Title: Nine Point Zero
Author: Ira Pasternack
Publication: The Outcrop, May 2011, p. 14, 19
Article Type: President’s Column
As I write this column, the grim news of the rising Japanese dead and missing toll has grown more tragic every day since the March 11th 9.0 magnitude earthquake and tsunami about three weeks ago. The nuclear crisis at the Fukushima Dai-ichi Tokyo Electric Power Plant has no resolution in sight as radiation levels continue to climb. My heart goes out to the Japanese people and I hope that the country’s recovery is swift and as painless as possible. While the battle to control the Fukushima Dai-ichi reactors continues, perhaps it would be appropriate to recall one of the greatest civil engineering disasters in our country’s history that remains the incident responsible for the second-greatest loss of life in California’s history (the greatest being the 1906 San Francisco earthquake and fire).
The St. Francis Dam, part of a massive aqueduct system designed to transport water from the northern California Owens Valley agricultural area to the burgeoning, thirsty southern California Los Angeles urban area, catastrophically failed three minutes before midnight on March 12, 1928. The failure released a 180-foot wall of water and debris down San Francisquito Canyon that killed an estimated 600 people on its way to the Pacific Ocean, 55 miles downstream. The exact number of casualties will never be known as many undocumented itinerant farm workers were camped along the path of destruction. Numerous commissions and studies have evaluated the many factors that might have contributed to the St. Francis Dam’s failure, but controversy remains over which of these was the primary culprit.
The city of Los Angeles began acquiring Owens Valley water rights around 1905, and commenced construction of a 237-mile long aqueduct system the following year. The chief engineer on the project, William Mulholland grew to national prominence during the aqueduct construction as it was the longest water-conveyance system developed in the world at the time. The aqueduct was completed in 1913, but due to persistent water shortages caused by regional drought, Mulholland devised plans for additional reservoirs to increase system water-storage capacity. The St. Francis Dam site along San Francisquito Canyon was one of Mulholland’s favorite locations for a reservoir due to ideal topography, and construction started in April 1924. The site is located about 40 miles north of downtown Los Angeles, near the city of Santa Clarita.
The initial plans for the St. Francis Dam specified a 500-foot radius of curvature, 175-foot tall solid concrete gravity structure with a maximum width at its base of 141 feet. Shortly before construction began, it was decided to raise the height of the dam 10 feet to increase the reservoir capacity. The height of the dam was raised another 10 feet during construction in July 1925 as the rate of Los Angeles growth accelerated and additional reservoir capacity was deemed necessary. This resulted in the dam’s height being raised 11 percent without any compensatory increase in the original design base width.
Two preeminent geologists of the time examined the St. Francis Dam site but failed to recognize that the linear San Francisquito Canyon was structurally controlled by a fault. The rocks on the eastern side of the fault are Precambrian Pelona Schist. Numerous landslides occur in the Pelona Schist due to the instability caused by the planes of schistosity paralleling the canyon walls. The rocks on the western side of the fault are the Oligocene Sespe Formation, an arkosic sandstone and conglomerate that appears competent because of the steep topography it forms in the semi-arid region and because it is very hard when struck with a rock hammer. Dropped into a glass of water overnight, however, a sample of the Sespe will dissolve completely, a property known among civil engineers as “slaking.” The contact between the Pelona Schist and the Sespe Formation is located about one-third of the way up the west side of the canyon wall at the St. Francis Dam site. The presence of approximately five feet of gouge and several feet of sheared and brecciated rock along the contact indicate it is a fault, but this was not recognized during construction of the dam.
The St. Francis Dam was completed in May 1926 and the reservoir began filling, but did not approach capacity until 1927. On March 7, 1928 the reservoir reached full capacity and Mulholland ordered no more water be allowed to flow into the reservoir. In the same week, a few motorists traveling on the road along the east (Pelona Schist) shore of the reservoir reported cracks and sagging in the roadbed near the dam’s east abutment. By one account, provided after the dam’s collapse, the roadbed had sagged almost one foot.
Throughout the filling process, several cracks and leaks appeared in the dam and were judged by Mulholland and his assistants to be within expectation for a concrete dam the size of the St. Francis. On the morning of March 12, the dam keeper discovered a new leak and immediately alerted Mulholland because he believed the muddy-brown color of the water to be an indication the leak was eroding away the dam’s foundation. Mulholland inspected the new leak and, convinced it was relatively minor and normal, pronounced the dam safe. Less than 12 hours later, the dam failed catastrophically breaking into several large pieces, some of which were transported almost a half mile downstream, while the center section of the dam remained standing.
Mulholland faced criminal prosecution by the Los Angeles County District Attorney and willingly accepted responsibility for the disaster stating: “Don’t blame anyone else; you just fasten it on me. Whatever fault there was on the job, put it on me. If there was an error in human judgment, I was the human, and I won’t try to fasten it on anyone else.” The jury determined responsibility for the disaster lay with the governmental organizations that oversaw the dam’s construction and cleared Mulholland of any criminal charges, because neither he nor anyone at the time was aware of the instability of the rock formations on which the dam was built. The hearings also recommended: “The construction and operation of a great dam should never be left to the sole judgment of one man, no matter how eminent.” Soon after the trial, Mulholland retired and retreated into a life of self-imposed isolation.
In response to the St. Francis Dam disaster, new standards were established for dam safety and design, professional engineer registration was enacted and engineering geologic input became standard practice. It will be interesting to observe what new regulations will be enacted to prevent future disasters similar to what is developing at the Fukushima Dai-ichi power plant.
Clements, T., 1966, St. Francis Dam failure of 1928, in Lung, R., and R. Proctor, eds., Engineering Geology in Southern California: Assoc. of Engineering Geologists, Arcadia, CA, p. 89-91.
Rogers, J.D., 2006, Lessons learned from the St. Francis Dam failure: Geo-Strata, v. 6, issue 2, p. 14-17. Downloaded from http://web.mst.edu/~rogersda/st_francis_dam/
U.S. Geological Survey photo archive of the St. Francis Dam disaster: http://libraryphoto.cr.usgs.gov/cgi-bin/search. cgi?search_mode=noPunct&free_ form=St.+Francis+Dam+Flood [link no longer active]
30-minute video on the St. Francis Dam disaster produced by the Santa Clarita Valley Historical Society featuring a TV interview with local historian Frank Rock: http://www.scvtv.com/html/legacyfrankrock.html [link no longer active]
Santa Clarita Valley Historical Society information on the St. Francis Dam disaster: http://www.scvhistory.com/scvhistory/stfrancis.htm