Revised_Drainage_Report_Johnson_8-6-12.pdfE R I C 1-4 0. 1 E TZ E A'N D A S S 0 C
arid ConsWtants f,
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City of Edmorids August 6, 2012
Engineering Division
121 5"' Avenue North
Edmonds, WA 98020
Subject. Revised Drainage Report for Garage and Deck Addition,, 843 T-Tindley Lane
'This report has been revised based on comments received frorn Jeriy Schuster, City Stoirnwater
Engineering Program Manager in a memo dated August 1, 2012 from Jennifer Lambert.
The site at 84.3 Hindley Lane proposes a new garage addition and removal of an existing deck and
construction of a new deck.
The project proposes construct 1,,193 SF of new impervious area, and mitigate 1,660 SF of
regulated improvements constructed in 1999 that have not been mitigated. In addition 657 SF of
existing non-regulated roof will be mitigated. The following summarizes, the existing and
proposed impervious areas.
Non-Reaulated Area (Prior to 1977): regulated (Post 1977,h
1974: Existing Vlouse Roof - 1,858 SF 1999: Existing Asphalt Drive — 1,215 SF
Existing Walk - 111 SF (Remove and Replace 1, 171 SF in Place)
Existing Concrete Drive - 663 SF Existing Gravel Drive - 543 SF (Pave)
Existing Asphalt Drive - 236 SF Total 1, 1714. SF
Total - 2,868 SF
FR Yj
New Construction
New Garage Roof - 768 SF
New Concrete Pad -1425 SF (Under New Deck) J,�
* 657 SF of Existing House Roof Will be Mitigated With Garage Roof
Total - 1,850 SF
The total amount of impervious area that will be mitigated is 3,564 SF and consists of 1,714 SF of
regulated, replaced areas; 1,193 SF of now area; and 657 SF of non-regulated area. Using Figure
B of Handout 4E72, Storinwaler Management (SWM) Drainage System Design and Erosion&
Sediment Control RequiremeniN and Handout HE72 B, Loiv Impact Development (LID)
Storinovaler Best Management Praetices (BMPs)Simplified Sizing, this prqject is classified as a
Small Site Category 1, Watershed Project since the amount of new plus regulated, replaced
impervious area (2,907) is between 2,000 and 5,000 SF and the project discharges to Hindley
Creek.
The impervious areas to be mitigated will have flow control provided by a new rain garden and
two new infiltration trenches. The soils of the site consist of Sandy Loam per the soils report by C
& H Designs (attached).
According to Table C-1, Recommended Infiltration Rates Based on USDA Soil Textural
Classification, Appendix C of Exhibit A Stormwater Code Supplement to Edmonds Community
Development Code Chapter 18.30, Sandy Loam has a short term infiltration rate of 1 in/hr and a
correction factor of 4 which results in a long term infiltration rate of 0.25 in/hr. Rain gardens are
designed using the short term infiltration rate of 1 in/hr. The infiltration trenches are designed
using the long term infiltration rate of 0.25 in/hr.
Table 5 of Handout #E72 B, Low Impact Development (LID) Stormwater Best Management
Practices (PMPs) Simplified Sizing was used to size the infiltration trenches and rain garden.
Table 5 only lists design infiltration rates of 0.25, 0.5 and 2.0 inches per hour. Sizing factors for
an infiltration rate of 1.0 in/hr is not shown, so the design infiltration rate of 1.0 in/hr for the rain
garden was rounded down to 0.5 in/hr, even though an interpolated value between 0.5 in/hr and
2.0 in/hr could have been used. The sizing factor for the rain garden is 6.8% for an infiltration
rate of 0.5 in/hr and a ponding depth of 6 inches. The sizing factor for the infiltration trenches
that was used is 9.1% based on an infiltration rate of 0.25 in/hr and 1.5 foot trench depth.
