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APPENDIX 1 – Minimum Technical Requirements for
New Development and Redevelopment
Section 1. Exemptions
Unless otherwise indicated in this Section 1, the practices described in this section are exempt
from the Minimum Requirements, even if such practices meet the definition of new development
or redevelopment.
Forest practices:
Forest practices regulated under Title 222 WAC, except for Class IV General forest practices
that are conversions from timberland to other uses, are exempt from the provisions of the
minimum requirements.
Commercial agriculture:
Commercial agriculture practices involving working the land for production are generally
exempt. However, the conversion from timberland to agriculture, and the construction of
impervious surfaces are not exempt.
Oil and Gas Field Activities or Operations:
Construction of drilling sites, waste management pits, and access roads, as well as construction
of transportation and treatment infrastructure such as pipelines natural gas treatment plants,
natural gas pipeline compressor stations, and crude oil pumping stations are exempt. Operators
are encouraged to implement and maintain Best Management Practices to minimize erosion and
control sediment during and after construction activities to help ensure protection of surface
water quality during storm events.
Pavement Maintenance:
The following pavement maintenance practices are exempt: pothole and square cut patching,
overlaying existing asphalt or concrete pavement with asphalt or concrete without expanding the
area of coverage, shoulder grading, reshaping/regrading drainage systems, crack sealing,
resurfacing with in-kind material without expanding the road prism, pavement preservation
activities that do not expand the road prism, and vegetation maintenance.
The following pavement maintenance practices are not categorically exempt. The extent to
which this Appendix applies is explained for each circumstance.
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• Removing and replacing a paved surface to base course or lower, or repairing the
pavement base: If impervious surfaces are not expanded, Minimum Requirements #1 - #5
apply.
• Extending the pavement edge without increasing the size of the road prism, or paving
graveled shoulders: These are considered new impervious surfaces and are subject to the
minimum requirements that are triggered when the thresholds identified for new or
redevelopment projects are met.
• Resurfacing by upgrading from dirt to gravel, asphalt, or concrete; upgrading from gravel
to asphalt, or concrete; or upgrading from a bituminous surface treatment (“chip seal”) to
asphalt or concrete: These are considered new impervious surfaces and are subject to the
minimum requirements that are triggered when the thresholds identified for new or
redevelopment projects are met.
Underground utility projects:
Underground utility projects that replace the ground surface with in-kind material or materials
with similar runoff characteristics are only subject to Minimum Requirement #2, Construction
Stormwater Pollution Prevention.
Section 2. Definitions Related to Minimum Requirements
Arterial – A road or street primarily for through traffic. The term generally includes roads or
streets considered collectors. It does not include local access roads which are generally limited to
providing access to abutting property. See also RCW 35.78.010, RCW 36.86.070, and RCW
47.05.021.
Bioretention – Engineered facilities that treat stormwater by passing it through a specified soil
profile, and either retain or detain the treated stormwater for flow attenuation. Refer to the
Stormwater Management Manual for Western Washington (SWMMWW), Chapter 7 of Volume V
for Bioretention BMP types and design specifications.
Certified Erosion and Sediment Control Lead (CESCL) – means an individual who has current
certification through an approved erosion and sediment control training program that meets the
minimum training standards established by the Washington Department of Ecology (Ecology)
(see BMP C160 in the Stormwater Management Manual for Western Washington (SWMMWW)).
A CESCL is knowledgeable in the principles and practices of erosion and sediment control. The
CESCL must have the skills to assess site conditions and construction activities that could impact
the quality of stormwater and, the effectiveness of erosion and sediment control measures used to
control the quality of stormwater discharges. Certification is obtained through an Ecology
approved erosion and sediment control course. Course listings are provided online at Ecology’s
website.
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Commercial Agriculture – means those activities conducted on lands defined in RCW
84.34.020(2) and activities involved in the production of crops or livestock for commercial trade.
An activity ceases to be considered commercial agriculture when the area on which it is
conducted is proposed for conversion to a nonagricultural use or has lain idle for more than five
years, unless the idle land is registered in a federal or state soils conservation program, or unless
the activity is maintenance of irrigation ditches, laterals, canals, or drainage ditches related to an
existing and ongoing agricultural activity.
Converted vegetation (areas) – The surfaces on a project site where native vegetation, pasture,
scrub/shrub, or unmaintained non-native vegetation (e.g., Himalayan blackberry, scotch broom)
are converted to lawn or landscaped areas, or where native vegetation is converted to pasture.
Discharge Point – the location where a discharge leaves the Permittee’s MS4 through the
Permittee’s MS4 facilities/BMPs designed to infiltrate.
Effective Impervious surface – Those impervious surfaces that are connected via sheet flow or
discrete conveyance to a drainage system. Impervious surfaces are considered ineffective if: 1)
the runoff is dispersed through at least one hundred feet of native vegetation in accordance with
BMP T5.30 – “Full Dispersion” as described in Chapter 5 of Volume V of the Stormwater
Management Manual for Western Washington (SWMMWW); 2) residential roof runoff is
infiltrated in accordance with Downspout Full Infiltration Systems in BMP T5.10A in Volume
III of the SWMMWW; or 3) approved continuous runoff modeling methods indicate that the
entire runoff file is infiltrated.
Erodible or leachable materials – Wastes, chemicals, or other substances that measurably alter
the physical or chemical characteristics of runoff when exposed to rainfall. Examples include
erodible soils that are stockpiled, uncovered process wastes, manure, fertilizers, oily substances,
ashes, kiln dust, and garbage dumpster leakage.
Hard Surface – An impervious surface, a permeable pavement, or a vegetated roof.
Highway – A main public road connecting towns and cities
Impervious surface – A non-vegetated surface area that either prevents or retards the entry of
water into the soil mantle as under natural conditions prior to development. A non-vegetated
surface area which causes water to run off the surface in greater quantities or at an increased rate
of flow from the flow present under natural conditions prior to development. Common
impervious surfaces include, but are not limited to, roof tops, walkways, patios, driveways,
parking lots or storage areas, concrete or asphalt paving, gravel roads, packed earthen materials,
and oiled, macadam or other surfaces which similarly impede the natural infiltration of
stormwater. Open, uncovered retention/detention facilities shall not be considered as impervious
surfaces for purposes of determining whether the thresholds for application of minimum
requirements are exceeded. Open, uncovered retention/detention facilities shall be considered
impervious surfaces for purposes of runoff modeling.
Land disturbing activity – Any activity that results in a change in the existing soil cover (both
vegetative and non-vegetative) and/or the existing soil topography. Land disturbing activities
include, but are not limited to clearing, grading, filling, and excavation. Compaction that is
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associated with stabilization of structures and road construction shall also be considered a land
disturbing activity. Vegetation maintenance practices, including landscape maintenance and
gardening, are not considered land-disturbing activity. Stormwater facility maintenance is not
considered land disturbing activity if conducted according to established standards and
procedures.
Low Impact Development (LID) – A stormwater and land use management strategy that strives
to mimic pre-disturbance hydrologic processes of infiltration, filtration, storage, evaporation and
transpiration by emphasizing conservation, use of on-site natural features, site planning, and
distributed stormwater management practices that are integrated into a project design.
LID Best Management Practices – Distributed stormwater management practices, integrated
into a project design, that emphasize pre-disturbance hydrologic processes of infiltration,
filtration, storage, evaporation and transpiration. LID BMPs include, but are not limited to,
bioretention,rain gardens, permeable pavements, roof downspout controls, dispersion, soil
quality and depth, minimal excavation foundations, vegetated roofs, and water re-use.
LID Principles – Land use management strategies that emphasize conservation, use of on-site
natural features, and site planning to minimize impervious surfaces, native vegetation loss, and
stormwater runoff.
