Response Letter.pdfDonna Breske
,& Associates
Civil Engineering & Land Use Planning
April 26, 2018
City of Edmonds
Attn: Jennifer Lambert, Engineering Technician
City Hall - second Floor
Edmonds, WA 98020
Subject: Response to Memorandum Request for Additional Information
21 Ave A, Suite 4
Snohomish, WA 98290
Telephone (360) 294-8941
www.donnabreske.com
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Donna Breske & Associates, LLC is in receipt of the City of Edmonds Request for Additional
Information for PLN20170065, Ballinger View 3 Lot Short Plat 23521 75t" Ave W. This letter was
prepared by Alex White, EIT of Donna Breske & Associates, LLC and reviewed and signed by Donna
L. Breske, PE. The responses to the referenced comments are as follows:
Response to Request for Additional Information:
GENERAL:
1. Show the collection of the street surface water runoff from the flow line of the new
curb/gutter being collected and directed to the the City storm system.
Response: The street surface water runoff from the flow line of the new curb/gutter is now shown on the
frontage improvement plan included in this projects sheet set. The plan shows the direction of flow
along the curb line and provide proposed point elevations consistent with the existing topography of the
current edge of pavement.
a. The curb line will need to move to the east to allow for an 11 ' lane width.
Response: The curb line has been moved to the east and is offset 11 ft east from the centerline of the
right of way.
b. Based on the rim elevation provided and the topographic lines, it appears that the flow line
could go both north and south. Please provide spot elevations so that it is clear which direction
the surface water will go.
Response: Flow arrows have been added to indicate the direction of flow along the curb line. A high
point in 75"' Ave W is located near the proposed drive access and is shown on the Frontage
Improvement Plan.
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Donna Breske 21 Ave A, Suite 4
A Associates Snohomish, WA 98290
Telephone (360) 294-8941
Civil Engineering &Land Use Planning www.donnabreske.com
2. If the pool is to be removed, please show the pool and all associated retaining structures
and fences to be removed from the right of way. If the pool is not to be removed and
encroachment will need to be reviewed and approved prior to civil approval.
Response: The pool is proposed to remain, and the retaining structures, fences and concrete within the
right of way are to be removed. The removal of these items is indicated on the Site Plan, and TESC
Plan.
3. Please show the tree located partially within the right-of-way as to be removed.
Response: The tree located partially within the right-of-way of 75t" Ave W is shown to be removed.
4. Please indicate whether the existing residence will be maintained or if it will be demolished.
Response: The plans now indicate that the existing residence is proposed to remain.
S. Based on the proposed driveway layout the City will not approve two driveways to be
constructed off of 75th Ave W. Please show all three homes accessing off of the private access
road.
Response: Per recommendation from Joanne Zulauf, the proposed driveway for Lot 1 would need to
move north of the proposed location on this review, and the private access road would need to be moved
5' south of the originally proposed location. The plans now show that the proposed driveway for Lot 1
has been positioned near the northwest corner of the site, with the driveway's north edge interesting the
west property boundary approximately 7.67 ft south of the northwest property corner. The proposed
private access road has been positioned 5' south of its original location, and its north edge is
approximately 48 ft from the south edge of Lot 1's driveway.
UTILITIES:
1. Show the location of existing and proposed dry utilities. There must be a minimum of 3'
horizontal separation from the dry utilities and sewer, storm and water.
Response: The location of existing and proposed dry utilities is shown on the plans. A minimum 3'
horizontal separation between the dry utilities, sewer and stormwater is shown and maintained on the
plans.
2. Show all City utilities within 75th Ave W (Sewer has not been shown in its entirety and water
has not been shown).
Response: All city utilities within 75"' Ave W are now shown on the plans. Sewer is now shown in its
entirety, and water is now shown.
