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Estimates put 4,445 people at risk in the case of flooding after a seismic event at Hagg Lake

by: NEWS-TIMES PHOTO: CHASE ALLGOOD - The water flowing through a broken Scoggins Dam would spread out along the flat valley floor and travel east to the Stimson Lumber mill (right) at a speed of about 3.5 miles per hour. After a 9.0-magnitude quake, the mill would likely already be in bad shape.Jack and Lois Killian live next to Joseph Gale Elementary School in Forest Grove. A mile from the Tualatin River, their home has never flooded in 35 years.

In the event of an earthquake-induced failure at Scoggins Dam, however, the Killians’ home just might become riverfront property — for a couple of days at least.

That’s what they learned at a Forest Grove open house last week hosted by the federal Bureau of Reclamation, which owns the 151-foot-high dam near Gaston, which forms Henry Hagg Lake. The bureau notified about 2,000 property owners in the highest-risk areas about the Jan. 9 open house.

The Killians were not among them and attended the latter event mainly out of interest, but were surprised and concerned by how close their property would be to the potentially catastrophic floodwaters.

“It looks like we’re right on the edge,” Lois Killian said. “We just had no idea that this was a possibility.”

They’re not alone. The massive, earthfill-style dam was constructed in 1975, when Oregonians were unaware of the danger posed by the offshore Cascadia Subduction Zone.

“It took scientists a very long time to figure out that we’re in an area that has high seismic hazards,” said Yumei Wang, a geohazards engineer for the Oregon Department of Geology and Mineral Industries (DOGAMI).

“Until then, a lot of structures weren’t designed for seismic shaking, so they’re vulnerable. Our building codes in Oregon were really very inadequate until 1994.”

Scientists now know that over the past 10,000 years, the Cascadia fault — which runs from northern California to British Columbia — has generated colossal, 9.0-magnitude quakes roughly every 500 years. Its last such event was on Jan. 26, 1700.

“The thing is like a conveyor belt,” said Wang, who presented a chart of the fault’s known history of major events for emphasis. “It could go in 10 years or it could go in 1,000 years, but the bottom line is that it will go.”

Such a quake would be widely expected to wreak devastation across western and central Oregon — particularly on unreinforced-masonry buildings (like many of the state’s older schools, civic buildings and downtown storefronts), bridges and other infrastructure.

The possible failure at Scoggins Dam would not be area residents’ only concern, but — depending on their proximity to the structure — it could pose a serious and far-reaching risk to their homes and livelihoods.

At the open house, the bureau and state officials shared large, full-color maps that detailed the worst-case scenario: a total failure of Scoggins Dam with the lake at full capacity (a level it reaches only a few months out of the year).

Christopher Regilski, dam safety coordinator for the bureau, said that such a scenario would funnel more than 53,000 acre-feet of Hagg Lake water through the valley east of the dam and into the land and communities beyond. The Stimson Lumber mill would be among those facing the most severe flooding, where the height of the water is predicted to be at least 25 feet and flowing at a peak volume of 550,000 cubic feet per second (cfs).

Regilski said a cubic foot is roughly the size of a basketball, except that it weighs almost 62.5 pounds, which means the weight of the floodwaters churning through the lumber mill at its peak would top 17,000 tons. For those who have seen Scoggins Dam’s spillway activated, its flow at elevation is only 14,000 cfs, barely one-fortieth of the volume that would roar out of the structure if it were fully compromised.

Regilski said an estimated 4,445 people would be “at risk” due to the flooding, which includes the employees at the mill as well as rural and some urban residents. He added that the estimates did not take evacuation procedures into account.

The maps used a color code to indicate the height of the waters in the areas at risk of flooding. Red meant over 25 feet. Orange was 15 to 20; yellow, 12 to 15. The red was limited just to the Scoggins Creek valley area, but orange and yellow were common outside it, particularly along the Tualatin River. Even near Forest Grove, flood depths as high as 20 feet were predicted at peak levels.

The surge would spread out considerably upon reaching Highway 47, a 3.5-mile trip which would take about an hour. The waters would split at that point. Some would flow south, flooding Wapato Lake and endangering the easternmost point of downtown Gaston.

The waters to the north would follow the path of the Tualatin River as it snakes toward Forest Grove, Cornelius and Hillsboro. Regilski said Forest Grove residents would have two hours before the lead waters threatened the southern part of the city, potentially reaching as far as Joseph Gale.

The volume and speed of the water would be greatly reduced by then, down to approximately 155,000 cfs and 2 feet per second, respectively. The Forest Grove Wastewater Treatment Facility, along with its associated lagoons, and a section of Highway 8 between Cornelius and Hillsboro, would be expected to flood, according to Regilski.

Gradually lessening impacts would be expected along the Tualatin all the way until where it meets with the Willamette at West Linn, a journey that will take even the fastest floodwaters 41 hours.

These open houses come on the heels of a three-year, federal seismic study of the dam, which was completed last year and recommended between $300 million and $400 million in upgrades. Regilski said the bureau and local jurisdictions that would be responsible for some of that cost, such as the Tualatin Valley Irrigation District, are still in the tentative planning stages, but felt that the informational sessions were their “moral responsibility.”

“We just felt that we knew about this risk, and we’re working to come up with a remedy, but even while we’re doing that, we knew that we needed to warn people,” he said. “It just seemed like the right thing to do.”

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