Technical Comments South Campus EstatesCOMMUNITY DEVELOPMENT DEPARTMENT 33325 8th Avenue South Federal Way, WA 98003-6325
253-835-7000
www.cityoffederalway.com
Jim Ferrell, Mayor
May 27, 2021
Mr. Scott Clark
Larson & Associates
9027 Pacific Avenue, Suite 4
Tacoma, WA 98444
sclark@rrlarson.com
Re: File #21-100244-00-SU; Technical Comment Letter
South Campus Estates Preliminary Plat, No Site Address, Federal Way
Dear Mr. Clark:
City of Federal Way staff has reviewed your submittal of documents related to the South Campus Estates
Preliminary Plat. City departments have provided the following comments that will need to be addressed
before moving forward with the review. Questions regarding the technical review comments should be
addressed to the referenced staff representative.
PUBLIC WORKS – TRAFFIC
Jason Kennedy, Senior Traffic Engineer, 253-835-2744, jason.kennedy@cityoffederalway.com
The Public Works Traffic Division has finished its review of the submitted materials and provides the
following technical review comments:
Plans Comments
1. An additional eight feet of right-of-way north of the indicated centerline on SW 366th Street needs to be
dedicated to incorporate the total required 20-foot pavement for half-street improvements. The items shown
in the plan that are north of the 20-foot pavement are not required and not recommended to be constructed
for this development as any future development taking place on the north side of SW 366th Street will be
required to complete the street improvements that are applicable for the Type T cross-section.
2. Show additional right-of-way to accommodate a three-foot utility strip that is now shown in the
conceptual drawing as 2.88 feet along 6th Avenue SW.
3. Provide the conceptual intersection illumination plan for intersection lighting at SW 366th Street and 6th
Avenue SW. The intersection illumination plan shall be designed by a professional engineer and will need
to be submitted as part of the engineering plan submittal.
Mr. Scott Clark
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May 27, 2021
21-100244-00-SU Doc. I.D. 81316
4. Show cross-section and dimensions of the private driveways on the plan that serve the flag/rear lots, per
Development Standard Drawing 3-2BB.
5. Show the driveway widths of all driveways, or add a note that states all driveways are the same width.
Driveways serving single households have a maximum width of 20 feet (Federal Way Revised Code [FWRC]
19.135.270).
6. If remaining in the plans, please identify/label the four-foot section in between the two-foot shoulder
and eight-foot ditch on the north side of SW 366th Street cross-section diagram that is not identified.
PUBLIC WORKS – DEVELOPMENT SERVICES
Kerry Murdock, 253-835-2746, kerry.murdock@cityoffederalway.com
The Public Works Development Services Division has finished its review of the submitted materials and
provides the following technical review comments:
1. VERTICAL DATUM shall be K.C.A.S. or NGVD-29, not NAVD 88.
2. The plat is supposed to survey into two existing city monuments and submit the City of Federal Way
Monument Record Standard Drawing 3-37 (see FWRC 18.60.120, “Monuments” and the City of Federal
Way Development Standards Section 3.9.5 “Survey Monuments” for additional information).
3. The plans and TIR indicate a five-foot-wide pervious asphalt pathway sidewalk (impervious at driveway
locations) within the right-of-way on the south side of SW 366th Street. The runoff from the roadway and
driveway approaches will be collected and treated utilizing infiltration from both the bioretention cells
and the pervious asphalt pathway reservoir course. The bioretention cell and the pervious asphalt
pathway reservoir course will be hydrologically connected by having the reservoir course directly adjacent
to the bioretention cell. The bioretention cells are located between the driveway approaches with the
infiltration trenches located under the driveway approaches. The city does not allow pervious asphalt
within the right-of-way due to maintenance concerns and the shorter lifecycles of these types of asphalts.
The five-foot sidewalk section will have to be constructed per Standard Detail 3-12; concrete shall be
class 3000, WSDOT specification 8-14.
4. The Geotechnical Report states that LID facilities such as rain gardens and bioretention swales will be
feasible in the upper weathered till soils using a design infiltration rate of 0.25 in/hr. Because the adjacent
sidewalk sections are required to be impervious and additional runoff from approximately 2,455 feet will
be diverted into the bioretention cells, remodeling is likely to determine if the bioretention cells can
support the additional capacity at the current modeled rates. The developed areas will have to be
amended in the TIR and statements regarding pervious/permeable asphalt within 366th.
