21-100244-SU-Response Letter (2) (7-6-2021)LARSON
AND ASSOCIATES dlLmjo*,d, P,Npeer& & AMN,PP6
July 2, 2021
City of Federal Way
RE: File #21-100244-00-SU; Technical Comment Letter
South Campus Estates Preliminary Plat
To: Ms. Natalie Kamieniecki
We have addressed the traffic comments from the City review letter dated May 27th, 2021 as follows:
Public Works —Traffic
Plan Comments:
1. This right of way on the north side of centerline on SW 3661h Street was previously dedicated to
provide the 30 feet of right of way which allows for the proposed improvements. As
recommended, we have removed the proposed bioretention swale and storm system from the
north side of SW 366th St. We have, however, proposed establishment of a 10 foot wide
vegetated flow path with amended soils following BMP T5.13 to be installed along the north
side of the road improvements for stormwater control purposes.
2. Additional right-of-way has been shown to accommodate the 3 foot utility strip along 6th Ave.
NW.
3. The conceptual intersection illumination plan prepared by PSE to be submitted separately once
available.
4. See attached updated conceptual storm plan including a Flag lot driveway cross-section detail.
5. Driveway width callouts have been provided on the updated conceptual storm plan, as
requested.
6. As discussed in item #1 above, we are proposing establishment of a 10 foot wide vegetated flow
path with amended soils following BMP T5.13 to be installed along the north side of the road
improvements for stormwater control purposes which is detailed in the updated conceptual
storm plan attached.
Public Works —Development Services
1. Noted. We have updated the Vertical Datum reference, as requested. See attached updated
Conceptual storm plan.
2. Noted. We will resolve this prior to final engineering document submittal.
As discussed with city staff, we will be submitting Variance Waiver request to allow for a
ballasted concrete sidewalk along 366th that will consist of traditional impervious concrete
surface with the reservoir rock base course below allowing for the hydraulic connectivity
between the bioretention cell and the sidewalk reservoir to maintain the storage volume within
the rock voids necessary to continue with this flow control stormwater design concept along
366th ROW fronting the project.
4. See response above regarding the submission of Variance Waiver request to allow for
implementation of ballasted concrete sidewalk along 366th to maintain stormwater storage
volume under sidewalk surface.
5. See updated geotechnical report that now describes the viability of lot infiltration trenches if the
infiltration trench design criteria is achieved following the 2014 Department of Ecology
stormwater manual. Some "filling" activities raising lot grades with appropriate "fill" material
may need to occur to ensure separation from bottom of storm facilities to impermeable soil
layer noted in geotech's soil's eval as specified in the updated Geotech. report.
6. Again, see updated geotechnical report that now describes the viability of lot infiltration
trenches if the infiltration trench design criteria is achieved following the 2014 Department of
Ecology stormwater manual. Some "filling" activities raising lot grades with appropriate "fill"
material may need to occur to ensure separation from bottom of storm facilities to
impermeable soil layer noted in geotech's soil's eval as specified in the updated Geotech.
report.
7. Again, see updated geotechnical report that now describes the viability of lot infiltration
trenches if the infiltration trench design criteria is achieved following the 2014 Department of
Ecology stormwater manual. Some "filling" activities raising lot grades with appropriate "fill"
material may need to occur to ensure separation from bottom of storm facilities to
impermeable soil layer noted in geotech's soil's eval as specified in the updated Geotech.
report.
8. We plan to utilize the native soils below and amended soils within the bioretention cells
following soil amendment BMPT5.13 and the current King County stormwater manual to
achieve appropriate water quality treatment of associated pollution generating impervious
surface runoff within the public right of way.
9. See updated conceptual storm report that provides additional detail on how bioretention cells
will meet the enhanced basic water quality treatment requirement. We have now added a note
that native underlying soils shall be tested as follows to ensure treatment properties are
available or the native soils will be "amended" and tested to ensure the following soil properties
are available along "cell' bottoms.
Please feel free to contact me at 253-474-3404 to discuss responses to your comments shown above.
Sincerely,,
GraVnVJtV--J��.MII i I on, PE
Larson & Ass o ates, Inc.