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Donna Breske 21 Ave A, Suite 4
& Associates Snohomish, WA 98290
Telephone (360) 294-8941
Civil Engineering & Land Use Planning www.donnabreske.com
3. Show all City utilities within 74th Ave W if any frontage improvements or utility work is to
be completed on 74th.
Response: All City Utilities within 74th Ave W are now shown on the plans, however frontage
improvements or utility work along 74tgh Ave W will not be required for this project.
4. Show the water service lines (existing and proposed) from the main to the meter and from the
meter to individual property line.
Response: The water service lines (existing and proposed) from the main to the meter and from the
meter to the individual property lines are now shown on the plans.
S. Show the sewers lines (existing and proposed) to each individual lot.
Response: The sewer lines (existing and proposed) to each individual lot are now shown on the plans.
a. Provide approximate invert elevations for side sewer at existing home (if relocating), property
lines and new connection to main to show feasibility.
Response: The approximate invert elevations for the side sewer at existing home, and property lines
have been shown on the plans. The side sewer at the existing home is not proposed to be relocated, and
will continue to be serviced by the sewer system within 74th Ave W.
b. The sewer must be a minimum have 10' horizontal separation from the water.
Response: The sewer is shown with a minimum 10' horizontal separation from the water.
6. Call out length and size of all pipe to verify slopes.
Response: The length and size of all pipes are called out with slopes shown on the plans.
7. Show all utility easements.
Response: All utility easements are shown on the plans.
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Donna Breske 21 Ave A, Suite 4
& Associates Snohomish, WA 98290
Telephone (360) 294-8941
Civil Engineering & Land Use Planning www.donnabreske.com
Response to Stormwater Review Comments dated Jan uary 29, 2018:
Reviewer: Zach Richardson, PE
1. General: It does not appear that the drainage report accounts for impervious surfaces added
or altered as a result of the required frontage improvements; update report as needed to include
and mitigate/address these surfaces.
Response: The drainage report now accounts for impervious surfaces added or altered as a result of the
required frontage improvements. These surfaces are proposed to be mitigated with a vegetated strip
within the right-of-way of 75th Ave W. The amount of new impervious surface totals 852 sf within the
right-of-way and is not anticipated to significantly change the existing drainage patterns. The new
impervious surfaces within the right of way are proposed to disperse within vegetation to the east of the
proposed sidewalk.
2. General: ECDC 18.30.D.5 (b.i) includes a requirement to treat 25% of the existing
unmitigated surfaces which are to remain; accordingly it appears that some mitigation of Lot 3
is required. Update plans and report to reflect this requirement or provide documentation of any
existing mitigation for these surfaces.
Response: Per ECDC 18.30.D.5 25% of the existing unmitigated surfaces to remain are proposed to be
treated with bioretention # 4 located on Lot 3. The total existing impervious surfaces to remain totaled
4,108 sf, requiring 1,027 sf (0.0236 Acres) of surface area to be treated by bioretention #4.
a. Surfaces included in this "retrofit requirement" are not considered toward other project
thresholds; therefore the addition of these Lot 3 surfaces alone will not trigger flow control
(though the frontage surfaces may).
Response: The total new and replaced impervious surfaces, including the frontage improvement areas is
8,776 sf and does not trigger the requirement for flow control. Additionally, this project proposes to
infiltrate the majority of impervious surface runoff.
3. General: It appears that dispersion is proposed, but it is not clear that dispersion is feasible:
Response: Dispersion is feasible in the area that it is proposed, and is the only feasible BMP for the
most eastern portion of the driving surface due to topographic constraints. The plans currently show
dispersion being used to mitigate runoff from 1,050 sf of the proposed replaced driving area providing
access to the existing garage.
a. Update plans and report to demonstrate or explain how/if the site can meet the dispersion
required ratio of 65:10 for native surface to impervious surface.
