2916_001.pdf7E
An Engineering Services Company
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November 12, 2019
City of Edmonds
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121 511 Ave. N.
Edmonds, WA 98020
Subject: Edmonds Crossing Apartments (BLD20190951)
Response to Stormwater Review Comments
PACE Project No. 410-002-18
Dear Zack:
Engineers I Planners I Surveyors
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We are pleased to submit this letter along with the accompanying plans/reports for further technical
review and approval. This submittal package is submitted in response to recent technical review letter
issued by the City's Stormwater Engineer on September 4, 2019. The review comments from that letter
are provided below (plain text), along with a brief response (bold italics) indicating how each comment
was addressed.
1. General: Structural plans and calculations are required for the detention vault; submit structural
documentation with future submittal.
Comment noted. Structural planslicalculations are provided with this submittal.
2. General: The crossing of public vs private stormwater and public vs private storm systems
requires additional attention; the proposed storm alignment is not acceptable as shown without
additional documentation that no other options exist or without providing actual mitigation for
those extra areas [comment truncated for brevity]
The drainage design has been revised to separate on -site and frontage runoff.
3. Drainage Report: Revise/clarify the final sentence of off -site analysis as it pertains to the
proposed grading plan, which appears to be potentially directing some of this upstream run-on
directly toward the proposed structure.
This section has been revised. After taking a closer look at this area, it appears that the
property immediately south of the project actually falls away from the project site, which
has now been documented in the report.
4. Drainage Report: Address concern in comment #1 above, and update the existing conditions
exhibit as needed to address:
See next page:
RESUB
NOV 18 2019
PACE Engineers, Inc. I SDA
1724 W. Marine View Drive I Suite 140
Everett, Washington 98201-2088
p 425.486.6533 1 f 425.486.6593
BUILDING DEPARTMENI
CITY OF EDMONnS
www.paceengrs.corn
November 12, 2019
Zack Richardson
City of Edmonds
Page 2 of 5
a. Include a legend to identify what hatching represents (impervious area).
The requested legends have been added.
b. Address why limits of basin do not appear to match basin delineation
Basin maps have been revised.
c. using the existing contours.
Basin maps have been revised.
wwwpaceengrs com
5. Drainage Report: Add a column for square feet to the table on the developed basin map and
separate out pollution generating and non -pollution generating pavements.
A column has been added to specify the square -footage of the constituent basin areas.
Runoff from the proposed sidewalk and roof areas will be combined with the rest of the
site runoff, so all of the site runoff is assumed to be ponution-generating for purposes of
the drainage design. A brief discussion to this effect has been added to the drainage
report.
a. Include a legend to identify what hatching represents (impervious area).
The requested legend has been added
b. Update hatching or add notes to clarify how the existing road surfaces are accounted for
in the areas listed on this sheet.
A legend has been added to correlate the table with the drawing's plan -view.
c. Address why limits of basin do not appear to match basin delineation using the existing
contours.
Basin limits have been adjusted and are now consistent with existing contours. Note
that the project proposes a reverse -slope to the fronting sidewalk to avoid
unmitigated discharge.
6. Drainage Report: Expand Section 6 (runoff treatment) to.
a. Explain how the roof was determined to be non -pollution generating (all metal material
treated, mechanical equipment includes self contained spill control, etc) and provide
description of effort the engineer made to coordinate with the project team to ensure this
condition is carried through construction.
The water quality treatment design for the project now assumes an site runoff to
originate from poflution-generating surfaces. Since treatment is now provided for all
roof runoff, this comment no longer appears to be applicable.
Now
November 12, 2019
Zack Richardson
City of Edmonds
Page 3 of 5
www paceengrs com
b. Specifically list within this section the amount/area of pollution generating materials
proposed, the minimum water quality volume calculated, and the amount of water quality
volume provided.
A brief summary table has been added to this section.
7. Drainage Report: Expand Section 7 (flow control) to specifically list within this section the
amount of impervious surfaces proposed, the minimum flow control volume calculated, and the
amount of water quality volume provided.
The requested flow control summary has been added
8. Drainage Report: Update MGS run to utilize Puget Sound 36 data per Edmonds Addendum.
Calculations updated as requested.
