CIVIL SUBMITTAL 4 - COVER LETTER RECD 5-6-19.pdfMay 1, 2019
GR0UP FOUR Inc.
P.O. Box 1059
Lake Stevens, Washington 98258
(425) 408-1152
Fax (425) 877-1341
e-mail: info@grp4.com
To: City of Edmonds
JoAnne Zulauf
Zack Richardson, PE
RE: Subdivision. File #: PLN20170057
Responses to First Review Comments
Group Four has revised our civil plan set and drainage report for the Spence 2-lot short plat at
17606 72nd Ave W, permit number PLN20170057, per the comments sent on April 26th, 2019.
The original comments are below with our responses in blue.
GENERAL
1. 4.10.19 Please revise bond worksheet as follows:
a. Sheet 2 - Add catch basin socks to TESC.
b. Sheet 7 -- Move infiltration trenches to be under "flow dispersal trench" as opposed to
"french drain" or provide a write in amount that is comparable to actual costs of materials
and installation. Drainage report states 4201f of infiltration trenching, the current
worksheet notes only 201f.
The catch basin socks have been added to the TESC quantity totals, 4 in total at approximately
100 dollars each. The now 300 if of infiltration trench has been added to the totals as flow
dispersal trench. The 201f french drain remains because there is a french drain proposed in the
Lot 2 driveway,
A Stormwater Covenant is required to be recorded against the property based on the proposed
stormwater facilities for the project. Please complete the covenant, including notarization and any
required exhibits and submit to the city. Ensure the stormwater covenant meets Snohomish
County Record Drawing requirements. The document must be complete, but will not be recorded
prior to permit issuance. Prior to construction, final location of facilities are to be verified (by field
inspection and asbuilt) and then the covenant will be recorded. Document available on the city
website at www.edmondswa.5 ov.
The,stormwater covenant for the infiltration facilities has been complete and is attached.
3. A Special Inspection Agreement is required for the inspection of subgrade and materials for the
infiltration trenches per note 5 on sheet 4. Form attached.
The client will arrange the special inspection.
4. The driveway approach for Lot 1 will need to accommodate the cars for the current sfr. Please
lengthen driveway to allow for parking on private property, the private driveway portion should
be a minimum of approximately 18' from the property line. How will the ped path look, call out
surface and width.
LAND SURVEYING CIVIL ENGINEERING LAND USE PLANNING PROJECT MANAGEMENT
The parking area in Lot 1 has been extended to 18' long. The pr d path is now called out as 3'
wide with a grass surface. A 4' rock wall has been added.
DRAINAGE
1. It does not appear that a drainage report was submitted; submit a drainage
memo describing the drainage plan and clarifying what is proposed under this
permit.
1st Update: The following comments require attention on the now included drainage
report:
a. The report states that no upstream areas are tributary to the site, but the
existing topography and proposed grading both appear to accept run-on
from 72nd Ave NE; update and/or clarify as needed.
2nd Update. Thickened edg.; and CBS added; how-, -i it is Laiclear why the CB
are set behind the flow line of the thickened edge. If not placed in flow line;
include text for why CB location(s) has to be non -typical and provide details
sufficient for construction. City preference is to remain at flow line as much
as feasible.
As discussed in our meeting the right of way CBs are set back from the EOP because
of conflicts with the existing water line.
i. Ensure detail address edge/taper/cut condition of the high -side of
the thickened edge, rock type and size, and extents of rock
(typically rock surrounds the entire CB in such uses)
Blow up of typical right of way CB has been added to the drainage plan. It includes
callouts for thickened edge saw cut, size and type of rock and the dimensions of rock
pad which now surrounds the entire catch basin.
b. Include description in the report of how the road drainage will flow after
construction and mitigate impacts as needed; it appears the project
proposes to allow for linear flow along the edge of widened pavement
which would be an erosion concern. A thickened edge, curbing, and/or
ditching appears to be needed to control runoff without increasing an
erosion nuisance.
2" d Update: It appz;aiz. uiu, 1761h west of tt,.V. FiLupuni-A CB is stili u., addre6sed; update to
address how flow to the west is handled/controlled or not a nuisance.
Per our meeting and following emails this will. not be required for this project only.
c. Update the mitigation table on page 5 to match the maximum surfaces used in
the design (per page 9); this table is likely what will be referenced during future
building permit drainage review and should list the maximum allowable
impervious surfaces.
2na Update: Address concerns with this table as notes:
i. The total of the propose Lot 2 and the frontage improvement impervious
areas appears to exceed 5,000 SF and may require additional drainage
mitigation as a Category 2 project; updated as needed.
