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21310 HIGHWAY 99
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TEMPORARY EROSION AND SEDIMENTATION
POND SIZING CALCULATIONS
39913
Edmonds Toyota
21300 Highway 99
Edmonds, Washington
Prepared for:
Lance Mueller & Associates
130 Lakeside
Suite #250
Seattle, WA 98122
June 29, 2007
Our Job No. 12888
STREET FILE 01EEPT
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CIVIL ENGINEERING, LAND PLANNING, SURVEYING, ENVIRONMENTAL SERVICES
18215 72ND AVENUE SOUTH KENT, WA 98032 (425) 251-6222 (425) 251-8782 FAX
BRANCH OFFICES + OLYMPIA, WA + TAcow, WA + SACRAMENTo, CA * TEmEcuLA, CA
www.barghausen.com
9 N to, $+
1.0 GENERAL INFORMATION
2.0 INPUT DATA SUMMARY
TABLE OF CONTENTS
APPENDIX
EXHIBITS
Vicinity Map
Basin Map
Isopluvial Map
Temporary Erosion and Sedimentation Control Pond Sizing Calculations
Soils Map
Geotechnical Report
Pages
1,2
3
12888.002.doc
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1.0 GENERAL INFORMATION
The proposed project site is located in a portion of thi-. Northwest quarter of Section 29,
Township 27 North, Range 4 East, Willamette Meridian, City of Edmonds, Snohomish County,
Washington. More specifically, the site is located near the intersection of Aurora Avenue North
and 212th Street.
The project consists of a single -story car dealership building, as well as a two-story parking
garage facility. The site will be approximately 90 pemmt impervious as well as having a
stormwater conveyance system, water quality vault, detention vault, and an off -site conveyance
system on the south side of the site.
ONSITE AREAS
TESC (to trap) = 1.59 acre3
Total Site Area = 1.59 acres
The temporary erosion control plans and final grading plans will be designed in accordance with
City of Edmonds standards. One sediment trap is being designed for this project. The trap is
sized in accordance with the 1992 DOE standards. The following pages contain calculations for
the sediment trap.
The existing soils include Alderwood Gravelly Sandy L6am, which is considered a Type C
classification. The temporary conditions during construction will be a graded condition and a
higher curve number was selected according to soil type.
Existing Condition Curve Numbers
DOE Table HI-1.3
Gravel Roads and Parking Lots (C) 89
Open Spaces (50-75% grass) (C) 90
Impervious Surfaces 98
Curve Numbers Under TESC Conditions
Graded Area
Stormshed Default DOE Table 111-1.3
91 N/A*
This project meets minimum Best Management Practices for erosion control, based on the
1992 Stormwater Management Manual for the Puget Sound Basin developed by the Department
of Ecology by providing a temporary rock construction entrance, temporary V-ditches with rock
check dams, silt fences around the perimeter of the site, and sediment traps These minimum Best
Management Practices meet established criteria for providing erosion control methodology for a
project site within the City of Edmonds.
The curve numbers provided by StormShed are higher and, therefore, provide a more conservative estimate of
on -site conditions than do closely related "Dirt Roads and Parking Lots" and "Fair Condition Open Spaces" shown
on DOE Table 111-1.3 in the Pond Sizing Exhibit.
12888.002.doc
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The computer model utilized to size the temporary sediment trap was the StormShed program
developed by Ingenious Systems, Inc., which utilizes the Saota Barbara Urban Hydrograph with a
Type IA rainfall distribution. The temporary erosion conuol phase of this development should
create a condition where no runoff leaves the developed portions of the site except where it is
discharged from the sediment ponds. The entire site shalli. have no untreated runoff nor allow
sediment -laden water to leave the site at any point.
Groundwater, if encountered in the utility trenches, will be allowed to be pumped into the
sediment trap. Baker tanks will not be used to collect groundwater. A high seasonal groundwater
table is known to exist in this area. Exfiltration of groundwater into shallow excavations may
occur during the wet seasonal months. Infiltration of runoff during the dry seasonal conditions
will occur in the same shallow excavations. The actual quantities are anticipated to be small to
negligible when compared to the surface water runoff inflow and discharge quantities.
No impact on the treatment and removal of sediments is anticipated given the high groundwater
tables.
Runoff values prepared by the Soil Conservation Service, i.e., CN numbers, include provisions
for shallow soils and high groundwater conditions. Final water quality runoff volume inflows
will not be affected by the known groundwater conditions.
It is anticipated that the introduction of fine silt clay partictes into the sedimentation and erosion
control ponds produce clogging of the pond bottom soil tayers. This resulting clogging will,
during the dryer seasonal conditions, inhibit the infiltration of runoff waters into the surrounding
subsoils and will ultimately provide some protection to the groundwater from runoff pollutants.
