King County Sinnema Quaale Upper Revetment Reconstruction Completed

Herrera is excited to congratulate King County on the completion of the construction of the Sinnema Quaale upper revetment reconstruction project.

The original Sinnema Quaale Upper Revetment on the Snoqualmie River between Duvall and Carnation, Washington was built out of repurposed automobiles, secured to the bank by timber piles. Then a separate rock revetment was constructed by King County in 1961. In recent years, flooding in combination with slope instability, led to the loss of the revetment and adjacent hill slope in places throughout a 1000-foot length of riverbank.

King County wanted to protect the Snoqualmie Valley Trail and a buried fiber-optic line buried within it, while improving conditions on nearby SR 203. Herrera assisted the County by evaluating twelve alternatives to increase slope stability and prevent future erosion into the existing infrastructure. Alternatives were compared in several areas that included: construction cost, ability to support the slope and improve habitat conditions and hydraulic and geomorphic performance. Ultimately, the County selected a layered design, with a river-facing, predominantly woody structure, secured by timber piles, backed by two steel sheet-pile walls and a structural earth wall that supports the reconstructed trail and embankment. Herrera completed the final engineering design plans and specifications in March 2015.

The first phase of construction survived several floods last year and the reconstruction of the revetment has already improved local habitat conditions, secured the slope and protected the bank from future erosion.

Additional information can be found on the King County website.