South Thornton Natural Drainage System (NDS) Project

Photo of a street with natural drainage system.
Example of a natural drainage system project a few years after installation

Project description

Improving Our Communities with Natural Drainage Systems
Seattle Public Utilities (SPU) will construct natural drainage systems (NDS) at 4 sites in the south Thornton Creek basin. NDS are built in the roadway shoulder (the space between the street edge and property line) and are filled with deep-rooted plants and spongy soils that temporarily hold and clean polluted stormwater from streets before it reaches Thornton Creek.

Natural drainage systems can offer multiple benefits to local neighborhoods and ecosystems including:

  • Increased landscaping
  • Lower risk of flooding
  • Creation of habitat along streets
  • Healthier creek ecosystems
  • Traffic calming
  • More street trees
  • Pedestrian safety improvements

We are partnering with the Seattle Department of Transportation (SDOT) to incorporate walkways into some of the streets that do not currently have formalized sidewalks, which aligns with community requests to install pedestrian improvements as a part of this project.

To provide a comfortable and accessible walking connection to Northgate Elementary School, SDOT is planning to construct a new sidewalk on the north side of N 117th St between Meridian Ave N and 1st Ave NE. This project will be constructed in coordination with the natural drainage systems on N 117th St and the redevelopment of Northgate Elementary.

We are also partnering with the City's Office of Arts and Culture to incorporate an art installation in the natural drainage system near Wedgwood Elementary School.

Check out the Project FAQ for more information.


Seattle Public Utilities (SPU) will construct natural drainage systems (NDS) at 4 sites in the south Thornton Creek basin. Selected sites can be viewed on this Thornton Creek map.


  • 41st Pl NE (NE 107th St to NE 105th St) (PDF)
  • 23rd Ave NE (NE Northgate Way to NE 103rd St) (PDF)
  • 117th St (Meridian Ave N to 1st Ave NE) (PDF) and  120th St (Meridian Ave N to 1st Ave NE) (PDF)
  • Wedgwood (NE 88th St, NE 87th St, 29th Ave NE, and 30th Ave NE) (PDF)

What's happening now?

We are finalizing the project design as we prepare to advertise and award a construction contract in early 2023. We anticipate that construction on the project will begin in mid-2023.

As we approach construction, we will share more information about construction start dates, duration, and anticipated impacts. Sign up for our project email list to stay updated on our next steps for the project.

Community benefits

Natural drainage systems will reduce pollution in Thornton Creek and Lake Washington and help manage stormwater. This project will also provide other community benefits such as reduced street flooding, improved traffic calming, new trees, and landscaping.

Community engagement

During the project’s design, we have engaged the community to gather input, answer questions, and incorporate feedback into the project design. Please sign up for our email list to stay informed of project updates and opportunities to engage with the project.

Seattle Public Utilities, in coordination with the City of Seattle Office of Arts & Culture is working to create an art installation for this project.

Proposed artwork to be installed in the NDS near Wedgewood Elementary School.

The artist, Leo Berk, developed an artwork to bring awareness to the ecological benefits of the natural drainage system that SPU is installing in Wedgwood. His design uses the visual metaphor of an oversized kitchen strainer perforated with pattern that illustrates the mechanism of this bioretention system. The pattern specifically highlights how these systems reduce the roadway pollutants that enter Thornton Creek and Lake Washington.

Leo Berk is a Seattle artist who has installed numerous public art projects and exhibited in galleries and museums around the Pacific Northwest. His artworks are fabricated in a wide range of materials with his preferred material being the one he hasn’t used yet.

2017 - 2018

  • Early project planning and site selection
  • Outreach to residents about potential sites

2019 - 2022

  • Design development for selected sites
  • Design phase outreach at key project milestones
  • Finalize design


  •  Advertise and award construction contract
  • Construction is tentatively scheduled to begin in mid-2023
  • SPU will provide information about start date, duration, and anticipated impacts prior to beginning work at each of the sites

Natural drainage systems are living systems and their appearance will change over time. The grasses, shrubs, and trees installed will grow and change as the garden matures. It may take up to three years for plants and shrubs in the natural drainage system to reach full maturity, and possibly longer for trees. The examples below show what a natural drainage system may look like over time.

Example before construction

Example newly planted

Example growing phase

Example mature phase

Maintenance and Care

SPU is responsible for all maintenance needs of the natural drainage systems, which will include general upkeep of the natural drainage system plantings, checking that the natural drainage system is functioning properly, removing sediment, and replacing or removing plantings and trees in the natural drainage system, if needed. You will not be asked to perform any maintenance related to the natural drainage system.  Maintenance of the planting strip that is not part of SPU’s natural drainage system remains the responsibility of the adjacent property owner.

Natural drainage systems slow stormwater and filter out pollutants before they can harm our creeks and other waterways. Keeping pollutants out of our water is important for fish, aquatic life, and people.
Building a natural drainage system in Thornton Creek basin is part of the Plan to Protect Seattle's Waterways. SPU used to call these "roadside rain gardens" but now calls them "natural drainage systems" or "NDS."
Why Thornton Creek? Thornton Creek is a salmon-bearing urban creek with degraded water quality. Stormwater picks up pollutants from streets—including oil, heavy metals, and fertilizers—and flows into the creek through ditches or pipes. This project will remove pollutants from the stormwater before it reaches the creek and reduce the quantity of stormwater going into the creek.


Public Utilities

Andrew Lee, General Manager and CEO
Address: 700 5th Avenue, Suite 4900, Seattle, WA, 98104
Mailing Address: PO Box 34018, Seattle, WA, 98124-5177
Phone: (206) 684-3000

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Seattle Public Utilities (SPU) is comprised of three major direct-service providing utilities: the Water Utility, the Drainage and Wastewater Utility, and the Solid Waste Utility.