Ordinance 37920006.90000
WSS/gjz
4/9/ 10
R:4/14/l Ogjz
ORDINANCE NO.3792
AN ORDINANCE OF THE CITY OF EDMONDS,
WASHINGTON, REPEALING AND REENACTING CHAPTER
18.30 STORM WATER MANAGEMENT OF THE EDMONDS
COMMUNITY DEVELOPMENT CODE, PROVIDING FOR
THE APPLICATION OF EXISTING CHAPTER 18.30 TO
VESTED PERMITS, AND FIXING A TIME WHEN THE SAME
SHALL BECOME EFFECTIVE.
WHEREAS, changes in state and federal regulatory law, most prominently the
Western Washington Phase II Municipal Storm Water Permit issued by the Department of
Ecology, require amendment of the City's storm water management policies; and
WHEREAS, public hearings were held regarding this matter on April 20, 2010,
NOW, THEREFORE,
THE CITY COUNCIL OF THE CITY OF EDMONDS, WASHINGTON, DO
ORDAIN AS FOLLOWS:
Section 1. The Edmonds Community Development Code, Chapter 18.30 Storm
Water Mana eg ment is hereby repealed (see Section 2 of this ordinance) and reenacted to read as
follows:
Sections:
18.30.000
18.30.010
18.30.020
18.30.030
Chapter 18.30
STORMWATER MANAGEMENT
Purpose
Definitions
Regulation
Applicability
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18.30.040
Administration
18.30.050
Project Classification
18.30.060
Stormwater Management Requirements
18.30.070
Exemptions, Exceptions, and Adjustments
18.30.080
Easements, Deeds, and Covenants
18.30.090
Inspection and Maintenance Roles and
Responsibilities
18.30.100
Enforcement Procedures
18.30.000 Purpose
A. To protect water resources, reduce the discharge of
pollutants to the maximum extent practicable, and satisfy the state
requirement under Chapter 90.48 RCW to apply all known,
available, and reasonable methods of prevention, control, and
treatment ("AKART") to stormwater runoff prior to discharge to
receiving waters.
B. To control stormwater runoff generated by development,
redevelopment, construction sites, or modifications to existing
stormwater systems that directly or indirectly discharge to the City
stormwater system, in a manner that complies with the Western
Washington Phase H Municipal Stormwater Permit issued by the
Department of Ecology.
C. To protect land and the ecological balance of receiving
water bodies near development sites from increased surface water
runoff rates and durations that could cause flooding or erosion,
scouring, and deposition of sediment (caused by development).
D. To protect private and public property and city streets and
rights -of -way (including easements) from flooding or erosion due
to development activity.
E. To provide for inspection and maintenance of stormwater
facilities in the city to ensure that these facilities perform as
designed.
F. To require that all public and private stormwater facilities
be operated, maintained, and repaired in a manner that conforms to
this chapter.
G. To establish the minimum standards that must be met for
compliance.
H. To provide guidelines for all who inspect and maintain
stormwater facilities.
I. To promote development practices that ensure the above
purposes are met.
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18.30.010 Definitions
For the purposes of this chapter, the following definitions shall
apply:
A. "Adjustment" means a variation in the application of a
Minimum Requirement to a particular project. Adjustments
provide substantially equivalent environmental protection.
B. "Approval" means the proposed work or completed work
conforming to this chapter as approved by the Public Works
Director or their designee.
C. "Applicant" means the owning individual(s) or
corporations or their representatives applying for the permits or
approvals described in this Chapter.
D. "Best management practice (BMP)" means the schedule of
activities, prohibition of practices, maintenance procedures, and
structural or managerial practices approved by the City that, when
used singly or in combination, prevent or reduce the release
pollutants and other adverse impacts to waters of Washington
State.
E. "City's Municipal Separate Storm Sewer System or MS4"
means a conveyance, or system of conveyances (including roads
with drainage systems, municipal streets, catch basins, curbs,
gutters, ditches, manmade channels, or storm drains) that are
owned or operated by the City of Edmonds, designed or used for
collecting or conveying stormwater, and are not a combined sewer
nor part of a Publicly Owned Treatment Works as defined in 40
Code of Federal Regulations (CFR) 122.2.
F. "Clearing" means the act of cutting or removing vegetation,
including grubbing.
G. "Common plan of development or sale" means a site where
multiple separate and distinct construction activities may take
place at different times on different schedules, but still under a
single plan. For example: phased projects and projects with
multiple filings or lots, even if the separate phases or filings/lots
will be constructed under separate contract or by separate owners
(e.g., a development where lots are sold to separate builders); a
development plan that may be phased over multiple years, but is
still under a consistent plan for long-term development; or projects
in a contiguous area that may be unrelated but still under the same
contract, such as construction of a building extension and a new
parking lot at the same facility.
H. "Construction Activity" means land -disturbing operations
including clearing, grading, or excavation that disturbs the surface
of the land. Such activities may include road construction,
construction of residential houses, office buildings, or industrial
buildings, and demolition activity.
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I. "Converted Pervious Surface" means the change in land
cover changed from native vegetation to lawn, landscape, or
pasture areas.
J. "Creek" is synonymous with "stream", which is defined in
ECDC 23.40.320.
K. "Critical areas" is defined in ECDC 23.40.320.
L. "Design storm" means a rainfall event or pattern of events
for use in analyzing and designing drainage facilities.
M. "Detention" means a facility for controlling stormwater
runoff for a prescribed design storm and releasing the stormwater
at a prescribed rate.
N. "Director" means the Public Works Director or a designee
with an appropriate background in engineering or another related
discipline.
O. "Earth material" means any rock, natural soil, fill, or any
combination thereof.
P. "Ecology" means the Washington State Department of
Ecology.
Q. "Effective impervious surface" means those impervious
surfaces that are connected via sheet flow or discrete conveyance
to a drainage system including the City's MS4. Impervious
surfaces on residential development sites are considered ineffective
if 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 (2005).
R. "Erosion" means the displacement of any earth material of
existing vegetation by rainfall, stormwater runoff, or seepage.
S. "Excavation" means the removal of any earth material.
T. "Exception" means relief from the application of a
Minimum Requirement to a project.
U. "Fill" means a deposit of earth material placed by artificial
means.
V. "Groundwater" means water in a saturated zone or stratum
beneath the land surface or below a water body.
W. "Highway" means a main public road connecting towns
and cities. In Edmonds, this includes State Highway 99 and
portions of State Highway 104, both classified as Principal
Arterials in the City's Comprehensive Transportation Plan.
X. "Illicit discharge" means any direct or indirect non-
stormwater discharge to the City's MS4, groundwaters, or a water
body, except as expressly allowed by Chapter 7.200 of Edmonds
City Code.
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Y. "Impervious surface" means a hard surface area that either
prevents or retards the entry of water into the soil mantle as it
occurs under natural conditions prior to development, resulting in
stormwater runoff from the surface in greater quantities or at an
increased rate of flow compared to stormwater runoff
characteristics under natural conditions prior to development.
Common impervious surfaces include (but are not limited to)
rooftops, walkways, patios, driveways, parking lots or storage
areas, concrete or asphalt paving, gravel roads, packed earthen
materials, and oiled macadam or other surfaces that similarly
impede the natural infiltration of stormwater. Open, uncovered
retention/detention facilities shall not be considered impervious
surfaces for purposes of determining whether the thresholds for
application of minimum requirements are exceeded. However,
open, uncovered retention/detention facilities shall be considered
impervious surfaces for purposes of runoff modeling. Outdoor
swimming pools shall be considered impervious surfaces in all
situations.
Z. "Lake" means an inland body of fresh water surrounded by
land.
AA. "Land -disturbing activity" means any activity that results in
movement of earth, or a change in the existing soil cover (both
vegetative and non -vegetative) or the existing soil topography.
Land -disturbing activities include, but are not limited to clearing,
grading, filling, and excavation. Compaction that is associated with
stabilization of structures and road construction shall also be
considered a land -disturbing activity. Vegetation maintenance
practices are not considered land -disturbing activity.
BB. "Low Impact Development" means development conducted
in a way that seeks to minimize or completely prevent alterations
to the natural hydrology of the site. Low impact development
includes site planning and design to reduce alterations of natural
soil and vegetation cover, minimize impervious surfaces, and
specific practices that help to replicate natural hydrology such as
permeable pavements, green roofs, soil amendments, bioretention
systems, and dispersion of runoff.
CC. "Maintenance" means repair and maintenance 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
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structure, as long as the functioning characteristics of the original
structure are not changed.
DD. "Maximum extent feasible" means the requirement is to be
fully implemented, constrained only by the physical limitations of
the site, practical considerations of engineering design, and
reasonable considerations of financial costs and environmental
impacts.
EE. "MS4" means the City's Municipal Separate Storm Sewer
System.
FF. "Native vegetation" means vegetation comprised of plant
species (other than noxious weeds) indigenous to the coastal region
of the Pacific Northwest which could have been reasonably
expected to occur naturally 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.
GG. "Natural drainage systems and outfalls" means the location
of the channels, swales, and other non -manmade conveyance
systems as defined by the earliest documented topographic
contours existing for the subject property, either from maps or
photographs, or such other means as appropriate.
HH. "New development" means land -disturbing activities,
including Class IV general forest practices that are conversions
from timber land to other uses per RCW 76.09.050; structural
development, including construction or installation of a building or
other structure; creation of impervious surfaces; and subdivision,
short 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. If the
project is part of a common development plan or sale, the disturbed
area of the entire plan shall be used in determining permit
requirements.
II. "New Impervious Surface" means impervious surface
created after July 6, 1977 (the effective date of the City's first
drainage control ordinance) that meets the conditions described in
the City of Edmonds Stormwater Supplement (see Chapter
18.30.060).
JJ. "Person" means any individual, partnership, corporation,
association, organization, cooperative, public or municipal
corporation, agency of the state, or local government unit, however
designated.
KK. "Project site" means that portion of a property, properties,
or right of way subject to land -disturbing activities, new
impervious surfaces, or replaced impervious surfaces. If the project
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is part of a common development plan or sale, the disturbed area of
the entire plan shall be used in determining permit requirements.
LL. "Receiving waters" means waterbodies or surface water
systems to which surface runoff is discharged via a point source of
stormwater or via sheet flow.
MM. "Redevelopment" means on a site that is already
substantially developed, the creation or addition of impervious
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 impervious surface that is not part of a
routine maintenance activity; and land -disturbing activities.
NN. "Replaced impervious surface" means for structures, the
removal and replacement of any exterior impervious surfaces or
foundation. For other impervious surfaces, means the removal
down to bare soil or base course and replacement. For sites with
existing single family dwelling units, (as defined in ECDC
21.90.080) a project that solely replaces the other impervious
surfaces in -kind (footprint and imperviousness of material does not
change), shall not be considered replaced impervious surface for
the purposes of this chapter, unless the project site has 1 acre or
greater of land -disturbing activities. (Note: These surfaces may
qualify as "new impervious surface" — see definition.)
00. "Roadway" means the traveled impervious portion of any
public or private road or street.
PP. "Site" means 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, or utility projects in the right-of-
way, the length of the project site and the right-of-way boundaries
define the site.
QQ. "Slope" means the degree of slant of a surface measured as
a numerical ratio, percent, or in degrees. Expressed as a ratio, the
first number is the horizontal distance (run) and the second is the
vertical distance (rise), as 2:1. A 2:1 slope is a 50 percent slope.
Expressed in degrees, the slope is the angle from the horizontal
plane, with a 90-degree slope being vertical (maximum) and 45
degrees being a 1:1 or 100 percent slope.
RR. "Soil" means the unconsolidated mantle of the earth that
serves as a natural medium for the growth of land plants.
SS. "Source control" means 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.
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TT. "Stormwater" means runoff during and following
precipitation and snowmelt events, including surface runoff and
drainage.
UU. "Stormwater facility" means a constructed component of a
stormwater drainage system, designed and constructed to perform
a particular function, or multiple functions. Stormwater facilities
include, but are not limited to, pipes, pumping systems, swales,
ditches, culverts, street gutters, catch basins, detention basins,
wetlands, infiltration devices, and pollutant removal devices.
VV. "Stormwater site plan" The report and associated plans
containing all of the technical information and analysis necessary
for the City to evaluate a proposed new development or
redevelopment project for compliance with stormwater
requirements. Contents of the Stormwater Site Plan will vary with
the type and size of the project, and individual site characteristics.
It may include a Construction Stormwater Pollution Prevention
Plan (Construction SWPPP) and a Permanent Stormwater Control
Plan (PSC Plan).
WW. "Substantially developed" means for sites in zone district
RS (as defined Title 16 ECDC) those that have an existing single
family dwelling unit (as defined in ECDC 21.90.080). For sites
with all other zone districts, substantially developed shall mean
those sites with 35 percent or more existing impervious surface
coverage.
XX. "Threshold discharge area" means an onsite area that drains
to either 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 presented in Volume I of the Stormwater
Management Manual for Western Washington (2005) illustrate this
definition. The purpose of this definition is to clarify how the
thresholds of this code are applied to project sites with multiple
discharge points.
YY. "Variance" means the same as "Exception".
ZZ. "Water body" means a surface water feature, whether
standing or flowing, including (but not limited to) sounds, lakes,
ponds, rivers, streams, and creeks including waters of the state.
AAA. "Watershed" means a geographic region within which
water drains into a particular river, stream, or water body.
BBB. "Waters of the state" includes those waters as defined as
"waters of the United States" in 40 CFR Subpart 122.2 within the
geographic boundaries of Washington State, and "waters of the
state" as defined in Chapter 90.48 RCW which includes lakes,
rivers, ponds, streams, inland waters, underground waters, salt
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waters and all other surface waters and water courses within the
jurisdiction of the State of Washington.
CCC. "Wetlands" are as defined in ECDC 23.40.320.
18.30.020 Regulation
A. The Public Works Director or a designee shall administer
this chapter and shall be referred to as the Director. The Director
shall have the authority to develop and implement procedures to
administer and enforce this chapter.
B. The Director shall approve and enforce procedures in
accordance with state and federal regulations, and compliance with
this chapter.
18.30.030 Applicability
A. This chapter applies if any of the following pertains to a
site:
1. Issuance of a City permit is required under all other
chapters of Titles 18 or 19 ECDC.
2. A Subdivision application is submitted per ECDC
20.75.040.
3. The proposed project:
a. Involves 500 square feet or more of land -disturbing
activity, new impervious surface, or replaced impervious surface.
Routine landscape maintenance practices outside of Critical Areas
(as defined in Title 23 ECDC) and ongoing farming or gardening
activities shall be excluded unless there is the potential for such an
activity to cause an illicit discharge to the City's MS4.
b. Is a utility or other construction projects consisting
of 500 lineal feet or more of trench excavation.
c. Is located in, adjacent to, or drains into (currently or
as a result of the project) a Critical Area or a Critical Area Buffer.
B. This chapter applies to actions whenever the Director
determines there is a potential for:
1. An illicit discharge or physical damage to the City's
MS4 or downstream properties, or
2. A violation of applicable City, State, or Federal
laws, regulations, or permits related to water quality.
C. If a City Permit has been issued or a subdivision
application submitted for a site, as provided in Section
18.30.030.A, the requirements of this chapter shall be administered
under those 'permits. If the site activities triggered in Section
18.30.030.A.3 or B of this subchapter do not necessitate City-
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issued permit, the requirements of this chapter shall be
administered under a Stormwater Permit as described in section
18.30.040.B.2.
D. The requirements of this chapter are minimum
requirements. They do not replace, repeal, abrogate, supersede or
affect any other more stringent requirements, rules, regulations,
covenants, standards, or restrictions. Where this chapter imposes
requirements that are more protective of human health or the
environment than those set forth elsewhere, the provisions of this
chapter shall prevail. When this chapter imposes requirements that
are less protective of human health or the environment than those
set forth elsewhere, the provisions of the more protective
requirements shall prevail.
E. The minimum requirements that apply to a project site will
differ based on (but are not limited to) the following: project scope
and configuration, physical site characteristics, site location, and
subsurface conditions. The combination of site activities and
physical characteristics will classify the type of project as
described in ECDC 18.30.050. Stormwater Management
Requirements are described in ECDC 18.30.060.
F. Approvals and permits granted under this chapter are not
waivers of the requirements of any other laws, nor do they indicate
compliance with any other laws. Compliance is still required with
all applicable federal, state and local laws and regulations,
including rules promulgated under authority of this chapter.
G. Compliance with the provisions of this chapter does not
necessarily mitigate all impacts to the environment. Thus,
compliance with this chapter should not be construed as mitigating
all drainage water or other environmental impacts, and additional
mitigation may be required to protect the environment pursuant to
other applicable laws and regulations. The primary obligation for
compliance with this chapter and for preventing environmental
harm on or from property is placed upon the applicant.
18.30.040 Administration
A. Applications. All stormwater review submittals shall
contain, in addition to the information required under any other
applicable City code, a Stormwater Site Plan as described in the
Stormwater Supplement (see Chapter 18.30.060) and other
information as the Director shall require.
B. Review
1. The Director shall review all plans for compliance with this
chapter.
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2. The Director may administratively, without hearing,
approve and issue a Stormwater permit when required by ECDC
18.30.030 if the proposed activity complies with requirements in
this chapter.
C. Inspection. All activities regulated by this chapter shall be
inspected by the Director. The Director shall inspect projects for
approval at various stages of the work to determine that they are
being constructed per the approved Stormwater Site Plans. Stages
of work requiring inspection include (but are not limited to)
preconstruction, installation of BMPs, land -disturbing activities,
installation of utilities, landscaping, and completion of project.
When reasonably required by the Director to accomplish the
purpose of this chapter or to comply with local, state or federal law
or regulation on stormwater, a special inspection or testing shall be
performed. The drainage system shall be installed concurrently
with site development and shall be completed as shown on the
approved plan before city approval of an occupancy permit or final
inspection.
D. Fees. Application, Review and Inspection fees as set in
Chapter 15.00 ECDC shall be paid.
18.30.050 Project Classification
For purposes of this chapter, projects are classified as Large Site,
Small Site, or Minor Site as described below, primarily based in
the extent of land -disturbing activities.
A. Large Site Project. Projects are Large Site Projects if they
involve:
1. 1 acre or more of land -disturbing activity, or
2. If the project disturbs less than 1 acre of land and it
is part of a larger common plan of development or sale where land -
disturbing activity involves 1 acre or more.
B. Small Site Project. Projects are Small Site Projects if are
they are not Large Site Projects and they involve:
1. 2,000 square feet or greater of new, replaced, or
new plus replaced impervious surface area or
2. At least 7,000 square feet of land -disturbing
activity or
3. 50 cubic yards or more of either grading, filling, or
excavating as described in ECDC 18.40.000.
C. Minor Site Project. Projects are Minor Site Projects if they
involve:
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I . 500 square feet or greater of new development or
redevelopment including land -disturbing activity and utility
projects that cause land disturbance and
2. Are not a Large or Small Site Project
18.30.060 Stormwater Management Requirements
A. General
1. All activities covered by this chapter shall comply
with the site planning and best management selection and design
criteria in the Stormwater Code Supplement to Edmonds
Community Development Code (ECDC) Chapter 18.30 (Exhibit
A), herein referred to as the Stormwater Supplement, to implement
the applicable minimum technical requirements listed in this
chapter.
2. The City may allow alternative or regional
approaches to treatment, flow control, or other minimum
requirements per the Basin/Watershed provisions of the Western
Washington Phase II Municipal Stormwater Permit.
B. Illicit Discharges and Connections. Non-stormwater illicit
discharges, including spills into the City's stormwater drainage
system, are prohibited per Chapter 7.200 ECC.
C. Low Impact Development. Low Impact Development
techniques shall be employed where feasible, reasonable, and
appropriate. When low impact development techniques are
employed, the design shall be consistent with the most recent
version of the Low Impact Development, Technical Guidance for
Puget Sound (Puget Sound Action Team and Washington State
University Pierce County Extension), other Low Impact
Development standards approved by Ecology, or the Stormwater
Supplement.
D. Minimum Technical Requirements by Project
Classification. The following lists the additional requirements that
apply to the specific project classes:
1. Large Site Projects. Large Site Projects shall meet
the minimum technical requirements outlined in Section 3 and
Section 4 of Appendix 1 of the Western Washington Phase II
Municipal Stormwater Permit) and the Stormwater Supplement.
