Nearly 30 years ago, the Portland District became the first and still only U.S. Army Corps of Engineers district in the continental United States to contend with an active volcano. The 1980 eruption of Mount St. Helens choked the Toutle, Cowlitz and Columbia rivers with massive mudflows and created a lifetime of work for District engineers.
The initial emergency response was as massive and swift as the mudflows that created the problems. The Corps responded immediately by raising levees and roads, removing debris and clearing blocked creeks between Castle Rock and Longview, Wash.
The Corps conducted emergency dredging for eight months, until the Columbia River navigation channel was fully restored and just over 100 million cubic yards of material had been removed from the three rivers.
By 1984, longer-term solutions were under way. A Corps project stabilized Spirit Lake, which had risen nearly 200 feet due to the debris, by tunneling 8,500 feet through Harry's Ridge and diverting some of the water to lower the lake's elevation.
At the end of 1989, the Corps completed construction of a Sediment Retention Structure on the north fork of the Toutle River. The 1,800-foot long, 184-foot high SRS was designed to slow the flow of water enough to allow sediment to drop out and be retained behind the structure.
The SRS handled the task pretty well for nearly two decades. While significant storage capacity remains, the SRS now retains sediment at about a quarter of the rate it did for the first 10 years. The 1985 decision document noted that this would happen, and suggested the Corps consider the cost of annual dredging versus raising the SRS once it did.
"What's happening with the SRS now is what's supposed to happen," said Jeremy Britton, the project's technical lead. "It's less efficient now that water is moving through the spillway, which is why we're looking at alternatives to manage the additional sediment."
Britton leads a technical team putting together a long-term sediment management plan. Unlike the emergency response in the 1980s, the Corps must evaluate alternatives with many variables in mind, such as more stringent environmental regulations.
The two main goals of the plan are to provide flood risk management for the communities of Longview, Kelso, Lexington and Castle Rock, Wash., and to maintain navigation on the Columbia River. Potential measures are also assessed to the degree that they minimize impacts to fish and wildlife, reduce operation and maintenance costs, protect cultural resources, are adaptable to changing conditions and are cost-effective, reliable and acceptable.
The team recently completed the first screening of alternatives, narrowing the field from 16 to eight viable options to manage sediment that will enter the system through 2035, the Congressionally authorized life of the project.
The team is now poised to begin the second-level screening on the remaining alternatives by taking a more in-depth look at each method, including conceptual design, modeling and estimated costs for each.
"Alternatives that were screened out in the first round are not necessarily off the table, but the level of effort for the second screening is much higher so it's beneficial to set aside some options based on feasibility," said Dylan Davis, project manager of the Mount St. Helens long-term study.
The measures now getting a closer look manage the sediment in a variety of ways. Raising the SRS or building multiple smaller-scale structures above the SRS would trap sediment close to the debris avalanche. Other possibilities are creating a sump in the Toutle River or dredging the Cowlitz River, both of which focus on handling the sediment much farther downstream.
The Corps is also considering using the Cowlitz River itself to handle the sediment by expanding the floodplain, releasing flushing flows from Mossyrock Dam and adding dikes to the mouth of the river to help push the sediment through.
The efficacy of some methods is more easily questioned. For instance, expanding the Cowlitz River floodplain would require a large-scale displacement of existing infrastructure and people.
"A closer look is warranted to calculate how effective it could be, but it is highly unlikely the benefits will outweigh the costs," Britton said.
On the other end of the spectrum, raising the SRS has been on the list as a long-term management alternative since it was built. While expensive, the SRS' previous performance suggests this would be highly effective.
"The preferred alternative will most likely be a combination of several measures selected based on their effectiveness, cost, adaptability and environmental considerations," said Britton.
The long-term sediment management plan is scheduled to be released by December 2009, and environmental coordination for the chosen methods will occur the following spring. But the Corps isn't waiting to address the problem.
The immense amount of debris from the eruption dramatically altered the hydraulic and hydrologic systems of the Cowlitz and Toutle rivers and reduced levels of protection provided by levees along the lower 20 miles of the Cowlitz River.
Construction of a levee cutoff wall along 1,700 feet of the Castle Rock levee begins soon, to ensure that it can withstand the increased flood risk. While flood risk is often thought of as overtopping of levees, this section of the levee is susceptible to underseepage: water could potentially seep under the levee and come out the other side. The cutoff wall will force any underseepage farther under the levee wall, reducing the force of the water and the flood risk along with it.
"Nothing has changed as far as the resistance of the levees; rather, the sediment load of the river and the uncertainty of flood frequency have increased," explained Britton. "We want to make sure the levee improvement is designed with this in mind."
Levees in the other communities along the lower Cowlitz River have decreased levels of protection as well, but they had more to begin with and are not at as much risk as the Castle Rock section.
Dredging planned for this November will benefit those communities and help to alleviate the burden on the levees. The Corps dredged the lower Cowlitz River for the first time in nearly 10 years in 2007 and again in 2008. The effort removed a great deal of sediment, but much more will need to be done to manage the sediment that continues to enter the system annually.
"The amount of material that needs to be dealt with is tremendous," said Tim Kuhn, Mount St. Helens project manager. "It's a very complex issue and there's no simple solution."
The debris avalanche was estimated at 3 billion cubic yards after the eruption. Only a small fraction of that sediment has been dealt with. The measures that come out of the long-term plan should manage sediment through 2035; however, the Corps is well aware that the problem won't end there.
"We know that what we do in the next few years won't be the final solution for this problem," said Britton, who was seven years old when Mount St. Helens erupted.
"It's not something we can fix and walk away from. This is an ongoing challenge that will need to be revisited by future generations."
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