San Luis Dam, also known as B.F. Sisk Dam, is 382 feet tall and 18,600 feet long, and impounds up to 2 million acre-feet of water. It is the largest offstream dam in the United States. Pursuant to the 1960 San Luis Act, San Luis Dam was constructed by the Bureau of Reclamation, and is still owned by the Bureau. The Bureau of Reclamation and the California Department of Water Resources entered into a Joint Use Agreement for San Luis in December 1961, which provides that the State of California operates and maintains the San Luis facilities. Because the dam is owned by the Bureau of Reclamation, it is not subject to oversight by DWR’s Division of Safety of Dams, or the Federal Energy Regulatory Commission’s Division of Dam Safety and Inspections.
1 San Luis Dam and Reservoir Source: Bureau of Reclamation
San Luis Dam is in a very seismically active area, and parts of the Ortigalita fault run through the reservoir. In the 2000’s, it was recognized that the Ortigalita fault could generate an earthquake of magnitude 7.1. The dam is also only 28 miles from the San Andreas Fault, and 23 miles from the Calaveras-Hayward Fault.
2 Faults near San Luis Dam Source: California Department of Conservation, 2012
It has been known since a large slide in 1981 that the earthen dam has serious structural and seismic deficiencies. The dam is currently the subject of a formal Bureau of Reclamation Corrective Action Study, begun in 2007. San Luis Dam is known to need an operating restriction of at least 50 feet to be within Reclamation’s Dam Safety Public Protection Guidelines. However, because the dam is operated by the California Department of Water Resources, there appears to have been no formal consideration of an interim operating restriction to protect lives and property.
1981 Embankment Slide
In September 1981, a 1,100 foot section of the upstream San Luis Dam embankment near the crest of the dam — 400,000 cubic yards — slid 177 feet. It was a deep seated failure which extended through the fill and into the native soil under the dam.
3 September 1981, San Luis Dam Embankment slide Source: J. Michael Duncan
A 1999 engineering analysis by the nationally recognized civil engineer J. Michael Duncan showed that the factor of safety for the dam had been reduced from 4.0 to 1.21 – too low to be safe in an earthquake. The table below is from Duncan’s analysis.
4 Safety factors after repair Source: Duncan, The use of back analysis to reduce slope failure risk (1999)
Known Seismic Hazards
Beginning in 1982, geologist Larry Anderson at the Bureau of Reclamation conducted a series of seismic studies which showed that the Ortigalita fault has higher rates of activity than had been previously recognized. The independently reviewed studies also showed that a large earthquake could lead to crest settlement and overtopping of the dam, which would result in large uncontrolled releases and likely dam failure. The Mid-Pacific Regional Office of the Bureau initiated a formal Corrective Action Study in 2007. In the letter to CVP contractors, the Mid-Pacific Regional Office stated:
Through the Dam Safety Program, Reclamation has identified several conditions at B.F. Sisk Dam that require action to reduce risks.
• The dam is located in a seismically active area, very close to the Ortigalita Fault
• Studies and deformation analyses conducted during 2005 indicate that, during a major earthquake, crest settlement greater than freeboard, or cracking associated with embankment deformation, could occur and lead to dam failure
• Failure of the dam could inundate hundreds of square miles including the town of Santa Nella, about 7 miles of Interstate 5, and numerous farms and houses along the San Joaquin River including some areas of Stockton
• An independent consultant review board agreed with Reclamation that the calculated crest settlement is possible for the estimated peak ground accelerations associated with the Ortigalita Fault
Under the current schedule, the Mid-Pacific Regional Office’s Corrective Action Study is not expected to be completed until May of this year. Construction on the proposed corrective action is not expected to begin until at least 2020, and funding has not yet been identified. The Mid-Pacific Regional Office published a draft appraisal for the San Luis Dam Corrective Action in 2013. The study indicated that a reservoir restriction of at least 50 feet is required to bring the seismic risk to within current Reclamation Public Protection Guidelines:
Since B.F. Sisk is an off-stream storage facility, a reservoir restriction is feasibly obtained by not filling (pumping into) the reservoir. This measure would consist only of a change in operations, so it is considered non-structural.
Based on current seismic deformation estimates (Reclamation 2013b) it appears that a permanent restriction of at least 50 feet would be required to reduce the risk to within current Reclamation Public Protection Guidelines. (emphasis added, p. 27)
However, Mid-Pacific Regional Office documents show no consideration of interim operating restriction on the dam, although such a restriction would normally have been considered under the Bureau’s 2011 Interim Dam Safety Public Protection Guidelines. The diagram of first priority risk reduction actions, from p. 30 of the 2011 Public Protection Guidelines, is shown below. The first priority actions clearly include taking interim steps for risk reduction.
5 Dam safety risk management actions Source: Reclamation Public Protection Guidelines
Because of California’s torrential rains this winter, San Luis reservoir filled. Failure to formally consider and implement an operating restriction has resulted in catastrophic risks, both to lives and to California’s water supply. If an earthquake were to occur, the earthen dam could slump and be overtopped. Failure of the dam would create an enormous path of destruction, almost 10 miles wide and over 80 miles long, from Los Banos to Antioch in the western Delta. The inundation would immediately hit Santa Nella (population 1,400) and Los Banos (population 37,000). It would flow through western Merced and Stanislaus counties, impacting West Modesto (population 5,600.) In San Joaquin County, the inundation would impact the western part of Manteca (population 72,000), the western part of Stockton (population 298,000.) In Contra Costa County, the inundation would impact Discovery Bay (population 14,000), and Brentwood (population 55,000.) In 2003, the San Joaquin County Office of Emergency Services estimated that 165,000 people were in the inundation path in San Joaquin County alone.
6San Luis Dam Inundation Map Source: City of Ripon
The dam inundation path also includes Clifton Court Forebay and the State Water Project and Central Valley Project Banks and Jones pumping plants. Potential impacts on Clifton Court Forebay and the SWP and CVP facilities is unknown. However, failure of San Luis dam would have catastrophic impacts on State Water Project and Central Valley Project water supplies.
7 State Water Project Banks Pumping Plant Source: The California Spigot
Although an operating restriction on San Luis reservoir would have a significant impact on State Water Project and Central Valley Project water supplies, most of the largest SWP and CVP contractors do have other storage available that does not have catastrophic risks to lives, property, and infrastructure. Metropolitan Water District has Diamond Valley Reservoir, with a capacity of 800,000 acre feet, as well as groundwater storage in the Semitropic groundwater bank. Santa Clara Valley Water District also has groundwater storage in Semitropic. Kern County Water Agency has groundwater storage available in the Kern Water Bank and other groundwater banks.