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Project update

July 9, 2018

Starting as early as July 25, the King County project team will begin drilling test holes along Coal Creek Parkway and in neighboring streets. The County’s project team is currently confirming a pipeline path that will reduce impacts to the Coal Creek Natural Area. The new sewer line will provide needed capacity for a growing community and allow us to move most of the pipeline away from its existing location in Coal Creek.

Drilling will occur at several places along Coal Creek Parkway and in three neighboring streets. It will take 2-4 days for crews to drill in each location. Construction of the new sewer line is currently scheduled to begin in 2021. View the project update for more details.

Coal Creek Natural Area

Watch for project updates as we make progress designing an upgrade to this important sewer pipeline. Once completed, this project and the new section of Coal Creek Trunk Sewer will provide service for decades to come.

View project library for past project updates.


Contact us

Contact Monica Van der Vieren community services lead at:


Project description

King County Wastewater Treatment Division (WTD) is planning to upgrade a regional sewer line that serves Bellevue and Newcastle. The existing pipeline was built between 1966 and 1991 in an area that looked different from today, with a smaller population. The pipeline is nearing its maximum capacity and needs to be upgraded to meet the growing needs of your community. When complete, the new sewer will provide safe and reliable service for another 50 years or more.

Protecting the Coal Creek Natural Area

The existing Coal Creek Trunk is approximately 2.5 miles long and primarily follows the banks of Coal Creek in the Coal Creek Natural Area between Newcastle and I-405. King County is evaluating options to move the pipe away from the Creek to protect this sensitive environmental area.

project location

Next steps

Additional information about the project area is needed before King County can finalize the route for the sewer upgrade. Over the next year, King County will study soils, groundwater, sensitive environmental areas and areas of cultural significance in the project area to confirm that the sewer can be built along the preferred route. We expect to finalize the route by late 2018.


  • Alternatives analysis: 2016 - 2017
  • Design: 2018 - 2020
  • Construction: 2021 – 2024

Where does your wastewater go?

The Coal Creek sewer line serves the City of Bellevue. The water you use in your home or business travels down the drain to the City’s sewer system. King County collects that wastewater from the City of Bellevue and routes it to South Treatment Plant in Renton.

STPaerial_600South Treatment Plant

Our commitment to you

Whether you live, work, recreate or commute in the area, we know you’ll have questions and concerns about this project. King County is committed to keeping you up-to-date. We will ask for your thoughts and input during the project. As this project gets underway, we’d like to get to know you. Our team is in the process of reaching out to neighbors and organizations in the project area. If you are interested in scheduling an individual meeting or briefing for your community organization, please contact Monica Van der Vieren at or 206-477-5502.