bld20110430-Farley_ENG1.pdf
City of EdmondsBLD20110430
TH
121 5AVENUE NORTHEDMONDS, WA 98020 • (425) 771-0220 FAX(425) 771-0221
Website: www.ci.edmonds.wa.us
PUBLIC WORK DEPARTMENT
Planning • Building • Engineering
Engineering Plan Review Comments
PermitApplication:Date:
#BLD20110430July 6, 2011
th
Project Name/Address:
Farley –8604-204St SW
Contact Person/Address, Fax or E-mail:
Timothy Farley –tjfarley@farleydimmock.com
Reviewer: JENNIFERLAMBERTDivision:
ENGINEERING
During review of the subject submittal, it was found that the following information, corrections, or
clarifications would need to be addressed. All Handouts referred to in these comments can be accessed at
our website: www.ci.edmonds.wa.usunder City Government / Development Services Department /
Engineering Division then scroll down to Handouts:
1)Please add the impervious surface chart(provided in the E72 Handout),as shown below, to the ‘Cut &
Fill / Drainage’ Plan.
LineTypeArea (square feet)
1.Non-Regulated
ExemptRegulated
2.Replaced
3.New (Post 1977)+
Total Regulated Impervious Area
=
4.
Mitigationrequired if in excess of 2000sf
-
5.Total Area Mitigated by Existing Stormwater Management System(s)
=
6.Regulated Area Not Yet Mitigated
=
7.Area Proposed to be Mitigated by Low Impact Development Techniques
=
8.Area Proposed to be Mitigated through Conventional SWM Techniques
2)Please show the year of construction on the Site Plan as shown in the Sample Site Plan provided(i.e.
existing home, additions, gravel areas, decks, driveway, etc).
3)Please show all impervious surface areas on the plans (i.e. walkways, gravel areas, shed, patios, etc) and
revise the impervious surface calculations accordingly.
4)The existing detention system is proposed to be relocated. Please show that the proposed location will
be no less than 10 feet from any structure.
DATE E-MAILED7/6/2011PAGE(1)
City of EdmondsBLD20110430
TH
121 5AVENUE NORTHEDMONDS, WA 98020 • (425) 771-0220 FAX(425) 771-0221
Website: www.ci.edmonds.wa.us
PUBLIC WORK DEPARTMENT
Planning • Building • Engineering
Engineering Plan Review Comments
5)The sizing of the existing detention system was installed under the previous version of the city
stormwater management code; therefore, the proposed connectionto the existing detention system does
not meet current city requirements. You may complete one of the following:
Option 1:
a.If you intend to connect the new impervious surface to the existing system, the
existing system will will need to be resizedper current city code (see attached sizinghandout) to
account for the proposed and existing impervious surfaces to be collected to the stormwater
system.
Option 2:
b.The new impervious surfaces may be connected to a separate detetion system, where
the new stormwater system will connect to the outfall line of the existing detention system. In
this option you will not need to modify the existing detention.
***Please note that it appears that the current detention system discharges to an infiltration
system. If you intend to discharge the runoff from the new impervious to the existing infiltration
system, you will need to either provide percolation rates obtained from a licensed geotechnical
engineer and enlarge the infiltration system or connect directly to the city storm system. ***
Option 3:
c.Connect the proposed impervious surface water runoffto an approved LID (Low
Impact Development) method; such as rain gardens (see
http://county.wsu.edu/mason/nrs/water/Documents/Raingarden_handbook.pdf), dispersion
trench, infitration, etc). If a new system is proposed that is designed for stormwater runoff to
infiltrate (i.e. raingarden, spreader trench, drywell, etc) into the ground a geotechnical report with
percolation rates may be required. Please note that if the impervious surface water runoff is less
than 2000 sq ft and is being discharged to a raingarden, then the infiltration rate can be
determined as shown in the handbook and a geotechnical engineer would not be required (applies
to raingardens ONLY).
Option 4:
d.Provide a design and calculations for alternative system from a licensed engineerthat
incorporatesthe existing system with a new system to allow for the additional impervious surface
water runoff per current city standards.
Option 5:
e.Propose another alternative for review and approval.
Please Note: All options above must be submitted for review and approval. Please provide the
necessary information to the City to adequately review the proposed system.
6)Please add a note to the ‘Cut & Fill / Drainage’ Plan that all disturbed soils will be compost amended.
7)Please show all trees within the construction area and label to be removed or remain.
a.If trees are to remain and near the construction area, please show a barrier fence around the tree
to protect during construction.
DATE E-MAILED7/6/2011PAGE(1)
City of EdmondsBLD20110430
TH
121 5AVENUE NORTHEDMONDS, WA 98020 • (425) 771-0220 FAX(425) 771-0221
Website: www.ci.edmonds.wa.us
PUBLIC WORK DEPARTMENT
Planning • Building • Engineering
Engineering Plan Review Comments
8)Show the location of the side sewer from the sewer main to the house.
9)On the ‘Cut & Fill / Drainage’ Plan there is a C/W line. I assume this line indicates the water service
line.
a.If this line is the water service line, please show the line connected to the water meter.
b.If this line is not the water service line please, clearly label what this line represents and please
show the water service line from the meter to the house.
10)Please show all fences, retaining walls, and rockeriesthat exist on the property.
Please resubmit 3 copies of the revised traffic control plansand associated documents with a written
response to each item to an Engineering Technician. Please contact me at 425-771-0220ext 1321or by e-
mail at jennifer.lambert@ci.edmonds.wa.usif you have specific questions regarding these plan
corrections.
DATE E-MAILED7/6/2011PAGE(1)
#E72
STORMWATER MANAGEMENT (SWM)
DRAINAGE SYSTEM DESIGN, AND
EROSION & SEDIMENT CONTROL REQUIREMENTS
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 Citys Website (www.ci.edmonds.wa.us). 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.
found on page 2 of this handout. For specific site
Step 1 Determine Applicability
requirements for each classification refer to
The first step is to determine if the Stormwater
Stormwater Code Supplement.
Management code applies to your project. Use
Figure-A, page 3, in this handout to establish
Step 4 Plan Your Site and Select BMPs
whether your project or site meets any of the
Plan your site and select BMPs as described in the
criteria for applicability.
Supplement to meet or exceed the minimum
requirements for your project.
Step 2 Determine Your Project Sites
Classification and Watershed
Step 5 Submit Reports and Plans for Review
Determine if your site is classified as a Large Site
Submit all required reports and plans for review.
Project by using Figure-B, page 4. If you have a
The handout for the appropriate classification of
Large Site Project, you do not have to fill out the
your project (Large, Small or Minor Site) will
Classification Worksheet, page 5 of this handout.
provide information regarding the submittal
If you do not have a Large Site Project, use the
requirements. The stormwater submittals will be
Classification Worksheet, page 5 and Figure-B,
reviewed concurrently with all other permit
page 4, to determine whether your project is
submittals. Revisions to reports and plans may be
considered a Small or Minor Site Project. For
required per City of Edmonds review.
