DRAINAGE ASSESSMENT.pdfDrainage Assessment
Kempers Residence ITY COPY
9303 Olympic View Drive
Edmonds, WA 98020
The proposed project Kempers Residence is located at 9303 Olympic View Drive
Edmonds, Washington. More generally, the site is located in Section 13, Township 27 North, and
Range 3 East of the Willamette Meridian in Snohomish County, Washington. The project is
considered for a "Category 2 Small Site Project" per figure City of Edmonds Handout #E72B.
The site contains 0.42 Acres and is currently undeveloped. The developed portion of the
site exists as lawn. The other half of the site contains steep slopes that is covered with heavy
vegetation. This portion of the site will be undisturbed. The site slopes to the northwest. Per SCC
survey of Snohomish County, the project site contains Alderwood type soils that have a
hydrologic classification of Type "C".
The project is zoned as RS -12 and the proposal is to construct a new single family home
on the southwestern portion of the site. The access will be from the drive aisle off of Olympic
View Drive. An existing detention pipe within the drive aisle off of Olympic View Drive was
designed to accommodate 5,000 SF of new impervious for the site. The total proposed
impervious for the site is 5,679 SF, therefore additional mitigation for 679 SF of impervious is
required. A 62 -ft long by 2 -ft wide by 1.5 -ft deep infiltration trench will be proposed to
accommodate for the additional mitigation. The runoff from the proposed driveway will be
collected into a yard drain that will be directed towards the infiltration trench. Refer to the
following calculations for the infiltration sizing. The remainder of the developed portions of the
site, including the walkways, patios and roof will be collected via roof and yard drains and
conveyed to the existing drainage pipe located under the drive aisle. The overflow from the
infiltration trench will be connected to the existing detention pipe. Refer to the following pages
for the supporting documents and calculations.
Sincerely,
Bran R. Kalab
P.O Box 1478 • Everett, WA 98206 • P: 425.303.9363
F: 425.303.9362 # info@insightengineering.net
ff 7— jos—
Ttlliltration 'rjr Lnch Sizing
The infiltration trench was sized per handout #E72B.
Impervious Area = 679 SF
Long term infiltration rate = 0.32 iph (Refer to attached geotechnical report)
Designed infiltration rate = 0.25 iph (Per Table 5)
Depth of trench = 1.5 ft
Site Category = 2
Sizing factor = 9.1% (Per Table 5)
Infiltration Trench Length (feet) = Impervious Area (Square Feet) x Sizing Factor (%)/100
679 SF x 9.1%/100 = 61.8 ft
Therefore a 62 LF infiltration trench per figure A-3 of the handout will be used to mitigate the
proposed driveway for the site.
Insight Engineering Co. — Drainage Assessment 11/9/15
In order to prevent erosion and trap sediments within the project site, the following
BMPs will be used approximately as shown on the Grading and TESC plan:
• Clearing limits will be marked by fencing or other means on the ground.
• The driveway will be constructed and graveled immediately. A rocked construction entrance
will be placed at the end of the driveway. Cleared areas accepting sheet flow from the driveway
and parking area will be seeded and mulched.
• Runoff will not be allowed to concentrate and no water will be allowed to point discharge onto
slopes.
• Silt fencing will be placed along slope contours at the downslope clearing limit.
• Mulch will be spread over all cleared areas of the site when they are not being worked. Mulch
will consist of air-dried straw and chipped site vegetation.
Insight Engineering Co. — Drainage Assessment 11/9/15
Table 5. Bioretention and Infiltration BMP Sizing Factors.
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.
Sizing factors developed to limit recurrence interval flow rates to: 2yr=0.07 cfs/acre; 10yr--0.25 cfs/acre; and I 00yr--0.45 cfs per acre of impervious
surface area.
BMP area is calculatedas a function of impervious area draining to it BMP Area (squarc feat) = laipervious Area (square feet) x Sizing Factor (%)/100.
Sizing factors are for bioretention facility bottom area. Total footprint arca may be calculated based an side slopes (31.1:1 V), ponding depth, and flLvboard.
