Chelan Wet Weather Storage

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.

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

• 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.

• 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

• 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.

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.