bld20110563_O'Donnell_ENG1.pdf
City of EdmondsBLD20110563
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:
#BLD20110546August 4, 2011
Project Name/Address:
O’Donnell –841 Elm Way
Contact Person/Address, Fax or E-mail:
Ron Johnson –ronmaile@comcast.net
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 revise the scale. It appears that the drawing is a 1”= 10’instead of 1”= 20’.
2)Please show where the construction entrance will be located and add a note that it will be installed per
Edmonds Standard detail E1.2.
3)Please add a note that the silt fence will be installed per Edmonds Standard Detail E1.1.
4)Pleaseadd a note that the filter socks will be added as needed per Edmonds Standard Detail E1.3.
5)Please provide grading calculations and show bottom of footings.
6)Please provide proposed and existing topographic lines.
7)Please add a note that all disturbed soils will be compost amended (see attached handout E72C).
*** Please note that if the Site Plan becomes unreadabledue to the amount of information provided,please
provide a separate Erosion Control / Grading Plan and include the proposed and exisiting topographic lines.
If plans are separated, only proposed topographic lines will need to be on the Site Plan / Utility Plan. ***
8)Please show the location of the sewer from the main to the house (see attached as-built).
9)Please show the water service line fromthe meter to the house.
DATE E-MAILED8/4/2011PAGE(1)
City of EdmondsBLD20110563
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
10)Please add the impervious surface chart(provided in the E72 Handout),asshown below, to the Site
Plan.
LineTypeArea (square feet)
1.Non-Regulated
ExemptRegulated
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 ImpactDevelopment Techniques
=
8.Area Proposed to be Mitigated through Conventional SWM Techniques
11)Please show the year of construction for all impervious surfaces as shown on the sample site plan
attached.
12)Please show how all impervious surfaces will be collected to the proposed systems.
a.Please provide proposed topographic lines for the driveway to verify that all surface water runoff
can be collected to the proposed detention system.
b.Please show the connection of the roof drains to the proposed stormsystem.
c.Please show how the surface water will be collected for any additional impervious surfaces.
13)Please provide rim and invert elevations for the city catch basin where the proposed storm system will be
connected.
14)Please show a short segment of the city storm main line.
15)Per Edmonds City Standardsstormpipe coverage must be a minimum of 1’ when placed in landscape
areas and 2’ under driveable areas. Pleasenote that some of the storm lines are proposed at the
minimum coverage requirement.
a.Please show proposed topographic lines so that pipe coverage can be verified.
DATE E-MAILED8/4/2011PAGE(2)
City of EdmondsBLD20110563
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
16)The Site Plan showsthe driveway with varying widths. Is this accurate?
a.If the driveway does not have varying widths, please accurately show the existing and proposed
drivewayswidths.
17)Please note that the driveway entrance (west entrance) for the proposed garage must meet current City
Standards. Per ECDC 18.80.060, driveways must be paved from edge of asphalt to the property line or
20’ (whichever is greater). Please revise plans accordingly.
18)Please obtain a city Right-Of-Way permit.
a.Please provide a Right-Of-Way application, signed by a contractor that is licensed and bonded to
work within the ROW.
b.Please provide a Traffic Control Plan. The Traffic Control Plan should also address how
construction vehicles will be entering and leaving the site.
19)Please show the easement along the east 12’ of the subject property (see attached easement #2121699).
Please resubmit 3 copies of the revised plansand associated documents with a written response to each
item to a Permit Coordinator. 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-MAILED8/4/2011PAGE(3)
#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.
- 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.
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