2024-2029 NPDES Stormwater General Permit Reissuance Ad Hoc
Committee – New Development, Redevelopment, and Construction,
Stormwater Management Manual (SWMM)
Background
Catalyzed by the 2018 Ad Hoc Process, the Washington State Department of Ecology
(Ecology) requested an Ad Hoc Process for the 2024 Permit Reissuances. The Ad Hoc
Process is meant to provide Ecology with a series of white papers that provide
suggestions on Permit changes.
The SWMM Ad Hoc Committee was made up of the following committee members:
Name
Jurisdiction
Deanna Seaman
Port of Tacoma
Mieke Hoppin
City of Tacoma
Shawn Christensen
Skagit County
Shannon Peterson
Thurston County
Cindy Cullen
City of Everett
Bill Lief
Snohomish County
Cory Olson
Snohomish County
Jane Dewell
Port of Seattle
Andy Wargo
Skagit County
Mark Wilgus
King County
Scott McQuary
City of Redmond
Jackie Caldwell
City of Camas
Timmie Maturan-Lee
Washington Military Department
Kevin Evans
City of Renton
Alex Nguyen
WSDOT
Olivia Torres Stagner
WSDOT
Garrett Starks
WSDOT
The SWMM Ad Hoc Committee participated in a series of 5 meetings with follow-up
email communication to develop this White Paper. The SWMM Ad Hoc Committee
discussed the following 12 Issue Statements. The Committee as a whole may not have
agreed on a specific issue or solution. To show the Committee’s agreements or
interest, attached to this White Paper are anonymous results from a poll where the
poll participants were asked to provide their agreement with the items as written
below. The poll is ranked from Strongly Agree to Strongly Disagree. Participants were
given the option to anonymously write comments within the poll as well. Those are
also attached for reference.
Issue #1: Current Best Management Practice (BMP) Options: Design, Constructability,
Maintainability
The 1992 SWMM for Puget Sound Basin (The Technical Manual) set the general design
standards for many BMPs that still exist in the SWMM. It is our understanding that
the design standards in the SWMM were developed based upon typical engineering
standards from the 1970s and 1980s, information from the International BMP
Database, information from other jurisdictions across the nation, and Ecology staff
knowledge and assumptions. Ecology has updated BMPs in the SWMM throughout
the years – in most instances to add clarifying language or tighten specifications.
There are concerns that the BMP selections did not fully consider constructability and
maintainability and did not rely on feedback from field personnel in a comprehensive
and systematic manner.
Further, the current manual drives designers and owners to select the least expensive
BMP option, not necessarily the BMP most appropriate for the project. What may be
inexpensive to design and install may not be maintainable in the future by the user(s)
and thus may not function to control flows and pollutants. The way the regulations
currently lead the design output does not always lead to a system that is constructible
or maintainable.
Ecology has not done a comprehensive review of BMPs in the SWMM with exception of
the Low Impact Development (LID) Practices when a separate LID Committee was
created to evaluate LID practices in 2007.
Suggestion to Address Issue #1
The Committee suggests that a comprehensive review of design standards,
constructability, maintainability, and cost be completed by a 3rd Party (aka a SWMM
Assessment). This assessment should include:
A review of other jurisdictions’ BMP Design Standards across the country
focusing on areas with similar rainfall distributions to Western Washington.
A series of meetings with both Permittee staff and private contractors to discuss
constructability and maintainability of BMPs.
A cost analysis of all BMPs including cost to construct and cost to maintain.
The results from the SWMM Assessment would be used as guidance for updating BMP
design standards contained in the SWMM and could be used to create a matrix
comparing the effectiveness, costs, constructability, and maintainability of various
BMPs.
BMP options in the SWMM should be robust, workable, and not drive developers to
choose the cheapest option or be overly driven by regulations to force choosing BMPs
that may not be easily constructible or maintainable.
This Committee suggests that a committee be created to review and approve the
Request for Proposal for the Outside Assessment Project and that a committee be
created to review the comprehensive assessment and provide suggestions based upon
the report. The committee should, at minimum, be made of a mix of Phase I, Phase II
jurisdictions (including Operation and Maintenance staff), and members of the
development community including project engineers and construction staff.