Maintenance – Repair and maintenance includes activities conducted on currently serviceable
structures, facilities, and equipment that involves no expansion or use beyond that previously
existing and results in no significant adverse hydrologic impact. It includes those usual activities
taken to prevent a decline, lapse, or cessation in the use of structures and systems. Those usual
activities may include replacement of dysfunctional facilities, including cases where
environmental permits require replacing an existing structure with a different type structure, as
long as the functioning characteristics of the original structure are not changed. One example is
the replacement of a collapsed, fish blocking, round culvert with a new box culvert under the
same span, or width, of roadway. In regard to stormwater facilities, maintenance includes
assessment to ensure ongoing proper operation, removal of built up pollutants (i.e.
sediments), replacement of failed or failing treatment media, and other actions taken to correct
defects as identified in the maintenance standards of Chapter 4, Volume V of the Stormwater
Management Manual for Western Washington (SWMMWW). See also Pavement Maintenance
exemptions in Section 1 of this Appendix.
Native vegetation – Vegetation comprised of plant species, other than noxious weeds, that are
indigenous to the coastal region of the Pacific Northwest and which reasonably could have been
expected to naturally occur on the site. Examples include trees such as Douglas Fir, western
hemlock, western red cedar, alder, big-leaf maple, and vine maple; shrubs such as willow,
elderberry, salmonberry, and salal; and herbaceous plants such as sword fern, foam flower, and
fireweed.
New development – Land disturbing activities, including Class IV -general forest practices that
are conversions from timber land to other uses; structural development, including construction or
installation of a building or other structure; creation of hard surfaces; and subdivision, short
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subdivision and binding site plans, as defined and applied in Chapter 58.17 RCW. Projects
meeting the definition of redevelopment shall not be considered new development.
Outfall – a point source as defined by 40 CFR 122.2 at the point where a discharge leaves the
permittee’s MS4 and enters a surface receiving waterbody or surface receiving waters. Outfall
does not include pipes, tunnels, or other conveyances which connect segments of the same
stream or other surface waters and are used to convey primarily surface waters (i.e., culverts).
On-site Stormwater Management BMPs: As used in this appendix, a synonym for Low Impact
Development BMPs.
Permeable pavement – Pervious concrete, porous asphalt, permeable pavers or other forms of
pervious or porous paving material intended to allow passage of water through the pavement
section. It often includes an aggregate base that provides structural support and acts as a
stormwater reservoir.
Pervious Surface – Any surface material that allows stormwater to infiltrate into the ground.
Examples include lawn, landscape, pasture, native vegetation areas, and permeable pavements.
Pollution-generating hard surface (PGHS) – Those hard surfaces considered to be a significant
source of pollutants in stormwater runoff. See the listing of surfaces under pollution-generating
impervious surface.
Pollution-generating impervious surface (PGIS) – Those impervious surfaces considered to be
a significant source of pollutants in stormwater runoff. Such surfaces include those which are
subject to: vehicular use; industrial activities (as further defined in the glossary of the
Stormwater Management Manual for Western Washington (SWMMWW)); storage of erodible or
leachable materials, wastes, or chemicals, and which receive direct rainfall or the run-on or
blow-in of rainfall; metal roofs unless they are coated with an inert, non-leachable material (e.g.,
baked-on enamel coating); or roofs that are subject to venting significant amounts of dusts, mists,
or fumes from manufacturing, commercial, or other indoor activities.
Pollution-generating pervious surfaces (PGPS) – Any non-impervious surface subject to
vehicular use, industrial activities (as further defined in the glossary of the Stormwater
Management Manual for Western Washington (SWMMWW); or storage of erodible or leachable
materials, wastes, or chemicals, and that receive direct rainfall or run-on or blow-in of rainfall,
use of pesticides and fertilizers, or loss of soil. Typical PGPS include permeable pavement
subject to vehicular use, lawns, and landscaped areas including: golf courses, parks, cemeteries,
and sports fields (natural and artificial turf).
Pre-developed condition – The native vegetation and soils that existed at a site prior to the
influence of Euro-American settlement. The pre-developed condition shall be assumed to be a
forested land cover unless reasonable, historic information is provided that indicates the site was
prairie prior to settlement.
Project site – That portion of a property, properties, or right of way subject to land disturbing
activities, new hard surfaces, or replaced hard surfaces.
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Rain Garden – A non-engineered shallow landscaped depression, with compost-amended native
soils and adapted plants. The depression is designed to pond and temporarily store stormwater
runoff from adjacent areas, and to allow stormwater to pass through the amended soil profile.
Receiving waterbody or Receiving waters – naturally and/or reconstructed naturally occurring
surface water bodies, such as creeks, streams, rivers, lakes, wetlands, estuaries, and marine
waters, or groundwater, to which a MS4 discharges.
Redevelopment – On a site that is already substantially developed (i.e., has 35% or more of
existing hard surface coverage), the creation or addition of hard surfaces; the expansion of a
building footprint or addition or replacement of a structure; structural development including
construction, installation or expansion of a building or other structure; replacement of hard
surface that is not part of a routine maintenance activity; and land disturbing activities.
Replaced hard surface – For structures, the removal and replacement of hard surfaces down to
the foundation. For other hard surfaces, the removal down to bare soil or base course and
replacement.
Replaced impervious surface – For structures, the removal and replacement of impervious
surfaces down to the foundation. For other impervious surfaces, the removal down to bare soil
or base course and replacement.
Site – The area defined by the legal boundaries of a parcel or parcels of land that is (are) subject
to new development or redevelopment. For road projects, the length of the project site and the
right-of-way boundaries define the site.
Source control BMP – A structure or operation that is intended to prevent pollutants from
coming into contact with stormwater through physical separation of areas or careful management
of activities that are sources of pollutants. The Stormwater Management Manual for Western
Washington (SWMMWW) separates source control BMPs into two types. Structural Source
Control BMPs are physical, structural, or mechanical devices, or facilities that are intended to
prevent pollutants from entering stormwater. Operational BMPs are non-structural practices that
prevent or reduce pollutants from entering stormwater. See Volume IV of the SWMMWW for
details.
Threshold Discharge Area – An on-site area draining to a single natural discharge location or
multiple natural discharge locations that combine within one-quarter mile downstream (as
determined by the shortest flowpath). The examples in Figure 2.1 below illustrate this definition.
The purpose of this definition is to clarify how the thresholds of this appendix are applied to
project sites with multiple discharge points.
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Vehicular Use – Regular use of an impervious or pervious surface by motor vehicles. The
following are subject to regular vehicular use: roads, un-vegetated road shoulders, bike lanes
within the traveled lane of a roadway, driveways, parking lots, unrestricted access fire lanes,
vehicular equipment storage yards, and airport runways.
The following are not considered subject to regular vehicular use: paved bicycle pathways
separated from and not subject to drainage from roads for motor vehicles, restricted access fire
lanes, and infrequently used maintenance access roads.
Wetland – Those areas that are inundated or saturated by surface or ground water at a frequency
and duration sufficient to support, and that under normal circumstances do support, a prevalence
of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include
swamps, marshes, bogs, and similar areas. Wetlands do not include those artificial wetlands
intentionally created from non-wetland sites, including, but not limited to, irrigation and drainage
ditches, grass-lined swales, canals, detention facilities, wastewater treatment facilities, farm
ponds, and landscape amenities, or those wetlands created after July 1, 1990, that were
unintentionally created as a result of the construction of a road, street, or highway. Wetlands may
include those artificial wetlands intentionally created from non-wetland areas to mitigate the
conversion of wetlands.
Figure 2.1 Threshold Discharge Areas
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Section 3. Applicability of the Minimum Requirements
3.1 Thresholds
Not all of the Minimum Requirements apply to every development or redevelopment
project. The applicability varies depending on the project type and size. This section
identifies thresholds that determine the applicability of the Minimum Requirements to
projects. Use the flow charts in Figures 3.1, 3.2, and 3.3 to determine which of the
Minimum Requirements apply. The Minimum Requirements themselves are presented in
Section 4 of this Appendix.
Use the thresholds in sections 3.2 and 3.3 at the time of application for a subdivision,
plat, short plat, building permit, or other construction permit. The plat or short plat
approval shall identify all stormwater BMPs that are required for each lot. For projects
involving only land disturbing activities, (e.g., clearing or grading), the thresholds apply
at the time of application for the permit allowing or authorizing that activity. Note the
exemption in Section 1 for forest practices other than Class IV General.
