The St. Francis Dam, or the San Francisquito Dam, was a concrete gravity-arch dam located in San Francisquito Canyon in northern Los Angeles County, California, United States, that was built between 1924 and 1926. The dam failed catastrophically in 1928, killing at least 431 people in the subsequent flood, in what is considered to have been one of the worst American civil engineering disasters of the 20th century and the third-greatest loss of life in California history."California's Worst Disasters Remembered" seecalifornia.com. Quote: "600 people in 1928 died in the St. Francis Dam flood in Santa Clarita. It was around midnight that Southern California suffered one of the worst disasters in the state's history, second only to the earthquake and fire that devastated San Francisco in 1906."
The dam was built to serve the growing water needs of the city of Los Angeles, creating a large regulating and storage reservoir that was an integral part of the Los Angeles Aqueduct. 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.
However, a defective soil foundation and design flaws led to the dam's collapse just two years after its completion. Its failure ended the career of William Mulholland, the general manager and chief engineer of the Bureau of Water Works and Supply (now the Los Angeles Department of Water and Power).
Background 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, thereby teaching himself geology and engineering. 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 water company and took control over the city's water supply. The Los Angeles City Council established the Water Department renamed the Bureau of Water Works and Supply in 1911 with Mulholland as its superintendent. Mulholland was concurrently named as the Bureau's chief engineer following the organization's name change.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. It used 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 incidents of sabotage by Owens Valley residents during the California Water Wars, the aqueduct has continued to operate well throughout its history and remains in operation today.
Planning and design
It was during the process of building the 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 where the dam would later be situated, Mulholland found the mid- and upper portion of the western hillside consisted mainly of a reddish-colored conglomerate and sandstone formation that had small veins 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. Later, although 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 and 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.
A surprising aspect of the early geologic exploration came later, when the need for a dam arose. Although Mulholland wrote of the unstable 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,Sixth Annual Report of the Bureau of the Los Angeles Aqueduct to the Board of Public Works, 1911 this fact was either misjudged or ignored by Stanley Dunham, the construction supervisor of the St. Francis Dam. 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 became the eastern abutment. His opinion was that this area was more than suitable for construction of the dam.
The population of Los Angeles had been increasing rapidly. In 1900, the population was slightly more than 100,000. By 1910, it had more than tripled to 320,000, and by 1920 it had reached 576,673. 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 the water bureau's largest of the time, the Lower San Fernando reservoir, 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 community now known as Sunland, in the northeast portion of the San Fernando Valley, but the high asking prices of the ranches and private land which had to be acquired were, in Mulholland's view, an attempted hold-up of the city. He ceased 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.
Construction and modification
The process of surveying the area and determining the location for the St. Francis Dam began in December 1922. Clearing of the site and construction began without any of the usual fanfare for a municipal project of this nature. The Los Angeles Aqueduct had become the target of frequent sabotage by angry farmers and landowners in the Owens Valley and the city was eager to avoid any repeat of these expensive and time-consuming repairs.
The St. Francis Dam, sometimes referred to as the San Francisquito Dam, was only the second concrete dam to be designed and built by the Bureau of Water Works and Supply. The first was the nearly dimensionally-identical Mulholland Dam, on which construction had begun one year earlier. The design of the St. Francis Dam was in fact an adaptation of the Mulholland Dam, with modifications made so as to suit the site. Most of the design profiles and computation figures of stress factors for the St. Francis Dam came from this adaptation of the plans and formulas which had been used in the constructing of the Mulholland Dam. This work was done by the engineering department within the Bureau of Water Works and Supply.
In describing the shape and type of the St. Francis Dam, the word curved is used although, by today's standards, due to the amount of curve in its radius, the dam would be considered arched, therefore making it of the arch-gravity design. It was not so called because the science of arch-gravity dams was still in its infancy. Little was known in the engineering community about the arch effect, how it worked and how loads were transmitted, other than that it did help with stability and support. As such, the dam was designed without any of the additional benefits given by the arch action, which led to its profile being considered conservative, given its size.Rogers, David J. "Impacts of the 1928 St. Francis Dam Failure on Geology, Civil Engineering, and America, p. 2"
Annually, as did most other city entities, the Bureau of Water Works and Supply and the ancillary departments reported to the Board of Public Service Commissioners on the prior fiscal year's activities. From these it is known that the preliminary studies of the area which became the site of the St. Francis Dam, and topographical surveys for the dam, were completed by June 1923. They called for a dam built to the elevation of above sea level, which is above the stream bed base. These early calculations for a reservoir created by the dam revealed it would have a capacity of approximately .Official Action Taken by the Board of Public Service Commissioners and Board of Water and Power Commissioners of the City of Los Angeles, Relative to the St. Francis Reservoir
On July 1, 1924, the same day Mulholland was to submit his annual report to the Board of Public Service Commissioners, Office Engineer W. W. Hurlbut informed him that all of the preliminary work on the dam had been completed. In his report presented to the Board, Mulholland wrote that the capacity of the reservoir would be (). Hurlbut, who also presented the board with his annual report, Report of the Office Engineer, gave a clarification for this change from the prior year's estimate. In his report he wrote that:
...at the St. Francis Reservoir the dam site has been cleared and the foundation trench started. All concrete placing equipment has been contracted for and it is expected actual work of pouring the concrete will start in approximately ninety days. Additional topographic surveys have been completed and disclose a storage capacity of 32,000 acre feet at elevation 1825 feet above sea level.
Construction of the dam itself began five weeks later, in early August, when the first concrete was poured.Annual Reports of the Board of Public Service Commissioners 1924–1925
In March 1925, prior to Mulholland's report to the Board of Public Service Commissioners, Hurlbut again reported to Mulholland on the progress of the St. Francis project. He stated the reservoir would now have a capacity of () and that the dam's height would be above stream bed level. Hurlbut wrote, in an explanation of these changes that was presented to the Board of Public Service Commissioners, that:
Additional surveys and changes in the plans for this reservoir have disclosed the fact that at crest elevation of 1835 feet above sea level the reservoir will have a capacity of 38,000 acre-feet.24th Annual Report of the Board of Public Service Commissioners, Report of the Office Engineer
This increase in the dam's height over the original plan of 1923 necessitated the construction of a wing dike along the top of the ridge adjacent to the western abutment in order to contain the enlarged reservoir.