Small Site Projects, you will further need to
Step 6 Construction
determine whether it is a Category 1 or a Category
Erosion & Sediment Control Measures must be
2 project. Next, identify the watershed for your
inspected and approved by the Citys Engineering
Small or Minor Site project using the City of
Division prior to the start of construction. The
Edmonds Watershed Map, Figure-C, page 7. This
project shall be constructed per approved plans.
will clarify some of the site-specific requirements
Any changes proposed to the approved plans must
in Step 3.
be submitted to the Engineering Division for
review and approval prior to construction.
Step 3 Determine the Minimum Requirements
Needed and Where on the Site They Apply
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
land disturbing activity
A project or overall development involving 1 acre or more of would
potentially require the following Minimum Requirements:
1. Preparation of Stormwater Site Plan 7. Flow Control
2. Construction Stormwater Pollution Prevention Plan 8. Wetland Protection
3. Source Control of Pollution 9. Operation and Maintenance
4. Preservation of Natural Drainage Systems and Outfalls 10. Offsite Analysis and Mitigation
5. Onsite Stormwater Management/Low Impact 11. Financial Liability
Development Techniques
6. Runoff Treatment
SMALL SITES
Category 1 and 2 - Stormwater Supplement Chapter 5
Small Site Minimum Requirements, #s 1-11 listed below, may be required for projects that involve:
newreplaced impervious surface; or
A) 2,000 square feet (sf) or more of regulated plus
land-disturbing activity; or
B) 7,000 sf or more of
or
C) 50 cubic yards (cy) or more of grading, fill or excavation;
or
D) Conversion of ¾ acre or more of native vegetation to lawn or landscaped area;
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 10. 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
(ECDC) may be viewed at www.ci.edmonds.wa.us/codes.stm. 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 or project site, the
Stormwater Management Code applies.
Does your Project Site require the issuance of a City
Yes
permit under any of the following:
ECDC Title 18 - Engineering Division
ECDC Title 19 - Building Division
2
A Stormwater Permit
No
Yes
Are you submitting a Subdivision application per
ECDC Chapter 20.75?
No
Does your Project site involve any of the following: Stormwater
Management
500 square feet or more of land-disturbing
Yes
requirements of ECDC
activity, new impervious surface, or replaced
Chapter 18.30 are
impervious surface?
applicable to your
A utility or other construction project
Project or Site.
consisting of 500 lineal feet or more of
trench excavation?
Is located in, adjacent to, or drains into
(currently or as a result of the project) a
Critical Area or a Critical Area Buffer?
No
Stormwater Management
requirements of ECDC Chapter
18.30 are not applicable to your
1
Project or Site
Note: The definitions of the terms in italics are found in ECDC Chapter 18.30.010 and the Stormwater Code Supplement.
______________________________
1
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.
2
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.
Yes
Does the project involve 1-
acre or more of land-
Large Site Project:
1
disturbing activity ?
See Stormwater Supplement
No
Chapter 4 for specific
Yes
Is the project part of a larger
requirements
.
common plan of development or
sale where the total disturbed area
for the entire plan will total 1-acre
Does the project create or add 5,000 square feet or
or more of land-disturbing activity?
more of new impervious surface, regulated
replaced impervious surface or new plus
No
regulated replaced impervious surface?
OR
Does the Project involve one or more of the
Convert ¾ acre or more of native vegetation to
following:
lawn or landscaped area
Yes
new
2,000 square feet (sf) or more of
OR
impervious surfaceregulated replaced
,
Through a combination of creating effective
impervious surface
or new plus regulated
impervious surface and converted pervious
replaced impervious surface?
surfaces, causes an increase of 0.1 cubic feet per
OR
second in the 100-year flow frequency from a
land-disturbing activity
7,000 sf or more of ?
threshold discharge area as estimated using an
OR
approved model?
50 cubic yards or more of either grading, fill, or
excavation as defined in Chapter 18.40.000
ECDC?
No
Yes
No
Minor Site Project: Category 1 Small Site Project: Category 2 Small Site Project:
See Stormwater Supplement See Stormwater Supplement See Stormwater Supplement
Chapter 6 for specific Chapter 5 or specific Chapter 5 for specific
requirements requirements
requirements
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 ECDC18.30.030.
________________________________
1
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 projects Site Classification will dictate the specific stormwater management
requirements applicable to your site. Completing this worksheet will help determine the amount of
Small Site
regulated impervious surface and whether your project falls into the classification of a
Minor Site
(Category 1 or Category 2), or a . Please reference the Glossary (pp. 10-11), Figures D and
E, (pp. 8-9), and Examples (pp. 12-13), to assist with completion of this worksheet.
1
1)Is Permeable Pavement Proposed For Use on this Site?
Yes No
Refer to Stormwater Supplement Chapter 5.1
YES
If , 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 1Non-Regulated Impervious Surface Area.
: Identify the
Line 2Replaced Impervious Surface Area
: Identify the , dividing the total between Exempt and Regulated; either or both
Replaced Impervious
may be zero. Note: For project classification purposes, may only be considered exempt
under certain conditions. Refer to the Glossary and Figure D.
Line 3New Impervious Surface Area
: Identify the 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 4plus
: Enter the sum of the total Replaced-Regulated 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 **
Line Type Area (square feet)
1. Non-Regulated
Exempt Regulated
2. Replaced
3. New (Post 1977) +
Total Regulated Impervious Area
=
4.
Mitigation required if in excess of 2000sf
5. Total Area Mitigated by Existing Stormwater Management System(s) -
6. Regulated Area Not Yet Mitigated =
7. Area Proposed to be Mitigated by Low Impact Development Techniques =
8. Area Proposed to be Mitigated through Conventional SWM Techniques =
1
(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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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 ¾ 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
PROJECT CLASSIFICATION CHART
AND THE (Figure B, pg 4)
Small Site - Category 1
Small Site - Category 2 Minor Site
Stormwater Supplement
Stormwater Supplement Stormwater Supplement
Chapter 5
Chapter 5Chapter 6
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Figure-C
2
City of Edmonds Watersheds
No warranty of any sort,
including accuracy,
fitness, or
merchantability
accompany this product.
1
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?
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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 Citys 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 Citys 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
3
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
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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into the City from Snohomish County. See Figure-E for exemptions for sites located outside of
Creek or Lake drainage basins.
Land-disturbing Activity:
Any activity that results in movement of earth, a change in the existing soil cover (both vegetative
and non-vegetative), or 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.
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EXAMPLES
Non-RegulatedExempt Replaced Regulated Replaced
The following are examples of how to determine the , ,
New
and 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 with no stormwater system installed
Proposal: Garage Addition 140 sf roofline
before
The 1972 House and Driveway were constructed to July 7,1977 so are considered Non-Regulated
impervious area. The 1999 House Addition and 2004 Detached Garage and proposed Garage Addition are
after )
considered New impervious surface since built July 7, 1977(zero replacedimpervious.
