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Project update

February 2022

Welcome to the Thornton Creek Basin Sewer Study and Upgrade webpage. The work to study the sewer basin began in Spring 2021 and it is expected to continue until Winter 2023. To learn more about this project, please read the Winter 2022 Fact Sheet here .

Learn more about this project: Washington Utility Dovetails Data Collection With Strong Community Engagement (Municipal Sewer & Water).

Project need

Through all our projects, King County Wastewater Treatment Division works to protect our region’s environment, public health, and waterways by collecting and treating wastewater. The Thornton Creek Basin Sewer Study and Upgrade Project will ensure that the Thornton Creek sewer pipe is able to carry wastewater flows so we can meet our clean water goals into the future.

The Thornton Creek sewer is an approximately 1.2-mile-long sewer pipe that collects wastewater from other pipes in our sewer system, serving 9.6 square miles of North Seattle and Shoreline. The pipe carries this wastewater to the Matthews Park Pump Station, where it is then conveyed and treated at the West Point Treatment Plant.

During heavy storms, the sewer pipe sometimes reaches its capacity. In order to avoid possible overflow, King County will need to either reduce the amount of stormwater and groundwater entering the system – known as infiltration and inflow (I/I) – or increase the size of the sewer pipe. Increasing the size of the sewer pipe may also require more costly upgrades to other parts of the regional wastewater system, including Matthews Park Pump Station.

Project benefits

This project can help provide benefits for:

  • Overall water quality in our region, by reducing wastewater overflows.
  • Local wastewater service reliability.
  • Resilience to climate change-related events.
  • Maintaining the regional wastewater system.

Project description

King County is studying the ways that stormwater and groundwater enter the sewer system in the area. Our team will look at local and regional sewer pipes, maintenance hole covers, and drainage connections, and we’ll identify the location and type of improvements that have the potential for the biggest impact to reduce I/I. The goal for this phase of the project is to identify and evaluate I/I reduction alternatives that can alleviate capacity constraints in the Thornton Creek sewer pipe.

During this phase, we’ll continue to monitor flows in the sewer system, and we’ll conduct surveys of residents who live in the project area. A future phase will compare the I/I reduction alternatives identified as part of this study to alternatives that would increase the size of the Thornton Creek sewer pipe. Together, these two study phases will help us identify a sustainable and cost-effective alternative to a major capital investment to the regional sewer system.

Sometimes during periods of heavy rainfall or major storms, additional water enters the sewer pipe, which can cause it to overflow. One reason for this problem is called infiltration and inflow (I/I), or excess water that enters into sewer pipes from groundwater and stormwater.

  • Groundwater (infiltration) seeps into sewer pipes through holes, cracks, joint failures, and faulty connections.
  • Stormwater (inflow) rapidly enters sewers via roof drain downspouts, foundation drains, storm drain cross-connections, and through holes in maintenance hole covers.

thornton infiltration and inflow graphic


Project timeline for the Infiltration and Inflow (I/I) Study. Spring 2021: identify project study area; data analysis. Summer 2021: community survey #1. Fall 2021 - Winter 2022: Field inspections. Spring 2022: identify I/I reduction alternatives. Summer 2022: community survey #2. Winter 2023: identify preferred I/I reduction alternative.

Community engagement

As the project moves forward, King County will consult the community to learn about your needs and priorities for improvements to our wastewater system.

Our teams will be reaching out to residents in areas with high I/I to learn more about sewer and drainage conditions in your neighborhood and on your property. In cases where we need to understand more about the conditions of sewer pipes, we may also request access to private property as we study the I/I problem and consider different solutions. If you may be affected, our team will reach out to you directly.

If you would like to be updated as the project progresses, please sign up for email updates, and we’ll let you know when new information becomes available. We appreciate your help as we all work together towards a clean water future.


Contact us

Contact Bibiana Ocheke-Ameh, community services lead, at:

If you’d like to receive email or text updates on the Thornton Creek Basin sewer study and upgrade project:

Project location

The study area covers the entire area served by the Thornton Creek sewer pipe. As we move forward with the project, our team will identify smaller areas that have the highest levels of infiltration and inflow for additional focus and study.

Thornton_project_mapProject Map


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