5. The plans and the TIR indicate the use of onsite infiltration trenches or “beds” of various sizes at depths
of 18” to infiltrate roof runoff via a downspout tight line system and within the ROW underneath the
driveway approaches. The Geotechnical Report has the following statements regarding the use of
infiltrative BMPs:
a. Field investigation indicated the soil conditions generally consist of six to eight inches of topsoil/duff
overlying weathered and un-weathered horizons of glacial till soils.
Mr. Scott Clark
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May 27, 2021
21-100244-00-SU Doc. I.D. 81316
b. Weathered till soils were observed in each test pit to depths ranging from 2.5 to 4 feet.
c. Test pits 1 & 2 showed mottling of the upper weathered till soils indicating groundwater is likely
perched within this layer on a seasonal basis. We expect the perched condition develops during and
shortly following the wet winter months.
d. The use of infiltration or drywells as a means of roof runoff flow control will not be feasible due to
the high fines of the upper weathered silty sand and silt soils, and the cemented nature of the
underlying till.
6. Appendix C and Chapter 5 of the King County Surface Water Design Manual (KCSWDM) state the following
regarding the use of Full Infiltration and Flow Control Design regarding trenches/alternative beds:
a. Many locations in King County have soils that are underlain by a compacted layer of soil (i.e., glacial
till or hardpan) which severely limits soaking capacity and causes water to perch on the relatively
impervious layer during the wet season. These factors can make full infiltration of runoff
impracticable, cost-prohibitive, unreliable, or deleterious to groundwater quality.
b. Existing soils must be coarse sands, cobbles, or medium sands and cannot be comprised of fill
materials where the infiltration device will be located.
c. The proposed trench bottom must be at least three feet above the seasonal high groundwater level
and three feet below the finished grade.
d. There must be at least three feet of permeable soil beneath the trench bottom.
e. For any optional or mandatory application of full infiltration, the distance measured down from the
bottom must be at least one foot for a gravel-filled infiltration system and three feet for a ground
surface depression. As indicated above, hardpan was discovered at a depth of 2.5 feet and the
proposed onsite infiltration beds have depths of 18 inches, so the required one-foot of distance
appears to have been met but conflicts with the minimum depth requirement of the distribution pipe.
f. Level six-inch minimum diameter rigid perforated distribution pipes shall extend the length of the
trench. Distribution pipe inverts shall be a minimum of two feet below the finished grade.
7. Please provide a more specific analysis on how the proposed infiltration trenches/beds onsite and within
the right-of-way driveway approaches meet the requirements/contradictions listed above.
8. Stormwater treatment and water quality treatment requirements apply to any improvements within the
public right-of-way. Because the frontage improvement requirements will exceed 5000 feet of new
impervious surface area, water quality treatment is typically required. Looking at one of the exemptions
requiring a water quality facility per the Enhanced Basic Water Quality Treatment Menu; Chapter 1 of the
KCSWDM states: “If the runoff from pollution-generating impervious and pollution generating pervious
surfaces is infiltrated in a facility per Section 5.2.1 in soils that meet the groundwater protection soil
quality, depth, and infiltration rate criteria given in Section 5.2.1; except for areas that are within one-
quarter-mile of a sensitive lake.”
9. The TIR states stormwater from the driveways will be treated via the native soil, which meets the CEC
and organic content required for treatment. Provide a more specific analysis on how the bioretention
cells meet the enhanced basic water quality treatment requirement.
Mr. Scott Clark
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May 27, 2021
21-100244-00-SU Doc. I.D. 81316
CLOSING
When resubmitting requested information, please contact the Permit Center for instructions on digital
resubmittal, or follow this link: https://www.cityoffederalway.com/page/permit-center. According to FWRC
19.15.050, if an applicant fails to provide additional information to the city within 180 days of being notified
that such information is requested, the application shall be deemed null and void and the city shall have no
duty to process, review, or issue any decisions concerning such an application.
If you have any questions regarding this letter or your development project, please contact me at
natalie.kamieniecki@cityoffederalway.com.
Sincerely,
Natalie Kamieniecki
Associate Planner
c: Kerry Murdock, Engineering Plan Reviewer
Jason Kennedy, Senior Traffic Engineer
Geoffrey P. Sherwin, P.E., geoff@jkmonarch.com