Response: The required ratio of 65:10 for native surface to impervious surface is only required for Full
Dispersion BMP T5.30. This project does not propose Full Dispersion BMP T5.30 for its surfaces and is
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ronna Breske 21 Ave A, Suite 4
Associates Snohomish, WA 98290
Telephone (360) 294-8941
Civil Engineering & Land Use Planning www.donnabreske.com
not required to maintain the 65:10 ratio. Basic Dispersion BMP T5.12 Sheet flow dispersion, only
requires a 10' vegetated flow path for every 20' of driving surface width.
b. Provide conceptual elevation information for the driveway enough to ensure driveway grading
can been done in a manner which maintains dispersion compatible slopes.
Response: Conceptual elevation information for the driveway has been provided on this project's
Grading plan sheet. The portion of driveway to be mitigated by BMP T5.12 has a max slope of 7.4% and
a cross slope of 2% allowing runoff to convey to the north edge of the pavement into the transition zone.
The land within the dispersion flow path is to be graded with a flow path slope of 3%. These slopes are
within the 15% slope threshold for sheet flow feasibility.
c. Engineer shall evaluate appropriateness of the proposed flow path which appears to direct
runoff north, directly toward the future structure on Lot I and two existing structures to the
north, on a site which appears to drain to the east currently.
Response: The flow path for portion of driving surface directly south of Lot 1 has been removed, and
this portion of the driving surface's runoff will be routed to bioretention #3 and #4. The cross slope of
the driving surface will allow the portion of dispersed driving surface runoff to flow in the northeasterly
direction. An asphalt berm will be used to direct and separate the runoff routed to sheet flow, and the
bioretention facilities.
4. Drainage Report: Update executive summary to include/note the proposed dispersion system
(section currently only mentions infiltration).
Response: The drainage report executive summary now states "A small portion of the driving surface
will be treated with sheet flow dispersion BMP T5.12".
5. Drainage Report: Expand upstream analysis to provide accurate picture of upstream areas
(size, location, etc) and where they currently drain.
Response: The upstream analysis now provides an accurate picture of the upstream areas, size, location,
and where they currently drain. It appears that the upstream area's drainage patterns and stormwater
conveyance systems primarily bypass the site.
a. Clarify where or how the current frontage drains and address impacts of the new curbing
which may alter the drainage course.
Response: The current frontage drains either north or south of the project site. The new curbing is
proposed to follow the existing topography of the existing edge of pavement, and will likely not alter the
current drainage course.
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A.Donna Breske 21 Ave A, Suite 4
& Associates Snohomish, WA 98290
Telephone (360) 294-8941
Civil Engineering &Land Use Planning www.donnabreske.com
6. Drainage Report: Expand downstream analysis to clam how the engineer determined that no
discharge was anticipated to leave the site when dispersion, a BMP generally assumed to
generate a site discharge, is the proposed mitigation for a large portion of the impervious
surfaces.
Response: The proposed dispersion BMP T5.12 demonstrates compliance the requirements of the 2014
Western Washington DOE manual and therefore the performance objectives related to on -site
stormwater management are anticipated to be achieved.
a. Update report to expand the downstream analysis effort as needed; current effort is
insufficient if a discharge is generated.
Response: The downstream analysis has been expanded. The proposed dispersion BMP T5.12
demonstrates compliance the requirements of the 2014 Western Washington DOE manual and therefore
the performance objectives related to on -site stormwater management are anticipated to be achieved.
7. Drainage Report: Expand downstream analysis to include a description of the potential
overflow routes from infiltration facilities.
Response: The proposed infiltration facilities are designed to infiltrate 100% of the impervious surface
runoff that is routed to them. Overflow routs have been designed to allow for gravity flow of overflow
runoff, and the outfalls will direct overflow to the eastern portion of the property due to topographical
constraints. Overflow runoff will likely disperse, discharge into 74th Ave W, and drain into the City
conveyance system located within 74th Ave W.
