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About the West Duwamish Wet Weather Storage project

Learn more about the West Duwamish Wet Weather Storage project schedule, need, and design elements.

Project schedule

Construction is anticipated to begin as soon as early 2026 and continue through summer 2028.

Timeline graphic showing that 2016 to early 2018 is planning, 2018 to 2025 is design, and 2026 to 2028 is construction.

Project need

Some 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 into local water bodies like the Duwamish River. These overflows are called Combined Sewer Overflows, or CSOs. CSOs help prevent sewer backups into streets and homes. However, they must be controlled because they carry harmful chemicals and bacteria into our water ecosystems. This project will reduce CSOs from the South Park drainage basin into the Duwamish River. We are committed to controlling the frequency of CSOs to no more than one untreated release per year over a 20-year average.

King County’s commitment to sustainability and Equity and Social Justice

King County prioritizes sustainable development and practices that support Equity and Social Justice (ESJ).

  • The County’s sustainability program provides a green building guide for infrastructure projects. This project will incorporate many sustainable elements including new trees, solar panels, and rain gardens.
  • King County promotes Equity and Social Justice in design, construction and management of infrastructure projects so communities can thrive. Community input helped to shape the look and feel of this project. It also helped us to identify priorities for community-driven initiatives that our team could support. Learn more on our Public Engagement page. 

Rendering of the storage facility and elements of design influenced by public input.

Rainwise!

We know that many South Park residents are excited about raingardens and other “green” solutions to keep polluted runoff or storm water out of the Duwamish. The RainWise program provides rebates that cover up to 100% of the cost of installing rain gardens and cisterns that will help manage runoff.

To learn more about the RainWise program, visit https://700milliongallons.org/

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