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Ballard natural drainage: systems of nature, networks of people

Guest post by Seattle Public Utilities’ Protect Our Waters team.

The Ballard area contributes about one-third of Seattle’s total combined sewer overflows (CSOs). CSOs occur in this area because the storm and sewer system is combined. During heavy rains the volume of stormwater runoff overwhelms this combined system, causing an overflow into local waterbodies.


Sidewalks covering the Silva Cells have a porous grating that allows water to flow into the adjacent roadside rain garden

The Ballard natural drainage system project is working to reduce this problem. The project will consist of “roadside rain gardens” (bio-retention using soils and plants) and a new tool, Silva Cells, to naturally absorb the stormwater that contributes to Ballard’s CSOs. Silva Cells are located under the sidewalks adjacent to the roadside rain gardens and are made from a blend of glass and polypropylene. They work like strong egg cartons to hold the weight of the sidewalks while leaving space to hold ultra-absorbent bio-retention soil. When coupled with the roadside rain gardens, Silva Cells increase the filtration area of the system. The project is estimated to infiltrate 5.8 million gallons of stormwater per year. Keeping all that runoff out of the pipes helps the system do its job to carry the toxic mix of sewage and runoff to the treatment plant.

A project like the Ballard natural drainage system takes communication, collaboration, and partnerships starting at the earliest stages of development. The Seattle Public Utilities (SPU) team began outreach in Ballard during the summer of 2012. Construction began in spring 2016 after a thorough planning and community outreach process. SPU also worked closely with the Seattle Department of Transportation’s Neighborhood Greenways Program and the Seattle Public Schools to promote the safety and walkability of the neighborhood during and after construction. Although it took four years to plan, the Ballard project will take less than a year to construct. Final plantings are scheduled to go in at the end of October and will mark completion of the project.

Protect Our Waters (POW) is the City of Seattle’s commitment to take actions and promote partnerships that protect and improve our creeks, lakes, the Duwamish River and Puget Sound. Read more in the most recent Protect Our Waters newsletter