LONGVIEW – Work to repair a failing intake gate at Spirit Lake will start this summer, briefly closing portions of the Truman Trail and re-establishing a 1980s-era access road to the site.
Repairs are intended to protect downstream communities from potentially catastrophic flooding.
The U.S. Forest Service handed down its final decision on the project Tuesday evening, choosing to build a 3.4-mile access road from Windy Ridge to the old pump station near the lake to complete the multimillion-dollar project.
While researchers on the Pumice Plain have expressed concern the road will disturb projects, some of which have been running for decades, Gifford Pinchot National Forest Supervisor Eric Veatch wrote in his decision, “there is a need to ensure the protection of public safety, health, property and the environment from a catastrophic breach of the Spirit Lake natural debris blockage.”
“Currently, (downstream communities’) safety relative to the stability of the debris blockage is based on assumptions derived from 35-year-old data,” he said. “Citizens of these communities are living and working downstream from a poorly understood natural debris blockage that utilizes a dated tunnel outlet infrastructure system that is subject to failure.”
The project previously had been estimated by the U.S. Army Corps of Engineers to cost between $5 million and $15 million, but Spirit Lake Project Manager Chris Strebig said the estimate is outdated and a new one won’t be available until contracts are sought.
The project is estimated to affect 4 percent of the designated 3,840-acre Class 1 Research Area, which includes 25 individual research plots. Researchers have said they not only fear that the road and drilling will disturb research plots, but broader risks include introducing invasive plant species to the natural laboratory and invite illegal use of the area, which is closed to the public to protect the research.
If the intake gate failed, it could release a flood of over 314,000 acre-feet yards of water – more than 100 million gallons. It also could spew out several billion cubic yards of sediment, which would rival the devastating mudflows of the original Mount St. Helens eruption.
Veach said a serious flood would inundate the Port of Longview for several months and temporarily close the ports of Portland, Vancouver and Kalama, as well as the railroad and Interstate 5. It also would flood out towns in the Cowlitz and Toutle rivers valleys.
There is an estimated $3.65 billion of property value in the potentially affected area, he said.
“I recognize that this decision may have short- and long-term consequences to some of the research occurring on the Pumice Plain,” Veach said. “My team and I have made every effort to design a project that reflects a balance between public safety concerns and effects to ongoing research.”
When Mount. St. Helens erupted in 1980, debris blocked the historic outflow of the lake, raising the water level 200 feet. Engineers built a tunnel in 1985 to drain the lake and prevent a catastrophic flood, but the 35-year-old tunnel needs repairs and upgrades, such as adding a second safety gate with emergency features.
The four- or five-year construction project will have four phases, Strebig said. Construction only can be done from June through October due to weather.
The plan
Phase 1 will take two to three years to complete and will include building the temporary access road, the staging areas and dredging about 7,100 cubic yards of spoils from the intake channel, which will be placed elsewhere in the lake.
Phase 2 will last a year and include construction of a cofferdam, a water-tight enclosure pumped dry to permit construction work below the waterline, and construction of the new gate.
Phase 3 will occur throughout phase 2 and include geotechnical research and core sampling, while the final phase is rehabilitation of the areas.
Core sampling would occur at about 30 locations within the same footprint of the 1982-1983 drilling site. Strebig said the drilling will provide important information about how much erosion of the ash layers is occurring, as well as groundwater movement. The data will be used to inform decisions about safe lake levels.
Afterward, seismic sensors will be placed in the drilled holes.
About 20 percent to 30 percent of the construction will be visible from the Windy Ridge viewing platform, and public trail access to 3 miles of the Truman Trail will be closed or partially closed during construction.
