Equivalent stormwater manuals for municipal stormwater permits

We provide information about Phase I Municipal Stormwater Permittee programs on controlling runoff for new development and redevelopment. This information is primarily intended for Western Washington Phase I permittees and other interested parties.

Historical and more information 

Historical information, current status, and links to the Phase I programs (including codes, ordinances, directors’ rules, public rules and manuals) are described below.

The Pollution Control Hearings Board (PCHB) issued a ruling on Jan. 5, 2011 that the Agreed Order #7273 entered in between Ecology and Clark County is unlawful.

Programs equivalent to the 2019 SWMMWW

All six Phase I permittees (Snohomish, King, Pierce, and Clark counties, and the cities of Seattle and Tacoma) submitted draft codes and manuals to Ecology for review. Our staff evaluated the submittals for equivalency with the 2019 Stormwater Management Manual for Western Washington, and worked through revisions with permittees.

Approved Programs

Please visit the PARIS link to locate the referenced program documents.

• Pierce County documents in PARIS
  • Pierce County Code Title 17A, as effective on July 1, 2021.
  • Pierce County Code Title 17B, as effective on July 1, 2021.
  • Pierce County Stormwater Management Manual and Site Development Manual, as effective July 1, 2021.
  • Pierce County Memorandum “Policy to Buy Into Regional Stormwater Ponds,” as effective October 27, 2015.

Special considerations for other jurisdictions
Pierce County completed an infiltration capacity analysis to justify placing Downspout Dispersion at the same list level as bioretention for sites that are underlain by Spanaway soils. The requirements, characteristics and descriptions of Spanaway Soils are defined and discussed in several locations in the approved Pierce County Stormwater Management and Site Development Manual (Pierce County’s Manual).

Other jurisdictions adopting Pierce County’s Manual will need to continue using the same information should they choose to adopt the Pierce County Manual. Downspout Dispersion being placed at the same level as bioretention is not approved for any other soil.

• Tacoma documents in PARIS
  • Tacoma Municipal Code Chapter 12.08 Wastewater and Surface Water Management as effective on July 1, 2021.
  • The 2021 Stormwater management Manual, as effective on July 1, 2021.
     
• Clark County documents in PARIS
  • The Clark County Stormwater Manual 2021 and amendments to Clark County Code Chapters 13.26A Water Quality and 40.386 Stormwater and Erosion Control as adopted June 1, 2021 (Ordinance 2021-06-02), effective July 1, 2021.

Special considerations for other jurisdictions
The Clark2012SG is not approved by Ecology for use outside of Clark County. Other jurisdictions outside of Clark County must continue using the standard WWHM2012 or another approved continuous runoff model, even if they choose to adopt this manual.

Other jurisdictions within Clark County may use the Clark2012SG in conjunction with other approved stormwater manuals, and are not constrained to using solely Clark County’s Stormwater Manual simply because they choose to use the Clark2012SG.

• Seattle documents in PARIS
  • Seattle Municipal Code Chapters 22.800 – 22.808 titled, “Stormwater Code” as adopted in May 2021 (Ordinance 126336).
  • Joint Seattle Public Utilities (SPU)/Seattle Department of Construction and Inspections (SDCI) Directors’ Rule titled, “Stormwater Manual” (Directors Rule SPU DWW-200/SDCI 10-2021) effective July 1, 2021.

Special considerations for other jurisdictions
Other jurisdictions considering adopting the City of Seattle’s Stormwater Manual Section 5.2 On-site Stormwater Management will need to ensure they also adopt the bioretention sizing requirements associated with the requirements in Section 5.2. This option may only be appropriate in the City of Seattle.

Additionally, many of the requirements in the City of Seattle’s Stormwater Program apply specifically to highly urbanized areas that have had at least 40% total impervious areas since 1985 (see I-3.4.7 MR7: Flow Control, in the SWMMWW). It may be inappropriate for other jurisdictions to adopt the City of Seattle’s program requirements designed to address these areas.