1,140 sf
Total Regulated New Impervious Surface: 400sf + 600sf + 140sf =
Example 2
:
1968 Single-Family House 1,500 sf roofline
Gravel Driveway 400 sf
Sidewalks/Patios 55 sf
Proposal: Pave driveway with asphalt, same footprint as gravel driveway (400 sf).
The existing gravel driveway is a combination of gravel and packed earthen material, this meets the definition
of impervious surface. Paving the gravel driveway counts as Exempt Replaced impervious surface (the
before
proposed paving in same footprint of the original driveway installed July 7, 1977).
0 sf
Total regulated New + replaced Impervious Surface:
Example 3a
:
1976 House 2,000 sf roofline
Concrete Driveway 200 sf
1999 Garage w/concrete slab (no stormwater system) 300 sf
Proposal: New garage (300 sf) demolish existing garage and slab; new garage will be same size and in same
location of the concrete slab and original garage to be replaced.
Proposed garage will be considered Regulated Replaced because the replaced impervious surface (garage &
after
slab) was first constructed July 7, 1977.
300 sf
Total Regulated New + Replaced: 0 + 300 sf =
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 original garage that will be replaced.
Proposed garage will be considered Exempt Replaced because the slab remains (not taken down to bare
prior to
ground or sub base), the new garage is in the same footprint as the old one that was first constructed
July 7, 1977.
0 sf
Total Regulated New + Replaced Impervious Surface:
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Example 4:
1979 House 1,200 sf
Gravel driveway 300 sf
Patio/Walkways 100 sf
1989 Garage 250 sf (No stormwater system on the site)
Proposal: Garage addition 200 sf and replacing gravel driveway with asphalt (300 sf)
after
House, patio/walkways, original garage and garage addition built July 7, 1977 with no storm system and
therefore are considered New impervious surface: 1,200+100+250 +200 sf = 1,750 sf. Driveway was
after
installed July 7, 1977. It is now changing from gravel to asphalt (both impervious) in same footprint it is
considered Regulated Replaced impervious surface: 300 sf
2,050 sf
Total Regulated New + Replaced Impervious: 1,750 sf + 300 sf =
Example 5:
1989 House 2,800 sf
Driveway/patio/walkways 700 sf
The 1989 construction was approved with a detention system sized for 3,500 sf of impervious surface that was
adequate under the provisions of the stormwater code effective at that time.
Proposal: 300 sf addition to house
Revised New impervious total 3,800 sf (zero replaced impervious).
after
The house, patio and walkways were installed July 7, 1977 with a storm system. The existing system does
not meet the sizing requirements of todays stormwater code for 3,500 sf of impervious surface. In this case,
there are two options available to meet the stormwater flow control requirements.
Option 1
Install a separate stormwater system (such as an infiltration trench or rain garden, if feasible)
that is sized only for the 300 sf new addition using current standards. Since the existing system was
approved at the time of construction for requirements of the code in effect at that time, resizing is not
required.
Option 2
Upgrade the existing system to meet to handle both the existing impervious surface of
(3,500 sf) and the proposed impervious surface (300 sf). The existing system would need to be sized
for 3,800 sf under the sizing requirements of the current code.
If, for example, a detention system is chosen as the flow control BMP:
Code requirements in effect in 1989 for 3,500sf of impervious surface (existing system) is 49 lineal ft
of 18 diameter pipe with ¾ orifice.
Current Code requirements for 3,800sf of impervious surface is 81 lineal ft of 18 diameter pipe with
¾ orifice.
Therefore applicant would need to add 32 lineal feet of 18 diameter pipe to existing detention system.
Other options include the Low Impact Development techniques such as pervious pavement or rain gardens.
13 of 13
Revised on 4/21/11 E72-SWM_Erosion_Control--04.21.11-FINAL Page
#E72 A
STORMWATER DETENTION SYSTEM SIMPLIFIED SIZING FOR
CATEGORY 1 SMALL SITE PROJECTS
Design and Construction Requirements
I
ntroduction
1
Generally, a Category 1 Small Site Project will have
Table 1 provides a lookup-table for sizing the detention
between 2,000 and 5,000 square feet of new plus
pipe and outlet orifice size based on the amount of
replaced impervious surface area that requires flow
impervious surface area draining to the system. This
control under Small Site Minimum Requirement #7.
2
table is based on a flow control standard for the 10-yr
recurrence storm (storm that has a 10% chance of
1
If your site is a Category 2 Small Site Project or a
occurring in any given year). To use the table, first
1
Large Site Project that will have stormwater detention
determine the impervious surface area draining to the
system, this handout cannot be used. See the Supplement
proposed detention system. If your value falls in
for more details.
between values in the first column, use the next highest
value (i.e. if you have 2,600 sf or impervious area, use
A detention system is a drainage system that collects
the sizing requirements for 3,000 sf). The length of
stormwater runoff from roofs, driveways, sidewalks,
pipe value in the body of the table varies based on the
patios, or other impervious surfaces, stores the runoff in
diameter of the pipe chosen for use. The diameter of the
a large underground pipe, then slowly releases the runoff
pipe chosen will be based on your assessment of site
though a small hole (orifice) into an approved discharge
specific factors such as slope, amount of cover over the
point (usually the Citys storm drainage system). Figure
system, physical constraints, and others. The different
1 shows a typical detention system for a Category 1
shading in the table provides information on the outlet
Small Site Project.
orifice size required for any given combination of pipe
length and pipe diameter to meet the flow control
Runoff enters the upstream catch basin where any
standard. Note that detention systems must be located
sediment or other solids sink to the bottom. Regular
entirely on private property and not in the Citys right-
cleaning of this and other aspects of the detention
of-way or easements.
system is very important to keep the system operating
properly (see Maintenance section below). The detention
Table 2 provides the approved material for use as
pipe is sloped towards the outlet to keep any solids that
stormwater detention pipes and the required catch
make it though the upstream catch basin moving though
basin/manhole sizes (upstream and downstream) based
the system to the downstream catch basin. This
on the size of the detention pipe connecting to it. Figure
downstream catch basin (or manhole) contains the flow
1 show a cross section of a typical detention system for
control structure. This flow control structure is a riser
Category 1 Small Site Projects. Figure 2 shows a detail
tee with a small hole or orifice on the bottom
of downstream catch basin/manhole and the flow control
connected to the outlet pipe. If the topography of the
structure.
property is such that is not possible to discharge from
the outlet catch basin by gravity to the approved
discharged point, then the flow control structure may be
replaced by a pump system, if approved by the City
Engineering Division. See the City Engineering
Division Pumping Policy for more details.
2
0.25 cubic feet per second per acre of impervious area (cfs/acre-
1
See Handout #E72 for site classification.
impervious).
