COMMITTEE AGENDA MEMORANDUM
Capital Improvement Program Committee
Government Code § 84308 Applies: Yes ☐ No ☒
(If “YES” Complete Attachment A - Gov. Code § 84308)
SUBJECT: title
Receive Information on the Coyote Dam Seismic Stability Project.
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RECOMMENDATION: Recommendation
Receive information on the conceptual analysis for the Coyote Dam Seismic Stability Project.
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SUMMARY:
Valley Water owns and operates 12 jurisdictional dams and 10 reservoirs in Santa Clara County. Dam safety regulatory requirements, Board policies, and obligations associated with dam ownership set the direction for the Dam Safety Program (Program). On January 26, 2026, the CIP Committee (Committee) requested that staff provide an update on the planning-level conceptual analysis for the Coyote Dam Seismic Stability Project (Project). On April 20, 2026, staff stated that a Project update would be presented at the October 2026 Committee Meeting.
Background
Coyote Dam is an earthfill dam constructed in 1936 and is one of only a handful of dams in the world built on an active fault. Coyote Reservoir stores watershed runoff and is located approximately three miles upstream of Anderson Reservoir on Coyote Creek, in which both reservoirs are operated as a system.
Since 1992, Valley Water and the California Division of Safety of Dams (DSOD) have been aware that a major earthquake could damage Coyote Dam, resulting in releases to Anderson Reservoir. Therefore, Coyote Dam has operated at a restricted water level approximately 19 feet below the spillway crest, which reduces the reservoir’s allowable storage to 53% of total capacity, or approximately 22,541 acre-feet.
A 2015 study identified several potential failure modes at Coyote Dam, including embankment overtopping during a Probable Maximum Flood (PMF) due to inadequate spillway capacity and rupture of the Calaveras fault beneath the dam. Fault rupture could potentially result in internal erosion of the core into the downstream gravel and rock zones. Subsequently, a 2024 study confirmed these potential failure modes and concluded that, during a Maximum Credible Earthquake (MCE), the existing downstream embankment is vulnerable to large deformations and cracking and that the existing spillway is not large enough to safely pass a PMF.
To address these safety concerns, it is recommended that the dam be modified to include a downstream filter and drainage system to mitigate potential cracking associated with seismic deformation and fault offset. The spillway would also be reconstructed to meet current safety and design standards. This study was shared with DSOD, and in late 2025, they concurred with its findings and recommendations.
Conceptual Alternatives Analysis
To ensure long-term dam safety and operational reliability, two conceptual alternatives have been developed to address both the seismic and fault-related vulnerabilities in the downstream embankment and inadequate spillway flood capacity, as follows:
Conceptual Alternative 1
• Dam Embankment Retrofit: Partial removal of the downstream embankment followed by reconstruction with a modern filter and drain system. The dam crest would be raised up to six feet to safely accommodate the updated PMF.
• Spillway Replacement: Construction of a new concrete spillway designed to meet current engineering standards and dam safety requirements.
Conceptual Alternative 2
• Dam Embankment Retrofit: Partial removal of the downstream embankment followed by reconstruction with a modern filter and drain system, but a dam crest raise would not be necessary to safely accommodate the updated PMF. Instead, the new spillway weir would be lengthened.
• Spillway Replacement: Construction of a new concrete spillway designed to meet current engineering standards and dam safety requirements.
The estimated 2025 planning-level construction cost is approximately $200 million for either conceptual alternative with a total Project cost of just over $400 million in the CIP FY27-31 Five-Year Plan. Acquisition of private property would not be required, but an agreement with the Santa Clara County Parks and Recreation Department would need to be negotiated, as the entire Project is adjacent to its lands.
Next Steps
Both conceptual alternatives will move to a feasible alternatives analysis, which will be shared with stakeholders and receive public feedback. Once the analysis is complete, a Feasible Alternatives Report will be prepared, and a Staff-Recommended Alternative will be selected. The Planning Study Report will follow, completing the planning phase. The Project will then advance to the design phase, with construction anticipated to begin in 2034.
1Assumes construction can only commence once the construction of the Anderson Dam Seismic Retrofit Project is complete
ENVIRONMENTAL JUSTICE IMPACT:
There is no environmental justice impact associated with this informational report.
ATTACHMENTS:
Attachment 1: PowerPoint
UNCLASSIFIED MANAGER:
Manager
Andrew Garcia, 408-630-2817