3.4 Drainage

Last Updated: 6-9-2017Print page

Cast iron drainage

Street design includes provision for the collection, treatment, and discharge of stormwater. Drainage system components such as pipe, catch basins, inlets and on-site stormwater management best management practices (BMPs), are considered integral street improvement elements, as are curbs, sidewalks, street trees and pavement. All of the Street Drainage, Storm Drains and Sewer design standards in this section are to be considered requirements.

When property development includes the installation of new or replaced paving or other hard surfaces that incurs additional storm or sewer flows, there may be a need to improve existing drainage systems in the right-of-way. The property development may also trigger a requirement to make grading improvements, or to extend or upgrade existing storm drains, inlets, catch basins and sanitary sewer pipes. Additionally there may be requirements for flow control and/or treatment facilities triggered by the 2016 Stormwater Code Seattle Municipal Code: Chapter 22.800. The amount of hard surface, the project location, site characteristics, the availability and capacity of existing infrastructure, and other variables play a role in determining flow control and treatment requirements.

All required drainage-related improvements shall be designed and constructed in accordance with the City of Seattle Stormwater Code, Stormwater Manual and Standard Plans and Specifications which establish acceptable materials, dimensions, locations, installation and testing requirements, and other requirements for pipelines, maintenance holes, connections, flow control and water quality treatment facilities and other system improvements. In addition, Seattle Public Utilities (SPU) CAM 1180 provides further guidance on the placement and design of drainage structures and conveyance facilities.

SPU is responsible for the plan review of new street drainage, stormwater treatment and flow control facilities, as well as for the review of project impacts to existing SPU infrastructure. This review typically happens through the Street Improvement Permit process for private development or during the project development phase of a Seattle Department of Transportation (SDOT) capital project. These reviews may result in a determination that a project has impacts that trigger the need to construct system improvements in order to maintain service levels. In some instances surface level treatment such as trench drains may require a Public Space permit.

The following design standards apply for storm drains and street drainage including drainage appurtenances, sanitary and combined sewers, on-site stormwater management BMPs and side sewers and service drains.

Stormwater Code and Manual Compliance

Project Type

The Stormwater Code differentiates between projects on private property and projects in the right-of-way for the purposes of determining thresholds and minimum requirements. See SDCI’s Stormwater Code Reference Page.  All work in the right-of-way is treated as a separate project when determining minimum requirements.  As such all mitigation for work in the right-of-way must occur in the right-of-way and all mitigation for work on private property shall occur on private property.  The three project types specific to the right-of-way are listed below and are defined in the Stormwater Seattle Municipal Code: Chapter 22.801 – Definitions:

  • Sidewalk Project
  • Roadway Project
  • Trail Project

Minimum Requirements for All Projects

See Stormwater Seattle Municipal Code: Chapter 22.805 Stormwater Minimum Requirements for All Projects.

Drainage Review Thresholds:

Generally, any activity that disturbs seven hundred fifty (750) square feet or more of land shall require drainage control review. Information is provided in the Stormwater Seattle Municipal Code: 22.807.020 – Drainage Control Review and Application Requirements.

Minimum Requirements for Flow Control and Treatment:

Information is provided in the Stormwater Manual: Volume 1 – Project Minimum Requirements.

Relevant Standard Plans and Specifications and other Resources:

Standard Plans 200 Series: Sewer-Drainage Appurtenances
Standard Plans 400 Series: 403 Pervious concrete, 425 pervious concrete sidewalk section notes
Standard Specifications Division 5: Surface Treatment and Pavements
Standard Specifications Division 7: Storm Drain, Sanitary and Combined Sewers, Water Mains and Related Structures
Standard Specifications Division 8: Miscellaneous Construction
Standard Specifications Division 9: Materials


Design Guidelines for Public Storm Drain Facilities: SPU CAM 1180 (2012)

Voluntary Roadside Rain Gardens and Bioretention:

SDOT CAM 2308 Voluntary Roadside Rain Gardens
SDOT CAM 2309 Voluntary Roadside Bioretention
Director’s Rule 17-2017/DWW-200 Stormwater Manual

2016 Stormwater Seattle Municipal Code:

Chapter 22.800
WSDOT Hydraulics Manual (2015)

Approval and Permit Procedures

SDOT Street Use: Street Improvement Permit

In accordance with SDOT CAM 2213 60% Complete Street Improvement Permit (SIP) Plan Approval Process (2009) and SDOT CAM 2214 90% Street Improvement Plan (SIP) Intake Appointment and 90% Complete SIP Acceptance Processes (2009), if on-site stormwater management BMPs are being considered, the project applicant must obtain 60% Complete SIP approval through an SDOT design guidance meeting with City staff to discuss possible site constraints, transportation needs, soil conditions, design and accessibility issues, and maintenance responsibilities before submitting a 90% complete set for formal review. The SDOT SIP Design Guidance process is described in SDOT CAM 2200 SDOT Street Improvement Permitting (SIP) Design Guidance (2016).