LDC response-01.30.2014.pdfJanuary 30 hl, 20, 3'
Jerry Shuster
Stoirrnwater Engineering Prograrn Manager
City of Edmonds
121 Fifth Avenue North,
Edinonds, WA 98020
Ph: 425371,0220 )(1323
RE: Stormilwater Review
Project:
Dear Mr. Shuster,
LDCH �F OVIC FN(ANEHflN6C.HOUV
BLD2,0130919
Meadowvlewv Estates
Please find below responses to all comments as received from City of Edr-nonds review on
1/14/14. All comments have been addressed.
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Section 4,FLow,�Qqjt[o analysis -- E)(hibit 3 shows an offsite contribution area for the
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City system to be 0.96 acres. This is not correct. In 21009, a new home was built just
downstream of this proposed development at 1563175TH PL W. The applicant had a
downstream capacity analysis performed to try and get a detention waver. That
analysis used a 14.6 acre drainage basin area for the City system. This estimate is in, line
with past City basin plans. That analysis also found that the downstream pipe with the
smallest capacity was the one between your node # 8 and node #9 at 4.2 cfs; compared
to your 5.1 cfs (see enclosed). Note that this was done under the previous stormwater
code (25 -year peak flow standard).
The analysis for the Halvorson site does not include: Time of Concentration
exhibits and calculations, rainfall values, and nodes I through 5 are not shown on
the exhibits. These should be included. Also, the manhole just west of node #0
receives flow from the south, which need to be accounted for in the calculations.
Response: The downstream analysis now includes the larger scope based on the
provided adjacent analysis. Based on our calculations, the total basin will have an
cfs. The Storm Drainage Analysis has been updated to reflect these calculations and
additional detail requested. The CBs corresponding to Nodes I through 5 have been
added to the Storm Drainage Analysis and sheet RD -02
Due the steepness of the proposed: storm pipes on the site (up to 15.4%) and where it
connect to the City system on 751h Ave N., high velocities of flow may cause surcharging
of Cl3s. Please calculate hydraulic grade lines for the system from the proposed
development down to riode H8.
11r1i I 4Y1h e "i' t a rii d a r (i
Response: The hydraulic grade lines have been included in the Storm Drainage
Analysis on page 4-19. All HGL elevations are below the rims of the structures.
RD -01— An additional set of catch basin should be placed on the driveway there it turns
towards the north and they should be connected to the proposed downstream system.
The spacing between catch basin is current too long for a 12%+ slope. This could cause
bypass and flooding down the proposed driveway to existing garage.
Response: An additional catch basin has been added at the bend in the proposed.
Since the driveway slopes to one side (the south/west), the addition of a catch basin
on the other side of the driveway is not necessary.
® RD -04 —The pipe material used should be on the plans.
Response: The pipe materials are now called out in the profile on sheets RD -03 and
RD -04..
® DT -01 -- The Stomfilter detail needs to include the data from the Drainage Report: the
number of cartridges, the media in the cartridges, and the design flow rate. This detail
does not include the 22 inch tall cartridges that can handle 9 gpm specified in the
Drainage Report.
Response: An updated Stormfilter detail has been included on sheet DT -02.
HWA Review:
RD -02: — The proposed grade of the storm drain section between CB -3 and CB -2 is
extremely steep (25.25%). It is likely that the trench backfill will act as a French drain
and collect and convey infiltration/shallow gorund water seepage down toward CB -2
where the inclination and direction of the trench changes abruptly creating the
potential for localized wet soil conditions that spread towards the steep slope. At this
location consideration might be given to inclusion of perforated trench drain tied into
CB -2.
Applicant Response: The proposed slope between the catch basins in question has been
reduced to below 20%. (HWA- we note that the gradient has been reduced to 15.39%
which is still relatively high. We still consider it likely that the trench will act as an
interceptor for shallow groundwater derived from locally infiltrated seasonal runoff
and consideration should be given to installing an under drain pipe in the trench that
is tied into CB -2. Does the geotechnical engineer have any concerns with this
scenario?)
Response. Bentonite or concrete check dams, have been added orithci proposed steep
pipesto prevent the possibility of acting as an interq.,eptor for shallow groundwater.
We believe that this r(-,!--subrTdttal addresses all City cornments. Let us know if you have any
questions or if you need any additional information,
Best Regards,
LDC Inc.
Dan Carmody, FI.E.
Project Engineer