Seattle Public Utilities

Swale on Yale – Capitol Hill Water Quality


Phase 1 of the Swale on Yale project was completed in 2013.

What & Why

SPU constructed a stormwater treatment system on four blocks on Yale Avenue North and Pontius Avenue North. The project consists of four biofiltration swales, a pre-treatment swirl concentrator, and a diversion structure. The project will greatly improve the quality of stormwater runoff entering South Lake Union.

What’s happening now?

The last phase of the “Swale on Yale” Capitol Hill Water Quality project moved into construction in April 2018. In this phase, Exxel Pacific constructed bioswales on the 300 blocks of Yale Avenue North and Pontius Avenue North (between Thomas and Harrison streets). This work was performed in conjunction with Vulcan Real Estate’s construction of the Sitka building.

In mid-December, we anticipate planting the newest bioswales and wrapping up construction. Planting is weather-dependent. At the end of this work, four city blocks will have stormwater treatment swales that remove pollutants before the water enters Lake Union.

Project goals and benefits

This project will treat an average of 190 million gallons of stormwater annually flowing from Capitol Hill into Lake Union, greatly reducing the amount of pollution flowing into the lake. It does this by diverting the stormwater into a series of extra-wide planting areas between the sidewalk and the roadway. These naturalistic, biofiltration “swales” are designed to slow the stormwater flow and remove pollutants before they reach the lake.

This project includes:

  • Four biofiltration swales: Slowing the stormwater flow by diverting it into swales allows sediments and pollutants to settle out before the water is discharged into Lake Union. Each swale will be approximately 270-feet long by approximately 10.5, 11.5 or 16.5-feet wide; they will be built along two blocks of Yale Avenue North and Pontius Avenues North.
  • Diversion vault: A large underground tank will be constructed under Yale Avenue North, between Stewart and John Streets, to divert the stormwater into the biofiltration swales.
  • Swirl concentrator: This equipment, also to be installed under Yale Avenue North, will spin the stormwater to create a vortex. When that happens, large solids and trash separate and collect in a sump. The sump will be cleaned regularly by SPU crews.
  • New storm drain: Approximately 2,000 feet of new storm drain will be built to convey untreated stormwater into the diversion vault, swirl concentrator, then the biofiltration swales. The treated stormwater then will go back into the storm drain for discharge into Lake Union.


Jason Sharpley, project manager
(206) 615-0030

Sign up for Swale on Yale project updates.