St. Francis Dam

St. Francis Dam
St Francis Dam crop.jpg
View of the dam looking north, with water in its reservoir
(February 1927)
LocationLos Angeles County, California, United States
Coordinates34°32′49″N 118°30′45″W / 34°32′49″N 118°30′45″W / 34.54694; -118.51250
Construction began1924
Opening date1926
Demolition date1929
Dam and spillways
ImpoundsLos Angeles Aqueduct
San Francisquito Creek
Height185 feet (56 m)
Height (foundation)205 feet (62 m)
Lengthmain dam 700 feet (210 m)
wing dike 588 feet (179 m)
Elevation at crestparapet 1,838 feet (560 m)
spillway 1,835 feet (559 m)
Width (crest)16 feet (4.9 m)
Width (base)170 feet (52 m)
Parapet width16 ft (4.9 m)
Hydraulic head182 ft (55 m)
Dam volumemain dam 130,446 cu yd (99,733 m3)
wing dike 3,826 cu yd (2,925 m3)
Spillway typeuncontrolled overflow
Reservoir
Total capacity38,168 acre⋅ft (47.080×10^6 m3)
Catchment area37.5 sq mi (97 km2)
Maximum length3 mi (4.8 km)
Maximum water depth182 ft (55 m)
Official nameSt. Francis Dam Disaster Site[1]
Reference no.919

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 40 miles (64 km) northwest of downtown Los Angeles, and approximately 10 miles (16 km) 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 11:57 p.m. on March 12, 1928, the dam catastrophically failed, and the resulting flood took the lives of what is estimated to be at least 431 people.[2][3] 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.[4]

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.[5]

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 was named as its Chief Engineer.[5][6]

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 233 miles (375 km) from the Owens Valley to Los Angeles.[7] 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.[8]

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 No. 1 and No. 2 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.[9]

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 where the dam would later 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.[10]

A surprising aspect of the early geologic exploration came later when the need for a dam arose. Although Mulholland wrote of the perilous nature of the face of schist on the eastern side of the canyon in his annual report to the Board of Public Works in 1911,[11] it was either misjudged or ignored by the construction supervisor of the St. Francis Dam, Stanley Dunham. Dunham testified, at the Coroner's Inquest, that tests which he had ordered yielded results which showed the rock to be hard and of the same nature throughout the entire area which would become the eastern abutment. His opinion was that this area was more than suitable for construction of the dam.[12]

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.[13] 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,[14] 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.[9][15]

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