Rain Garden Design:
The contributing impervious area to the rain garden will be:
New Garage Roof — 768 SF
Existing House Roof — 657 SF
Replaced Asphalt Drive — 623 SF
Pave Existing Gravel Drive — 543 SF
Total — 2,537 SF
Per Table 5, the sizing factor for a Bioretention Cell (Rain Garden) with a 6 -inch ponding depth, a
native soil infiltration rate of 0.5 in/hr is 6.8% X contributing impervious area = 0.068 X 2,591 =
176.2 SF of rain garden bottom area. The area provided will be 176.2 SF. The rain garden will
have 1 FT of bioretention soil, a 6 -inch ponding depth and 6 inches of freeboard. Overflow will
occur to a 2 -foot wide, grass -lined swale having a slope of 1.0 percent.
Infiltration Trench 41 Design:
The contributing impervious area to this trench is 425 SF of new concrete pad under a new deck.
Per Table 5, the sizing factor for an Infiltration Trench having a 1.5 foot depth and a native soil
infiltration rate of 0.25 in/hr is 9.1% X the contributing impervious area = 0.091 X 425 = 38.7 LF.
This trench length presumes a trench width of 2 feet; resulting in a total trench surface area of
77.4 SF. The actual trench will be 2.75 ft wide by 28.25 LF for a total surface are of 77.7 SF.
The trench depth will be 1.5 feet deep.
Infiltration Trench #2 Design:
The contributing impervious area to this trench is 548 SF of replaced asphalt drive. Per table 5,
the sizing factor for an infiltration Trench having a 1.5 foot depth, a native soil infiltration rate of
0.25 in/hr is 9.1% X the contributing impervious area = 0.091 X 548 = 49.9 LF. This trench
length presumes a width of 2 feet; resulting in a total trench surface area of 99.8 SF. The actual
trench will be 3.5 ft wide by and average length of 28.5 LF for a total surface area of 99.8 SF.
The trench depth will be 1.5 feet deep.
This report and the civil construction drawings satisfy the requirements of the City Engineering
Division with regard to drainage and erosion control.
Sincerely,
ERICH O. TIETZE AND ASSOCIATES
Erich O. Tietze, PE
ROBB JOHNSON
843 HINDLEY LANE
EDMONDS, WA 98020
PARCEI # 2-7032400211000
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PLOT' PLAN ROBB JOHNSON
84.3 HINDLEY LANE
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EDMONDS, WA 98020
425-771-3036
72? SF
0 Emci.i 0. TCONSULT
piETZE AND ASSOCIATES
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1714 NW 200TH LANE, SHI WA 98177 (206) 542 2065 EOTA@CLFARWIRC,NET
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PLOT' PLAN ROBB JOHNSON
84.3 HINDLEY LANE
EDMONDS, WA 98020
425-771-3036
0 Emci.i 0. TCONSULT
piETZE AND ASSOCIATES
A
ENGINEERS ND t.
....... . .
1714 NW 200TH LANE, SHI WA 98177 (206) 542 2065 EOTA@CLFARWIRC,NET
SOILS FOR INFILTRATION
Alder wood Series., Type 4 soils.
Sia 1.) 0-14" Dark Brown Sandy Loam.,14-22"Light brown sandy loam.,
22--48" tight fine silty sand., water 6 48".
SL 2.) 0-6" Dark Brown Sandy Loam.,6-24" Light Drown Sandy loam.,
SCALE 24-30" mottled clay loam. 30" hard.
none SL 3.) 0-17" light brown sandy loam., 17-24" brown sandy loam.,
24-30" mottled clay loam. 30" hard.
SL 4.) 0-22" Dark Brown Loamy Sand,22-32 reddish brown sandy loam.
32-36" Mottled Silt Loam., 36" hard.
A 3 A 2 A I
existing single family residence
A 4
M
843 Hindley Lane
C & H DeS'I nS Robb Johnson a'1a jnr/i2
Po BOX 234 'Add- 343 lfindley lane tax #
Stemwood WA 98292
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EXPIRES
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