12-7-18 Staff Report Attachments 6 - 8.pdfCUSTOMER EUC
CONTACT
Form
A coordinated utility environment which mØcimizes j oint utility opportunities to provide quality service for the citizens of Edmonds
YOU WILL BE GIVEN A CONFIRN{ATION NUMBER BY MCHUTILITY ONCE
rou HAVE INFORMED THEM OFYOURPROJECT.
SITE ADDRESS:
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THIS FORM MUST BE COMPLETED AND GIVEN TO DEVELOPMENT
SERVICES STAFF AT THE TIME OF PERMIT SUBMITTAL Rev.10/1/13
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Mary McAllister - 425-67037t6
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You will need to provide PUD with a site plan and a completed New Service
Questionnaire.
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New Customer Construction
Department - I-88832t-7 77 9
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Call to verifu gas availability and to coordinate serwice install. You will need to
provide parcel number' contact phone number and mailing address.
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Jeremy Fallt - 425-2634024
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You will need to provide Frontier with the location of your project, total line
requirement, and the date in which service is required. A copy of your
development plan may be required.
John \ü/arrick 425 263-5328
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You will need to provide Comcast with the location of your project, a copy o{
your development and site plan (digitat copy if available.) The date in which
service is required and a list of contact names, phone numbers & mailing
addresses.
425.774-7769
For custotners in Olyrnpic View Service Area
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You will need to provide OWVSD with the survey map of your site and
complete a development in{ormation form.
ATTACHMENT 6
Ðonna Breske
& Associates
Civil Engineering & Land Use Planning
July 17,20lt
City of Edmonds
121 5ùAveN.
Edmonds, WA 98026
Subject: Driveway Slope Deviation Request
Reference: Ryker Young 2-lot Short Plat located at20ll4 8
21 Avenue A, Suite 4
Pb: 360-2294-8941
Snohomish, V/A 98290
e-mail : domab@)don{rabreske. qom
RECEåVE[J
JUL 20 2ri8
ÐEVELoËH[fü-sEn/lcE$
3rd' Ave W Edmonds, V/A 98026
A deviation is requested to allow a l5% driveway slope for the referenced 2-lot short
plat.
Per Chapter 20.01 of the Edmonds City Development Code, the grade of the driveway may not
exceecl 14% unless special approval is granted by the Public Works Director.
In accordance with the Ciff of Edmonds Development Standards, the Public Works Director may
authorize a driveway slope of more than l4o/o and no more than 20% if it is detennined that:
a. The driveway is the only economical and environmentally reasonable altemative;
b. The driveway will not present a traffic, pedestrian, bicycle or safety hazardor otherwise
negatively impact public safety;
c. The police and fire chief concur in allowing the increased driveway slope; and
d. The public health, safety and general welfare will not be adversely affected.
Reason supporting approval ofthe deviation are:
1. 83rd Ave West is a sloped sfi'eet. The elevation differences along the frontage range fi'om a low
elevation of 372,to a high elevation of 386. To minimize the on-site slope of the j o ì n t
access drive aisle, the access starts at the high elevation of 386. Q.{ote: This is also the location of the
driveway for the existing house, i.e. on the uphill side of the frontage). Placing the dliveway at the
elevation of 386 is a reasonable altemative to placing it at the low side.
2. The new joint access will be placed in the location of the existing driveway. Slope and location are
consistent with the existing driveway serving a single family house. A vertical curve is designed at the
frontage to smoothly transition from the slope of 83'd Ave S.W. to the l5Yo slopes. There is no indication
that two houses taking access in the same location will present a safety hazard. ln fact, the new joint
driveway should provide a stronger visual identifier to pedestrians walking along the asphalt walkway that
a vehicle access point exists.
3 . The joint access length is 1 13 feet in length. The standard length at which a turn-around is needed is
150 feet. Additionally, a sag vertical curve is designed a frontage to smoothly transition frorn the slope of
83'd Ave S.W. to fhe 15% slope. The joint driveway will provide a smooth joint access road for use by
police and fire.
4. The Grading plan results in a rockery height of 8 ft. at the end of the joint access. Reducing the driveway
slope to 14% would result in unnecessary grading and an unnecessary increased height to the rockery of
about 1.5 in height. Less grading means less impact to public health and safety.
Sincerely,
fu,** ,ç, 8"t þÅ
Donna L. Breske, P.E.
Mobile Phone: 206-715-9582 ATTACHMENT 7
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PREPARED BY:
DONNA BRESKE & ASSOCIATES, LLC
21 AVE A, SUITE 4
SNOHOMISH, WA 98290
PHONE: (425) 334-9950
D O N N AB R ES K ECÐ C O M CAST. N ET
15% DRIVEWAY SLOPE EXHIBIT
TAX N0. 00qt200000402 SITE ADDRESS: 20114 83RD AW. W,
IR\JECT 4RO\ONENT: R\KER yWNe EDMoNDS, WA 98026_6727
ISSUE DAIE: 7-12-20'18
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CITY OF EDMONDS
121 5th Avenue North, Edmonds WA 98020
Phone: 425.771.0220 • Fax: 425.771.0221 • Web: www.edmondswa.gov
DEVELOPMENT SERVICES DEPARTMENT • PLANNING DIVISION
August 17, 2018
Ryker Young
Real Property Funding Group
750 Roosevelt Way NE
Seattle, WA 98115
Ryker@rpare.com
Subject: Letter of Completeness and Request of Additional Information
Young SP 2-Lot Short Subdivision
Application No. PLN20180050
Mr. Young,
On behalf of the City of Edmonds Planning Division, I have reviewed the application for a 2-lot
short subdivision at 20114 83rd Ave. W., for completeness pursuant to Edmonds Community
Development Code (ECDC) 20.02.002 and 20.75.040. The City has determined that the
application meets the procedural submission requirements and therefore is complete. Please
accept this letter as the City’s notice to applicant of determination of completeness pursuant to
ECDC 20.02.003.