Will the project discharge
stormwater either directly or
indirectly into an MS4 owned or
operated by the Permittee?
Permittee is not required
to apply the Minimum
Requirements to the
project.
Continue with Figure 3.2 and 3.3
No
Yes
Figure 3.1 Flow Chart for Determining Whether
the Permittee Must Regulate the Project
Is the Project exempt according to
Section 1 of this Appendix?
No
Yes
START
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Yes
Yes Yes
No
No
Yes No
Yes
No
Does the project convert
¾ acres or more of
vegetation to lawn or
landscaped areas, or
convert 2.5 acres or more
of native vegetation to
pasture?
See Redevelopment
Minimum
Requirements and
Flow Chart
(Figure 3.3)
Minimum
Requirement #2
applies.
Does the project
result in 5,000
square feet, or
greater, of new plus
replaced hard
surface area?
All Minimum
Requirements apply
to the new and
replaced hard surfaces
and converted
vegetation areas.
Does the project
result in 2,000 square
feet, or greater, of
new plus replaced
hard surface area?
Minimum Requirements
#1 through #5 apply to
the new and replaced
hard surfaces and the
land disturbed.
Does the project have
land disturbing
activities of 7,000
square feet or greater?
Start Here
No
Does the site have
35% or more of
existing impervious
coverage?
Figure 3.2 Flow Chart for Determining Requirements for New Development
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All Minimum Requirements apply to the new and
replaced hard surfaces and converted vegetation areas.
Yes
No
Next Question
Yes
Next
Question
No
Yes
No
Minimum Requirements #1 through #5 apply to
the new and replaced hard surfaces and the land
disturbed.
Minimum Requirements #2 applies.
Does the project add 5,000 square feet or more of new hard surfaces?
OR
Convert ¾ acres or more of vegetation to lawn or landscaped areas?
OR
Convert 2.5 acres or more of native vegetation to pasture?
All Minimum Requirements apply to the
new hard surfaces and the converted
vegetation areas.
Is this a road
related project?
Does the project add 5,000 square feet or more of new hard surfaces?
Yes
Yes
Yes
No
No
No
Do new hard surfaces add 50% or
more to the existing hard surfaces
within the project limits?
No additional
requirements
No additional
requirements
Figure 3.3 Flow Chart for Determining Requirements for Redevelopment
Does the project result in 2,000 square feet, or more, of new plus replaced hard surface area?
OR
Does the land disturbing activity total 7,000 square feet or greater?
Is the total of new plus replaced hard surfaces
5,000 square feet or more, AND does the value
of the proposed improvements – including
interior improvements – exceed 50% of the
assessed value (or replacement value) of the
existing site improvements?
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3.2 New Development
All new development shall be required to comply with Minimum Requirement #2.
The following new development shall comply with Minimum Requirements #1 through
#5 for the new and replaced hard surfaces and the land disturbed:
• Results in 2,000 square feet, or greater, of new plus replaced hard surface area, or
• Has land disturbing activity of 7,000 square feet or greater.
The following new development shall comply with Minimum Requirements #1 through
#9 for the new and replaced hard surfaces and the converted vegetation areas:
• Results in 5,000 square feet, or greater, of new plus replaced hard surface area, or
• Converts ¾ acres, or more, of vegetation to lawn or landscaped areas, or
• Converts 2.5 acres, or more, of native vegetation to pasture.
3.3 Redevelopment
All redevelopment shall be required to comply with Minimum Requirement #2.
The following redevelopment shall comply with Minimum Requirements #1 through #5
for the new and replaced hard surfaces and the land disturbed:
• Results in 2,000 square feet, or more, of new plus replaced hard surface area, or
• Has land disturbing activity of 7,000 square feet or greater.
The following redevelopment shall comply with Minimum Requirements #1 through #9
for the new hard surfaces and converted vegetation areas:
• Adds 5,000 square feet or more of new hard surfaces or,
• Converts ¾ acres, or more, of vegetation to lawn or landscaped areas, or
• Converts 2.5 acres, or more, of native vegetation to pasture.
The local government may allow the Minimum Requirements to be met for an equivalent
(flow and pollution characteristics) area within the same site. For public road projects, the
equivalent area does not have to be within the project limits, but must drain to the same
receiving water.
3.4 Additional Requirements for Re-development Project Sites
For road-related projects, runoff from the replaced and new hard surfaces (including
pavement, shoulders, curbs, and sidewalks) and the converted vegetation areas shall meet
all the Minimum Requirements if the new hard surfaces total 5,000 square feet or more
and total 50% or more of the existing hard surfaces within the project limits. The project
limits shall be defined by the length of the project and the width of the right-of-way.
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Other types of redevelopment projects shall comply with Minimum Requirements #1
through #9 for the new and replaced hard surfaces and the converted vegetation areas if
the total of new plus replaced hard surfaces is 5,000 square feet or more, and the
valuation of proposed improvements – including interior improvements – exceeds 50% of
the assessed value of the existing site improvements.
The Permittee may exempt or institute a stop-loss provision for redevelopment projects
from compliance with Minimum Requirement #5 On-site Stormwater Management,
Minimum Requirement #6 Runoff Treatment, Minimum Requirement #7 Flow Control
and/or Minimum Requirement #8 Wetlands Protection as applied to the replaced hard
surfaces if the Permittee has adopted a plan and a schedule that fulfills those requirements
in regional facilities.
The Permittee may grant a variance/exception to the application of the flow control
requirements to replaced impervious surfaces if such application imposes a severe
economic hardship. See Section 6 of this Appendix.
3.5 Modification of the Minimum Requirements
Basin Planning is encouraged and may be used to tailor Minimum Requirement #5
On-site Stormwater Management, Minimum Requirement #6 Runoff Treatment,
Minimum Requirement #7 Flow Control, and/or Minimum Requirement #8 Wetlands
Protection. Basin planning may also be used to demonstrate an equivalent level of
treatment, flow control, and/or wetland protection through the construction and use of
regional stormwater facilities. See Section 7 of this Appendix for details on Basin
Planning and how Permittees may use basin planning to modify the Minimum
Requirements in Section 4.
Section 4. Minimum Requirements
This Section describes the Minimum Requirements for stormwater management at new
development and redevelopment sites. Section 3 of this Appendix should be consulted to
determine which of the minimum requirements below apply to any given project. Figures
3.2 and 3.3 should be consulted to determine whether the minimum requirements apply to
new surfaces, replaced surfaces, or new and replaced surfaces.
4.1 Minimum Requirement #1: Preparation of Stormwater Site Plans
The permittee shall require a Stormwater Site Plan from all projects meeting the
thresholds in Section 3.1 of this Appendix. Stormwater Site Plans shall use site-
appropriate development principles, as required and encouraged by local development
codes, to retain native vegetation and minimize impervious surfaces to the extent feasible.
Stormwater Site Plans shall be prepared in accordance with Chapter 3 of Volume 1 of the
Stormwater Management Manual for Western Washington (SWMMWW).
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4.2 Minimum Requirement #2: Construction Stormwater Pollution Prevention Plan
(SWPPP)
Permittees may choose to allow compliance with this Minimum Requirement to be
achieved for an individual site if the site is covered under Ecology’s General NPDES
Permit for Stormwater Discharges Associated with Construction Activities and fully
implementing the requirements of that permit.
Thresholds
All new development and redevelopment projects are responsible for preventing erosion
and discharge of sediment and other pollutants into receiving waters.
Permittees must require a Construction Stormwater Pollution Prevention Plan (SWPPP)
for all projects which result in 2,000 sq. ft. or more of new plus replaced hard surface
area, or which disturb 7,000 sq. ft. or more of land.
Projects below those thresholds are not required to prepare a Construction SWPPP, but
must consider all of the Elements listed below for Construction SWPPPs and develop
controls for all elements that pertain to the project site. The Permittee may develop an
abbreviated SWPPP format to meet the SWPPP requirement under this permit for project
sites that will disturb less than 1 acre.