The drainage report now.addresses this project as a Category 2. It now addresses
minimum requirements 1-9. The infiltration gallery has been modeled in WWHM
2012 showing that 300 if of trenching is sufficient to infiltrate runoff from all targeted
surfaces.
ii. Update the roadway quantity, explanation, or exhibit to clarify separate
quantities for (1) the areas actually draining to the system,
(2) the areas requiring mitigation, and (3) areas by-passing mitigation.
By-pass area may need to be checked against the peak -flow
increase to confirm flow control is adequate under revised category
2 condition.
A note has been added to the targeted impervious areas table on page 5 to clarify
the impervious area requiring,mitigation.
iii. The project may not take retrofit credit for portions of the driveway which
are required to be removed as part of the plat project; accordingly the
project may not consider the portions of the existing driveway to be
removed on Lot 2 as mitigation for Lot 1 surfaces to remain. Removal of
surfaces which otherwise were permitted to remain, is required for this
condition. Update as needed.
As discussed in our meeting, the 25% retrofit requirement is met by the 1,300 sf of
driveway removed from lot 1. The remaining 1,000 sf of driveway in lot 2 will also be
removed but this is not part of the retrofit requirement for the existing impervious surfaces
in lot 1.
d. It is not clear that the impervious surfaces for road improvements include the
replaced surfaces; update as needed to clarify.
2nd Updated: Still unctvai, table shows i,j00 SF of froinagv, exhibu snows 1,770 SF, and
MR section references 1,700 SF. The MR #5 section further suggests that the trench
sizing was solely based on the Lot 2 areas (3,944 SF) and does not provide a calculation
for sizing infiltration to account for the frontage improvements. The quantity of bypass
area does not appear to be reported anywhere either. Update and clarify as needed.
A note has been added to the table on page 5 to clarify this. As discussed in' our meeting
upstream bypass is not required because we are mitigating runoff from those surfaces.
e. It is not clear how the engineer has determined the 1,000 SF equivalent area for
treatment from the frontage; provide exhibit or additional information to show how
it was determined that a CB at this location would collect flow from 1,000 SF (or
whatever value its adjusted to in future iterations) as designed.
2nd Update: Exhibit provided, but it does not appear that the by-pass area is ever
quantified; provide a square footage of the surface requiring mitigation which are
bypass said mitigation and/or being address by flow swap.
As discussed in our meeting upstream bypass is not required because we are mitigatir rr
runoff from those surfaces.
2. It appears that the proposed driveway surface will well -exceed 3% in slope and, therefore,
requires check -damns; update plans and provide additional details as needed.
1st Update: The driveway slopes are now under 3%, but the north driveway on 176th no longer
aligns with the garage it is intended to serve, appears to be graded steeper than 2:1 from the
edges up to existing grades, and creates a 4' to 5' cut. It is unclear how this driveway will
provide any functional use of the area without corresponding pedestrian improvements or
adjoining the garage directly.
a. Update plans to as needed to provide a 2:1 maximum slope around, the driveway
and specify stabilization of the slopes as needed.
b. Identify pedestrian route between home and required parking spaces and identify
proposed improvements and corresponding mitigation as needed.
c. It appears that the cut required to construct this may put the bottom of the infiltration
facility deeper than the geotechnical information can support (boring only went 4' down);
update report to address this concern or adjust design as needed.
2nd Update: Adequacy of driveway and pathway is deterred to the engineering reviewer, but
ensure drainage design and quantities are updated for any necessary revisions or addition
of feasible pedestrian pathways as needed. Nothing further, if revisions are not required for
driveway and/or ped path.
The driveway has been lengthened per the general comments. The additional length was
included in the WWHM model but runoff increase is negligible.
3. Nothing further.
4. Nothing further.
5, Sheet 6: It appears the gravel driveway on Lot 2 is proposed to remain and represents an
impervious surface subject to the 25% retrofit requirement; update plans to include mitigation
and/or remove the gravel surfacing on Lot 2 (similar to the portion shown on Lot 1). Jpdate:
See l.c.ii above
6. Sheet 6 (New): It is unclear how the frontage drainage is being control;' update plans to
better demonstrate how flows from the roadway surfaces will be managed. Update: See Lc
& Ld above
7. Nothing further.
8. Nothing further.
9. Nothing further.
10. New: Add note/callouts to sheet 6 to clarify that at least the last CB prior to runoff entering the a
infiltration trench, must have the riser as shown in SD-636.
a. As currently configured, a riser is required in each CB
b. Alternatively, proposed CBs can drain to a single CB with riser if
reconfigured.
A note has been added to sheet 6 stating that all proposed catch basins are required to ha,,,e a.
riser per SD-636.
Sincerely,
Curt Iffrig, EIT
LAND SURVEYING CIVIL ENGINEERING LAND USE PLANNING PROJECT MANAGEMENT