• Snohomish County documents in PARIS
  • Snohomish County Code Chapter 30.63A, as amended by Amended Ordinance No. 21-025, adopted on June 16, 2021.
  • Snohomish County Code Chapter 30.63B, as amended by Amended Ordinance No. 21-025, adopted on June 16, 2021.
  • The 2021 Snohomish County Drainage Manual, as adopted on June 28, 2021.
  • Snohomish County Engineering Design and Development Standards, as adopted January 22, 2016, Chapters 1, 5, and 11.

• King County documents in PARIS
  • King County Code Chapter 9.04 and Chapter 9.12 as amended by Ordinance 18257, adopted on March 15, 2016.
  • King County Code Chapter 16.82
  • 2021 Surface Water Design Manual, including appendices and references, effective July 23, 2021.

Special Considerations for other jurisdictions
Other jurisdictions considering adopting King County’s Core Requirement #9: Flow Control BMPS in King County’s 2021 Surface Water Design Manual will need to ensure they also adopt the bioretention sizing requirements in King County’s Surface Water Design Manual. This option may only be appropriate in King County.

WSDOT Highway Runoff Manual

We completed our review of the 2019 Washington State Department of Transportation Highway Runoff Manual (The 2019 HRM) and found that it meets the BMP selection, design, infeasibility criteria, and limitations for public road projects equivalent to Ecology's 2019 SWMMWW and 2019 Stormwater Management Manual for Eastern Washington.

These are primarily located within Chapter 5 and Chapter 6 of the 2019 HRM. Other sections of the 2019 HRM contain WSDOT-specific guidance that may not be deemed part of an equivalent program for a municipality.

Jump to:

• Compliance options
• Combining different manuals
• WSDOT Highway Runoff Manual

Permit requirements

The Phase I Municipal Stormwater Permit (special condition S5.C.5.a) and the Western Washington Phase II Municipal Stormwater Permit (WWA Phase II Permit) (special condition S5.C.6.a) require permitted cities and counties to establish enforceable programs to manage stormwater from new development, redevelopment, and construction activities.

There are two parts to the enforceable program requirement:

Part one

The first part of the program must describe the minimum technical requirements, thresholds, definitions, adjustments, and variance criteria to be used for new development, redevelopment, and construction sites.

From S5.C.6.b.i of the Wester Washington Phase II Permit, an ordinance or other enforceable mechanism must include: 

"The Minimum Requirements, thresholds, and definitions in Appendix 1, or the 2013 Appendix 1 amended to include the changes identified in Appendix 10, or Phase I program approved by Ecology and amended to include Appendix 10, for new development, redevelopment, and construction sites."

Both permits contain an identical Appendix 1, including a basin planning option (Section 7), which permittees may use (with Ecology’s approval) to tailor certain minimum requirements or demonstrate an equivalent level of protection through the use of regional facilities.

Part two

The second part of the program required in both permits is the implementation of site planning requirements; Best Management Practice (BMP) selection, design, and infeasibility criteria; BMP limitations; as well as Low Impact Development (LID) competing needs criteria.

These components work together to protect water quality, reduce the discharge of pollutants to the Maximum Extent Practicable (MEP) and satisfy Washington state requirements for applying AKART.

From S5.C.6.b.ii of the WWA Phase II Permit: 

"The local requirements shall include the following requirements, limitations, and criteria that, when used to implement the minimum requirements in Appendix 1 (or program approved by Ecology under the 2019 Phase I Permit) will protect water quality, reduce the discharge of pollutants to the MEP, and satisfy the State requirement under Chapter 90.48 RCW to apply AKART prior to discharge:

1. Site planning requirements
2. BMP selection criteria
3. BMP design criteria
4. BMP infeasibility criteria
5. LID competing needs criteria
6. BMP limitations"

The primary distinction between the two permits is that Phase I permittees must submit their draft enforceable requirements, technical standards, and manuals to Ecology for review and approval, whereas Phase II permittees are not given the option to do so.

The Phase I process is generally referred to as an “equivalency review.” During this review, we evaluate each Phase I program for its functional equivalency to the requirements described in Appendix 1 and the relevant portions of Ecology’s Stormwater Management Manual for Western Washington (SWMMWW). We do not have the capacity to review Phase II programs in detail, and thus rely on the Phase II Permit and Appendix 1 to ensure the minimum requirements, thresholds, and definitions are consistently applied to projects in Phase II coverage areas.