- 1 -
Table 1
Detention Pipe Look-Up Table
Category 1 Small Site Projects
Contributing New Plus Length of Pipe (ft) For Given Inside Diameter
Replaced Impervious
Surface Area (sf)
18-inch 24-inch 30-inch 36-inch
2,000 41 22 15 11
2,500 49 30 18 12
3,000 61 33 23 16
3,500 74 38 24 17
4,000 91 45 29 20
4,500 95 49 32 23
5,000 112 56 35 25
sf square feet
Orifice Size:
= 0.5 (1/2) inch diameter = 0.75 (3/4) inch diameter
= 0.625 (5/8) inch diameter = 0.875 (7/8) inch diameter
- Outlet pipes (including in the flow control structure) must be 8 in diameter for 2,000 to 4,000 square feet of new plus replaced impervious surface area.
- Outlet pipes (including in the flow control structure) must 10 in diameter for greater than 4,000 square feet of new plus replaced impervious surface area.
Note: Multiple pipes in parallel connected together (manifolded) can be used to meet length requirements, if site has constraints.
Table 2
Approved Detention Pipe Material and Catch Basin/Manhole Structure Sizing
Detention Pipe
Inside Diameter WDOT
(inches) Detention Pipe Material Catch Basin/Manhole
18 All types in list below table Type 1L Catch basin
24 All types in list below table Type 2, 48-inch diameter manhole
30 All Metal & RPP Type 2, 48-inch diameter manhole
All Concrete & CPEP Type 2, 54-inch diameter manhole
PVCP Type 2, 60-inch diameter manhole
36 Metal & RPP Type 2, 54-inch diameter manhole
All concrete, CPEP, & PVCP Type 2, 60-inch diameter manhole
WDOT Washington Department of Transportation
List of Approved Pipe Materials for Detention
Metal: Plastic:
ACSP PVCP
Aluminized Corrugated Steel Pipe (Type 2 Polyvinyl Chloride Pipe (SDR 35 or better)
CPEP
meets AASTO designations M274 and M36) Corrugated Polyethylene Pipe (smooth
®
ASRP
- Aluminum Spiral Ribbed Pipe (16 gauge or interior wall, or N-12 pipe)
RPP
better) - Ribbed Polyvinylchloride Pi
CAP
Corrugated Aluminum Pipe (16 gauge or
better)
DIP
Ductile Iron Pipe (Class 50 or better)
Concrete:
PCP
Plain Concrete pipe
RCP
Reinforced Concrete Pipe
- 2 -
- 3 -
Maintenance Requirements
ECDC Section 18.30.090 requires privately-owned
stormwater management facilities, such as detention
systems, to be properly maintained. The owner of
the property is the responsible party for such
maintenance. The system must be kept in good
working order by regularly removing build-up of
sand, silt or other solids that may accumulate in the
catch basins or the detention pipe. The entire system
should be inspected once per year and cleaned as
necessary. Keeping the flow control orifice clear of
debris is of particular importance and it should be
inspected on a regular basis and cleared, if
necessary. A plugged orifice can defeat the function
of a detention system and may cause private
property or street flooding.
The City may make periodic inspections of
detention facilities to ensure they are operating
properly. ECDC Section 18.30.100 contains the
enforcement provisions the City can use to ensure
the system is properly maintained.
References
City of Edmonds. 2010. Edmonds Community
Development Code (ECDC) Chapter 18.30 (including
Exhibit A: Edmonds Stormwater Code Supplement).
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.
Washington Department of Transportation. 2010a.
Standard Specifications for Road and Bridge
Construction.
M 41-10.
Washington Department of Transportation.
2010b. Standard Plans. M 21-01. August 2, 2010.
- 4 -
#E72 B
LOW IMPACT DEVELOPMENT (LID) STORMWATER BEST MANAGEMENT
1
PRACTICES (BMPs) SIMPLIFIED SIZING
INTRODUCTION impervious surface area, it triggers Permit requirements.
See Section 4.7 the Stormwater Supplement (Edmonds
This handout helps you to:
2010) and Section 4.7 in Appendix I of the Permit to see
1.Determine the applicability of the simplified
if your site triggers this requirement. If your Large Site
sizing methods to your project.
Project exceeds one of thresholds for MR#7, the project
2.Learn about the pre-designed LID BMPs.
is subject to a different flow control standard than is
3.Use the flow control credits and sizing factors to
not
used in this handout, and you may use the simplified
assist in the design of these LID BMPs for your
sizing method contain herein.
project.
For supporting information, see the reference section at Project Size
the end of this handout. Small Site Projects and Large Site Projects subject to the
Edmonds-specific flow control standards are categorized
APPLICABILITY
in this handout by size:
2
The simplified BMP sizing applies to projects classified
Category 1 Standard: projects with 2,000 to 5,000 sf of
as Small Site Projects and many classified as Large Site
new plus replaced impervious surface area
Projects. The approach described in this handout can be
Category 2 Standard: projects with more than 5,000 sf
used to demonstrate compliance with both Minimum
but less than 10,000 sf of new plus replaced
Requirement (MR) #5, Onsite Stormwater
impervious surface area.
Management/LID Techniques for Controlling Runoff
Project Drainage Basin
and MR #7, Flow Control.
There are two types of drainage basins in Edmonds:
2
creek/lake, and direct discharge into Puget Sound. The
The simplified sizing method only applies to projects
type of drainage basin dictates the flow control standard
that are subject to the Edmonds-specific flow control
and, therefore, the sizing of the LID BMP.
standards. To determine which flow control standards
apply to your project, you must know your project
Edmonds-specific Flow Control Standard
2
classification, and drainage basin.
The Edmonds-specific flow control standards for LID
and other infiltration BMPs are provided in Table 1.
Project Classification
Requirements vary by project size and the drainage
Projects are classified as either Large Site Projects,
basin in which the project site is located. Note that the
Small Site Projects, or Minor Site Projects. Large Site
Stormwater Supplement has the LID Credit Option
Projects in creek or lake basins may trigger the flow
that allows mitigation of less than 100% of the new and
control requirements (MR#7) in the Western
replaced impervious surface area if LID BMPs are used
Washington Phase II Municipal Stormwater Permit
(see the Stormwater Supplement for details).
(Ecology 2009) (Permit). Generally, if your site has
10,000 square feet (sf) or more or new plus replaced
It is important to note that these flow control standards
are based on the assumption that all disturbed and
1
Defined as a schedule of activities, prohibition of practices, maintenance
converted pervious surfaces have compost-amended
procedures, and structural and/or managerial practices that, when used singly
or in combination, prevent or reduce the release pollutants and other adverse soil (including compost/soil mix) to City standards
(See
impacts to waters of Washington State.
.
handout XX for further explanation)
2
See Handout #E72 for determining project classification, drainage basin, and
other relevant site-specific issues.
- 1 -
.
