PG&E Admits Scott Dam Faces Serious Seismic Risks
In a March 16, 2023 press release, PG&E has admitted that Scott Dam faces previously unacknowledged risks from seismic activity. Those risks are so great that the utility has announced that it will never again raise the gates atop Scott Dam, reducing the capacity of the Lake Pillsbury reservoir by about 20,000 acre feet.
PG&E also hints strongly that the utility will remove Scott Dam sooner rather than later, stating “(t)he company plans to continue to develop long-term mitigation measures which could include expedited partial or full removal of Scott Dam.” (emphasis added)
For years Friends of the Eel River has been researching and drawing attention to dam safety concerns at Scott Dam. The information PG&E has developed which evidently shows an unacceptable risk of dam failure is classified by the company as Critical Energy Infrastructure Information (CEII), and is thus unavailable to the public. Fortunately, our research is readily available and helps fill in the gaps.
“We’ve been raising concerns about dam safety and reliability at the Potter Valley Project for years, so frankly this latest development is somewhat validating,” said Alicia Hamann, Executive Director for Friends of the Eel River. “We know that the latest research shows the Bartlett Spring Fault, which runs right through the reservoir, is capable of producing earthquakes up to a magnitude 7 – far greater than the magnitude 5.9 PG&E used in their previous seismic safety modeling for Scott Dam.”
We have been raising concerns about Scott Dam’s location next to the Bartlett Springs fault, which appears to be capable of generating earthquakes of M 7. In 2018 we commissioned and published a study by Miller Pacific Engineering of the active ancient landslide above Scott Dam’s southern abutment.
And of course there’s the story of Scott Dam’s unusual design. Back in the early 1920s, engineers were forced to redesign the dam mid-construction when dam builders attempted to secure the foundation of the southern abutment to bedrock and found they were securing it to a large and slowly sliding boulder. That boulder, named “the knocker,” now sits just behind the dam and is the reason for the unusual sharp angle of Scott Dam’s southern flank. This seat-of-the-pants design change may well have resulted in a weaker structure. We have at least one report of a crack in the dam in that angle.
In a document filed today with the Federal Energy Regulatory Commission, PG&E outlines the process they began last spring to reevaluate Scott Dam engineering. The company received a memo from their consultants on March 14, 2023 presenting results of a simplified seismic stability analysis. The contents of that memo (which are classified as CEII) were apparently alarming enough for the company to take immediate action, though they will continue a more complete analysis. The filing goes on to state that “the dam may become structurally unstable when subjected to seismic loading from the updated ground motions.”
“The fact that PG&E will never again raise the gates atop Scott Dam tells us they have done some careful math and identified a real tipping point,” says Scott Greacen, Friends of the Eel River’s Conservation Director, “This is a very big deal. We think this is just the start of important changes in PG&E’s approach to removing Scott and Cape Horn dams.”
“The opportunity for salmon and steelhead recovery on the Eel River is significant, but time is of the essence. We need to get our native fish back to their headwaters, and that means removing both Scott and Cape Horn dams,” said Hamann, “As part of their license surrender and decommissioning process, PG&E is due to file a draft plan at the end of this year. Given the latest developments, and the urgency for both salmon recovery and eliminating the risk of dam failure, I expect that plan to outline a method for rapid dam removal.”