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Mount St. Helens Long-Term Sediment Management Plan for Flood Risk Reduction

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Progress rept.

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When the sediment retention structure SRS on the North Fork Toutle River became a run-of-river project in 1998, with all rows of outlet works pipes closed and all flow passing the spillway, a significantly larger amount of sediment began passing the structure. Some of this sediment deposits in the Cowlitz River where it increases flood risk. The 1985 Mount St. Helens, Washington, Decision Document, Toutle, Cowlitz and Columbia Rivers, prepared by the U.S. Army Corps of Engineers Corps and recommended construction of the SRS, identified dredging in the Cowlitz River as a means to maintain flood risk levels once the SRS became a run-of-river project, as well as providing the option for assessing other long-term alternatives. The conditions in and around the Cowlitz River are different now than from what they were in 1985. Endangered Species Act issues and a lack of readily available dredge disposal sites make dredging the river difficult and expensive. As a result, a long-term sediment management plan for flood risk reduction was initiated to re-evaluate the sediment conditions and sediment management alternatives. The Water Resources Development Act of 2000 authorized the Corps to maintain the 1985 Decision Document levels of flood protection for Castle Rock, Lexington, Longview, and Kelso on the Cowlitz River through the year 2035. Shown below are the level of protection LOP values authorized by Congress, LOP values in 1996 prior to the SRS becoming run-of-river, and current LOP values. The current LOP values were impacted by both sedimentation in the Cowlitz River and an updated evaluation of hydrology.

Subject Categories:

  • Physical and Dynamic Oceanography
  • Civil Engineering

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