Smith Island Setback Levee & Estuary Restoration

CLIENT Snohomish County

Location Everett, WA

The Smith Island project restored 400 acres of estuary wetlands and tidal channels on the Snohomish River delta along Union Slough.

The plan involved construction of a new 5,000-foot-long setback levee to reintroduce tidal and riverine hydrology, restore estuarine wetland conditions, and improve environmental conditions for fish habitat.

Shannon & Wilson completed an exploration program, design, plans, and specifications for the 1.2-mile, 15-foot-high setback levee.  To overcome challenges presented by soft and variable alluvial soils, the design included a seepage berm, numerous geosynthetics, and carefully monitored fill placement to avoid stability failures.

Three feet of settlement was expected due to levee fill.  Shannon & Wilson used survey monitoring data to revise the settlement estimates during construction, reduce fill volumes, and streamline the construction schedule to meet project deadlines.

In addition, the site is located within the fallout area of the former Everett Smelter, with both lead and arsenic contamination.  Shannon & Wilson performed site characterization and developed a soil management plan that permitted the County to retain all the soil on the island using the areawide contamination guidance.  This allowed the county to avoid exportation of soil off-island, an expensive and logistically difficult process.

water Dams / Levees
Laboratory Geotechnical Engineering Contamination / Remediation Geologic Hazards Natural Resources Water Resources


2019 Local Outstanding Civil Engineering Project, Water Resources Category—American Society of Civil Engineers (ASCE), Seattle Section 


Stan Boyle
Senior Vice President

Brian Reznick
Vice President

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