Table 1. Edmonds-Specific flow Control Standards for LID and Infiltration BMPs
a
Edmonds-specific Flow Control Standards
Project Location
Category 1 Sites Category 2 Sites
Creek or Lake Basin
The post-development 10-year recurrence The post-development 2-, 10-, and 100-year
interval flow shall not exceed 0.25 cfs per recurrence interval flows shall not exceed 0.07, 0.25,
acre of impervious surface area. and 0.45 cfs per acre of impervious surface area,
respectively.
b
Direct Discharge Basin
The post-development 10- and 100-year recurrence interval flow shall not exceed 0.25 and 0.45 cfs
per acre of impervious surface area, respectively.
Notes:
cfs cubic feet per second
a
Large Site Projects in creek or lake basins that exceed the Phase II Permit thresholds are subject to a different flow control standard and may not
use the simplified sizing method. Thresholds for Large sites are based on area of new and replaced impervious surfaces, area of converted pervious
surfaces, and increases in the 100-year recurrence interval flow (see Stormwater Supplement).
b
Small Site Projects and Large Site Projects in direct discharge basins may be exempt from Minimum Requirement #7 and the Edmonds-specific
flow control requirements if a downstream capacity and erosion analysis per Minimum Requirement #10 shows no unacceptable downstream
impact, or an alternative mitigation plan is approved (see Stormwater Supplement).
Site Soil Conditions While other BMPs may be used to achieve Edmonds-
The simplified sizing approach applies to sites with a specific stormwater flow control requirements, you
native soil design infiltration rates shown in the last must use a continuous hydrologic simulation model to
2
column of Table 2. The infiltration rate(s) on your size them as described in Stormwater Supplement. The
project site can be evaluated using one of three design configuration, sizing method, and range of site
approved methods: the USDA Soil Textural infiltration rates for the pre-designed flow control
Classification, the ASTM Gradation Testing (D BMPs are summarized in Table 2.
10
approach), or the Modified Pilot Infiltration Testing
(PIT) Method. See Appendix C of the Stormwater To apply the simplified method, BMPs must meet the
Supplement for more information regarding soil testing specific prescribed design requirements (such as side
requirements. slopes, ponding depth, and soil or gravel depth)
presented in this section. To determine if a BMP is
Important:
the short-term or measured infiltration rate suitable for your project site, consult the Stormwater
for the native soil on your project site must be Supplement and the Low Impact Development
corrected to acknowledge the effects of site variability Technical Guidance Manual for Puget Sound (WSU
and long-term clogging due to siltation and biomass 2005). Site considerations include (but are not limited
buildup in the infiltration facility. The corrected or to) spatial constraints, slopes, and feasibility of
design rate must be used to design stormwater flow infiltration.
control facilities using the simplified method presented
BMP Descriptions
in this handout. For further guidance, see the
Stormwater Supplement, Appendix C.
The BMPs with predetermined sizing factors and flow
control credits are described below.
PRE-DESIGNED FLOW CONTROL BMPS
The simplified sizing method was developed for the
Permeable Pavement
following LID BMPs:
Permeable pavement is a paving system that allows
Permeable pavement surfaces
·
rainfall to percolate into an underlying aggregate
Bioretention cells (rain gardens)
·
storage reservoir where stormwater is stored and
Gravelless chambers
·
infiltrated to the underlying soil. A permeable
Infiltration trenches
·
pavement system consists of a pervious wearing
Drywells
course (e.g., porous asphalt, pervious concrete, paver
·
blocks,concrete open celled paving grids, or plastic
lattices filled with turf or stone) and an aggregate
subbase course installed over native soil.
2
If the design infiltration rate is less than 0.13 in/hr, the LID BMPs in this
handout are considered not feasible. If the design infiltration rate for your
site is 0.13 in/hr or greater and not represented in the ranges presented in
Table 2 and you want to use one or more of the LID BMPs in this handout,
contact the City Engineering Division.
- 2 -
Table 2. LID BMPs and Configurations for Simplified Sizing Method.
Design Infiltration Rate
BMP Design Configuration BMP Sizing Method (inches per hour)
LID Runoff Reduction Methods
Permeable Pavement 0 to 5% Subgrade Slope Flow Control Credit 0.13
LID Facilities
Rain Gardens (Bioretention) 6- and 12-inch Ponding Depth Sizing Factor 0.25 2.0
Infiltration Trench 1.5 and 3-foot Depth Sizing Factor 0.25 2.0
Gravelless Chamber Sizing Factor 0.25 2.0
Drywell 4 and 6-foot Depth Sizing Factor 0.5 2.0
typical chamber design is shown in Figure A-4. This
The permeable pavement BMP described here is
BMP consists of a buried chamber, typically made of
designed to manage only the water that falls upon it,
durable plastic or other prefabricated material, within
and is not intended to take stormwater run-on from
which collected stormwater is temporarily stored and
other areas. A typical pavement design is shown in
then infiltrated into underlying soil. Gravelless
Figure A-1. For more specific design considerations,
chambers create an underground cavity that can
see the City Engineering Divisions Permeable
provide a greater void volume than infiltration
Pavement Policy.
trenches, and often require a smaller footprint because
Rain Gardens (Bioretention)
they can store more runoff than a trench filled with
drain rock in a comparable space. Per Kitsap County
Bioretention facilities (also known as rain gardens) are
requirements, the chamber must have a minimum void
shallow depressions with a designed soil mix and
volume of 2.6 cubic feet per linear foot, and a
plants adapted to the local climate and soil moisture
minimum infiltrative surface of 2.8 square feet per
conditions. The healthy soil structure and vegetation
linear foot. These same requirements are suitable for
promote infiltration, water storage, and slow release of
applications in Edmonds.
stormwater flows to more closely mimic natural
conditions. The simplified sizing approach applies
Drywell
only to bioretention facilities that do not have an
The drywell BMP is based on guidance for downspout
underdrain to intercept infiltrated runoff, or an
impermeable liner impeding infiltration to underlying infiltration drywell design, presented in the Ecology
soil. Sizing factors are provided for two design Manual (Ecology 2005). A typical drywell design is
shown in Figure A-5. This BMP consists of an
variations: a 6-inch and 12-inch ponding depth. A
aggregate-filled hole where collected stormwater is
typical bioretention design is shown in Figure A-2.
temporarily stored and then infiltrated into the
Infiltration Trench underlying soil. Drywells are similar to infiltration
trenches but are typically deeper and require less site
The infiltration trench BMP is based on guidance for
area. Sizing factors are provided for two drywell
downspout infiltration trench designs, presented in the
design variations: a 4-foot and a 6-foot aggregate
Ecology Manual (Ecology 2005). A typical trench
depth.
design is shown in Figure A-3. This BMP consists of
an aggregate-filled trench where collected stormwater
BMP Design Requirements
is temporarily stored and then infiltrated into the
To use the simplified sizing method, the BMP design
underlying soil. Sizing factors are provided for two
requirements listed in Table 3 must be met. Additional
design variations: a 1.5-foot and 3-foot aggregate
requirements that you must account for in your design
depth in the trench. For both trench depths, the trench
(including infiltration rate testing methods, infiltration
is 24 inches wide.
rate correction factors, setbacks, and vertical
Gravelless Chamber
separation from the bottom of the facility to the
underlying water table) are presented in the
The gravelless chamber BMP is based on guidance
Stormwater Supplement and the Ecology Manual
presented in the Kitsap County Stormwater
(Ecology 2005).
