Blue Bridge Scour Evaluation and Countermeasure Design

CLIENT: Snohomish County Public Works, Washington

Significant damage was inflicted on the east end of Blue Bridge during major floods in 2003 and 2006. Emergency repairs were made to the revetment during and after both floods. While investigating bank erosion at the east bridge approach for Snohomish County, NHC determined that the west end was also vulnerable to lateral erosion and local scour.

Snohomish County contracted NHC to evaluate the risk of erosion and scour at the west end of the bridge and to assist in designing countermeasures to protect the crossing. NHC also supported the County in securing the necessary permits.

NHC developed a 2D hydraulic model of the South Fork Stillaguamish River encompassing Blue Bridge. The model was used to understand the forces acting on the bridge during floods and to guide design of the protective measures. A suite of alternative countermeasures was evaluated using criteria such as cost, protection level, impacts to the river, and ability to permit. A final design was selected and evaluated in the 2D model and also in a 1D model to facilitate a FEMA zero-rise analysis and CLOMR review. NHC provided countermeasure construction oversight, which helped lead to a successful project completion.