Background
Mount St. Helens erupted May 18, 1980, blasting more than 3 billion cubic yards of volcanic ash and debris 14 miles into the sky and thundering down the mountain in an immense landslide of mud and rock. The extraordinary natural disaster killed 57 people and impacted the lives of thousands more.
In the months following the eruption, the U.S. Army Corps of Engineers managed to offset the impacts of the flow of debris into the Toutle, Cowlitz and Columbia rivers. Congress assigned the Portland District the responsibility and authority to find long-term solutions to manage the continuing flow of sediment and reduce flooding. The Spirit Lake Tunnel, completed in 1985, helped stabilize the lake’s water levels. The Sediment Retention Structure, completed in 1989, keeps hundreds of millions of cubic yards of sediment from rushing down the Toutle River, preventing significant flooding and navigation problems.
Grade building structures
Today, sediment from the Mount St. Helens debris avalanche continues to cause flooding concerns to residents of Castle Rock, Kelso, Lexington and Longview, Wash. One strategy the Corps is testing is the grade building structures that were constructed in the North ForkToutle River, about one mile upstream of the SRS, in 2010.
The goals of the pilot project were to test the constructability and effectiveness of the grade building structures measure in order to inform the alternatives evaluation process. The pilot project consisted of:
The Cross Valley Structure A “step-weir and baffle” system, constructed of posts and panels, that slowed down the river and caused a pool to form during high-flow events. The slowing of the river allowed the sediments in the water to drop out and be retained upstream of the structure and now has pushed flows around itself, due to sediment retention.
Island Forming Structures Constructed as “engineered log jams” created of posts and logs, these produced a “protecting” effect as water flowed around them, causing more stable channels while creating relatively protected islands behind each log jam.
A Diversion Berm Built with a core of a sediment-filled geotextile tube, the diversion berm directed the majority of flow over the structures in the test area for the duration of the monitoring period and also allowed testing methodologies of the manipulation of the river system on the sediment plain.
The Corps will monitor the pilot project’s performance through 2012. The monitoring plan includes visual recording of the size and type of sediment masses being formed, gathering and analyzing sediment samples to verify the type of sediments being trapped and the use of Light Detection and Ranging (LiDAR) imagery to measure volumes of sediment. This monitoring effort has already helped engineers determine both the quantity of sediment being retained by the grade building structures and provided vital lessons about the behavior of the river system on the sediment plain. The results we’ve seen indicate that these structures could be an important component of our long-term sediment management plan, which will be developed with public input in the next few years.