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Anderson Dam and Reservoir

Anderson Dam Seismic Retrofit Project – Community Update Meeting Wednesday, Sept. 13, 2017

Where: Morgan Hill Community and Cultural Center Hiram Room
17000 Monterey Road
Morgan Hill, CA 95037

Time: 6 – 7:30 p.m.

For information, please contact Community Liaison
Peggy Lam at plam@valleywater.org 


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

For information, please contact Community Liaison
Peggy Lam at plam@valleywater.org 


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. 

Aerial view of Anderson DamFindings 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.

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.

Boating and Recreational Activities
For boating and recreational activities, contact the Santa Clara County Park at 408-355-2200 or www.parkhere.org

Answers to most commonly asked questions about the project can be found in the following link:

Anderson publications

Informational materials:

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You can also use "Access Valley Water" to submit questions, complaints or compliments directly from your computer to a water district staff person who can help you.




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