Response to City Storm Comments Memo_02-28-18.pdf10135 SE Sunnyside Rd t 503.659.9500
Suite 200 f 503.659.2227
Clackamas, OR 97015 www.pacland.com
Memorandum
Date: 02/28/18
To: Zack Richardson
From: Shawn Nguy - PACLAND
Subject: Response to Stormnwater Engineering Review Comments
Application #: bld20171246
Project: Shops at Aurora Village
Project Address: 8431 244th St SW
The following are responses to each of the comments from your review:
Review Comments:
1. General: Drainage report & civil plans must be stamped and signed by the engineer of record at next,
and all future, submittals.
Response: Plans and Drainage Report are stamped and signed.
2. General: DOE general use designation for Kristar/Oldcastle filters requires manufacture approval of
the site plan; prior to final permit approval, submit verification of manufacturer approval with dated
references to the site plan reviewed.
Response: Noted. See attached letter from Kristar/Oldcastle.
a. City reviewer has concerns about proposal of 84" manhole lid designed very near to the
required minimum depth (ie. no room for cones/risers) across a 5%-7% paved slope; provide
additional details, grading info, or written response to show how the lid will be accounted for.
Response: The storm filter structure has been relocated to allow more depth for lid
adjustments across grades. Kristar/Oldcastle has customized the structure for this site.
3. General: It is not clear that the detention system has been adequately sized; update plans and reports as
needed to address the following:
a. It is not clear that the design has correctly accounted for the internal walls and gaps within the vault;
provide calculation (and additional dimensions) as needed to verify actual storage volume provided
in vault.
Response: The storm vault has been revised to account for internal walls. The required
minimum volume for the vault based on WWHM is 13,219 cf. Including the 6" of sediment
storage, the total minimum required volume is 14,160 cf.
In order to account for internal walls and sediment storage and the use of precast modules,
the size of the vault footprint has increased to 40'x64', with a total available storage volume
of 15,606 cf, which exceeds the required minimum volume of the vault. See updated plans.
Page 2 of 4
City of Edmonds
February 2018
b. It is not clear that the vault has been design to account for the required minimum 6" of
sediment storage; update plans as needed.
Response: Completed. The storm vault has been revised to account for the required 6" of
sediment storage. The interior bottom of the vault is set at elevation 412.50" which is 6
inches below the lowest pipe invert elevation of 413.00. See updated plans.
c. It appears that the mitigated 50-year peak exceeds the pre -developed 50- year peak; update plans
and/or report, or address in written response, as needed.
Response: Per our coordination with Clear Creek Solutions, who is the developer of the WWHM
software, the flow control criteria is based on flow duration, not flow frequency. In WWHM2012
frequency is calculated using the Log Pearson Type III method. This method is based on the log of
the mean, standard deviation, and skew of the annual peak values. Sometimes the Mitigated
scenario produces peak values with a higher standard deviation and skew than the
Predevelopment scenario. This can sometime result in the Mitigated 50-yr and 100-yr values being
higher than the Predeveloped. This is okay and is allowed by Ecology.
d. Several dimensions in the report and plans may differ; ensure all dimensions are consistent at
resubmittal.
Response: Noted. See updated plans and stormwater report.
4. Drainage Report: Expand the minimum requirement #5 section of the report to list all potential BMPs
and a specific infeasibility criteria (per ECDC 18.30 addendum Appendix A) for each BMP, or
implement BMPs as required.
Response: Noted. Refer to the updated drainage report.
5. Drainage Report: It appears that a full detention vault will backwater the filter system; address impacts
to water quality treatment as a result of this design and/or update design as needed.
Response: Revised. The water quality structure has been relocated to allow for free discharge
without backwater effects from the detention vault. See updated plans.
CSWPP: It is unclear that the CSWPP is adequate for the site; update plan and text as needed to address
the following:
a. The project appears to be more than 1 acres in size and will, therefore, require a general construction
stormwater permit from DOE; the report text should be updated to reflect this requirement, OR more
specifically restrict the clearing and grading limits to demonstrate that less than an acre of
land disturbing activity is proposed.
Response: The limit of disturbance has been revised and is now less than 1 acre. A construction
stormwater permit from DOE is not required.
b. The timing and limits of pavement removal are left unclear and could significantly impact the erosion
control needs for the site is construction traffic and parking could utilize the existing stable surfaces.
Update plans to clarify timing and limits of pavement removal.
Page 3 of 4
City of Edmonds
February 2018
Response: It is the intent of the contractor and developer to phase the project in such a manner as
to minimize exposure of the subgrade at any one time. The entire site will not be stripped at once.
Existing pavement will be maintained as long as possible to facilitate vehicular access around the
site during construction. It is the intent to start construction and conduct earth disturbing activities
during the summer months and dry weather conditions. Refer to additional notes and timing
provided on the plans.
c. If the entire site is to be cleared at once, it is unclear that silt fencing alone can provide adequate
sediment control; expand text and/or provide additional calculation to demonstrate that adequate
sediment control will be provided.
