180th Street Corridor Improvements Snohomish County, Washington

For one of the fastest-growing areas of Snohomish County, plans for additional transit options and improvements to the arterial transportation system were considered a top priority. As a main route in the county’s southeast, the 180th corridor would be reconstructed with a multimodal design that addresses capacity needs while also enhancing multiple adjacent wetland habitats.

Increased Roadway Capacity, Fully-Mitigated Environmental Impact

In meeting the county’s five-lane urban arterial design stands, improvements to the 180th Street corridor includes the widening of the road with two new lanes to improve capacity and reduce congestion. Sidewalks on both sides of the road and bike lanes are part of multimodal design. With much of the work occurring at the confluence of two fish-bearing streams, significant changes to that system were needed. Realigning a portion of Thompson Creek and the removal of a roadside ditch creates a more natural channel and habitat. A new stormwater conveyance system adds modular wetlands for stormwater treatment, and stormwater detention vaults for flow control. The removal of hydrologic barriers along with wetland enhancements through grading, soil amendments, and planting were part of fully-mitigating all impacts on site. Otak designed two culvert replacements, a restored stream channel, stormwater facilities, retaining walls, and utility relocation to support the County’s roadway improvement goals.

Improved Fish Passage from an Redesigned Culvert System

Replacing an undesireable five-culvert system with two separate precast concrete box culverts to create a fish-passable stream channel below the 180th Street corridor. The culverts include a 90-foot long, 14-foot wide stream simulation culvert for Silver Creek and a 195-foot long, 14-foot wide stream simulation culvert for Thompson Creek. The stream channel restoration includes 700 linear feet of restored stream channel, including riffle-pool bed forms, streambed gravel, and large wood structures within the creeks. The new system opened addition al spawning habitat in the upstream watershed.

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Nico Vanderhorst

Regional Director, Puget Sound

Natural Systems Design
“I enjoy working for an employer that has cultivated a culture which encourages employees to think big and to pursue our dreams.”

Russell Gaston

Director of Water & Natural Resources

Structural Engineering
“The structural group touches a lot of projects and our role in each of them can vary greatly... I think one thing I do well is keeping a lot of tasks moving at the same time.”

Bob Doherty


Senior Project Manager