BMP length is calculated as a function of impervious area draining to it: BMP Length (feet) = Impervious Area (squire fcet) x Sizing Factor
(%)/100.
Revised 10/15/2015 E72B - BMP Sizing Handout page 9 of 16
Sizing
Factor
Native Soil
(% of contributing
BMP
Facility Overflow Depth
Design
impervious
area)
Sizing Equation
Infiltration Rate
(inches/hour)
Category I
Category
Sites a2
Sites b
9.7%
9.7%
i 0 .5
6.8%
6.8%
6 inch ponding depth
1.0
5.0%
5.2%
1.5
4.1%
4.4%
2.0
3.6%
3.9%
Bioretention Bottom Area (square
Bioretention
. . . ..........
feet) = Impervious Area (square
Cell
0.25
6.2%
6.2%
1 feet) x Sizing Factor (%)/100
0.5
4.2%
4.2%
12 inch ponding depth
1.0
3.1%
-2.5%
3.2%
1.5
72.6%
2.0
0.25
2.2%
9.1%
2.3%
0.5
6.2%
6.2%
1.5 foot depth of aggregate
1.0
3.8%
4.0%
1 1.5
2.9%
3.1%
2.0
2.3%
2.9%
Infiltration Trench Length (feet)
Infiltration
Impervious Area (square feet) x
5.4%
5.4%
Trench e0.25
Sizing Factor (%)/100
,5
0.5
'00
4.0%
4
4.0%
3 foot depth of aggregate
1.0
0
2.6%
2.6X
2.6%
1.5
1 5
2
2.1%
2.1%
2.0
1.8%
11 2.0%
0.25
3.7%
3.7%
. .... . .....
0.5
2.7%
2.7%
Gravelless Chamber Length (feet)
Gravelless
NA
1.0
1.9%
1.9%
Impervious Area (square feet) x
Chamber'
Sizing Factor (%)/100
1.5
1.5%
1.6%
2.0
1.3%
1.4%
0.5
6.2%
6.2%
4 foot depth
.......
1.5
3.5%
3.6%
Dry ell Area (square feet)
Impervious Area (square feet) x
3.0%
1 3.1%
Sizing Factor (%)/100
Drywell e2.0
0.5
4.7%
4.8%
1.0
3.4%
3.5%
6 foot depth
1
1.5
2.8%
2.9%
2.0
. . . . ...........
2.4%
2.5%
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.
Sizing factors developed to limit recurrence interval flow rates to: 2yr=0.07 cfs/acre; 10yr--0.25 cfs/acre; and I 00yr--0.45 cfs per acre of impervious
surface area.
BMP area is calculatedas a function of impervious area draining to it BMP Area (squarc feat) = laipervious Area (square feet) x Sizing Factor (%)/100.
Sizing factors are for bioretention facility bottom area. Total footprint arca may be calculated based an side slopes (31.1:1 V), ponding depth, and flLvboard.
BMP length is calculated as a function of impervious area draining to it: BMP Length (feet) = Impervious Area (squire fcet) x Sizing Factor
(%)/100.
Revised 10/15/2015 E72B - BMP Sizing Handout page 9 of 16
ME
November 4, 2015
Northwest, In,c,
Keith Kemper
7417 Meadowdale Beach Rd
Edmonds, WA 98026
Subject: INFILTRATION EVALUATION
PROPOSED NEW RESIDENCE
9303 OLYMPIC VIEW DR
EDMONDS, WASHINGTON
Dear Mr. Kemper:
Geotechnical Engineers, Geologists
& Environmental Scientists
G-3978
GEO Group Northwest, Inc. is pleased to submit this evaluation for the proposed infiltration of
stormwater at the project site. We recommend that the following recommendations be
incorporated into the construction plans for the new residence.
PROJECT DESCRIPTION
Based upon information provided by Insight Engineering we understand that the proposed
development at the site is to include a new single family residence. A driveway which includes
permeable pavers is proposed for the northeast comer of the property. We understand that the
majority of site stormwater will be discharged to an existing stormwater system at the existing
driveway. About 750 square feet of impervious runoff from the driveway is proposed to be
infiltrated into the existing site soils at a new infiltration trench to be located at the east side of
the property.
