Stormwater Code

See also: Grading Code, Grading PermitSide Sewer Code, Side Sewer Permit

We have several trainings related to the 2021 Stormwater Code and Manual update. See the Training section.

What Is It?

The Seattle Stormwater Code and Manual regulate stormwater runoff in Seattle. The 2021 Seattle Stormwater Code and Manual will become effective on July 1, 2021.

Visit the Stormwater Codes and Rules Archive for the 2009 and 2016 Code and Manual versions and associated forms and documents.

Read the Code

The Stormwater code is Title 22, Subtitle VIII of the Seattle Municipal Code (SMC). You can find the stormwater code on the Seattle Municipal Code website.  

Read the Manual

The 2021 City of Seattle Stormwater Manual consists of five volumes and a set of appendices. Below is the Stormwater Manual, broken into sections.

Seattle Public Utilities' New Public Drainage System Requirements

This new SPU Public Drainage System Requirements Director's Rule (SPU DWW-210) includes Storm Mainline Extension, Approved Point of Discharge, and Ensuring Sufficient Capacity Requirements as well as the Public Drainage System Design Requirements that were in SPU CAM 1180. This rule replaces SDCI's Small Project Point of Discharge protocol and prohibits any new drainage discharges to the "formerly combined" public sanitary sewers and requires mainline extensions for certain projects in the combined sewer and ditch and culvert areas.

Forms and Documents

Submittal documents: If drainage review is required, some or all the following documents must be submitted with the building or grading permit application.

Infiltration Checklists: If your project requires On-site Stormwater Management (OSM), Flow Control, or Water Quality you must first investigate the feasibility of infiltration facilities (such as a dry well) to meet these standards. Submit the applicable infiltration checklist below with supporting documentation.

King County Sewage Treatment Capacity Charge Forms: Submit the applicable form(s) below for projects that add or replace dwelling units or buildings or that convert buildings to a new use. See sewage treatment capacity charge - King County for more information on capacity charges. Note: these documents are required even if drainage review is not required.

Additional Forms: You may also be required to submit one or more of these forms.

Note: If your project has 5,000 square feet, or greater, of new plus replaced hard surface area, your application must be prepared by a professional engineer. See the "For Professional Engineers" section below.

Do I Need Drainage Review?

You need to get a drainage review for your SDCI construction or grading permit if one or more of the following apply:

  • You are disturbing more than 5,000 square feet of land
  • You are adding or replacing more than 750 square feet of hard surface, such as pavement
  • You are adding or replacing more than 750 square feet of a building (as measured by the roof outline)
  • A grading permit is required for your project
  • You are applying for a subdivision, short plat, unit lot subdivision, or lot boundary adjustment
  • You are applying for a master use permit (MUP) for development on a property that has non-disturbance areas associated with environmentally critical areas (ECA) such as steep slopes, landslide prone areas, riparian corridors, wetlands buffers, etc.
  • Your project is within a category I peat settlement-prone area per SMC 25.09.020
  • Other less common criteria as described in SMC 22.807.020.A / Stormwater Manual Volume 1, Chapter 8

We use the Drainge Review Checklist as an aid when reviewing permit applications. While it is not comprehensive and does not include all requirements, applicants can use it as a reference to ensure that their drainage plans address the most common requirements to expedite our review.

Training

We have several opportunities for you to learn about the 2021 Stormwater Code.

Recorded Video Trainings

Reference Materials

For Professional Engineers

Important Updates to Stormwater Modeling Requirements for 2021 Stormwater Manual

The following are a few of the stormwater modeling requirements that have changed with the 2021 Seattle Stormwater Manual. This is not a comprehensive list, but highlights some of the more impactful changes.

  • The Peak Flow Control Standard has changed and will result in facilities that are larger. See Volume 1, Section 5.3.5. Note: the Pre-sized Flow Control Factors for projects with less than 10,000 square feet of hard surface did not increase significantly.
  • The Pervious Land Segment (PERLND) and Impervious Land Segment (IMPLND) Runoff Parameters are required to be adjusted based on the project site characteristics. See the HSPS Parameter Modification section in Appendix F, Section F-4 and Table F.11.
  • At the time of publication of the 2021 Seattle Stormwater Manual, the approval of MGS Flood is limited and was not approved for modeling bioretention (infiltrating or non-infiltrating) by Ecology. Refer to the Approval Status of Continuous Simulation Models section of the SWMMWW for a list of currently approved models and limitations.

Drainage Reports

A drainage report prepared by a licensed civil engineer is required for projects with 5,000 square feet or more of new plus replaced hard surface or 1 acre or more of land disturbing activity. See Volume 1, Chapter 8 for comprehensive drainage review submittal requirements and Appendix B of the Seattle Stormwater Manual for drainage report requirements.

Pre-Sized Versus Stormwater Modeling for Flow Control and Water Quality

Projects less than 10,000 square feet of new and replaced hard surface may use the Pre-Sized Flow Control Calculator or the pre-sized equations in Volume 3 of the Stormwater Manual. The pre-sized equations are in the in the BMP Sizing section for most of the BMPs in Volume 3, Chapter 5.

If there is more than 10,000 square feet of new and replaced impervious surface, then continuous runoff modeling with an approved stormwater model using the Seattle 158-year Precip / Evap series is required.

Guidance for the Seattle 158-Year Precipitation / Evaporation Time Series

The Seattle 158-year Precip / Evap series is included as an option in the 2012 Western Washington Hydrology Model (WWHM 2012) and MGS Flood continuous runoff models.

  • WWHM 2012 Time Series Selection Instructions
    • On the “Mapping Information Screen”, select the “Seattle” map
    • Next, select the “SPU 158 Year 5min” time series option
    • In the “View” pull down menu in the task bar, select “Options”, and go to the “Timestep” tab
    • Then select the appropriate time step for the type of facility that is being designed (see Table F.12, Appendix F, Section F-4 of the 2016 Seattle Stormwater Manual)
  • MGS Flood Time Series Selection Instructions
    • On the “Project Information” screen, in the “Select Climate Region” pull down box, select “32. Seattle 38 MAP”
    • Next, go to the “Simulate” screen
    • In the “Computational Timestep” pull down bar, select the appropriate time step for the type of facility that is being designed (see Table F.12, Appendix F, Section F-4 of the 2016 Seattle Stormwater Manual)

Note: We do not provide technical support for the approved continuous rainfall hydrologic models or in the application of the model to specific projects. Consultants who would like training in the use of the approved models may contact these companies directly:

Do You Need a Construction or Industrial Stormwater General Permit?

A Department of Ecology Construction Stormwater General Permit is required for projects disturbing one or more acres of land, or for any sized project that has the potential to be a significant contributor of pollutants (e.g. contaminated sites).

See the Department of Ecology Industrial Stormwater General Permit webpage to determine if an Industrial Stormwater General Permit is required for your site.