General Requirements
The SWPPP shall include a narrative and drawings. All BMPs shall be clearly referenced
in the narrative and marked on the drawings. The SWPPP narrative shall include
documentation to explain and justify the pollution prevention decisions made for the
project. Each of the thirteen elements listed below must be considered and included in the
SWPPP unless site conditions render the element unnecessary and the exemption from
that element is clearly justified in the narrative of the SWPPP.
Clearing and grading activities for developments shall be permitted only if conducted
pursuant to an approved site development plan (e.g., subdivision approval) that establishes
permitted areas of clearing, grading, cutting, and filling. These permitted clearing and
grading areas and any other areas required to preserve critical or sensitive areas, buffers,
native growth protection easements, or tree retention areas as may be required by local
jurisdictions, shall be delineated on the site plans and the development site.
The SWPPP shall be implemented beginning with initial land disturbance and until final
stabilization. Sediment and Erosion control BMPs shall be consistent with the BMPs
contained in chapter 4 of Volume II of the Stormwater Management Manual for Western
Washington (SWMMWW).
Seasonal Work Limitations - From October 1 through April 30, clearing, grading, and
other soil disturbing activities may only be authorized by the Permittee if silt-laden runoff
will be prevented from leaving the site through a combination of the following:
1. Site conditions including existing vegetative coverage, slope, soil type and
proximity to receiving waters; and
2. Limitations on activities and the extent of disturbed areas; and
3. Proposed erosion and sediment control measures.
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Based on the information provided and/or local weather conditions, the Permittee may
expand or restrict the seasonal limitation on site disturbance. The following activities are
exempt from the seasonal clearing and grading limitations:
1. Routine maintenance and necessary repair of erosion and sediment control BMPs,
2. Routine maintenance of public facilities or existing utility structures that do not
expose the soil or result in the removal of the vegetative cover to soil, and
3. Activities where there is one hundred percent infiltration of surface water runoff
within the site in approved and installed erosion and sediment control facilities.
Construction Stormwater Pollution Prevention Plan (SWPPP) Elements
1. Preserve Vegetation/Mark Clearing Limits:
a. Before beginning land disturbing activities, including clearing and grading,
clearly mark all clearing limits, sensitive areas and their buffers, and trees that are
to be preserved within the construction area.
b. Retain the duff layer, native top soil, and natural vegetation in an undisturbed
state to the maximum degree practicable.
2. Establish Construction Access:
a. Limit construction vehicle access and exit to one route, if possible.
b. Stabilize access points with a pad of quarry spalls, crushed rock, or other
equivalent BMPs, to minimize tracking of sediment onto public roads.
c. Locate wheel wash or tire baths on-site, if the stabilized constructions entrance is
not effective in preventing tracking sediment onto roads.
d. If sediment is tracked off site, clean the affected roadways thoroughly at the end
of each day, or more frequently as necessary (for example, during wet weather).
Remove sediment from roads by shoveling, sweeping, or pick up and transport the
sediment to a controlled sediment disposal area.
e. Conduct street washing only after sediment is removed in accordance with 2.d,
above.
f. Control street wash wastewater by pumping back on-site, or otherwise prevent it
from discharging into systems tributary to waters of the State.
3. Control Flow Rates:
a. Protect properties and waterways downstream of development sites from erosion
and the associated discharge of turbid waters due to increases in the velocity and
peak volumetric flow rate of stormwater runoff from the project site.
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b. Where necessary to comply with 3.a, above, construct stormwater retention or
detention facilities as one of the first steps in grading. Assure that detention
facilities function properly before constructing site improvements (e.g.,
impervious surfaces).
c. If permanent infiltration ponds are used for flow control during construction,
protect these facilities from siltation during the construction phase.
4. Install Sediment Controls:
a. Design, install, and maintain effective erosion controls and sediment controls to
minimize the discharge of pollutants.
b. Construct sediment control BMPs (sediment ponds, traps, filters, etc.) as one of
the first steps in grading. These BMPs shall be functional before other land
disturbing activities take place.
c. Minimize sediment discharges from the site. The design, installation and
maintenance of erosion and sediment controls must address factors such as the
amount, frequency, intensity and duration of precipitation, the nature of resulting
stormwater runoff, and soil characteristics, including the range of soil particle
sizes expected to be present on the site.
d. Direct stormwater runoff from disturbed areas through a sediment pond or other
appropriate sediment removal BMP, before the runoff leaves a construction site or
before discharge to an infiltration facility. Runoff from fully stabilized areas may
be discharged without a sediment removal BMP, but must meet the flow control
performance standard in 3.a, above.
e. Locate BMPs intended to trap sediment on-site in a manner to avoid interference
with the movement of juvenile salmonids attempting to enter off-channel areas or
drainages.
f. Where feasible, design outlet structures that withdraw impounded stormwater
from the surface to avoid discharging sediment that is still suspended lower in the
water column.
5. Stabilize Soils:
a. Stabilize exposed and unworked soils by application of effective BMPs that
prevent erosion. Applicable BMPs include, but are not limited to: temporary and
permanent seeding, sodding, mulching, plastic covering, erosion control fabrics
and matting, soil application of polyacrylamide (PAM), the early application of
gravel base early on areas to be paved, and dust control.
b. Control stormwater volume and velocity within the site to minimize soil erosion.
c. Control stormwater discharges, including both peak flow rates and total
stormwater volume, to minimize erosion at outlets and to minimize downstream
channel and stream bank erosion.
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d. Soils must not remain exposed and unworked for more than the time periods set
forth below to prevent erosion:
• During the dry season (May 1 – September 30): 7 days
• During the wet season (October 1 – April 30): 2 days
e. Stabilize soils at the end of the shift before a holiday or weekend if needed based
on the weather forecast.
f. Stabilize soil stockpiles from erosion, protect with sediment trapping measures,
and where possible, locate away from storm drain inlets, waterways and drainage
channels.
g. Minimize the amount of soil exposed during construction activity.
h. Minimize the disturbance of steep slopes.
i. Minimize soil compaction and, unless infeasible, preserve topsoil.
6. Protect Slopes:
a. Design and construct cut-and-fill slopes in a manner to minimize erosion.
Applicable practices include, but are not limited to, reducing continuous length of
slope with terracing and diversions, reducing slope steepness, and roughening
slope surfaces (for example, track walking).
b. Divert off-site stormwater (run-on) or ground water away from slopes and
disturbed areas with interceptor dikes, pipes and/or swales. Off-site stormwater
should be managed separately from stormwater generated on the site.
c. At the top of slopes, collect drainage in pipe slope drains or protected channels to
prevent erosion.
• Temporary pipe slope drains must handle the peak volumetric flow rate
calculated using a 10-minute time step from a Type 1A, 10-year, 24-hour
frequency storm for the developed condition. Alternatively, the 10-year 1-
hour flow rate predicted by an approved continuous runoff model, increased
by a factor of 1.6, may be used. The hydrologic analysis must use the existing
land cover condition for predicting flow rates from tributary areas outside the
project limits. For tributary areas on the project site, the analysis must use the
temporary or permanent project land cover condition, whichever will produce
the highest flow rates. If using the Western Washington Hydrology Model to
predict flows, bare soil areas should be modeled as “landscaped area.”
d. Place excavated material on the uphill side of trenches, consistent with safety and
space considerations.
e. Place check dams at regular intervals within constructed channels that are cut
down a slope.
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7. Protect Drain Inlets:
a. Protect storm drain inlets made operable during construction so that stormwater
runoff does not enter the conveyance system without first being filtered or treated
to remove sediment.
b. Clean or remove and replace inlet protection devices when sediment has filled
one-third of the available storage (unless a different standard is specified by the
product manufacturer).