While the application is procedurally complete, it was determined that additional
information/clarification is necessary in order to continue review of the application. Please
provide responses to the following items at your earliest convenience so that staff’s review of the
proposal may continue:
Planning:
1. Under the legal description/title report notes on the preliminary short plat, the following is
stated; easements, restrictions, or reservation of record which would be disclosed by the title
report are shown per First American Commitment for title File No. 4229-3036831 dated
March 28, 2018. However; the tittle report provided states commitment date of May 29,
2018, please correct if this is a typo or provide the document as referenced.
2. The title report must be prepared within 30 days of application submittal. The title report
submitted with your application is dated May 29, 2018, which exceeds 30 days before the
application submittal date (received July 20, 2018). Therefore, please provide an updated
title report. Please be sure to provide any referenced documents along with the title report.
ATTACHMENT 8
3. The preliminary short plat indicates gross lot areas for proposed lots 1 and 2 please provide
the net lot are for proposed lot 1. Net area is equivalent to gross area excluding the area of
any vehicular access easements. Please update the gross and net areas of the proposed lots
accordingly.
4. Per ECDC 20.75.060.M, please show existing structures within 25 feet of the proposed short
plat. It appears that a structure located on the adjacent property to the south may be within 25
feet, if so please provide on the plans.
5. The preliminary short plat does not show the correct setback for proposed lot 2, lot 2 is a flag
lot and requires a 7.5 foot setback from the existing and proposed property lines. Please
revise accordingly.
6. Please note that the preliminary plans reference the maximum allowed height as well as lot
coverage information. Height calculations, and lot coverage are not reviewed nor approved as
part of a short plat application (they are reviewed at the time of building permit application
review). Additionally proposed structure setbacks will be reviewed in greater detail with a
building permit.
7. Per ECDC 20.75.085.A.2, and per ECDC 23.80.070.A.4.c, the proposal shall be designed to
minimize grading by using shared driveways and by relating street, house site and lot
placement to the existing topography. Proposed retaining walls of up to 10 feet in height are
shown on the plans within the proposed side, rear and front yard of the proposed lots. Other
than the proposed access area, retaining walls of up to 10 feet in height do not meet the
requirements of the code provisions. Please revise plans to be consistent with the code
requirement.
8. Sheets 1 thru 5 do not reference the same lot dimensions as provided on the proposed
preliminary site plan for proposed lots 1 and 2, please make the plans consistent.
Engineering:
1. Refer to the enclosed memorandums from JoAnne Zulauf, Engineering Technician dated
August 9, 2018, and Zack Richardson, Stormwater Engineer, dated August 1, 2018. As
part of your response, please include a cover letter stating how each comment was
addressed. Any questions on the comments in the enclosed memorandum may be
directed to Zack Richardson, PE at Zachary.Richardson@edmondswa.gov or (425) 771-
0220 or JoAnne Zulauf at JoAnne.Zulauf@edmondswa.gov.
Please submit the above information to the Planning Division as soon as possible so that staff
may continue processing your application. Please provide a complete response to the comments
including a cover letter stating how each comment was addressed.
Since the application has been determined to be complete, a notice of application will be posted
on the subject property and mailed to adjacent property owners within the next two weeks
consistent with ECDC 20.03.002.
According to ECDC 20.02.003.D, the above requested information must be submitted within
90 days (or by November 15, 2018) or the application will expire.
I will be the main staff contact for your application. If you have any questions at any point
during the review process, you may reach me at Michele.Szafran@edmondswa.gov or (425) 771-
0220.
Sincerely,
Development Services Department - Planning Division
Michele Q. Szafran / Planner
Cc: JoAnne Zulauf, Engineering Technician,
Zack Richardson, Stormwater Engineer
City of Edmonds
Date: August 9, 2018
To: Michele Szafran, Associate Planner
From: JoAnne Zulauf, Engineering Technician
Subject:
PLN20180050, Riker Young 2 lot short plat
20114 83rd Ave W
The comments provided below are based upon review of the preliminary civil plans &
documents for the subject short plat. Additional information is requested from the applicant
at this time in order to continue review of the application and provide preliminary approval of
the short plat. Please ask the applicant to provide a written response to each of the comments
below and revise and resubmit plans accordingly.
GENERAL
1. Rockeries are prohibited in the right of way and those located on private property must be
located no closer to the right of way than a distance equal to that of the wall height. Please
confirm this distance at the property line will be met. Add some top and bottom of wall
elevations to provide a clear picture. Please see ECDC 18.40.020 for more information.
