Drilling for data.
BLUE RIVER - A drilling rig soon will be boring deep into Cougar Dam, about 50 miles east of Eugene, so federal engineers can better determine just how well it would hold up during a massive earthquake.
The 452-foot earthen dam holding back the waters of Cougar Reservoir along the South Fork of the McKenzie River undergoes annual checks as well as more thorough inspections every five years. But the drilling will provide much deeper understanding about the state of the dam. The U.S. Army Corps of Engineers' Portland District maintains 19 dams, and officials selected five to be the first to be drilled.
Of the five dams, four are in the Willamette Basin, and three are upstream of Eugene and Springfield. Crews already have drilled into Hills Creek and Lookout Point dams. They collected samples to understand the layers of material, and they installed monitoring gear. Engineers still are evaluating the data, according to the agency, with nothing alarming discovered immediately.
"There is inherent risk to all dams (though)," said Erik Petersen, corps operations manager in Eugene.
Experts agree a major failure of the large dams upriver of Eugene- Springfield could spell catastrophe for the metro area. Hence the attention to the dams' condition. One fear is that in an earthquake, landslides into the lake behind a dam could push water over the top of the dam, eroding and rupturing it.
Recognizing this risk, the Lane County Office of Emergency Management created an evacuation route map based on inundation maps created by the corps. The office posted the map online last week after The Register-Guard inquired about it.
The map shows what would happen if all corps dams in the lower Willamette Valley failed at once.
"The map is intended to provide a generalized view of everywhere in Lane County that water could possibly go and nothing more (no water depths, time arrivals, etc.)," Lane County Emergency Manager Linda Cook wrote in an email to the newspaper.
Where the water would go
Comparing the Lane County map to topographic maps, it appears flood waters could reach 20 to 40 feet in lower lying parts of Eugene, Springfield and surrounding communities, including Cottage Grove and Creswell, before spreading out into the floodplain of the south Willamette Valley. The county's map shows floodwaters covering downtown Eugene, the University of Oregon campus, downtown Springfield and Gateway. Skinner Butte would temporarily be an island.
Individual maps the corps has disclosed in the past have showed that a failure of the Hills Creek, Fall Creek and Lookout Point dams, all on the Middle Fork of the Willamette River, would send a wall of water into Eugene-Springfield reaching up to the fourth floor of Sacred Heart Medical Center at RiverBend in Springfield.
The corps rarely publicly shares its own maps of where floodwaters would rise during a worst-case scenario such as a huge earthquake or other calamity. The agency held a couple of public meetings in Lane County where it displayed its maps in 2011, but it asked meeting attendees not to take photos and didn't offer copies of the maps.
A near disaster in Northern California this February served as a reminder of the danger of dams. After heavy rains, part of the Oroville Dam's spillway washed away, prompting the temporary evacuation of 188,000 residents living below the earthen dam. The dam did not fail despite dramatic damage, sparing nearby cities of flooding. A concrete spillway is part of a dam that allows excess water to escape over the dam without eroding a dam's earthen structure.
Hundreds of thousands of people live downstream of the dams in Lane County, but none of the dams hold back as much water as Oroville Dam.
"It's just a totally different situation from a risk standpoint," Petersen said.
71 million gallons of water
When full, Cougar Lake holds 219,000 acre-feet of water, or enough water to submerge 219,000 acres of land a foot deep. One acre-foot is 325,851 gallons of water, so when full, the lake would have more than 71 million gallons of water. Thanks to a rainy winter, as of Friday, Cougar Reservoir was full, according to the corps.
At 770 feet tall, Oroville Dam is the tallest dam in the United States, according to the California Department of Water Resources. When full, it holds 16 times as much water as Cougar Lake, more than 3.5 million acre-feet.
Major earthquakes around the world in the past decade and increasing interest in the Cascadia subduction zone also contribute to corps engineers wanting to know more about their dams. Geologists expect a magnitude 9 earthquake in the Cascadia subduction zone, off the coasts of Oregon, Washington and Northern California, sometime in the next 500 years.
The magnitude 9 Japan quake in 2011 and the magnitude 8.8 Chile quake in 2010 gave scientists plenty of real-world data about how dams withstood the shaking.
Only one small dam failed in Japan and none in Chile, said Silas Sanderson, corps team leader for the drilling research of Cougar Dam. Overall, he said earthen dams have done well even in major earthquakes.
"We expect this dam to perform very well," Sanderson said during a Wednesday visit to Cougar Dam.
The annual dam checks, five-year inspections and drilling studies by the corps all try to answer the same question: "What could possibly go wrong?" Petersen said.
Starting this week, crews will drill 15 holes into Cougar Dam, some as deep as 300 feet. They'll then install monitoring equipment, including sensors that collect data about water pressure on the dam and how the large pile of rocks that form the dam's base continue to settle. Built in the early 1960s, Cougar Dam primarily consists of basalt rock blasted from the surrounding ridges by dam builders.
Last week, a team of engineers performed the five-year inspection of Cougar Dam. Members of the team hiked through the dam's spillway while a couple of their colleagues took a close look at the large gates holding back water by dangling in a basket from a crane. They also did not find anything alarming.
"Dam safety is our top priority," Petersen said, "and we take it very, very seriously."
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|Publication:||The Register-Guard (Eugene, OR)|
|Date:||May 29, 2017|
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