APPROVED STM RESUB 2 Drainage Memo - Revisedne
011ilEGA
ENGINEERING, INC.
March 24, 2021
City of Edmonds
Engineering Division
RESUB
Mar 25 2021
CITY OF EDMONDS
DEVELOPMENT SERVICES
DEPARTMENT
2707 Wetmore Avenue
Everett, WA 98201
t 425_387.3820
f 425_259.'1958
APPLIES WITH APPLICABLE
CITY STROMWATER CODE
/01 /2021
RE: Building Permit Document Review — by Omega Engineering, Inc. for Ritter SP — Lot B.
Permit No. — BLD2020-0865
Dear Staff,
This letter is intended to confirm that the information shown on the site plan and SWPPP/TESC plan for
the. Lot B building permit application is consistent with the approved drainage design for the Ritter SP
(PLN20120021).
Site Plan
have reviewed the current site plan being submitted to the City for the building permit on Lot B. The
improvements- shown for this project including 4he -work-within the joint access easement including
drainage and other utilities matches the approved short plat documents. The following area assumptions
were used for the approved short plat drainage design.
Total On -Site Impervious Surfaces: 8,050 sf
Access Road Win Easement & R/W: 1,511 sf
The following area the approved areas for Lot A and the proposed areas for Lot B:
Lot A — Approved under Building Permit BLD20181030
Total Impervious Surfaces: '. - 2,361• sf {Roof & D/W)
Lot B — Proposed Under Building Permit BLD2020-0865
Total Impervious Surfaces: 3,716 sf (Roof & D/W)
Total Project Impervious Proposed: 1,511 + 2,361 + 3,716 = 7,588 sf
As shown in the area summary above, the short plat improvements along with the approved and
proposed improvements on'Lot A and B will total462 sf less than the maximum allowed based on the
approved short plat drainage design. If acceptable to the City, a document can be recorded for both
parcels identifying they each lot will have an ability to construct a maximum of 2,591 sf and 3,947 sf of
impervious surfaces for Lot A and B respectively. In addition, the document should note that the
impervious improvements within the access easement on Lot A do not count toward the 2,591 sf allowed
on Lot A.
On-Site'Drainage' Mitigation
As requested by staff, the current drainage code requirements for the proposed improvements on Lot B
need to show how they meet the current City Stormwater Regulations. The proposed building perming on
Lot B proposes to construct 3,716 sf of new impervious surfaces that were originally accounted for in the
drainage mitigation for the short plat. All runoff from the new hard surfaces will be directed to the
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ENGINEERING, INC. t .
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proposed detention pipe located under the access within the easement on Lot A. However, prior to
release of the runoff from Lot B the City of Edmonds Appendix A — On -Site Stormwater Management
BMP Infeasibility,,Criter-ia was reviewed to: determine: the feasibility of the BMPs in the order listed.
Lawn and Landscaped Areas:
Post -Construction Soil Quality and Depth: This option was elected, and the specifications have been
referenced on the site plan along with a note to apply this to any area of Lot B that was graded or
compacted.
Roof
Full Dispersion: Not .Feasible due to Geotechnical findings of erosion hazard soils on this site and the
requisite 65:35 Retained Native Vegetation: Cleared Area ratio could not be provided on this site.
Downspout Infiltration: Not Feasible due to the Geotechnical findings of erosion hazard soils on this site.
Bioretention or Rain Garden: Not Feasible due to the Geotechnical findings of erosion hazard soils on this
site.
Downspout Dispersion Systems: Not Feasible due to the Geotechnical findings of erosion hazard soils on
this site and the on -site grades and flow paths that cannot meet the BMP requirements.
Perforated Stubout Connection: Selected for all roof runoff prior to connection to the short plat
conveyance system and detention system to the west. Location shown on the plans along with a
reference to the City's detail.
Other Hard Surfaces
Full, Dispersion: Not Feasible due to Geotechnical findings of erosion hazard soils on -this site and -the
requisite 65:35 Retained Native Vegetation: Cleared Area ratio could not be provided on this site.
Permeable Pavement: Not Feasible due to the Geotechnical findings of erosion hazard soils on this site.
Bioretention or Rain Garden: Not Feasible due to the Geotechnical findings of erosion hazard soils on this
site.
Sheet Flow Dispersion: Not Feasible due to the Geotechnical findings of erosion hazard soils on this site
and the on -site grades and flow paths that cannot meet the BMP requirements.
Concentrated Flow Dispersion: Not Feasible due to the Geotechnical findings of erosion hazard soils on
this site and the on -site grades and flow paths that cannot meet the BMP requirements.
Detention Vaults and Pipes: Not Feasible. "Where there is less than 1,000 sf of contributing site
impervious surfaces that are not proposed to be managed by other on -site stormwater management
BMPs." The total driveway/walk area is 759 sf and the roof area is mitigated as noted above. However,
all runoff-from"the`impervious surfacesJot this site are',thbutaryto" the detention pipe designed/approved
under the original Short Plat.
In addition, the Native Growth Protection Area is shown per the approved short plan and the site plan
shows no work within the area to be preserved/protected.
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SWPPP/TESC
The. area shown,to..,be.-cleared/disturbed for the Lot B building permit along with the improvements for
utility connections to the City R/W match the approved plans and or assumptions for the Ritter SP.
Summary
Based on the information reviewed the Ritter SP — Lot B plans are consistent with the approved short plat
design for this project while also incorporating the new on -site LID BMPs as requested by City Staff.
Thank you for taking the time to review the updated submittal package for this project. Please feel free to
contact me with any questions you may have.
Sin e�ely,
o e M. Smeby, P.E.
Civil Design Engineer