The design of the 90,400-af, 240-foot-high Anderson Dam retrofit is a critical step in the District’s goals of providing a reliable water supply, reliable flood protection, and a healthy ecosystem to the residents of Santa Clara County. The high-hazard dam is within three miles of the Calaveras Fault and under DSOD and FERC jurisdiction.
AECOM is providing services for the seismic retrofit of Anderson Dam, a 240-foot-high zoned rockfill embankment founded on alluvium, older unconsolidated deposits, and Franciscan Complex bedrock. The dam is near the active Calaveras Fault, and the site straddles the conditionally active Coyote Creek-Range Front Fault System, with traces mapped crossing the dam footprint and the outlet works alignment. The scope of services includes the development of the basis of design and preparation of basis of design contract documents including technical memoranda and reports, cost estimates, and plans and specifications.
The seismic retrofit project comprises multiple aspects, including stabilizing the embankment for the maximum credible earthquakes on the Calaveras and CCRF faults; replacing the outlet works to mitigate the fault rupture risks, meeting current California Division of Safety of Dams drawdown requirements, providing additional flood management capability for District use; and reconstructing the spillway to meet current regulatory requirements.
In its recent seismic stability evaluation completed for the District, AECOM concluded that the dam could become unstable due to liquefaction of a lower zone of the downstream shell, the foundation alluvium, and the upstream shell during the maximum earthquakes on the Calaveras and CCRF faults. The CCRF faults were characterized as ‘Conditionally Active,’ with a potential fault offset of up to four feet. This offset would shear the outlet pipe, rendering it inoperable and potentially leading to uncontrolled release of the reservoir. In view of those deficiencies, DSOD restricted the maximum operating reservoir elevation to a level about 45 feet below the dam crest.
Because of their size and vulnerability, the outlet works do not meet DSOD requirements for emergency reservoir drawdown. Also, a recent study has shown that the spillway discharge capacity is insufficient to pass the Probable Maximum Flood (PMF) as updated in accordance with HMR 58/59 guidance and the spillway foundation does not meet current criteria.
Project objectives:
- Build a replacement embankment that can resist the MCE on the Calaveras and CCRF faults, and can accommodate potential fault offset in the foundation.
- Replace the outlet works to: (1) mitigate the fault rupture risk from the MCE on the CCRF fault zone, (2) meet current DSOD drawdown requirements, and (3) provide additional flood management capability for District use.
- Replace the existing spillway to meet current foundation criteria and to pass the updated PMF.
- Upgrade the unlined channel located downstream of the spillway to pass the PMF.
- Address other dam safety deficiencies identified through the planning and design phases of project delivery and restore the full operational capacity of the reservoir.
- In winter of 2020, FERC ordered that the reservoir be drawn down and maintained at the lowest level possible. As a result, Valley Water decided to split the low-level outlet design into a separate fast-tracked project to improve their control over the reservoir level as soon as possible while work continued on the replacement dam design.
The AECOM team will complete the design of the project, including the design of dam embankment stabilization measures, design of the replacement outlet works system to accommodate shaking and fault offset, and design of a replacement spillway.