GEOLOGIC CONDITIONS
According to the geologic map for the site vicinity the surficial soils at the site consist of the
Transitional Beds (Qtb). The Transitional Beds typically consist of silty soils which may or may
not be glacial in origin and which lie stratigraphically below the Vashon Advance Outwash
(Qva). Advance Outwash soils typically are sands and gravels deposited by meltwater during the
glacial advance.
13240 NE 20th Street, Suite 10 Bellevue, Washington 98005
Phone 425/649-8757 Fax 425/649-8758
November 4, 2015 G-3978
9303 Olympic View Dr, Edmonds,. Washington Page 2
SUBSURFACE CONDITIONS
On October 29, 2015 a GEO Group Northwest, Inc., engineer visited the subject site and
excavated two hand -auger borings at the locations shown on the attached Plate 2 - Site Plan.
Soils encountered at the hand -auger borings consist of silty soils with crushed rock and quarry
spall fills to a depth of around 10 -inches below ground surface (bgs). Underlying these fills we
encountered a dense to very dense well -graded gravelly silty SAND with occasional cobbles to
the bottom of the borings. No groundwater seepage was observed at the boring locations.
Both borings were advanced to the maximum depth possible with hand -drilling equipment.
Dense conditions, gravels and cobbles prevented further exploration. Details regarding our
exploration are attached as Plates Al & A2 - Hand-Augered Boring Logs & Soil Legend.
We viewed the high bank at the west side of the site with the hope that we would be able to note
any seeps present at the slope, thereby determining the approximate groundwater level. Due to
heavy vegetation at the bank and the very steep inclination we could not determine a seepage
level at the bank or whether one is even present.
REVIEW OF ONLINE GEOLOGIC DATA
We reviewed online publicly available subsurface information for the site vicinity (previously
provided by GeoMapNW and now hosted by the DNR). Based upon this information test pits
performed by others in the site vicinity rarely noted the presence of groundwater seepage and
where it was noted it was at a deeper depth such as at 9.5 -feet below ground surface.
GRADATIONAL ANALYSIS
We performed gradational analysis on one soil sample from a depth of 2 -feet bgs at the HA -1
location, at the proposed infiltration facility location and depth. This analysis confirmed the soil
description as a gravelly silty SAND with occasional cobbles. The silt content finer than the
#200 sieve was determined to be 18.7 percent. Extrapolation of the gradation curve as shown on
the attached Grain Size Distribution Curve - Plate A3 indicates that the D,o value for the
proposed infiltration soils to be 0.025 mm.
GEO Group Northwest, Inc.
November 4, 2015 G-3978
9303 Olympic View Dr, Edmonds, Washington Page 3
INFILTRATION EVALUATION
Based upon our subsurface investigation the underlying site soils consist of gravelly silty SAND
with occasional cobbles. These soils may be the Advance Outwash soils which are oftentimes
located stratigraphically above the Transitional Beds anticipated for the project site. Advance
Outwash soils are sometimes relatively free draining soils. But in this case the soils observed are
not free -draining, having less than 5% passing the No. 200 sieve, however they will still drain
with a slower infiltration rate. The proposed stormwater from the 750 -square feet of impervious
surface may be collected and infiltrated via the proposed infiltration trench. We recommend that
a catchbasin with sump be installed between the driveway and the infiltration trench in order to
mitigate the risk of sediment clogging at the infiltration system. The proposed infiltration trench
detail included on the Plate 1 - Site Plan is appropriate for the subject site and specific proposed
development extent.
INFILTRATION RATE
From Darcy's Law for permeability we calculate that the initial infiltration rate for the soils at the
2 -foot deep level to range between 0.88 in/hr and 1.33 in/hr. We recommend that the designer
use an initial infiltration rate of 1 in/hr and apply the following correction factors Site Variability
CF„ = 0.9, Test Method Cft = 0.4, and degree of influent control for siltation and bio -build-up
Cf. = 0.9. This results in a long term design infiltration rate of 0.32 in/hr. Maintenance should
be required to clean-out the catchbasin sump pit and to allow the infiltration facility to continue
to function for the infiltration system design lifetime.
