SEATTLE – To safeguard the structural integrity of the Cedar River I-type floodwall, located near the municipal airport in Renton, Wash., the U.S. Army Corps of Engineers, Seattle District, is clearing all woody vegetation within 15 feet of the I-wall starting next week.
The $65,000 federally-funded project will bring the structure into compliance with Corps guidance focused on the structural performance and reliability of I-wall flood risk projects by mandating a 15-foot vegetation-free zone.
A May 2006 Seattle District inspection using the Corps’ national criteria found the structure did not meet the Corps current standards for I-walls because of numerous trees growing in the 15-foot zone on the riverside of the levee. Due to the nature of the structure, the Seattle District scrutinizes vegetation proximate to I-walls particularly closely. Seattle District conducted Endangered Species Act consultation and the National Environmental Policy Act process leading to a Final Environmental Assessment and Finding of No Significant Impact completed in October 2012.
In addition, the Corps conducted a variance viability investigation July-August 2012 to determine if some vegetation might be allowed under a variance, if approved. During field work, tree roots were found penetrating I-Wall joints and joint seals in the concrete cap. Additional analysis on the effects of vegetation on I-Walls established a methodology for estimating root spread, which found that trees within 15 feet of the wall can easily send out roots with the potential to impact the wall’s structural integrity.
Vegetation clearing is expected to begin Monday and be complete by the end of the month. Work involves the removal of 220 trees, 136 of which are 6 inches or larger in diameter at breast height (DBH), and clearing all vegetation within 15 feet of the I-wall. Outside the 15-foot vegetation-free zone, approximately 40 trees will remain to provide limited riparian habitat to species in the Cedar River.
To help offset impacts of vegetation removal, willow stakes are being planted at 12-inch spacing along 200 feet at the project site along the upstream portion of the floodwall.
In addition, two planting areas have been identified to offset the loss of tree cover at the project site. Willow stakes will be planted at 12-inch spacing along 1,620 feet on the right bank opposite the vegetation removal. The second planting site is upstream from the project site at the city of Renton’s Ron Regis Park with some plantings along the river and some along the Elliott Spawning Channel. The proposed off-site plan includes planting 408 trees to provide a 3-to-1 replacement ratio for the trees larger than 6 inches DBH that are being removed. Proposed mitigation planting is expected to be complete by the end of November 2012.
Throughout the planning process, the Corps coordinated with the City of Renton, U.S. Fish and Wildlife Service, National Marine Fisheries Service, the Washington State Department of Ecology, Washington Department of Fish and Wildlife and the Muckleshoot Indian Tribe.
The Cedar River I-wall is designed to provide flood defense up to a 100-year flood event level, or a flood with a 1-in-100 chance of occurring in any given year. Prior to its completion in 2000, damages started occurring on the left bank in a 4-year flood event, or a 1-in-4 chance of occurrence; and at 8-year level on the right bank, or a 1-in-8 chance of occurrence.
The floodwall protects the Renton municipal airport and a Boeing assembly plant along with other area businesses and infrastructure.