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Environmental Scoping Meeting
September 2009

B.F. Sisk Dam Description

B.F. Sisk Dam is a 300-foot-high zoned compacted earthfill embankment located on the west side of California's Central Valley approximately 12 miles west of Los Banos, California.  The dam is over 3 ½ miles long and impounds San Luis Reservoir which has a total capacity of over 2 million acre-feet. 

The dam was built between 1963 and 1967 to provide supplemental irrigation water storage for the Federal Central Valley Project and municipal and industrial water for the California State Water Project.  Water is lifted into the reservoir for storage by the Gianelli Pumping – Generating Plant from the California Aqueduct and from the Delta-Mendota Canal via O’Neill Forebay. 

B.F. Sisk Dam (also known as San Luis Dam) is owned by the Bureau of Reclamation (Reclamation) and operated by the California Department of Water Resources (DWR).  Reservoir storage space is allotted 55 percent State and 45 percent Federal.

Safety of Dams Investigations Background

A series of studies and analyses culminating in the seismic risk analysis that was completed in 2006 determined that there is justification to take action to reduce risk to the downstream public.  Consequently, Reclamation, with collaboration from DWR, initiated a Corrective Action Study to investigate and determine a course of action to mitigate risk.  This decision was a result of the following developments:

• Increases in the estimates of both the severity of ground shaking due to nearby earthquake faults, primarily the Ortigalita fault, which crosses San Luis Reservoir and the probability of a large event on this fault.
• New understanding of the properties of the dam’s foundation materials and their ability to resist deformation when subjected to severe shaking.
• Advances in computer-based analysis methods, which allow better assessment of the behavior of the dam under seismic loading.

Current Investigations and Analyses

Geology:  Reclamation and DWR have completed several geologic investigations to better understand:

• The foundation materials beneath the dam
• As-built placement of embankment zoning
• Borrow area locations and materials

Engineering:  Appraisal level designs have been completed and development of feasibility level designs are underway.  All structural alternatives would include at least one downstream berm to stabilize the embankment if exposed to earthquake shaking.  Each alternative would also include new zoning in the downstream side of the crest to protect against cracking.  A structural alternative with a lower construction cost which would require a reservoir restriction is also under consideration. 

Economics:  An economic analysis to compare life cycle cost of each alternative is underway.  The analysis is being performed using the Common Assumptions Workgroup that includes, Reclamation, DWR, and water user representatives.

Environmental:  Environmental studies that will result in an Environmental Impact Statement  (EIS)/ Environmental Impact Report (EIR) is being conducted by North State Resources under contract from Reclamation.  DWR and Reclamation are participating in this task which will be in compliance with state and federal guidance.

Corrective Action Study:  This phase of the Safety of Dams project is referred to as a Corrective Action Study (CAS).  Completion of the CAS is expected in 2013.  It will result in feasibility level designs, environmental documentation, selection of the preferred alternative, and a Modification Report to the federal Office of Management and Budget and to Congress.  Congressional acceptance of the Modification Report allows funding for construction.