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City Policies Requiring and Related to using GSI

City of Seattle Stormwater Code
The City of Seattle’s Stormwater Code contained in Seattle Municipal Code (SMC) 22.800-22.808 requires projects to implement green stormwater infrastructure (GSI) to the maximum extent feasible (MEF).

City of Seattle Stormwater Flow Control and Water Quality Treatment Technical Requirements Manual (pdf)

  • Chapter 4.3 presents the general requirements for all Infiltration facilities, including setbacks and site restrictions, and subsurface evaluation requirements.
  • Chapter 4.4 outlines the design guidelines and criteria for use of GSI BMPS. Information includes engineering limitations, sizing requirements, and submittal requirements for the implementation of green stormwater infrastructure as a part of stormwater code compliance. 
  • Appendix D provides the Facility Operations and Maintenance Requirements, including the 2-year warranty period reporting requirements for GSI in the right-of-way. 
  • Appendix E provides the required protocol for determining native soil infiltration rate as part of the detailed subsurface evaluation.

Directors Rule on Green Stormwater Infrastructure to the Maximum Extent Feasible for SFR and Parcel Based Projects (pdf)

Directors Rule on Green Stormwater Infrastructure to the Maximum Extent Feasible for Roadway, Trail, and Sidewalk Projects (pdf)

All projects are required to implement green stormwater infrastructure (GSI) to the maximum extent feasible. Per the stormwater code maximum extent feasible (MEF) means the requirement is to be fully implemented, constrained only by the physical limitations of the site, practical considerations of engineering design, and reasonable considerations of financial costs and environmental impacts. Clarification of Seattle’s interpretation of code requirement is provided in the following Directors Rules.

Right-of-Way Improvement Manual
The Right-of-Way Improvement manual, Chapter 6.4, provides information on rules specific to the use of GSI Facilities within the Right-of-way (ROW).

Stormwater Modeling
General Stormwater Modeling requirements and useful tools are provided on the Department of Planning and Development website.