2. The proposed contours as shown on the plans indicate a driveway slope in excess of 20%
in several locations. The Edmonds Community Development Code 18.80.070 does not
allow for driveway slopes to exceed 14% without first obtaining approval by the City
Engineer. The maximum allowed driveway slope is 20%. Adjust the slope of the
driveway to reflect a 14% grade and/or maximum 20% grade along with a request for
waiver that provides an explanation as to why the required 14% slope cannot be achieved.
A request for waiver requires noticing per a Type II decision.
UTILITIES
1. Show the location of existing and proposed dry utilities.
Thank you.
MEMORANDUM
REQUEST FOR ADDITIONAL INFORMATION
Page 1 of 2
STORMWATER REVIEW COMMENTS
City of Edmonds
Engineering Division
To: Applicant
Date: August 1, 2018
Project Name: Ryker 2-Lot SP
Permit Number: PLN20180050
Address: 20114 83rd Ave W
Review Type: Preliminary Plat (Feasibility)
Submittal Date: 7/25/2018
Reviewer: Zack Richardson, PE
City of Edmonds, Stormwater Engineer
Recommendation: I recommend that PLN20180050 be withheld until the comments
below are adequately addressed.
Review Comments:
1. It is not clear that the infiltration rates have been adequately measured and
reported; update geotechnical information to ensure infiltration rate is obtained
consistent with Edmonds Addendum Appendix B (attached) and includes the
following information:
a. Location & approximate elevation of borings completed
b. Boring logs, extending a minimum of 1’ beyond the infiltration surface
c. Description of infiltration testing including acknowledgement of following
the methods outlined in Appendix B (pre-soak period, method of adding
water, etc) and identification of the depth the test was performed at.
d. Correction factors used to calculate long-term infiltration rate.
e. The stamp and signature of the preparer (applies to septic designers as
well as engineers).
2. It appears that the performance standard is being used for MR #5; update MR #5
section to explicitly state the performance standard is being achieved, and
include additional WWHM outputs to verify LID flows rates requirements are met.
a. If list approach is being used, explicitly state as much, and expand MR #5
section to explain how bioretention was selected as the BMP and how
other BMPs before bioretention were determined infeasible.
Page 2 of 2
3. The project proposes two wall/footing drain outlets and a LID overflow which will
direct runoff across a public walkway, which is not permitted. Update plans to
provide connection to the existing subsurface system which will not impact
pedestrian walkways.
a. Additional concerns exist for the south most wall/footing drain which
appears to discharge toward an adjacent parcel before reaching the
walkway and may be a steep slope/erosion concern.
4. It is not clear that the bioretention facility has been sized correctly; update report
and calculations as needed to address the following:
a. It does not appear the correct rain data/gage was used; revise rain gage
to “Puget East 36” (as required per Edmonds Addendum 2.1) and re-run
model.
b. The rounding of acres appears to have resulted in the loss of 0.011 acres
(~480’ SF) within the model; adjust model to reflect the 0.45 acres total
parcel size stated in introduction.
c. The assumed impervious areas for each lot are below the DOE assumed
minimum value of 4,000 SF, and anecdotally, 36% for total lot coverage
seems restrictively low. Adjust model to minimum value (or actual
anticipated value above minimum), OR update plans to clearly state that
additional impervious surface restrictions are required as part of the plat
process (SWMMWW Supplemental Guidance in I-2.4.1)
Engineering/Civil Plan Development Comments:
The following comments were noted by the reviewer as issues that will likely require revision prior to
construction approval, but do not necessarily required attention prior to feasibility approval.
A. Prior to issuance of a construction permit, the engineer shall assess potential
impacts of the proximity of the storm drain system to the wall footing/bottom, and
address concerns for the pipe being in the influence zone of the wall, if needed.
B. Prior to issuance of a construction permit, the engineer shall address
pretreatment for the driveway surfaces; at a minimum a down turned elbow or
vertical tee will be required.
C. Provide additional break down of the distribution of assumed impervious surface;
the shared driveway surfaces, the pollution generating impervious surface, and
the non-pollution generating impervious surface assumed for each lot shall be
listed separately.
JUNE 2017 APPENDIX B – METHODS FOR DETERMINING DESIGN INFILTRATION RATES
B-1. DETERMINING DESIGN INFILTRATION RATES – OVERVIEW 1
Appendix B – Methods for Determining
Design Infiltration Rates
B.1 Determining Design Infiltration Rates – Overview
The following methods are based on the requirements outlined in the SWMMWW, Volume III,
Section 3.3.6. However, to improve clarity and usability and to remove inconsistencies in the
SWMMWW, the information in this appendix has been organized in a slightly different structure.
Therefore, although the following is equivalent to the SWMMWW, users shall use this appendix for work
in the city.
B.1.1 Methods
There are two acceptable methods for estimating the measured (a.k.a., initial) saturated hydraulic
conductivity (Ksat) rate at a site, outlined below and described in detail in Sections 3 and 4 of this
appendix. A correction factor is applied to the initial rate to determine the design (long-term) infiltration
rate (see Section 2). The design (long-term) infiltration rate is then used for routing and sizing the
infiltration facility, and for checking for compliance with the maximum drawdown time of 48 hours. Note
that the subgrade correction factors in this appendix (see Section 2) may not apply to bioretention,
permeable pavement, and rain gardens. Refer to SWMMWW Volume III, Section 3.4 and Addendum
Checklist 5 for additional guidance on infiltration testing methods and application of appropriate
correction factors specific to bioretention, rain gardens, and permeable pavement.
