161205 MAD (E72-SWM Form).pdfSTORMWATER MANAGEMENT (SWM)
DRAINAGE SYSTEM DESIGN, AND
EROSION & SEDIMENT CONTROL REQUIREMENTS
Stormwater runoff from development, redevelopment, and construction sites is regulated
under Edmonds Community Development Code (ECDC) Chapter 18.30, including
Exhibit A: Edmonds Stormwater Code Supplement (Supplement) and
2005 Department of Ecology Manual
Overall Stormwater Management Approach
This handout contains an overall summary of the requirements and options available to assist an applicant in
designing, constructing, and maintaining stormwater Best Management Practices (BMPs) in the City of
Edmonds to meet the intent of ECDC 18.30. The Stormwater Code Supplement is available for use in our
Development Services Department as well as on the City's Website (www.ed ondswa.gov). Please refer to
the Supplement for a more detailed explanation of each requirement. Applicants are encouraged to use the
Classification Worksheet on page 5 of this handout to help determine the requirements for their project.
Step 1— Determine Applicability
The first step is to determine if the Stormwater
Management code applies to your project. Use
Figure -A, page 3, in this handout to establish
whether your project or site meets any of the
criteria for "applicability".
Step 2 — Determine Your Project Site's
Classification and Watershed
Determine if your site is classified as a Large Site
Project by using Figure-B, page 4. If you have a
Large Site Project, you do not have to fill out the
Classification Worksheet, page 5 of this handout.
If you do not have a Large Site Project, use the
Classification Worksheet, page 5 and Figure-B,
page 4, to determine whether your project is
considered a Small or Minor Site Project. For
Small Site Projects, you will further need to
determine whether it is a Category 1 or a Category
2 project. Next, identify the watershed for your
Small or Minor Site project using the City of
Edmonds Watershed Map, Figure-C, page 7. This
will clarify some of the site -specific requirements
in Step 3.
Step 3 — Determine the Minimum Requirements
Needed and Where on the Site They Apply
found on page 2 of this handout. For specific site
requirements for each classification refer to
Stormwater Code Supplement.
Step 4 — Plan Your Site and Select BMPs
Plan your site and select BMPs as described in the
Supplement to meet or exceed the minimum
requirements for your project.
Step 5 — Submit Reports and Plans for Review
Submit all required reports and plans for review.
The handout for the appropriate classification of
your project (Large, Small or Minor Site) will
provide information regarding the submittal
requirements. The stormwater submittals will be
reviewed concurrently with all other permit
submittals. Revisions to reports and plans may be
required per City of Edmonds review.
Step 6 — Construction
Erosion & Sediment Control Measures must be
inspected and approved by the City's Engineering
Division prior to the start of construction. The
project shall be constructed per approved plans.
Any changes proposed to the approved plans must
be submitted to the Engineering Division for
review and approval prior to construction.
Step 7 — Operate and Maintain BMPs
The minimum requirements are based on the Operate and maintain BMPs as required by the
project classification determined in Step 2. A approved submittals.
summary of the Minimum Requirements can be
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Overview of Stormwater Management Minimum Requirements
Depending on the type, location, and size of the proposed project, different combinations of these
minimum requirements apply. The following is a brief list of the overall Minimum Requirements for
Large, Small, and Minor sites. Please refer to the Stormwater Code Supplement for additional
information.
LARGE SITES — Stormwater Supplement Chapter 4
A project or overall development involving 1 acre or more of land disturbing activity would
potentially require the following Minimum Requirements:
1. Preparation of Stormwater Site Plan
2. Construction Stormwater Pollution Prevention Plan
3. Source Control of Pollution
4. Preservation of Natural Drainage Systems and OutfalIs
5. Onsite Stormwater Management/Low Impact
Development Techniques
6. Runoff Treatment
7. Flow Control
8. Wetland Protection
9. Operation and Maintenance
10. Offsite Analysis and Mitigation
1.1. Financial Liability
SMALL SITES — Category 1 and 2 - Stormwater Supplement Chapter S
Small Site Minimum Requirements, #'s 1-11 listed below, may be required for projects that involve:
A) 2,000 square feet (sf) or more of regulated new plus replaced impervious surface; or
B) 7,000 sf or more of land -disturbing activity; or
C) 50 cubic yards (cy) or more of grading, fill or excavation; or
D) Conversion of 3/4 acre or more of native vegetation to lawn or landscaped area; or
E) Causes an increase of 0.1 cubic feet per second (cf/s) or more in the 100 year flow frequency
from a threshold discharge area.
