On March 22, 2014, a catastrophic landslide near Oso, Washington flowed thousands of feet across the Stillaguamish River valley, obliterating dozens of homes and spreading 10 million cubic yards of mud, trees and debris over a half mile area. The slide buried approximately one mile of SR 530- a critical highway that connects a string of rural communities- with up to 20 feet of debris, closing it for four months.
SDA collaborated with the Washington State Department of Transportation to develop a unique application of Best Management Practice that provided enhanced treatment within the constrained footprint available for treatment. This was a particular challenge as the roadway alignment was severely constrained by the right-of-way, restricting the project footprint and limiting the amount of space available to treat run-off. Under these conditions, traditional treatment options were not viable.
SDA was the drainage and TESC lead for the design-build team that reconstructed this portion of the highway. The reconstruction required the construction of 6 fish passable culverts, treatment systems, conveyance systems and subsurface drainage. SDA was part a design build team that delivered on an aggressive schedule which restored transportation on this corridor 3 months before the contract required and only 3 months after the slide.
The fish passage structures were especially difficult to design, because some or all of the historic channels were obliterated and the downstream channel (Stillaguamish River) had not reached is final postslide elevation. Predictions were made using a combination of pre and post slide lidar data, upstream reconnaissance, hydraulics modeling by the State and Corps of Engineers. SDA coordinated the work with the internal design team, the contractor, Snohomish County, the DOT, WDFW, and the Tulalip and Stillaguamish Tribes. SDA was proud to be a part a team that travelled to Washington DC to accept a National ACEC Award for the project.