8. Stabilize Channels and Outlets:
a. Design, construct, and stabilize all on-site conveyance channels to prevent erosion
from the following expected peak flows:
• Channels must handle the peak volumetric flow rate calculated using a 10-
minute time step from a Type 1A, 10-year, 24-hour frequency storm for the
developed condition. Alternatively, the 10-year, 1-hour flow rate indicated by
an approved continuous runoff model, increased by a factor of 1.6, may be
used. The hydrologic analysis must use the existing land cover condition for
predicting flow rates from tributary areas outside the project limits. For
tributary areas on the project site, the analysis shall use the temporary or
permanent project land cover condition, whichever will produce the highest
flow rates. If using the Western Washington Hydrology Model to predict
flows, bare soil areas should be modeled as “landscaped area.”
b. Provide stabilization, including armoring material, adequate to prevent erosion of
outlets, adjacent stream banks, slopes, and downstream reaches at the outlets of
all conveyance systems.
9. Control Pollutants:
a. Design, install, implement and maintain effective pollution prevention measures
to minimize the discharge of pollutants.
b. Handle and dispose all pollutants, including waste materials and demolition debris
that occur on-site in a manner that does not cause contamination of stormwater.
c. Provide cover, containment, and protection from vandalism for all chemicals,
liquid products, petroleum products, and other materials that have the potential to
pose a threat to human health or the environment. On-site fueling tanks must
include secondary containment. Secondary containment means placing tanks or
containers within an impervious structure capable of containing 110% of the
volume contained in the largest tank within the containment structure. Double-
walled tanks do not require additional secondary containment.
d. Conduct maintenance, fueling and repair of heavy equipment and vehicles using
spill prevention and control measures. Clean contaminated surfaces immediately
following any spill incident.
e. Discharge wheel wash or tire bath wastewater to a separate on-site treatment
system that prevents discharge to surface water, such as closed-loop recirculation
or upland application, or to the sanitary sewer, with local sewer district approval.
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f. Apply fertilizers and pesticides in a manner and at application rates that will not
result in loss of chemical to stormwater runoff. Follow manufacturers’ label
requirements for application rates and procedures.
g. Use BMPs to prevent contamination of stormwater runoff by pH modifying
sources. The sources for this contamination include, but are not limited to: bulk
cement, cement kiln dust, fly ash, new concrete washing and curing waters, waste
streams generated from concrete grinding and sawing, exposed aggregate
processes, dewatering concrete vaults, concrete pumping and mixer washout
waters.
h. Adjust the pH of stormwater if necessary to prevent violations of water quality
standards.
i. Assure that washout of concrete trucks is performed off-site or in designated
concrete washout areas only. Do not wash out concrete trucks onto the ground, or
into storm drains, open ditches, streets, or streams. Do not dump excess concrete
on-site, except in designated concrete washout areas. Concrete spillage or
concrete discharge to surface waters of the State is prohibited.
j. Obtain written approval from Ecology before using chemical treatment other than
CO2 or dry ice to adjust pH.
10. Control De-Watering:
a. Discharge foundation, vault, and trench de-watering water, which have similar
characteristics to stormwater runoff at the site, into a controlled conveyance
system before discharge to a sediment trap or sediment pond.
b. Discharge clean, non-turbid de-watering water, such as well-point ground water,
to systems tributary to, or directly into surface waters of the State, as specified in
8, above, provided the de-watering flow does not cause erosion or flooding of
receiving waters. Do not route clean dewatering water through stormwater
sediment ponds. Note that “surface waters of the State” may exist on a
construction site as well as off site; for example, a creek running through a site.
c. Handle highly turbid or otherwise contaminated dewatering water separately from
stormwater.
d. Other treatment or disposal options may include:
(i) Infiltration
(ii) Transport off-site in vehicle, such as a vacuum flush truck, for legal
disposal in a manner that does not pollute state waters.
(iii) Ecology-approved on-site chemical treatment or other suitable treatment
technologies.
(iv) Sanitary or combined sewer discharge with local sewer district approval, if
there is no other option.
(v) Use of a sedimentation bag that discharges to a ditch or swale for small
volumes of localized dewatering.
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11. Maintain BMPs:
a. Maintain and repair all temporary and permanent erosion and sediment control
BMPs as needed to assure continued performance of their intended function in
accordance with BMP specifications.
b. Remove all temporary erosion and sediment control BMPs within 30 days after
achieving final site stabilization or after the temporary BMPs are no longer
needed.
12. Manage the Project:
a. Phase development projects to the maximum degree practicable and take into
account seasonal work limitations.
b. Inspection and monitoring – Inspect, maintain, and repair all BMPs as needed to
assure continued performance of their intended function.
c. Maintaining an updated construction SWPPP – Maintain, update, and implement
the SWPPP.
d. Projects that disturb one or more acres must have site inspections conducted by a
Certified Erosion and Sediment Control Lead (CESCL). Project sites disturbing
less than one acre may have a CESCL or a person without CESCL certification
conduct inspections. By the initiation of construction, the SWPPP must identify
the CESCL or inspector, who must be present on-site or on-call at all times.
13. Protect Low Impact Development BMPs
a. Protect all Bioretention and Rain Garden BMPs from sedimentation through
installation and maintenance of erosion and sediment control BMPs on portions of
the site that drain into the Bioretention and/or Rain Garden BMPs. Restore the
BMPs to their fully functioning condition if they accumulate sediment during
construction. Restoring the BMP must include removal of sediment and any
sediment-laden Bioretention/rain garden soils, and replacing the removed soils
with soils meeting the design specification.
b. Prevent compacting Bioretention and Rain Garden BMPs by excluding
construction equipment and foot traffic. Protect completed lawn and landscaped
areas from compaction due to construction equipment.
c. Control erosion and avoid introducing sediment from surrounding land uses onto
permeable pavements. Do not allow muddy construction equipment on the base
material or pavement. Do not allow sediment-laden runoff onto permeable
pavements or base materials.
d. Pavements fouled with sediments or no longer passing an initial infiltration test
must be cleaned using procedures from the local stormwater manual or the
manufacturer’s procedures.
e. Keep all heavy equipment off existing soils under LID BMPs that have been
excavated to final grade to retain the infiltration rate of the soils.
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4.3 Minimum Requirement #3: Source Control of Pollution
All known, available and reasonable source control BMPs must be required for all
projects approved by the Permittee. Source control BMPs must be selected, designed, and
maintained in accordance with Volume IV of the Stormwater Management Manual for
Western Washington or an approved equivalent manual approved by Ecology.
4.4 Minimum Requirement #4: Preservation of Natural Drainage Systems and Outfalls
Natural drainage patterns shall be maintained, and discharges from the project site shall
occur at the natural location, to the maximum extent practicable. The manner by which
runoff is discharged from the project site must not cause a significant adverse impact to
downstream receiving waters and down gradient properties. All outfalls require energy
dissipation.
4.5 Minimum Requirement #5: On-site Stormwater Management
Applicability
Except as provided below, the Permittee must require On-site Stormwater Management
BMPs in accordance with the following project thresholds, standards, and lists to
infiltrate, disperse, and retain stormwater runoff on-site to the extent feasible without
causing flooding or erosion impacts.
Projects qualifying as flow control exempt in accordance with Section 4.7 of this
Appendix do not have to achieve the LID performance standard, nor consider
bioretention, rain gardens, permeable pavement, and full dispersion if using List #1 or
List #2. However, those projects must implement BMP T5.13; BMPs T5.10A, B, or C;
and BMP T5.11or T5.12, if feasible.
Project Thresholds
1. Projects triggering only Minimum Requirements #1 through #5 shall either:
a. Use On-site Stormwater Management BMPs from List #1 for all surfaces
within each type of surface in List #1; or
b. Demonstrate compliance with the LID Performance Standard. Projects
selecting this option cannot use Rain Gardens. They may choose to use
Bioretention BMPs as described in the SWMMWW1.
2. Projects triggering Minimum Requirements #1 through #9 must meet the
requirements in Table 4.1
1 All references to the Stormwater Management Manual for Western Washington are to the 2014 amended version.
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Table 4.1: On-site Stormwater Management Requirements for Projects Triggering
Minimum Requirements #1 - #9
Project Type and Location Requirement
New development on any parcel inside the
UGA, or new development outside the
UGA on a parcel less than 5 acres
Low Impact Development Performance
Standard and BMP T5.13; or List #2
(applicant option).
