Increasing the Resiliency of the City’s Water Supply System

For more than 50 years, the City of Anacortes relied on a single raw water pipeline to supply their treatment plant. The 42-inch-diameter concrete pipe extends under the Skagit River but posed a single point of failure, which would leave the City without clean drinking water.

The City is a major provider of water to over 60,000 people in the region. Therefore, the project significantly increases the resiliency of the City’s water supply system. It also strengthens the City’s position as a reliable, responsible long-term water provider for its citizens and wholesale customers and enables sustainable social and economic development in Skagit County and the northern portion of Island County.

The new Anacortes Water Treatment Plant Resiliency project adds a 54.9-million-gallon second raw water line under the river and a 2-million-gallon clearwell to operate in parallel with the facility's current treatment operations. The new pipeline can carry the full capacity of the City's need while providing operators flexibility and allowing the original pipeline to be shut down for inspections. As the City’s project engineer, HDR provided planning, alignment analyses, trenchless pipe installation analysis, preliminary engineering, cost analyses, design, and construction management and observation for the pipeline.

The new 42-inch-diameter welded steel pipeline measures approximately 3,100 lineal feet and consists of three segments. Two of the segments, one on each side of the river, were installed using typical open trench pipe construction. The third segment, extending for 1,960 lineal feet under the river from near the intake pump station to the WTP, was installed using horizontal directional drilling. This was critical as the Skagit River is a major salmon habitat and highly popular fishing destination.

The project team overcame significant design challenges with the directional drilling, including being deep enough to minimize inadvertent hydraulic fracturing, providing vertical clearance under the dikes and utilities, and locating the entrance and exit away from structures. To satisfy the design constraints, the team evaluated three alignments and then combined the new pipeline and clearwell tank into a single construction project, which helped reduce the City's administrative effort and risks.

Completed 20% under budget and in-service within five months of the notice to proceed, the new water pipeline and clearwell provide additional resiliency and reliability to Anacortes' system.