Civil Submittal 1 - Draft Stormwater Report.pdf
T ECHNICAL I NFORMATION R EPORT
T RANG S HORT P LAT
TH
2313575P LACE W EST;E DMONDS,WA98026
The Concept Group
4701 SW Admiral Way, Ste 353 Seattle, WA 98116 (206) 446-1291
Technical Information Report
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23135 Ï 75 Place West; Edmonds, WA 98026
I certify that this technical information report and all attachments were prepared
either by me or my technical staff working directly under my supervision.
Date Description
March 27, 2018 Original Submission
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July 24, 2018 1 Revision
Appendices
Appendix A: Soil Data
Appendix B: Civil Plans
Trang Short Plat ÎTechnical Information Report July 2018
MR1ÏP REPARATION OF S TORMWATER S ITE P LAN
Project Overview
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The property is located at 23135 Ï 75 Place West in the City of Edmonds, Washington.
The site is currently developed with a SFR that will remain. The project proposes to short
plat the property into a total of two (2) parcels and construct a single-family structure,
driveway and associated utilities on the newly platted lot.
This Technical Information Report and attached Stormwater Site Plan submittal is required
for the approval of the short plat and building permit for the above referenced subject
property. The current land-use designation of the property is single-family residential. The
proposed land-use is single-family residential. The Stormwater Site Plan has been prepared
in accordance the City of Edmonds 2017 Stormwater Addendum to EcologyÔs 2014
Stormwater Management Manual for Western Washington.
Site Information
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Address: 23135 Ï 75 Place West; Edmonds, WA
City, County, State: Edmonds, King County, Washington
Governing Agency: City of Edmonds
Figure 1 Ï Vicinity Map / Site Location
Not-to-Scale
Soils Information
Soils Survey
The Soils Conservation Service (SCS) mapped the soils information in the buildable project
area as Alderwood/Urban Land (5/6). Refer to Figure 2 below and Appendix A for the
detailed soil map.
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Trang Short Plat ÎTechnical Information Report July 2018
Figure 2 Ï Soils Map
Not-to-Scale
Existing Conditions
The site is currently developed with a SFR. The general topography of the site slopes from
west to east. Elevations on the site vary from a high point of 306 feet at the western o portion
of the property to 296 feet at the eastern property line. Table 1 details the existing impervious
surface areas.
Table 1 Ï Existing Impervious Surface (SF)
Roof 2,578
Driveway/Walkway 1,340
Total 3,918
Developed Conditions
The project proposes to construct a single-family structure, driveway and associated utilities.
Table 2 details the proposed impervious surface areas.
Table 2 Ï Impervious Surface (SF)
Lot 2 Roof 2,800
Lot 2 Driveway 680
Lot 1 Driveway 345
Frontage Improvement (widening) 521
Frontage Improvement (sidewalk) 512
Total 4,858
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Trang Short Plat ÎTechnical Information Report July 2018
Threshold Determination
The City of Edmonds 2017 Stormwater Addendum to EcologyÔs 2014 Stormwater
Management Manual for Western Washington sets forth the drainage requirements for this
project. Based on the flowchart of Figure 3.1, ÑFlow Chart for Determining Requirements
for Development,Ò Minimum Requirements (MR) 1 through 5 will apply to the new and
replaced impervious surfaces and converted pervious surfaces.
Edmonds Stormwater Addendum, Figure 3.1
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Trang Short Plat ÎTechnical Information Report July 2018
MR2ÏCSWPPP
A Construction Stormwater Pollution Prevention Plan (CSWPPP) has been prepared for this
project under a separate cover. The project will comply with the thirteen (13) SWPPP
elements during construction. All erosion and sediment control measures shall be governed
by the requirements of the City of Edmonds.
Construction Sequence and Procedure
Prior to the start of any grading activity upon the site, all erosion control measures, including
installation of a stabilized construction entrance, shall be installed in accordance with the
construction documents.
The best construction practice will be employed to properly clear and grade the site and to
schedule construction activities. The planned construction sequence for the construction of
the site is as follows:
1.Flag or fence clearing limits.
2.Install catch basin protection if required.
3.Grade and install construction entrance(s).
4.Install perimeter protection (silt fence, brush barrier, etc.).
5.Construct sediment ponds and traps.
6.Grade and stabilize construction roads.
7.Construct surface water controls (interceptor dikes, pipe slope drains, etc.)
simultaneously with clearing and grading for project development.
8.Maintain erosion control measures in accordance with City of Edmonds standards
and manufacturerÔs recommendations.
