Today, sediment from the Mount St. Helens debris avalanche continues to cause flooding concerns to residents of Castle Rock, Kelso, Lexington and Longview, Wash.
In 2010, the Corps constructed a series of grade building structures designed to trap sediment flowing from the Mount St. Helens volcano, preventing it from making its way into the Cowlitz River system. Engineers believe these structures, located upstream from the SRS, combined with other actions, will help reduce the need for frequent dredging while still providing effective flood risk management.
Long-term sediment management
The Corps is developing a long-term sediment management plan to limit the amount of sediment flowing into the Toutle, Cowlitz and Columbia rivers. A range of prospective actions are being reviewed to develop long-term solutions for the vast amount of Mount St. Helens sediment. For more information on this plan, please visit the Sediment Retention Structure.
The Corps is working with the Cowlitz Tribe, state and federal resource agencies to ensure the Corps’ near-term sediment management actions will not impact downstream juvenile and adult fish passage, and ensure that long-term actions will provide the opportunity for upstream volitional fish passage in the North Fork Toutle River.
The goals of the pilot project were to test the feasibility of construction 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.
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 is monitoring the GBS performance. The monitoring 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 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 over the next few years.
In 2012, the Corps raised the SRS spillway 7 feet to increase sediment storage capacity behind the existing structure to address the near-term sediment management needs. The 7-foot spillway raise forms a shallow pool behind the SRS of about 2 million cubic yards and 290 acres. Sediment will fill to the elevated spillway crest within one to two years. The spillway raise is expected to increase the overall trapping efficiency, because the raised crest will reduce the slope behind the SRS, which should cause more sediment to settle out before passing over the spillway.