Vine Basin Combined Sewer Overflow Control

Combined sewer overflow structure with Elliott Bay in the foreground and buildings in the background. — 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

Heavy rains in Belltown can send pollutants and untreated wastewater into Elliott Bay, making regional waterways less safe for people and wildlife. To fix this problem, Seattle Public Utilities (SPU) is planning a combined sewer overflow (CSO) control project in the neighborhood.

Combined sewers receive two types of water: Wastewater which includes untreated sewage from homes and businesses and stormwater which includes untreated runoff from streets, sidewalks, and rooftops.

Both types of water are combined into a single system which carries them to a treatment plant to be cleaned before being discharged into a natural body of water – in this case, Elliott Bay.

Because heavy rain events can overwhelm such systems, the Vine Basin combined sewer system was built with an emergency overflow near the intersection of Vine Street and Alaskan Way which allows excess untreated stormwater and wastewater to occasionally overflow into Elliott Bay.

This project aims to reduce the number of overflows to an average of one per year by adding capacity to the system and coordinating with King County to receive and treat additional combined sewer flows during large storm events.

Key project facts

Combined sewer system design upgrades are expected to include:

New 36-inch combined sewer pipe below Elliott Ave between Vine and Bay streets.
New connection to King County's Elliott Bay Interceptor pipe which will carry untreated wastewater and stormwater to a King County wastewater treatment facility.
Temporary repairs to streets and sidewalks which are disrupted for this work.

This work was scheduled to take place before an upcoming Seattle Department of Transportation project in the area.

While the Vine Basin CSO Control Project will make temporary street and sidewalk repairs, the SDOT Elliott Ave and Western Ave Paving and Mobility project will permanently pave the two streets and make other upgrades to improve safety and mobility for people walking and biking in the area.

Location

Vine Basin project area. — The Vine Basin takes in untreated water from approximately 40 city blocks — stretching all the way Pike Place Market to Denny Way. The exact location of improvements will be determined as design progresses, but construction activities are expected to be take place primarily along Elliott Ave between Vine St and Bay St.

Project schedule

SPU is reviewing preliminary design plans. Based on these preliminary plans, SPU and other agencies including the Washington State Department of Ecology, King County, the Seattle Department of Transportation (SDOT) will offer feedback, propose changes, and begin refining important details like construction schedule, traffic control plans and how to maintain access for residents and businesses during construction.

These refinements will be included in the next round of designs. Final designs are expected to be completed in 2027 when SPU will select a contractor who is expected to begin construction work in 2028. This schedule is subject to change.

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. By coordinating our work with SDOT, we are minimizing disruptions and maximizing our community’s investments in infrastructure.

Other benefits include the following:

Cleaner water in Elliott Bay, Puget Sound, and the Duwamish River.
Compliance with the State of Washington Department of Ecology consent decree.
Healthier communities by reducing bacteria and pollution that could make people sick.
Protection for fish and wildlife, including salmon and other marine life.
Safer places to swim, kayak, and fish for residents and visitors.
Less sewage in waterways during heavy rainstorms.
Long-term investment in a reliable sewer system that works as the city grows.
Support for climate resilience, helping Seattle handle heavier rains in the future.

CSOs in Seattle

Approximately one third of Seattle has a Combined Sewer System where stormwater and wastewater are combined. Since 2008, we have invested almost $1 billion to reduce CSOs. We are reducing pollution in our waters. Learn more about SPU’s commitment to reducing CSOs all over the city

Stormwater flows from roof gutters and storm drains mixing with wastewater flows from a home. The graphic is set on a rainy day indicating the relationship between heavy rain events and combined sewer overflows. — In many Seattle neighborhoods, the same sewer pipes carrying sewage (wastewater) also carry untreated rainwater (stormwater) from roofs, drains, and streets. During heavy rains, if the amount of wastewater and stormwater exceeds the capacity of the sewer system, the excess flow is released into nearby water bodies through an outfall pipe. This is called a combined sewer overflow (CSO), which can harm fish, wildlife, and swimmers.

Community engagement

SPU is committed to providing timely information and updates on project activities. Stay tuned for future community engagement opportunities as we move forward with project design.

In 2019, SPU held an in-person and online open house to share project information and collect feedback from the community. Community input from this outreach effort helped inform the options analysis process.