Management Design Manual (Kitsap County 1997). A
- 3 -
Table 3. BMP Design Requirements for the Simplified Sizing Method
Design Criteria for All BMPs
These BMPS shall not be used in:
3
- The North Edmonds Earth Subsidence and Landslide Hazard Area (ESLHA)
- Upgradient (east) of the ESLHA
4
- In a Landslide Hazard Area
- Within 5 feet of property lines and easements
- Within 10 feet from underground storage tanks
- Within 50 feet from proposed or existing septic systems or drain fields.
- Where the depth to the top of the seasonal groundwater level is less than 1 foot from the bottom of the
facility.
The BMP must meet the following setback requirements from buildings:
- If runoff is from an area less than 5,000 square feet, setback is 5 feet from a structure without a
basement and 10 feet for a structure with a basement.
- If runoff is from an area greater than or equal to 5,000 square feet, a structure shall not intersect
with a 1Horizontal:1Vertical slope from the bottom edge of the facility. Minimum 5 feet from a
structure without a basement and 10 feet for a structure with a basement.
An overflow path must be specified such that runoff above the facilitys design capacity does not cause:
- Flooding of a building or emergency access
- Erosion or downstream sedimentation
- Slope failure.
Permeable Pavement
The pavement shall not receive stormwater run-on from other areas
Aggregate depth (for storage) must be at least 3 inches.
For sites with more than 5,000 sf of impervious area with subgrade slopes greater than 2 percent, the flow
control standard is not achieved. The mitigated area shall be calculated using the flow control credit (Table 4).
Aggregate shall be washed gravel with a minimum void volume of 20 percent.
Slope of the subgrade underlying the permeable pavement shall be less than 5 percent.
No underdrain or impermeable layer shall be used.
For subgrade slopes greater than 2 percent, higher flow control credit may be achieved if you design the system
with subsurface, low permeability check dams to ensure water storage along the sloping subbase.
Permeable Concrete Permeable asphalt
3
1 See Edmonds Community Development Code (ECDC) Chapter 19.10.
4
2 See ECDC Chapter 23.80.
- 4 -
Table 3 (continued). BMP Design Requirements for the Simplified Sizing Method
Bioretention (Rain Garden)
The site drainage area contributing runoff to an individual bioretention facility shall be sized to collect runoff
5
from an area no larger than 5,000 square feet square feet of impervious surface.
Bioretention bottom area shall be sized using the simplified sizing method.
Ponding depth shall be as specified (6 or 12 inches).
Top area (total facility footprint) will be larger than the bottom area. You can calculate top area as a function of
the bottom area, the side slopes, and the total facility depth (e.g., ponding and freeboard depth).
Bottom area shall be flat (0 percent slope).
The minimum freeboard measured from the invert of the overflow channel to the of the cell overtopping
elevation shall be 6 inches.
Rain garden shall not be placed on a slope 5% or greater
Side slopes shall be no steeper than a ratio of 3H (horizontal):1V (vertical).
Imported bioretention soil per the specification in Attachment 2 shall be used. This specification is included in
this handout. This soil mix meets the Ecology treatment soil requirements, has a design infiltration rate of 3.0
inches per hour, and has 40 percent porosity.
Because imported bioretention soil is used, the design infiltration rate of the underlying native soil does not
require a correction factor (i.e., the design, or long-term infiltration rate is the same as the initial infiltration
rate).
Bioretention soil depth shall be a minimum of 12 inches for flow control. If your bioretention facility will be
used for water quality treatment in addition to flow control, the soil depth shall be a minimum of 18 inches.
No underdrain or impermeable layer shall be used.
Raingarden
5
The area limitation is to ensure that bioretention facilities are small-scale and distributed. Also, the assumed infiltration rate correction factor applied
bioretention soil mix is based on a contributing area that does not exceed 5,000 square feet of impervious surface.
- 5 -
Table 3 (continued). BMP Design Requirements for the Simplified Sizing Method
Infiltration Trench
Infiltration trench length shall be determined using the simplified sizing method.
The infiltration rate used to determine the required length shall be the design, or long-term, rate and must be
calculated using correction factors in the Stormwater Supplement.
Trench cross section shall be 24 inches wide by 18 inches deep, or 36 inches deep.
One pipe per trench. If multiple trenches are used, the spacing shall not be closer than 10 feet, measured on
center.
Trench aggregate shall have a minimum void volume of 30 percent (1-1/2 to 1-3/4 inch washed rock).
The location of the overflow shall be above the washed rock trench.
Trenches located under driving areas must have 2 feet of cover from top of wearing course to the top pipe.
Gravelless Chamber
Chamber length shall be determined using the simplified sizing method.
The infiltration rate used to determine the required length shall be the design, or long-term, rate and must be
calculated using correction factors in the Ecology Manual.
Void (storage) volume provided by the chamber shall be at least 2.6 cubic feet per linear foot and infiltrative
surface under the chamber footprint shall be at least 2.8 square feet per linear foot.
Invert elevation of the overflow conveyance shall be set above the storage reservoir.
Drywell
Drywell area shall be determined using the simplified sizing method.
The infiltration rate used to determine the required area shall be the design, or long-term, rate and must be
calculated using correction factors per the Ecology Manual.
Minimum aggregate depth before overflow shall be as specified (4 or 6 feet).
Drywell aggregate shall have a minimum void volume of 30 percent (1-1/2 to 1-3/4 inch washed rock).
6
subsurface soil. In these situations the mitigated
FLOW CONTROL CREDITS AND SIMPLIFIED
impervious surface area is calculated as the product of
BMP SIZING
the flow control credit and the permeable pavement
area (Table 4). The area mitigated for Category 2
Pre-designed flow control BMPs can be evaluated
Small sites is calculated as follows:
using flow control credits (runoff reduction credits) or
sizing equations (relating the facility size to the
Area Mitigated = 84% X Permeable Pavement Area
impervious area mitigated).
Note that full credit is not achieved in this example,
Flow Control Credits and your site design would require additional flow
control measures for the permeable pavement area to
Flow control credits apply only to the use of
meet the flow control standard. The effective
permeable pavement. The flow control credit values in
impervious area to be additionally mitigated will be
Table 4 are based on how this BMP achieves the
16% of the permeable pavement area. One alternative
Edmonds-specific flow control standard. To use the
would be to use another BMP that a will achieve the
credits, your permeable pavement system must meet
full standard instead of the permeable pavement with
the specific design requirements presented in Table 3.
greater than 2% slope. Contact the City Engineering
Division for additional options
Permeable pavement installed with subgrade slopes
.
exceeding 2% do not receive 100 % credit for
Category 2 Small Site Projects because of increased
potential for lateral flow through the aggregate storage
reservoir along the top of the lower permeability
6
An exception to this is when permeable pavement installations on subgrade
slopes between 2 and 5 percent are designed to include measures such as
impermeable berms perpendicular to subgrade slope. Contact the
Engineering Division for more information on this subject.
