Study shows Lake Oswego, including Oswego Lake, probably spared

by: PAMPLIN MEDIA GROUP: CHASE ALLGOOD - In a worst-case scenario, a 25-foot (or more) wall of water would roar out of Hagg Lake and down Scoggins Valley if Scoggins dam were fully breached during an earthquake.More than 100 area residents, local officials and other interested parties became the first members of the general public to see the predicted effects of a seismically induced failure at Scoggins Dam on Hagg Lake.

The widespread flooding across a wide swath of the southern and western metro area that would be expected in such an event was illustrated on seven full-color maps. The data and the maps were the result of a study by the Bureau of Reclamation, which owns the 151-foot-high dam near Gaston, and unveiled the maps to the public for the first time at two open houses held Jan. 9 at Hillsboro and Forest Grove.

In the worst-case scenario, both Rivergrove and West Linn could be impacted by floodwaters coming down the Tualatin River from Hagg Lake. The study indicated both Lake Oswego and Oswego Lake probably would be spared.

The open houses came 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.

Christopher Regilski, dam safety coordinator for the bureau, said his agency 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.”

Lake Oswego Fire Chief Ed Wilson was among the emergency management officials who met at a “tabletop drill” prior to the open houses to discuss the inundation maps. Wilson said he and the other officials went through “a brief scenario,” but the local fire and emergency management departments are waiting to receive copies of the maps from the Bureau of Reclamation before they can prepare more detailed plans.

“The maps weren’t as detailed as I’d like to see,” he said. “Once we get those, we’ll put our tools together.” by: PAMPLIN MEDIA GROUP: 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.

In the meantime, Wilson recommended that local residents — particularly those who own property near Tualatin River — sign up for Code Red, the free emergency alert service offered by the city that uses landline and cellular phones. 

Based on the maps Wilson did see, he commented on the bleak picture presented for those residents who live closer to Scoggins Dam. 

“Walking away from it, my sense was that the damage upstream is going to be devastating,” he said.

Lake Oswego residents can contact the fire department for more information about the issue.

Approximately 2,000 property owners, who were deemed to be in the highest-risk areas, were sent mailed notices about the sessions. Several of the residents who did attend remarked that they had been previously unaware about the extent of the risk posed by the dam in the event of a catastrophic earthquake.

They were not alone. According to Yumei Wang, a geohazards engineer for the Oregon Department of Geology and Mineral Industries, the massive, earthfill-style dam was constructed in 1975, when most 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,” Wang said. “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. The last such event was on Jan. 26, 1700.

“The thing is like a conveyor belt,” Wang said, indicating 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 sweeping risk to their homes and livelihoods.

The maps and information shared at the open houses 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).

Regilski 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, including Rivergrove and West Linn. 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, but those estimates did not take evacuation procedures into account, he added.

The surge would spread out considerably upon reaching Highway 47, 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, eventually terminating at the Willamette near West Linn.

The bureau’s study indicated that residents and property owners in both Rivergrove and West Linn would be threatened by the potential flood, though Oswego Lake and the city of Lake Oswego would be spared.

Rivergrove’s and West Linn’s proximity to the Tualatin may put them more at risk than other communities that are farther from the river; however, this risk will be mediated by the fact that it would take hours for the flood to reach either town, and its velocity, volume and depths will all have been significantly reduced by that time.

The study gave Rivergrove approximately 17 hours before it would see the flood’s lead waters, and the southernmost portion of West Linn would stay dry for as long as 41 hours after the failure. The waters in Rivergrove would raise the Tualatin’s depth approximately 15 feet, compared with 10-12 in the West Linn area, though the flooding was not expected to spread very far outside the river’s banks in either community.

“There’s no loss of life associated with that height,” Regilski said, referring to the predicted depths in West Linn. “But it’s still a serious flood. It’s something that we’re concerned about.”

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