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Project update

September 2021

The West Duwamish CSO Control Project is moving forward with a storage tank to hold stormwater and wastewater during large storms. The tank will prevent raw sewage from spilling into the Duwamish River during these storms. After the storm, the water will flow to West Point Treatment Plant for cleaning.

The project is currently in the design phase. The project schedule will be influenced by factors including the federal consent decree to reduce combined sewer overflows, permit requirements, and the Clean Water Plan.

Watch for more information this fall about the project’s history and current plans, and how you can provide input.

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.

Project description

Every drainage basin is unique, so over the last year we’ve been focused on learning the specifics of South Park, including studying soils, the groundwater table and how redevelopment has influenced the way water flows in the neighborhood.

We looked at a range of options to reduce CSOs in South Park, including:


Using plants, trees and soil to soak up water is called green stormwater infrastructure (GSI). GSI techniques could include roadside rain gardens or permeable pavement that allows stormwater to soak through to the ground below.


Traditional infrastructure solutions use pipes and underground storage tanks to store wastewater. Tanks and pipes hold extra sewage and stormwater until there is room in the pipes again. If a storage pipe or tank is selected, that project could take place outside of South Park.


A separated stormwater system consisting of two pipes: one that carries stormwater and one that carries sewage from homes and businesses. If a separated system is selected, it would include the design and installation of new pipes underground.

The project team considered technical needs, operation and maintenance requirements, land use and permitting, community impacts and cost. We have determined that an underground storage tank is the most effective solution to minimize CSOs in South Park. We are still working out details on location, but we anticipate that construction will take place near the 1st Avenue South Bridge.

Some of the benefits of a storage tank include:

  • Building a storage tank allows us minimize construction impacts in residential areas of South Park
  • Most storage tank construction will take place off the road, so we can keep traffic moving
  • Lowest construction and operations cost of the options studied


Contact us

Contact Kristine Cramer, community services lead at:


Amelia Bahr at:

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

Project location

West Duwamish CSO control map

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Combined sewer systemWhy does sewage overflow on the rainiest days?

Person talking to a King County staff person at information boothKing County RainWise team at the 2021 Duwamish River Festival