B.F. Sisk Dam (SCCAO)

B.F. Sisk Dam is a 380-foot-high zoned compacted earthfill embankment located on the west side of California's Central Valley 12 miles west of Los Banos, California. The dam is more than 3.5 miles long and impounds San Luis Reservoir which has a total capacity of more than 2 million acre-feet.

The dam was built between 1963 and 1967 to provide supplemental irrigation water storage for the federal Central Valley Project (CVP) and municipal and industrial water for the California State Water Project (SWP). 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 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 (CAS) 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 are performing and 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: Feasibility level designs have been completed and development of final designs for a structural modification to address seismic risks are underway. The structural alternative includes raising the dam to increase available freeboard and several downstream berms to stabilize the embankment if exposed to earthquake shaking. This alternative also includes new zoning along the downstream side of the crest to protect against cracking. Feasibility level designs for a reservoir restriction and full breach of the reservoir have been completed.

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: Reclamation has issued a solicitation for environmental and cultural resources compliance services. DWR and Reclamation are participating in this task which will be in compliance with state and federal guidance. Public review of the draft Environmental Impact Statement / Environmental Impact Report (EIS/EIR) is scheduled for July 2018.

Corrective Action Study: This phase of this Safety of Dams project that developed alternatives, produced feasibility level designs, and selected a preferred alternative was completed in April 2018.

Final Design Phase: The current phase of this Safety of Dams project is referred to as the final design phase. Tasks in the final design phase include performing additional analyses and design; preparing drawings and specifications for the structural modification; preparing environmental documentation; and preparing and delivering a modification report to the federal Office of Management and Budget and to Congress. Congressional acceptance of the modification report allows funding for construction.

Last Updated: 4/30/18