8. Drainage Report: It appears that the WWHM model for the bioretention cells may have a
flaw; the reviewer believes this may be a result of missing depth/height dimensions within the
modelled bioretention cell. Update WWHM report (include additional screenshots, etc) as
needed to demonstrate that the model includes an element which will limit storage to the
maximum design ponding depth.
Response: The WWHM model for the bioretention cells did not have a flaw, the cells were originally
modeled to have a ponding depth of 12", modeled as "freeboard" to ensure 100% infiltration with an
assumed overflow being a spillway at the "rim" of the bioretention pond. The maximum designed
ponding depth of a bioretention cell (not including the 6" freeboard) is 12" per the 2014 DOE Manual.
The current design calls for a maximum ponding depth of 6" and screenshots of the WWHM inputs have
been included in the report.
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Donna Breske 21 Ave A, Suite 4
& Associates Snohomish, WA 98290
Telephone (360) 294-8941
Civil Engineering & Land Use Planning www.donnabreske.com
9. Drainage Report: It appears that the infiltration facilities are only sized for the impervious
areas which will drain to them; while only the impervious areas require mitigation, facilities
must be sized to account for all areas which can contribute runoff to them. Update plans and
report as needed to include pervious areas potentially draining to these facilities or show how
they will be isolated from other contributing areas.
Response: The plans now specify that the graded areas surrounding the bioretention facilities must have
4" berm added along the uphill facing top edge of the facilities to ensure that pervious surface runoff
does not sheet flow or drain into them. Additionally, the on -site driving surface is to have an extruded
curb installed along the edge of pavement adjacent to uphill areas to prevent pervious area surface flows
from draining into the proposed private conveyance system located within the paved drive access.
a. If isolated somehow, impacts of runoff from the pervious areas to adjacent parcels/ROW must
be addressed within the report as well (i.e. a discharge will be generated).
Response: The pervious runoff will be isolated from the runoff routed to bioretention, however all
pervious surfaces are proposed meet the requirements of BMP T5.13 Post Construction Topsoil Quality
and Depth, and demonstrate compliance with the 2014 Western Washington DOE Manual, and therefore
are assumed to meet the performance objectives related to on -site stormwater management. The effects
of runoff from pervious surfaces to adjacent parcels/ROW are anticipated to meet the criteria of the
DOE.
10. Drainage Report: Geotechnical report recommends that infiltration systems for this site be
designed with a designated overflow system; update plans or report to show or explain how this
condition is considered/addressed.
Response: The infiltration systems for this site have now been designed with overflow risers and outfall
locations that will allow for gravity flow.
11. Plans: It appears tree removal as a result of the proposed storm drain alignments may not be
reflected on the plans; update tree removal symbology and calculations as needed.
Response: The plans show all trees that will require removal as a result of stormdrain alignments. The
Site plan, and TESC plan indicate tree removal.
Note for future design: Prior to construction approval, the bioretention section(s) may need to be
altered to ensure they reach the native layer proposed as the infiltration receptor; this layer may
be moderately deeper than currently proposed unless the facilities are relocated near the west
end of the site.
Response: The bioretention section/detail shown on the drainage plan, shows the existing soil strata for
infiltration pits 1 and 2 per the Geotechnical report. It appears that the native layer proposed as the
infiltration receptor is encountered depths between 2.3' and 3'below the ground surface. To allow for
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Donna Breske 21 Ave A, Suite 4
& Associates Snohomish, WA 98290
Telephone (360) 294-8941
Civil Engineering & Land Use Planning www.donnabreske.com
gravity flow, the depth of the proposed bottom layer of the bioretention facilities will be approximately
5.5' to 6' below the existing ground surface and will penetrate the native soils required for infiltration.
Please see the detail on the drainage plan displaying proposed depths of the bioretention layers.
Sincerely,
" -, � ///, ! 6- �-
Alex White, EIT
425-208-9744
Donna L. Breske, P.E.
Phone: 360-294-8941
Mobile: 206-715-9582
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