2. Small Site Projects. Small Site Projects are further
defined as Category 1 or Category 2. All new development or
redevelopment projects shall, at a minimum, comply with the
applicable requirements for Category 1 or Category 2 as found in
the Stormwater Supplement. Any or all of small site minimum
requirements (SSMRs) may be required on any Small Site Project
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by the Director to meet the purpose of this Chapter based on site
specific factors including, but not limed to, location, soil
conditions, slope, and designated use. These requirements are
summarized as follows:
a. Category 1: For Small Site Projects with less than
5,000 square feet of new, replaced, or new plus replaced
impervious surface area the following SSMRs shall apply:
i. SSMR #1 — Preparation of Stormwater Site
Plan
ii. SSMR #2 — Construction Stormwater
Pollution Prevention Plan
iii. SSMR #3 — Source Control of Pollution
iv. SSMR #4 — Preservation of Natural
Drainage Systems and Outfalls
V. SSMR #5 — Onsite Stormwater Management
vi. SSMR #7 - Flow Control - This SSMR may
be waived by the Director based on the extent of application of
SSMR #5 to infiltrate, disperse, and retain stormwater runoff on
site without causing flooding or erosion impact, per the criteria in
the Stormwater Supplement.
vii. SSMR #8 — Wetland Protection
viii. SSMR#9 — Operation and
Maintenance
ix. SSMR#11 — Financial Liability — Applies to
stormwater systems constructed in or adjacent to Critical Areas or
Critical Area buffers.
b. Category 2: For Small Site Projects that include
5,000 square feet or greater of new, replaced, or new plus replaced
impervious surface area; or convert % acre or more of native
vegetation to lawn or landscaped area; or, through a combination
of creating effective impervious surface and converted pervious
surfaces, causes a 0.1 cubic feet per second increase in the 100-
year flow frequency from a threshold discharge area as estimated
using an approved model. The following SSMRs shall apply:
i SSMR #1 - Preparation of Stormwater Site
Plan
ii. SSMR #2 — Construction Stormwater
Pollution Prevention Plan
i. SSMR #3 — Source Control of Pollution
ii. SSMR #4 — Preservation of Natural
Drainage Systems and Outfalls
iii. SSMR #5 — Onsite Stormwater Management
iv. SSMR #6 — Runoff Treatment.
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V. SSMR #7 — Flow Control
vi. SSMR #8 — Wetland Protection
vii. SSMR #9 — Operation and Maintenance
viii. SSMR #10 — Off -Site Analysis and
Mitigation
ix. SSMR #11 — Financial Liability
c. Any or all of SSMRs may be required on any Small
Site Project by the Director to meet the purpose of this Chapter
based on site specific factors including, but not limed to, location,
soil conditions, slope, and designated use.
3. Minor Site Projects The following Minimum
Requirements apply to Minor Site Projects:
a. Minor Site Construction Stormwater Pollution
Prevention Practices as described in the Stormwater Supplement.
b. Additional requirements may be imposed by the
Director or designee on Minor Project Sites to meet the purpose of
this Chapter based on site specific factors including, but not limed
to, location, soil conditions, slope, and designated use.
18.30.070 Exemptions, Exceptions, Adjustments and
Appeal
A. Exemptions. The following land uses and land -disturbing
activities are exempt from the provisions of this chapter:
1. Forest practices regulated under Title 222 WAC,
except for Class IV general forest practices that are conversions
from timber land to other uses, are exempt from the provisions of
the minimum requirements.
2. Commercial agriculture practices that involve
working land for production are generally exempt. However, land
conversion from timberland to agriculture, and the construction of
impervious surfaces are not exempt.
3. Construction of drilling sites, waste management
pits, and associated access roads, and 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.
4. Roadway Projects. The following roadway
maintenance practices or activities are exempt: pothole and square -
cut patching, overlaying existing asphalt or concrete pavement
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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, and roadside vegetation maintenance.
a. For Large Site Projects only, the following road
maintenance practices or activities are considered redevelopment,
and therefore are not categorically exempt. The extent to which
this exemption applies is explained for each circumstance.
i. Removing and replacing a paved surface to base
course or a lower level, or repairing the roadway base: If
impervious surfaces are not expanded, Large Site Project
Minimum Requirements #1 - #5 apply. However, in most cases,
only Large Site Project Minimum Requirement #2, Construction
Stormwater Pollution Prevention, shall be required. Where
appropriate, project proponents are encouraged to look for
opportunities to use permeable and porous pavements.
ii. Extending the pavement edge without increasing the
size of the road prism, or paving graveled shoulders are considered
new impervious surfaces and are subject to the minimum
requirements that are triggered when the thresholds identified for
redevelopment projects are met.
iii. Resurfacing by upgrading from dirt to gravel,
asphalt, or concrete; or 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 redevelopment projects
are met.
5. 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.
6. With respect to replaced impervious surfaces, a
redevelopment project may be exempt from compliance with
SSMR 6 (treatment), SSMR 7 (flow control), and SSMR 8
(wetlands protection) (or the associated applicable minimum
requirements for large sites) should the City adopt a plan and
schedule that fulfills those requirements through a regional
drainage control plan (e.g., via a regional facility or facilities,
stream restoration, or basin -specific development requirements).
7. City capital and maintenance projects are exempt
from the financial liability minimum requirement.
{WSS781085.DOC;1\00006.900000\ }- - 15 -
B. Exceptions
1. The Director may approve a request for an
exception to the minimum requirements of this chapter following
legal public notice of an application for an exception and of the
Director's decision on the application. All legal public notice
related to this request for an exception shall be in the manner
prescribed in Section 20.03.002 ECDC and the applicant shall pay
all costs to publish the legal public notices required by this
provision. The Director shall provide and keep a written findings
of fact of the decision.
2. The approval of the exception shall only be granted
when the applicant demonstrates that the exception will not
increase risks to public health, safety, and welfare, or to water
quality, or to public and private property in the vicinity or
downstream of the property, and the exception shall be the least
possible exception that could be granted and still provide
compliance with the intent of the minimum requirements. In
addition, the exception shall only be granted when it has been
determined by the director that one or more of the following
applies:
a. The requirement would cause a severe and
unexpected financial hardship that outweighs the requirement's
benefits, and the criteria for an adjustment cannot be met; or
b. The requirement would cause harm or a significant
threat of harm to public health, safety, and welfare, the
environment, or public and private property, and the criteria for an
adjustment cannot be met; or
c. The requirement is not technically feasible, and the
criteria for an adjustment cannot be met; or
d. An emergency situation exists that necessitates
approval of the exception.
3. An application for an exception on the grounds of
severe and unexpected financial hardship shall describe, at a
minimum, all of the following:
a. The current, pre -project use of the site; and
b. How application of the requirement(s) for which an
exception is being requested restricts the proposed use of the site
compared to the restrictions that existed prior to adoption of this
chapter; and
c. The possible remaining uses of the site if the
exception were not granted; and
d. The possible uses of the site that would have been
allowed prior to the adoption of this chapter; and
{WSS781085.DOC;1\00006.900000\ }- - 16 -
e. A comparison of the estimated amount and
percentage of value loss as a result of the requirements versus the
estimated amount and percentage of value loss as a result of
requirements that existed prior to adoption of the requirements of
this chapter; and
f. The feasibility of the applicant to alter the project to
apply the requirements of this chapter
4. An exception to the requirements shall only be
granted to the extent necessary to provide relief from the economic
hardship as determined by the Director, to alleviate the harm or
threat of harm to the degree that compliance with the requirement
becomes technically feasible, or to perform the emergency work
that the Director determines is warranted.
5. The Director may require an applicant to provide
additional information at the applicant's expense, including (but
not limited to) an engineer's report or analysis.
6. When an exception is granted, the Director may
impose new or additional requirements to offset or mitigate harm
or the threat of harm that may be caused by granting the exception,
or that would have been prevented if the exception had not been
granted.
C. Adjustments
1. The Director may approve a request for adjustments
to the requirements of this chapter when the Director finds that:
a. The adjustment provides substantially equivalent
environmental protection; and
b. The objectives of safety, function, environmental
protection, and facility maintenance are met, based on sound
engineering practices.
2. During construction, the Director may require, or
the applicant may request, that the construction of drainage control
facilities and associated project designs be adjusted if physical
conditions are discovered on the site that are inconsistent with the
assumptions on which the approval was based, including (but not
limited to) unexpected soil or water conditions, weather generated
problems, or changes in the design of the improved areas; and
3. A request by the applicant for an adjustment shall
be submitted to the Director for review and approval prior to
implementation. The request shall be in writing and shall provide
facts substantiating the requirements of subsection C.1, and if
made during construction, the factors in subsection C.2. Any such
modifications made during the construction of drainage control
{WSS781085.DOC;1\00006.900000\ }- - 17 -
facilities shall be included with the final approved drainage control
plan.
D. Appeal
1. The Director's decision on an application for an
exception or adjustment may appeal to the Hearing Examiner in
accordance with a Type II appeal process in Chapter 20.06 ECDC.
2. The applicant shall carry the burden of proof.
3. The decision of the Hearing Examiner is appealable
to Superior Court in accordance with Chapter 36.70C RCW.
18.30.080 Easements, Deeds, and Covenants
A. Easements. A Public Storm Drainage Inspection Easement
shall be required where:
1. Stormwater facilities identified on project plans will
be located on property owned by a party other than the owner of
the project site or
2. Access is needed to structural or non structural
stormwater facilities for inspection by the City to ensure that these
stormwater best management practices continue to function as
designed.
Easements shall be as specified in Engineering Division documents
or approved by the Director, and recorded with Snohomish County
and on all proper deeds.
B. Deeds and Covenants for Low Impact Development. Deed
restrictions and covenants shall be required for all sites using low
impact development techniques to ensure that these stormwater
best management practices continue to function as designed. The
deed restrictions or covenants shall address or append requirements
and responsibilities for long term management and maintenance of
these best management practices.
18.30.090 Inspection and Maintenance Roles, and
Responsibilities
Proper inspection and maintenance of stormwater facilities
(including construction BMPs) is essential for the protection of the
City's MS4 and the environment. Inspection and maintenance of
all stormwater facilities shall be required in accordance with the
Stormwater Supplement.
A. Stormwater Maintenance and Inspection Standards
Stormwater facilities shall be inspected and maintained per the
requirements of the Stormwater Supplement. For systems which
{WSS781085.DOC; 1 \00006.900000\ )- - 18 -
do not have a maintenance standard, the owner shall develop a
standard based on guidelines from the manufacturer, designer, or a
registered professional engineer and submit the standards to the
Director for approval.
B. Ownership. Stormwater facilities are either privately or
publicly owned and maintained. All stormwater facilities that
serve commercial and industrial sites are private. Storm drainage
facilities or controls that are privately owned by a homeowner's
association or similar organization also are private.
C. Maintenance and Inspection. All privately owned storm
drainage facilities or controls shall be maintained by the owner, or
the homeowner or owner association ("Owner") if one is
established as part of a residential or commercial development.
All private storm drainage facilities shall be regularly inspected to
ensure proper operation and shall monitor the facility or control as
required or as set forth in the Stormwater Supplement. The Owner
shall maintain records of inspection and maintenance, disposal
receipts, and monitoring results. The records shall catalog the
action taken, the person who took it, the date said action was taken,
how it was done, results of any monitoring effort, and any
problems encountered or follow-up actions required. The records
shall be made available to the City upon request. The Owner shall
maintain a copy of the Stormwater Operations and Maintenance
Manual (if required) on site, and shall make reference to such
document in real property records filed with Snohomish County, so
others who acquire real property served by the privately owned
storm drainage facilities or controls are notified of their obligation
to maintain such facilities or controls.
D. When an inspection identifies an exceedance of the
maintenance standard, maintenance shall be performed:
1. Within 1 year for wet pool facilities and retention/detention
facilities.
2. Within 6 months for typical maintenance.
3. Within 9 months for maintenance requiring re -vegetation,
and
4. Within 2 years for maintenance that requires capital
construction of less than $25,000.
E. Disposal of Waste from Maintenance Activities Disposal
of waste from maintenance activities shall be conducted in
accordance with the minimum Functional Standards for Solid
Waste Handling, Chapter 173-304 WAC, guidelines for disposal of
waste materials from storm water maintenance activities, and
where appropriate, the Dangerous Waste Regulations, Chapter
173-303 WAC.
{WSS781085.DOC;1\00006.900000\ }- - 19 -
F. City Inspection The regular inspection of privately owned
storm drainage facilities or controls is essential to enable the City
to evaluate the proper operation of the City's MS4 and the
environment. The City shall have access to private stormwater
facilities for inspection to ensure they are properly operated and
maintained in accordance with ECC 7.200,100. The City may offer
an incentive program to owners to encourage the proper
maintenance of private storm drainage facilities.
18.30.100 Enforcement Procedures
A. General Enforcement action shall be in accordance with
this chapter whenever a person has violated any provision of this
chapter. The choice of enforcement action is at the discretion of
the City. The severity of any penalty shall be based on the nature
of the violation, the damage, or risk to the public or to public
resources, or the degree of bad faith of the person subject to the
enforcement action.
B. Notice of Violation and Order to Correct. The Director
shall have the authority to serve a person a Notice of Violation and
Order to Correct (NOV/OTC) if an action is being undertaken in
violation of this chapter. Issuance of any other warning, notice, or
order is not a condition precedent for the Director to issue an
NOV/OTC.
1. Content of NOV/OTC — the NOV/OTC shall
contain:
a. A description of the specific nature, extent, and time
of violation and the damage or potential damage;
b. A notice that the violation or the potential violation
cease and desist, and, in appropriate cases, the specific corrective
action to be taken within a given time;
c. A civil penalty under ECDC 18.30.090(C) below
may be issued with the order.
d. Appeal rights.
2. Notice. An NOV/OTC may be imposed by a notice
in writing, either by certified mail with return receipt requested, or
by personal service, to the person(s) shown on the rolls of the
Snohomish County assessor as the owner of the site, noted as the
applicant on any application for development approval or observed
doing regulated activity on the site.
3. Effective date. The stop work order issued under
this section shall become effective immediately upon receipt by the
person to whom the order is directed.
4. Compliance. Failure to comply with the terms of an
NOV/OTC shall result in additional enforcement actions including
{WSS781085.DOC;1\00006.900000\ }- - 20 -
(but not limited to) criminal prosecution, the issuance of additional
civil penalty and abatement.
C. Maintenance Orders. The Director shall have the authority
to issue to an owner or person an order to maintain or repair a
component of a Sormwater facility or BMP to bring it into
compliance with this chapter, the Stormwater Supplement, and the
Edmonds Community Development Code. The order shall include:
1. A description of the specific nature, extent and time
of the violation and the damage or potential damage that
reasonably might occur;
2. A notice that the violation or the potential violation
cease and desist and, in appropriate cases, the specific corrective
actions to be taken; and
3. A reasonable time to comply, depending on the
circumstances.
D. Civil Penalty. A person who fails to comply with the
requirements of this chapter, who fails to conform to an approval
or order issued, who undertakes new development without first
obtaining }approval, or who fails to comply with a stop work issued
under these regulations shall be subject to a civil penalty levied in
accordance with the provisions of Chapter 20.110 ECDC,
provided, however, that the appeal process shall commence with a
notice of violation as provided in ECDC 20.110.040(B).
1. Civil penalties for code violations shall be imposed
in accordance with the provisions of Chapter 20.110 ECDC,
provided, however, that in addition to the penalties set forth in that
chapter, the hearing examiner is authorized to levy a penalty of up
to Twenty Thousand Dollars ($20,000) per occurrence based upon
an assessment of the following factors. Where such factors are
present, the hearing examiner is authorized to levy such penalty
after taking into consideration the full impact of the violation and
any mitigating circumstances (see 2 below):
a. The violation created a risk to public health and the
significance of the risk.
b. The violation damaged the environment and the
significance of the damage.
c. The violation caused damage to public and private
property and the significance of such damage.
d. A history of similar violations, if any.
e. The economic benefit of the violations, if any, to
the person or entity responsible for the violations.
{WSS781085.DOC;1\00006.900000\ )- - 21 -
2. Mitigating circumstances which may be used to
offset or reduce the time resulting from the application of the
preceding factors are limited to:
a. Full compliance with a Voluntary Compliance
Agreement and no history of similar violations.
b. Full compliance with a Voluntary Compliance
Agreement and a history of one or two similar violations (lesser
reduction).
C. A "Voluntary Compliance Agreement" is defined as
a legally binding agreement entered into between the City and the
alleged violators, by which the violator(s) acknowledge the
existence of the violation, waive all appeal rights, and agree to and
does pay a fine in an amount stipulated to between the violator and
the City.
3. If the violation(s) are not corrected as ordered, or a
Voluntary Compliance Agreement is not entered into within that
time period and no appeal is filed for the next 15-day period shall
be 150 percent of the initial penalties, and the penalties for the next
15-day period shall be 200 percent of the initial penalties. The
intent of this subsection is to increase penalties beyond the
maximum penalties stated as an additional means to achieve timely
compliance.
4. Unless otherwise provided in a Voluntary
Compliance Agreement, civil penalties shall be paid within 30
days of service of the notice and order or stop work order if not
appealed. Payment of the civil penalties assessed under this chapter
does not relieve a person found to be responsible for a code
violation of his or her duty to correct the violation or to pay any
and all civil penalties or other cost assessments issued pursuant to
this chapter.
5. The city may suspend immediate payment of civil
penalties if the person responsible for a code violation has entered
into a Voluntary Compliance Agreement. Penalties shall begin to
accrue again pursuant to the terms of the Voluntary Compliance
Agreement if any necessary permits applied for are denied,
canceled or not pursued, if corrective action identified in the
voluntary compliance agreement is not completed as specified, or
if the property is allowed to return to a condition similar to that
condition which gave rise to the Voluntary Compliance
Agreement, provided, however, that additional penalties shall not
be imposed until additional notice and opportunity for hearing
have been provided in accordance with Chapter 20.110 ECDC.
6. Civil penalties assessed create a joint and several
personal obligations in all persons responsible for a code violation.
{WSS781085.DOC;1\00006.900000\ }- - 22 -
E. The determination of the hearing examiner issued in
accordance with Chapter 20.110 ECDC shall be appealable to the
Snohomish County Superior Court in accordance with the
provisions of Chapter 36.70C RCW.
F. The remedies provided for in this section shall not be
exclusive. The city may also use other civil and administrative
remedies available to it, including but not limited to, the remedies
provided in ECDC Title 19 and the State Building and Dangerous
Building Code.
Section 2. The amendments set forth in Section 1 shall be effective June 1, 2010,
provided, however, that the existing Chapter 18.30 shall remain in full force and effect with
respect to any permit application filed prior to June 1st and which is vested in accordance with
Washington state law and which under Washington law must continue to be processed in
accordance with the provisions of the existing Chapter.
Section 3. Effective Date. This ordinance, being an exercise of a power specifi-
cally delegated to the City legislative body, is not subject to referendum, and shall take effect
five (5) days after passage and publication of an approved summary thereof consisting of the
title.
APPROV D:
M O G RY IIAAKENSON
ATTEST/AUTHENTICATED:
CITY CLERK, SANDRA S. CHASE
APPROVED AS TO FORM:
OFFICE OF CJAY ATT RNEY:
BY
W. SCOTT SNYDER
{WSS781085.DOC;1\00006.900000\ 1- - 23 -
FILED WITH THE CITY CLERK: 04-16-2010
PASSED BY THE CITY COUNCIL: 04-20-2010
PUBLISHED: 04-26-2010
EFFECTIVE DATE: 05-01-2010
ORDINANCE NO. 3792
{WSS781085.DOC;1\00006.900000\ }- - 24 -
SUMMARY OF ORDINANCE NO.3792
of the City of Edmonds, Washington
On the 20th day of April, 2010, the City Council of the City of Edmonds, passed
Ordinance No. 3792. A summary of the content of said ordinance, consisting of the title,
provides as follows:
AN ORDINANCE OF THE CITY OF EDMONDS, WASHINGTON, REPEALING AND
REENACTING CHAPTER 18.30 STORM WATER MANAGEMENT OF THE EDMONDS
COMMUNITY DEVELOPMENT CODE, PROVIDING FOR THE APPLICATION OF
EXISTING CHAPTER 18.30 TO VESTED PERMITS, AND FIXING A TIME WHEN THE
SAME SHALL BECOME EFFECTIVE.
The full text of this Ordinance will be mailed upon request.
DATED this 21 st day of April, 2010.
CITY CLERK, SANDRA S. CHASE
(WSS781085.DOC;1\00006.900000\ )- - 25 -
STORMWATER CODE SUPPLEMENT TO EDMONDS
COMMUNITY DEVELOPMENT CODE CHAPTER 18.30
V D-Allf
April 20, 2010
1.0 Introduction................................................................................................................................1
2.0 Classifying Projects..................................................................................................................3
2.1 Project Classifications.....................................................................................................3
2.2 Determining New Impervious Surface Area...................................................................3
2.3 Determining Replaced Impervious Surface Area............................................................3
2.3 Determining Project Basin Type.....................................................................................4
3.0 Low Impact Development........................................................................................................7
3.1 LID and this Stormwater Supplement.............................................................................7
4.0 Large Site Requirements...........................................................................................................9
4.1.
