By Ms. Michelle Helms (USACE)April 23, 2010
TOUTLE, Wash. - When you nudge it with your toe, it moves but doesn't break - kind of like the skin that forms on the top of pudding after it's been sitting for a while. That's what the still-moist sediment gathering behind the upstream base of the Sediment Retention Structure on the North Fork Toutle River feels like.
The drier, more solid ash and sand closer to the structure is covered in native vegetation and inhabited by the creatures who make their home on Mount St. Helens - signs of life in an area still recovering from one of the greatest natural disasters in the continental United States.
Mount St. Helens erupted May 18, 1980, blasting more than three billion cubic yards of volcanic ash and debris into the sky and down the mountain in an immense landslide of mud and rock. The SRS is one of the tools the Corps of Engineers uses to manage the avalanche that is still, 30 years later, making its way down the mountain.
A little history
The extraordinary natural disaster killed 57 people and impacted the lives of thousands more. It was an event of such magnitude that the mere mention of it sparks vivid memories for people living in the Northwest when it happened.
"I call it my 'Independence Day' movie experience," said Jerry Christensen, deputy chief, Engineering and Construction Division. In May 1980 Christensen lived in Troutdale and was a soils engineer in the Engineering Division.
"I was just getting started for the day and headed out to get my Sunday morning Oregonian newspaper. As I walked towards the paper box everyone on the street was looking to the north and there it was...Mt. St. Helens billowing ash to 40,000 or 60,000 feet."
"It was frightening to have been anywhere close to the event and being totally helpless," said Don Chambers, chief, Engineering and Construction Division.
Chambers, a structural engineer in the Engineering and Planning Division in 1980, said the sheer power of the eruption was impressive.
"It blew off the top and the side of the mountain, instantly launching billions of cubic yards of rock and soil."
In the months following the eruption, the Corps found fast, effective solutions to offset the impacts of the huge flow of debris into the Toutle, Cowlitz and Columbia rivers.
"The Toutle and Cowlitz rivers looked like chocolate milkshakes," said Janice Sorenson, paralegal specialist in the Office of Counsel.
Within a few years, Congress tasked Portland District with finding long-term solutions to manage the continuing flow of sediment and reduce flooding. The Spirit Lake Tunnel, completed in 1985, provided much needed stabilizing of the lake's levels.
The SRS, completed in 1989, keeps hundreds of millions of cubic yards of sediment from rushing down the Toutle River and causing significant flooding and navigation problems.
Mount St. Helens is still active and the avalanche triggered by the eruption 30 years ago is still moving, so the Corps still needs a long-term plan to address flood protection. Plans to provide that protection are under development.
A long-term commitment
Some Corps experts say the hardest part of the project is determining the size of the problem.
The avalanche has stabilized some, but material is still coming down the mountain. The ever-changing conditions make it tough for engineers to estimate how much more sediment will flow out of the mountain, let alone find solid solutions.
"We will never stop all of the material," said Tim Kuhn, Mount St. Helens Project Manager. "It's more of a matter of how to best manage the sediment that directly impacts the lower Cowlitz River and maintain the congressionally authorized levels of flood damage reduction."
When Congress entrusted the Corps with flood damage reduction for the Washington communities of Longview, Kelso, Lexington and Castle Rock, engineers recognized dredging would be part of the plan. Despite the SRS, a tremendous amount of sediment still makes its way downstream to the lower Cowlitz.
As important as it is, dredging is somewhat inefficient and costly. Since 2007, more than 3.5 million cubic yards of sediment have been dredged from the Cowlitz, at a cost of $7.7 million. Once the material is removed from riverbeds it has to go somewhere, and finding areas to place it is challenging.
A balancing act
Another difficult aspect of the project is finding the right balance between protecting human life and property, and allowing the life and beauty of the mountain to thrive.
"We want to identify and implement an effective long-term strategy to manage the massive volumes of sediment, while also working to protect and potentially enhance the ecosystem," said Kuhn.
One challenge for the Corps is to find ways to protect lives and property downstream that follow the requirements of environmental policies put in place since the 1980s.
That's a challenge a new generation of Corps members are ready to meet.
"As a native Washingtonian, Mount St. Helens was often referred to in school; it's significant to state history and I have a personal interest in the project," said Alison Burcham who was born nearly a year after the eruption and who recently joined the Mount St. Helens team. "I have an environmental engineering background, so the concept of returning the area to equilibrium is the kind of thing I'm interested in."
"Environmental awareness and regulations have evolved immensely over the past 30 years," said Jim Stengle, environmental resource specialist. "Many actions and responses that occurred in the 1980s likely would not be permitted today."
Any plan for managing the debris has to pass muster with at least 16 federal laws and executive orders, as well as any state and local regulations, before it can begin being carried out.
A new approach
These days the Corps is studying new ways to manage sediment.
"We're taking an adaptive strategy approach," said Kuhn. "Building smaller structures will allow us to trap sand on a smaller scale than the SRS without having to build a large permanent structure."
Engineers believe a series of structures upstream from the SRS will help reduce the need for frequent dredging while still providing effective flood management and plan to test the idea this summer.
"The grade or slope of the sediment plain is already increasing due to deposition," said Jeremy Britton, Geotechnical Team Lead for the Mount St. Helens project. "The goal of the structures is to increase the rate of deposition, adding roughness and slowing down the flow of water, which will encourage deposition of sediment."
The plan has built-in flexibility: structures would be placed when and where they're needed, based on the changing conditions. The structures undergoing testing include large woody debris, pile dike structures and low-height wood weirs. Their size and shape can be adapted to meet the challenges of an evolving environment.
The test program is undergoing a series of reviews and is on track to begin construction this summer.
While it is obvious the mountain is still adapting to the massive amount of ash and debris coming out of the crater, visitors to Mount St. Helens can see nature fighting back against the devastation of the eruption. Trees planted a few years after the volcano exploded now tower over elk and other wildlife nibbling on the vegetation taking root in the ash and sand, leaving footprints in the pudding-like sediment.