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C1: Anderson Dam Seismic Retrofit*

About This Project

At Valley Water, we are working to protect our community’s drinking water, keep residents and businesses safe from flooding and natural disasters, repair and replace aging dams, reservoirs and pipelines, while reducing toxins, hazards and contaminants in our waterways.

Anderson Reservoir is currently limited to about 58% of its capacity due to seismic concerns (see News and Updates tab below for current operating capacity), 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.

*This project was voter approved as part of the Safe, Clean Water and Natural Flood Protection Program.

Anderson Dam
Datapoints
Status
On Target
Location
Morgan Hill
Phase
Planning
Schedule
Start FY 2014 / Finish FY 2031
Funding
Safe, Clean Water Fund ($66.1 million); Water Utility Enterprise Fund
News and UpdatesNews and Updates
Reports and DocumentsReports and Documents
Environmental and Community BenefitsEnvironmental and Community Benefits
History and BackgroundHistory and Background
News & Updates

Geotechnical investigations
Geotechnical investigations within Anderson Reservoir began Sept. 8, 2020. The work had been originally scheduled to begin on Sept. 1, 2020, but was postponed due to the California wildfires.

Geotechnical studies collect, test and analyze soil characteristics, in addition to testing strength of the soil and rock's compressibility.

Lowering of Anderson Reservoir
In adherence to a directive issued in February 2020 by FERC, the lowering of water levels at Anderson Reservoir will start on Oct. 1, 2020. Valley Water will drain the reservoir at the rate of about 200-acre feet, or 65 million gallons a day, the equivalent of 98 Olympic swimming pools every 24 hours. The reservoir could be near empty by December 2020 or early 2021, but if winter storms occur, full draining could take until April 2021.

Valley Water’s 42-page draining plan can be found here. The plan includes building a 1,700-foot-long tunnel, up to 24-feet in diameter, on the left side of the dam looking towards the reservoir. The work will start soon after the reservoir is drained. The new tunnel will increase the release of water during an emergency by five-fold.

Valley Water requested recreational closure of Anderson Reservoir beginning Oct. 1 and running through the life of the project.

Current Restricted Operating Capacity
As of Aug. 10, 2020, Anderson Reservoir is at 24 percent of reservoir capacity. By the time draining begins on Oct. 1, 2020, the reservoir will be about 11 percent full due to public consumption and storage in local groundwater aquifers. The full capacity of the reservoir will be restored after the construction is completed.

Coyote Creek fish rescue
In response to the planned lowering of Anderson Reservoir, Valley Water relocated steelhead and other sensitive species from Coyote Creek during the week of Aug. 17, 2020. Biologists transferred the fish to an area of the Coyote Watershed that is more suitable for this species while the work is occurring.

The decision to relocate the fish was the result of discussions between California Department of Fish and Wildlife, National Marine Fisheries Service and Valley Water.

Valley Water staff busy relocating fish from Coyote Creek.

Missed one of our public meetings?

Valley Water videos each of its meetings, virtual and the ones in person. The public can view the meetings through Valley Water’s You Tube channel at https://www.youtube.com/channel/UC03PpRJryXF_5aO8LoOA2Vg

 

Updated September 2020

 

Reports & Documents

Informational materials:

Anderson Dam Virtual Meeting Materials September 17, 2020

Anderson Dam Virtual Meeting Materials May 28, 2020

Materials from Past Anderson Dam Seismic Retrofit Public Meetings

Materials from Community Update Meeting Wednesday, April 10, 2019:

Materials from Community Update Meeting Monday, June 4, 2018:

Materials from Community Update Meeting, Wednesday, October 24, 2018:

Materials from Community Update Meeting Wednesday, Sept. 13, 2017:

Materials from the Anderson Dam Seismic Retrofit Project 
Public Meeting on March 22, 2017:

Environmental & Community Benefits

Key Performance Indicator for the Safe, Clean Water Program

  1. 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. 

Benefits

  • 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 

Countywide 

 

History & Background

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. Valley Water’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 Valley Water staff 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. Valley Water staff 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 Valley Water 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 Valley Water to continue work on this critical infrastructure project.

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 Valley Water 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, Valley Water 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.

View the Safe, Clean Water Program’s annual reports, annual IMC audit reports, and independent audits, including a staff response, on the Valley Water website.

Facilities
Anderson Dam