Dam Safety Series: Faults

Hold PG&E At Fault

Underlying issues threatening Scott Dam

Scott Dam’s proximity to the Bartlett Springs Fault is a major safety concern. The Bartlett Springs fault was not identified by geologists as “a regionally significant seismic source” until the 1980s, nearly sixty years after Scott Dam was constructed. Scott Dam is not only old, outdated, and inadequately designed; it’s poorly located too.

Location, Location, Location

The Bartlett Springs Fault system, see Figure 1., is an extension of the San Andreas Fault Complex that runs within a mile of Scott Dam under Lake Pillsbury reservoir. The best available science indicates this fault zone is capable of producing earthquakes in excess of magnitude 7, “Our models produce a wide magnitude range: from M6.32 to M7.24.” Lozos et al, 2015.

Figure 1. Scott Dam and Bartlett Springs Fault, click to enlarge.

Earthquake safety is a key element of life in California and FOER has repeatedly requested that PG&E reveal the real risks to dam safety associated with seismic activity at Scott Dam.

Why was Scott Dam constructed in a fault zone? Building a dam near a fault zone is actually fairly common. Where river channels and fault zones cross, there tends to be a narrow river canyon downstream of a wide expanse. As the two sides of a fault zone move relative to each other, the shifting landscape pinches the river canyon, forming a narrow place that is easier to dam. Meanwhile upstream, sections of the riverbed are stretched laterally, away from the narrow river canyon, forming a wide valley ideal for creating a man-made body of water.

Scott Dam is located where the narrow river channel crosses a fault zone, a seemingly ideal location for a dam.

Knowledge is key

The other reason Scott Dam is located in a fault zone is that engineers would not fully comprehend the underlying issues of the Bartlett Springs Fault until several decades later. When Scott Dam was being built, geophysicist Alfred Wegener had just introduced his hypothesis of continental drift. It took nearly 50 more years of research and advanced technology to prove and refine his hypothesis into the theory of plate tectonics.

The reality is that downstream communities must be prepared for the worst-case scenario: dam failure. Both Scott Dam and Cape Horn Dam are rated high-hazard. As discussed in our previous post, Dam Safety Series: Aging Structure, this means that loss of life is likely in the event of a dam failure.

PG&E knows the seismicity issues at Potter Valley Project are a threat to public health and safety. Scott Dam has a questionable design, is outdated, and is poorly located in an active fault zone. Scott Dam needs engineering analysis of the dam’s capacity to withstand earthquakes that is publicly available, so downstream communities can develop adequate plans.

References

Lienkaemper, James J., 2010, Recently active traces of the Bartlett Springs Fault, California; a digital database: U.S. Geological Survey Data Series 541, 10 p. and data [http://pubs.usgs.gov/ds/541/].

Lozos, Julian C.; Harris, Ruth A.; Murray, Jessica R.; and Lienkaemper, James J., “Dynamic rupture models of earthquakes on the Bartlett Springs Fault, Northern California” (2015). USGS Staff — Published Research. 864.

Pacific Gas and Electric Company, 2017. Potter Valley Hydroelectric Project, FERC Project No. 77, Relicensing Pre-Application Document (PAD) Volume 1: Public Information, Sections 1-7.