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Project update

Winter 2022

King County has completed field inspections for the Thornton Creek Basin Sewer Study and Upgrade Project. In July and August, we conducted smoke testing in five small geographic areas within the project study area.

Thank you to the residents in these areas for your cooperation during smoke testing. Smoke testing provided important data about potential problem areas within the local sewer system, which connects to the County’s Thornton Creek sewer pipe. This is a safe, efficient, and cost-effective method that will help us minimize environmental damage that occurs when the sewer system is overfilled with stormwater and can lead to sewer overflows. Read the Smoke Testing frequently asked questions to learn more about smoke testing.

For those that requested it, we mailed you video footage from the side sewer inspections, which you should have received by November 12, 2022.

We have now completed the study.

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 has been studying the ways that stormwater and groundwater enter the sewer system in the area served by the Thornton Creek sewer pipe. Our team looked at local and regional sewer pipes, maintenance hole covers, and drainage connections. We made progress in identifying 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 was to identify and evaluate I/I reduction alternatives that can alleviate capacity constraints in the Thornton Creek sewer pipe

In fall 2021, we sent a survey to 9,971 addresses within the study area and 700 people responded. The survey asked about sump pumps, foundation drains, downspouts, side sewers, surface water and storm drainages, community benefits and communication preferences. This information has informed our field inspections, happening since spring 2022

Our final step was to compare 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 guide us towards 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 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 - Summer 2022: Field inspections. Spring 2022 - Fall 2022: identify I/I reduction alternatives. Winter 2023: identify preferred I/I reduction alternative.

Community engagement

In fall 2021, we surveyed residents in areas with high I/I to learn more about sewer and drainage conditions in their neighborhoods and properties. Thank you for taking the survey and sharing your feedback!

Read more about the engagement effort here .

In summer 2022, we conducted smoke testing in five small geographic areas. We were in the neighborhoods while the smoke testing was going on and able to respond to questions and concerns in real time. Thank you to the people who live in the impacted areas for your cooperation and patience while we conducted the tests and gathered data.

Learn more


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. The map below highlights the areas where smoke testing will occur.

Map displays project area and highlights where smoke testing will occurProject map

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