1. Preparation of Stormwater Site Plan including: 7. Flow Control
2. Erosion and Sediment Control Plan 8. Wetland Protection
3. Source Control of Pollution 9. Operation and Maintenance
4. Preservation of Natural Drainage Systems and Outfalls 1.0. Offsite Analysis and Mitigation
5. Onsite Stormwater Management/Low Impact 11. Financial Liability
Development Techniques
6. Runoff Treatment
MINOR SITES — Stormwater Supplement Chapter 6
A project that involves 500 sf or more of new development or redevelopment including land -
disturbing activity or a utility project that causes land disturbance, and is not considered a Large or
Small Site Project, must employ Minor Site Construction Stormwater Pollution Prevention Practices.
Additional Requirements may be imposed to meet the intent of the Stormwater Code based on site
specific factors.
Note: Terms in bold italics are described in the Glossary on pages 10-11. Definitions are found in ECDC Chapter 18.30.010 and the
Stormwater Code Supplement.
The information presented in this handout (E72) should not be used as a substitute for City codes and regulations. The Edmonds Community Development Code
(F,CDC) may be viewed at www.edm2gdswa.gov The applicant is responsible for ensuring compliance with the fees and regulations that are applicable at the
time of submittal. If you have a specific question about a certain aspect of your project, please contact the Engineering Division at 121 Fifth Avenue North, (425)
771-0220. Please note that other local, state, and/or federal permits or approvals may be required.
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Figure -A
Determining Applicability of Stormwater Management Code ECDC Chapter 18.30
If any of the descriptions in the Blue Boxes apply to your project t or project site, the
Stormwater Management Code applies.
Note: The definitions of the terms in italics are found in ECDC Chapter 18.30.010 and the Stormwater Code Supplement.
This chart provides an initial screening for determining the applicability of ECDC Chapter 18.30. The results from using this chart
do not substitute for a determination of applicability by the Public Works Director or Designee per ECDC Chapter 18.30.030 and
the relevant portions of the Supplement.
If ECDC Chapter 18.30 is applicable to the proposed project and it does not require any other City -issued permit, a Stormwater
Permit and associated fees will be required.
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Figure-B
Project Classification
If you have determined that the Stormwater Management Code
applies to your project (Figure -A), follow the Blue boxes in the chart
below to determine the Classification of your project.
Notes:
Terms in bold italics are described in the Glossary on pages 10-11. The definitions of the all terms in italics are found in ECDC
Chapter 18.30.010 and the Stormwater Code Supplement
The Classification flowchart assumes the project in question meets the applicability requirements of ECDC1830.030.
Land -disturbing Activity: Any activity that results in the 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 grading,
filling, excavation, and compaction associated with stabilization of structures and/or road construction.
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The project's Site Classification will dictate the specific stormwater management
requirements applicable to your site. Completing this worksheet will help determine the amount of
regulated impervious surface and whether your project falls into the classification of a Small Site
(Category 1 or Category 2), or a Minor Site. Please reference the Glossary (pp. 10-11), Figures D and
E, (pp. 8-9), and Examples (pp. 11-12), to assist with completion of this worksheet.
1) Is Permeable Pavement' Proposed For Use on this Site? ❑ Yes ❑ No
Refer to Stormwater Supplement Chapter 5.1
If YES, the subject area is to be considered impervious for initial site classification purposes. Include total
permeable pavement area in the calculation of Non -Regulated, Replaced and/or New impervious surface
areas in the table below.
2) Determine the Amount and Type of Existing & Proposed Impervious Surface for the Site
Refer to Stormwater Supplement Chapter 2 and Fig. C
Line l: Identify the Non -Regulated Impervious Surface Area.
Line 2: Identify the Replaced Impervious Surface Area, dividing the total between Exempt and Regulated; either or both
may be zero. Note: For project classification purposes, Replaced Impervious may only be considered exempt
under certain conditions. Refer to the Glossary and Figure D.
Line 3: Identify the New Impervious Surface Area for your project. All impervious areas created post -July 7, 1977 or
after the date of annexation into the City are regulated & should be included in this total unless they can be
categorized separately as a Replaced -Regulated area.
Line 4: Enter the sum of the total Replaced -Regulated _ plus the total New impervious areas.
Line 5: Identify the total area currently mitigated by an existing city -approved stormwater management system.
Line 6: Enter the sum of the value in Line 4 less the value in Line 5 to identify the total Regulated area in which
stormwater controls have not yet been applied.
Line 7: Identify the total area proposed to be mitigated through the use of Low Impact Development Techniques.
Line 8: Identify the total area proposed to be mitigated through conventional Stormwater Management Techniques.