Minimum Requirement #1 — Preparation of Stormwater Site Plan.................................9
4.2
Minimum Requirement #2 — Construction Stormwater Pollution Prevention
Plan................................................................................................................................10
4.3
Minimum Requirement #3 — Source Control of Pollution............................................10
4.4
Minimum Requirement #4 — Preservation of Natural Drainage Systems and
Outfalls..........................................................................................................................11
4.5
Minimum Requirement #5 — Ofsite Stormwater Management....................................11
4.5.1 BMP Selection...............................................................................................12
4.5.2 Onsite/LID Techniques and Infiltration..........................................................12
4.6
Minimum Requirement #6 — Runoff Treatment............................................................14
4.6.1 Project Thresholds..........................................................................................14
4.6.2 Runoff Treatment Standards...........................................................................15
4.6.3 Runoff Treatment BMP Selection..................................................................15
4.7
Minimum Requirement #7 — Flow Control...................................................................17
4.7.1 Project Thresholds and Applicability.............................................................17
4.7.2 Flow Control Standards..................................................................................18
4.7.3 Flow Control Facility Sizing..........................................................................22
4.7.4 Flow Control BMP Selection..........................................................................23
4.8
Minimum Requirement #8 — Wetland Protection.........................................................24
4.9
Minimum Requirement #9 — Operation and Maintenance............................................24
4.10
Minimum Requirement #10 — Offsite Analysis and Mitigation....................................24
4.10.1 Qualitative Analysis........................................................................................25
4.10.2 Quantitative Analysis......................................................................................25
4.11
Minimum Requirement #I I —Financial Liability.........................................................26
5.0 Small Site Requirements.........................................................................................................29
5.1 Small Site Minimum Requirement # 1 —Preparation of Stormwater Site Plan .............30
5.2 Small Site Minimum Requirement #2 — Construction Stormwater Pollution
PreventionPlan..............................................................................................................31
5.3 Small Site Minimum Requirement #3 — Source Control of Pollution ..........................31
::7TABLEOFCONTENTS
APPJL2010
5.4 Small Site Minimum Requirement #4 — Preservation of Natural Drainage
Systemsand Outfalls.....................................................................................................32
5.5
Small Site Minimum Requirement #5 — Onsite Stormwater Management...................32
5.5.1 Onsite/LID BMP Selection.............................................................................32
5.5.2 Onsite/LID Techniques and Infiltration..........................................................34
5.5.3 Bioretention Cells or Rain Gardens for Single Family Residential
Sites................................................................................................................35
5.6
Small Site Minimum Requirement #6 — Runoff Treatment..........................................35
5.6.1 Project Thresholds..........................................................................................35
5.6.2 Runoff Treatment Standards...........................................................................36
5.6.3 Runoff Treatment BMP Selection..................................................................36
5.7
Small Site Minimum Requirement #7 — Flow Control.................................................37
5.7.1 Project Thresholds and Applicability.............................................................37
5.7.2 Flow Control Standards..................................................................................38
5.7.3 Flow Control Facility Sizing..........................................................................41
5.7.4 Flow Control BMP Selection..........................................................................42
5.8
Small Site Minimum Requirement #8 — Wetland Protection........................................43
5.9
Small Site Minimum Requirement #9 — Operation and Maintenance ..........................43
5.10
Small Site Minimum Requirement # 10 — Offsite Analysis and Mitigation ..................43
5.10.1 Qualitative Analysis........................................................................................44
5.10.2 Quantitative Analysis......................................................................................44
5.11
Small Site Minimum Requirement # 11 — Financial Liability.......................................44
6.0 Minor Site Requirements........................................................................................................47
6.1 Minor Site Minimum Requirement — Construction Stormwater Pollution
PreventionPlan..............................................................................................................47
7.0 Operation and Maintenance Requirements.............................................................................49
8.0 Terminology...........................................................................................................................51
9.0 References...............................................................................................................................55
Appendix A — Synopsis of the Western Washington Phase I1 Municipal Stormwater
Permit......................................................................................................................................57
Appendix B — Supplemental Technical Information.....................................................................59
EdmondsWatersheds..............................................................................................................59
EdmondsSoils and Slopes......................................................................................................59
Appendix C — Approved Methods for Obtaining Design Infiltration Rates..................................67
USDA Textural Classification................................................................................................67
Modified Pilot Infiltration Testing (PIT) Method..................................................................68
InfiltrationTest..............................................................................................................69
DataAnalysis................................................................................................................70
ApplyCorrection Factor...............................................................................................70
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TABLEOFCONTENTS H
aPFZa.2mo
Table 4-1: Site Planning and Design Techniques to Reduce the Amount of
Stormwater Runoff Generated...............................................................................10
Table 4-2:
Commonly Used Onsite/LID Stormwater Management Techniques ....................13
Table 4-3:
Runoff Treatment Best Management Practices.....................................................16
Table 4-4:
City of Edmonds flow control requirements for Large Site Projects
Triggering Phase I1 Permit Requirements..............................................................18
Table 4-5:
City of Edmonds Flow Control Requirements for Large Site Projects
Creekor Lake Basinsa............................................................................................21
Table 4-6:
City of Edmonds Flow Control Requirements for All Other Large Site
Projects Direct Discharge Basins..........................................................................22
Table 4-7:
Flow Control Best Management Practices.............................................................24
Table 5-1:
Site Planning and Design Techniques to Reduce the Amount of
Stormwater Runoff Generated...............................................................................31
Table 5-2:
Commonly Used Onsite/LID Stormwater Management Techniques ....................33
Table 5-3:
City of Edmonds Flow Control Requirements for Small Site Projects —
Creekor Lake Basins a...........................................................................................40
Table 5-4:
City of Edmonds Flow Control Requirements for Small Site Projects —
Direct Discharge Basins a.......................................................................................41
Table 5-5:
Flow Control Best Management Practices.............................................................43
Table C-1:
Recommended Infiltration Rates Based on USDA Soil Textural
Classification..........................................................................................................67
Table C-2:
Correction Factors to be Used With In -Situ Infiltration Measurements to
Estimate Long -Term Design Infiltration Rates......................................................70
di agnatd0l?!it7zd,„,,,,,tsst,,,,n,n./,,.,;u,tx,ocna„L/nr„A.,,,: TABLE OFCONTENTS W
APRIL2010 EDM ONDS STORNM/ATER CODE SU DRfMEM
List of Figures
Figure 2-1: Project Classification........................................................................................5
Figure 2-2: Determining What Qualifies as "New Impervious Surface" for a
Project...............................................................................................................6
Figure 4-1: Flow Chart for Determining Minimum Requirements for New
Development on Large Site Projects..............................................................27
Figure 4-2: Flow Chart for Determining Minimum Requirements for
Redevelopment on Large Site Projects..........................................................28
Figure B-1: City of Edmonds Watersheds.........................................................................61
Figure B-2: City of Edmonds Soil Map Units...................................................................63
Figure B-3: City of Edmonds Slopes Greater Than 15% and North Edmonds Earth
Subsidence & Landslide Hazard Area............................................................65
Figure C-1: Soil Classification Based on the Textural Triangle........................................68
r!j,rt.ltso.t,>tu,,d,,;,,ous+t:,;,:;:a:r.F;,;;;,a,,.,,td;-at-io;: T'ABLEOFCONTENTS IV
aPRII-2mo
1.0 Introduction
This Stormwater Code Supplement (Supplement) provides direction for implementing the City of
Edmonds Community Development Code (ECDC) Chapter 18.30, Stormwater Management. Review
ECDC Chapter 18.30.030 to determine if the City's Stormwater Management code and this Supplement
apply to your project.
The contents of this Supplement come from several primary sources, including:
• Stormwater Management Manual for Western Washington (Ecology 2005) (Stormwater
Manual) and manuals deemed equivalent to it by the Washington State Department of
Ecology (Ecology).
• Western Washington Phase II Municipal Stormwater Permit (Ecology 2009) (Phase II
Permit). See Appendix A for more information.
• Low Impact Development, Technical Guidance Manual for Puget Sound (Puget Sound
Action Team and WSU Pierce County Extension 2005) (LID Manual).
• Basis for Updated Stormwater Management Standards for New Development and
Redevelopment Projects in Edmonds (Herrera Environmental Consultants 2009).
Where this Supplement conflicts with the Stormwater Manual, this Supplement shall apply. Where
provisions of this Supplement conflict with other City of Edmonds Code requirements or state and federal
requirements, the document with the more stringent provisions will apply.
The rest of this Supplement is organized as follows:
• Chapter 2 - Project Classifications: How to determine the stormwater requirements that
apply to your site.
• Chapter 3 — Low Impact Development (LID): Introduction to LID stormwater
management practices and where they are found in this Supplement
• Chapter 4 — Large Site Requirements: Direction on the applicable minimum requirements
and best management practice (BMP) selection and design criteria for Large Site
Projects, including low impact development (LID) techniques and specific runoff
treatment and flow control measures.
• Chapter 5 — Small Site Requirements: Direction on the applicable minimum
requirements, and BMP selection/design criteria for Category 1 and Category 2 Small
Site Projects, including the use of LID techniques and specific runoff treatment and
flow control measures.
• Chapter 6 — Minor Site Requirements: Direction on the applicable minimum
requirements and BMP selection/design criteria for Minor Site Projects
• Chapter 7 — Operations and Maintenance: Operations and maintenance (O&M) direction
for the BMPs referenced in this Supplement.
di I.RV4!=f0-e70&1'-;,1,rtistw.nrva;eia;%uFten+rnUina;.Ilo,; 1.0INTRODUCTION 1
APRIL2OlO
• Chapter 8 — Terminology: Definitions of key terms used in this Supplement.
• Chapter 9 — References.
The Appendices provide additional background information on stormwater management in the City.
• Appendix A summarizes provisions of the Western Washington Phase II Municipal
Stormwater Permit (Phase II Permit) that affect City stormwater management
requirements.
• Appendix B provides a map of Edmonds' watersheds, areas with steep slopes, and soils
information.
• Appendix C describes the two approved methods for obtaining design infiltration rates.
Throughout the remainder of this document, words in the definitions section of ECDC Chapter 18.30 and
in Chapter 8 of this Supplement are in bold italics when first used.
-' r„t-ctr,nt„r,rr.,,,<,.._,. s„nlf:-,,...,t/>,,,t.a:: 7.0INTRODUCTION 2
APRIL2010 Q]MONDSSTOFU T TERCOCESUPPLENIEM
2.0 Classifying Projects
This chapter helps you classify a project per ECDC Chapter
18.30.050. It also helps you classify impervious surfaces as Terms in bold italics are defined I
existing, new, or replaced, which is necessary for determining in the Terminology section of
which minimum requirements apply to a project.
this document or in ECDC
Chapter 18.30.
2.1 Project Classifications
New development, redevelopment, and construction projects are classified as Large Site Projects (see
Chapter 4), Small Site Projects (see Chapter 5), or Minor Site Projects (see Chapter 6). Small Site
Projects are further divided into Category 1 and Category 2. See Figure 2-1 (located at the end of this
chapter) and the text below to determine the classification (and the category, if a Small Site Project) of
your project. The classification is largely based on the extent to which a project has the following:
• Land -disturbing activity (including clearing, grading, and excavating)
• New impervious surface and replaced impervious surface area — Figure 2-2 and
Section 2.2 provide direction on determining whether any or all of the impervious
surface area on a site is considered new impervious area. Section 2.3 provides direction
on determining the extent of replaced impervious surface area (if any).
• Conversion of native vegetation to lawn or landscaped area.
2.2 Determining New Impervious Surface Area
Determining a project's classification and applicable minimum requirements is based in part on the area
of new impervious surface generated. "New impervious surfaces" are those impervious surfaces
(currently pervious) that are proposed for the project, but also include existing impervious surface on the
site if the following condition applies: the existing impervious surface was created after July 6, 1977 (the
effective date of the City's first drainage control ordinance) without stormwater controls (with noted
exceptions).
Figure 2-2 presents a process for determining what portions of a project site are considered to be new
impervious surface for the purposes of stormwater management.
2.3 Determining Replaced Impervious Surface Area
Determining a project's classification and applicable minimum requirements is based in part on the area
of replaced impervious surface. "Replaced impervious surfaces" are exterior impervious surfaces
proposed for the project where existing surfaces are also impervious. More specifically, "replaced
impervious surface" means:
di C,tU.4.t-9UU!iti�rMnar�d<sm,.nr�eaysu;.rpremrrrt firr.;:.eur. 2.0 CLASSIFYING PROJECTS 3
"For structures, the removal and replacement of any exterior impervious surfaces or foundation.
For other impervious surfaces, the removal down to bare soil or base course and replacement. For
sites with existing single family dwelling units, (as defined in Chapter 21.90.080 ECDC) a project
that solely replaces impervious surfaces in -kind (footprint and imperviousness of material does
not change, but not a structural foundation), shall not be considered replaced impervious surface
for the purposes of this chapter, unless the project site has I acre or greater of land -disturbing
activities."
Therefore, in -kind replacement of exterior impervious surfaces alone for Small Site Projects and Minor
Site Projects (that are not part of a larger scale site redevelopment) are not considered to be replaced
impervious surfaces. In these cases, the impervious surfaces are classified as existing impervious surfaces.
These existing surfaces, however, may meet the definition of "new impervious surface" if they were
constructed after July 6, 1977, as described in Section 2.2.
2.3 Determining Project Basin Type
Projects are also classified by the drainage basin type: "Creek or Lake Basin" or "Direct Discharge
Basin" sites. These categories are defined below.
1. Creek or Lake Basin sites: Those that eventually discharge into a surface water body
such as a creek, wetland or pond, prior to discharging into Puget Sound; or that discharge
into a storm drain or surface water body such as a creek prior to discharging into Lake
Ballinger.
2. Direct Discharge Basin sites: Those that discharge runoff directly to Puget Sound via a
pipe system, ditch, or other direct means without first entering a creek or other water
body. Sites located in Direct Discharge Basins may have a different standard for flow
control or may be exempt from flow control (See Sections 4.7 and 5.7).
A map of watersheds in Edmonds is presented as Figure B-1 in Appendix B to this Supplement. Direct
Discharge Basins are those labeled "Puget Sound," Puget Sound Piped," and "Edmonds Way." All other
basins are considered as Creek or Lake Basins. Approximately 83 percent of the land area of the City is
within a Creek or Lake Basin. An applicant with site -specific information that is contrary to the basin
designations shown in Figure B-1 can present this information to the Public Works Director or designee
for a possible change in basin designation. The Public Works Director or designee will make a
determination on any requests for a site -specific change in basin designation.
r f ?_lJ CLASSIFYING F'Vi01ECTS
a-=
Start'
Figure 2-1: Project Classification
Does the project involve 1- Yes
acre or more of land -
disturbing activity?
No
Yes
Is the project part of a larger
common plan of development
or sale where the total
disturbed area for the entire
plan will total 1-acre or more
of land -disturbing activity?
No
Does the Project involve one or more of the Yes
following:
2,000 square feet or more of new impervious
surface, replaced impervious surface or new
plus replaced impervious surface?
OR
7,000 square feet of land -disturbing activity?
OR
50 cubic yards or more of either grading, fill,
or excavation as defined in Chapter 18.40.000
ECDC?
No
Minor Site Project:
Read Chapter 3 and see
Chapter 6 for requirements
Large Site Project:
Read Chapter 3 and see
Chapter 4 for
requirements
Does the project create or add 5,000 square feet of
new impervious surface, replaced impervious surface
or new plus replaced impervious surface?
OR
Convert % acre or more of native vegetation to lawn
or landscaped area
OR
Through a combination of creating effective
impervious surface and converted pervious
surfaces, causes an increase of 0.1 cubic feet per
second in the 100-year flow frequency from a
threshold discharge area as estimated using an
approved model?
No I Yes
Category 1 Small Site Project:
Read Chapter 3 and see Chapter 5
for requirements
1 Assumes the project in question meets applicability requirements of ECDC18.30.030.
Category 2 Small Site Project:
Read Chapter 3 and see Chapter
5 for requirements
di 2.0 CLASSIFYING PROJECTS 5
Figure 2-2: Determining What Qualifies as "New Impervious Surface" for a Project
START
Only the
Does the site have existing
impervious
impervious surfaces that
Is this existing
sur ace
were created after July 6,
Yes impervious surface
proposed with
1977'?
currently connected
the project is
considered
No
to a City -approved
New
stormwater facility?Z
Impervious
Yes
Surface3.
Was a waiver granted
for a stormwater
No
Yes
facility for this
existing impervious
area?
No
Is the site zoned as single
Yes
family residential (Zone RS)?
No
No
Existing impervious surface
Is the site
within a Creek or
created since July 6, 1977
Lake drainage
basin? Yes
plus proposed impervious
surface is considered New
Impervious Surface.
' This is the effective date of the City's first drainage control ordinance. The intent of this cumulative impacts mitigation
requirement is to adequately mitigate for impervious surfaces on project sites that are submitted under separate permits. The
separate submittals could have project areas that do not meet thresholds, but would meet thresholds if the projects are
combined as one project.
For parcels that were annexed to the City after this date, the date of annexation shall substitute for the effective date of the
City's first drainage control ordinance. For annexed parcels, a functioning Snohomish County -approved stormwater
management facility can substitute for a City -approved facility.
2 For the purpose of this flowchart, it is assumed that all existing impervious surface will remain after the proposed project is
complete. If any existing impervious surface will be demolished for this project, the project may contain a combination of new
and replaced impervious surface. Consult the City's Engineering Division for direction.
s If there is an existing stormwater management system on site, contact the City's Engineering Division to discuss whether the
existing system has the capacity for the new impervious surface area.
4 See Figure 13-1 and Section 2.3.
_y' e.4.0.1t90t)tltlar„;r„;r.<s!am;a;uur5/ug.�$.n„)n:a2u.d+u- 2.0MASSIFYINGPROJECTS 6
APRIL201O EDMONDSSTCR WTERCODESUPPI.EMENT
3.0 Low Impact Development
Low impact development (LID) is a relatively new approach to stormwater management. Formulation of
LID principles began in Prince George's County, Maryland in the late- 1980s to address the growing
economic and environmental limitations of conventional stormwater management practices. For the
purposes of this document, LID is defined as:
"Stormwater management and land development strategies applied at the parcel and subdivision
scale that emphasizes the conservation and use of onsite natural features integrated with
engineered, small scale hydrologic controls to more closely mimic the predevelopment
hydrologic functions" (PSAT and WSU, 2005).
The goal of LID is to mimic a site's predevelopment hydrology by using design techniques that infiltrate,
filter, store, evaporate, and detain runoff close to its source in small, decentralized facilities. Instead of
managing and treating stormwater in large, costly, end -of -pipe facilities at the bottom of drainage areas,
LID seeks to manage stormwater using small, cost-effective landscape features at the lot level. Well -
designed and appropriately sited LID measures can also improve the habitat and aesthetics of a developed
site.
3.1 LID and this Stormwater Supplement
This Supplement allows the use of LID techniques or BMPs on Large Site Projects, when feasible and as
required by the Phase II Permit. The Permit requires that the City identify and summarize barriers to the
use of LID techniques. To accomplish this, the Supplement requires that Large Site Projects consider the
use of LID techniques and document the decision -making processes used to select their application. More
information on this requirement can be found in Sections 4.5 and 4.7. While small sites are not required to
consider or use LID techniques, the use of LID is allowed by the City, where feasible.
LID BMPs are found throughout this Supplement. These BMPs emphasize minimizing the volume and
rate of stormwater runoff from a site both during and after construction. Use caution when using some
LID BMPs: site conditions, such as the presence of "hardpan " or till soils, steep slopes, and proximity of
adjacent proprieties may preclude the use of LID BMPs due the possibility of causing flooding or erosion
impacts to nearby properties.
Minimizing the stormwater runoff that leaves the site during the construction phase of a project is
discussed in Minimum Requirement #2 (Construction Stormwater Pollution Prevention Plan) for Large,
Small, and Minor Site Projects (Sections 4.2, 5.2, or 6.1). Minimizing stormwater runoff that leaves a site
after construction with a permanent stormwater control plan is accomplished in two main ways, in the
following order:
1. Minimize the amount of rainwater that becomes stormwater runoff through site
design practices that use LID planning methods, including those that minimize
.J 3.0 LOW IMPACT DEVELOPMENT 7
impervious surface area using Minimum Requirement #1, Stormwater Site Plan (Sections
4.1 or 5.1).
2. Minimize or eliminate runoff going off site with LID techniques, if site conditions
allow by implenting Minimum Requirement #5, Onsite Stormwater Management
(Sections 4.5 or 5.5).
LID methods that are allowed and encouraged in Edmonds include:
• Retaining or restoring native forest cover, other trees, and site vegetation to capture,
infiltrate, and evaporate all or part of the precipitation falling on the site.
• Developing a site with the smallest impervious footprint possible and minimize land -
disturbing activities such as clearing and grading.
• Preserving or restoring the health and water -holding capacity of the soils by compost -
amending.
Other LID methods that are allowed but that should only be implemented if site conditions allow are:
• Designing with runoff reduction methods such as vegetated roofs, rainwater harvesting
and permeable pavement.
• Managing stormwater runoff using infiltration, bioretention, and dispersion to the extent
practicable.
Once these LID practices have been explored for applicability, stormwater runoff that cannot be managed
onsite and flows offsite must meet applicable flow control and water quality requirements: Minimum
Requirement #6, Runoff treatment and Minimum Requirement #7, Flow Control (Sections 4.6 and 4.7 or
5.6 and 5.7).
Alf 0.1140-000e r....fs tnn,,,,.,,,.. s<;n r,:m,..+t;im,,.do;; 3.0 LOW IMPACT DEVELOPMENT 8
.1 1 �� • I•. • •� >
4.0 Large Site Requirements
This chapter explains how to comply with the Large Site Project minimum requirements (ECDC
Chapter 18.30.060A-D1). For all large sites, the Thresholds, Definitions, Minimum Requirements and
Exceptions, Adjustment and Variance Criteria found in Appendix I of the NPDES Phase II Municipal
Stormwater Permit are applicable per Chapter 18.30 ECDC, as well as the mandatory incorporated
provisions of the Stormwater Manual.
Figures 4-1 and 4-2 (located at the end of this chapter) will help you determine whether a Large Site
Project is considered new development or redevelopment, which minimum requirements apply, and to
which site surfaces (new impervious, replaced impervious, land disturbed, and/or converted pervious) the
requirements apply. The minimum requirements for Large Site Projects are described below. Appendix B
contains supplemental technical information on watershed boundaries and the location of steep slopes and
soil types to assist with a stormwater site plan.