LIMITATIONS
This report has been prepared for the specific application to this site for the exclusive use of Mr.
Keith Kemper and his authorized representatives. Any use of this report by other parties is solely
at that party's own risk. We recommend that this report be included in its entirety in the project
contract documents for reference during construction.
Our findings and recommendations stated herein are based on field observations, our experience
and judgement. The recommendations are our professional opinion derived in a manner
consistent with the level of care and skill ordinarily exercised by other members of the profession
GEO Group Northwest, Inc.
November 4, 2015 G-3978
9303 Olympic View Dr, Edmonds, Washington Page 4
currently practicing under similar conditions in this area and within the budget constraint. No
warranty is expressed or implied. In the event that soil conditions not anticipated in this design
are encountered during site development, GEO Group Northwest, Inc., should be notified and the
above recommendations should be re-evaluated.
If you have any questions, or if we may be of further service, please do not hesitate to contact us..
Sincerely,
GEO GROUP NORTHWEST, INC.
al(4� 41�'�
Adam Gaston
Project Engineer
Attachments:
Plate 1
Site Plan
Plate Al -
Soil Legend
Plate A2 -
Hand Augered Boring Logs
Plate A3 -
Grain Size Distribution
cc: Mr. Santhosh Moolayil - sarithosh@ ins i erg i.ncerin .riet
Mr. Craig Chaney - chaney.cs@gmail.com
GEO Group Northwest, Inc.
LEGEND
OF SOIL CLASSIFICATION AND PENETRATION TEST
UNIFIED SOIL CLASSIFICATION SYSTEM (USCS)
MAJOR
DIVISION
GROUP
TYPICAL DESCRIPTION
LABORATORY CLASSIFICATION CRITERIA
SYMBOL
WELL GRADED GRAVELS, GRAVEL -SAND
Cu = (D60 / 010) greater than 4
CLEAN
GW
MIXTURE, LITTLE OR NO FINES
Cc = (D302 ) / (D10 ' D60) between 1 and 3
DETERMINE
GRAVELS''..
PERCENTAGES .
GRAVELS
(little or no
GP
POORLY GRADED GRAVELS AND GRAVEL -SAND
GRAVEL AND AND
NOT MEETING ABOVE REQUIREMENTS
(Mare Than Half
lines)
MIXTURES LITTLE OR NO FINES
FROM GRAIN SIZE
COARSE -
Coarse Grains
- ��-
� ....�.�.--�
•-�••-.�- -•�,.• �.. -.- ---- .. -�.�.- -_ ....
DISTRIBUTION _ .._..._........ � �, .......
GRAINED SOILS
Larger Than No_ 4
CURVE ATTERBERG LIMITS BELOW
Sieve)
DIRTY
GM
SILTY GRAVELS, GRAVEL -SAND -SILT MIXTURES
'A' UNE.
GRAVELS
CONTENTor P.I. LESS THAN 4
_.,,...,...._____._
_..�_ ..------- .v...-. _........-.....
OF FINES
(with Sonia
CLAYEY GRAVELS, GRAVEL -SAND -CLAY
EXCEEDS 12% ATTERBERG LIMITS ABOVE
'A'
lines)
GC
MIXTURES
COARSE GRAINED UNE.
SOILS ARE or P.I. MORE THAN 7
.. ._..� ....
.. ...
�- _ ...WELL
.. -
CLASSIFIED AS
GRADED SANDS, GRAVELLY.._
, NDS, WILE
Cu = (060 / D10) greater than 6
FOLLOWS: (m0/D1�
SANDS
CLEAN
SW
OR FINESSAN
(030') / (D10' 060) between 1 and 3
SANDS
(More Than Half
Coarse Grains
(the or no
,-,.m,,,
.�.._ ....�..
