Mulholland continued as Superintendent and named as its Chief Engineer.〔Water and Power Associates Inc. ("William Mulholland Biography" )〕〔Water and Power Associates Inc. ("DWP - Name Change Chronology" )〕 Mulholland achieved great recognition among members of the engineering community when he supervised the design and construction of the Los Angeles Aqueduct, which at the time was the longest aqueduct in the world and uses gravity alone to bring the water from the Owens Valley to Los Angeles.〔American Society of Civil Engineers ("First Owens River - Los Angeles Aqueduct" )〕 The project was completed in 1913, on time and under budget, despite several setbacks. Excluding the incidents of sabotage by Owens Valley residents in the early years, the aqueduct has continued to operate well throughout its history and remains in operation today. It was during the process of building the Los Angeles Aqueduct that Mulholland first considered sections of San Francisquito Canyon as a potential dam site. He felt that there should be a reservoir of sufficient size to provide water for Los Angeles for an extended period in the event of a drought or if the aqueduct were damaged by an earthquake. In particular he favored the area between where the hydroelectric power plants Powerhouses and were to be built, with what he perceived as favorable topography, a natural narrowing of the canyon downstream of a wide, upstream platform which would allow the creation of a large reservoir area with a minimum possible dam. A large camp had been set up to house the workers near this area and Mulholland used his spare time becoming familiar with the area's geological features. In the area, which later the dam would be situated, he found the mid and upper portion of the western hillside consisted mainly of a reddish colored conglomerate and sandstone formation that had small strings of gypsum interspersed within it. Below the red conglomerate, down the remaining portion of the western hillside, crossing the canyon floor and up the eastern wall, a drastically different rock composition prevailed. These areas were made up of mica schist that was severely laminated, cross-faulted in many areas and interspersed with talc. Although later many geologists disagreed on the exact location of the area of contact between the two formations, a majority opinion placed it at the inactive San Francisquito Fault line. Mulholland ordered exploratory tunnels and shafts excavated into the red conglomerate hillside to determine its characteristics. He also had water percolation tests performed. The results convinced him that the hill would make a satisfactory abutment for a dam should the need ever arise. The surprising part of this early geologic exploration came later when the need for a dam arose. This is because it appears that Mulholland either misjudged or ignored the perilous nature of the schist on the eastern side of the canyon although he was well aware of it and he wrote distinctly of its characteristics in his annual report to the Board of Public Works in 1911.〔Sixth Annual Report of the Bureau of the Los Angeles Aqueduct to the Board of Public Works, 1911〕 The population of Los Angeles was increasing rapidly. In 1900 the population was slightly over 100,000. By 1910 it had become more than three times that number at 320,000, and by 1920 the figure reached 576,673.〔(【引用サイトリンク】title= Historical Resident Population City & County of Los Angeles, 1850 to 2000 )〕 This unexpectedly rapid growth brought a demand for a larger water supply. Between 1920 and 1926, seven smaller reservoirs were built and modifications were made to raise the height of its largest of the time, the Lower San Fernando, by seven feet, but the need for a still larger reservoir was clear. Originally, the planned site of this new large reservoir was to be in Big Tujunga Canyon, above the city now known as Sunland, in the northeast portion of the San Fernando Valley, but the high value placed on the ranches and private land which would be needed were, in Mulholland's view, an attempted hold-up of the city. He ceased the attempts at purchasing those lands and, either forgetful of or disregarding his earlier acknowledgement of geological problems at the site, renewed his interest in the area he had explored twelve years earlier, the federally owned and far less expensive private land in San Francisquito Canyon. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「St. Francis Dam」の詳細全文を読む スポンサード リンク
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