New development outside the UGA on a
parcel of 5 acres or larger
Low Impact Development Performance
Standard and BMP T5.13.
Redevelopment on any parcel inside the
UGA, or redevelopment outside the UGA
on a parcel less than 5 acres
Low Impact Development Performance
Standard and BMP T5.13; or List #2
(applicant option).
Redevelopment outside the UGA on a
parcel of 5 acres or larger
Low Impact Development Performance
Standard and BMP T5.13.
NOTE: This table refers to the Urban Growth Area (UGA) as designated under the
Growth Management Act (GMA) (chapter 36.70A RCW) of the State of Washington. If
the Permittee is located in a county that is not subject to planning under the GMA, the
city limits shall be used instead.
Low Impact Development Performance Standard
Stormwater discharges shall match developed discharge durations to pre-developed
durations for the range of pre-developed discharge rates from 8% of the 2-year peak flow
to 50% of the 2-year peak flow. Refer to the Standard Flow Control Requirement section
in Minimum Requirement #7 for information about the assignment of the pre-developed
condition. Project sites that must also meet minimum requirement #7 shall match flow
durations between 8% of the 2-year flow through the full 50-year flow.
List #1: On-site Stormwater Management BMPs for Projects Triggering Minimum
Requirements #1 through #5
For each surface, consider the BMP’s in the order listed for that type of surface. Use the
first BMP that is considered feasible. No other On-site Stormwater Management BMP is
necessary for that surface. Feasibility shall be determined by evaluation against:
1. Design criteria, limitations, and infeasibility criteria identified for each BMP in
the SWMMWW; and
2. Competing Needs Criteria listed in Chapter 5 of Volume V of the SWMMWW.
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Lawn and landscaped areas:
• Post-Construction Soil Quality and Depth in accordance with BMP T5.13 in
Chapter 5 of Volume V of the SWMMWW
Roofs:
1. Full Dispersion in accordance with BMP T5.30 in Chapter 5 of Volume V of the
SWMMWW, or Downspout Full Infiltration Systems in accordance with BMP
T5.10A in Section 3.1.1 of Volume III of the SWMMWW.
2. Rain Gardens in accordance with BMP T5.14A in Chapter 5 of Volume V, or
Bioretention in accordance with Chapter 7 of Volume V of the SWMMWW. The
rain garden or bioretention facility must have a minimum horizontal projected
surface area below the overflow which is at least 5% of the area draining to it.
3. Downspout Dispersion Systems in accordance with BMP T5.10B in Section 3.1.2
of Volume III of the SWMMWW.
4. Perforated Stub-out Connections in accordance with BMP T5.10C in Section
3.1.3 of Volume III of the SWMMWW.
Other Hard Surfaces:
1. Full Dispersion in accordance with BMP T5.30 in Chapter 5 of Volume V of the
SWMMWW.
2. Permeable pavement 2 in accordance with BMP T5.15 in Chapter 5 of Volume V
of the SWMMWW, or Rain Gardens in accordance with BMP T5.14A in Chapter 5
of Volume V, or Bioretention in accordance with Chapter 7 of Volume V of the
SWMMWW. The rain garden or bioretention facility must have a minimum
horizontal projected surface area below the overflow which is at least 5% of the
area draining to it.
3. Sheet Flow Dispersion in accordance with BMP T5.12, or Concentrated Flow
Dispersion in accordance with BMP T5.11 in Chapter 5 of Volume V of the
SWMMWW.
List #2: On-site Stormwater Management BMPs for Projects Triggering Minimum
Requirements #1 through #9
For each surface, consider the BMPs in the order listed for that type of surface. Use the
first BMP that is considered feasible. No other On-site Stormwater Management BMP is
necessary for that surface. Feasibility shall be determined by evaluation against:
1. Design criteria, limitations, and infeasibility criteria identified for each BMP in
the SWMMWW; and
2. Competing Needs Criteria listed in Chapter 5 of Volume V of the SWMMWW.
2 This is not a requirement to pave these surfaces. Where pavement is proposed, it must be permeable to the extent
feasible unless full dispersion is employed.
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Lawn and landscaped areas:
• Post-Construction Soil Quality and Depth in accordance with BMP T5.13 in
Chapter 5 of Volume V of the SWMMWW
Roofs:
1. Full Dispersion in accordance with BMP T5.30 in Chapter 5 of Volume V of the
SWMMWW, or Downspout Full Infiltration Systems in accordance with BMP
T5.10A in Section 3.1.1 of Volume III of the SWMMWW
2. Bioretention (See Chapter 7 of Volume V of the SWMMWW) facilities that have a
minimum horizontally projected surface area below the overflow which is at least
5% of the of the total surface area draining to it
3. Downspout Dispersion Systems in accordance with BMP T5.10B in Section 3.1.2
of Volume III of the SWMMWW
4. Perforated Stub-out Connections in accordance with BMP T5.10C in Section
3.1.3 of Volume III of the SWMMWW.
Other Hard Surfaces:
1. Full Dispersion in accordance with BMP T5.30 in Chapter 5 of Volume V of the
SWMMWW
2. Permeable pavement2 in accordance with BMP T5.15 in Chapter 5 of Volume V
of the SWMMWW
3. Bioretention (See Chapter 7, Volume V of the S WMMWW) facilities that have a
minimum horizontally projected surface area below the overflow which is at least
5% of the total surface area draining to it.
4. Sheet Flow Dispersion in accordance with BMP T5.12, or Concentrated Flow
Dispersion in accordance with BMP T5.11 in Chapter 5 of Volume V of the
SWMMWW
4.6 Minimum Requirement #6: Runoff Treatment
Project Thresholds
When assessing a project against the following thresholds, only consider those hard
and pervious surfaces that are subject to this minimum requirement as determined in
Section 3 of this Appendix.
The following require construction of stormwater treatment facilities:
• Projects in which the total of pollution-generating hard surface (PGHS) is 5,000
square feet or more in a threshold discharge area of the project, or
• Projects in which the total of pollution-generating pervious surfaces (PGPS) – not
including permeable pavements - is three-quarters (3/4) of an acre or more in a
threshold discharge area, and from which there will be a surface discharge in a
natural or man-made conveyance system from the site.
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Treatment-Type Thresholds
1. Oil Control:
Treatment to achieve Oil Control applies to projects that have “high-use sites.”
High-use sites are those that typically generate high concentrations of oil due to
high traffic turnover or the frequent transfer of oil. High-use sites include:
a. An area of a commercial or industrial site subject to an expected average
daily traffic (ADT) count equal to or greater than 100 vehicles per 1,000
square feet of gross building area;
b. An area of a commercial or industrial site subject to petroleum storage and
transfer in excess of 1,500 gallons per year, not including routinely delivered
heating oil;
c. An area of a commercial or industrial site subject to parking, storage or
maintenance of 25 or more vehicles that are over 10 tons gross weight
(trucks, buses, trains, heavy equipment, etc.);
d. A 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.
2. Phosphorus Treatment:
The requirement to provide phosphorous control is determined by the local
government with jurisdiction (e.g., through a lake management plan), or the
Department of Ecology (e.g., through a waste load allocation). The local
government may have developed a management plan and implementing
ordinances or regulations for control of phosphorus from new/redevelopment for
the receiving water(s) of the stormwater drainage. The local government can use
the following sources of information for pursuing plans and implementing
ordinances and/or regulations:
a. Those waterbodies reported under section 305(b) of the Clean Water Act, and
designated as not supporting beneficial uses due to phosphorous;
b. Those listed in Washington State's Nonpoint Source Assessment required
under section 319(a) of the Clean Water Act due to nutrients.