- 6 -
Table 4. Flow control credits for Permeable Pavement.
a
Flow Control Credit
BMP
Category 1 Sites Category 2 Sites
b
Permeable Pavement (up to 2% subgrade slope) 100% 100%
bcd
Permeable Pavement (2-5% subgrade slope) 100% 84%
a
Impervious area mitigated by a BMP is calculated as: \[Flow Control Credit (%)/100\] x \[Dispersed Area or Pavement Area\]. Permeable pavement at
a slope exceeding 2 percent does not achieve full (100%) credit, and as such the site design would require additional flow control measures to meet
the flow control standard.
b
Aggregate depth (for storage) must be at least 3 inches.
c
Actual modeled credit is 94%, but 100% is granted
d
If you wish to receive full flow control credit for a permeable pavement BMP on a slope between 2 and 5 percent, include subsurface berms in your
design to provide a minimum of 3 inches of subsurface ponding in the storage aggregate reservoir. Permeable pavement installations on subgrade
slopes greater than 5 percent do not receive flow control credit.
To use the sizing factors in Table 5 your BMP design
Sizing Factors
must meet the specific design requirements in Table
3. To be conservative, design infiltration rates for the
Sizing factors for LID and traditional infiltration
native soils must be rounded down to the nearest rate
facilities are provided by design soil infiltration rates
evaluated (e.g., if your site has a design infiltration
in Table 5. You can use these factors to calculate the
rate of 0.75 inches per hour, the sizing factors for 0.5
BMP size as a function of the impervious area
inches per hour shall be used). You may linearly
draining to it, as follows:
interpolate between the design depths evaluated (e.g.,
BMP Area (square feet) = Impervious Area (square
you can interpolate the required bottom area of your
feet) x Sizing Factor (%)/100, or
bioretention facility if the ponding depth will be
between 6 and 12 inches using the values listed for
BMP Length (feet) = Impervious Area (square feet) x
those pre-sized ponding depths in Table 5), but you
Sizing Factor (%)/100
may not extrapolate beyond the design parameters
For example, the size of a bioretention cell with 6
used in the pre-sizing calculations (e.g., you cannot
inches of ponding storage depth at a Category 1 site
extrapolate the required bottom area for design
with a native soil design infiltration rate of 0.5 inches
ponding depths less than 6 inches or greater than 12
per hour would be calculated as 6.8 % of the
inches for bioretention facilities).
impervious area draining to it. For 1,200 square feet
of contributing area, the bioretention cell area is
Water Quality Treatment using Infiltration
calculated as:
Facilities
Bioretention Bottom Area = Impervious Area
When sized to achieve either Category 1 or Category
(1,200 square feet) x Sizing Factor (6.8/100) =
2 site standards (Table 5), infiltration facilities
81.6 square feet.
infiltrate at least 98 percent of the influent runoff.
Therefore, these facilities will also meet the Citys
Important
: this bioretention area calculated in Table
water quality treatment standards when the native or
5, is the bottom area. The top area (total facility
imported soil on your site meets the treatment soil
footprint) will be larger than the bottom area. You
requirements set forth in the Ecology Manual (2005).
can calculate top area as a function of the pre-sized
bottom area, the side slopes, and the total facility
MAINTENANCE REQUIREMENTS
depth (accounting for design ponding depth and
ECDC Section 18.30.090 requires privately-owned
additional freeboard depth).
stormwater management facilities, such as LID
BMPs be properly maintained. The owner of the
Similarly, the length of an infiltration trench with 3
property is the responsible party for such
feet of aggregate storage depth where the native soil
maintenance. The system must be kept in good
design infiltration rate is 0.25 inches per hour would
working order. The entire system should be
be calculated as 5.4 percent of the impervious area
inspected once per year. An improperly maintained
draining to it. Area must be expressed in square feet,
BMP may cause private property or street flooding.
and length must be expressed in feet.
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Contact the City Engineering Division for
WSU. 2009. Bioretention Soil Mix Review And
maintenance information.
Recommendations For Western Washington. Prepared
for: Puget Sound Partnership. Prepared by: Curtis
The City may make periodic inspections of BMPs to
Hinman WSU Extension Faculty. Laboratory analysis
ensure they are operating properly. ECDC Section
performed by Shannon and Wilson,. Compost
18.30.100 contains the enforcement provisions the
Specifications provided by David MacDonald Seattle
City can use to ensure the system is properly
maintained.
Public Utilities. January.
.
WSU. 2005. Low Impact Development Technical
REFERENCES
Guidance Manual for Puget Sound. Washington State
University (WSU) Pierce County Extension and Puget
Ecology. 2005. Stormwater Management Manual for
Sound Action Team. January.
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.
Edmonds. 2010. Stormwater Code Supplement to
Edmonds Community Development Code Chapter
18.30. City of Edmonds. April.
Herrera. 2010. Stormwater Best Management Practices
Simplified Sizing Tool. Technical Memorandum
prepared for City of Edmonds. Herrera Environmental
Consultants. April 29.
Kitsap County. 1997. Kitsap County Stormwater
Management Design Manual. Kitsap County
Department of Public Works. April.
Seattle, 2009. Stormwater Manual Vol. 3 of 4:
Stormwater Flow Control and Water Quality
Treatment Technical Requirements Manual,
Department of Planning and Development, Directors
Rule 17-2009 and Seattle Public Utilities, Director s
Rule 2009-005.
WDOT.2010. Standard Specifications for Road,
Bridge and Municipal Construction. M-41-10.
Washington Department of Transportation.
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Table 5. Bioretention and Infiltration BMP Sizing Factors.
