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Project update

May 2023

Potholing near 1st Ave S Bridge beginning May 25

Starting as early as May 25, King County contractor will spend up to four business days potholing, or digging small holes, to better understand the locations and condition of utility lines including sewer, power, water, telephone, and gas lines. View project flyer to learn more.

This work will occur at the storage facility site and near the following locations:

  • 2nd Ave SW between SW Michigan Street and Highland Park Way SW
  • SW Michigan Street east of 2nd Ave SW
  • Highland Parkway SW near the intersection with 2nd Ave SW
  • W Marginal Way SW near the intersection with SW Front Street

Map displaying locations of potholing (where a contractor will dig small holes to better understand the locations and condition of utility lines including sewer, power, water, telephone, and gas lines) near the future storage facility.

What you can expect

  • Work hours 8 a.m. to 4 p.m. on business days between May 25 and May 30. Potholing on Highland Park Way SW may require night work due to high traffic volumes in the area.
  • Temporary lane closures with flaggers on site to assist vehicle and pedestrian movement. Lane closures will not be concurrent, and each will last up to three hours.
  • One drill rig and up to four service vehicles on site.
  • Noise and slight vibrations during work hours, including engine noise and periodic hammering.
  • Occasional truck traffic accessing the site during work hours.

Please watch for equipment and signs in the area, travel slowly to stay safe and follow direction from flaggers.

To share your input, ask questions about this project, or for language assistance needs, reach out to Demmelash Adera at

Project overview

King County will build a 1.25-million-gallon underground storage facility near the intersection of SW Michigan St and 2nd Avenue SW to hold stormwater and wastewater that currently overflows into the Duwamish River during large storms. The water in the storage facility will then flow to the West Point Treatment Plant for treatment before it is safely released into Puget Sound, reducing risks to public health and the environment. This storage facility will reduce overflows and pollution into the Duwamish River. The County will also build supporting infrastructure underneath West Marginal Way SW.

In addition to the new storage facility, the County will upgrade existing infrastructure at the corner of Highland Park Way SW and West Marginal Way SW at the edge of the Duwamish Greenbelt.

The project has recently reached the 60 per cent design milestone. At 60 per cent design, the project details are determined including equipment, architecture, landscaping, and contractor documents (design drawings and specifications). The project team is now focused on finalizing design details and preparing for construction, which will begin in 2025.

Project timeline

Read the most recent project newsletter to learn more.

Project need

Like many cities around the country, older parts of King County’s sewer system use the same set of pipes to carry both sewage and stormwater to a treatment plant. During storms, the pipes can fill with stormwater that runs off roofs, driveways and streets. When the system is overwhelmed, it is designed to overflow. These overflows are called Combined Sewer Overflows, or CSOs.

To keep the sewer system working and to prevent sewer backups, the excess water and sewage is released into our local water bodies through CSOs. However, CSOs pose a risk to public health and the environment. Over the past several years, King County has been planning a project to reduce CSOs from the South Park drainage basin into the Duwamish River. Our requirement is to reduce CSOs to no more than one overflow per year on average.

Our commitment to the community

Community leaders and partners identified high priority neighborhood goals around water quality education, greening the neighborhoods and supporting workforce development.

Through this project, our team will seek opportunities to support community-driven initiatives in the nearby neighborhoods.

Person talking to a King County staff person at information boothProject team at Duwamish River Festival, 2022. Photo credit: West Seattle Blog.

Read our most recent project newsletter to learn about some of the community benefit projects we supported in 2022 and the community priorities we heard during our recent project survey.

Additionally, the project is part of the Combined Sewer Overflow (CSO) Art Master Plan. The theme for this public art project will be "the end of the line." The end of the line is where the built system meets the natural one at our regional bodies of water. This topic considers our relationship to the point of connection between built and natural systems.

Our commitment to sustainability

The Wet Weather Storage Facility and surrounding site will incorporate many sustainable elements. Some examples of design elements already added include new trees, solar panels, and rain gardens.



Contact us

Demmelash Adera at:

If you’d like to receive email or text updates on the West Duwamish CSO Control project:

Project location

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Combined sewer systemWhy does sewage overflow on the rainiest days?