Stormwater Review 1.pdfy OV U0410
v STORMWATER REVIEW COMMENTS
City of Edmonds
Engineering Division
To: Engineering Reviewer
Date: November 20, 2017
Project Name: Mietzner (244th)
Permit Number: BLD20171333
Address: 8609 244th St SW
Review Type: Civil Construction
Submittal Date: 9/28/2017
Reviewer: Zack Richardson, PE
City of Edmonds, Sto mwater Engineer
Recommendation: I recommend that BLD20170133 be withheld until the comments
below are adequately addressed. For the design as currently proposed, the applicant
shall review Edmonds SWMMWW Addendum Checklists #4 & #11 prior to re -submittal.
Review Comments:
1. Geotechnical Report: It is not clear what testing method was used to determine
the design infiltration rate or that the design infiltration rate includes the correct
adjustment factors. Update report to include more thorough description of
infiltration testing and ensure design rate calculations are consistent with City of
Edmonds SWMMWW Addendum Checklist #4.
a. It is also unclear how the engineer determined varied rates for different
BMP types; provide calculations to support each rate provided or remove
non -pertinent rates.
2. Drainage Report: It is unclear why the designer omitted all pervious areas from
the hydrologic modelling; update modelling to reflect entire site or provide
clarification as needed.
a. If areas are to by-pass the infiltration facility, the calculations must be
updated to demonstrate that the flow control and LID requirements are
met at a downstream point of compliance which considers the by-pass
flows.
3. Drainage Report: It is unclear why the modelled 0.66 acres was further reduced
to 0.44 acres for the mitigated WWHM run (page 4 of WWHM report); update or
provide clarification as needed.
4. Drainage Report: Engineer shall provide explanations of how the void ratios of
each material were selected for the WWHM model.
a. Per SWMMWW, porous asphalts are typically 16% - 25% (page 929)
b. The 0.4 value used for the rock layer is typical of rounded (Type 26) rock;
crushed rock would typically be assumed to have less void space than
rounded rock.
5. Drainage Report: It appears that the engineer included both rock layers in the
storage volume of the pervious pavement facility; typically the structural or
choker course are not included in this volume. Engineer shall update text of
report to address potential impacts for planning on stored water within this course
layer (or update design to only utilize lower storage course).
6. Drainage Report: The modelled slope of the pervious pavement does not appear
to match the plan sheet; update calculations as needed.
a. Note that check damns are required for slopes exceeding 3% which may
further reduce capacity of the system; update plans and calculations
accordingly.
7. C2.0: The SWPPP notes a sediment trap/pond in several locations which is not
currently shown on the plans; show sediment trap/pond and sizing calculation or
revise SWPPP for consistency.
a. If pond or trap is not provided, expand drainage report to explain why
engineer believes no flow control is needed and a why silt fence, alone, is
anticipated to be sufficient for sediment control.
8. C2.0: It is unclear how the infiltration areas will be protected or sequenced during
construction; update plans and notes as needed to provide direction to contractor
to ensure adequate infiltration capabilities at end of construction.
a. Since soil remediation is required and will remove the top 2' from the site,
it is recommended that over excavation of the driveway areas and soil
remediation work occur at the very tail end of the construction process.
9. C2.0: It appears that there is no proposed control for runoff form pervious areas;
address runoff form pervious areas or update drainage report to address impacts
of drainage to adjacent parcel from these areas (and remove all statement of "no
discharge from the site").
10. C3.0: Include or reference a dispersion trench detail as needed for construction
of the facility.
11. C3.0: Utilities installed within or under infiltration facilities must include trench
check -damns to prevent the horizontal movement of runoff along the pipe
trenches; update plans and provide details as needed. (Also recommended in the
"infiltration trenches" part of the geotechnical report).
12. C3.0: It is unclear what is proposed for discharge of the footing drains; update
plans as needed to show how footing drains will outlet.
a. If discharging to the infiltration system, provide elevations sufficient to
verify that footing drains will not be backwatered by the infiltration facility.
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13. C3.0: It appears form the information provided, that the yard drain outlet pipes do
not have the minimum 2' cover required under traffic rates surfaces; update to
provide adequate cover and/or provide additional info to demonstrate adequate
pipe strength (manufacturers spec, geotechnical evaluation, etc.).
14. C3.0: Update permeable asphalt section to reference note #4 (instead of note #5)
for soils information.
15. C3.0: Provide full specification for contractor to meet the soil water quality
treatment requirement (spec particular product type, mixing ratio, etc); current
direction is too vague to ensure organic content/cat-ion exchange rate is
achieved.
16. C3.1: As noted in checklist #11, check damns are required for pervious
pavement slopes exceeding 3%; update plans to provide check damns and
details as needed.
17. C3.1: The proposed low spot of Road A does not align with the crossing flow line
of Road B; update plans as needed and/or provide additional spot elevations on
the grading plan for all "non -typical" areas of grading/sloping.
18. C3.1: It appears the outlet pipe from CB #1 does not have the minimum 2' cover
required under traffic rates surfaces; update to provide adequate cover and/or
provide additional info to demonstrate adequate pipe strength (manufacturers
spec, geotechnical evaluation, etc.).
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