Issue #2: Additional BMP Guidance Needed
Jurisdictions face questions from development staff (both internal and external) on
subjects not contained in the SWMM but related to new and redevelopment
stormwater mitigation. Some examples include:
Many schools and parks that undergo redevelopment upgrade from natural
fields to turf fields. The manual should provide guidance on modeling artificial
turf and evaluating artificial turf per the Minimum Requirement thresholds.
Temporary sediment tanks are a common TESC BMP used but Ecology has not
provided design guidance. The manual should provide design guidance for
temporary sediment tanks.
It is ideal for projects to attempt to utilize existing stormwater features on a site
when they are available to minimize site disturbance and utilize existing
facilities that are already budgeted for inspection and maintenance. The
Manual should provide guidance to appropriately model usable capacity within
existing flow control and/or stormwater treatment BMPs constructed using
previously approved standards.
Suggestion to Address Issue #2
Ecology creates a web-based location where jurisdictions can suggest subjects that
need additional guidance, describe the best way to receive that guidance (how to
document, language in the SWMM, etc.), and level of need. Based upon the need,
Ecology creates guidance (not additional requirements) that further clarifies Ecology
intent regarding a range of subjects that jurisdictions commonly need further
clarification.
It is this Committee’s understanding that at times Ecology provides guidance to
jurisdictions in terms of both manual intent and “how to guidance”. It is suggested
that any guidance Ecology provides be made into formal intent or how to guidance and
be made readily available on Ecology’s website. It may not be necessary to include
this guidance in the SWMM itself.
Issue #3: Minimum Requirement #8 – Wetland Protection
There are concerns that as written, flow control standards may not be appropriately
installed because MR#8 takes precedence over MR#7. For example, if a wetland is
already altered due to existing development, should MR #7 take precedence with a
project?
Wetland protection standards may not protect wetlands depending on proximity of
project to the wetland.
Do incremental flow control standards placed on small projects actually protect
wetlands given the overall wetland watershed size? For example, if the project is only
5% of the total contributing basin to the wetland does the project really affect the
wetland?
Suggestion to Address Issue #3
Ecology should conduct a comprehensive analysis of several wetlands of varying size
to see if small project mitigation relative to overall wetland watershed size affects
wetland function over time.
Create limits as to when MR#8 applies in terms of distance from project to wetland.
Allow alternative modeling schemes such as a comparison to a theoretical
predeveloped condition. If there are multiple projects impacting the same wetland,
allow for a combined effect and shared mitigation apportioned by %flow of each project
to the wetland.
Issue #4 – Terms and Definitions
The terms and definitions contained in the SWMM conflict with terms and conditions
in other permits, including the Phase I, Phase 2, and the Industrial Stormwater
General Permit
Examples:
Outfall is used in many instances in the SWMM to refer to discharge locations –
but not specifically to discharge into receiving waters.
The use of the terms public and private are confusing. Are these based on the
stormwater system or entity or another standard?
Suggestion to Address Issue #4
Complete a comprehensive search of all terms used within the Permit to ensure their
meaning is consistent in the SWMM or that terms are appropriately defined within
each context. Ensure consistency in all terms throughout the SWMM (ex. Stormwater,
discharge, stormwater discharge, etc.) and throughout the various Ecology issued
Permits as appropriate. Ensure consistency with Fact Sheets, Guidance documents,
and FAQs. This eliminates misinterpretation and cherry picking the definition that fits
the need of the user rather than the actual regulatory / Ecology intent.
Issue #5: Climate Impacts
Should Ecology consider future climate impacts and require designers to analyze and
incorporate climate impacts or is current BMP design sufficient?
Suggestion to Address Issue #5
Ecology should continue to support the study of defensible methods for predicting
future rainfall increases and sea level rise due to climate impacts and what impacts
those increases are likely to cause in the future to constructed and natural systems.
Ecology should support a study to determine how impacts might be addressed via
regulations, capital projects, and/or land use/zoning changes. Any new regulations
should carefully consider the timeline of projected impacts, the design life of various
BMPs, and how to manage aging infrastructure currently in the built environment.