POORLY GRADED SANDS, GRAVELLY SANDS,
.,..,,,...,.�,�.._ ...........m,.. _ ._..,,�,,.,,,-,...-.............. _ ......,
<5% Fine Grained:
NOT MEETING ABOVE REQUIREMENTS
More Than Hall by
Smaller Than No. 4
fines)
SP
UTILE OR NO FINES
GW, GP, SW, SP '....
Welght Larger
Than No, 200
Sieve)
_..�,..w.r._
._.._..,........_...
_................�... _.._.._......_.�....._..-----_
>....,........ ...............�,.m,�
ATTERBERG LIMITS BELOW
''.
Sieve
DIRTY
SM
SILTY SANDS, SAND -SILT MIXTURES
> 12% Fine Grained:
'A' LINE
SANDS
GM, GC, SM, SC'... CONTENT OF with P I LESS THAN 4
•__�_
..._.... _ _ _
FINES
9C
5 to 12% Fine LIMITS ABOVE
ne EXCEEDS 12°/
(with some
CLAYEY SANDS, SAND -CLAY MIXTURES
Grained: use dual A'mmm
UNE
lines)
symbols with P.I. MORE THAN 7
Liquid Limit
SILTS, ROCK FLOUR, SANDY SILTS
SILTS
<50%
MLINORGANIC
OF SLIGHT PLASTICITY
60 •: •- --••� °-- - °•• -
(Below A-Une on
low
PLASTICITY CHART A-Une
FIN 5 RAINED
Plasticity Chart,
Negligible Organic)
Li uidUmk.........
q
MH
INORGANIC SILTS, MICACEOUS OR
50 FOR SOIL PASSING
NO. """""•'"""
> 50%
DIATOMACEOUS, FlNE SANDY OR SILTY SOIL
40 SIEVE
..
._.......--,.._.,.
-. ......�,w......
._.......,,...,. ..,,._.....•-,....
o CH or OH
Liquid Umk
Uqu
UCL
INORGANIC CLAYS OF LOW PLASTICITY,
X 40
CLAYS
GRAVELLY, SANDY, OR SILTY CLAYS, CLEAN30%
ul
(Above A -Una on
<id
CLAYS
Placticity Chan,
yZ 30,
F .a w
Negligible Organic)CH
Liquid Umk
INORGANIC CLAYS OF HIGH PLASTICITY, FAT
U
> 50%
CLAYS
CL or OL
20 ... mm _ ._.
More Than Hall
.... m..,....,_._. _.- ..... _ _._ •
Weight Smaller
Liquid Umk
p
O
ORGANIC SILTS AND ORGANIC SILTYLLCLAYS OF
MH or OH
Than No. 200
Sieve
.ORGANIC SILTS &
CLAYS
c 50°/,
LOW PLASTICITY
^^^^ mm,m
10 -•••-
7 -M x ML
I, (Below A-Une an
4
Plastic' Chan)ORGANIC
rUql
b0"h k
OH....._
CLAYS OF HIGH PLASTICITY
i .._...
0
0 10 20 30 40 0 60_..70 ao 90 1o0 1 10
..,,
_.d,,.. ..._.m...m,,..mw ._.._._..�._. .,.,
LIQUID LIMIT (%)
HIGHLY
ORGANIC SOILS
Pt
PEAT AND OTHER HIGHLY ORGANIC SOILS
SOIL PARTICLE SIZE
GENERAL GUIDANCE OF SOIL ENGINEERING PROPERTIES FROM STANDARD PENETRATION TEST (SPT)
U.S. STANDARD SIEVE
FRACTION
Passing
.........
Retained
SANDY SOILS
SILTY & CLAYEY SOILS
........
Slave Sim
„... »[.....
�k2W
Slave Size
...._. �...�...!
