Stormwater Memo.pdfRAM Engineering
to Memorandum
This memo provides the design analysis of the stormwater drainage control system for Lot 3 of the Cheeks Short Plat
(22924102nd PI W). A gravelless chamber system is proposed to control the proposed addition of 3,350 sf of
impervious area for Lot 3. 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 = 3,350 sf
Building Rooftop, Covered Deck and Patio (roof line) = 2,830 sf
Uncovered Patio and Walks = 110 sf
Driveway (pollution generating) = 410 sf
Individual Lot Gravelless Chamber Sizing Summary:
Individual Lot
Total Impervious = 3,350 sf
Required Chamber Length (3,350 sf x 0.014)1 46.9 ft
Required Chamber Storage (46.9 ft @ 2.6 cf/ft) 121.9 cf
Required Chamber Surface Area (46.9 ft @ 2.8 sf/ft) 131.3 sf
Proposed System Hancor Arc 36 HC Storage 60.0 ft
Provided Chamber Storage [60.0 ft @ 2.04 cf/ft (10.7 cf / 5.25 ft)z ] 122.4 cf
Provided Chamber Surface Area [60.0 ft @ 2.34 cf/ft (12.27 cf / 5.25 ft)z ] 140.4 sf
Gravelless Chamber Size: 60.0 ft long Hancor Arc 36 HCz
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 #E728:
Design Criteria for All BMPs
M Thew BMPS shall not be used in:
The North Edmonds Earth Subsidence and Landslide Hazard Area (ESLI-IA)3
Upgradient (east) of the ESLHA
In a Landslide Ha lard Area"
Within 5 feel of propcity lines and easements
Within 10 feet from underground storage tanks
Within 50 feet from proposed or existing seplic systems or drain fields.
Where the depth to the top of the seasonal groundwater level is less than I toot front the bottom of the
facility.
The BMP must meet the following setback requirenients front buildings:
- 11'rttuoff is from an area less than.5,(W square Icer, setback is 5 feet froth a structure without a
basement and 10 feet for a structure with a basement.
- If runoff is front an area greater than or equal to 50M square feet. a su tiClUre shall not intersect
with a [Horizontal: [Vertical slope front the bottom edge of the facility. Mininunn 5 icet front a
siructure without a basement and 10 feet for a structure with a basentent.
M An overflow path must he specified such that runolT above the facilily's design capacity does not cause:
Flooding of a building or emergency access
Erosion or downstreamt sedimentation
Slope failure.
101,71,16
Attachments:
A. Edmond's Handout ##E72B, Figure A-4 Typical Grovelless Chamber Detail
B. Hancor Arc 36 HC System Brochure (2 pages).
"M Engineering 1653113`hAve W,suite A108 Lynnwood WA 98037 (425)678-6960 RAMengineeringinc.com
Siring Factor
(`'L of contributing®
Native Soil Design
impervious area)
Infiltration Rate
Categon' Category
WHIP Facility Overflow Depth (ineheJhotn-)I
Sites 7 Siles b
Sizing Equation
0.15
3-70m 17%
.. ,
04
.._
..
7
w.-. 1 7%
Grarelless Chaznber Length (feet)
t @Ilea$ �.
N�1 0
1.9°o
= Imperi"ious Area (square feet) x
Chamber'
Sizing Factor (°0)'100
LS
15% l.G°o
0a 1 1�VW
BMP length is calculated as a funclion of imixrvious arca draining to it: BMP
Length Ifeet)= Intlxrvious
Arca (square teal x Sizing Factor
1 �/, )I I W,
Design Criteria for All BMPs
M Thew BMPS shall not be used in:
The North Edmonds Earth Subsidence and Landslide Hazard Area (ESLI-IA)3
Upgradient (east) of the ESLHA
In a Landslide Ha lard Area"
Within 5 feel of propcity lines and easements
Within 10 feet from underground storage tanks
Within 50 feet from proposed or existing seplic systems or drain fields.
Where the depth to the top of the seasonal groundwater level is less than I toot front the bottom of the
facility.
The BMP must meet the following setback requirenients front buildings:
- 11'rttuoff is from an area less than.5,(W square Icer, setback is 5 feet froth a structure without a
basement and 10 feet for a structure with a basement.
- If runoff is front an area greater than or equal to 50M square feet. a su tiClUre shall not intersect
with a [Horizontal: [Vertical slope front the bottom edge of the facility. Mininunn 5 icet front a
siructure without a basement and 10 feet for a structure with a basentent.
M An overflow path must he specified such that runolT above the facilily's design capacity does not cause:
Flooding of a building or emergency access
Erosion or downstreamt sedimentation
Slope failure.
101,71,16
Attachments:
A. Edmond's Handout ##E72B, Figure A-4 Typical Grovelless Chamber Detail
B. Hancor Arc 36 HC System Brochure (2 pages).
"M Engineering 1653113`hAve W,suite A108 Lynnwood WA 98037 (425)678-6960 RAMengineeringinc.com
Attachment A. Edmond's Handout #E728, Figure A-4 Typical Gravelless Chamber Detail
Profile View
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The. toliewing products are known to meet these giavelles! c}�� *n criteria
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h".nviroCIvarnbori, Bi h Cespacity by TrIn rr or, ]
StvxnAechC SC.310 by Infiltrator SyVems, Inc
Figure A-4. Tyical gravelless chalnber (Figure 5.27 in Appendix 5A of the Mtsap CountN Slormirater
,i'tllanageal entDei Manual jKitsap County 19971).
A-1
RAM Engineering 1653113" Ave W, Suite A108 Lynnwood WA 98037 (425)678-6960 RAMengineeringinc.com
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intrusive installations.
AM11 111 11 , I , I J
The Are 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 sac6ficing performance,
This unique combination allows for increased effluent dispersal
installation and simplified contouring capabilities.
FEATURES & BENEFITS:
• Injection -molded from High Density polyethylene
(HDPE) for lightweight and sturdy design
• 20 -degree iliiintegral aill iliirticulaii ting nt that is ideal
for either straight or contoured septic leachfield
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• True corrugated chamber design eliminates
flat surfaces and provides increased load
bearing capability in the trench
• Designed to accommodate both gravity -fed
and 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-tD-remove 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
'i WI
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B-1
RAM Engineering 1653113th Ave W, Suite A108 Lynnwood WA 98037 (425) 678-6960 RAM engineeringi nc.corn
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