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March 28, 2022
Project: Anacortes Water Treatment Plant clearwell
Client: City of Anacortes
The city of Anacortes owns and operates an intake pump station on one side of the Skagit River that supplies water to the city’s water treatment plant on the opposite side of the river through a 42-inch-diameter concrete cylinder pipe, buried within the riverbed. The original pipeline, installed in 1969, has been in continuous use and has never been inspected or serviced but is believed to be in good condition.
Recent climate change has increased the potential risks to the original pipeline associated with more frequent and intense high river flows, causing concern that this original concrete pipeline was a single point of failure for the city’s water supply. To significantly increase the reliability of uninterrupted water supply, the city of Anacortes planned to construct a second raw water pipeline providing for full redundancy for the original pipeline, which will remain in service.
The city of Anacortes is a significant provider of water in the region, supplying water to over 60,000 people, including those in Anacortes, and its wholesale customers: the town of La Conner, the Swinomish Indian Tribal Community, the city of Oak Harbor, two major petroleum refineries in Skagit County, and the Naval Air Station on Whidbey Island.
The HDR team provided planning, alignment analyses, trenchless pipe installation analysis, preliminary engineering, cost analyses, design, and construction management for the pipeline project. Engineers designed a second pipeline that could carry the total capacity of the city’s 54.9 million gallons per day water right from the Skagit River combined with an additional 2.1 million-gallon clearwell tank to double the disinfection contacting time for treated water produced by the water treatment plant. Combining the new raw water pipeline and the second tank into a single construction project reduced costs, administrative effort, and risks.
The new 42-inch-diameter welded steel pipeline has a total length of approximately 3,100 lineal feet and consists of three segments. Two 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 between the two facilities, was installed using the horizontal directional drill method. The scale of this pipeline was immense, drilling approximately 60 feet below the riverbed at its deepest point, for 1,950 feet underground.
The river crossing segment was staged and assembled in sections within a temporary construction easement across adjacent farm property. After almost a month of drilling, the borehole was complete at a diameter of about 48 inches. Assembly and testing of the entire pipe string requiring field welding of 33 pipe segments happened concurrently with the boring operation. The pipe pull-in took 10 hours on Oct. 27, 2020.
The Skagit River is a significant salmon habitat and popular fishing destination, with dikes that extend along the project area providing critical flood protection for a large portion of Skagit County. These physical and construction constraints at the river crossing posed significant complexity to the design of the boring to work within the protective dikes, around the existing water treatment plant, and below the river. The horizontal directional drill construction method provided several advantages. It would not require any work within the ordinary high-water level for the river and the depth of the boring meant no impact to the existing dikes on either side of the river. These factors significantly reduced the timeline and complexity of the project permitting process and increased the allowable construction window, saving both time and costs.
The addition of the second raw water pipeline provides for full redundancy for the original pipeline, which will remain in service, significantly increasing the reliability of uninterrupted water supply to the city and its wholesale water customers. Having a second pipeline also allows inspection and service of either pipeline without loss of water service.