Engineering Prelim Comments-3.pdfMEMORANDUM
REQUEST FOR ADDITIONAL INFORMATION
Date: February 7, 2019
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.
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-buildupCF.
= 0.9
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
City of Edmonds
site variability, which would require a relatively high degree of certainty in the
uniformity of site conditions.
2. Nothing further.
3. Nothing further.
4. Drainage Report: Update MR #5 section to state how the proposed bypass
area satisfies MR #5, 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 #5. 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 #5 to explicitly state why other infiltration
BMP (beside a bio-retention area) are infeasible. Note: As a new impervious
surface, the code does not include an exception based on quantity of flow
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).
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).
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"?).
7. Nothing further.
8. Nothing further.
9. Nothing further.
Please contact Zack Richardson, Stormwater Engineer, directly with any comments
or questions at 425-771-0220.
Thank you.