PORTLAND, Ore. – The U.S. Army Corps of Engineers is seeking public comments on proposed updates to its decades-old long-term sediment management plan for Mount St. Helens.
In an updated draft of the supplemental environmental impact statement for its plan, the Corps proposes the phased construction of two incremental spillway crest raises at the sediment retention structure on the North Fork Toutle River, followed by the construction of grade-building structures on the sediment plain upstream from the main structure. Dredging of the lower Cowlitz River would also be performed as needed.
The Corps has been fighting sediment erosion at Mount St. Helens since the volcano’s 1980 eruption triggered a debris avalanche, which deposited more than three billion cubic yards of sediment into the Toutle River basin. Recognizing that erosion from the debris avalanche would result in elevated sediment loads for several decades, the Corps completed a long-term management plan.
From this 1985 plan, the Corps was authorized to build, operate and maintain a sediment retention structure at North Fork Toutle River and perform associated actions to help reduce flood risk to communities downstream. Because the structure blocks upstream fish passage, the Corps also constructed a fish collection facility just downstream. Fish collected at the facility are transported by tank truck for release upstream from the structure.
The 1985 plan recognized that additional actions would be needed in the future to maintain the congressionally authorized levels of flood risk protection. The revised draft document describes why the Corps’ ongoing involvement is critical and lays out its plan for the coming decade and beyond.
“Our proposed measures take into account numerous changes to conditions in and around the project area since the original 1985 decision document was published and will allow the Corps to continue managing flood risk for communities along the lower Cowlitz River through the year 2035,” said Mike Turaski, project manager for the long-term sediment management plan.
To ensure the proposed sediment management plan does not jeopardize any endangered or threatened species or adversely modify their designated critical habitat, the draft document has been revised to include an evaluation of fish conservation measures. Such measures include establishing a third upstream fish release site at Deer Creek and the modification or replacement of the Washington Department of Fish and Wildlife fish collection facility just downstream from the retention structure. Any changes to the facility would be done in partnership with the state of Washington.
“Our proposed measures take into account numerous changes to conditions ... and will allow the Corps to continue managing flood risk for communities along the lower Cowlitz River through the year 2035.”
The two spillway crest raises described in the phased construction proposal would increase the total height of the spillway by up to 23 feet. Once the spillway crest has been fully raised, grade-building structures described in the same proposal could be built well upstream of the retention structure to further assist in sediment management.
The adaptive nature of the phased construction proposal is part of a cost-effective strategy for managing flood risk based on sediment in-filling conditions behind the retention structure, conditions in the lower Cowlitz River and the budgeting cycle for funding, explained Turaski. The order in which the phases would be implemented was deliberately chosen to maximize efficiency and avoid overbuilding.
“Raising the spillway crest increases the sediment trapping efficiency at the sediment retention structure,” he said. “Meanwhile, grade-building structures will help increase the rate of sediment accumulation, further raising the efficiency of the main structure.”
For each of the three phases of construction, the decision to begin construction would depend on the results of the Corps’ flood protection level monitoring in leveed areas along the lower Cowlitz River — namely, in the Washington cities of Castle Rock, Lexington, Kelso and Longview. Each construction phase would only be launched if the authorized flood protection level for any of these leveed areas is not being met and the Corps determines the level of protection could not recover naturally.
As part of the revised proposal, the Corps accounted for known affected interests and its consultation with the National Marine Fisheries Service since the original draft document was opened to public review in August 2014. The Corps is now holding a second public comment period to give the public the opportunity to comment on these revisions.
The public comment period is intended to provide those interested in the proposed actions an opportunity to make their concerns or comments known. Upon completion of the public comment period, the Corps will consider all comments submitted on both the original and this updated draft of the supplemental environmental impact statement before issuing the final document. The final document will include responses to comments received on both the original and this revised draft.
Public involvement is a vital part of the Corps’ planning process because it allows the agency to work with the public to better inform its decisions. The participation of interested citizens in this process ensures management decisions can be made based on all available information. Public comments allow the Corps to meet its public service goals while lowering the chances of unintended consequences to affected interests.
The revised draft of the supplemental environmental impact statement is available for review and comment at the Mount St. Helens section of the Portland District website at www.nwp.usace.army.mil/mount-st-helens.
Written comments may be sent via email to MSHLongTermPlan@usace.army.mil or by regular mail to:
U.S. Army Corps of Engineers, CENWP-PM
ATTN: Ann Hodgson
P.O. Box 2946
Portland, OR 97208-2946
The Corps will consider all comments received or postmarked by Nov. 6. Please reference public notice CENWP-PM-E-17-03 in all written correspondence. All comments received will become part of the administrative record and are subject to public release under the Freedom of Information Act, including any personally identifiable information such as name, phone numbers and addresses.