Engineering Prelim Comments-2.pdfMEMORANDUM
REQUEST FOR ADDITIONAL INFORMATION
Date: December 10, 2018
To: Kernen Lien, Planning
From: JoAnne Zulauf, Engineering
Subject: PLN20180057, Groset 2 lot short plat
22027 96th Ave West
The comments provided below are based upon review of the preliminary civil plans &
documents for the subject short plat. Additional information is requested from the applicant
at this time in order to continue review of the application and provide preliminary approval of
the short plat. Please ask the applicant to provide a written response to each of the
comments below and revise and resubmit plans accordingly.
Stormwater Engineer 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).
Ksatdzs,b„ = Ksat,,,;u,i x CFI• x CFr x CF„
Table B.1. Correction Factors to be Used With In -Situ Saturated Hydraulic Conductivity
Measurements to Estimate Design Rates.
Issue
Partial Correction Factor
Site variability and number of locations tested (CF)
CF = 0.33 to 1.0
Test Method (CFT)
CFT
• Large-scale PIT
• CFT = 0.75 for Large-scale PIT
• Small-scale PIT
• CFT = O.S for Small-scale PIT
• Other small-scale (e.g., Double ring, falling head)
• CFT = 0.4 for other small-scale test
• Grain Size Method
• CFT = 0.4 for Grain Size Method
Degree of influent control to prevent siltation and bio-buildupCFna
= 0.9
2. Drainage Report: Paragraph 3 of the executive summary appears to have
some errant text stating full infiltration is not feasible due to flow paths; clean-
up and clarify as needed.
3. Drainage Report: Report states several times that site will be designed to
infiltrate 100%; however, there appears to be a by-pass area of driveway
City of Edmonds
proposed which will discharge surface runoff without mitigation. Update text
and downstream analysis as needed.
4. Drainage Report: Update MR #5 section to state how the proposed bypass
area satisfies MR #5, and/or provide mitigation BMPs as required.
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.
6. Sheet 1: List the vertical datum used for the topo/design.
a. Note: Listed elevations vary rather significantly (15'+) from the City
contours; engineer shall verify all elevations were translated to the
project datum correctly.
7. Sheet 2: Show a storm stub for collection of the roof runoff from the shop
building and/or clarify how the structure will drain to the rain garden.
8. Sheet 2: It appears that the proposed drywells for footing drains do not comply
with setback requirements for infiltration facilities; updated as needed to
maintain required setbacks.
9. Sheet 2: While the rain garden detail has a dimension and label for 6"
freeboard, it appears the south -side (left side of detail) does not actually come
up high enough to provide 6" freeboard as shown; update as needed to
provide required 6" freeboard.
a. Note: The model appears to have assumed a 6" ponding depth but a
12" ponding depth is currently shown; it may be that elevations internal
to the rain garden need adjustment without adjusting the grading.
Please contact Zack Richardson, Stormwater Engineer, directly with any comments
or questions at 425-771-0220.
Thank you.