• Method 1. Field Testing Procedures:
o Large-Scale Pilot Infiltration Test (PIT). This test applies to infiltration facilities with
drainage areas greater than 1 acre (i.e., projects that are using the “Detailed Method” – see
Addendum Checklist 6 and SWMMWW Volume III, Section 3.3.5) and may be used to
demonstrate infeasibility of bioretention, rain gardens, or permeable pavement in meeting
Minimum Requirement No. 5.
o Small-Scale PIT. This test applies to infiltration facilities with drainage areas less than 1 acre
(i.e., projects that are using the “Simple Method” – see Addendum Checklist 6 and
SWMMWW Volume III, Section 3.3.5) and may be used to demonstrate infeasibility of
bioretention, rain gardens, or permeable pavement in meeting Minimum Requirement No. 5.
o U.S. EPA Falling Head Percolation Test Procedure (as Modified for the City of Edmonds).
This test may only be used for BMP performance verification testing. This test may not be
used for BMP design; or to demonstrate infeasibility of bioretention, rain gardens, or
permeable pavement in meeting Minimum Requirement No. 5.
• Method 2. Soil Grain Size Analysis: This method may only be used at project sites that are
underlain by soils not consolidated by glacial advance (e.g., recessional outwash soils) and may
not be used to demonstrate infeasibility of bioretention, rain gardens, or permeable pavement in
meeting Minimum Requirement No. 5.
APPENDIX B – METHODS FOR DETERMINING DESIGN INFILTRATION RATES JUNE 2017
2 B.1. DETERMINING DESIGN INFILTRATION RATES – OVERVIEW
B.1.2 Number and Location of Tests
The following summarizes requirements related to the number and location of infiltration tests for specific
BMPs. Users should refer to the individual BMP descriptions in SWMMWW Volumes III and V for
additional details.
In addition to the requirements outlined in this section, note that for all BMPs, the depth and number of
test holes or test pits and samples should be increased, if in the judgment of a professional engineer with
geotechnical expertise (licensed in the State of Washington), a licensed geologist, licensed engineering
geologist registered in the State of Washington, or hydrogeologist, the conditions are highly variable and
such increases are necessary to accurately estimate the performance of the infiltration system. The
exploration program may also be decreased if, in the opinion of a professional engineer licensed in the
State of Washington in civil engineering, or licensed engineering geologist registered in the State of
Washington, the conditions are relatively uniform and the borings/test pits omitted will not influence the
design or successful operation of the facility. In high water table sites, the subsurface exploration
sampling need not be conducted lower than 2 feet below the ground water table.
Projects shall perform the type and number of tests in accordance with specific BMP requirements in the
SWMMWW, summarized below:
• Allowable methods:
o As noted in Section 1, only small-scale and large-scale PITs are allowed to demonstrate
infeasibility of bioretention, permeable pavement, and rain gardens (in accordance with
Minimum Requirement No. 5). Other methods outlined in this appendix (and SWMMWW
Volume III, Section 3.3.6) ma y be used to determine the design infiltration rate of underlying
soils for these BMPs, but may not be used to demonstrate infeasibility. Therefore, the
summaries below refer to PIT methods as a first preference for measuring infiltration rates for
these three BMPs. Projects should consider the specific BMPs proposed (or anticipated) for a
site, and the associated testing requirements, when planning and performing infiltration
testing.
o When using grain size analyses to measure infiltration rates, perform the number of test pits
described below. Conduct at least one grain size analysis per soil stratum in each test hole
within 2.5 times the maximum design water depth, but not less than 10 feet below the
proposed base of the infiltration facility.
• Infiltration Basins: at least one test pit or test hole per 5,000 square feet of basin infiltrating
surface (in no case less than two per basin).
• Infiltration Trenches (does not include downspout infiltration): at least one test pit or test hole per
200 feet of trench length (in no case less than two per trench).
• Bioretention:
o For small bioretention cells (bioretention areas receiving water from one or two individual
lots or <0.25 acre of pavement or other impervious surface): at least one small-scale PIT, or
other method outlined in this appendix, at each potential bioretention site.
JUNE 2017 APPENDIX B – METHODS FOR DETERMINING DESIGN INFILTRATION RATES
B-1. DETERMINING DESIGN INFILTRATION RATES – OVERVIEW 3
o For large bioretention cells (bioretention areas receiving water from several lots or 0.25 acre
or more of pavement or other impervious surface): at least one small-scale PIT, or other
method outlined in this appendix, per 5,000 square feet of bioretention area.
o For bioretention swales or long, narrow bioretention areas: at least one small-scale PIT, or
other method outlined in this appendix, every 200 linear feet and within each length of road
with varying subsurface characteristics.