4.1. Minimum Requirement #1 - Preparation of Stormwater Site Plan
Stormwater Site Plans shall be prepared for all Large Site Projects in accordance with Volume 1,
Chapter 3 of the Stormwater Manual. The Stormwater Site Plan will document compliance with all
applicable minimum requirements and the design of all best management practices (BMPs). For all
Large Site Projects that involve 5,000 square feet or more of new or replaced impervious surface,
stormwater best management practices must be designed by a civil engineer. See handouts prepared by
the City's Engineering Division for specific submittal requirements.
The Stormwater Site Plan shall include the submittal requirements in the handout, reflect consideration of
site planning and design measures intended to reduce project impact on stormwater quality and quantity,
and must consider the use of LID approaches.
The use of LID should also include approaches that minimize the effective impervious surface area (area
directly connected to the City's drainage system or surface waters) therefore reducing the amount of
impervious surface area that requires mitigation using stormwater management BMPs. By reducing the
amount of stormwater runoff generated, LID site planning and design techniques can also be used to
satisfy the flow control requirements of Minimum Requirement #7. See Section 4.7.2 for criteria.
Simplified design tools are available to encourage the use of LID and allow easy evaluation of LID flow
control benefits (see Section 4.7.3.2).
Table 4-1 summarizes some potential LID site planning measures.
Large Site Minimum Requirement #5 (Section 4.5) discusses LID techniques for managing stormwater
runoff on site to reduce or eliminate the amount of runoff that flows offsite to receiving waters.
(j Aq Vr 001,41,x. ,,—,-zennrv.,.
4.0 LARGE SITE REQUIREMENTS 9
Table 4-1: Site Planning and Design Techniques to Reduce the Amount of Stormwater Runoff Generated
BMP #
Stormwater Management Technique
Reference(s)/Design Guidance
T5.21
Better Site Design (e.g., ,Reduce Effective
Volume V Stormwater Manual; Chapter 3
Impervious Areas Associated with Roads, Shared
LID Manual
Accesses, Alleys, Sidewalks, Driveways, and
Parkin Areas
T520
Native Vegetation Protection, Reforestation, and
Volume V Stormwater Manual; Chapter 4
Maintenance
LID Manual
--
Minimize disturbance area
Volume V Stormwater Manual; Section 5
LID Manual
-
Vegetated Roofs
City Building Division
--
Rainwater Harvesting
LID Manual/City Building Divison
Permeable Pavement (asphalt, concrete; paving
Sectioni6.3 LID Manual or Volume 3 Seattle
blocks, "grass -Crete")
Manual 2009 (base course requirements)
Stormwater Manual = Stormwater Management Manual for Western Washington (Ecology 2005)
LID Manual = Low Impact Development: Technical Guidance Manual for Puget Sound.
Volume 3 Seattle Manual = Stormwater Flow Control & Water Quality Treatment Technical Requirements Manual.
4.2 Minimum Requirement #2 - Construction Stormwater Pollution
Prevention Plan
All new development and redevelopment projects are responsible for preventing erosion and discharge of
sediment and other pollutants into receiving waters during construction. See Section 4.2, Appendix 1 of
the Phase II Permit and Volume II of the Stormwater Manual for a complete description of minimum
requirement and direction on BMP selection and design. Compliance with this minimum requirement
may be achieved for an individual site if the site is covered under Ecology's General NPDES Permit for
Stormwater Discharges Associated with Construction Activities, if the provisions of such permit are fully
implemented, and such a permit is approved by the Public Works Director or designee. The City does not
approve erosivity waivers, as described in Section 4.2, Appendix 1 of the Phase II Permit.
4.3 Minimum Requirement #3 - Source Control of Pollution
All known, available, and reasonable source control BMPs shall be applied to control pollution. Source
control BMPs shall be selected, designed, and maintained according to the Stormwater Manual. See
Volume IV of the Stormwater Manual for guidance on the applicable source control BMP selection and
implementation.
Specific source controls are not required for single family residential sites. General requirements for these
sites include preventing the discharge of pollutants to the City's storm drainage system per Edmonds City
Code Chapter 7.200 (Illicit Discharges). This includes common household items such as pesticides,
herbicides, fertilizers, detergents and fluids from vehicle maintenance.
4.0 LARGE SITE REQUIREMENTS 10
1 1 IMM19.1 .7
4.4 Minimum Requirement #4 - Preservation of Natural Drainage
Systems and Outfalls
Natural drainage patterns shall be maintained. Run-on (from a potential upstream site) and runoff
discharges from the project site shall occur at the location of the natural drainage system and outfall, to
the maximum extent practicable. In most cases, the "natural location" is the existing discharge location on
the site. For some redevelopment projects, the natural location for runoff discharge may have been
previously altered. Contact City Engineering staff with questions about the natural discharge location.
The manner by which runoff is discharged from the project site must not cause an adverse impact to
downstream receiving waters and downgradient properties. All outfalls to creeks, ditches, or other open
channels require energy dissipation to prevent erosion. Minimum Requirement #10 describes offsite
analysis that is required to identify potential downstream concerns so that adverse impacts are avoided
through appropriate onsite design.
See Section 2.5.4 of Volume I of the Stormwater Manual for guidance on complying with this minimum
requirement. The supplemental requirement in 2.5.4 of Volume I of the Stormwater Manual apply to
Large Site Projects.
4.5 Minimum Requirement #5 - Onsite Stormwater Management
Onsite stormwater management BMPs described in this section are considered LID techniques. These
LID BMPs must be used to infiltrate, disperse, and retain stormwater runoff on site to the maximum
extent feasible without causing flooding or erosion impacts, in accordance with Section 4.5, Appendix 1
of the Phase lI Permit. These onsite/LID BMPs can be used for flow control and/or runoff treatment to
reduce the size of, or eliminate the need for, additional facilities designed for Minimum Requirements #6
and #7, if applicable. All Large Site Projects are required to consider the use of LID practices and
document the decision —making processes used to select or screen out their application.
As described in Minimum Requirement #1, the use of LID should begin in the site planning stage to
minimize the amount of stormwater released from the site. Once site planning and design is complete and
the generation of offsite runoff is minimized, stormwater runoff from the planned impervious surface and
converted pervious surface areas must be mitigated.
All sites are required to implement:
Roof downspout control BMPs, (either infiltration or dispersion) functionally equivalent
to those described in Chapter 3 of Volume III of the Stormwater Manual to reduce the
hydrologic disruption of the developed site to the maximum extent feasible without
causing flooding or erosion impacts.
The soil quality and depth BMP T5.13 for compost -amending, in Chapter 5 of Volume V
of the Stormwater Manual for all disturbed pervious surface areas.
Art;.a rrn ..,.r..... n;„,„,,,,,. »,..r, ,,......_o,. 4.0 LARGE SITE REQUIREMENTS 11
APRIL2010
For some Large Site Projects, reducing the amount of stormwater runoff generated using LID planning
techniques (Section 4.1) and using LID BMPs can satisfy the flow control requirements of Minimum
Requirement #7 as well as Minimum Requirement #5. See Section 4.7.2 for criteria. Simplified design
tools are available to encourage the use of LID and allow easy evaluation of LID flow control benefits
(see Section 4.7.3.2).
4.5.1 BMP Selection
Common onsite/LID practices include but are not limited to:
• Bioretention cells (also known as rain gardens).
• Small-scale "traditional" infiltration facilities (such as dry wells, gravelless chambers,
and trenches).
• Permeable pavement (asphalt, concrete, paving blocks, "grass-crete").
• Dispersion BMPs such as wheel strip driveways or other pavement sloped to drain to
onsite vegetation adequate for dispersion, or downspout or sheet flow dispersion.
Onsite/LID BMP design requirements shall be those in the LID Manual or the Washington State
Department of Transportation's Highway Runoff Manual (HRM). BMP design requirements presented in
other stormwater management manuals or standard documents approved by Ecology such as the City of
Seattle and King County stormwater manuals are also acceptable when modified for local conditions such
as precipitation and soil conditions. Table 4-2 presents a summary of onsite/LID measures that could be
used to manage stormwater on site, along with references for design guidance information. Additional
LID BMPs can be proposed as long as they are found in an Ecology -approved stormwater manual, are
appropriate for site conditions, and do not cause on- or offsite flooding or erosion impacts.
LID techniques that rely on infiltration should be designed with caution, due to the prevalence of till or
"hardpan" soil and steep slopes in Edmonds. Section 4.5.2 discusses this matter further.
See Section 5.5.3 for a discussion of installing rain gardens on single-family residential sites without the
services of a geotechnical professional to assess soil infiltration rates.
4.5.2 Onsite/LID Techniques and Infiltration
Onsite/LID features that rely on infiltration (such as bioretention) are not appropriate for every site.
Limitations and considerations include:
• Infiltration is prohibited in the Earth Subsidence and Landslide Hazard Area (ESLHA)
(ECDC Chapter 19.10), upgradient of the ESLHA, and other areas with geological
instability (landslide hazard areas). See Figure B-3 in Appendix B.
• Infiltration is prohibited in "steep slope areas" with average ground surface slopes equal
to or greater than 15 percent. See Figure B-3 in Appendix B.
r„ r,.:rsn-4nn,a..»..,.,,,-, o» ,,, ,,.,.,;;:f-r,..» .,; r,-,.;r.do;: 4.0 LARGE SITE, REQUIREMENTS 12
APRIL2010 STORMWATERCODESUPPLENENT
• Infiltration is prohibited within setbacks from the top of a designated landslide hazard or
steep slope areas.
• Infiltration should be applied in areas that have higher infiltration rates (especially in
areas with soils classified as Everett gravelly sandy loam).
When not restricted as noted above, infiltration should be used as much as possible in the Greater Lake
Ballinger Watershed and in all other creek basins.
Table 4-2: Commonly Used Onsite/LID Stormwater Management Techniques
BMP #
Onsite Stormwater Management Technique
Reference(s)/Design Guidance
Infiltration'BMPs
--
Bioretention Facilities Raingardens)
Section 6.1 LID Manual and City Pre -Sizing Tables
Infiltration Trench (downspout or other
City Pre -Sizing Tables
--
Drywell
City Pre -Sizing Tables
-
Gravelless Chamber
City Pre -Sizing Tables
T5.13
Preserve and Restore Soil Quality and Depth
Volume V Stormwater Manual; "Soils for Salmon"
website, Section 6.1 LID Manual
Dispersion BMPs
C.2.9.3
Wheel Strip Driveway
King County Manual. Appendix C.
TS.10,
T511 &
75.12
Downspout or Sheet Flow Dispersion Systems
Volume V Stormwater Manual, City; Pre -Sizing
Tables
T5.30
Full Dispersion
Volume V Stormwater Manual
LID Manual = Low Impact Development: Technical Guidance Manual for Puget Sound.
City Pre -Sizing Tables are available from the City's Engineering Division
Stormwater Manual = Stormwater Management Manual for Western Washington.
King County Manual = 2009 King County Surface Water Design Manual.
Soils reports are required where infiltration is proposed; soil reports for individual lots must include at
least two soil logs for each proposed infiltration location. Each log shall correspond to soil conditions
extending a minimum of 4 feet depth below ground surface (6 feet for drywells). The report shall describe
the Soil Conservation Service (SCS, now called the Natural Resource Conservation Service or NRCS)
series of the soil and the textural class of each horizon through the depth of the log, and shall note any
evidence of a high groundwater table, such as mottling. Reports solely using the Unified Soil
Classification System (USCS) will not be accepted. Soils reports must be prepared by or under the
direction of a licensed onsite sewage system designer, civil engineer, engineering geologist, or
geotechnical engineer.
A soils report produced for siting and design of an onsite sewage system may also be used to satisfy this
soils report requirement, provided that (a) the depth of the soil log(s) is at least 4 feet, (b) the depth to
seasonal high water table is determined, and (c) the location of the soil logs is adequate to determine the
feasibility of the infiltration system.
d/ D.1ttdt.u4.r?!Hier,,,,l;4s,!car,,:;,s:,,:,;:n;-f,a,,,;,ta,,,,,.4;,�;• 4.0 LARGE SITE REQUIREMENTS 13
APPJL2010
Field infiltration rates can be evaluated using one of two approved methods: USDA Textural
Classification, or the Modified Pilot Infiltration Testing (PIT) Method. These methods are described in
Appendix C to this Supplement.
A correction factor shall be applied to the measured infiltration rate to convert to the design infiltration
rate. This correction factor shall be different based on the time of year of the test. If the test is conducted
from May 1 through October 31, the correction factor shall be two times the correction factor listed in
Table C-1 (USDA method) or Table C-2 (modified PIT method) in Appendix C of this Supplement. For
infiltration rates tested from November 1 — April 30, the correction factor shall be equal to the correction
factor listed in Table C-1 (USDA method) or Table C-2 (modified PIT method) in Appendix C of this
Supplement.
In certain situations, BMPs that rely on infiltration are classified as Underground Injection Controls
(UICs) and may be regulated by Ecology under the UIC Program (Washington Administrative Code
[WAC] 173 218). For more information on Underground Injection Control (UIC) see the 2006 Ecology
document titled Guidance for UIC Wells that Manage Stormwater. This document is available online at
<hqp://www.ecy.wa.gov/biblio/0510067.html>.
4.6 Minimum Requirement #6 - Runoff Treatment
Runoff treatment BMPs must handle the water quality design storm volume or flow rate using the
appropriate treatment type (basic, enhanced, oil control, or phosphorus treatment) for all new and
replaced project pollution -generating surfaces. BMPs to meet this minimum requirement must be
designed by a civil engineer.
4.6.1 Project Thresholds
Stormwater treatment facilities are required for the following:
• Projects in which the total of new and replaced effective, pollution generating
impervious surface (PGIS) is 5,000 square feet or more in a threshold discharge area
of the project.
• Projects in which the total of pollution generating pervious surfaces (PGPS) is three-
quarters (3/4) of an acre or more in a threshold discharge area, and from which there is a
surface discharge from the site into a natural or man-made conveyance system.
That portion of any development project in which the PGIS or PGPS thresholds listed above is not
exceeded in a threshold discharge area shall apply onsite stormwater management BMPs in accordance
with Minimum Requirement #5 to the maximum extent practicable.
:ry C,rc.P.t1D-tiuUimm...... is.rro»»;,,,,.�.�.;r,:,1er»,..:U»r,v:.do;: 4,0 LARGE SITE REQUIREMENTS 14
APRIL2010
4.6.2 Runoff Treatment Standards
Runoff treatment standards shall be implemented per Section 4.6, Appendix 1 of the Phase II Permit. In
addition, the following Edmonds -specific requirements apply.
4.6.2.1 Oil Control
All projects in areas not zoned as single family residential that collect runoff from 5 or more parking
spaces shall install oil containment catch basins if another approved oil control system is not employed.
The outlet pipe of these catch basins shall have a downturned 90 degree elbow to restrict the outflow of
oil and other floatables. There should be at least 9 inches of clearance between the elbow and the inside
wall of the catch basin to facilitate removal of floatables. A maintenance schedule for the removal of oil
and other floatables from these catch basins must be submitted with all plans for review.
4.6.2.2 Phosphorus Treatment
Per Section 4.6.2, Appendix 1 of the Phase II Permit, phosphorus treatment is required for projects that
discharge to a nutrient -critical receiving water. Currently this requirement applies to projects in the
Greater Lake Ballinger Watershed (see Appendix B) that has a Total Maximum Daily Load (TMDL) for
phosphorus. Bioretention systems with overflows (i.e., systems not providing 100 percent infiltration)
should not be used in this basin due to potential phosphorus export in the treated runoff discharged
ultimately to the lake.
4.6.3 Runoff Treatment BMP Selection
A sampling of runoff treatment BMPs is summarized in Table 4-3, with the treatment levels and design
criteria references. Additional BMPs may be approved by the City's Engineering Division on a case -by -
case basis. BMPs shall be selected from the list according to the treatment type required, and according to
the following general prioritization guidelines:
• Bioretention systems should be implemented where feasible in creek basins. If infiltration
of runoff is not feasible at the site, consider use of bioretention systems with
underdrains.
• Onsite Stormwater Management Measures (see Section 4.5 above) are preferred, if
feasible, over larger runoff treatment facilities. Be sure that these onsite techniques are
being applied to the maximum extent practicable before selecting runoff treatment
BMPs.
<y a.r-; srav >r1n,: ox,,,dsstw„ :-..:a«,;, -r,r,,,,,,,r!rn,::.roc 4.0 LARGE SITE REQUIREMENTS 15
Table 4-3: Runoff Treatment Best Management Practices
Runoff Treatment
Treatment Type
Reference(s)/Design
Basic Enhanced Phosphorus
Oil
BMP #
BMP
Treatment
Treatment
Treatment
Control
Guidance
-
Bioretention Facilities
X
X
Section 3.1 LID
(Rain Gardens)
Manual
T7.10
Infiltration Basins
*
*
*
Volume III Stormwater
Manual
T7.20
Infiltration Trenches
*
*
*
Volume III Stormwater
- Manual
T7.30
Bioinfiltration swale
*
*
*
Volume V Stormwater
Manual
T9.10
Basic Biofiltration
X '
Volume V Stormwater
Swale
Manual
T9.20
Wet Biofiltration Swale
X
Volume V Stormwater
Manual
T9.30
Continuous Inflow
X
Volume V Stormwater
Biofiltration Swale
Manual
T9.40
Basic Filter Strip
X
Volume V Stormwater
Manual
T9.50
Narrow area filter strip
X
Volume V Stormwater
Manual'
RT.02
Compost -Amended
X
X
X
HRM
Vegetated Filter Strip
T1010
Wetponds!(basic), .
X,
Volume V Stormwater
Manual
T10.10
Wetponds (large)
X
X
Volume V Stormwater
Manual
T1020
Wet vaults
X
*
Volume V Stormwater
Manual
T10.30
Stormwater treatment
X
X
Volume V Stormwater
wetland
Manual
T10.40
Combined
X'
*
*
Volume V Stormwater
detention/weVool
Manual
facilities
--
Sand Filter Basin
X
*
*
Volume V Stormwater
Manual
T8.10
Sand Filter Vault
X '
*
*
Volume V Stormwater
Manual
T8.20
Linear Sand Filter
X
*
*
X
Volume V Stormwater
Manual
RT.07
Media Filter Drain
X
X
X
HRM
T11.10
API (Baffle type)
X
Volume V Stormwater
Separator Bay
Manual
Tl la 1
Coalescing Plate (CP)
X
Volume V Stormwater
Separator;Bay
Manual
RT.22
Oil Control Booms
X
HRM
X = BMP meets this treatment type
* = BMP can be designed to meet this treatment type
LID Manual = Low Impact Development: Technical Guidance Manual for Puget Sound
Stormwater Manual = Stormwater Management Manual for Western Washington
HRM = WSDOT Highway Runoff Manual
r„*aa?aar,>t,>,:r„;.,,...,to„n„-,,.,.,.,,;rc^r,.,,,..,;r..r.ao;: 4.0 LARGE SITE REQUIREMENTS 16
1 1 7• • �� • a �' •� 7
4.7 Minimum Requirement #7 - Flow Control
Flow control BMPs shall be implemented for all effective impervious surface area that is new and
replaced and converted pervious surfaces per Section 4.7, Appendix I of the Phase II Permit to reduce
the impacts of stormwater runoff from impervious surfaces and land cover conversions. BMPs to meet
this minimum requirement must be designed by a civil engineer. Large Site Projects that fall below the
Phase II Permit thresholds for flow control but have 2,000 square feet or more effective impervious
surface must comply with the Edmonds -specific flow controls standard described in Section 4.7.2.
The use of LID techniques under Minimum Requirements #1 and #5 can satisfy this minimum
requirement for some Large Site projects. Simplified design tools are available to encourage the use of
LID and allow easy evaluation of LID flow control benefits (see Section 4.7.3.2).
4.7.1 Project Thresholds and Applicability
The applicability of this minimum requirement is dependent upon the drainage basin of the project site.
See Chapter 2 for drainage basin classifications. If flow control is not required, onsite stormwater
management techniques must still be applied to the maximum extent practicable (see Section 4.5).
Creek or Lake Basin Projects
Large Site Projects in Creek or Lake Basins must meet Phase II Permit flow control standards if any of
the following Phase II permit thresholds are met or exceeded:
• Projects in which the total new and replaced effective impervious surface in a threshold
discharge area is greater than 10,000 square feet.
• The native vegetation area converted to lawn or landscaping is 3/4 acres or more, or the native
vegetation area converted to pasture is 2.5 acres or more in a threshold discharge area and from
which there is surface discharge in natural or man-made conveyance systems from the site.
• Where through a combination of effective impervious surface and converted pervious surfaces
the project causes a 0.1 cubic feet per second or greater increase in the 100-year recurrence peak
flow from a threshold discharge area as estimated by an Ecology -approved continuous simulation
model.
Direct Discharge Basin Projects
Large Site Projects in Direct Discharge Basins can be exempt from Small Site Minimum Requirement #7
under one of the following circumstances:
• A quantitative Offsite Analysis as described in Large Site Minimum Requirement #10 is
performed by the applicant and no unacceptable downstream issues (such as a capacity
or erosion issue), are found.
