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Surface Water Design Standards

Frequently Asked Questions

Why are the new Surface Water Design Standards needed? 

The Washington State Department of Ecology (DOE) issued the Western Washington National Pollutant Discharge Elimination System (NPDES) Phase II Municipal Stormwater Permit on February 16, 2007, under authority delegated to it by the US Environmental Protection Agency, pursuant to the Federal Clean Water Act.  Like many jurisdictions in Washington, Renton is required to adopt surface water standards equivalent to the standards in the 2005 Ecology Stormwater Management Manual for Western Washington.

The adoption of new Stormwater Design standards is needed for compliance with the NPDES Phase II Permit.  The full permit may be viewed at the Department of Ecology's website. 

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When will the revised regulations go into effect?

The Renton City Council adopted Ordinance 5526 amending the City stormwater Code for new developments, redevelopments and construction sites. The Surface Water Design Standards became effective February 10, 2010.

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What are the revised Surface Water Design Standards in the City of Renton?

The City of Renton Surface Water Design Manual consists of the following:

  • City of Renton Amendments to the King County Surface Water Design Manual. The City of Renton Amendments to the King County Surface Water Design Manual clarifies requirements that are specific to Renton and are different from the county manual.
  • 2009 King County Surface Water Design Manual, these standards apply to all new developments, redevelopments and construction sites in Renton. 

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Where can I find the City of Renton Surface Water Design Standards?

For your convenience, there is a link at the top of this page on the left side menu tab that will take you to the design standards. 

Otherwise, you may find the City of Renton Surface Water Design Standards located on the website under Government, City Departments and Services, Public Works, Utility Systems Division, Surface Water Utility Engineering, Surface Water Design Standards (left side menu tab). 

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What are the NEW thresholds for drainage reviews and what are the types of drainage review in Renton?

Drainage review is required for any proposed project subject to a City of Renton development permit or approval AND that meets any one of the following conditions:

  • The project results in 2,000 square feet or more of new plus replaced impervious surface; OR
  • The project proposes 7,000 square feet or more of land disturbing activity; OR
  • The project proposes to construct or modify a drainage pipe/ditch that is 12 inches or more in size; OR
  • The project contains or is adjacent to a flood, erosion, or steep hazard area, or landslide hazard drainage area.

The types of drainage reviews in Renton are

  • Small Project Drainage Review
  • Targeted Project Drainage Review
  • Full Project Drainage Review
  • Large Project Drainage Review

The drainage requirements will vary depending on project and site characteristics.  Refer to Chapter 1 of the City of Renton Amendments to the King County Surface Water Design Manual for project classification, descriptions and storm requirement details.

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What are the NEW Erosion Control requirements in Renton?

All projects are required to submit an Erosion and Sediment Control (ESC) Plan, an ESC Report, and designate an ESC Supervisor.  Refer to section 2.3.1.3 in the Surface Water Design Manual for general ESC requirements.

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What is the NEW soil amendment requirement?

If the project's new pervious surface exceeds 7,000 square feet, the soil moisture holding capacity of the new pervious surface must be protected.  Refer to the Chapter 1 of Surface Water Design Manual for additional requirements. 

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What are the new requirements to conveyance systems?

Projects constructing new or replacing conveyance systems receiving runoff from pollution generating impervious surface (PGIS) shall provide spill control. Allowable options for spill control are:

  • A tee section
  • A wall section
  • A baffle or coalescing plate

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What are the changes to Water Quality Treatment requirements? 

For new developments, water quality treatment is still required when a project results in 5,000 square feet or more of pollution generating impervious surface area (PGIS).  Water Quality Treatment is now also required unless exempt for:

  • New development or redevelopment that creates 35,000 square feet or more of new pollution generating pervious surface.
  • Redevelopment projects that result in more than 5,000 square feet of new plus replaced PGIS.

In addition, the level of water quality treatment has changed for certain land uses.  Multi-family residential, commercial, and industrial project sites will be required to provide enhanced basic water quality treatment that targets the removal of metals (such as copper and zinc).  Refer to Chapter 6 of the Surface Water Design Manual for more design requirements.

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What are the changes to flow control requirements? 

Flow control will no longer be based just on new impervious surface area; it will be based on "target impervious surface" which is a combination of new and replaced impervious surface.  In addition, all projects subject to flow control requirements must apply at least one flow control BMP to the target impervious surface. Refer to the Surface Water Design Manual for specific flow control requirements and exemptions. 

The flow control facility requirement will vary according to the flow control area within which the project is located.  There are four such flow control areas depicted in the City of Renton Flow Control Application Map (reference 11-A of the SWDM).  The area specific flow control requirements are:

  • Flow Control Duration Standard – Matching Forested Site Conditions
  • Flow Control Duration Standards – Matching Existing Site Conditions
  • Peak Rate Flow Control Standard – Matching Existing Site Conditions
  • Flood Problem Flow Control Standard

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What is a Flow Control BMP?

Flow Control BMP’s are methods or designs for dispersing, infiltrating or otherwise reducing or preventing development-related increases in surface and storm water runoff at, or near, the sources of those increases. Flow control BMPs include, but are not limited to: dispersion; infiltration; rain gardens; roof downspout infiltration; permeable pavements; rainwater harvesting; and reduction of development footprint.

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What are the benefits of providing a flow control BMP?

Projects that implement flow control BMPs, whether required or optional, may use the flow control BMP
credits as indicated in Table 5.2.2.A of the Surface Water Design Manual to:

  • Compute post-development 100-year peak flows when assessing the 0.1-cfs exceptions from providing a flow control facility.
  • Reduce the size of the flow control facility, when required.

The application of BMP‘s into project developments will help reduce runoff; recharge ground water; and protect environmental resources.

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Is a Rain Garden considered a Water Quality Treatment facility?

Yes, Rain Gardens meet basic and enhanced basic Water Quality Treatment requirements if designed to infiltrate 91% of the influent runoff resulting from the new plus replaced pollution generating impervious surface.   

For more design requirements, see section 6.7.1 of the Surface Water Design Manual.

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What are the Changes to Oil Control Requirements?

If a proposed project develops a site that will have high-use site characteristics, then the project must provide oil control treatment to the high use portion of the site.

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What is the definition of high use site?

High-use site means a commercial or industrial site that:

  • Has an expected average daily traffic (ADT) count equal to or greater than 100 vehicles per 1,000 square feet of gross building area;
  • Is subject to petroleum storage or transfer in excess of 1,500 gallons per year, not including delivered heating oil; or
  • Is subject to use, storage, or maintenance of a fleet of 25 or more vehicles that are over 10 tons net weight (trucks, buses, trains, heavy equipment, etc.)

Also included is any road intersection with a measured ADT count of 25,000 vehicles or more on the main roadway and 15,000 vehicles or more on any intersecting roadway, excluding projects proposing primarily pedestrian or bicycle use improvements. 

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Why are the Surface Water Design Standards good for the community?

The new Surface Water Design Standards will produce the following benefits:

  • Reduce flooding
  • Reduce pollution
  • Reduce erosion
  • Protects Surface Water quality
  • Protects environmental resources and habitat
  • Increase groundwater recharge
  • Protect the recreation and aesthetics values of the city’s water resources

For questions or comments, please contact:

Ronald J. Straka
Surface Water Utility Engineering Supervisor
Renton City Hall - 5th Floor
1055 South Grady Way
Renton WA 98057
425-430-7248 (Phone)
425-430-7241 (Fax)

Email to the Surface Water Utility Supervisor