• Permeable Pavement:
o For projects subject for Minimum Requirement No. 1 through 5: at least one small-scale PIT,
or other method outlined in this appendix, per 5,000 square feet of permeable pavement (in
no case less than one test per site).
o For projects subject for Minimum Requirement No. 1 through 9, commercial property: at
least one small-scale PIT, or other method outlined in this appendix, per 5,000 square feet of
permeable pavement (in no case less than one test per site).
o For projects subject for Minimum Requirement No. 1 through 9, residential development: at
least one small-scale PIT, or other method outlined in this appendix, at every proposed lot,
every 200 linear feet of roadway, and within each length of road with significant differences
in subsurface characteristics.
• Rain Gardens: at least one small-scale PIT, or other method outlined in this appendix, at each
potential rain garden site.
APPENDIX B – METHODS FOR DETERMINING DESIGN INFILTRATION RATES JUNE 2017
4 B-2. CORRECTION FACTORS
B.2 Correction Factors
The Ksat obtained from the field tests or soil grain analyses is a measured (initial) rate. This measured
rate must be reduced through correction factors that are appropriate for the design situation to produce an
acceptable design infiltration rate. This adjustment is made in Step 5 of the Design of Infiltration
Facilities (SWMMWW Volume III, Section 3.3.4 Steps for the Design of Infiltration Facilities –
Simplified Approach [also summarized in Addendum Checklist 6]). Note that the correction factors below
may not apply to the infiltration testing conducted for bioretention, permeable pavement, and/or rain
gardens (see Volume III, Section 3.4 and Addendum Checklist 5 for additional information).
The following equation estimates the maximum design infiltration rate (Ksatdesign) using correction factors
to account for site variability, number of tests conducted, uncertainty of test method, and the potential for
long-term clogging due to siltation and bio-build up. The specific correction factors used shall be
determined based on the professional judgment of the site designer, considering all issues that may affect
the infiltration rate over the long term, subject to the approval of the City.
Ksatdesign = Ksatinitial x CFV x CFT x CFM
Table B.1. Correction Factors to be Used With In-Situ Saturated Hydraulic Conductivity
Measurements to Estimate Design Rates.
Issue Partial Correction Factor
Site variability and number of locations tested (CFv) CFv = 0.33 to 1.0
Test Method (CFT)
• Large-scale PIT
• Small-scale PIT
• Other small-scale (e.g., Double ring, falling head)
• Grain Size Method
CFT:
• CFT = 0.75 for Large-scale PIT
• CFT = 0.5 for Small-scale PIT
• CFT = 0.4 for other small-scale test
• CFT = 0.4 for Grain Size Method
Degree of influent control to prevent siltation and bio-buildup CFM = 0.9
Site Variability and Number of Locations Tested (CFv):
The number of locations tested must be capable of producing a picture of the subsurface conditions that
fully represents the conditions throughout the facility site. The partial correction factor used for this issue
depends on the level of uncertainty that adverse subsurface conditions may occur. If the range of
uncertainty is low – – for example, conditions are known to be uniform through previous exploration and
site geological factors – – one field test (or grain size analysis location) may be adequate to justify a
partial correction factor at the high end of the range.
If the level of uncertainty is high, a partial correction factor near the low end of the range may be
appropriate. This might be the case where the site conditions are highly variable due to conditions such as
a deposit of ancient landslide debris, or buried stream channels. In these cases, even with many
explorations and several field tests (or several grain size test locations), the level of uncertainty may still
be high. A partial correction factor near the low end of the range could be assigned where conditions have
a more typical variability, but few explorations and only one field test (or one grain size analysis location)
is conducted. That is, the number of explorations and tests conducted do not match the degree of site
variability anticipated.
JUNE 2017 APPENDIX B – METHODS FOR DETERMINING DESIGN INFILTRATION RATES
B-2. CORRECTION FACTORS 5
Uncertainty of Test Method (CFT):
CFT accounts for uncertainties in the testing methods. For the full-scale PIT method, CFT = 0.75; for the
small-scale PIT, CFT = 0.50; for smaller-scale infiltration tests such as the falling head percolation test
method, CFT = 0.4; for grain size analysis, CFT = 0.40. These values are intended to represent the
difference in each test’s ability to estimate the actual saturated hydraulic conductivity. The assumption is
the larger the scale of the test, the more reliable the result.
Degree of Influent Control to Prevent Siltation and Bio-Buildup (CFM):
Even with a pre-settling basin or a basic treatment facility for pre-treatment, the soil’s initial infiltration
rate will gradually decline as more and more stormwater, with some amount of suspended material,
passes through the soil profile. The maintenance schedule calls for removing sediment when the facility is
infiltrating at only 90 percent of its design capacity. Therefore, a correction factor, CFM, of 0.9 is called
for.
APPENDIX B – METHODS FOR DETERMINING DESIGN INFILTRATION RATES JUNE 2017
6 B-3. METHOD 1 – FIELD TESTING PROCEDURES
B.3 Method 1 – Field Testing Procedures
B.3.1 Large-Scale Pilot Infiltration Test (PIT)
Large-scale in-situ infiltration measurements, using the PIT method described below, is the preferred
method for estimating the measured (initial) saturated hydraulic conductivity (Ksat) of the soil profile
beneath the proposed infiltration facility. The PIT method reduces some of the potential scale errors
associated with relatively small-scale tests (such as the modified falling head percolation test, double ring
infiltrometer, or “stove-pipe” infiltration tests). It is not a standard test but rather a practical field
procedure recommended by the Washington Department of Ecology’s Technical Advisory Committee.