4.0 LARGE SITE REQUIREMENTS 17
APRIL2010 EDIy1ONDSSTCRMWATERCODE SLPPLE MENT
• A quantitative Offsite Analysis as described in Large Site Minimum Requirement #10 is
performed by the applicant, and one or more unacceptable downstream issues are
discovered. A plan is proposed by the applicant to mitigate for the unacceptable
downstream issue and the mitigation plan is approved by the Public Works Director or
designee.
Any Large Site Project in a Direct Discharge Basin can be exempt from Large Site Minimum
Requirement # 10 by meeting the applicable flow controls standard for sites in Creek or Lake Basins (see
Section 4.7.2).
4.7.2 Flow Control Standards
Flow control standards for Large Site Projects vary by basin type and the amount of effective impervious
surface. These standards for projects that must meet Phase II permit requirements are summarized in
Table 4-4, and for all other projects in Table 4-5 (Lake or Creek Basin) or Table 4-6 (Direct Discharge
Basin).
Table 4-4: City of Edmonds flow control requirements for Large Site Projects Triggering Phase 11 Permit
Requirements
> 10,000 sf Effective Impervious Area fl
Numerical Standard:
Match 1/2 the 2-year to 50-year flow durations to predeveloped forest condition
Applies to:.
New and replaced impervious surface and disturbed pervious areas
Computational
Methods:
Continuous hydrologic modeling with MGS precipitation data b,
Options to Meet Numerical Standard
LID Credit Option
(If feasible):
Credit given for meeting the flow control standard using LID techniques described in the
Stormwater Manual
Traditional Option:
If it is not feasible to achieve the standard using LID only, larger -scale infiltration or
detention may be used as described'in the Stormwater Manual
sf square feet
LID: low impact development
Or at least 3/4 acres conversion to lawn/landscape, at least 2.5 acres conversion to pasture, or a greater than 0.1 cubic feet per second increase
in the 100-year flood frequency.
MGS precipitation is the Puget East 36 precipitation time series developed for the Washington State Department of Transportation by MGS
Engineering Consultants, Inc. and available in WWHM and MGS Flood hydrologic modeling software.
` For infiltration facilities, flood frequency calculations (i.e., determination of recurrence interval peak flows) must be performed using an
approach capable of properly evaluating zero flow years as explained in Section 4.7.3.1 below.
Phase 11 Permit Flow Control Standard
Large Site Projects in Creek or Lake Basins that meet one or more of the Phase II permit thresholds listed
above must meet the following flow control standard for all new and replaced impervious surface and
converted pervious surface: match 1 /2 the 2-year to 50-year recurrence flow durations to the predeveloped
condition as determined using a continuous hydrologic model.
cj CA-Jd..t4DDttthervanassmr:r,�n;:nrsu,�»rJraaa+lliaaf.dv:; 4.0 LARGE SITE REQUIREMENTS 18
Edmonds -Specific Flow Control Standards
Large Site Projects in Creek or Lake Basins or in Direct Discharge Basins that do not meet one or more of
the Phase II permit thresholds must meet the City -specific flow control standards. Large Site Projects in
Direct Discharge Basins can be exempt from these flow control standards if a quantitative off -site
analysis indicates that the uncontrolled flows from the site will not cause a flooding, erosion, or other
problem downstream (see Section 4.10).
Recurrence interval flows for the developed condition shall be determined using a continuous hydrologic
model. For sites with 2,000 square feet or greater (and less than or equal to 5,000 square feet) of new plus
replaced impervious surface area the post -development 10-year recurrence interval flow shall not exceed
0.25 cubic feet per second (cfs) per acre of impervious surface area for all flow control BMPs.
For sites with greater than 5,000 square feet of new plus replaced impervious surface area, limits for the
post -development peak flows depend upon the BMP type selected:
• For detention BMPs:
■ Creek or Lake Basins: the post -development _2-, 10-, and 100-year
recurrence interval peak flows shall not exceed 0.07, 0.14, and 0.33 cfs per
acre of impervious surface area, respectively.
■ Direct Discharge Basins: the post-development_10-, and 100-year
recurrence interval peak flows shall not exceed, 0.25 and 0.45 cfs per acre
of impervious surface area, respectively.
• For infiltration BMPs:
■ Creek or Lake Basins: the post-development_2-, 10-, and 100-year
recurrence interval peak overflows shall not exceed 0.07, 0.25, and 0.45
cfs per acre impervious surface area, respectively.
■ Direct Discharge Basins: the post-development-10-, and 100-year
recurrence interval peak overflows shall not exceed 0.25, and 0.45 cfs per
acre impervious surface area, respectively.
There are two methods to satisfy Minimum Requirement #7 for sites subject to the City -specific Standard:
the "LID Credit Option" and the "Traditional Option". Large Site Projects shall use the LID Credit
Option to implement flow control, if feasible. The LID Credit Option allows an applicant to mitigate only
a portion of the new and replaced impervious surface area if only LID site planning techniques and LID
BMPs are used. The requirements for the LID Credit Option are presented below.
• For projects that generate greater than 2,000 square feet (but less than or equal to 5,000
square feet) of new plus replaced effective impervious surface area, the greater of 1)
2,000 square feet or 2) 85 percent of new plus replaced impervious surface area is
mitigated to meet the flow control standard using LID measures.
• For projects that generate greater than 5,000 square feet of new plus replaced effective
impervious surface area, the greater of 1) 5,000 square feet or 2) 90 percent of new plus
d; 4.0 LARGE SITE REQUIREMENTS 19
replaced impervious surface area is mitigated to meet the flow control standard using
LID measures.
If it is not feasible to use the LID Credit Option to meet Minimum Requirement V, the project shall meet
the flow control standard for all new and replaced impervious surface and converted pervious surface. In
this case, the "Traditional Option", larger -scale infiltration BMPs or detention BMPs may be used to
provide flow control for the areas not mitigated using LID measures.
,: ..,n..,;an.;..sb;: 4.0 LARGE SITE REQUIREMENTS 20
§
■
�
0
a
k
§
4.
\
\ E
± ca
�j
§ 0
0
,
2
\
§
2
%
�
u
�
±
k
U.:�1>IE:i�i,'
Table 4-6: City of Edmonds Flow Control Requirements for All Other Large Site Projects
Direct Discharge Basins
>_ 2,000 A
Effective Impervious Surface a
Numerical
For detention and infiltration BMPs:
Standard: '
10-year = 025 cfs/acre impervious, and
100-year= 0.45 cfs/acre impervious
An exemption from these flow control standards may be allowed by the Public Works Director or designee
if, a quantitative offsite analysis per Large Site Minimum Requirement # 10 (see Section 4.10) indicates no
unacceptable downstream issue (suchas a capacity or erosion issue). If an unacceptable downstream issue
is found, the applicant can either propose mitigation of the issue to the Public Works Director or designee
(whom will consider the proposal but not necessarily approve it) or design and implement flow control
measures to the Lake or Creek Basin Standard (see Section 4.7.1).
Applies to
New and replaced effective impervious surface
Computational
Simplified Sizing Approach or continuous hydrologic modeling with MGS precipitation data b, °
Methods
Options to Meet Numerical Standard
LID Credit
For sites with 2,000 sf or more (but less than 5,000 sf) of effective impervious surface: If 2,000
Option (If
sf or 85% of new plus replaced impervious surface area (whichever is greater) is mitigated to the
feasible)
numerical standard using Onsite/LID techniques (Minimum Requirement #5), the requirement is
met.
For sites with 5,000 sf or more of effective impervious surface: If 5,000 sf or 90% of new plus
replaced impervious surface area (whichever is greater) is mitigated to the numerical standard
using Onsite/LID techniques (Minimum Requirement #5), the requirement is met.
Traditional
If OnsiteLLID techniques are not feasible; larger -scale infiltration or detention may used to
Option
triitigate capacity ;impact to the extent necessary - See Section 4.7.4.
sf. square feet
But less than 10,000 square feet of effective impervious area.
n MGS precipitation is the Puget East 36 precipitation time series developed for the Washington State Department of Transportation by
MGS Engineering Consultants, Inc. and available in WWHM and MGS Flood hydrologic modeling software.
` For infiltration facilities, flood frequency calculations (i.e., determination of recurrence interval peak flows) must be performed using an
approach capable of properly evaluating zero flow years as explained in Section 4.7.3.1 below.
4.7.3 Flow Control Facility Sizing
All flow control facilities for Large Site Projects must be designed by a civil engineer.
4.7.3.1 Modeling Requirements
Flow control facilities must be sized to meet the standards described above using an approved continuous
runoff simulation model. Guidance on application of continuous runoff models for flow control BMP
sizing is provided in Volume III of the Stormwater Manual. Supplemental guidance is provided in this
section.
Approved continuous runoff models include Western Washington Hydrologic Model (WWHM) and
MGS-Flood. Flow control sizing must be conducted using the "Puget East 36" precipitation time series
developed by MGS Engineering Consultants (MGS 2002). This time series is available in WWHM
(labeled "DOT data"), and is one of the default precipitation time series in MGS-Flood.
cy ,r.L» .:Dtkk1,M»....as 4.0 LARGE SITE REQUIREMENTS 22
In addition, for infiltration facilities, flow frequency calculations (i.e., determination of recurrence
interval peak flows) must be performed using an approach capable of properly evaluating zero flow years.
This is important because infiltration facilities sized to meet the recommended flow control standards will
infiltrate approximately 98 percent of the runoff volume, resulting in an annual peak flow time series that
contains several zero or very low peak flow values. Currently the MGS-Flood model properly calculates
flood frequency for these scenarios, while WWHM does not. Until WWHM integrates a more flexible
flow frequency routine, it is recommended that annual peak flow datasets with multiple low or zero
annual peak flow values be exported from WWHM and analyzed using a spreadsheet method to
determine flow frequency (i.e., 2-, 10-, and 100-year recurrence interval flows). The Gringorten plotting
position formula (Maidment 1993), which is non -parametric (i.e., does not depend on the mean, standard
deviation, or skew of the data), works well for these applications.
4.7.3.2 Simplified Sizing Approach
A pre -sized approach for sizing flow control BMPs has been developed for Large Site Projects in
Edmonds with between 2,000 and 10,000 square feet of new and replaced impervious surface area. This
approach allows the designer to size selected BMPs to meet the numerical flow control standards without
the use of a continuous hydrologic model. Guidance for using the simplified sizing approach for flow
control BMPs is provided in City of Edmonds Handouts available from the Engineering Division.
4.7.4 Flow Control BMP Selection
This section discusses infiltration and flow control BMPs. If onsite/LID is not used, flow control can be
achieved using larger -scale infiltration or detention.
4.7.4.1 Larger -Scale Infiltration
If LID is not feasible, larger -scale, centralized infiltration facilities may be used (as opposed to the small
and distributed infiltration facilities associated with LID). Infiltration is the preferred method of
stormwater flow control relative to detention, but only in certain areas of Edmonds where infiltration of
large amounts of runoff in a concentrated area will not contribute to flooding or offsite slope stability
problems. The feasibility of infiltration as a means to meet stormwater flow control and treatment
requirements will depend upon specific project site locations due to the variations in soil and topography
across the City. See Chapter 4.5.2 for guidelines pertaining to the applicability and testing required for
implementing infiltration for flow control.
Examples of infiltration BMPs for flow control are summarized in Table 4-7.
4.7.4.2 Detention
If infiltration BMPs are not feasible to meet flow control requirements for a site, detention BMPs can be
used. The minimum orifice size for detention facility outlet control structures is one-half inch (1/2 inch).
4.0 LARGE SITE REQUIREMENTS 23
APRIL2010
Table 4-9 summarizes some available BMPs for flow control, including several detention options. The
City's Building Division should be contacted prior to designing roof detention systems.
Table 4-7: Flow Control Best Management Practices
BMP #
Flow Control BMP
I Reference(s)/Design Guidance
Large - Scale Infiltration BMPs
T7.10
Infiltration Basins
Volume III Stormwater Manual
T7.20
Infiltration Trenches
Volume IIIStormwater Manual
T7.30
Infiltration Vaults
Volume III Stormwater Manual
--
Gravelless Chambers
Kitsap County Stormwater Management Design Manual_(Kitsap
County 1997),
Detention BMPs
Detention Ponds
Volume III Stormwater Manual
--
Detention Tanks
Volume III Stormwater Manual
-
Detention Vaults
Vol ume'III Stormwater Manual
--
Use of Parking Lots for Detention
Volume III Stormwater Manual
--
Use of Roofs for Detention
Volume III Stormwater Manual
Stormwater Manual = Stormwater Management Manual for Western Washington.
4.8 Minimum Requirement #8 - Wetland Protection
Volume I, Section 2.5.8 of the Stormwater Manual has guidance on meeting Minimum Requirement #8.
Refer also to ECDC Chapter 23.40.
4.9 Minimum Requirement #9 - Operation and Maintenance
BMPs shall be maintained in accordance with the provisions in Chapter 7 of this Supplement.
4.10 Minimum Requirement #10 - Offsite Analysis and Mitigation
Large Site Projects that discharge stormwater off site shall submit as part of their Stormwater Site Plan
an offsite analysis that assesses the potential offsite impacts of stormwater discharge. Offsite analyses
assess upstream and downstream conditions, including the conveyance capacity and erosion potential in
the downstream system. If a problem is found, mitigation is required to prevent worsening of that
problem.
The existing or potential impacts to be evaluated and mitigated as part of any off- site/downstream
analysis shall include:
• Conveyance system capacity problems.
• Localized flooding.
• Wetlands impacts such as siltation or other damage.
• Upland erosion impacts, including landslide hazards.
q M1LJt;D00fle...»a»;is ,,iph.,',",.,.;4. ,,, 4.0LARGE SITE REQUIREMENTS 24
• Stream channel erosion in the channel or at the outfall location.
• Locations where surface water enters and exits the site.
All projects shall perform a qualitative analysis downstream from the site. A quantitative analysis may
also be required by the Director or his/her designee when there is a potential for downstream flooding or
erosion. Each of these types of analysis is described further below.
4.10.1 Qualitative Analysis
Project applicants shall submit a qualitative analysis of each upstream drainage system entering a site
(run-on) and each downstream drainage system leaving a site (runoff). The upstream analysis shall
identify and describe points where water enters the site and the tributary area. The qualitative analysis
shall extend downstream for the entire flow path, from the project site to the receiving water, or up to
one -quarter mile, whichever is less. A map defining the onsite and offsite areas that contribute stormwater
runoff to the site shall be provided by the applicant. The map shall be prepared with a defined scale.
Upon review of this analysis, the Public Works Director or Designee may require a qualitative analysis
extending further downstream, mitigation measures deemed adequate to address the problems, or a
quantitative analysis, depending upon the presence of existing or predicted flooding, erosion, or water
quality problems, and on the proposed design of the onsite drainage facilities.
4.10.2 Quantitative Analysis
If required, a civil engineer or professional land surveyor must field survey all existing storm drainage
systems downstream of the project site for a minimum distance of 1/4-mile from the point of connection
to the existing public drainage system. If the ultimate discharge point is to Puget Sound via a culvert
owned by BNSF Railway, the analysis must be followed through the drainage system all the way to Puget
Sound. The goal of the inspection and analysis is to evaluate whether the capacity of the drainage
system(s) is adequate to handle the existing flows, flows generated by the project, flooding problems,
erosion damage or potential, amount of freeboard in channels and pipes, and storage potential within the
system.
All existing and proposed offsite surface water conveyance systems shall be sized to convey flows
without surcharging the City's storm system or the BNSF culvert under the railroad tracks (if applicable)
during the 50-year recurrence event. If the project is in the North Edmonds Earth Subsidence and
Landslide Hazard Area (ESLHA, see Appendix B), contact the City Engineering Division for
requirements.
If a capacity problem, streambank erosion problem, or other problem with the downstream system is
encountered in the analysis, the runoff flow from the project site will be restricted per Minimum
a7 es-o teo.?1L'tLednr,,;;to.rmrnnva:e.;:uu,-r,:vr„rnticatnon 4.0LARGE S11'EREQUIREMENTS 25
Requirement #7 — Flow Control or other mitigation may be proposed by the applicant subject to review
by the Public Works Director or Designee.
4.11 Minimum Requirement #11 -Financial Liability
A performance bond shall be posted as a financial guarantee for required stormwater systems and BMPs
related to Construction Stormwater Pollution Prevention Plans for the following Large Site Projects:
• Single family residential projects located within Critical Areas, Critical Area buffers
including the ESLHA.
• Multi -family residential and commercial projects
• Subdivisions involving 7,000 square feet or more of [and disturbing activity.
4,0 LARGE SITE REQUIREMENTS 26
APRL2010
Figure 4-1: Flow Chart for Determining Minimum Requirements for New Development on Large Site Projects
Start
Does the site have 35% or YES See Redevelopment
more of existing impervious 0 Minimum Requirements
coverage I Flow Chart (Figure 4-2)
NO
Does the project add SJDOD IYES
square feet or more of new
impervious surface?
NO
Does the project convert
3/4 acres or more of native
vegetation to lawn or
landscaped areas, or
YES
All Minimum Requirements
(#l-M1) apply to new AND
replaced impervious
convert 2.5 acres or more
surfacesAND converted
of native vegetation to
pervious surfaces.
pasture?
NO
Does the project have
2,000 square feet or more YES
of new impervious
surfaces?
NO
4
Minimum Requirements#1
Does the project have land- YES
through #i5 apply to new
disturbing activities of7p00
AND replaced impervious
square feet or more?
surfacesAND the land
disturbed.
NO
Apply Minimum Requirement
#2, ConstructiorrStormwater
Pollution Prevention'
f 4.OLARGE SITE REQUIREMENTS 27
Figure 4-2: Flow Chart for Determining Minimum Requirements for Redevelopment on Large Site Projects
From Figure 4-1
Does the area of new, replaced, or new plus replaced impervious surfaces total
2,000 square feet or more?
OR Do es the area of land -disturbing a ctivity total 7,000 square feet or more?
YES I I NO
Minimum Requirements #1 through #5 apply to
new AND replaced impervious surfacesAND the
land disturbed.
Next Question
Apply Minimum Requirements #2, Construction
Stormwater Pollution Prevention
Does the project add 5 P00 square feet or more of new impervious surfaces?
OR Convert 3/4 acres or more of native vegetation to lawn or landscaped areas?
OR Convert 2.5 acres or more of native vegetation to pasture?
YES INO
All Minimum Requirements (#1-#11) apply to new Next
AND replaced impervious surfaces AND Question Is this aroad-related
convertedpervious surfaces: project?
YES NO
Does the project add 5p00 square feet or more to the
existing impervious surfaces within the project limits?
YES 1 1140
Do new impervious surfaces
add 50% or more to the
existing impervious surfaces
within project limits?
YES
No additional:
requirements
All Minimum Requirements ,(#1
#11) apply to new AND replaced
impervious surfaces.
Is the total of new plus replaced
impervious surfaces5,000 square
feet or more,AND does the does
NQ the value of the proposed
improvements — including interior
improvements — exceed 50% of the
assessed value (or replacement
value) or the existing site
improvements?
YES
d/ t„?-C9t-1D-ttun,:e,noru.,tn,,,r..,,,.r..;n,,"..,<..,tb,a...ac 4.0LARGE SITE REQUIREMENTS 28
APPJL2010 EDNONDSSTORMWATERCODESU'PLEMENT
5.0 Small Site Requirements
This section provides specific guidance on how to comply with the minimum requirements for Small Site
Projects. Appendix B contains supplemental technical information on watershed boundaries and the
location of steep slopes and soil types.
The requirements applicable to Small Site Projects differ for Category 1 and Category 2 project types.
For an explanation of how to classify a Small Site project, see Chapter 2.
Category 1 Small Site Projects are subject to the following Minimum Requirements for all new and
replaced impervious surfaces, converted pervious surfaces, and disturbed pervious surface areas:
• Small Site Minimum Requirement #1 — Preparation of Stormwater Site Plan (Section
5.1)
• Small Site Minimum Requirement #2 — Construction Stormwater Pollution Prevention
Plan (Section 5.2)
• Small Site Minimum Requirement #3 — Source Control of Pollutants (Section 5.3, not
required for single family residential sites)
• Small Site Minimum Requirement #4 — Preservation of Natural Drainage Systems and
Outfalls (Section 5.4)
• Small Site Minimum Requirement #5 — Onsite Stormwater Management (Section 5.5)
• Small Site Minimum Requirement #7 — Flow Control (Section 5.7, may be waived
under certain circumstances)
• Small Site Minimum Requirement #8 — Wetland Protection (Section 5.8, if wetlands
are on the site or the site drains to certain wetlands)
• Small Site Minimum Requirement #9 — Operation and Maintenance (Section 5.9 if a
structural BMP is installed)
• Small Site Minimum Requirement #11 —Financial Liability (Section 5.1 ]) for single
family residential sites, applies only to stormwater systems constructed in or adjacent to
Critical Areas or Critical Area buffers).
All nine of these Minimum Requirements may a not be applicable to any particular project. In most cases,
Category I single family residential projects are subject to Minimum Requirements #1, #2 , #4 , #9
(possibly), and either Minimum Requirement #5 or #7 or a combination of both #5 and V.
