Fixing Anderson Dam remains the top priority for Valley Water

Valley Water is dedicated to working as quickly as possible to complete the Anderson Dam Seismic Retrofit Project so it can safely withstand a large earthquake. It is vital for the health and safety of our community, water supply in Santa Clara County, and our natural environment.

Studies have shown a large earthquake on the Calaveras or Coyote Creek faults could damage Anderson Dam, causing damage or failure and an uncontrolled release of water that could inundate cities and rural areas from San Francisco Bay south to Monterey Bay, including much of Silicon Valley.

In 2019, Valley Water produced a draft of a video demonstrating a worst-case scenario should a filled-to-capacity Anderson Dam fail during a major earthquake. It is frightening to watch but showcases exactly why Valley Water has made this project a top priority and has been working diligently to get this project completed.

While the scenario is very unlikely, Valley Water has taken precautions to reduce the risk of this improbable event even further. This event would have to occur when the reservoir is full because of torrential rains that have produced the highest flood possible at the exact time a maximum earthquake strikes along a nearby fault line.

This draft video has been used by Valley Water to better explain to lawmakers and regulators the level of urgency needed to move the Anderson Dam Seismic Retrofit Project forward. Valley Water has made flood inundation maps available to the public and partners for many years but felt a video would illustrate more clearly the impact of a worst-case-scenario event.

Currently Anderson Reservoir is operating below the level required by the Federal Energy Regulatory Commission (FERC) of 35% capacity. FERC is now demanding Valley Water to begin a full drawdown of the reservoir by Oct. 1, 2020.

The requirement to empty Anderson Reservoir could result in unsafe consequences:

  • A top concern is the potential to damage the intake structure during the drawdown, which would give us no way to control water flows out of the reservoir, potentially impacting downstream communities. While Valley Water will take every precaution to make sure that does not happen, it is a potential consequence.
  • If the intake structure is damaged during the emptying of Anderson Reservoir, there is a potential for the damage to be exacerbated during an earthquake. A damaged intake structure would mean Valley Water loses control of maintaining water levels within the reservoir in the future.
  • Maintaining the reservoir at the level mandated by FERC as of Feb. 20, 2020, as opposed to a full drawdown beginning in October as ordered, will provide Valley Water an additional ability to control water flows out of the reservoir.
  • A bill has been introduced in the California Legislature, Assembly Bill 3005 authored by Assembly Member Robert Rivas, to speed up the regulatory process and move this project along as quickly as possible. Valley Water continues to look at ways to engage local, state and federal lawmakers to help deliver this project in a timely manner.

Fixing Anderson Dam is a priority for Valley Water. Project details are available at https://www.valleywater.org/anderson-dam-project.

3 comments

  1. Thank you for the info and out reach. It doesn’t seem to me that it’s necessary to lower the damn 100%. The wildlife and waterfowl need that as a source for their environment. It seems that you could certainly do work you need without completely draining the reservoir.

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    1. The “intake” refers to upstream portion of the dam’s current outlet structure through which water in the reservoir is released from the dam out into Coyote Creek below. Based on our current engineering assessment, the loss of water pressure against the dam embankment from a draw-down of the reservoir to Deadpool increases the risk that the existing intake structure could crumple or deform, resulting in less (or no) water being able to be released from the dam than it can presently release. Land subsidence could also result.
      Valley Water sympathizes with your concerns regarding completely draining the reservoir. However, FERC has ordered us to do this beginning no later than October 1, 2020, and FERC regulates this dam.

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