The PIT method is performed as follows:
Infiltration Test:
1. Excavate the test pit to the depth of the bottom of the proposed infiltration facility. Lay back the
slopes sufficiently to avoid caving and erosion during the test. Alternatively, consider shoring the
sides of the test pit.
2. The horizontal surface area of the bottom of the test pit should be approximately 100 square feet.
3. Accurately document the size and geometry of the test pit.
4. Install a vertical measuring rod (minimum 5 feet long) marked in half-inch increments in the
center of the pit bottom.
5. Use a rigid 6-inch-diameter pipe with a splash plate on the bottom to convey water to the pit and
reduce side wall erosion or excessive disturbance of the pond bottom. Excessive erosion and
bottom disturbance will result in clogging of the infiltration receptor and yield lower than actual
infiltration rates.
6. Add water to the pit at a rate that will maintain a water level between 6 and 12 inches above the
bottom of the pit. A rotameter can be used to measure the flow rate into the pit.
Note: For infiltration facilities serving large drainage areas, designs with multiple feet of standing
water can have infiltration tests with greater than 1 foot of standing water. The depth must not
exceed the proposed maximum depth of water expected in the completed facility.
7. Every 15 to 30 minutes, record the cumulative volume and instantaneous flow rate in gallons per
minute necessary to maintain the water level at the same point on the measuring rod.
8. Keep adding water to the pit until 1 hour after the flow rate into the pit has stabilized (constant
flow rate; a goal of 5 percent variation or less variation in the total flow) while maintaining the
same pond water level. The total of the pre-soak time plus 1 hour after the flow rate has
stabilized should be no less than 6 hours.
9. After the flow rate has stabilized for at least 1 hour, turn off the water and record the rate of
infiltration (the drop rate of the standing water) in inches per hour from the measuring rod data,
until the pit is empty. Consider running this falling head phase of the test several times to estimate
the dependency of infiltration rate with head.
JUNE 2017 APPENDIX B – METHODS FOR DETERMINING DESIGN INFILTRATION RATES
B-3. METHOD 1 – FIELD TESTING PROCEDURES 7
10. Within 24 hours after the conclusion of testing, over-excavate the pit to see if the test water is
mounded on shallow restrictive layers or if it has continued to flow deep into the subsurface. The
depth of excavation varies depending on soil type and depth to hydraulic restricting layer, and is
determined by the engineer or certified soils professional. Mounding is an indication that a
mounding analysis is necessary.
Data Analysis:
1. Calculate and record the infiltration rate in inches per hour in 30 minutes or 1-hour increments
until 1 hour after the flow has stabilized.
Note: Use statistical/trend analysis to obtain the hourly flow rate when the flow stabilizes. This
would be the lowest hourly flow rate.
2. To compute the design infiltration rate (Ksatdesign), apply appropriate correction factors outlined
previously.
Example:
The area of the bottom of the test pit is 8.5 feet by 11.5 feet.
Water flow rate was measured and recorded at intervals ranging from 15 to 30 minutes throughout the
test. Between 400 minutes and 1,000 minutes, the flow rate stabilized between 10 and 12.5 gallons per
minute or 600 to 750 gallons per hour, or an average of (9.8 + 12.3) / 2 = 11.1 inches per hour.
To compute the design infiltration rate (Ksatdesign), the infiltration rate must then be adjusted by the
appropriate correction factors outlined previously.
B.3.2 Small-Scale Pilot Infiltration Test
A smaller-scale PIT can be used in any of the following instances:
• The drainage area to the infiltration site is less than 1 acre.
• The testing is for bioretention areas or permeable pavement surfaces that either serve small
drainage areas and/or are widely dispersed throughout a project site.
• The site has a high infiltration rate, making a large-scale PIT difficult, and the site geotechnical
investigation suggests uniform subsurface characteristics.
Infiltration Test:
1. Excavate the test pit to the estimated bottom of the proposed infiltration facility. In the case of
bioretention, excavate to the estimated elevation at which the imported soil mix will lie on top of
the underlying native soil. For permeable pavement, excavate to the elevation at which the
imported subgrade materials, or the pavement itself, will contact the underlying native soil. If the
native soils (road subgrade) will have to meet a minimum subgrade compaction requirement,
compact the native soil to that requirement prior to testing. Note that the permeable pavement
design guidance recommends compaction not exceed 90 to 92 percent. Finally, lay back the
slopes sufficiently to avoid caving and erosion during the test. Alternatively, consider shoring the
sides of the test pit.
APPENDIX B – METHODS FOR DETERMINING DESIGN INFILTRATION RATES JUNE 2017
8 B-3. METHOD 1 – FIELD TESTING PROCEDURES
2. The horizontal surface area of the bottom of the test pit should be 12 to 32 square feet. It may be
circular or rectangular, but accurately document the size and geometry of the test pit.
3. Install a vertical measuring rod adequate to measure the ponded water depth and that is marked in
half-inch increments in the center of the pit bottom.
