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Anderson Dam Seismic Retrofit - Reservoir Bank and Rim Stability

Anderson Dam Seismic Retrofit

The Anderson Reservoir dam in Morgan Hill, California is getting a safety upgrade. On October 1, 2020, Valley Water began lowering the water levels in order to allow construction on the dam. The project is intended to improve the dam’s strength for increased safety, minimize the risk of uncontrolled water release - flooding, as well as restore its original capacity.


The dam is located to the immediate east of the city of Morgan Hill holding up to 91,300 acre feet of water. The Earthen dam was built in 1950 to facilitate the developing water needs of a growing population.

The Anderson Dam Seismic Retrofit Project (ADSRP) will correct dam seismic deficiencies and otherwise meet all current Federal Energy Regulatory Commission (FERC) and California Department of Water Resources, Division of Safety of Dams (DSOD), dam safety design standards.

The reservoir drawdown to dead pool phase initiated on October 1, 2020. Considerations for the project:

  • Reservoir rim bank instability.

  • Tunnel construction

  • Dam tunnel allows a safer method for more efficient and safer inflows, seismically robust, function after earthquake.

  • Allows release of water during storm events to prevent flooding

Reservoir Drawdown to Deadpool

One component of the project are the considerations for bank and rim stability compromise during drawdown. As the FERC Order concludes, until full remediation through the ADSRP is completed, the dam safety risk at Anderson is high. A full drawdown of the reservoir to deadpool began on October 1, 2020, and maintenance of deadpool to the extent feasible, reduces the risk of an uncontrolled downstream releases as much as possible given the current condition of the dam. Because drawing down the reservoir to deadpool may result in reservoir rim and bank instability, Valley Water will implement monitoring and implementation of geotechnical stabilization measures to address those potential adverse impacts. In addition, drawdown to, and maintenance of deadpool may adversely affect water supplies, land surface elevations, and groundwater recharge, which must also be addressed by avoidance, minimization and mitigation measures.

Avoidance and Minimization Measures

Reservoir Bank and Rim Stability Improvements may need to be implemented. These structural improvements would protect the existing Anderson Reservoir rim from potential landslides due to drawdown.

In order to monitor these potential landslide events, Eyasco, Inc. is providing data management by utilizing SiteHawk software to display and track movement and changes at specific locations along the reservoir bank. This critical data allows the geotechnical team to determine if and where stability remediation may be necessary.

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