3. Enhanced Treatment:
Except where specified below under “4. Basic Treatment”, Enhanced treatment
for reduction in dissolved metals is required for the following project sites that: 1)
discharge directly to fresh waters or conveyance systems tributary to fresh waters
designated for aquatic life use or that have an existing aquatic life use; or 2) use
infiltration strictly for flow control – not treatment – and the discharge is within ¼
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mile of a fresh water designated for aquatic life use or that has an existing aquatic
life use:
Industrial project sites,
Commercial project sites,
Multi-family project sites, and
High AADT roads as follows:
Within Urban Growth Management Areas:
• Fully controlled and partially controlled limited access highways with
Annual Average Daily Traffic (AADT) counts of 15,000 or more
• All other roads with an AADT of 7,500 or greater
Outside of Urban Growth Management Areas:
• Roads with an AADT of 15,000 or greater unless discharging to a 4th
Strahler order stream or larger;
• Roads with an AADT of 30,000 or greater if discharging to a 4th Strahler
order stream or larger (as determined using 1:24,000 scale maps to
delineate stream order).
Any areas of the above-listed project sites that are identified as subject to Basic
Treatment requirements (below), are not also subject to Enhanced Treatment
requirements. For developments with a mix of land use types, the Enhanced
Treatment requirement shall apply when the runoff from the areas subject to the
Enhanced Treatment requirement comprise 50% or more of the total runoff within
a threshold discharge area.
4. Basic Treatment:
Basic Treatment is required in the following circumstances:
• Project sites that discharge to the ground, UNLESS:
1) The soil suitability criteria for infiltration treatment are met (See
Chapter 3, Volume III of the SWMMWW), and alternative pretreatment
is provided (see Chapter 6, Volume V of the SWMMWW); or
2) The project site uses infiltration strictly for flow control – not treatment
- and the discharge is within ¼-mile of a phosphorus sensitive lake
(use a Phosphorus Treatment facility), or
3) The project site is industrial, commercial, multi-family residential, or a
high AADT road (consistent with the Enhanced Treatment-type
thresholds listed above) and is within ¼ mile of a fresh water
designated for aquatic life use or that has an existing aquatic life
use.(use an Enhanced Treatment facility).
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• Residential projects not otherwise needing phosphorus control as
designated by USEPA, the Department of Ecology, or by the Permittee;
• Project sites discharging directly (or indirectly through a municipal
separate storm sewer system) to Basic Treatment Receiving Waters
(Appendix I-C of the SWMMWW);
• Project sites that drain to fresh water that is not designated for aquatic life
use, and does not have an existing aquatic life use; and project sites that
drain to waters not tributary to waters designated for aquatic life use or
that have an existing aquatic life use;
• Landscaped areas of industrial, commercial, and multi-family project sites,
and parking lots of industrial and commercial project sites that do not
involve pollution-generating sources (e.g., industrial activities, customer
parking, storage of erodible or leachable material, wastes or chemicals)
other than parking of employees’ private vehicles. For developments with
a mix of land use types, the Basic Treatment requirement shall apply when
the runoff from the areas subject to the Basic Treatment requirement
comprise 50% or more of the total runoff within a threshold discharge
area.
Treatment Facility Sizing
Size stormwater treatment facilities for the entire area that drains to them, even if some of
those areas are not pollution-generating, or were not included in the project site threshold
decisions (Section 3 of this appendix) or the treatment threshold decisions of this
minimum requirement.
Water Quality Design Storm Volume: The volume of runoff predicted from a 24-hour
storm with a 6-month return frequency (a.k.a., 6-month, 24-hour storm). Wetpool
facilities are sized based upon the volume of runoff predicted through use of the Natural
Resource Conservation Service curve number equations in Chapter 2 of Volume III of the
SWMMWW), for the 6-month, 24-hour storm. Alternatively, when using an approved
continuous runoff model, the water quality design storm volume shall be equal to the
simulated daily volume that represents the upper limit of the range of daily volumes that
accounts for 91% of the entire runoff volume over a multi-decade period of record.
Water Quality Design Flow Rate
1. Preceding Detention Facilities or when Detention Facilities are not required:
The flow rate at or below which 91% of the runoff volume, as estimated by an
approved continuous runoff model, will be treated. Design criteria for treatment
facilities are assigned to achieve the applicable performance goal (e.g., 80% TSS
removal) at the water quality design flow rate. At a minimum, 91% of the total
runoff volume, as estimated by an approved continuous runoff model, must pass
through the treatment facility(ies) at or below the approved hydraulic loading rate
for the facility(ies).
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2. Downstream of Detention Facilities:
The water quality design flow rate must be the full 2-year release rate from the
detention facility.
Treatment Facility Selection, Design, and Maintenance
Stormwater treatment facilities shall be:
• Selected in accordance with the process identified in Chapter 4 of Volume I, and
Chapter 2 of Volume V of the SWMMWW,
• Designed in accordance with the design criteria in Volume V of the SWMMWW,
and
• Maintained in accordance with the maintenance schedule in Volume V of the
SWMMWW.
Additional Requirements
The discharge of untreated stormwater from pollution-generating hard surfaces to ground
water must not be authorized by the Permittee, except for the discharge achieved by
infiltration or dispersion of runoff through use of On-site Stormwater Management
BMPs in accordance with Chapter 5, Volume V and Chapter 7, Volume V of the
SWMMWW; or by infiltration through soils meeting the soil suitability criteria in Chapter
3 of Volume III of the SWMMWW.
4.7 Minimum Requirement #7: Flow Control
Applicability
Except as provided below, the Permittee must require all projects provide flow control to
reduce the impacts of stormwater runoff from hard surfaces and land cover conversions.
The requirement below applies to projects that discharge stormwater directly, or
indirectly through a conveyance system, into a fresh water body.
Flow control is not required for projects that discharge directly to, or indirectly through
an MS4 to a water listed in Appendix I-E of the SWMMWW subject to the following
restrictions:
• Direct discharge to the exempt receiving water does not result in the diversion of
drainage from any perennial stream classified as Types 1, 2, 3, or 4 in the State of
Washington Interim Water Typing System, or Types “S”, “F”, or “Np” in the
Permanent Water Typing System, or from any category I, II, or III wetland; and
• Flow splitting devices or drainage BMP’s are applied to route natural runoff volumes
from the project site to any downstream Type 5 stream or category IV wetland:
o Design of flow splitting devices or drainage BMP’s will be based on
continuous hydrologic modeling analysis. The design will assure that flows
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delivered to Type 5 stream reaches will approximate, but in no case exceed,
durations ranging from 50% of the 2-year to the 50-year peak flow.
o Flow splitting devices or drainage BMP’s that deliver flow to category IV
wetlands will also be designed using continuous hydrologic modeling to
preserve pre-project wetland hydrologic conditions unless specifically waived
or exempted by regulatory agencies with permitting jurisdiction; and
• The project site must be drained by a conveyance system that is comprised entirely of
manmade conveyance elements (e.g., pipes, ditches, outfall protection) and extends to
the ordinary high water line of the exempt receiving water; and
• The conveyance system between the project site and the exempt receiving water shall
have sufficient hydraulic capacity to convey discharges from future build-out
conditions (under current zoning) of the site, and the existing condition from non-
project areas from which runoff is or will be collected; and
• Any erodible elements of the manmade conveyance system must be adequately
stabilized to prevent erosion under the conditions noted above.
If the discharge is to a stream that leads to a wetland, or to a wetland that has an outflow
to a stream, both this minimum requirement (Minimum Requirement #7) and Minimum
Requirement #8 apply.
Permittees may petition Ecology to exempt projects in additional areas. A petition must
justify the proposed exemption based upon a hydrologic analysis that demonstrates that
the potential stormwater runoff from the exempted area will not significantly increase the
erosion forces on the stream channel nor have near-field impacts.
Thresholds
When assessing a project against the following thresholds, consider only those
impervious, hard, and pervious surfaces that are subject to this minimum requirement as
determined in Section 3 of this Appendix.