4. Use a rigid pipe with a splash plate on the bottom to convey water to the pit and reduce side wall
erosion or excessive disturbance of the pond bottom. Excessive erosion and bottom disturbance
will result in clogging of the infiltration receptor and yield lower than actual infiltration rates. Use
a 3-inch-diameter pipe for pits on the smaller end of the recommended surface area, and a 4-inch
pipe for pits on the larger end of the recommended surface area.
5. Pre-soak period: Add water to the pit so that there is standing water for at least 6 hours. Maintain
the pre-soak water level at least 12 inches above the bottom of the pit.
6. At the end of the pre-soak period, add water to the pit at a rate that will maintain a fixed water
level between 6 and 12 inches above the bottom of the pit over a full hour. The depth must not
exceed the proposed maximum depth of water expected in the completed facility.
7. Every 15 minutes, record the cumulative volume and instantaneous flow rate in gallons per
minute necessary to maintain the water level at the same point (between 6 to 12 inches) on the
measuring rod. The specific depth should be the same as the maximum designed ponding depth
(usually 6 to 12 inches).
8. After 1 hour, turn off the water and record the rate of infiltration (the drop rate of the standing
water) in inches per hour from the measuring rod data, until the pit is empty.
9. A self-logging pressure sensor may also be used to determine water depth and drain-down.
10. Within 24 hours after the conclusion of testing, over-excavate the pit to see if the test water is
mounded on shallow restrictive layers or if it has continued to flow deep into the subsurface. The
depth of excavation varies depending on soil type and depth to hydraulic restricting layer, and is
determined by the engineer or certified soils professional. The soils professional should judge
whether a mounding analysis is necessary.
Data Analysis:
Use the guidance for the large-scale PIT method outlined above.
B.3.3 Falling Head Percolation Test Procedure (as modified for the City of
Edmonds) (Source: U.S. EPA, On-site Wastewater Treatment and
Disposal Systems, 1980)
Note: This test may only be used for performance verification testing, as outlined previously in this
appendix. This test may not be used for BMP design, or to demonstrate infeasibility of meeting Minimum
Requirement No. 5.
1. Location of Tests
Tests shall be spaced uniformly throughout the area. If soil conditions are highly variable, more
tests may be required.
JUNE 2017 APPENDIX B – METHODS FOR DETERMINING DESIGN INFILTRATION RATES
B-3. METHOD 1 – FIELD TESTING PROCEDURES 9
2. Preparation of Test Hole (as modified for the City of Edmonds)
The diameter of each test hole is 8 inches, dug or bored to the proposed depths of the absorption
systems or to the most limiting soil horizon. To expose a natural soil surface, the bottom of the
hole is scratched with a sharp pointed instrument and the loose material is removed from the test
hole. A 6-inch-inner-diameter, 4-foot-long, PVC pipe is set into the hole and pressed into the soil
6 inches, and then 2 inches of one-half to three-fourths-inch rock are placed in the pipe to protect
the bottom from scouring when water is added.
3. Soaking Period
The pipe is carefully filled with at least 12 inches of clear water. The depth of water must be
maintained for at least 4 hours and preferably overnight if clay soils are present. A funnel with an
attached hose or similar device may be used to prevent water from washing down the sides of the
hole. Automatic siphons or float valves may be employed to automatically maintain the water
level during the soaking period. It is extremely important that the soil be allowed to soak for a
sufficiently long period of time to allow the soil to swell if accurate results are to be obtained.
In sandy soils with little or no clay, soaking is not necessary. If, after filling the pipe twice with
12 inches of water, the water seeps completely away in less than 10 minutes, the test can proceed
immediately.
4. Measurement of the Percolation Rate
Except for sandy soils, percolation rate measurements are made 15 hours, but no more than
30 hours after the soaking period began. The water level is adjusted to 6 inches above the gravel
(or 8 inches above the bottom of the hole). At no time during the test is the water level allowed to
rise more than 6 inches above the gravel. Immediately after adjustment, the water level is
measured from a fixed reference point to the nearest one-sixteenth inch at 30 minute intervals.
The test is continued until two successive water level drops do not vary by more than one-
sixteenth inch within a 90-minute period. At least three measurements are to be made.
After each measurement, the water level is readjusted to the 6-inch level. The last water level
drop is used to calculate the percolation rate.
In sandy soils or soils in which the first 6 inches of water added after the soaking period seeps
away in less than 30 minutes, water level measurements are made at 10 minute intervals for a
1-hour period. The last water level drop is used to calculate the percolation rate.
5. Calculation of the Percolation Rate
The percolation rate is calculated for each test site by dividing the time interval used between
measurements by the magnitude of the last water level drop. This calculation results in a
percolation rate in terms of minutes/inch. To determine the percolation rate for the area, the rates
obtained from each hole are averaged. (If tests in the area vary by more than 20 minutes/inch,
variations in soil type are indicated. Under these circumstances, percolation rates should not be
averaged.) To compute the design infiltration rate (Ksatdesign), the final percolation rates
must then be adjusted by the appropriate correction factors outlined previously.
Example: If the last measured drop in water level after 30 minutes is five-eighths-inch, then:
percolation rate = (30 minutes)/(5/8 inch) = 48 minutes/inch. (At a minimum, a correction factor
“CFT” of 0.4 shall be applied to all field methods for determining infiltration rates.)
