Vine Basin Combined Sewer Overflow Control

Photo the west end of Vine Street near Elliott Bay and the outfall structure
The west end of Vine Street near Elliott Bay and the outfall structure where overflows occur. The Vine Basin CSO Control project will make improvements in the Vine Basin to limit the number of polluted combined stormwater and sewage overflows into the Bay.

Project description

Sewer pipes in Seattle carry sewage (wastewater) away from homes and buildings for treatment at King County's treatment plants before discharging into Puget Sound. In some neighborhoods, like Belltown, the same sewer pipes also carry untreated rainwater (stormwater) from roofs, drains and streets. During heavy rains, if the amount of sewage and stormwater exceeds the sewer system capacity, the excess flow is released into nearby water bodies through an outfall pipe. This is called a combined sewer overflow (CSO) and can harm fish, wildlife, and swimmers.

The Vine Basin CSO Control project will make changes to the combined wastewater and stormwater system in the Vine Basin to reduce CSO’s in Elliot Bay, improving water quality and public health.


This project will make improvements in the Vine Basin, which is in Belltown. The exact location of improvements will be determined in design, but we'll likely have impacts on Elliott Ave between Vine and Bay streets.

What's happening now?

In 2019, we received feedback from several community groups, completed sewer system flows modeling work, and analyzed the cost-effectiveness and efficiency of several preliminary design options. We are now moving forward with the recommended King County partnership option. This solution involves sending more City sewer system flow to King County's Elliott Bay Interceptor sewer pipe and will likely include:

  • Installing new sewer pipe under Elliott Ave between Vine and Bay streets
  • Installing a new connection to King County's Interceptor pipe
  • Restoring the right-of-way to Seattle Department of Transportation's standards

Now that a preferred option has been selected, the project is preparing to enter the early design phase. We are working on selecting and onboarding a consultant design firm to design the project.

Community benefits

By reducing and controlling CSO’s in the Vine Basin this project will improve water quality and support public health while reducing the risk of harm to fish and wildlife. Areas impacted by construction will be restored to existing or better condition including hard surfaces, vegetation, and pedestrian walkways.

Community engagement

SPU is committed to providing timely information and updates on project activities. Updates may be available in multiple formats: website, emails, drop-in sessions, briefings, and/or public meetings. There will be several opportunities for the public to engage and provide feedback throughout the project. Check back for more information about upcoming opportunities to get engaged and sign up for the project listserv.

SPU previously held an in-person and online open house to share project info and collect feedback from the community. See the 2019 feedback summary for details about this previous outreach. Community input from this outreach effort helped inform the options analysis process.

Construction could begin as early as 2027, pending input from our regulatory agencies, such as the Department of Ecology, as well as completion of the project design and permitting processes. The design phase will begin in 2025, and we will keep the community informed as the schedule develops.

Public Utilities

Andrew Lee, General Manager and CEO
Address: 700 5th Avenue, Suite 4900, Seattle, WA, 98104
Mailing Address: PO Box 34018, Seattle, WA, 98124-5177
Phone: (206) 684-3000

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Seattle Public Utilities (SPU) is comprised of three major direct-service providing utilities: the Water Utility, the Drainage and Wastewater Utility, and the Solid Waste Utility.