** Provide a copy of the following table on the drainage plan sheet for the proposed project **
i (e.g. porous asphalt, porous concrete, paver blocks, concrete open celled paving grids, or plastic lattices filled with turf or stone)
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EdmondsCity of
Site Classffi�catkw—Warks4el54
3) Determine the Total Area of Land Disturbing Activity sf
Refer to Stormwater Supplement Chapter 8
4) Determine the Quantity of Grading, Fill and/or Excavation cy
5) Will the project convert 3/4 Acre or More of Native Vegetation to Lawn or ❑ yes ❑ No
Landscaped Area?
6) Identify the Watershed the Existing Site Runoff Discharges to
Refer to Stormwater Supplement Chapter 2.3
Based on Site Location and Watershed Map — Figure-C. Check all that apply.
A. ❑ Direct Discharge B. ❑ Creek or Lake Basin
❑ Edmonds Way Basin
❑ Puget Sound Basin
❑ Puget Sound Piped Basin
DETERMINE PROJECT CLASSIFICATION USING THE INFORMATION ABOVE
AND THE PROJECT CLASSIFICATION CHART (Figure B, pg 4)
I Small Site - Category 1 1 Small Site - Category 2 1 Minor Site
Stormwater Supplement Stormwater Supplement Stormwater Supplement
Chapter 5 Chapter 5 Chapter 6
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Figure-C
City of Edmonds Watersheds
A larger version of the Edmonds Watershed Map is available in the Development Services Office and on the City of Edmonds
website at ci.edmonds.wa.us.
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Figure-D
What Qualifies as Replaced Impervious Surface?
DeterminingReplaced Impervious
(Small and Minor Sites Only)
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Figure-E
Regulation of NEW Impervious Surface Areas
for Determining Site Classification
Note: 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. See Glossary, p. 10-11.
1 Mitigation requirements are based on the total cumulative impervious area created on a project site since July 6, 1977, the effective date of
the City's first drainage control ordinance.
2 For parcels that were annexed into the City after this date, the date of annexation shall substitute for the effective date of the City's first
drainage control ordinance.
3 For annexed parcels, a functioning Snohomish County -approved stormwater management facility can substitute for a City -approved
facility.
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Glossary
This glossary provides explanations of the terminology used in this handout. The actual definitions can be
found in ECDC Chapter 18.30.010 or the Supplement.
Impervious Surface: Hard surface area that either prevents or retards the entry of water into the ground as it
would occur in natural, undeveloped conditions. Impervious surfaces include, but are not limited to,
rooftops, driveways, walkways, concrete, asphalt, and packed earthen materials. Decks will be considered
impervious if one or more of the following apply:
• Deck is made of "solid" material that does not allow rain water to run through it
• Deck has "slots" where rainwater can run through but the ground under the deck does not allow the
rainwater to infiltrate into the ground.
Open, uncovered retention/detention facilities shall not be considered impervious surfaces for purpose 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.
For the purpose of determining stormwater control requirements, impervious surfaces are divided into
four categories: "Non -Regulated", "Exempt Replaced", "Regulated Replaced", and "New". See
explanations below:
Non -Regulated Impervious Surface:
All impervious surface area installed prior to July 7, 1977 or prior to the date the parcel was
annexed into the City from Snohomish County that will remain unchanged during the site
redevelopment.
Replaced Impervious Surface:
All impervious surface area on the project site existing at the time of application that is proposed to
be removed and re-established in the same footprint. This does not include impervious surface
removed for the sole purpose of installing utilities or performing maintenance.
"Exempt Replaced" - Some of the replaced impervious surface may be "Exempt' if it meets
the following condition:
On single-family residential parcels only the replacement of a driveway, walkway,
or similar surface in the same footprint in which it was originally installed prior to
July 7, 1977 or prior to the date the parcel was annexed into the City from
Snohomish County.
"Regulated Replaced" - All other replaced impervious surfaces on any parcel that do not
qualify as `Exempt' are considered "Regulated."
New Impervious Surface:
All areas converted from pervious surfaces (native vegetation, etc.) to impervious surfaces and
permeable pavements on a project site on or after July 7, 1977 or the date the parcel was annexed
into the City from Snohomish County. See Figure-E for exemptions for sites located outside of
Creek or Lake drainage basins.
3 Removed refers to the removal of building down to bare soil or subgrade. Roof replacement such as re -shingling, is considered to be a maintenance
activity and not considered replacing an impervious surface. For others surfaces, such as pavement, "removed" means the removal of concrete (PCC)
or asphalt (AC) slabs, driveways, sidewalks or similar surfaces down to bare soil or base course. The pulverization in -place and replacement with a
similar wearing course is considered replaced impervious surface. The partial grinding of surfaces for overlay (not down to base soil or base course) is
considered to be a maintenance activity.