Blow Relative Friction
Counts Density Angle
Blow Unconfined i
Description Counts Strength Description
� SILT/CLAY
0.075
N % �, degree
N u, Isf
q
AN
0-4 0 -15
Very Loose'2 < 0.25 Very sok
FINE
440 0.425
4200
0.075
4-10 15-35 26-30
Loose 2-4 4 0.25 - 0.50 Soft
MEDIUM
410 2
440
0.425
10-30 35-65 28-35
Medium Dense 4-8 0.50-1.00 Medium Stiff
COARSE
44 4.75
410
2
30-50 66-85 35-42
Dense 8- 15 100-2.00 Stiff
> 50 as - 100 38-46
Very Dense 15 - 30 2100-4.00 Very Stiff
FINE
19
84
4.75
> 30 > 4.00 Hard
COARSE
''. 76
19
COBBLES
. .,-.aw__ ........,_.
76 mm to 203 mm
GroupNorthwest, Inc.
BOULDERS
> 203 mm
s
GeotachnEal Engineers, Geologists, &
ROCK
Environmental Scientists
FRAGMENTS
13240 NE 20th Street. Suite 10
III
Bellevue, WA 96005
ROCK
>0.76 Cubic meter In Valume
Phone (425) 649757
Fax (425) 649758 IPLATE Al
HAND AUGERED BORING NO: HA -1
LOGGED BY AG LOG DATE: 10/29/2015
DEPTH
ft.
I
USCS
SOIL DESCRIPTION
SAMPLE
No.
Water
%
OTHER TESTS/
COMMENTS
SAMPLE Water OTHER TESTS/
ft.
SM
Crushed rock and sandy soil, moist, dense to very dense (FILL)
SM
Probe 0"
1
---,
--------------------------------------------------------
SM
Tan well graded gravelly silty SAND with occ, cobbles and roots, dry,
I
2
Probe <1 "
S-1 Probe <1"
dense to very dense
3
2
Total depth of boring = 36" bgs (below ground surface)
q
Hand -Auger refusal due to gravels and cobbles at 36" bgs
Probe 1-2"
No groundwater seepage observed
5
S-1
6
3
7
Total depth of boring = 30" bgs (below ground surface)
NAND AUGERED BORING LOGS
Group Northwest, hie.
Hand -auger refusal due to gravels and cobbles at 30" bgs
9303 OLYMPIC VIEW DRIVE
_
No groundwater seepage observed
EDMONDS, WASHINGTON
a
JOB NO. G-3978
DATE 11/2/15 PLATE A2
5
6
7
HAND AUGERED BORING NO: HA -2
LOGGED BY
AG LOG DATE: 10/29/2015
DEPTH
SAMPLE Water OTHER TESTS/
ft.
USCS
SOIL DESCRIPTION No. % COMMENTS
SM
Dark brown silty SAND with quarry spall rock, moist, dense Probe 1-2"
-- -
-_--------�---_____-__...__.._,___--_-__--------,_-,
...
Probe <1 "
SM
--
Tan wel I graded gravelly silty SAND with occ. cobbles, , dense
2
S-1 Probe <1"
Probe 1-2"
3
Total depth of boring = 36" bgs (below ground surface)
q
Hand -Auger refusal due to gravels and cobbles at 36" bgs
No groundwater seepage observed
5
6
7
NAND AUGERED BORING LOGS
Group Northwest, hie.
PROPOSED DEVELOPMENT
9303 OLYMPIC VIEW DRIVE
_
Geolechnical Engineers, Geologists, &
EDMONDS, WASHINGTON
Environmental Scientists
JOB NO. G-3978
DATE 11/2/15 PLATE A2
UNITED STATES DEPARTMENT OF'AGRICULTURE
30
20
10
1000
100 10
10 .,.
UNITED STATED DEPARTMENT OF AGRICULTURE.
TEXTURAL TRIANGLE SUMMARY
Dlo
) O is mm SMIo
mm SLT s
Do
mm dAY •
9�
THIS SOD. IS CLA39IPIED AS HYDROLOGIC SOIL GROUP
IT IS TYPICALLY DESCRIBED AS rY f*4A/ 4AWb (4., , h
.0001
SAMPLE DESCRIPTIONS
I�CJ►Tl�i � / �' a !rS
DATE ~L®: (o - 2
GRAIN Std DtMIBU nON
4,olP6R
5303 oL%rMPic. V 1 La Zu2tVK
Flo WA
C ATE