New regulations should not be developed or considered until sound science regarding
climate impacts is developed and agreed upon at a national level. For communities
that are already mostly built, new regulations on new development will not have a
large impact on mitigating impacts of rainfall increases or sea level rise.
One option to support the study of defensible methods might be for Ecology to model
various scenarios based on completed climate action plan scenarios developed in
Pierce, King, Snohomish, and Clark counties and see if BMP design changes
significantly enough given different hydrologic information.
Ecology should consider updating historical storm records used in WWHM on a
regular and defined basis – such as every Permit term.
Issue #6: The LID List Approach
The LID List Approach (versus the LID Performance Standard) is a commonly used
approach by many developers because in most cases a Professional Engineer is not
needed. There are concerns that the list hierarchy does not appropriately consider
facility function, constructability, and maintenance and may provide preference to
facilities that do not provide greater hydrologic benefits. The hierarchical approach
results in higher cost BMPs that are not as easy to maintain (ex. Rain
gardens/permeable pavement vs infiltration trenches) and that in the long run will
hydrologically perform worse given maintainability and constructability issues than
lower ranked BMPs that are easier to install and maintain and are less often replaced
by homeowners.
Suggestion to Address Issue #6
Reconsider the LID List Approach hierarchy by adding in additional factors such as
inspection, constructability and maintainability considering factors such as evaluation
over time (ex. Are specialized consultants or tests needed to ensure function?).
Conduct real world analyses of performance of MR #5 BMPs over time. There are
many real-world scenarios of preferred facilities failing over short periods of time
(especially permeable pavement) due to constructability issues.
Issue #7: SWMM BMPs and Facilities Covered Under other NPDES Permits
New development, redevelopment, and construction site projects located on sites that
already fall under separate NPDES Permits – such as the Industrial Stormwater
General Permit (ISGP) or individual permits should be allowed greater flexibility in
BMP choice and design.
Suggestion to Address Issue #7
Incorporate appropriate language in various Permits to require greater coordination
amongst the various Permits.
Ecology should develop a comprehensive guidance document regarding BMP selection
on sites that have existing site specific BMPs installed and designed to treat specific
contaminants of concern for the site and how new and redevelopment projects may be
allowed to utilize those BMPs. This may require the TAPE program to more rapidly
consider approved technologies for different permits. An example of this is Chitosan
Enhanced Sand Filtration; this technology is general use level designation (GULD) for
the Construction stormwater general permit but requires specific Ecology approval for
use on a site with an ISGP.
Update TAPE designations to allow use of proven technologies on any permit
addressing the pollutant for which the technology was approved.
Issue #8: SWMM Usability and Plain Talk
The SWMM is written as a technical engineering manual which makes sense for many
BMPs but may not be appropriate for others. In particular, the source control BMPs
are not usable for their audience. The SWMM needs to be accessible for one-time
users and non-engineers.
Suggestion to Address Issue #8
Make source control BMPs available as individual handouts. Transcreate all source
control BMPs for the most common communities in Western Washington, and plain
talk all for usability.
Create more companion documents such as Tip Sheets, Activity Sheets, Calculators,
case studies etc. to make the SWMM an easier to use document. These likely do not
need to be created from scratch but seeing what other jurisdictions have available and
adapting for widespread use.
Issue #9: SWMM Equivalency Process
The 2007-2012, 2012-2018, and 2019-2024 Permits included provisions that require
approval of an equivalent manual by Ecology.
It does not appear that Ecology ever formalized the process in which Ecology
determines a Manual to be equivalent. Because of this, the equivalency process is
different for each jurisdiction dependent upon the Ecology staff who complete the
equivalency process. This difference in process was very apparent during the 2020
Equivalency Process.
Suggestion to Address Issue #9
Have an independent council review the Permit and Permit history to see if there are
any legal requirements for the equivalency process. The PCHB did require that
Ecology include a public comment period for the public to provide comment on
proposed equivalent manuals, but it is unclear if that also warrants Ecology’s review
and approval of these manuals.
Have the independent council create an equivalency review procedure document that
clearly shows Ecology and Permittee responsibility and ensures that reviews are
equitable amongst jurisdictions.
Issue #10: Requirement to inspect residential developments every 6 Months until 90%
of lots…