20190314 Resub Response Letter.pdfDonna Breske
& Associates
Civil Engineering & Land Use Planning
March 13, 2019
City of Edmonds
Development Services Department
Planning Division
Attn: Kernen Lien, JoAnne Zulauf, and Zachary Richardson
City Hall - Second Floor
Edmonds, WA 98020
Subject: Request for Additional,
Information Groset Two -Lot Short Platt
File number PLN20180057
21 Ave A, Suite 4
Snohomish, WA 98290
Telephone (360) 294-8941
www.donnabreske.com
Donna Breske & Associates, LLC is in receipt of the Request for Additional Information Comments for
Groset Two -Lot Short Plat PLN201280057 dated February 7, 2019. This letter was prepared by Alex
White, EIT, at Donna Breske & Associates, LLC and reviewed and signed by Donna L. Breske, PE.
Review Comments:
1. Geotechnical Report: It is not clear that the required correction factors were applied to the measured
infiltration rate to obtain the long-term design infiltration rate; provide or show calculation for
converting between the two rates using the required correction factors (copied below).
Update: The addendum to the geotechnical report only applied a single correction factor for test method
and not all three noted above. It is the site variability correction factor which requires professional
judgement and needs to be provided; based on the information provided thus far, the infiltration rate
could be anywhere between 2.15 in/hr and 6.5 in/hr. The utilized infiltration rate would require that the
geotech recommends a 0.92 correction factor for site variability, which would require a relatively high
degree of certainty in the uniformity of site conditions.
Response: The geotechnical engineer has now specified a site variability factor of 1.0 and has provided
a design infiltration rate of 6.3 in/hr.
2. Nothing further.
3. Nothing further.
4. Drainage Report: Update MR #S section to state how the proposed bypass area satisfies MR #S,
and/or provide mitigation BMPs as required.
Update: We disagree with the conclusion that a portion of the driveway is infeasible for treatment with
MR #S. Infiltration by way of trench, drywell, or pervious pavements all appear feasible and could be
constructed within the footprint of the new impervious surface (though additional utility improvements
or relocation may be needed). Update to provide mitigation for driveway surface or expand the section
for MR #S to explicitly state why other infiltration BMP (beside a bio-retention area) are infeasible.
Note: As a new impervious sup face, the code does not include an exception based on quantity of flow
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Donna Breske
& Associates
Civil Engineering & Land Use Planning
21 Ave A, Suite 4
Snohomish, WA 98290
Telephone (360) 294-8941
www.donnabreske.com
produced, but this may afford opportunities for smaller infiltration systems (such as the same drywell
detail as footing drains, but with an open grate; the vertical pipe would even carry the flow below most
utility elevations if left solid).
Response: The design now includes an infiltration trench that will collect runoff from the bypass area,
and the WWHM2012 calculations indicate that this project will meet the low impact development
performance standard.
5. Drainage Report: It does not appear that the `retro-fit' requirement of ECDC 18. 30. 060.D.5. b. i has
been addressed; update plans and report as needed to provide mitigation for 25% of the existing
unmitigated surfaces to remain.
Update: Splash block and vegetate flow path now shown and impervious area exhibit looks good and
clear; however it does not appear the flow paths can be achieved as shown. It appears that both of the
west blocks are short of the required 50' length, and none of the 3 drawn flow path arrows appear to
follow existing contours (ie. flow downhill). Update as needed to provide mitigation; if dispersion is
proposed, ensure and note in report text that flow path lengths can be achieved, flow path surface meets
the conditions for a vegetated flow path, and ensure flow paths are directed downhill (without
rechanneling the flows or overlapping other flow paths).
Response: Splash blocks have been removed from the stormwater design. The existing drving surface
will be mitigated by an infiltration trench.
6. Nothing further for preliminary approval. The vertical benchmark information was the information
requested previously; ensure this information is carried onto civil construction drawings and updated to
reflect the correct project
("southeast project corner"?)
Response: The vertical benchmark is shown on the plans, with a note indicating that the vertical datum
is assumed.
7. Nothing further.
8. Nothing further.
9. Nothing further.
Sincerely,
Alex White, EIT
425-208-9744
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Donna Breske, PE
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