Compliance options

Western Washington Phase II cities and counties essentially have three compliance pathways for implementing the relevant requirements of S5.C.6:

1. Adopt the minimum technical requirements, thresholds, definitions, adjustment and variance criteria found in Appendix 1, or
2. Develop and adopt tailored minimum technical requirements and implementing BMPs* based on an Ecology-approved basin plan or other similar water quality and quantity planning effort that provides equal or similar protection of receiving waters and equal or similar levels of pollutant control as compared to Appendix 1. For this option, Ecology must review and approve the basin plan or similar planning effort prior to the permittee’s adoption and use of the tailored requirements, or
3. Adopt the minimum technical requirements, thresholds, definitions, adjustment and variance criteria found in an Ecology-approved equivalent Phase I program.

*Note that adjustment and variance criteria as written in Appendix 1 or equivalent to those in Appendix 1 must be adopted. It is unlikely that an Ecology-approved basin plan would justify changes to the Appendix 1 adjustment and variance criteria.

Special Condition S5.C.6.b.ii requires permittees to develop site planning requirements, BMP selection, design and infeasibility criteria, BMP limitations, and LID competing needs criteria that when used to implement the minimum technical requirements of Appendix 1, protect water quality, reduce the discharge of pollutants to the MEP, and satisfy AKART prior to discharge.

Documenting the pathway to compliance

Permittees must document that their programs meet these objectives through one of the following compliance pathways:

1. Choose and cite the requirements, limitations, and criteria in the SWMMWW, or
2. Choose and cite the requirements, limitations, and criteria in an Ecology-approved Phase I program, or
3. Develop and adopt requirements, limitations, and criteria addressing S5.C.4.a.ii.(a)-(f) that protects water quality, reduces the discharge of pollutants to the MEP, and satisfies Washington state requirements for applying AKART. For this option, the burden is on the permittee to document how the local program accomplishes these objectives prior to discharge.

Combining different manuals

It is important to keep in mind that a Phase II permittee may only establish alternative requirements to the minimum technical requirements, thresholds, and definitions contained in Appendix 1 (S5.c.6.b.i) through adoption of an Ecology-approved basin plan (Section 7 of Appendix 1) or through the adoption of a single Ecology-approved Phase I program. Some Phase II permittees have expressed interest in selecting specific components of several different Phase I equivalent programs and/or the SWMMWW, and combining these requirements into a unique local program.

We expect that Phase II permittees adopting an Ecology-approved Phase I program (i.e., in lieu of Appendix 1) are also adopting the associated site planning requirements, BMP selection, design, infeasibility criteria and limitations, and LID competing needs criteria from that same Ecology-approved Phase I program. If a Phase I program has received our approval for alternatives to the minimum technical requirements, thresholds, definitions, adjustment and variance criteria in Appendix 1, such alternatives are only appropriate when implemented with the requirements, limitations and criteria addressing S5.C.6.b.ii.(a)-(f) that are part of that Ecology-approved program, and any additional conditions that we place on such an approval.

If a Phase II permittee chooses to select different BMP limitations or criteria, or site planning requirements from several different Phase I programs, the onus is on the permittee to document how this composite manual meets the permit requirements as described above. This approach is essentially a version of the third compliance pathway listed above for S5.C.6.b.ii. In this case, a permittee cannot rely on our determination of equivalency associated with the Phase I programs because our equivalency determination is based on a review of the individual Phase I program as a whole.

It is incorrect to assume that individual elements of approved equivalent Phase I programs can be isolated, recombined, and still be equivalent to Appendix 1 and the SWMMWW. Permittees are cautioned that this practice may create potential liability. Furthermore, it may be inappropriate to adopt a Phase I program designed to address specific general conditions in a Phase I community (e.g., highly urbanized, fully built-out city) in a jurisdictions with significantly different characteristics (large-lot suburbs at the fringe of an urbanized area).

