About

Storing combined stormwater and sewage on rainy days is one way to control combined sewer overflows. With this project, extra water will flow into a storage tank when pipes get too full. The tank will hold the extra stormwater and sewage. This helps keep it out of the Duwamish River.

After the rain slows, pumps will move the water back into the sewer system. The water will then go to West Point Treatment Plant for full treatment. The storage tank will work in big storms, which is when we need it most to protect Puget Sound.

This project includes an underground storage tank. The proposed site of the storage tank is on West Marginal Way Southwest near Southwest Dakota Street.

This project is part of King County’s Mouth of Duwamish Wet Weather Facilities program. The program will reduce combined sewer overflows from five combined sewer outfall pipes. It will keep more than 400 million gallons of polluted water out of the Duwamish River and Elliott Bay each year. This work helps King County meet state and federal clean water laws. Learn more about the program at kingcounty.gov/MDWetWeatherFacilities.

The Chelan Wet Weather Storage project includes these proposed clean water facilities:

• An underground storage tank near North Delridge on West Marginal Way Southwest. During heavy storms, the storage tank will hold extra water until the treatment plant is ready for it. After the storm, the water will be sent by pipe to West Point Treatment Plant for treatment.
• New pipes and an upgraded support building. They will connect the storage tank to the regional sewer system and help everything work together.

All communities deserve a fair chance to do well. To support this, King County makes equity and fairness a priority in every stage of our projects.

• We listen to people and work with neighbors. We ask for feedback and use it in our plans when possible.
• We focus on communities most affected by our work.
• We plan and build projects that protect public health and the environment.

We share regular project updates and provide opportunities for community feedback. We do this in each phase of the project.

We looked at many different ways to design and place these facilities. We compared each option using the same set of factors: 

Location matters. The proposed plan places the storage tank close to existing sewer pipes and the Duwamish River. This means we need fewer new pipes. Fewer new pipes means less time spent building, lower costs, and fewer disruptions to nearby roads and neighborhoods. 

Before construction, you may see our team gathering information about the project area. This may include soil and groundwater testing. We will notify nearby businesses and community members before this work starts. We usually do this notification 48 to 72 hours in advance. 

Construction could start as early as 2029 and last about five years. The facilities could begin operating as soon as 2034. As construction gets closer, we will share updates about what to expect and how we will stay in touch. 

This project is currently in the design phase.

During the design phase, we will:

• Create detailed plans for building each facility
• Apply for permits to start building
• Write contracts that will explain the work and who will do it
• Study soil and groundwater samples to learn more about the project area
• Identify environmental impacts and plan ways to address them