9. Drainage Report: Revise conveyance calculations as noted:
a. It appears that the conveyance check only includes the portions upstream of the vault
and assumes free -flow at low end of system, but in fact the capacity of this system is
decreased when the detention system builds up. The system must past the 1 00-year
flow in an emergency (ie. fully clogged orifices); demonstrate that the system has
adequate capacity when the vault is full, adjust plan or elevations as needed.
The requested conveyance calculations have been provided, and a brief
description of the analysis has been added to the report.
b. It does not appear any information is provided for the frontage conveyance system;
demonstrate that a 0.6% sloped 12" pipe is sufficient for the frontage system or update
plans as needed (simple Manning's calculation usually sufficient for this step).
The requested conveyance calculations have been provided, and a brief
description of the analysis has been added to the report.
c. The outfall pipe from the vault appears to be the flattest pipe onsite but is not included in
the conveyance analysis; confirm that the outfall (12" @0.6%) outfall is sufficient capacity
to pass the 1 00-year flow (simple Manning's calculation usually sufficient for this step).
The requested conveyance calculations have been provided, and a brief
description of the analysis has been added to the report.
10. C4.0: Show/identify where the building footing drains are to discharge; if discharging to vault,
provide minimum elevation for the perforated portions which is at least 6" above the design
water surface of the vault.
The plans call for the rooflfooting drain discharge location, with the anticipated elevation.
won
November 12, 2019
Zack Richardson
City of Edmonds
Page 4 of 5
www.paceengrs.com
11. C4.0: Show/identify where the vault footing drain will discharge; include details as needed for
construction of any special systems, if proposed.
Vault footing drain has been added to the plans.
12. C4.0: Review, and address/respond as needed, if the proposed configuration of CB #5 with the
proposed curb and wall is feasible for proper alignment of the grate and access.
The referenced CB has been moved further from the wall for easier constructability and is
specified with a solid locking lid so it will not need to align with the curb.
13. C4. 1: Identify the elevations of crossing utilities on the profiles and check for conflicts; address
any crossings with less than 1' of vertical separation, as needed.
All utility crossings have been identified.
14. C4. 1: Adjust invert at CB #4 to provide 2' cover between top of pipe and bottom of flexible
pavement; it does not appear the thickness of the pipe or the pavement are accounted for
currently.
Structures have been adjusted to provide the required cover. The trench drain discharge
line will have less than 2-ft of cover since it originates from a shallow, proprietary catch
basin unit. However, it is not expected to cross beneath pavement, and has been
specified as a ductile iron pipe since it has relatively low cover.
15. C4.1: Add to the CIB #2 callout a reference to the control structure detail on sheet C4.3.
The requested callout has been added.
16. C4.1: It appears the project results in standing water in the CIB within the ROW when the
detention system is retaining water; this is generally not permitted unless all other options are
demonstrated to be infeasible Update or address as needed.
The design has been revised and no standing water is expected to remain in the right-of-
way improvements.
17. C4.3: The sediment storage in the wet pond area needs to be increased to 1' (per BMP T1 0.40).
The vault has been revised as requested
18. C4.3: Add, callout, and provide details as needed to allow for one-man access at the 5x1 0 grate
over the outlet (typically done with a lockable 3'x3' hinged access).
The access grate detail has been revised as requested
'November 12, 2019
Zack Richardson
City of Edmonds
Page 5 of 5
www.paceengrs.com
19. C4.3: Update the CB #2 detail to clarify that two access points are proposed (add "typ" to
existing callout, add 2nd callout, etc) and update the section view to be consistent.
The detail has been revised as requested.
20. Plans: On appropriate sheet, please add notes or callouts to state that BMP T5.13 (amended
soils) is required for all disturbed pervious areas and provide detail (SID-642) or specification
sufficient enough for contractor to meet the requirements.
The requested notes/details have been added.
This concludes our response to the recent review comments. Please feel free to call with any questions
or if you need anything further.
Sincerely,
PACE Engineers, Inc.
Kenneth J. McIntyre, PE
Project Manager
CP=E
19.Q 'A