The following circumstances require achievement of the standard flow control
requirement for western Washington:
• Projects in which the total of effective impervious surfaces is 10,000 square feet or
more in a threshold discharge area, or
• Projects that convert ¾ acres or more of vegetation to lawn or landscape, or convert
2.5 acres or more of native vegetation to pasture in a threshold discharge area, and
from which there is a surface discharge in a natural or man-made conveyance system
from the site, or
• Projects that through a combination of hard surfaces and converted vegetation areas
cause a 0.10 cubic feet per second (cfs) increase or greater in the 100-year flow
frequency from a threshold discharge area as estimated using the Western
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Washington Hydrology Model or other approved model and one-hour time steps (or a
0.15 cfs increase or greater using 15-minute time steps).3
Standard Flow Control Requirement
Stormwater discharges shall match developed discharge durations to pre-developed
durations for the range of pre-developed discharge rates from 50% of the 2-year peak
flow up to the full 50-year peak flow. The pre-developed condition to be matched shall be
a forested land cover unless:
• Reasonable, historic information is available that indicates the site was prairie prior to
settlement (modeled as “pasture” in the Western Washington Hydrology Model); or
• The drainage area of the immediate stream and all subsequent downstream basins
have had at least 40% total impervious area since 1985. In this case, the pre-
developed condition to be matched shall be the existing land cover condition. The
map in Appendix I-G of the SWMMWW depicts those areas which meet this
criterion. Where basin-specific studies determine a stream channel to be unstable,
even though the above criterion is met, the pre-developed condition assumption shall
be the “historic” land cover condition, or a land cover condition commensurate with
achieving a target flow regime identified by an approved basin study.
This standard requirement is waived for sites that will reliably infiltrate all the runoff
from hard surfaces and converted vegetation areas.
Western Washington Alternative Requirement
An alternative requirement may be established through application of watershed-scale
hydrological modeling and supporting field observations. Possible reasons for an
alternative flow control requirement include:
• Establishment of a stream–specific threshold of significant bedload movement other
than the assumed 50% of the 2-year peak flow;
• Zoning and Land Clearing Ordinance restrictions that, in combination with an
alternative flow control standard, maintain or reduce the naturally occurring erosive
forces on the stream channel; or
• A duration control standard is not necessary for protection, maintenance, or
restoration of designated and existing beneficial uses or Clean Water Act compliance.
See Section 7 Basin/Watershed Planning of this Appendix for details on how alternative
flow control requirements may be established.
3 The 0.10 cfs (one-hour time steps) or 0.15 cfs (15-minute time steps) increase should be a comparison of the post-
project runoff to the existing condition runoff. For the purpose of applying this threshold, the existing condition is
either the pre-project land cover, or the land cover that existed at the site as of a date when the local jurisdiction first
adopted flow control requirements into code or rules.
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Additional Requirement
Flow Control BMPs shall be selected, designed, and maintained in accordance with
Volume III of the SWMMWW or an approved equivalent.
4.8 Minimum Requirement #8: Wetlands Protection
Applicability
The requirements below apply only to projects whose stormwater discharges into a
wetland, either directly or indirectly through a conveyance system.
Thresholds
The thresholds identified in Minimum Requirement #6 – Runoff Treatment, and
Minimum Requirement #7 – Flow Control shall also be applied to determine the
applicability of this requirement to discharges to wetlands.
Standard Requirement
Projects shall comply with Guide Sheets #1 through #3 in Appendix I-D of the
SWMMWW. The hydrologic analysis shall use the existing land cover condition to
determine the existing hydrologic conditions unless directed otherwise by a regulatory
agency with jurisdiction.
Additional Requirements
Stormwater treatment and flow control facilities shall not be built within a natural
vegetated buffer, except for:
• Necessary conveyance systems as approved by the Permittee; or
• As allowed in wetlands approved for hydrologic modification and/or treatment in
accordance with Guide Sheet 2 in Appendix I-D of the SWMMWW.
An adopted and implemented basin plan prepared in accordance with the provisions of
Section 7 of this Appendix may be used to develop requirements for wetlands that are
tailored to a specific basin.
4.9 Minimum Requirement #9: Operation and Maintenance
Permittees must require an operation and maintenance manual that is consistent with the
provisions in Volume V of the SWMMWW for proposed stormwater facilities and BMPs.
The party (or parties) responsible for maintenance and operation shall be identified in the
operation and maintenance manual. For private facilities approved by the Permittee, a
copy of the operation and maintenance manual shall be retained on-site or within
reasonable access to the site, and shall be transferred with the property to the new owner.
For public facilities, a copy of the operation and maintenance manual shall be retained in
the appropriate department. A log of maintenance activity that indicates what actions
were taken shall be kept and be available for inspection by the local government.
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Section 5. Adjustments
Adjustments to the Minimum Requirements may be granted by the Permittee provided that a
written finding of fact is prepared, that addresses the following:
• The adjustment provides substantially equivalent environmental protection.
• Based on sound Engineering practices, the objectives of safety, function,
environmental protection and facility maintenance, are met.
Section 6. Exceptions/Variances
Exceptions/variances (exceptions) to the Minimum Requirements may be granted by the
Permittee following legal public notice of an application for an exception or variance, legal
public notice of the Permittee’s decision on the application, and written findings of fact that
documents the Permittees determination to grant an exception. Permittees shall keep records,
including the written findings of fact, of all local exceptions to the Minimum Requirements.
Project-specific design exceptions based on site-specific conditions do not require prior approval
of Ecology. The Permittee must seek prior approval by Ecology for any jurisdiction-wide
exception.
The Permittee may grant an exception to the minimum requirements if such application imposes
a severe and unexpected economic hardship. To determine whether the application imposes a
severe and unexpected economic hardship on the project applicant, the Permittee must consider
and document with written findings of fact the following:
• The current (pre-project) use of the site, and
• How the application of the minimum requirement(s) restricts the proposed use of
the site compared to the restrictions that existed prior to the adoption of the
minimum requirements; and
• The possible remaining uses of the site if the exception were not granted; and
• The uses of the site that would have been allowed prior to the adoption of the
minimum requirements; and
• A comparison of the estimated amount and percentage of value loss as a result of
the minimum requirements versus the estimated amount and percentage of value
loss as a result of requirements that existed prior to adoption of the minimum
requirements; and
• The feasibility for the owner to alter the project to apply the minimum
requirements.
In addition any exception must meet the following criteria:
• The exception will not increase risk to the public health and welfare, nor be
injurious to other properties in the vicinity and/or downstream, and to the quality
of waters of the state; and
• The exception is the least possible exception that could be granted to comply with
the intent of the Minimum Requirements.
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Section 7. Basin/Watershed Planning
Basin/Watershed planning may be used by the Permittee to tailor Minimum Requirement #5
On-site Stormwater Management, Minimum Requirement #6 Runoff Treatment, Minimum
Requirement #7 Flow Control, and/or Minimum Requirement #8 Wetlands Protection. Basin
planning may also be used to demonstrate an equivalent level of treatment, flow control, and/or
wetland protection through the construction and use of regional stormwater facilities.
Basin planning provides a mechanism by which the minimum requirements and implementing
BMP’s can be evaluated and refined based on an analysis of a basin or watershed. Basin plans
may be used to develop control strategies to address impacts from future development and to
correct specific problems whose sources are known or suspected. Basin plans can be effective at
addressing both long-term cumulative impacts of pollutant loads and short-term acute impacts of
pollutant concentrations, as well as hydrologic impacts to streams, wetlands, and ground water
resources.
Basin planning will require the use of continuous runoff computer models and field work to
verify and support the models. Permittees who are considering the use of basin/watershed plans
to modify or tailor one or more of the minimum requirements are encouraged to contact Ecology
early in the planning stage.
Some examples of how Basin Planning can alter the minimum requirements are given in
Appendix I-A from the SWMMWW.
In order for a basin plan to serve as a means of modifying the minimum requirements the
following conditions must be met:
• The plan must be formally adopted by all jurisdictions with responsibilities under
the plan; and
• All ordinances or regulations called for by the plan must be in effect; and
• The basin plan must be reviewed and approved by Ecology.