Seattle Public Utilities Mami Hara, General Manager/CEO

Sidewalk Rain Gardens

You can use rain gardens to manage runoff from sidewalks to meet the "On-Site List Approach" if your project:

  • Has at least 2,000-square-feet (e.g., approximately 330-linear-feet of 6 foot-wide sidewalk) of new plus replaced hard surface; or
    Has an activity that disturbs 7,000-square-feet or more of land.
  • Your project is a “trail and sidewalk project” or “roadway project.”
  • The rain garden will receive a minimal amount of runoff from pollution-generating hard surfaces.
  • The designated street type or classification is not:
    • Downtown
    • Urban center connector
    • Principal arterial

Find information about street types in the Right-of-Way Improvement Manual.

You can’t use rain gardens to meet:

  • Flow control requirements
  • Water quality treatment requirements
  • On-site performance requirements

Find detailed requirements in the Seattle Stormwater Code and Manual.

Are rain gardens feasible for your project? Use the Seattle Stormwater Manual On-Site List Infeasibility Criteria (Volume III, Appendix C).

How do rain gardens manage sidewalk runoff?

When a rain garden is installed within a planting strip, runoff from the sidewalk flows to the planting strip. It is then collected and conveyed to the rain garden. Runoff slowly filters through the compost amended soil or bioretention soil, supplying water to the rain garden vegetation and recharges ground water. Ultimately, the rain garden reduces the quantity of runoff that would otherwise flow into the public drainage system.

During larger storm events, sidewalk runoff may exceed the rain garden’s infiltration capacity. In these instances, excess runoff overflows to the gutter over the curb or a depressed curb.

Does this apply to my project?

Your project must meet the following criteria to cover typical rain garden installations used for sidewalk runoff:

  1. Street has existing or new curbs to prevent roadway runoff from entering the rain garden.
  2. Top of curb is at least 1-inch lower than the adjacent sidewalk.
  3. Planting strip width is at least 5-feet-wide.
  4. Street longitudinal slope does not exceed 4%.
  5. Rain gardens have sloped facility sides and a maximum ponding depth of 3-inches.

For a full description of requirements including alternate rain garden configurations, see the Seattle Stormwater Code and Manual.

Additional information

CAM 1190 – Rain gardens for on-site stormwater management of sidewalk runoff (pdf) contains more information about:

  • Considerations for rain garden siting and layout
  • Determining rain garden geometry and sizing
  • Managing additional site drainage