North Thornton Natural Drainage Systems

Example of a natural drainage system a few years after installation

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 a 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. (Project Map) 

What's Happening Now?

This project is currently in the early design phase. We anticipate reaching our 30% design milestone in Spring 2023.

We originally identified 22 blocks where natural drainage systems might be built. We are now evaluating these blocks to determine where natural drainage systems will be most effective and feasible and where we can coordinate with other City projects in your neighborhood. Based on this evaluation, we will select about 13 blocks to move forward to construction.

While we are still in the process of evaluating natural drainage system locations, we have narrowed our list of potential blocks. The following blocks will no longer be considered for NDS installation:

• NE 135th St, from 17th Ave NE to 22nd Ave NE
• 32nd Ave NE, from NE 137th St to NE 140th St
• NE 127th St between 27th Ave NE and 28th Ave NE
• NE 133rd St between 23rd Pl NE and 25th Ave NE
• 12th Ave NE between NE 125th St and NE 127th St

Based on our evaluation, we are not moving forward with these blocks due to design challenges, utility conflicts, and community feedback.

In 2023, we will share an update with more information about where we propose to install the natural drainage systems. Following this block selection announcement, we will conduct in-person engagement with residents and community members on the selected blocks as we further refine the project design.

Community Benefits

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

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

How does SPU decide which blocks to choose?

SPU picks potential project locations by working with other city departments (for example, with planned street or pedestrian improvements), by looking for areas where this project may address a drainage issue, and by collecting information from the community.

The final project location depends on a lot of factors:

• How much stormwater can be managed at a particular location
• Location of nearby adverse conditions (steep slopes, high groundwater, or contaminated sites)
• Potential impacts to existing trees
• Existing soil conditions
• Location and condition of existing utilities
• Community input
• Width of existing public right-of-way
• Presence of driveways
• Existing parking congestion and availability of off-street parking
• Project construction, operation, and maintenance cost

Community Engagement

We will incorporate community input into our project planning, site selection, design, and final decisions as much as possible. In addition to community input, we will evaluate the sites to identify where it is technically feasible for us to build the natural drainage system.
 
The project’s first community feedback survey was completed in June 2020. Since then, we’ve been working to understand where natural drainage systems can have the most water quality and community benefit and have identified 22 blocks where a natural drainage system might be built. In spring of 2022, we asked neighbors living and working on the 22 blocks under consideration to share their thoughts through an online survey.
 
We are planning additional opportunities for SPU to learn more about the community's priorities and how we can work together to improve water quality in Thornton Creek, reduce drainage issues, and make other improvements in your neighborhood. Please sign up for the project email list to receive updates about the project and about opportunities to share your input.