Response: The site will not be cleared at once. It is the intent of the contractor and developer to
phase the project in such a manner as to minimize exposure of the subgrade at any one time.
C2.0: Revise all lynch type catch basins and 30" ADS drain basins to be Type 1 catch basins or Type 2
manholes, as appropriate, per Edmonds details.
Response: Per our coordination, the use of lynch type catch basins is allowed to provide oil control at all
the collection points through the site. The oil control measures provided by the elbow traps inside the
lynch basins will help with overall long-term maintenance of the storm system. Each of the Lynch CBs
proposed are less than 5ft in depth and the CBs are not located within designated pedestrian and ADA
paths. Type 2 manholes with grate are used where the structure depth exceeds 5ft.
C2.0: Number all catch basins and manholes (30" ADS drains are not currently numbered).
Response: Revised. See updated plan.
C2.0: Inverts at the following catch basins appear to have insufficient cover: CB #1, CB #2, & the 30" ADS
drain upstream of the water quality filter. Revise plans to provide minimum 2' cover to the bottom of the
base course for traffic areas, or provide alternative pipe material (ductile iron, etc).
Response: The stormwater system has been revised to provide a minimum 2ft of cover throughout the
storm network.
10. C2.0: It is unclear what is proposed for the vault and building footing drain systems; show footing
drain piping and outlet location and provide additional (vault) details as needed.
Response: Footing drain has been added to the perimeter of the storm vault.
11. C2.0: It appears that the areas around the south-east corner of the building (sidewalk to south and east, 2-3
southern -most parking stalls, and entrance drive isle) will flow down to the existing drive isle and therefore
bypass the proposed facilities; update grading plan to direct runoff to the proposed storm system and/or
revise drainage design to account for bypass areas.
Response: Grades have been adjusted to direct flows from the site to the catch basins to the maximum
extent feasible.
12. C2.0: It appears that the west curb of the north island east of the building is proposed to be flat (ie. no
north/south sloping) with runoff directed to it from the west, potentially resulting in a ponded area;
update grading plan as needed to direct runoff to a drain, and/or add new drain as needed.
a. This island appears to be a good location for a rain -garden or bio-retention with curb cuts.
Page 4 of 4
City of Edmonds
February 2018
Response: Grades have been adjusted to eliminate ponding at the location in question. With the increased
vault size, there is not enough room to feasibility fit a rain garden type structures. Infiltration of runoff is
not proposed onsite per geotechnical recommendations.
13. C4.0: Ensure the invert elevations on the flow control structure are all listed correctly; it appears the
invert out has two inverts listed (it must be at or above the bottom of detention vault).
Response: Revised.
14. C4.1: Update vault section/profile to show and callout the top of the required sediment storage and
the top of the design water surface.
Response: Revised.
0
Oldcastle
Precast'
February 16, 2018
Shawn Nguy, PE
PACLAND Engineering & Development Consulting Services
10135 SE Sunnyside Rd, Suite 200
Clackamas, OR 97015
RE: The Shops @ Aurora Village — PerkFilterTm Review
Dear Shawn:
5331 SW Macadam Ave., #376
Portland, OR 97239
Oldcastle Precast - Stormwater has reviewed the design of the proposed media filtration device for
the Shops @ Aurora Village project located in Edmonds, WA. The PerkFilter is to be placed
downstream of a detention facility, and Oldcastle Precast — Stormwater performed both a flow -
based and mass -based sizing analysis using site information provided by PACLAND (the designer).
The system sizing is governed by the anticipated annual mass -loading. Oldcastle Precast -
Stormwater recommends a 72" PerkFilter manhole with five (5) 12" cartridges. Peak storms will
bypass the PerkFilter treatment chamber via the internal weir on the inlet/bypass assembly. The
Location of the filtration system on the site and the depth of the system appears to allow for
adequate maintenance access and internal clearance.
The PerkFilter system has been designed in accordance with the Washington State Department of
Ecology General Use Level Designation for the PerkFilter to operate at a cartridge hydraulic loading
rate of 1.5 gpm/sf of media surface area using zeolite-perlite-carbon (ZPC) filter media.
In summary, the PerkFilter system appears to be a good treatment solution for this project and has
been designed to operate within Oldcastle Precast - Stormwater's specifications. Please note, the
hydraulics of the collection system upstream or downstream of the filtration unit has not be
reviewed by Oldcastle Precast - Stormwater. If you have any questions or require any additional
information, please do not hesitate to contact me.
Regards,
1. wVlll lt,l
9V(
Matthew Thompson, EIT
Engineer -In -Training
Matthew.Thompson@oldcastle.com
(800) 579-8819
0Ldcastlestormwater.com 16 Stormwater