Construction sequencing provides for the logical installation. of sedimentation, erosion control,
and other features that may lend to site stabilization and dewaitering of the subsoils during the
early stages of site development. The proposal includes the utilization of the final stormwater
quality treatment areas for sediment trap construction when possible. This minimizes excavation
activities, allows for the stabilization of surfaces, and control of the erosion and sedimentation
process by construction sequences. Site grading proposed will allow for the construction of final
storm drainage facilities. This grading will include positive slopes for drainage to the interim
treatment facilities, removal of stockpiles, and contouring for roadway base materials after utility
installations.
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2.0 INPUT DATA SUMMARY
Sizing Sediment Traps:
To Sediment Trap*1
1.59 acres for both pre -developed and graded condition
Pre -developed CN
88.7
0.47 acre
CN = 86
Soil T!Te C
0. 11 acre
CN = 98
Soil Type C
1.01 acres
CN = 89
Soil Type C
Graded CN
91
1.59 acres
CN = 91
Soil TiTe C
2-year Precipitation
1.5 inches
ONS-ITE AREAS
TESC (to trap)
� 1.59 acres
Total Site Area
= 1.59 acres
tt Area tributary to sediment trap does not include the entire 4.18-acre site.
12888.002.doc
Appended on: 15:01:14 Friday, June 29, 2007
Pretesc Event Summary
Record Id: Pretesc
gnm Method __Jj SBUH IRainfall type
Intv 1 10.0 .tor
IF- IlAbstraction Coeff
Area _J1 0.47 ac JDCIA
F-
CN DC CN
TC 2.7( DC —TC
E
Pervious CN Calc
TYPEIA
484.00
0.20
1. 12 ac
89.88
5.16 min
Description SubArea Sub cn
Open spaces, lawns,parks (>75% grass)
Pervious Composited CN (AMC 2)
Pervious TC Calc
I Description
Fs —he —et -�Oense
grasses : 0.24
6. o o A
F5 o--.o—o
Fo. —24-6 5— - 1
12.76 min
Pervious TC
irectIv Connected CN Calc
2.76 min
.11 Description 11, SubArea Sub cn
Impervious surfaces (pavements, roofs, etc)
Gravel Roads & Parking Lots 1.01 ac
DC Composited CN (AMC 2)
F— Directly Connected TC Calc
Description I Fs7lTpe7]
FS-h-e-e-t-7lSmooth Surfaces.: 0.011 1300.00 ft! F. 5 0 % 1 Fo --o —n 67 F --5-6—i 7n W-7 8—m- —in-
Shallow aved and gravel areas (n=0.012) IE��EE,-=50% L0120]=
F- Directly Connected TC E�
Licensed to: Barghausen Engineers
Appended on: 10: 07: 10 Friday, June 29, 2007
tesc Event Summary
JPeak Q (cf:s)] IRain
I]) _17fi)
—
0.0880 1 7E;��K]Fs_B_U_H_ � FTY—PE-17A
E�
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1�1
0.7119 �F
8.00
Record Id: tesc
hod IE _SBUH JERainfall type A
10.00 min ng Factor
jHyd Intv
I IlAbstraction Coeff 11=_-0.
lPervious Area C I
'N 1F_ gToo I
Ifervious TC I IFDC T iin
Directly Connected CN Calc
Description Area Sub cn
Newly graded area 1.57 ac IL 91.00
DC Composited CN (AMC 2) 91.00
Directly Connected TC Calc
Type Description gth I Slope I Coeff I Misc TT
F2.00O/o FO06�0O 11.50 1113.15
Cultivated Soil w/ residue cover 225.00
Sheet 00
'iv
I F(s<=20%).: 0.06 i1ft in min
Channel Earth -lined (n--0.025) 135.00
(interm) 11 ft
III Directly Connected TC I 13.56min
Licensed to: Barghausen Engineers
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I Job No. 128881
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111-1-44 FEBRU—A—RY,1992
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124 123 122 121
111-1-46 FEBRUARY, 1992
TEMPORARY EROSION AND S",',EDIMENTATION
CONTROL POND SIZING C�kl,CULATIONS
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Edmond Toyota
Erosion/Sedimentation Control and Sedimentation Pond Sizing
Using DOE 1992 SWMMPS section 115.8.6 BMP E3.40 design guidelines
Using software program to determine Q2
Given:
2-year 24-hour interim developed storm event, assuming bare soil conditions
Required surface area formula SA 1.2 x Q2/.00096 or 1,250 sq. ft. per cfs inflow
Q2 = 0.22 cfs
SA = 275 sq. ft.
Required Sediment Storage depth 1.5 ft. plus sediment settling depth 2 ft. = 3.5 required depth
Approximately required total volume based upon surface area
Required Surface
Provided Surface
Depth
Area
Area
Volume Provided
Pond
(ft.)
(sq. ft)
(sq. ft.)
(cu. ft.)
TESC Sediment Trap
3.5
275
475
1,662.5
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12888.002.doc