Vine Basin CSO Control

What & Why

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 (Vine Basin), the same sewer pipes also carry untreated rain water (stormwater) from roofs, drains and streets. During heavy rains, if the amount of sewage and stormwater exceeds the sewer system capacity, the excess flows overflow into nearby water bodies through an outfall pipe. These overflows can harm fish, wildlife and swimmers. This is called a combined sewer overflow (CSO).

The Washington State Department of Ecology (Ecology) requires SPU to limit the number of combined sewer overflows at each of its outfall pipes to no more than one per year on average. The Vine Basin outfall (Outfall 69), located at the base of Vine Street, does not currently meet this requirement.

The Vine Basin CSO Control project will make improvements to the combined wastewater and stormwater system in the Vine Basin so the frequency of overflows at the outfall meets Ecology's requirements.

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 design solutions. At this time, we're recommending moving forward with a King County partnership solution. This solution involves sending more City sewer system flow to King County's Elliott Bay Interceptor sewer pipe and would 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

Additionally, we may partner with other City agencies to install additional community benefits like green stormwater infrastructure and pedestrian improvements. We'll continue to work with the community throughout design.

Project goals

To improve water quality in the region and meet the commitments made to Ecology and detailed in the Plan to Protect Seattle’s Waterways.