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Land -disturbing Activity:
Activity that results in the movement of earth, a change in the existing soil cover (both vegetative
and non -vegetative), or a change in the existing soil topography. Land -disturbing activities include
but are not limited to clearing, grading, filling, excavation, and compaction of soils associated with
structure stabilization and road construction.
Pervious Surface:
Any surface that allows the entry of water into the ground as it would occur in nature.
Jf 1/ ' D y
The following are examples of how to determine the "Non -Regulated," "Exempt Replaced ", "Regulated Replaced"
and "New" impervious surface totals for a specific project. If you have any questions, please contact an Engineering
Technician.
Example 1
1972 House 1,200 square feet (sf) roofline
Driveway (paved) 60 sf
1999 House Addition 400 sf
2004 Detached Garage 600 sf (no stormwater system has been installed)
Proposal: Garage Addition which adds 140 sf to the roofline
Total Non -Regulated: 1260sf - 1972 House and Driveway (both constructed prior to July 7, 1977)
Total New Impervious: I40sf — 1999 House Addition, 2004 Garage Addition and Proposed Garage Addition
(ail built after July 7, 1977 and do not meet the definition of replaced).
Example 2:
1968 Single -Family House 1,500 sf roofline
Gravel Driveway 400 sf
Sidewalks/Patios 55 sf
Proposal: Pave existing gravel driveway with asphalt in same footprint (400 sf).
Total Non -Regulated: 1555sf — 1968 House and Sidewalks/Patios
Total Exempt Replaced: 400sf — 1968/Proposed Driveway area. The existing (pre-1977) gravel driveway is a
combination of gravel and packed earthen material and therefore meets the definition of impervious surface.
Paving the driveway in the same footprint as the original gravel driveway (constructed pre-1977) qualifies as
"Exempt Replaced" impervious.
Example 3a:
1976 House 2,000 sf roofline
Concrete Driveway 200 sf
1999 Garage w/concrete slab 300 sf
Proposal: New garage (300 sf) — demolish existing garage and slab; new garage will be same size and in same
location as the original garage to be replaced. There are currently no on site stormwater management systems.
Total Non -Regulated: 2200sf — 1976 House and concrete driveway
Total Regulated Replaced: 300sf — The original garage & slab were built after July 7, 1977 and the existing
surfaces were removed in their entirety (down to bare soil), therefore, no exemptions apply.
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Example 3b:
1976 House 2,000 sf roofline
Concrete Driveway 200 sf
Garage w/concrete slab 300 sf
Proposal: New garage (300 sf) — demolish existing garage (leave slab); new garage will be same size and in
same location as the original garage to be replaced.
Total Non -Regulated: 2200sf— 1976 House and concrete driveway
Total Exempt Replaced: 300sf — The original garage & slab were built prior to July 7, 1977 and only the
structure is being removed, not the slab. As the existing impervious surfaces are not being removed down to
bare soil the proposal qualifies as "Exempt Replaced" impervious.
Example 4•
1979 House
1,200 sf
Gravel driveway
300 sf
Patio/Walkways
100 sf
1989 Garage
250 sf
Proposal: Garage addition (200 sf) and pave existing gravel driveway with asphalt in same footprint (300 sf).
There are currently no on site stormwater management systems.
Total Regulated Replaced: 300sf — The original gravel driveway is considered to be an impervious surface and
was constructed after July 7, 1977, therefore, no exemptions apply.
Total New Impervious: 1750sf - 1979 House, patio/walkways, original garage and garage addition all built or
proposed to be built after July 7, 1977.
Example 5•
1989 House 2,800 sf
Driveway/patio/walkways 700 sf
Proposal: House addition (300 sf). The 1989 improvements were approved with a detention system sized for
3,500 sf of impervious stnface that was adequate under the provisions of the stormwater code in effect at that
time.
Total New Impervious: 3800sf — 1989 House, driveway/patios/walkways and proposed 300sf house addition.
The 1989 house, driveway, patio and walkways were installed after July 7, 1977 with an approved on -site
stormwater management system The existing system does not meet the sizing requirements of today's
stormwater code for 3,500 sf of impervious surface area. In this case, there are two options available to meet
the stormwater flow control requirements.
Option 1: Install a separate stormwater management system (such as a storm detention system,
infiltration trench or rain garden) sized to comply with current code for the new 300 sf house addition.
If a separate system were to be installed to handle the runoff from the new addition then no changes to
the existing system would be required.
Option 2: Direct all storm flows (existing and proposed) to the existing storm detention system and
revise the existing system to comply with current code for all 3800sf of impervious surface area. The
changes required in this scenario, for example, might result in adding additional pipe sections to the
existing detention system.
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