9.Relocate erosion control measures or install new measures so that as site conditions
change the erosion and sediment control is always in accordance with the City of
Edmonds Erosion and Sediment Control Standards.
10.Cover all areas that will be unworked for more than seven days during the dry season
or two days during the wet season with straw, wood fiber mulch, compost, plastic
sheeting or equivalent.
11.Stabilize all areas that reach final grade within seven days.
12.Seed or sod any areas to remain unworked for more than 30 days.
13.Upon completion of the project, all disturbed areas must be stabilized and BMPs
removed if appropriate.
Trapping Sediment
Structural control measures will be used to reduce erosion and retain sediment on the
construction site. The control measures will be selected to fit specific site and seasonal
conditions.
The following items will be used to control erosion and sedimentation processes:
Temporary gravel construction entrance
Filter fabric fences (Silt fences)
Ground cover measures such as straw cover and/or hydroseeding
Inlet protection
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Trang Short Plat ÎTechnical Information Report July 2018
Vehicle tracking of mud off-site shall be avoided. Installation of a gravel construction
entrance will be installed at a location to enter the site. The entrances are a minimum
requirement and may be supplemented if tracking of mud onto public streets becomes
excessive.
MR3 Ï S OURCE C ONTROL OF P OLLUTION
BMPs for mobile fueling of vehicles and heavy equipment and for concrete handling will is
included in the CSWPPP, prepared under a separate cover.
MR4 Ï P RESERVATION OF N ATURAL D RAINAGE S YSTEMS &O UTFALLS
The property is located within the Hall Creek basin which drains to Lake Ballinger. The
entire project site currently drains to this one basin. In the developed condition, stormwater
will continue to drain to Lake Washington.
MR5 Ï O N-SITE S TORM W ATER M ANAGEMENT
The Project utilizes Better Site Design by generally matching existing grades. Existing trees
and other vegetation around the Site perimeter will be preserved where possible to help
minimize disturbance to the hydrologic cycle. Native soils in all disturbed pervious areas
will be amended with compost.
Roof Surface
1.Full dispersion of runoff from impervious surfaces is not feasible due to the lack of
native vegetation on the site.
2.Full infiltration of runoff from impervious surfaces is feasible because on-site
infiltration test yielded measured infiltration rates of 20.0 inches per hour. No other
.
BMPs are required to be evaluated
The continuous simulation model, the Western Washington Hydrology Model, WWHM,
was used to size the infiltration trench. WWHM, version 2012 was used to determine the
required infiltration trench size. Results from the WWHM model are included with this
memorandum. The following parameters were used to model the facility:
Measured Infiltration rate = 20 inch/hour
Correction factor (per infiltration memorandum) = 0.225
Design Infiltration rate = 4.5 inch/hour
Furthermore, the project addresses the CityÔs ÓretrofitÔ requirement per ECDC
18.30.060.D.5.b.i; by providing mitigation for, at minimum, 25% of the currently
unmitigated impervious surface on the Lot 1 that is to remain. The project proposes to
mitigate approximately 1,289 SF of the existing roof, which accounts for 32.9% of the total
existing unmitigated impervious surface on Lot 1 with an infiltration trench.
Infiltration trenches area proposed to mitigate onsite runoff, infiltrating 100% of the 25-year
developed flow, with an overflow release rate of 0.00 cfs. Refer to Table 3 for the trench
dimensions.
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Table 3 - Infiltration Trench Design
Impervious Roof Required Trench Provided Trench
Lot 1: 1,289 SF 66 SF x 3Ô D 17 LF x 4Ô W x 3Ô D = 68 SF x 3Ô D
Lot 2: 2,800 SF 139 SF x 3Ô D 28 LF x 5Ô W x 3Ô D = 140 SF x 3Ô D
Driveway Surface
1.Runoff from the proposed driveway for Lot 2 is mitigated with permeable pavement
surfaces to infiltrate stormwater runoff.
2.Runoff from the proposed driveway for Lot 1 is mitigated with a gravel dispersion
trench to manage stormwater runoff.
Lawn and Landscape Area Soil Management Plan
1.Within the limits of site disturbance, duff and topsoil will be retained in an
undisturbed state and stockpiled for later use to stabilize and amend soils throughout
the Site. Postconstruction soil amendment will meet the requirements of BMP T5.13
Post-Construction Soil Quality and Depth.