Category 2 Small Site Projects are subject to the Small Site Minimum Requirements listed for Category
1 Small Site Projects, as well as evaluating the applicability of the following for all new and replaced
impervious surfaces, converted pervious surfaces, and disturbed pervious surface areas:
• Small Site Minimum Requirement #6 — Runoff Treatment (Section 5.6)
il' ......,..,ion„„.,,,... .rn..n..r,...ab:: 5.0SMALL SITE REQUIREMENTS 29
• Small Site Minimum Requirement #8 — Wetland Protection (Section 5.8)
• Small Site Minimum Requirement #10 — Offsite Analysis and Mitigation (Section
5.10).
All 11 of these Minimum Requirements may a not be applicable to any particular project. In most cases,
Category 2 single family residential projects are subject to Minimum Requirements #1, #2, #4, 99 and
either Minimum Requirement #5 or #7 or a combination of both #5 and #7
5.1 Small Site Minimum Requirement #1 - Preparation of Stormwater
Site Plan
Stormwater Site Plans shall be prepared for all Small Site Projects, The Stormwater Site Plan shall
document compliance with all applicable minimum requirements and the design of all BMPs. For all
Category 2 Small Site Projects that involve 5,000 square feet or more of new or replaced impervious
surface (Category 2), stormwater BMPs must be designed by a civil engineer.
This Stormwater Site Plan shall include the following:
• A complete dimension site plan, drawn to scale, showing area of proposed construction,
modifications or repair, topographical information and a tally of the new and replaced
impervious surface.
• Construction plan, details, and specifications when applicable of all stormwater control
features.
Additional information may be required depending on the site conditions and proposed development. See
handouts prepared by the City's Engineering Division for guidance and details on the submittal
requirements.
The Stormwater Site Plan shall include the submittal requirements in the handout and reflect
consideration of site planning and design measures intended to reduce the impact of the project on
stormwater quality and quantity and should strongly consider the use of LID approaches as outlined in
Section 3.0.
Table 5-1 summarizes some LID site planning measures that can be implemented, with references to
guidance documents for further information.
Small Site Minimum Requirement #5 (Section 5.5) discusses LID techniques for managing stormwater
runoff on site to reduce or eliminate the amount that flows offsite to receiving waters.
5.0SMALL, SITE REQUIREM ENTS 30
APRIL2010
Table 5-1: Site Planning and Design Techniques to Reduce the Amount of Stormwater Runoff Generated
BMP #
Stormwater Management Technique
Reference(s)/Design Guidance
T5-21
Better Site Design (e.g., ,Reduce Effective Impervious
Volume V Stormwater Manual; Chapter 3
Areas Associated with Roads, Shared Accesses, Alleys,
LID Manual
Sidewalks, Driveways, and Parkin Areas
T5.20
Native Vegetation Protection, Reforestation, and
Volume V Stormwater Manual; Chapter 4
Maintenance
LID Manual `<
--
Minimize disturbance area
Volume V Stormwater Manual; Section 5
LID Manual
Ve etated Roofs
City Buildin „ Division
--
Rainwater Harvesting
LID Manual
--
Permeable Pavement (asphalt, concrete,, paving blocks,
Section 6.3 LID Manual or Volume 3
"grass -Crete")
Seattle Manual (base course requirements)
LID Manual = Low Impact Development: Technical Guidance Manual for Puget Sound.
Volume 3 Seattle Manual = Stormwater Flow Control & Water Quality Treatment Technical Requirements Manual.
Stormwater Manual = Stormwater Management Manual for Western Washington.
5.2 Small Site Minimum Requirement #2 - Construction Stormwater
Pollution Prevention Plan
A Construction Stormwater Pollution Prevention Plan contains two parts: an Erosion and Sediment
Control Plan and a Stormwater Spill Prevention Plan. Most Small Site Projects are only required to have
an Erosion and Sediment Control Plan. Additional pollution prevention measures may be required based
on site specific conditions or construction methods proposed.
All new development and redevelopment projects are responsible for preventing erosion and discharge of
sediment and other pollutants into receiving waters during construction. See handouts prepared by the
City's Engineering Division for guidance on preparing a stormwater pollution prevention plan for small
sites. See Volume II of the Stormwater Manual for direction on BMP selection and design. The City of
Edmonds does not approve erosivity waivers, as described in Section 4.2, Appendix 1 of the Phase II
Permit.
5.3 Small Site Minimum Requirement #3 - Source Control of Pollution
All known, available, and reasonable source control BMPs shall be applied to control pollution. Source
control BMPs shall be selected, designed, and maintained in according to the Stormwater Manual. See
Volume IV of the Stormwater Manual for guidance on the applicable source control BMP selection and
implementation.
Specific source controls are not required for single family residential sites. General requirements for these
sites include preventing the discharge of pollutants to the City's storm drainage system per Edmonds City
c! C,504.?40-000........ .a,sto»:raww,c,r,;—,", no:: 5,0SMALL SITE REQUIREMENTS 31
APRUDIO
Code Chapter 7.200 (Illicit Discharges). These pollutants include common household items such as
pesticides, herbicides, fertilizers, detergents, and fluids from vehicle maintenance.
5.4 Small Site Minimum Requirement #4 - Preservation of Natural
Drainage Systems and Outfalls
Natural drainage patterns shall be maintained, with run-on (from a potential upstream site) and runoff
from effective impervious surface on the project site discharging via the location of the natural drainage
system and outfall, to the maximum extent practicable. In most cases, the "natural location" is the
existing discharge location at the site. For some redevelopment projects, the natural location for runoff
discharge may have been previously altered. If there is a question as to the natural discharge location,
contact City Engineering Division staff.
The manner in which runoff is discharged from the project site must not cause an adverse impact to
downstream receiving waters and down -gradient properties. All outfalls require energy dissipation to
prevent erosion. Minimum Requirement #10 describes offsite analysis that is required for certain
Category 2 Small Site Projects to identify potential downstream concerns so that adverse impacts are
avoided through appropriate design.
5.5 Small Site Minimum Requirement #5 - Onsite Stormwater
Management
Onsite stormwater management BMPs described in this section are considered LID techniques. These
onsite/LID techniques should be considered at all small sites to infiltrate, disperse, and retain stormwater
runoff on site without causing flooding or erosion impacts. If properly designed in the right soil
conditions, these onsite/LID BMPs can be used for flow control and/or runoff treatment to reduce the size
of, or eliminate the need for, additional facilities designed to comply with Minimum Requirements #6
(Runoff treatment) and #7 (Flow Control), if applicable.
As stated in Small Site Minimum Requirement # 1, the use of LID approaches should begin at the site
planning stage to minimize the amount of stormwater runoff that is released from a site. Once site
planning and design are complete and the generation of offsite stormwater runoff is minimized, the
stormwater runoff from the planned impervious surface and converted pervious surface areas may be
mitigated.
The soil quality and depth BMP T5.13 for compost -amending, in Chapter 5 of Volume V of the
Stormwater Manual is required for all disturbed pervious surface areas to restore the water holding
capacity of these areas.
5.5.1 Onsite/LID BMP Selection
Common LID practices include but are not limited to:
5.0 SMALL SITE REQUIREMENTS 32
• Bioretention cells (also known as rain gardens).
• Small-scale "traditional" infiltration techniques (such as trenches, dry wells, and
gravelless chambers).
• Permeable pavement (asphalt, concrete, paving blocks, "grass-crete").
• Dispersion BMPs such as wheel strip driveways or other pavement sloped to drain to
drain to onsite vegetation adequate for dispersion, or downspout or sheet flow
dispersion.
Onsite/LID BMP design requirements shall be those in the LID Manual or the Washington State
Department of Transportation's Highway Runoff Manual. BMP design requirements presented in other
stormwater management manuals or standard documents approved by Ecology such as the City of Seattle
and King County stormwater manuals are also acceptable when modified for local conditions such as
precipitation and soil conditions. Table 5-2 presents a summary of Onsite/LID measures that could be
used to manage stormwater onsite, along with references for design guidance information. Additional LID
BMPs can be proposed as long as they are found in an Ecology -approved stormwater manual, appropriate
for site conditions, and do not cause flooding or erosion impacts onsite or offsite.
LID techniques that rely on infiltration should be designed with caution, due to the prevalence of till or
"hardpan" soil and steep slopes in Edmonds. Section 5.5.2 discusses this matter further.
Table 5-2: Commonly Used Onsite/LID Stormwater Management Techniques
Onsite Stormwater Management
BMP # Technique Reference(s)/Design Guidance
,Infiltration BMPs
Bioretention Facilities Raingardens) Section 6.1 LID Manual and City Pre -Sizing Tables
Infiltration Trench" (downspout or other City Pie -Sizing Tables
Drywell City Pre -Sizing Tables
Gravelless Chamber; City Pre -Sizing Tables
T5.13 Post -Construction Soil Quality and Depth Volume V Stormwater Manual; Section 6.1 LID
(compost -amended) Manual
BMPs
C.29.3 Whee1'Stri Drivewa" Kifig,'Cotin,,MahAial'.'Aopendix C.
Downspout or Sheet Flow"Dspersiori City Pre -Sizing Tables
Systgrns
LID Manual = Low Impact Development: Technical Guidance Manual for Puget Sound.
City Pre -Sizing Tables are available from the City's Engineering Division
Stormwater Manual = Stormwater Management Manual for Western Washington.
King County Manual = 2009 Surface Water Design Manual.
dy 0,�0414611?011ed„;,,.;;41.sro,,,;;,-,,ru.4ur,•r,a„?„rrr,rtdo;: 5.0 SMALL SITE REQUIREMENTS 33
5.5.2 Onsite/LID Techniques and Infiltration
Onsite/LID features that rely on infiltration are not appropriate for every site. Limitations and
considerations include:
• Infiltration is prohibited in the North Edmonds Earth Subsidence and Landslide Hazard
Area (ESLHA, see Appendix 13) (ECDC Chapter 19.10), upgradient of the ESLHA, and
other areas with geological instability (landslide hazard areas).
• Infiltration is prohibited in "steep slope areas" with average ground surface slopes equal
to or greater than 15 percent (see Appendix 13).
• Infiltration is prohibited within setbacks from the top of a designated landslide hazard or
steep slope areas.
• Infiltration should be applied in areas that have higher infiltration rates (including Everett
gravelly sandy loam soils).
When not restricted as noted above, infiltration should be used as much as possible in the Greater Lake
Ballinger Watershed and in all other creek basins.
Soils reports are required where infiltration is proposed, and for individual lots they must include at least
two soils logs for each proposed infiltration location. Each log shall correspond to soil conditions
extending a minimum of 4 feet depth below ground surface (6 feet for drywells). The report shall describe
the Soil Conservation Service (SCS, now called the Natural Resource Conservation Service or NRCS)
series of the soil and the textural class of each horizon through the depth of the log, and it shall include
notes of any evidence of a high groundwater table, such as mottling. Reports solely using the Unified
Soil Classification System (USCS) will not be accepted. Soils reports must be prepared by or under the
direction of a licensed onsite sewage system designer, civil engineer, engineering geologist, or
geotechnical engineer.
A soils report produced for siting and design of an onsite sewage system may also be used to satisfy this
soils report requirement, provided that (a) the depth of the soil log(s) is at least 4 feet, (b) the depth to
seasonal high water table is determined, and (c) the location of the soil logs is adequate to determine the
feasibility of the infiltration system.
Field infiltration rates can be evaluated using one of two approved methods: the USDA Textural
Classification, or the Modified Pilot Infiltration Testing (PIT) Method.
A correction factor shall be applied to the measured infiltration rate to convert to the design infiltration
rate. This correction factor shall be different based on the time of year of the test. If the test is conducted
from May I through October 31, the correction factor shall be two times the correction factor listed in
Table C-1 (USDA method) or Table C-2 (modified PIT method) in Appendix C of this Supplement. For
infiltration rates tested from November 1 — April 30, the correction factor shall be equal to the correction
factor listed in Table C-1 (USDA method) or Table C-2 (modified PIT method) in Appendix C of this
Supplement.
d/ 2�idJs?4D.0!Nla.^.',»nnru+-t�»,nv.3a.Ys,,n„�ra»,1;≠.do,: 5.0 SMALL SITE REQUIREMENTS 34
f�G 71yii(� �%�:�:���:iyC�e��nU�!`i�Z��XT,:T�To��i�►
In certain situations, BMPs that rely on infiltration are classified as Underground Injection Controls
(UICs) and may be regulated by Ecology under the UIC Program (Washington Administrative Code
[WAC] 173 218). For more information on Underground Injection Control (UIC) see the 2006 Ecology
document titled Guidance for UIC Wells that Manage Stormwater. This document is available online at
<http://www.ecy.wa.fzov/biblio/0510067.html>.
5.5.3 Bioretention Cells or Rain Gardens for Single Family Residential Sites
Rain gardens may be designed using the Rain Garden Handbook for Western Washington Homeowners
(WSU Extension 2009) (without the services of certified geotechnical professional) under the following
circumstances:
• The area of land -disturbing activity on the site is below the regulatory threshold (1 acre
of land -disturbing activity).
• The site is zoned as single-family residential.
• The rain garden(s) will receive runoff from less than 2,000 square feet of impervious
surface.
If the amount of new, replaced, or new plus replaced impervious surface area (regulated impervious area)
is 2,000 square feet or greater, the applicant may subtract the amount of regulated impervious surface area
that will be served by the rain garden(s) from the total regulated impervious surface area. The balance of
the regulated impervious surface area will still be subject to all applicable minimum requirements (even if
this balance is less than 2,000 square feet). For example, suppose a Category I single-family residential
Small Site Project has 3,700 square feet of regulated impervious surface area. An owner plans to use two
rain gardens designed per the Rain Garden Handbook for Western Washington Homeowners, each
handling 950 square feet of regulated impervious surface area. The owner must apply other BMPs to the
remaining 1,800 square feet of regulated impervious surface area to meet this minimum requirement.
If a rain garden is removed from service at any time, the impervious surface draining to it becomes new
impervious surface subject to applicable stormwater requirements.
5.6 Small Site Minimum Requirement #6 - Runoff Treatment
Runoff treatment BMPs, if applicable, must handle the water quality design storm volume or peak flow
rate using the appropriate treatment type (basic, enhanced, oil control, and/or phosphorus treatment) for
all new and replaced project pollution generating surfaces. BMPs to meet this minimum requirement must
be designed by a civil engineer.
5.6.1 Project Thresholds
Stormwater treatment facilities are required for the following Category 2 Small Site Projects:
o�jn,r-na, U.orrfa, ,,,,,,,;r<.,m,,,;,- re+,: n; r.-,, ,rle.adnr. :>.0 SMALL SITE REQUIREMENTS 35
APRL2010
fZWAMWOI e,rjq_`1iaTrr 2u
• Projects in which the total of new and replaced effective, pollution -generating
impervious surface (PGIS) is 5,000 square feet or more in a threshold discharge area
of the project.
• Projects in which the total of pollution -generating pervious surfaces (PGPS) is three-
quarters (3/4) of an acre or more in a threshold discharge area, and from which there is a
surface discharge from the site into a natural or man-made conveyance system from the
site.
That portion of any development project in which the PGIS or PGPS thresholds listed above are not
exceeded in a threshold discharge area should consider the use onsite stormwater management BMPs in
accordance with Small Site Minimum Requirement #5 to the maximum extent practicable.
5.6.2 Runoff Treatment Standards
Runoff treatment standards shall be implemented per Section 4.6, Appendix 1 of the Phase 11 Permit. In
addition, the following Edmonds -specific requirements apply.
5.6.2.1 Oil Control
All projects in areas not zoned as single family residential that collect runoff from 5 or more parking
spaces shall install oil containment catch basins, if other approved oil control systems are not employed.
The outlet pipe of these catch basins shall have a downturned 90 degree elbow to restrict the outflow of
oil and other floatables. There should be at least 9 inches of clearance between the elbow and the inside
wall of the catch basin to facilitate removal of floatables. A maintenance schedule for the removal of oil
and other floatables from these catch basins must be submitted with all plans for review.
5.6.2.2 Phosphorus Treatment
Per Section 4.6.2, Appendix 1 of the Phase II Permit, phosphorus treatment is required for projects that
discharge to a nutrient -critical receiving water. Currently, this applies to projects in the Greater Lake
Ballinger Watershed that have a total maximum daily load (TMDL) for phosphorus. Bioretention
systems with overflows (i.e., systems not providing 100 percent infiltration) should not be used in this
basin due to potential phosphorus export.
5.6.3 Runoff Treatment BMP Selection
A sampling of runoff treatment BMPs is summarized in Table 4-3, Section 4.6, with the treatment levels
and design criteria references. BMPs shall be selected from the list according to the treatment type
required, and according to the following general prioritization guidelines:
• Bioretention systems should be implemented where feasible in creek basins. If infiltration
of runoff is not feasible at the site, consider use of bioretention systems with
underdrains.
5,0 SMALL SITE REQUIREMENTS 36
APRL2010
• Onsite stormwater management measures (see Section 5.5 above) are preferred, if
feasible, over larger runoff treatment facilities. Be sure that these onsite techniques are
being applied to the maximum extent practicable before selecting runoff treatment
BMPs.
5.7 Small Site Minimum Requirement #7 - Flow Control
To reduce the impacts of stormwater runoff from impervious surfaces and land cover conversions, flow
control BMPs shall be implemented for all effective impervious surface that is new and replaced and
converted pervious surfaces. To meet this minimum requirement, flow control BMPs for Category 2
Small Site Projects must be designed by a civil engineer. As discussed in Section 5.5.1, the use of LID
techniques under Small Site Minimum Requirements #1 and #5 can reduce or eliminate the need for
compliance with this minimum requirement.
5.7.1 Project Thresholds and Applicability
The applicability of this minimum requirement is dependent upon the drainage basin of the project site.
See Chapter 2 for drainage basin classifications.
Creek or Lake Basin Projects
For Small Site Projects in Creek or Lake Basins, Edmonds -specific flow control standards must be met if
the total new and replaced impervious surface area exceeds 2,000 square feet.
Direct Discharge Basin Projects
For Small Site Projects in Direct Discharge Basins Edmonds -specific flow control standards must be met
if the total new and replaced impervious surface area exceeds 2,000 square feet. Small Site Projects in
Direct Discharge Basins can be exempt from Small Site Minimum Requirement #7 under any one of the
following circumstances:
• A quantitative offsite analysis as described in Small Site Minimum Requirement # 10 is
performed by the applicant and no unacceptable downstream issues are found.
• A quantitative offsite analysis as described in Small Site Minimum Requirement #10 is
performed by the applicant and one or more unacceptable downstream issue is
discovered. A plan is proposed by the applicant to mitigate for the unacceptable
downstream issue and the mitigation plan is approved by the Public Works Director or
designee.
Any Small Site Project in a Direct Discharge Basin can be exempt from Small Site Minimum
Requirement # 10 by meeting the applicable flow control standards for sites in Creek or Lake Basins (see
Section 5.7.2).
r.:/ 5.0 SMALL SITE REQUIREMENTS 37
APRL2010
5.7.2 Flow Control Standards
Flow control standards for Small Site Projects vary by basin type and the amount of new and replaced
effective impervious surface area and whether the project is in a Creek or Lake Basin or a Direct
Discharge Basin. These standards are summarized in Table 5-3 and 5-4 below.
Small Site Projects in Creek or Lake Basins must meet the Edmonds -specific flow control standards.
Recurrence interval flows for the developed condition shall be determined using a continuous hydrologic
model. For sites with 2,000 square feet or greater (and less than or equal to 5,000 square feet) of new plus
replaced effective impervious surface area the post -development 10-year recurrence interval flow shall
not exceed 0.25 cubic feet per second (cfs) per acre of impervious surface area.
For sites with greater than 5,000 square feet of new plus replaced impervious surface area, limits for the
post -development peak flows depend upon the BMP type selected:
• For detention BMPs:
■ Creek or Lake Basins: the post -development 2-, 10-, and 100-year
recurrence interval peak flows shall not exceed 0.07, 0.14, and 0.33 cfs per
acre of impervious surface area, respectively.
■ Direct Discharge Basins: the post -development 10-, and 100-year
recurrence interval peak flows shall not exceed, 0.25 and 0.45 cfs per acre
of impervious surface area, respectively.
• For infiltration BMPs:
■ Creek or Lake Basins: the post -development 2-, 10-, and 100-year
recurrence interval peak overflows shall not exceed 0.07, 0.25, and 0.45
cfs per acre impervious surface area, respectively.
■ Direct Discharge Basins: the post -development _10-, and 100-year
recurrence interval peak overflows shall not exceed 0.25, and 0.45 cfs per
acre impervious surface area, respectively.
There are two methods to satisfy Minimum Requirement #7 for sites subject to the Edmonds -specific
standard: the "LID Credit Option" and the "Traditional Option". Small Site Projects should strongly
consider the LID Credit Option to implement flow control, if feasible. The LID Credit Option allows an
applicant to mitigate only a portion of the new and replaced impervious surface area if only LID site
planning techniques and LID BMPs are used. The requirements for the LID Credit Option are presented
below:
• For projects that generate greater than 2,000 square feet (but less than or equal to 5,000
square feet) of new plus replaced effective impervious surface area, the greater of 1)
2,000 square feet or 2) 85 percent of new plus replaced impervious surface area is
mitigated to meet the flow control standard using LID measures.
• For projects that generate greater than 5,000 square feet of new plus replaced effective
impervious surface area, the greater of 1) 5,000 square feet or 2) 90 percent of new plus
::/ AB04140 &ten.',,;r,.,,fs.,te,,;;;,.,;;,,,,;r,.,r,:-n,r,,:nr,;Ldo;: 5.0SMALL SITE REQUIREMENTS 38
APRIL2010
replaced impervious surface area is mitigated to meet the flow control standard using
LID measures.