Here are a few examples of how this may happen:

1. Specific elements of a Phase I equivalent manual may be tied to other specific program elements in order to make the full suite of conditions/requirements equivalent to the minimum requirements of the permit (as described in Appendix 1 and as referenced to the SWMMWW). Selecting one program element without the others may create a situation where a requirement does not meet MEP or AKART. 
2. Some requirements may exist in sections of local code outside of the stormwater code. For example, requirements associated with Appendix 1, Minimum Requirement #8, Wetlands, may exist in the Phase I permittee’s critical areas code and not in their stormwater code. 
3. Specific procedural requirements or steps in a Phase I program may not be available to another jurisdiction. For example, Washington State Department of Transportation’s (WSDOT) Highway Runoff Manual contains provisions for project-specific alternatives subject to approval by a “Demonstrative Approach Team.” This procedural requirement is not available to Phase I or Phase II cities and counties. 
4. Some aspects of a Phase I program may be based on the conditions present in that jurisdiction. For example, a Phase I program designed to address redevelopment inside the Urban Growth Boundary (UGA) will not be appropriate for new development outside the UGA. 

Without our approval and an associated determination of equivalency of a local code and manual, the local jurisdiction must rely on its own justification and documentation that the local program meets the permit requirements. Ecology, EPA, or a third party may review such documentation to evaluate compliance with the permit.

We encourage any permittee who is considering using some element(s) of an Ecology-approved equivalent Phase I program to review the comments and resolution documentation associated with that Phase I program review. These materials will be available on the Phase I equivalent manual page Phase I equivalent manual page when they are completed. Note that a Phase I program is not considered “equivalent” until after we have modified the Phase I permit (Appendix 10) following public review and comment. We may consider the program “approvable” until such time as the permit modification is completed and the determination of “equivalent” can be made.

Washington Department of Transportation’s Highway Runoff Manual (HRM)

We have determined the HRM to be equivalent to both of Ecology’s Western and Eastern Washington stormwater management manuals for minimum design requirements and best management practices for public road projects. Permittees may adopt and employ these equivalent sections (only) for use within their jurisdictions.

Please note that there are additional provisions (or exemptions) in the HRM that apply only to WSDOT and are not appropriate for local governments to follow and implement. The HRM was revised and updated on April 30, 2019; the conditions of equivalency for the HRM will be described in the Phase I Municipal Stormwater Permit, Appendix 10, and on the website cited above.

The Washington state municipal stormwater permits require permitted local governments to establish programs that prevent and control the impacts of stormwater runoff from new development, redevelopment, and construction sites.

Minimum requirements, thresholds, and definitions are in Appendix 1 of the municipal stormwater permit. In addition, local governments in Eastern Washington must adopt a site planning process, and criteria for selecting and designing best management practices (BMPs) similar to those described in Ecology’s Stormwater Management Manual for Eastern Washington. 

Eastern Washington Phase II Permittees have the option of either adopting portions of Ecology’s Stormwater Manual or portions of a regional manual determined to be equivalent to Ecology’s Stormwater Manual. We will update this page as additional information becomes available.

Spokane County Regional Stormwater Manual

On Oct. 8, 2007, Ecology approved the Spokane County Regional Stormwater Manual as equivalent to the minimum requirements of the Eastern Washington Phase II Municipal Stormwater Permit.

Yakima County Regional Stormwater Manual

On Feb. 2, 2010, Ecology approved the Yakima County Regional Stormwater Manual as equivalent to the minimum requirements of the Eastern Washington Phase II Municipal Stormwater Permit. 

Washington Department of Transportation’s Highway Runoff Manual (HRM)

We have determined the HRM to be equivalent to both of Ecology’s Western and Eastern Washington stormwater management manuals for minimum design requirements and best management practices for public road projects. Permittees may adopt and employ these equivalent sections (only) for use within their jurisdictions.

Please note that there are additional provisions (or exemptions) in the HRM that apply only to WSDOT and are not appropriate for local governments to follow and implement. The HRM was revised and updated on April 30, 2019; the conditions of equivalency for the HRM will be described in the Phase I Municipal Stormwater Permit, Appendix 10, and on the website cited above.