PLN20160035_721 Edmonds Way_Stormwater Review2.pdf-i OV F I)p
v "N STORMWATER REVIEW COMMENTS
'7 City of Edmonds
„ Engineering Division
To: Engineering Reviewer
Date: December 22, 2017
Project Name: Chung (3-lot short plat)
Permit Number: PLN20160035
Address: 721 Edmonds Way
Review Type: Civil Plans
Submittal Date: 12/28/2017
Reviewer: Zack Richardson, PE
City of Edmonds, Stormwater Engineer
Recommendation: I recommend that PLN20160035 be withheld until the comments
below are adequately addressed.
Review Comments:
General: As noted in the previous comments, the engineer does not appear to
have correctly adjusted the direct discharge requirement for the size of the
project.
a. The prescribed flowrates provided in the Edmonds supplement (2010) are
applied per acre of impervious; projects must divide the project generated
flow rate by the total acres of the project impervious in order to adjust the
flow rate to an equivalent rate for comparison.
b. As currently modelled: 10-year rate = 0.120266 / 0.248 acres = 0.485 >
0.25 4 Does not meet direct discharge requirement.
c. As currently modelled: 100-year rate = .193843 / 0.248 = 0.782 > 0.45 4
4 Does not meet direct discharge requirement.
d. See additional comments below about adequacy of the currently modelled
surfaces; additional revision may be required.
2. Drainage Report: Revise the developed site conditions and/or provide an
developed conditions exhibit which addresses the following comments:
a. It is not clear that the impervious surfaces for the joint driveway have
correctly accounted for the additional road width required for the extruded
curb or thickened edge. Update report to clarify if this is included in the
calculated impervious surfaces; the plans will need to be updated to
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Stormwater Review
specify the proposed drainage control mechanism (thickened edge or
extruded curb) in order to be consistent with the report.
b. It does not appear that the modelling includes the impervious surfaces
generated for the frontage improvements; update report to include these
impervious surfaces in all drainage calculations.
c. The currently proposed limits on the future lots will require recording of a
covenant to guarantee that the impervious limits are adhered to in the
future; update report to note this requirement.
i. Note that currently proposed surfaces may severely restrict
buildability down the road; most notably reflected in the driveway
shown on Lot 3 which already exceeds the assumed impervious
limit for the lot driveway total.
3. Drainage Report: It appears that the impervious surfaces from the frontage
improvements and all portions of the joint driveway below CB #2 will not be
collected in the proposed system and will bypass; update report as needed to
account for bypass area.
a. Address impacts of potential surface flow increases within the ROW
b. Modell as a bypass basin with a point of compliance analysis, as needed
(ie. if a facility is provided)
4. Drainage Report: Significantly expand the MR #5 section to provide distinct
infeasibility for each BMP type and expand explanation as noted below:
a. Steep slopes are currently cited as infeasibility but existing site conditions
describe the site as "average slope of 4%" and the "steepest portion... is
8%"; update report for consistency or apply BMPs as required.
b. Setbacks are currently sited as infeasibility but no utilities appear to be
proposed within the joint driveway easement area which appears to be a
potential location for application of BMPs; update to provide explanation of
infeasibility within joint driveway or provide BMPs as required.
c. It does not appear that a site specific soils investigation has been
conducted and therefore, it does not appear that infiltration can be
adequately determined infeasible; provide additional soils information
and/or explanation of infeasibility.
5. Drainage Report: It appears that the conveyance calculation only considered
runoff from impervious surfaces and not from the entire site; update calculations
to reflect all runoff anticipated from the site.
6. Cl: Update impervious area table to match the drainage report and include joint
driveway surface area. Ensure values are consistent within the table itself.
7. C4: It is unclear if CB #1 (in Edmonds Way) is aligned to collect runoff from the
flow line; provide additional notes or details as needed to ensure grate is aligned
within the flow line of the curb & gutter.
a. Existing CB's in Edmonds Way appear to be spaced —457' apart which
exceed the WSDOT maximum allowable 300 feet; therefore it appears
that a collection CB is required.
8. C4: Update sheet to eliminate overlapping text, or remove text that is shown on
other sheets.
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9. C4: Update and/or provide additional details as needed to clarify what kind of
structures CB# SDCO #1 & CB# SDCO #2 actually are; profile suggest these are
"cylindrical junction structures" but no details are provided for such a structure.
10. C4: Update and/or provide additional details as needed to clarify what kind of
structures CB# Lot 1 Stub, CB# Lot 2 Stub, & CB# Lot 3 Stub actually are; profile
suggest these are structures of some sort (i.e. not just capped stubs) but no
details are provided.
11. C4: Add missing callout for CB #2 & CB #3 to this sheet.
12. C4: Add a downturned elbow, frop tee, or approved equal to CB #2 for floatables
screening before discharging to the ROW system.
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