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Windermere Basin

  • Tank under-drain system completed – April 2013
  • Completed excavation for the 2.05 million gallon underground storage tank in Magnuson Park – March 2013.
  • Large tree stumps found during tank excavation.
  • Large tree stumps found during tank excavation.
  • Tree stumps on display along Sportsfield Drive.
  • Construction of 2.05 million gallon underground storage tank in Magnuson Park taken in February 2013.
  • Construction of 2.05 million gallon underground storage tank in Magnuson Park taken in January 2013.

Sewer and Stormwater Pollution

Currently, during normal weather conditions in Seattle, existing barriers in the sewer system in the Windermere Basin prevent combinations of wastewater and stormwater from overflowing into nearby Lake Washington. However, during episodes of heavy rainfall, these mixtures overcome the sewer barriers and flow into the lake.

Project Description and Benefits

The Windermere Sewage Overflow Prevention Project will improve these existing sewer systems to reduce the frequency and volume of sewer overflows into Lake Washington during periods of heavy rainfall.

Near Magnuson Park on the south side of NE 65th Street, the project team will install:

  • A 2.05 million gallon (MG) underground storage tank
  • A facility vault with mechanical and electrical equipment

Beneath NE 65th Street and Sand Point Way NE, the project team will install:

  • A 2,250-foot-long gravity sewer and parallel forcemain
  • Motor-operated gates to control the flow of wastewater into the tank
Map of construction area with pipeline sections color-coded by installation method.

View an area map of where construction will occur (right). The staging area, tank site, and pipeline alignment are all displayed here. Sections of the pipeline are color-coded according to installation method.

After project completion, the local sewer system will divert sewage and stormwater into the new storage tank during heavy downpours. Once the rainfall subsides, the storage tank will empty the contents back into the sewer system for regular processing. Consequently, this new sewer system will reduce sewage and stormwater pollution in Lake Washington.

View Engineering Plans for the project (pdf)

Project Schedule

Onsite activities will last approximately two years, from September 2012 onwards. Level of onsite activities and potential temporary impacts in the community will vary by season and construction phase; the overall construction schedule is subject to change. Below is a visual sequence of when project activities begin.

Windermere Basin construction-process

Community Engagement

For more than two years prior to construction, the project team has reached out to Windermere residents, businesses, and community groups to discuss how the project will reduce sewer and stormwater pollution in Lake Washington. For a detailed list of these efforts, view the History of Windermere Basin Community Engagement (pdf).

Project Brochure

To learn more about this pollution prevention project, download the Windermere Basin project brochure (pdf).

Contact Us

Email the Windermere CSO project team at SPU_WCSO@seattle.gov to join our mailing list to receive regular updates or call us at (206) 386-9778 if you have specific questions or concerns.