Anderson Reservoir is currently limited to about 52% of its capacity due to seismic concerns, costing Santa Clara County valuable drinking water resources. This project covers earthquake retrofitting of Anderson Dam to improve reliability and safety, and returns the reservoir to its original storage capacity.
Anderson Dam creates the county’s largest surface water reservoir—Anderson Reservoir— which stores local rainfall runoff and imported water from the Central Valley Project. The reservoir is an important water source for treatment plants and the recharge of the groundwater basin. Besides restoring drinking water supplies, the upgrade also supports compliance with environmental regulations. The District’s regular reservoir releases ensure that downstream habitat has healthy flows and temperatures to sustain wildlife.
A breach of Anderson Dam at full capacity could have catastrophic consequences, including inundation of surrounding land more than 30 miles northwest to San Francisco Bay, and more than 40 miles southeast to Monterey Bay.
In December 2016, the board was informed by the district that findings from the geotechnical and geologic investigations performed during the project’s design phase led to the conclusion that a more extensive dam retrofit than had originally been envisioned would have to be performed. Further, the Board was informed that the more extensive retrofit work would double the previous project’s estimated cost. The district presented the Board with a water supply cost-benefit analysis that showed the benefits of the more extensive retrofit project significantly outweighed the cost of not proceeding with the retrofit, which would require the district to purchase additional imported water every year to make up for the loss of long-term storage at Anderson Reservoir. Based upon this information and analysis, the board directed the district to continue work on this critical infrastructure project.
The Santa Clara Valley Water District is moving forward on the project design for the Anderson Dam Seismic Retrofit Project. The design work thus far has been undergoing a series of independent reviews to ensure best outcomes. We are working to complete a draft Environmental Impact Report for public review by summer 2018. The Anderson Dam project is one of our largest capital projects to date, and is a critical public safety project. We are committed to ensuring the best design for maximum protection, and are aiming to break ground in 2020.
Progress of the Anderson Dam Seismic Retrofit Project work
Following the seismic stability evaluation, the water district initiated the Anderson Dam Seismic Retrofit Project in 2012 as a permanent fix to the risks identified by the seismic study. In addition to seismically retrofitting the dam embankment, the planning phase of the project identified the need to:
- Replace the existing outlet pipe that runs below the dam to improve capacity and reliability
- Increase the wall height of the concrete spillway to approximately 9 ft and the height of the dam crest to 7 ft to provide more freeboard required to pass the revised Probable Maximum Flood (PMF)
This project is currently in the design phase. Geotechnical and geologic investigations have been performed in many areas around the dam to collect data for the retrofit project and to complete the design of the additional project elements. As part of this investigation previously unidentified seismic deficiencies were discovered:
- The upstream embankment shell materials are also susceptible to liquefaction during a MCE. Liquefaction describes a phenomenon whereby a soil loses strength in response to earthquake shaking, causing it to behave like a liquid.
- The existing Transition Zones (special material placed between the clay core and the rockfill shells to protect the core) were determined to be inadequate to prevent failure due to possible fault offset leading to seepage and erosion through the bedrock foundation beneath Anderson Dam during a major earthquake
These findings have resulted in modifications to the reservoir restriction and the extent of necessary retrofit work to stabilize the dam. The maximum water surface elevation in the reservoir was restricted an additional 10 feet in January 2017 to accommodate greater potential deformation due to liquefaction of the upstream shell. The retrofit project which was originally planned to include large upstream and downstream buttresses has been modified to a nearly complete replacement of Anderson Dam in place. This replacement will ensure the post-project facility has removed all liquefiable material in and beneath the embankments and will be built to the most modern design standards and with rigorous quality control. Finally, a new high level outlet will also be constructed to allow rapid drawdown of the upper portion of the reservoir in case of an emergency.
The water district is working closely with the State’s Division of Safety of Dams and the Federal Energy Regulatory Commission. Both agencies have jurisdictional authority over the dam and reservoir, and the water district must obtain their review and approval for all project design plans. In addition, the project is continuously overseen by an independent panel of dam experts. Environmental documents will be prepared to comply with federal and state regulations, and permits will be obtained from several regulatory agencies for water diversion activities during construction, including full dewatering of the reservoir.
Completion of the design, obtaining all regulatory approval and acquisition of permits will be completed before construction commences, currently planned for 2020.
