Stormwater Memo.pdfRAS Engineerin
To: JoAnne Zulauf
City of Edmonds
Engineering Division
From: Rob Long, PE
RAM Engineering
Subject: Cheeks Short Plat- Lot 2— 22922102nd pl W
Stormwater Memo
RAM Job No. 15-016
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Date: October 7, 2016
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This memo provides the design analysis of the stormwater drainage control system for Lot 2 of the Cheeks Short Plat
(22922 102nd PI W). A gravelless chamber system is proposed to control the proposed addition of 3,500 sf of
impervious area for Lot 2. A site specific 2.0 in/hr design infiltration rate was determined by the geotechnical engineer,
The stormwater gravelless chamber has been sized using the City of Edmonds Simplified Sizing Tables (handout #E72 -B)..
The below tables provide the sizing information for the infiltration drywell systems.
Project Area Summary:
Proposed Impervious = 3,500 sf
Building Rooftop, Covered Deck and Patio (roof line) = 2,860 sf
Uncovered Patio and Walks = 120 sf"
Driveway (pollution generating) = 520 sf EGINEE I
Individual Lot Gravelless Chamber Sizing Summary:
Individual Lot
Total Impervious = 3,500 sf
Required Chamber Length (3,500 sf x 0.014)1 49.0 ft
Required Chamber Storage (49.0 ft @ 2.6 cf/ft) 127.4 cf
Required Chamber Surface Area (49.0 ft @ 2.8 sf/ft) 137.2 sf
Proposed System Hancor Arc 36 HC Storage 63.0 ft
Provided Chamber Storage [63.0 ft @ 2.04 cf/ft (10.7 cf / 5.25 ft)z ] 128.5 cf
Provided Chamber Surface Area [63.0 ft @ 2.34 cf/ft (12.27 cf / 5.25 ft)Z ] 147.4 sf
Gravelless Chamber Size: 63.0 ft long Hancor Arc 36 HC2
1. 1.4% impervious surface sizing factor per Edmonds simplified sizing charts (with 2.0 in/hr soils and Category 2 Site)
2. See Hancor Arc 36 HC specification brochure (segment length 63 in., capacity 10.7 cf, open bottom area 12.27 sf)
Infiltration Trench Design Criteria per Edmond's Handout #E72B:
Design Criteria for All BNNIPs
• These BMPS shall not N used in:
The North Edmonds Earth Suhsidence and Landslide Hazard Area (ESLHA)'
Upgradient (east) of the ESLHA
In a Landslide Hazard Areal
Within 5 feet ol'property lines and easements
Within 10 feat from underground storage ranks
Within 50 feet from proposed or existing septic sysleins or drain fields.
Where the depth io the top of tie: seasonal groundwater level is less than I foot from the bottom of the
facility.
• The BNIP most inect the following setback requirements front buildings:
- if runtiffis front an area less than 50) square feet, setback is 5 feet from .1 SITUClUre without a
basement and 10 fee[ fora sirticuire with a basement.
- If runoff is front ;in area greater than or equal to 5.0)0 square feet. a structure shall not interact
with a 114orizontal: lVerlical slope from the bolloin edge ofthe facility. Minimum 5 ket froin a
structure without a basement and 10 feet for a structure with it basenient.
•
An overflow path intisi be specified such that runoffabove the racilky's design capacity does not cause:
Flooding of building or emergency access
Erosion or downstream sedi inentalion
Slope failure,
101-7116
Attachments:
A. Edmond's Handout #E72B, Figure A-4 Typical Gravelless Chamber Detail
B. Hancor Arc 36 HC System Brochure (2 pages).
RAM Engineering 1653113 1h Ave W, Suite A108 Lynnwood WA 98037 (425) 678-6960 RAMengineeringinc corn
Sizing Factor
(`<c of contributi1w,
Native Soil Desi -0n
imperviousarea)Infiltration
Rate
Category Category
BMP Facility Overflow Depth (inche 4 -hour)
I Sites, 2 Sites
Sizing Equation
0.25
3.7% 3.746
. ....................... . . . .........
. ..... ......... .. .