Sizing Factor
(% of contributing
impervious area)
Native Soil Design
Infiltration Rate
Category Category
BMP Facility Overflow Depth (inches/hour) a b Sizing Equation
1 Sites 2 Sites
Bioretention 6 inch ponding depth 0.25 9.7% 9.7% Bioretention Bottom Area (square
c,d
Cell feet) = Impervious Area (square
0.5 6.8% 6.8%
feet) x Sizing Factor (%)/100
2.0 3.6% 3.9%
12 inch ponding depth 0.25 6.2% 6.2%
0.5 4.2% 4.2%
2.0 2.2% 2.3%
Infiltration 1.5 foot depth 0.25 9.1% 9.1% Infiltration Trench Length (feet) =
e
Trench Impervious Area (square feet) x
0.5 6.2% 6.2%
Sizing Factor (%)/100
2.0 2.3% 2.9%
3 foot depth 0.25 5.4% 5.4%
0.5 4.0% 4.0%
2.0 1.8% 2.0%
Gravelless NA 0.25 3.7% 3.7% Gravelless Chamber Length (feet)
e
Chamber = Impervious Area (square feet) x
0.5 2.7% 2.7%
Sizing Factor (%)/100
2.0 1.3% 1.4%
e
Drywell 4 foot depth 0.5 6.2% 6.2% Drywell Area (square feet) =
Impervious Area (square feet) x
2.0 3.0% 3.1%
Sizing Factor (%)/100
6 foot depth 0.5 4.7% 4.8%
2.0 2.4% 2.5%
a
Sizing factors developed to limit the post-development 10-year recurrence interval flow to 0.25 cubic feet per second (cfs) per acre of impervious
surface area.
b
Sizing factors developed to limit recurrence interval flow rates to: 2yr=0.07 cfs/acre; 10yr=0.25 cfs/acre; and 100yr=0.45 cfs per acre of
impervious surface area.
c
BMP area is calculated as a function of impervious area draining to it: BMP Area (square feet) = Impervious Area (square feet) x Sizing Factor
(%)/100.
d
Sizing factors are for bioretention facility bottom area. Total footprint area may be calculated based on side slopes (3H:1V), ponding depth, and
freeboard.
e
BMP length is calculated as a function of impervious area draining to it: BMP Length (feet) = Impervious Area (square feet) x Sizing Factor
(%)/100.
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Attachment 1
Typical Designs of Low Impact Development Best Management Practices
Note: These are not standard details and are for reference only
Figure A-1. Typical permeable pavement.
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Figure A-2. Typical bioretention (rain garden).
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Varies
Varies
Varies
Varies
Figure A-3. Typical downspout infiltration trench (Figure 3.2 in Volume III of Washington
State Department of Ecologys Stormwater Management Manual for Western
Washington \[Ecology 2005\]).
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Solid lid with surface access
Figure A-4. Typical gravelless chamber (Figure 5.27 in Appendix 5A of the Kitsap County
Stormwater Management Design Manual \[Kitsap County 1997\]).
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Varies
Figure A-5. Typical drywell (Figure 3.4 in Volume III of Washington State Department of
Ecologys Stormwater Management Manual for Western Washington \[Ecology
2005\])
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Attachment 2
RAIN GARDEN (BIORETENTION) SOIL MIX SPECIFICATIONS
Bioretention Soil shall consist of a well-blended mixture by volume of 3 parts mineral aggregate and 2 parts
compost meeting the requirements below.
MINERAL AGGREGATE
Mineral aggregate shall be analyzed by an accredited lab using #200, #100, #60, #40, #20, #10, #4, 3/8 and
1-inch sieves; and meets the following:
Sieve Size Percent
Passing
3/8 100
No. 4 95-100
No. 10 75-90
No. 40 25-40
No. 100 4-10
No. 200 2-5
7
Efforts should be made to use aggregate with gradation meeting Coefficient of Uniformity (CU) equal to 6
8
or above; and Coefficient of Curvature (CC) of 1 to 3.
As a substitute Fine Aggregate for Portland Concrete, 90-3.1(2)B, Class 1 (WDOT 2010) can be used.
COMPOST
Compost is defined by the Washington Dept. of Ecology as: the result of the biological degradation of Type I
or III feedstocks, under controlled conditions designed to promote aerobic decomposition, per WAC 173-
350-220, and meet the following physical criteria. The Department of Ecology has a current list of
composting facilities that meet this standard. Go to http://www.ecy.wa.gov/programs/swfa/compost/ and
look for Current Composting Facilities in Washington. As of December 2010, the following suppliers were
on Ecologys list of approved compost suppliers in Snohomish County:
Bailand Farms Yardwaste (Bailey) Compost
·
Cedar Grove Composting Co.
·
Lenz Enterprises Inc
·
Misich Farms/Riverside Topsoils
·
Pacific Topsoils (PTI)
·
Some of these suppliers compost can be found at local home improvement stores in 1 or 1.5 cubic feet bags.
For additional information go to: http://www.soilsforsalmon.org/pdf/Soil_BMP_Manual.pdf.
D
60
7
CU=
The ratio of the diameter of a grain size, in millimeters, indicated by the gradation curve at the 60-percent passing (by weight), to the
D
10
the diameter of a grain size, in millimeters, indicated by the gradation curve at the 60-percent passing (by weight).
2
()
D
30
8
CC=
D is the grain size, in millimeters, indicated by the gradation curve at the 30-percent passing level. D and D as
30 1060
D´D
1060
.
described in footnote no. 1
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#E72 C
COMPOST-AMENDED SOIL: HOW TO MEET THE STORMWATER
MANAGEMENT REQUIREMENT FOR SMALL SITE PROJECTS
INTRODUCTION to City standards. For more information on
whether your site is required to meet these
This handout helps you to:
requirements see Handout #E72, ECDC Chapter
1.Determine which areas of your site, if any,
18.30 and the Stormwater Code Supplement.
require compost-amending.
2.Provides two options for meeting the
DEFINITIONS
requirement including detailed description
Disturbed pervious surface
: Any part of a
of how to compost-amend the soil and what
pervious area that is disturbed during a
materials to use.
development or redevelopment project, but
The requirements in this handout only apply to
remains pervious after the project is completed.
11
Small Site projects. Large Site Projects that are
subject to the requirements of the Western
Converted pervious surface
: Land cover changed
Washington Phase II Municipal Stormwater
from native vegetation to lawn, landscape, or
Permit (Ecology 2009) should not use this
pasture areas.
handout. If soil amending is required at a Large
Site Projects, the soil quality and depth BMP
OPTIONS FOR MEETING THE
T5.13 for compost-amending, in Chapter 5 of
STANDARD
Volume V of the Stormwater Manual must be
used.
Table 1 provides two options:
1.Compost-amending existing soils
APPLICABILITY
2.Importing a topsoil/compost mix.
Small site projects that need to meet Minimum
Requirement (MR) #5, Onsite Stormwater
Either method is acceptable if the requirements in
Management/Low Impact Development
the table are met. The table is divided into two
Techniques for Controlling Runoff and MR #7,
columns: the first column provides the
Flow Control must also compost-amend soils in
information on how to comply with the
disturbed pervious surfaceconverted
all and
requirement in turf (grass or lawn) areas; the
pervious surface
areas (see definitions below).
second column provides the information on how
This Best Management Practice (BMP) is
to comply with the requirement for planting bed
required to restore the water holding capacity of
areas. Figures 1 and 2 are representative
the soil in these areas. The standards for MR#5
examples of a cross-section of the final product
and/or MR #7 are based on the assumption that
using compost-amending option.
all disturbed pervious surface and converted
pervious surface areas will be compost-amended
1
See Handout #E72 for site classification.
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