Updated January 2018
- Anderson Dam Seismic Retrofit Project Fact Sheet
- Anderson Dam Seismic Retrofit Project FAQ
- Anderson Dam Seismic Study and Retrofit Project
- Notice of Preparation and Initial Study
- Anderson Dam Planning Study Report
- Reservoir Operating Restrictions
- Seismic Stability Evaluation FAQ
- Dam Safety Program
- Inundation Map (2016)
Materials from Past Anderson Dam Seismic Retrofit Public Meetings
Materials from Community Update Meeting Wednesday, Sept. 13, 2017
- Project Description
- Updated Project FAQ
- PowerPoint Presentation (PDF)
- Construction Sequence animation (PPT)
- Click here to view a video of the meeting
Materials from the Anderson Dam Seismic Retrofit Project
Public Meeting on March 22, 2017:
- Project Description
- Project FAQ
- PowerPoint Presentation
- Download the Dec. 13, 2016 update presentation
- Responses to questions posed at the January 2013 public meeting
Key Performance Indicator for the Safe, Clean Water Program
Provide portion of funds, up to $45 million, to help restore full operating reservoir capacity of 90,373 acre-feet.
In May 2015, the Board approved, as part of its annual groundwater production charge setting, that $14 million of Safe, Clean Water funding be transferred to the Water Utility Enterprise Fund as reimbursement for the Anderson Dam project in FY16 instead of the Safe, Clean Water-scheduled $15 million transfer in FY18. The purpose of this earlier transfer was to offset the groundwater production charge increase for FY16. The next fund transfer is scheduled for FY28.
Brings the dam into compliance with today’s seismic standards
Increases reliability and safety of our area’s largest reservoir by protecting it from earthquakes
Eliminates operational restrictions issued by the state Division of Safety of Dams (DSOD) which would restore Anderson Reservoir to its full capacity of approximately 90,373 acre-feet, regaining 48% or about 43,500 acre-feet of water storage for our current and future water supply
Ensures compliance with environmental laws requiring reservoir releases that maintain appropriate flows and temperatures to support downstream wildlife habitat
Minimizes the risk of uncontrollable releases from the reservoir which could cause downstream flooding
Geographic Area of Benefit
Anderson Reservoir is the largest of the 10 water district reservoirs and provides a reliable supply of water to Santa Clara County. It has a total storage capacity of 89,073 acre-feet (one acre-foot is 325,851 gallons of water, enough to serve two households of five for one year). Anderson Dam was built in 1950 and named after the key founder and first president of the water district, Leroy Anderson. A long, deep natural gorge located three miles east of U.S. 101 in Morgan Hill provided a suitable dam site.
Findings of the original seismic stability evaluation completed in 2011 on Anderson Dam indicated that the downstream and upstream embankments could become unstable during a very large magnitude earthquake and the rupture of faults underlying the dam may have adverse impact on the outlet pipe and intake structure.
A storage restriction of about 55 feet below the dam crest has been put in place to protect the public, reducing the allowed storage capacity to 52,553 acre-feet. This voluntary restriction exceeds the 45-foot restriction approved by the regulatory agencies (California Division of Safety of Dams and the Federal Energy Regulatory Commission) and was instituted by the District in response to additional findings during the design phase of the Anderson Dam Seismic Retrofit Project (ADSRP). The water district and regulatory agencies believe that this restriction will prevent the uncontrolled release of water in case the dam is structurally damaged after a major earthquake.
About the Safe, Clean Water and Natural Flood Protection Program
In November 2012 the voters of Santa Clara County overwhelmingly approved Measure B, the Safe, Clean Water and Natural Flood Protection Program, as a countywide special parcel tax for 15 years with a sunset date of June 30, 2028. This Program replaced the Clean, Safe Creeks and Natural Flood Protection Plan, which voters approved in November 2000.
The Safe, Clean Water Program was developed with input from more than 16,000 residents and stakeholders and was created to match the community’s needs and values. The voters of Santa Clara County identified five priorities:
Priority A: Ensure a Safe, Reliable Water Supply
Priority B: Reduce Toxins, Hazards and Contaminants in our Waterways
Priority C: Protect our Water Supply from Earthquakes and Natural Disasters
Priority D: Restore Wildlife Habitat and Provide Open Space
Priority E: Provide Flood Protection to Homes, Businesses, Schools and Highways
Other: Six projects from the Clean, Safe, Creeks Plan have been carried forward into the Safe, Clean Water Program.
Each year, the District prepares a report providing a progress update for each of these Program priorities, along with fiscal year accomplishments.
To ensure transparency and accountability to the voters, the ballot measure also created an Independent Monitoring Committee, appointed by the Santa Clara Valley Water District Board of Directors. The Independent Monitoring Committee annually reviews the Program’s progress to ensure the outcomes are achieved in a cost-efficient manner and reports its findings to the Board.
In addition, the Program requires three independent audits, the first of which was conducted in FY 2017.