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King County Washington home

Chelan Wet Weather Storage

King County plans to build an underground storage tank in the North Delridge neighborhood of West Seattle. During large storms, the 7-million-gallon tank will hold extra stormwater and sewage. This helps prevent untreated water from entering the Duwamish River.

Project update

July 2026

This project is currently in the design phase.

Field survey work continues

We are doing on-the-ground field surveys in the project area. This work helps us plan our design and prepare for future construction. The surveys show us what is underground near our planned facilities. They also help us identify places that may be culturally or historically important. 

This survey work began in summer 2025. We expect it to continue through summer 2026. 

If you see survey activity near North Delridge, here’s what to expect: 

  • Standard work hours are 7:30 a.m. to 5 p.m. We will reach out to nearby businesses in advance if we plan work outside these hours. 
  • Crews typically use one truck and one survey vehicle  
  • You may hear engine noise or hammering while crews are working. You may also feel slight vibrations. 
  • Crews may need to set up detours. If they do, flaggers will assist drivers and people walking or biking.  
  • Crews will remove equipment and restore disturbed surfaces once work is complete.  

Thank you for your continued coordination as our crews complete this work. 

Find more information about this upcoming work. 

Upcoming community events

Come meet our team this summer and learn more about Mouth of Duwamish Wet Weather Facilities. 

SODO Business Improvement District National Night Out

When: Tuesday, August 4, 2026, from 4 – 6 pm  

Where: Canine Companions Puget Sound Field Office - 2454 Occidental Avenue South  

What: Connect with community members, business owners, and local law enforcement over cold drinks and tasty bites. Register here.  

SOPASUPA x Duwamish River Fest x River City Skatepark

When: August 8, 2026, from 12 pm – 9 pm  

Where: 14th Avenue South and Cloverdale Street South / Duwamish River People’s Park – 8700 Dallas Street South  

What: Join your neighbors for an epic day of free music, kid’s activities, skateboarding, beer garden, cultural traditions, and more. Learn more about the event here.  

Stay informed

We will share updates as design continues.

Sign up for email and text updates for the Chelan Wet Weather Storage project.

Contact the project team

Questions or comments are always welcome. Contact Broch Bender using one of the channels below.

Map showing the Chelan Wet Weather Storage project area. The upgrades include new pipes, new support building and a storage tank. The project helps prevent polluted water from entering the Duwamish River during storms.
Map showing the Chelan Wet Weather Storage project area. The upgrades include new pipes, new support building and a storage tank. The project helps prevent polluted water from entering the Duwamish River during storms. 

Project description

Heavy storms can send millions of gallons of stormwater through Seattle’s sewer system. When the pipes fill up, this wastewater can overflow into the Duwamish River and Elliott Bay.

In North Delridge, most of this overflow comes from an outfall pipe called the Chelan Avenue pipe. During large storms, this pipe releases water that has not been treated. This project builds an underground tank to temporarily hold extra wastewater on rainy days until it can be treated.

This project also includes upgrades to a support building and new underground pipes.

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The image above shows what the site could look like. The building design and site features, including the fence, is in progress and may change as work continues. The storage tank will be underground, with only support buildings above ground.

Why this project matters

  • The tank will hold extra stormwater and sewage during large storms.
  • This helps keep untreated water out of the Duwamish River and Elliott Bay.
  • Cleaner water is better for people, fish, and wildlife.

How the system works

The steps below show how the sewer system works on dry days, during storms, and after the rain slows.

On dry days or during light rain

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  • Local sewer pipes collect water and sewage from homes, businesses, and streets.
  • These pipes connect to larger pipes called trunk sewers.
  • Regulator stations control how water moves into these larger pipes. Think of them like traffic lights that help keep water flowing through the system.
  • Water then flows underground through trunk sewers to West Point Treatment Plant.
  • West Point Treatment Plant cleans the water before it enters Puget Sound.

During heavy rain

  • Large amounts of rainwater run off streets and rooftops into storm drains. This sends much more water into the sewer system than on a typical day.
  • The combined sewer pipes in the area can fill faster than the system can send water to the treatment plant.
 

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  • When the pipes fill up, the system sends extra stormwater and sewage to the Chelan Avenue combined sewer outfall (CSO). The outfall is located along the Duwamish River.
  • This releases a mix of stormwater and sewage into nearby waterways.

In the future

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  • This project will store extra stormwater and sewage in a new underground storage tank.
  • The tank will hold extra wastewater during storms.
  • After the rain slows, pumps will send the stored wastewater back into the sewer system.
  • The wastewater will travel to West Point Treatment Plant. There, it will be treated before it enters Puget Sound.
  • New pipes and upgrades to a support building will connect the underground 7-million-gallon tank to the sewer system. The new pipes will be under West Marginal Way Southwest. They will run from Chelan Avenue Southwest to Southwest Dakota Street.
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Project funding

King County Wastewater Treatment Division pays for this project. Ratepayers help fund this work through sewer charges on their local sewer bill.

The current estimated cost for this project is $700 million. This estimate is subject to change.