If the LID Credit Option is not used to meet Minimum Requirement V, the project shall meet the flow
control standard for all new and replaced impervious surface and converted pervious surface. In this case,
the "Traditional Option", larger -scale infiltration BMPs or detention BMPs may be used to provide flow
control for the areas not mitigated using LID techniques.
dj-9.041_1UOClo+admondsstm»r+s;nisu-rf,re:,,antfrc„taoi: 5.0 SMALL SITE REQUIREMENTS 39
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Table 5-4: City of Edmonds Flow Control Requirements for Small Site Projects - Direct Discharge Basinsa
> 2,000 sf
Effective Impervious Area
Numerical
For detention and infiltration BMPs:
Standard:
10-year= 0.25 cfs/acre impervious, and
100-year 0.45 cfs/acre impervious
An,exemption from these flow control standards may be allowed by the Public Works Director or
designee'if a quantitative offsite analysis per Large Site Minimum Requirement # 10 (see Section 4,10)
indicates no unacceptable downstream issue (such as a capacity or erosion issue). If an unacceptable
downstream issue is found, the applicant can either propose mitigation of the issue to the Public Works
Director or designee (whom will consider the proposal but not necessarily approve it) or design and
implement flow control measures to the Lake or Creek Basin Standard (see Section 5.7.1)..
Applies to
New and replaced impervious surface and disturbed pervious areas
Computational
Simplified Sizing Approach or continuous hydrologic modeling with MGS precipitation data b
Methods
Options to Meet Numerical Standard
LID Credit
For sites with 2,000 sf or more (but less than 5,000 sf) of effective impervious surface: If 2,000
Option (If
sf or 85% of new plus replaced impervious surface area (whichever is greater) is mitigated to
feasible)
the numerical standard using Onsite/LID techniques (Minimum Requirement #5), the
requirement is met.
For sites with 5,000 sf or more of effective impervious surface: If 5,000 sf or 90% of new plus
replaced impervious surface area (whichever is greater) is mitigated to the numerical standard
using Onsite/LID techniques (Minimum Requirement #5), the requirement is met.
Traditional
If Onsite/LID techniques are not feasible, ;larger -scale infiltration or detention may be used to
Option
mitigate capacity impact to the extent necessary, - See Section 5.7.4.
sf square feet
° MGS precipitation is the Puget East 36 precipitation time series developed for the Washington State Department of Transportation by MGS
Engineering Consultants, Inc. and available in WWHM and MGS Flood hydrologic modeling software.
n For infiltration facilities, flood frequency calculations (i.e., determination of recurrence interval peak flows) must be performed using an
approach capable of properly evaluating zero flow years as explained in Section 5.7.3.1 below
5.7.3 Flow Control Facility Sizing
5.7.3.1 Modeling Requirements
Flow control facilities must be sized to meet the standards described above using an approved continuous
simulation runoff model. Guidance on application of continuous runoff models for flow control BMP
sizing is provided in Volume III of the Stormwater Manual. Supplemental guidance is provided in this
section.
Approved continuous runoff models include the Western Washington Hydrologic Model (WWHM) and
MGS-Flood. Flow control sizing must be conducted using the "Puget East 36" precipitation time series
developed by MGS Engineering Consultants (MGS 2002). This time series is available in WWHM
(labeled "DOT data"), and is one of the default precipitation time series in MGS-Flood.
Flood frequency calculations (i.e., determination of recurrence interval peak flows) must also be
performed for infiltration facilities, using an approach capable of properly evaluating zero flow years.
d! �.r�et.toJt7t1 .,;rr;:.,ro.r;,+o„_;;unsar,.r:t s... ,u: 5.0 SMALL SITE REQUIREMENTS 41
FEURIPOW4
This is important, because infiltration facilities sized to meet the recommended flow control standards
will infiltrate approximately 98 percent of the runoff volume, resulting in an annual peak flow time series
that contains several zero or very low peak flow values.
Currently, the MGS-Flood model properly calculates flood frequency for these scenarios, but WWHM
does not. Until WWHM integrates a more flexible flood frequency routine, it is recommended that annual
peak flow datasets with multiple low or zero annual peak flow values be exported from WWHM and
analyzed using a spreadsheet method to determine flood frequency (i.e., 2-, 10-, and 100-year recurrence
interval flows). The Gringorten plotting position formula (Maidment 1993), which is non -parametric (i.e.,
does not depend on the mean, standard deviation, or skew of the data), works well for these applications.
5.7.3.2 Simplified Sizing Approach
A pre -sized approach for sizing flow control BMPs has been developed for Small Site Projects in
Edmonds with between 2,000 and 10,000 square feet of new and replaced impervious surface area. This
approach allows the designer to size selected BMPs to meet the numerical flow control standards without
the use of a continuous hydrologic model. Guidance for using the simplified sizing approach for flow
control BMPs is provided in handouts available from the City Engineering Division.
5.7.4 Flow Control BMP Selection
This section discusses infiltration and flow control BMPs. If onsite/LID techniques are not used, flow
control can be achieved using larger -scale infiltration or detention facilities.
5.7.4.1 Larger Scale Infiltration
If LID techniques are not feasible to meet this minimum requirement, larger -scale, centralized infiltration
facilities may be used (as opposed to the small and distributed infiltration facilities associated with LID).
Infiltration is a preferred method of stormwater flow control relative to detention, but only in certain areas
of Edmonds where infiltration of large amounts of runoff in a concentrated area will not contribute to
flooding or offsite slope stability problems. The feasibility of infiltration as a means to meet stormwater
flow control and treatment requirements will depend upon specific project site locations due to the
variations in soil and topography across the City. See Chapter 5.5.2 for guidelines pertaining to the
applicability and testing required for implementing infiltration for flow control.
A sampling of infiltration BMPs for flow control is summarized in Table 5-5.
5.7.4.2 Detention
If infiltration BMPs are not feasible to meet flow control requirements for a site, detention BMPs can be
used. The minimum orifice size for a detention control structure is one-half inch (1/2 inch). Table 5-4
summarizes some available detention BMPs for flow control.
�1 OadJ9l40l70tI edrnunJs s[ennrc.;;o� sr„,vlprrnrnf fieal dax;
5.0 SMALL SITE REQUIREMENTS 42
APPJL2010
5.8 Small Site Minimum Requirement #8 - Wetland Protection
Section 2.5.8 of Volume I of the Stormwater Manual has guidance on meeting Small Site Minimum
Requirement #8. Refer also to ECDC Chapter 23.40.
5.9 Small Site Minimum Requirement #9 - Operation and Maintenance
BMPs shall be maintained in accordance with the provisions in Chapter 7 of this Supplement.
Table 5-5: Flow Control Best Management Practices
BMP #
Flow Control BMP
I Reference(s)/Design Guidance
Larger -Scale Infiltration BNIPs
T7.10
Infiltration Basins
Volume III Stormwater Manual
T7.20
Infiltration Trenches
Volume III Stormwater Manual
T7.30
Infiltration Vaults
Volume III Stormwater Manual
Gravelless Chambers
Kitsap County Stormwater Management Design Manual
(Kitsap County 1997) ;
Detention BMPs
Detention Ponds
Volume III Stormwater Manual
--
Detention Tanks
Volume III Stormwater Manual
--
Detention Vaults
Volume III Stormwater Manual
--
Use of Parkin Lots for Detention
Volume III Stormwater Manual
Use of Roofs for Detention
Volume III Stormwater Manual and City Building Division
Stormwater Manual = Stormwater Management Manual for Western Washington.
5.10 Small Site Minimum Requirement #10 - Offsite Analysis and
Mitigation
Offsite analyses assess upstream and downstream conditions, including conveyance capacity limitations
or erosion potential in the downstream system. A qualitative or quantitative analysis may be required by
the Public Works Director or a designee when there is a potential for downstream flooding or erosion. If a
problem is found, mitigation is required to prevent worsening the problem.
The existing or potential impacts to be evaluated and mitigated as part of any offsite/downstream analysis
shall include:
• Conveyance system capacity problems.
• Localized flooding.
• Wetlands impacts such as siltation or other damage.
• Upland erosion impacts, including landslide hazards.
• Stream channel erosion in the channel or at the outfall location.
i at so-r tr'rr^mr,n;fss!n»,,,,,,,.,, ,,; rar» ..,rt-,.,,.. <>;: M SMALL SITE REQUIREMENTS 43
APRIL2010 EWONDS STORMWATERCODE &FREINENT
• Locations where surface water enters and exits the site.
5.10.1 Qualitative Analysis
If required, project applicants shall submit a qualitative analysis of each upstream drainage system
entering a site (run-on) and each downstream drainage system leaving a site (runoff). The upstream
analysis shall identify and describe points where water enters the site and the tributary area. The
qualitative analysis shall extend downstream for the entire flow path, from the project site to the receiving
water, or up to one -quarter mile, whichever is less. A map defining the onsite and offsite areas that
contribute stormwater runoff to the site shall be provided by the applicant. The map shall be prepared
with a defined scale.
Upon review of this analysis, the Public Works Director or Designee may require a qualitative analysis
extending further downstream, mitigation measures deemed adequate to address the problems, or a
quantitative analysis, depending upon the presence of existing or predicted flooding, erosion, or water
quality problems, and on the proposed design of the onsite drainage facilities.
5.10.2 Quantitative Analysis
If required, a civil engineer or professional land surveyor must field survey all existing storm drainage
systems downstream of the project site for a minimum distance of 1/4-mile from the point of connection
to the existing public drainage system. If the ultimate discharge point is to Puget Sound via a culvert
owned by BNSF Railway, the analysis must be followed through the drainage system all the way to Puget
Sound.
The goal of the inspection and analysis is to evaluate whether the capacity of the drainage system(s) is
adequate to handle the existing flows, flows generated by the project, flooding problems, erosion damage
or potential, amount of freeboard in channels and pipes, and storage potential within the system. All
existing and proposed offsite surface water conveyance systems shall be sized to convey flows without
surcharging the City's storm system or the BNSF culvert under the railroad tracks (if applicable) during
the 50-year recurrence event. If the project is in the ESLHA (see Appendix B), contact the City
Engineering Division for requirements.
If a capacity problem, streambank erosion problem, or other problem with the downstream system is
encountered in the analysis, the flow from the project shall be restricted per Minimum Requirement #7 —
Flow Control or other mitigation may be proposed by the applicant subject to review by the Public Works
Director or designee..
5.11 Small Site Minimum Requirement #11 -Financial Liability
A performance bond shall be posted as a financial guarantee for required stormwater systems and BMPs
related to Construction Stormwater Pollution Prevention Plans for the following Small Site Projects:
::7 0s041so1?t?O,�d,,,,,,,,.:.,ra,n,,;nvs,;;,,•,e-rnr:rrfr,r.u:.ao,: 5.0SMALL SITE REQUIREMENTS 44
APRIL2010
• Single family residential projects located within Critical Areas (including the ESLHA)
and critical area buffers.
• Multi -family residential and commercial projects
• Subdivisions involving 7,000 square feet or more of land disturbing activity.
, Off0*4.1D-:7Ut7arr,:::;<srm::;:r;;:e�su;:Fgra+�r:tfin,+trio:: 5.0 SMALL SITE REQUIREMENTS 45
FFUN—ol1 7• • A ••� •.� •• 0
6.0 Minor Site Requirements
Minor Site Projects must comply with Minor Site Construction Stormwater Pollution Prevention
Practices.
6.1 Minor Site Minimum Requirement - Construction Stormwater
Pollution Prevention Plan
All new development, redevelopment, and construction projects are responsible for preventing erosion
and discharge of sediment and other pollutants into the City's municipal storm drainage system and
receiving waters. For Minor Site Projects, a shorter Minor Site Construction Stormwater Pollution
Prevention Plan can be prepared.
The following BMPs must be applied:
For Sites with Clearing and Grading:
• Plan and implement proper clearing and grading of the site. It is most important to clear
only the areas needed, preserve vegetation, protect soils, and keep exposed areas to a
minimum.
• Re -vegetate, cover with straw, or otherwise stabilize all exposed soil as soon as possible.
• Locate any soil piles away from drainage systems. Temporary stabilization or covering of
soil piles must occur at the end of each work day or other best management practices
must be implemented to prevent discharges of soil or sediment to the City's storm
drainage system or surface waters or adjacent properties.
• Provide storm drain inlet protection within the project area and downstream as required
to prevent sediment and pollutants from entering the City's storm drainage system. Inlet
protection must be maintained throughout the life of the project and not cause any
discharge of sediment or localized flooding issues. All inlet protection must be removed
and properly disposed of immediately after project completion.
Additional Requirements for Underground Utility Projects
• Areas within the paved City right-of-way that have soil or sediment deposited during
construction must be swept or otherwise removed and properly managed at the end of
each work day, unless other arrangements have been made with the City's Engineering
Division.
• Water -tight trucks must be used to transport any saturated soils from the site.
• All contractors must a have a spill containment and clean up kit onsite at all times that is
compatible with the applicable pollutants
• All utility workers must exercise proper pollution prevention techniques during all
construction activities including the proper storage of any hazardous materials.
q, a,v-as,rn-nerved»„»u<,toy»;r,-,:: ,.nnrjar,,,,.,,traal.d,>,: 6.0 MINOR SITE REQUIREMENTS 47
APPJL2010
De -watering shall be controlled per Element # 10, Section 3.2.3, Volume II, of the Stormwater Manual.
All discharges into the City -owned storm system or water of the state shall be in compliance with ECC
7.200, Illicit Discharges. Sanitary sewer discharges must have the approval from the Edmonds Treatment
Plant and Engineering Division.
See Volume II of the Stormwater Manual for additional guidance on the selection and design of
construction -phase best management practices designed to prevent erosion and sedimentation.
::, nl;isl-w-run),-_,,,,,,,.,,,r, „ :,,,,"..,,.._�,,.(�o;; U MINOR SITE REQUIREMENTS 48
APRIL2010
7.0 Operation and Maintenance
Requirements
See the LID Manual and Volume III and Volume V of the Stormwater Manual for guidance on operation
and maintenance measures for BMPs.
7.0 OPERATION AND MAINTENANCE REQUIREMENTS 49
a=
8.0Terminology
This section explains terms used in this Supplement. Additional definitions for stormwater requirements
are found in ECDC 18.30.010 and the Stormwater Manual.
Basin plan: A plan or study to manage the quality and/or quantity of stormwater or water bodies within a
watershed that has been formally adopted by the City of Edmonds.
Civil engineer: A professional engineer licensed in the state of Washington in civil engineering.
Direct discharge basin: A drainage basin that discharges runoff directly to Puget Sound via a pipe
system, ditch, or other direct means without first entering a creek or other water body.
Disturbed pervious surface: Any part of a pervious area that is disturbed during a development or
redevelopment project, but remains pervious after the project is completed.
Effective impervious surface: Those impervious surfaces that are connected via sheet flow or discrete
conveyance to the City's municipal separate storm sewer system.
Grade: The slope of a surface such as a road, channel, or natural ground.
Grading: Any one or a combination of the following: excavating, filling, or disturbance of that portion of
the soil profile that contains decaying organic matter.
Greater Lake Ballinger Watershed: That portion of the City of Edmonds that discharges stormwater
runoff directly or indirectly into Chase Lake, Hall Creek, or Lake Ballinger.
Mitigation: Implement best management practices to reduce or eliminate the impacts of development on
the City's stormwater system and receiving waters.
Nutrient -critical receiving water: Any surface water or water segment listed as Category 5 (impaired)
under Section 303(d) of the Clean Water Act for total phosphorus through the State of Washington's
Water Quality Assessment program and approved by EPA. Lake Ballinger is considered a nutrient -critical
receiving water.
Pollution -generating impervious surfaces (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 defined in the Stormwater Manual (Volume I, Glossary- p. 26)
• Storage of erodible or leachable materials, wastes, or chemicals
• Receiving direct rainfall or the run-on or blow-in of rainfall.
;:j A,904Jro-un„>,;,,,,,f,zo»,;r,,f,r,:,;,;-J,r-„-rti>;;::dna 8.OTERMINOLOGY 51
F
Erodible or leachable materials, wastes, or chemicals are those substances which, when exposed to
rainfall, measurably alter the physical or chemical characteristics of the rainfall runoff. Examples include
erodible soils that are stockpiled, uncovered process wastes, manure, fertilizers, oily substances, ashes,
kiln dust, and garbage dumpster leakage.
Metal roofs are also considered PGIS unless they are coated with an inert, non -leachable material (e.g.,
baked -on enamel coating). A surface, whether paved or not, shall be considered subject to vehicular use if
it is regularly used by motor vehicles. The following are considered regularly -used surfaces:
• Roads
• Unvegetated road shoulders
• Bike lanes within the traveled lane of a roadway
• Driveways
• Parking lots
• Unfenced fire lanes
• Vehicular equipment storage yards
• Airport runways.
The following are not considered regularly used surfaces:
• Paved bicycle pathways separated from and not subject to drainage from roads for motor
vehicles
• Fenced fire lanes
• Infrequently used maintenance access roads
• Sidewalks separated from and not subject to drainage from roads for motor vehicles.
Pollution -generating pervious surfaces (PGPS): Any non -impervious surface subject to application of
pesticides or fertilizers, or loss of soil. Typical PGPS include lawns, landscaped areas, golf courses, parks,
cemeteries, and sports fields.
Predevelopment condition: For sites that meet or exceed the regulatory threshold, and exceed the
thresholds that require flow control in Table 4.2, Section 4.7 of the Phase II Permit, shall use the
appropriate predevelopment condition described in said section of the Phase II Permit.
Regulatory Threshold: When referring to site size, those that have one acre or more of land -disturbing
activity, including projects less than one acre of land -disturbing activity that are part of a larger common
plan of the development or sale.
Total Maximum Daily Load (TMDL): A TMDL is a calculation of the maximum amount of a pollutant
that a water body can receive and still meet water quality standards, and an allocation of that amount to
:ry r0a?aDr>rut,„;,,,,:f:.tir,,,;,n,.,.,.,;;;r,r„i..r<re.,t.da;; 8,OTERMINOLOGY 52
APPoL2010
the pollutant's sources. A TMDL is the sum of the allowable loads of a single pollutant from all
contributing point and nonpoint sources. The calculation must include a margin of safety to ensure that
the water body can be used for the purposes the state has designated. The calculation must also account
for seasonable variation in water quality. Water quality standards are set by states, territories, and tribes.
They identify the uses for each water body, for example, drinking water supply, contact recreation
(swimming), and aquatic life support (fishing), and the scientific criteria to support that use. The Clean
Water Act, section 303, establishes the water quality standards and TMDL programs. In Washington
State, the Department of Ecology establishes water cleanup plans to guide TMDL implementation.
Lijas-n !enotrrt,r,,,,,,;is.,zo,,,, ;,,r,,,,a„rr,,,:. o:: 8.01EKMINOLOGY 53
RIIIVWXJI#� 'KIWWAZM>
9.0 References
Ecology. 2005. Stormwater Management Manual for Western Washington. 5 vols. Ecology Publications 05-
10-029 through 05-10-033. Washington State Department of Ecology, Water Quality Program. February
2005.
Ecology. 2009. Western Washington Phase II Municipal Stormwater Permit. National Pollutant Discharge
Elimination System and State Waste Discharge General Permit for Discharges from Small Municipal
Separate Storm Sewers in Western Washington. Modified June 17 2009.
Herrera. 2009. Basis for Updated Stormwater Management Standards for New Development and
Redevelopment Projects in the City of Edmonds. Technical Memorandum prepared for City of Edmonds.
Herrera Environmental Consultants. July 10, 2009.
King County. 2009. King County Surface Water Design Manual. King County Department of Natural
Resources and Parks, Water and Land Resources Division.
Kitsap County. 1997. Kitsap County Stormwater Management Design Manual,
Maidment, D.R. 1993. Handbook of Hydrology. McGraw-Hill, Inc., New York.
PSAT and WSU. 2005. Low Impact Development: Technical Guidance Manual for the Puget Sound. Puget
Sound Action Team, Olympia, Washington and Washington State University Pierce County Extension,
Tacoma, Washington.
SCS. 1983. Soil Survey of Snohomish County Area, Washington. U.S. Soil Conservation Service, July 1983.
Seattle. 2009. Stormwater Manual, Volume 3, Stormwater Flow Control & Water Quality Treatment
Technical Requirements Manual, Director Rules 2009-005 SPU, 17-2009 DPD, November 2009.
WSU Extension. 2009. Rain Garden Handbook for Western Washington Homeowners: Designing your
Landscape to Protect our Streams, Lakes, Bays, and Wetlands. Washington State University Pierce County
Extension, Tacoma, Washington.
d/ ...... ,;;;;-ar»aar rr„r.av;: 9.0REFERENCEs 55
APRIL2010
Appendix A - Synopsis of the Western
Washington Phase II Municipal
Stormwater Permit
The City of Edmonds has a population of approximately 41,000 and owns and operates a municipal
stormwater system that is separate from the sanitary sewer system. As such, the City is regulated by the
Washington Department of Ecology (Ecology) to comply with the provisions of the Western Washington
Phase II Municipal Stormwater Permit (Phase II Permit) for stormwater entering into and discharging
from this municipal stormwater system.
