North Thornton Natural Drainage Systems

Example of a natural drainage system a few years after installation

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Project description

Improving creek water quality, reducing drainage issues, and providing other community benefits.

More than 12 million pounds of pollution are carried into our water systems by stormwater runoff every year. Stormwater that falls on hard surfaces in your neighborhood - like roofs, streets, and parking lots - can pick up harmful pollutants before it runs into Thornton Creek, which flows into Lake Washington and eventually into Puget Sound. The pollutants can harm fish, wildlife, and our ecosystems.

SPU is working to minimize pollution at its source. Natural drainage systems (NDS) are shallow depressions built in the roadway shoulder (the public space between the street edge and property line). They are filled with special soil and deep-rooted plants to temporarily hold and filter stormwater, reducing pollution in Lake Washington and Puget Sound.

The North Thornton Natural Drainage Systems Project aims to:

• Build natural drainage systems to reduce the amount of pollution in Thornton Creek and Lake Washington and improve stormwater drainage in your neighborhood.
• Provide other community benefits, which include reduction of drainage issues, traffic calming, more street trees, and added landscaping.

Location

The North Thornton Natural Drainage System project is focused on reducing pollution in the north branch of Thornton Creek. (Map of Thornton Creek Watershed)

As part of this project, SPU will build natural drainage systems at two sites in Lake City neighborhoods. Check out the links below for the proposed designs at each project location.

• Little Brook
  • 32nd Ave NE between NE 135th St and NE 137th St
• Olympic Hills
  • 25th Ave NE between NE 127th St and NE 130th St
  • NE 127th St between 25th Ave NE and 27th Ave NE
  • 27th Ave NE between NE 125th St and NE 127th St

This map shows the locations selected for natural drainage systems.

What's happening now?

Project Update

SPU has completed our technical feasibility studies in the North Thorton Creek basin and has selected two project sites to move forward into the project’s design phase:

• Little Brook
  • 32nd Ave NE between NE 135th St and NE 137th St
• Olympic Hills
  
  • 25th Ave NE between NE 127th St and NE 130th St
  • NE 127th St between 25th Ave NE and 27th Ave NE
  • 27th Ave NE between NE 125th St and NE 127th St

Originally, we planned to include more blocks in this project. However, through our technical analysis we discovered that installing natural drainage systems in the North Thornton Creek basin is challenging because of high groundwater elevation and underground utilities. Additionally, some of the blocks we studied would not be able to treat enough stormwater to be cost-effective. Due to these factors, we eliminated many blocks and ultimately chose two sites to move forward into design.

The project is in the mid-design phase, with construction tentatively expected to begin in 2026. Over the next several months, the project team will be working on:

• Community and Business Engagement: Continue coordinating with local businesses and homeowners to gather design feedback and address easement needs
• Design Advancement: Progress the design, incorporating feedback from the community, Seattle Public Utilities (SPU) stakeholders, and permitting agencies
• Community Outreach Planning: Prepare for the next round of community outreach, tentatively scheduled for spring 2025

Community benefits

Natural drainage systems will reduce pollution in Thornton Creek and Lake Washington and help manage stormwater. This project will provide additional benefits to local neighborhoods and ecosystems, including:

• More plants and plant diversity
• Reduced drainage issues
• Healthier creek ecosystems
• Improved pedestrian safety and mobility
• More street trees

Community engagement

Thank you to everyone who shared their valuable feedback during the project’s planning and early design phases. During these project phases, we administered online and mailed surveys and learned about specific concerns and interests from neighbors around the project area. Here are some common themes we heard from community members: 

• Parking: There is concern about the loss of parking adjacent to businesses and residences
• Business access: There is concern about construction and long-term impacts on business access, especially related to deliveries 
• Sidewalks: There is support and excitement for new sidewalks in the project area. Neighbors would like partial-block sidewalks extended to the full block and further, if possible. 
• Trees and plants: Neighbors requested that the project minimizes impacts to trees to the extent feasible
• Traffic circles: Neighbors would prefer that the plantings remain in the traffic circle at the intersection of NE 127th St and 25th Ave NE
• Drainage concerns: Neighbors shared observations of drainage concerns on streets, specifically on the east side of 25th Ave NE between NE 127th St and NE 130th St

Community engagement will continue to be important during the project’s design and construction phases. In 2024 and 2025, we will reach out to community members in the project area to ask for feedback on our proposed design. Please sign up for the project email list to receive updates about opportunities to share your feedback.