APPENDIX B – METHODS FOR DETERMINING DESIGN INFILTRATION RATES JUNE 2017
10 B-4. METHOD 2 – SOIL GRAIN SIZE ANALYSIS METHOD
B.4 Method 2 – Soil Grain Size Analysis Method
For infiltration basins and trenches, for each defined layer below the infiltration facility to a depth below
the facility bottom of 2.5 times the maximum depth of water in the facility, but not less than 10 feet,
estimate the initial saturated hydraulic conductivity (Ksat) in cm/sec using the following relationship (see
Massman 2003). For large infiltration facilities serving drainage areas of 10 acres or more, soil grain size
analyses should be performed on layers up to 50 feet deep (or no more than 10 feet below the water
table).
(Equation 1):
Where, D10, D60, and D90 are the grain sizes in mm for which 10 percent, 60 percent, and 90 percent of the
sample is more fine and ffines is the fraction of the soil (by weight) that passes the U.S. #200 sieve (Ksat is
in cm/s).
For bioretention areas, analyze each defined layer below the top of the final bioretention area subgrade to
a depth of at least 3 times the maximum ponding depth, but not less than 3 feet (1 meter). For permeable
pavement, analyze for each defined layer below the top of the final subgrade to a depth of at least 3 times
the maximum ponding depth within the base (reservoir) course, but not less than 3 feet (1 meter).
If the licensed professional conducting the investigation determines that deeper layers will influence the
rate of infiltration for the facility, soil layers at greater depths must be considered when assessing the
site’s hydraulic conductivity characteristics. Massman (2003) indicates that where the water table is deep,
soil or rock strata up to 100 feet below an infiltration facility can influence the rate of infiltration. Note
that only the layers near and above the water table or low permeability zone (e.g., a clay, dense glacial till,
or rock layer) need to be considered, as the layers below the groundwater table or low permeability zone
do not significantly influence the rate of infiltration. Also note that this equation for estimating Ksat
assumes minimal compaction consistent with the use of tracked (i.e., low to moderate ground pressure)
excavation equipment.
If the soil layer being characterized has been exposed to heavy compaction (e.g., due to heavy equipment
with narrow tracks, narrow tires, or large lugged, high pressure tires) the hydraulic conductivity for the
layer could be approximately an order of magnitude less than what would be estimated based on grain
size characteristics alone (Pitt 2003). In such cases, compaction effects must be taken into account when
estimating hydraulic conductivity.
For clean, uniformly graded sands and gravels, the reduction in Ksat due to compaction will be much less
than an order of magnitude. For well-graded sands and gravels with moderate to high silt content, the
reduction in Ksat will be close to an order of magnitude. For soils that contain clay, the reduction in Ksat
could be greater than an order of magnitude.
If greater certainty is desired, the in-situ saturated conductivity of a specific layer can be obtained through
the use of a PIT. Note that these field tests generally provide a Ksat combined with a hydraulic gradient.
In some of these tests, the hydraulic gradient may be close to 1.0; therefore, in effect, the test infiltration
rate result is the same as the hydraulic conductivity. In other cases, the hydraulic gradient may be close to
the gradient that is likely to occur in the full-scale infiltration facility. The hydraulic gradient will need to
be evaluated on a case-by-case basis when interpreting the results of field tests. It is important to
fines906010102.08f- 0.013 - 0.015+ 1.90+-1.57)(log DDDKsat=
JUNE 2017 APPENDIX B – METHODS FOR DETERMINING DESIGN INFILTRATION RATES
B-4. METHOD 2 – SOIL GRAIN SIZE ANALYSIS METHOD 11
recognize that the gradient in the test may not be the same as the gradient likely to occur in the full-scale
infiltration facility in the long-term (i.e., when groundwater mounding is fully developed).
Once the Ksat for each layer has been identified, determine the effective average Ksat below the pond.
Ksat estimates from different layers can be combined using the harmonic mean:
(Equation 2):
Where, d is the total depth of the soil column, di is the thickness of layer “i” in the soil column, and Ki is
the saturated hydraulic conductivity of layer “i” in the soil column. The depth of the soil column, d,
typically would include all layers between the pond bottom and the water table. However, for sites with
very deep water tables (>100 feet) where groundwater mounding to the base of the pond is not likely to
occur, it is recommended that the total depth of the soil column in Equation 2 be limited to approximately
20 times the depth of pond, but not more than 50 feet. This is to ensure that the most important and
relevant layers are included in the hydraulic conductivity calculations. Deep layers that are not likely to
affect the infiltration rate near the pond bottom need not be included in Equation 2.
Equation 2 may over-estimate the effective Ksat value at sites with low conductivity layers immediately
beneath the infiltration basin. For sites where the lowest conductivity layer is within 5 feet of the base of
the pond, it is suggested that this lowest Ksat value be used as the equivalent hydraulic conductivity rather
than the value from Equation 2. Using the layer with the lowest Ksat is advised for designing bioretention
areas or permeable pavement surfaces. The harmonic mean given by Equation 2 is the appropriate
effective hydraulic conductivity for flow that is perpendicular to stratigraphic layers, and will produce
conservative results when flow has a significant horizontal component such as could occur due to
groundwater mounding.
∑=
i
i
equiv
K
d
dK