The complete text of the NPDES Phase Il Permit is available on Ecology's website I . This section
provides a summary of permit provisions that relate to the City's management of stormwater.
Section S5.C.4 of the Phase II Permit prescribes how the City should regulate runoff from new
development, redevelopment, and construction sites. For projects that involve I acre or more of land -
disturbing activity ("regulatory threshold"), the Phase 11 Permit requires compliance with certain
minimum technical requirements. The City has adopted these minimum technical requirements from
Appendix 1 of the Phase II Permit for sites that meet or exceed the regulatory threshold (Large Site
Projects). Appendix 1 of the Phase 1I Permit also modifies or changes portions of the Stormwater Manual
related to certain minimum requirements and exceptions/variances. These changes/additions have been
incorporated into ECDC Chapter 18.30 and this Supplement and will be noted as such.
The Phase II Permit requires the City of Edmonds stormwater regulations to have a site planning process
and BMP selection and design criteria that, when used to implement the Permit's minimum requirements
(Appendix 1), will protect water quality, reduce the discharge of pollutants to the maximum extent
practicable and satisfy the State requirement under Chapter 90.48 RCW to apply all known, available and
reasonable methods of prevention, control and treatment (AKART) prior to discharge. The Phase II
Permit also states that Permittees who choose to use the site planning process and BMP selection and
design criteria in the Stormwater Manual, or an equivalent manual approved by Ecology under the Phase I
Permit, may cite this choice as their sole documentation to meet this requirement. For Large Site Projects,
the City of Edmonds has chosen to use the site planning process and BMP selection and design criteria in
the Stormwater Manual, as modified by other equivalent manuals approved by the Department under the
Phase I Permit. This Supplement provides details on implementing those requirements in Edmonds on
sites of all sizes.
I <http://www.ecy.wa.gov/programs/wq/stormwater/municipal/phaseIIww/wwphi ipermit.html>
2 Including projects less than 1 acre of land -disturbing activity that are part of a larger common plan of the
development or sale.
%:j C,7J,?.11P-C!?[7::dno»;fs siom:�ra; err .p '.. 'frr•.v:.don
APPENDIXA 57
For sites that fall below regulatory threshold, the Phase II Permit gives regulated jurisdictions a choice: 1)
either continue to apply the local stormwater requirements in effect at the time of permit issuance
(February 2007) or 2) apply the same minimum requirements as for the Large Site Projects. The City of
Edmonds has chosen to continue using the current stormwater requirements for projects that fall below
the regulatory threshold but to clarify the requirements and provide more choices (Best Management
Practices or BMPs) for achieving these requirements including the use of Low Impact Development
(LID) techniques. The most recent stormwater requirements for City of Edmonds were adopted in March
1995. The field of stormwater management has advanced substantially in the almost 15 years since
adopting these requirements. This Supplement incorporates advancements that aim to strike a balance
between encouraging desirable development and providing environmental protection.
Most development projects in Edmonds will only be subject to the Small Site Project or Minor Site
Project requirements since very few parcels in the City are 1 acre or greater. The Small Site Project
requirements are tailored to primarily address stormwater management issues related to development and
redevelopment of single family residential projects in the urban environment of Edmonds. These projects
make up the majority of the development activity in the City. These requirements apply to non -single-
family residential projects as long as these sites meet the definition of a Small Site Project.
d'ir:rnatan-erut.............. ......:,,,.,s,n;�re�,,,..r..,...,..,i<,,. APPFND(XA 58
AML2010
Appendix B - Supplemental Technical
Information
Edmonds Watersheds
Broadly speaking, the stormwater runoff in Edmonds either travels west to Puget Sound (either via a
creek or directly piped) or to the east to Lake Ballinger or Hall Creek, which discharges to Lake Ballinger
(Figure B-1).
Lake Ballinger has a Department of Ecology -mandated limit on the amount of phosphorus (a chemical
element) that can enter the lake. Excess phosphorus from stormwater runoff is the primary driver for
unwanted algae blooms in the lake. Due to this limit (or TMDL-total maximum daily load) for total
phosphorus imposed by Ecology, special water quality treatment requirements are included under
Minimum Requirement # 6 or Small Site Minimum Requirement #6. Private properties on the lake
experience periodic flooding during large storm events. The City of Edmonds in conjunction with its
watershed partners (Cities of Lynnwood, Mountlake Terrace, Shoreline and Lake Forest Park and
Snohomish County) have developed a Strategic Action Plan for the greater Lake Ballinger/McAleer
Creek Watershed to begin to address flooding a water quality issues in the basin.
Edmonds Soils and Slopes
Edmonds is fortunate to be located adjacent to Puget Sound and possess topography that facilitates
desirable views. The underlying soils and relatively steep slopes, however, complicate the application
stormwater management techniques.
Prior to logging and subsequent development of the Edmonds area, trees and the forest duff layer above
the soil surface (consisting or primarily or needles, leaves, branches, bark, and stems, in various stages of
decomposition) covered the City. The tree canopy stored a large amount of the rainfall and the duff layer
acted as a giant sponge storing and soaking up rainfall. Virtually all of this rainfall was stored in the tree
canopy and the duff layer, absorbed by the roots of trees or other vegetation, or slowly moved through the
soil on top of less permeable layers, forming the headwaters of Edmonds creeks. There was little or no
"runoff' from these forests.
With logging came the elimination of the majority of the tree canopy and the duff layer and the
elimination of the associated water -holding capacity. The soils that remain, in the vast majority of the
City, once the forest is removed, is till or hard pan that does not store or absorb the rainwater. Rain water,
instead, primarily becomes runoff that flows off the impermeable soils and the hardscape or impermeable
surfaces built during development (roofs, parking lots, roads, sidewalks, etc.). This runoff is collected in
ditches or pipes and these concentrated flows are discharged to creeks, Lake Ballinger, or Puget Sound
d/ APPENDIX B 59
APRL2D10
(without treatment). By planting trees and using compost -amended soils the water -holding capacity of a
developed site can improve, to a limited extent.
The U.S. Department of Agriculture, Natural Resource Conservation Service or USDA-NRCS (formerly
the Soil Conservation Service) has mapped soil types in Edmonds (Figure B-2). Based on the most recent
data from 1983, approximately 82 percent of the land area in Edmonds is underlain by Alderwood series
soils (till or hardpan). Approximately 12 percent of Edmonds has Type A or permeable soils. Given this
soil regime, infiltration, and low impact development techniques that primarily rely on infiltration, are
challenging to implement in the majority of Edmonds.
Approximately 25 percent of the land area of Edmonds has a slope of 15 percent or greater or is in an
Earth Subsidence and Landslide Hazard Area (the Meadowdale area in the northernmost portion of the
City) (Figure B-3). Geologic hazards in these areas can be increased when stormwater runoff from
impervious surfaces percolates into the soil.
Planners and designers should use the information on Figures B-2 and B-3 as guidelines since soil and
slope conditions on an individual parcel may vary from the information presented. Nevertheless, careful
consideration should be applied when infiltration or LID techniques relying on infiltration are considered
in areas of till soils or steep slopes.
There are areas in the City where infiltration and low impact development techniques that rely on
infiltration are feasible based on favorable soil type and relatively flat slopes; Southwest Edmonds, for
example. In these areas infiltration and LID techniques are encouraged over more conventional
stormwater management techniques.
<i, .^,.r_;aru e?tl?) ed„,,,,.;,:,.„»:y,;rr,.,;;r;..,,,.r...,,;r.o,;r APPENDIX I3 60
�FIG U s s
r
CITY OF EDMONDS
WATERSHEDS
goDeer Creek Ilk- 5, Perrinville
Edmonds Marsh Puget Sound
� Edmonds Way i,, s r Puget Sound Piped
Fruitdale Shell Creek
Good Hope Pond Shellabarger
Halls Creek
Southwest Edmonds A
Hindley Creek
j� '� Southwest Edmonds B
;lake Ballinger
' Stilthouse Creek
,7m Lund's Gulch
Talbot Park A
' Tf
�a, MeadowdaleA�
Talbot Park B
j'it; Meadowdale ByP
Terrace Creek
Northstream
;Westgate Pond
Outfall Creek
1 Willow Creek
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4,000 6,000 8,000
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/°c`x9° Soil Map Units 4SCS 1953
LEGEND
--- City of Edmonds Boundary
Soil Map Units
SOIL NAME
Alderwood Gravelly sandy loam, 2-8 % slopes
Alderwood Gravelly sandy loam, 8-15 % slopes
Alderwood Gravelly sandy loam, 15-25 % slopes
Alderwood Urban land complex, 2-8 % slopes
Alderwood Urban land complex, 8-15 % slopes
Alderwood-Everett Gravelly sandy loams,'25-70
-
Custer fine sandy loam
Everett gravelly, sand loam, 0-8 % slopes ;
Everett gravelly sand loam, 8-15 % slopes
ens%
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Fluvaquents, tidal
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Pits
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Appendix C - Approved Methods for
Obtaining Design Infiltration Rates
This appendix supplements the infiltration requirements found in Sections 4.5.2 and 5.5.2 of this
document. The Unites States Department of Agriculture (USDA) textural classification or the modified
pilot infiltration test (PIT) may be used to obtain a site design infiltration rate. The City may require the
modified PIT procedure on large sites and/or on sites that demonstrate considerable variability between
samples using the USDA textural classification.
USDA Textural Classification
Short-term infiltration rates may be estimated from soil grain size distribution (gradation) data using the
USDA textural analysis approach. This estimated short-term infiltration rate can then be converted to a
design infiltration rate using information in the Stormwater Manual.
The soil particle size distribution and soil texture analysis to obtain a short-term infiltration rate should be
implemented in accordance with the USDA (Soil Survey Manual, USDA, October 1993, Chapter 3 pages
60-65).
This manual only considers soil passing the #10 sieve (2 mm) (U.S. Standard) to determine percentages
of sand, silt, and clay for use in Figure C-1. These short-term (field) infiltration rates should be used with
the required correction factors in the Stormwater Manual to obtain the design (long-term) infiltration rates
(Table C-1 below from the Volume 3 of the Stormwater Manual). With prior approval by the City of
Edmonds Engineering Division, the correction factors may be reduced (to a minimum of 2.0) if there is
little soil variability and there will be a high degree of long-term facility maintenance and adequate
pretreatment to reduce total suspended solids in influent stormwater.
Table C-1: Recommended Infiltration Rates Based on USDA Soil Textural Classification
Estimated Design
*Short -Term
Correction
(Long-term) Infiltration
Infiltration Rate
Factor
Rate
(in./hr)
CF
(in./hr)
Clean sandy gravels and gravelly sands (i.e.,
101,
2
10
90% of the total soil sample is retained in the
#10. sieve)
Sand
8
4
2
Loamy Sand
2
4
0.5
Sandy Loam
1
4
0.25
Loam
0.5 <
4
0.13
Source: Stormwater Management Manual for Western Washington (Ecology 2005).
* From WEF/ASCE (1998).
9/ r1-04140000 sio-," erAPPENDIX 67
APRIL2010 EDVK NDSSTOR "TER CODE SLPPLEMENT
Figure C-1: Soil Classification Based on the Textural Triangle
Textural Triangle U.S.D.A.
wox clay
90
IQ
6o
so
70 Cla
30
/. 60
- 40
• �
4
ono
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lay
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/
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60
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Silt
90
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Slit
100% sand 90 60 70 60 60
40 30
20 10 100x 611t
�..� Pwe**t sari
Shaded area is applicable for design of infiltration BMPs
Modified Pilot Infiltration Testing (PIT) Method
The Pilot Infiltration Test (PIT) consists of a relatively large-scale infiltration test to better measure
infiltration rates for design of stormwater infiltration facilities. The PIT reduces some of the scale errors
associated with relatively small-scale double ring infiltrometer or "stove -pipe" infiltration tests. It is not a
standard test but rather a practical field procedure based on the methods recommended by Ecology's
Technical Advisory Committee. Correction factors must be applied to the infiltration rate measured using
PIT to establish a design infiltration rate for BMP sizing.
APPENDIXC 68
IT i ell 1
For infiltration basins, there shall be one test pit per 5,000 square feet of basin infiltrating surface with a
minimum of two per basin, regardless of basin size. For bioretention facilities and permeable pavement
facilities, there shall be one test pit per 5,000 square feet of contributing area. For infiltration trenches,
there shall be one test pit per 50 feet of trench length. For drywells, there shall be at least one test pit per
well.
Prepare detailed logs for each test pit and a map showing the location of the test pits. Logs must include
the depth, depth to water, evidence of seasonal high groundwater elevation, existing ground surface
elevation, proposed facility bottom elevation, and presence of stratification that may impact the
infiltration design.
PIT reports shall be stamped by a Professional Engineer or prepared by an onsite wastewater treatment
designer licensed with the State of Washington.
Infiltration Test
• Excavate the test pit to the depth of the bottom of the proposed infiltration facility. Lay
back or otherwise stabilize the slopes sufficiently to avoid caving and erosion during the
test. A pre -formed, bottomless box made of plywood can be useful for limiting the size
of the excavation.
• The size of the bottom of the test pit should be as close to the size of the planned
infiltration facility as possible, but not less than 2 feet by 2 feet. Where water
availability is a problem, smaller areas may be considered as determined by the site
professional.
• Accurately document the size and geometry of the test pit.
• Install a device capable of measuring the water level in the pit during the test. This may
be a pressure transducer (automatic measurements) or a vertical measuring rod
(minimum 5 feet long) marked in half -inch increments in the center of the pit bottom
(manual measurements).
• Use a rigid 6-inch-diameter pipe with a splash plate or some other device on the bottom
of the pit to reduce side -wall erosion and excessive disturbance of the pit bottom.
Excessive erosion and disturbance may result in clogging and yield lower than actual
infiltration rates.
• Add water to the pit at a rate that will maintain a water level between 3 and 4 feet above
the bottom of the pit.
Note: A water level of 3 to 4 feet provides for easier measurement and flow stabilization control.
However, the depth should not exceed the proposed maximum depth of water expected in the completed
facility.
Every 15 to 30 minutes, record the cumulative volume and instantaneous flow rate in gallons per minute
necessary to maintain the water level at the same point (between 3 and 4 feet) on the measuring rod. This
d/ APPENDIX 69
APRIL2010
can best be accomplished with an in -flow meter. It can also be accomplished by timing how long it takes
to fill a known volume such as a 5-gallon bucket.
Add water to the pit until 1 hour after the flow rate into the pit has stabilized (constant flow rate) while
maintaining the same pond water level (usually 17 hours).
After the flow rate has stabilized, turn off the water and record the rate of infiltration in inches per hour
using the pressure transducer or measuring rod, until the pit is empty.
Data Analysis
Calculate and record the infiltration rate in inches per hour until 1 hour after the flow has stabilized.
Note: Use statistical/trend analysis to obtain the hourly flow rate when the flow stabilizes. This would be
the lowest hourly flow rate.
Apply Correction Factor
The infiltration rate obtained from the PIT shall be considered to be a short-term rate. This "short-term"
rate must be reduced through correction factors to account for site variability and number of tests
conducted, degree of long-term maintenance and influent pretreatment/control, and potential for long-
term clogging due to siltation and bio-buildup. The corrected infiltration rate is considered the "long-
term" or "design" infiltration rate and is used for all BMP sizing calculations.
One exception to the requirement for a correction factor applies to bioretention facilities. Specifically,
when imported bioretention soil is used, no correction factor is required for the infiltration rate of the
underlying native soil.
A minimum infiltration rate correction factor of 2.0 is required for all facilities designed using the
PIT method. Correction factors greater than 2.0 should be considered for situations where long-term
maintenance will be difficult to implement, where little or no pretreatment is anticipated, or where site
conditions are highly variable or uncertain. These situations require the use of best professional judgment
by the site engineer and the approval by the City of Edmonds. The typical range of correction factors to
account for these issues, based on Ecology's guidance, is summarized in Table C-2. In no case shall the
design infiltration rate exceed 10 inches per hour.
Table C-2: Correction Factors to be Used With In -Situ Infiltration Measurements to Estimate Long -Term
Design Infiltration Rates
Issue
Partial Correction Factor
Site variability and number of locations tested
CFv =1.5 to 6
Degree of long-term maintenance to prevent siltation and bio-buildupCFm
= 2 to 6
Degree of influent control to prevent siltation and bio-buildup
CH = 2 to 6
Total Correction Factor (CF) = CFv + CFm + CF;
ct OwV414)-000,'cAf f'ENDIXc 70
APRUD10
The following discussions are to provide assistance in determining the partial correction factors that may
apply.
Site variability and number of locations tested — The number of locations tested must be capable of
producing a picture of the subsurface conditions that fully represents the conditions throughout the facility
site. The partial correction factor used for this issue depends on the level of uncertainty that adverse
subsurface conditions may occur. If the range of uncertainty is low —for example, conditions are known
to be uniform through previous exploration and site geological factors —one pilot infiltration test may be
adequate to justify a partial correction factor at the low end of the range. If the level of uncertainty is high,
a partial correction factor near the high end of the range may be appropriate. This might be the case where
the site conditions are highly variable due to a deposit of ancient landslide debris, or buried stream
channels. In these cases, even with many explorations and several pilot infiltration tests, the level of
uncertainty may still be high. A partial correction factor near the high end of the range could be assigned
where conditions have a more typical variability, but few explorations and only one pilot infiltration test
is conducted. That is, the number of explorations and tests conducted do not match the degree of site
variability anticipated.
Degree of long-term maintenance to prevent siltation and bio-buildup — a partial correction factor at
the low end of the range may be used if there is certainty that maintenance requirements will be carried
out consistently. If there is a high degree of uncertainty that long-term maintenance will be carried out
consistently, or if the maintenance plan is poorly defined, a partial correction factor near the high end of
the range may be justified.
Degree of influent control to prevent siltation and bio-buildup — A partial correction factor near the
high end of the range may be justified under the following circumstances:
• If the infiltration facility is located in a shady area where moss buildup or litter fall
buildup from the surrounding vegetation is likely and cannot be easily controlled
through long-term maintenance
• If there is minimal pre-treatment, and the influent is likely to contain moderately high
TSS levels.
If influent into the facility can be well controlled such that the planned long-term maintenance can easily
control siltation and biomass buildup, then a partial correction factor near the low end of the range may be
justified.
The determination of long-term design infiltration rates from in -situ infiltration test data involves a
considerable amount of engineering judgment. Therefore, when reviewing or determining the final long-
term design infiltration rate, the local jurisdictional authority should consider the results of both textural
analyses and in -situ infiltration tests results when available.
n,rr.f sD-tt[tr7,..,»;,ms,ro,,,;m,:nr:,,;,•.,errmrm.rna.r!s:: APPENDIXC 71
APRIL2010
Example:
The area of the bottom of the test pit is 8.5 feet by 11.5 feet.
ar • r. • + - ••r •• a e
Water flow rate was measured and recorded at intervals ranging from 15 to 30 minutes throughout the
test. Between 400 minutes and 1,000 minutes, the flow rate stabilized between 10 and 12.5 gallons per
minute or 600 to 750 gallons per hour, or an average of (9.8 + 12.3) / 2 = 11.1 inches per hour.
Applying at least the minimum correction factor of 2.0 (example only) the design long-term infiltration
rate becomes 5.6 inches per hour, anticipating adequate maintenance and pre-treatment.
::/.^sv9tJD..lt7,ar.,,r,.,,,.,;to», ..,.rs.ar,=»c:tfirs;:.dac APPENDIXc 72
STATE OF W ASHINGTON,
COUNTY OF SNOHOMISH
r enr
SUMMARY OF ORDINANCE NO.'3792
o the City of Edmonds, Wash.ngton
On the 20th day of April, 2010, the City Council of the City of
Edmonds, passed Ordinance No. 3792.: A summary of the content
of said ordinance, consisting of the title, provides as follows:
AN ORDINANCE OF THE CITY OF EDMONDS, WASHINGTON,
REPEALING AND REENACTING CHAPTER 18.30 STORM WA-
TER MANAGEMENT OF THE EDMONDS COMMUNITY D V L-
:C D OVIDING.FOR THE APPLICATION OF EX-
ISTING CHAPTER 18.30 TO VESTED PERMITS, AND FIXING A
E TIMWHEN THE SAME_ SHALL BECOME EFFECTIVE.
The full text of this Ordinance will be mailed upon request.
DATED this 21st day of April; 2010.
CITY CLERK, SANDRA S.:CHASE
Published: April 26. 2010. -
R
APR 2 9 2010
s7 l tp ? a
Account Name: City of Edmonds
Affidavit of Publication
S.S.
The undersigned, being first duly swom on oath deposes and says that she is Principal Clerk of
THE HERALD, a daily newspaper printed and published in the City of Everett, County of
Snohomish, and State of Washington; that said newspaper is a newspaper of general
circulation in said County and State; that said newspaper has been approved as a legal
newspaper by order of the Superior Courtof Snohomish County and that the notice
Summary of Ordinance No. 3792
Storm Management
a printed copy of which is hereunto attached, was published in said newspaper proper and not in
supplement form, in the regular and entire edition of said paper on the following days and
times, namely:
April 26, 2010
and that said newspaper was
and swom to before me this
day of April, 2010
Notary Public in and r the tate ot�
County. ;
i
i
_ am_.
Account Number. 101416 Y,
subscribers during all of said period.
saw
26th
o
` b7 Wumber: 0001693595
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