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Appendix A
Soils Data
The Concept Group Appendices
M EMORANDUM
4701 SW Admiral Way, Ste 353 Seattle, WA 98116 T 206.446.1291 www.ConceptBusinessGroup.com
Date: March 8, 2018
July 24, 2018 (Revision 1)
To: City of Edmonds
Public Works Department
From: Anna Nguyen, P.E.
Subject: Drainage Calculations & Subsurface Report
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Project Address: 23135 75 Place W
Geologic Setting
The Web Soil Survey, provided by the USDA Natural Resources Conservation Service was
referenced for the geologic and soil conditions of the project site. According to this publication, the
surficial soil unit at the project site and its immediate vicinity is mapped as Alderwood/Urban Land
(5/6).
project
location
Figure 1 Ï Geologic Map (Not-to-Scale)
ANEW DIMENSION OF DESIGNPERCEPTION
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23135 7 Place West Page 2 of 4
Small Scale Pilot Infiltration Test
On March 7, 2018, we performed a field infiltration test. The test pit location is shown in Figure
2. The small-scale infiltration test was performed in accordance with the specifications provided in
the 2014 DOE Stormwater Management Manual for Western Washington.
The test consists of excavating a test pit to the design bottom of the proposed infiltration facility (5
feet deep) with a minimum of 12 square feet bottom area. The test pit is then pre-soaked with at
least 12 inches of water above the bottom for a period of 6 hours. At the end of pre-soak period,
the pit is filled with 12 inches of water and a constant head is maintained for the constant head
testing period of one hour. The volume of water per time unit needed to maintain 12 inches of
constant head is recorded.
The field infiltration rate is then calculated based on the final measured volume per time unit and
the surface area of the hole. In addition, at the end of the constant head test, we shut the water off
and measured the falling head infiltration rate by recording the drop in water level between regular
time intervals. An additional test pit was excavated to a depth of 7 feet below the existing grade to
verify the any presence of groundwater and soil conditions.
Based on the surface area of the test hole, and the measured volume of water per hour needed to
maintain the constant head, we calculated a field infiltration rate of 20 inches per hour.
Groundwater/ Hydraulic Restrictive Layer
This test was conducted during the wet-season. The test pit was excavated to a total depth of 7 FT.
The subsurface exploration did not encounter a restrictive/hardpan clay layer or groundwater.
Infiltration test pit
location
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Figure 2 - Soils Test Pit Location (Not-to-Scale)
T HE C ONCEPT G ROUP
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23135 7 Place West Page 3 of 4
Figure 3 - Soils Log (Not-to-Scale)
Correction Factor
The correction factor was calculated based on Table B.1 of the 2017 City of Edmonds Stormwater
Manual. The test was conducted during the wet-season in the vicinity of the proposed stormwater
BMP; therefore, the minimum correction factor is recommended for this site.
Design Infiltration Rate = Measured Infiltration Rate x CF v + CF T + CF m
Where:
CF v =0.50
CF T=0.50
CF m =0.90
Design Infiltration Rate = 20.0 x 0.50 x 0.50 x 0.90 = 4.5 inches/hour
Figure 4 Ï Table B.1 (2017 Edmonds Stormwater Addendum)
T HE C ONCEPT G ROUP
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23135 7 Place West Page 4 of 4
Recommendations
Based on our infiltration testing on the site, it is our opinion infiltration of the development storm
water on the site is feasible.
Limitations
The conclusions and recommendations contained in this report are based on site conditions as they
existed at the time of our exploration and assume that the soil and groundwater conditions
encountered in the test pits are representative of subsurface conditions on the site. If the subsurface
conditions encountered during construction are significantly different from those observed in our
explorations, we should be advised at once so that we can review these conditions and reconsider
our recommendations where necessary.
Unanticipated soil conditions are commonly encountered on construction sites and cannot be fully
anticipated by merely taking soil samples in test pits. Subsurface conditions can also vary between
exploration locations. Such unexpected conditions frequently require making additional
expenditures to attain a properly constructed project.
This report has been prepared for the exclusive use of property owners and its representatives for
specific application to this project and site. Our recommendations and conclusions are based on
observed site materials and engineering analyses. Our conclusions and recommendations are
professional opinions derived in accordance with current standards of practice within the scope of
our services and within budget and time constraints.
No warranty is expressed or implied. The scope of our services does not include services related to
construction safety precautions, and our recommendations are not intended to direct the
contractor's methods, techniques, sequences, or procedures, except as specifically described in our
report for consideration in stormwater design.
T HE C ONCEPT G ROUP
Trang Short Plat ÎTechnical Information Report
Appendix B
Civil Plan Set
The Concept Group Appendices