'The Big One' prompts drilling at dam
The federal government began drilling in Scoggins Valley north of Gaston last week to help determine the best way to improve the earthquake resistance of Scoggins Dam.
The dam was built in the 1970s to create Henry Hagg Lake, a major county water source. But it's not designed to withstand intense earthquakes such as "The Big One," a devastating 9.0 magnitude quake that some scientists say has a 15 to 20 percent chance of happening in the next 50 years.
"Scoggins Dam is at threat of failure in a major earthquake just like much of the infrastructure in the Northwest," said Mark Jockers, a spokesman for local water utility Clean Water Services (CWS).
If the dam collapsed in the event of such a quake, it would not only cause massive flooding but severely damage access to water, which is a "lifeline resource" after an earthquake, Jockers said.
While CWS, Forest Grove, Cornelius, Hillsboro and other cities all use water from the lake, the dam itself is run by the federal Bureau of Reclamation. Of all the dams controlled by that bureau, Scoggins is the farthest west, and the closest to the Cascadia Subduction Zone. That's made it a top concern for the feds.
Funding for an upgrade was included in the omnibus spending package signed by President Obama in 2015. CWS plans to piggyback on that seismic project with a related effortto increase the water supply for Washington County, where population is growing quickly, by raising the height of the dam.
At the moment, engineers are considering two options: 1) raise and reinforce the existing dam or 2) build a second dam downstream.
"Initially we were working with the federal government to get permission to pile our dirt on their dirt... simply to raise the existing dam," said Jockers.
But a downstream dam at a point where the valley is much narrower could accommodate a "smaller, shorter, stable, concrete facility," Jockers said, making it more cost-effective.
Last week, crews began drilling, testing and collecting samples and will continue for several months.
The eight holes being drilled at the current dam site will help determine important construction details such as what kind of rock is there and how far down beneath the soil it is, said Tom VanderPlaat, Scoggins project manager for Clean Water Services.
Crews will also install monitors in the ground around the current dam to get an idea of the groundwater level there. If the water table is too close to a dam's spillway, its pressure could lift and distort the concrete surface unless drainage systems or pumps are installed to relieve the pressure.
The Bureau of Reclamation will also drill a mile and a half downstream where the new dam site is proposed, although access roads must first be built so the drill rig can reach that location.
In addition, the bureau will drill near the Scoggins Creek Picnic Area, where the road circling the lake hits its lowest point. A raised dam would likely flood the road so a bridge would be needed to keep it intact and the drilling will gather information for such future construction.
It could take as long as three years of study before planners choose a dam option and another three years before any construction begins, with three years beyond that before all upgrades are completed.
The long timeline is due to the uniquely powerful and complex threat posed by "The Big One," according to Reclamation engineer Chris Regilski.
"It's not like the most standard kind of work that we've done before," said Regilski. "It's very difficult to model, to analyze, and to make sure that we do the most cost-effective method."
Meanwhile, Jockers said, the current dam is structurally sound, and the upgrades are needed only because of relatively new information that shortened the timeline for the likelihood of "The Big One."
"The dam is safe as it's designed," he said. "The challenge is a major Cascadia Subduction Zone earthquake, which this dam was never designed for."
KOIN 6 News is a news partner of the News-Times.