U.._
2,706 2-7%
.......... - , --
Gsavelles,� Chamber Lerielli, (feet)
NA 1.0
1.9% 1.9%
Impel viotis Area (square feet) x
Chamber
— - - - - ----------
Sizitig Factor (%)100
I.i% 1.6%
.......... .
BMP leng th is calcu hied as a function of i nijxrvious area drai a ing to i 0: BMP
Length (1,M) = Impervious
Area (situare feel i x Sizing Faclor
Design Criteria for All BNNIPs
• These BMPS shall not N used in:
The North Edmonds Earth Suhsidence and Landslide Hazard Area (ESLHA)'
Upgradient (east) of the ESLHA
In a Landslide Hazard Areal
Within 5 feet ol'property lines and easements
Within 10 feat from underground storage ranks
Within 50 feet from proposed or existing septic sysleins or drain fields.
Where the depth io the top of tie: seasonal groundwater level is less than I foot from the bottom of the
facility.
• The BNIP most inect the following setback requirements front buildings:
- if runtiffis front an area less than 50) square feet, setback is 5 feet from .1 SITUClUre without a
basement and 10 fee[ fora sirticuire with a basement.
- If runoff is front ;in area greater than or equal to 5.0)0 square feet. a structure shall not interact
with a 114orizontal: lVerlical slope from the bolloin edge ofthe facility. Minimum 5 ket froin a
structure without a basement and 10 feet for a structure with it basenient.
•
An overflow path intisi be specified such that runoffabove the racilky's design capacity does not cause:
Flooding of building or emergency access
Erosion or downstream sedi inentalion
Slope failure,
101-7116
Attachments:
A. Edmond's Handout #E72B, Figure A-4 Typical Gravelless Chamber Detail
B. Hancor Arc 36 HC System Brochure (2 pages).
RAM Engineering 1653113 1h Ave W, Suite A108 Lynnwood WA 98037 (425) 678-6960 RAMengineeringinc corn
Attachment A. Edmond's Handout #E728, Figure A-4 Typical Gravelless Chamber Detail
Profile View
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The following products are known to mcet these glavelless chamber ciitwis
i
Carfiltsaalaal 11-8�1a CalA.-ity by lnfilga ato% systems, Inc.
I rt tc t, ba tt#zcrt Nigh Capacity by k aort or, Inc.
Stormtechtr SC -310 by In£tltralor Systems. Inc
Figure A-4. Tvpical grarelless chamber (Figure 5.27 in Appendix 5A of the Kitsap Count. Storialroler
Mrrnagenrent Design.,Vemmil [ itsap "ount% 1.9971.).
A-1
RAM Engineering 1653113th Ave W, Suite A108 Lynnwood WA 98037 (425) 678-6960 RAMenginee ringinc.conn
intrusive installations.
The Arc 36 HC septic leaching chamber is a sturdy, lightweight plastic
unit that combines maximized infiltrative surface area and storage
capacity with an improved structural design to handle most any
conventional leachfield system challenge without sacrificing performance
This unique combination allows for increased effluent dispersal
installation and simplified contouring capabilities,
UUFTNIT���
• Injection -molded from High Density Polyethylene
(HDPE) for lightweight and sturdy design
• 20 -degree integral articulating joint that is ideal
for either straight or contoured septic leachfield
applications
• True corrugated chamber design eliminates
flat surfaces and provides increased load
bearing capability in the trench
• Designed to accommodate both gravity -fed
anA pressure -dosed systems
• 'Lock and Drop' joint provides a more positive
connection during installation and backfill
• A universal inlet/outlet end cap
• Inspection vent ports on every unit with
easy-to-4emove knockouts for maximum
job site flexibility
• Convenient five-foot lengths are easy to handle
• Quickly installed by one person into three-foot
wide trench or bed applications
component which snaps in place to allow side
entry at any joint throughout the trench line
Diamond plate texture increases slip resistance
and enhances ease of installation
B-1
RAM Engineering 1653113 1h Ave W, Suite A108 Lynnwood WA 98037 (425) 678-6960 RAMengineeringinc.com
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