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 the nearby Puget Sound 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.

Location

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 2025, we contracted with an engineering firm to start the design of the recommended option to increase the flow capacity of the Vine Basin sewer-storm water system. This option involves working with King County as they will need to receive higher flows in their Elliott Bay Interceptor sewer pipe during large storm events. Sewer-storm water system design upgrades are expected to include:

Installing a new 36-inch sewer pipe under Elliott Ave between Vine and Bay streets
Installing a new connection to King County's Elliott Bay Interceptor pipe
Connecting the new 36-inch sewer pipe to existing sewer pipes near the intersections where Elliott Ave crosses Vine, Cedar, Clay, and Broad streets
Restoring the right-of-way to Seattle Department of Transportation's (SDOT) standards. This restoration work will occur under a separate SDOT street improvement project that immediately precedes the installation of the new 36-inch sewer pipeline.

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 which helped to inform the options analysis process.

Schedule

Construction is anticipated to begin in the fall of 2027, pending input from our regulatory agencies, such as the Department of Ecology, as well as completion of the project design and permitting processes. We will keep the community informed as the design of this project progresses and more details are known.