Dam Safety Series: Aging Structure

Dams and the Consequences of Time 

Dam safety concerns of Scott Dam’s aging structure

This year Scott Dam turned 100 years old, and as is the case with any century-old infrastructure, there are some concerns about its safety. Cape Horn Dam, 12 miles downstream, is 114 years old. Both were designed during a time when engineers assumed and built for a stable, predictable climate with reliable water resources. Both structures are also rated as “high hazard” structures, meaning that loss of life is likely in the event of dam failure.

The lifespan of older dams
Worldwide, most dams constructed in the last century were built to last 50 – 100 years. These older dams are often not engineered with modern safety standards or constructed to withstand intensified weather events of our changing climate. These older dams were built in a time when engineers based their designs on climate predictability and false notions of an infinitely wet world. Furthermore, depending on where older dams are located and the type of materials used for construction, these aging structures are not built to withstand significant seismic events. The aging structure of older dams poses significant safety concerns for downstream communities and environments, as was witnessed in the 2017 Oroville Dam failure emergency.

The recent report, Ageing Water Infrastructure: An Emerging Global Risk, by the United Nations University Institute for Water, Environment and Health (UNU-INWEH), notes that most of the large dams constructed between 1930 and 1970 worldwide were designed to have a lifespan of 50 to 100 years. According to this global study, “These aged structures incur rapidly rising maintenance needs and costs while simultaneously declining their effectiveness and posing potential threats to human safety and the environment.” Furthermore, at 50 years a large concrete dam, such as Scott Dam, “would most probably begin to express signs of aging.”

The dams of the Potter Valley Project are due for a health check-up.

High-hazard potential
Scott Dam and Cape Horn are both over 100 years old and built of concrete, and rated “high-hazard potential” dams by FEMA’s Federal Guidelines for Dam Safety. If either one of these dams fails, they are likely to cause loss of life and “significant economic losses, including damages to downstream property or critical infrastructure, environmental damage, or disruption of lifeline facilities”. According to the American Society of Civil Engineers 2019 Infrastructure Report Card, over half of California’s 1,476 state, federal, and privately-owned dams are considered high-hazard potential dams. Approximately 70% of these dams are greater than 50 years old.

A much-needed paradigm shift
Most dams older than 50 years were not built to current dam safety standards and need to be evaluated under even stricter safety standards designed to ensure the continuing safety of aging structures. Following the Oroville Dam crisis, UC Berkeley dam safety expert Dr. Robert Bea specifically recommended the Federal Energy Regulatory Commission (FERC) develop additional criteria to assess the safety of aging structures. Unfortunately, FERC refuses to address dam safety questions when it relicenses dams, insisting its separate dam safety program ensures all licensed structures are safe. It is very hard for citizens to verify these assurances, however, as most specific information about dam safety issues is hidden from the public as Critical Energy Infrastructure Information, i.e., information that might be useful to someone planning to attack energy infrastructure.

Failed measures of safety at Scott Dam
The inherent safety of a dam is determined by several factors including the capacity of the reservoir behind the dam and the condition of the dam’s components. Mark Ogden, project manager with the Association of State Dam Safety Officials noted, “It’s not like an expiration date for your milk, but the components that make up that dam do have a lifespan.”

The water storage capacity of the reservoir behind Scott Dam has declined significantly due to sedimentation. Lake Pillsbury reservoir originally held about 95,000-acre-feet of water and is now partially filled in with sediment. As of 2019, the reservoir held 75,000-acre-feet of water, but only 65,000-acre-feet can safely be released or sediment will clog the downstream outlet, as discussed in our previous blog in our Dam Safety Series: Sediment.

Over the years, PG&E has been required to report the declining condition of Scott Dam’s aging structure to FERC. Several reports filed by PG&E suggest serious issues that may be associated with Scott Dam’s aging structure. These include decaying concrete, cracks in the structure’s face, pipes with unknown connections, spraying leaks, and the vulnerability of the dam’s only low-water outlet to clog with sediment.

A spraying leak on the downstream face of Scott Dam was first observed in April 2016.

And this is just what we have been able to uncover. What other liabilities is PG&E hiding under the guise of Classified Critical Energy Infrastructure Information?