07.07.2011-Blueline Stormwater Response Comments.pdfJuly 7, 2011
Jeanie McConnell, Engineering Program Manager
City of Edmonds
121 Stn Ave N
Edmonds, WA 95020
RE: Edmonds Way Apartments; Application No. bld2011O410
Blueline Job No. 10-105
Ms. McConnell:
Of 1
This letter is in response to your review comments of the Edmonds Way Apartments project dated July 7,
2011. The plans have been revised per your requested revisions. Below is a list of each comment needing
addressed with our responses in bold.
STORM DRAINAGE REPORT
1. Minimum Requirement ##2: Construction Stormwater Pollution Prevention Plan, page 4-1:
1. Since this site is over the 1 -acre threshold, a Construction Stormwater Pollution Prevention Plan
(CSWPPP) needs to be prepared and submitted to the City of Edmonds per the requirements in the
Phase II Municipal Stormwater permit. Notes for a Temporary Erosion and Sediment Control (TESL) plan
on the referenced drawing will not suffice.
CSWPPP was submitted to Jeanie McConnell on July 11th, 2011.
2. As noted in the document, the project has applied to the Dept. of Ecology for coverage under the
Construction Stormwater General Permit (CSWGP). The CSWPPP prepared for the Ecology CSWGP will
also satisfy the City's Minimum Requirement #2. Please submit the CSWPPP prepared for the
Ecology CSWGP for review to meet this minimum requirement. The information in Section 6 of the
Drainage Report should be included as well as an evaluation of the site's erosion potential (see
attachment). Please note that the Edmonds Way stormwater system discharges to a marine near -shore
habitat.
Noted. CSWPPP was submitted to Jeanie McConnell on July 11th, 2011.
3. Under our Phase II Municipal permit, the City is required to inspect the site before, during, and after
construction, focusing on pollution prevention and erosion/sediment control issues. The initial
inspection is to assess the erosion potential for the site. During construction, the inspection will look to
see that the approved plans are being properly implemented. The post -construction inspection looks to
assess whether the CSWPPP provision have been properly removed and the site is stabilized. Please
work with the City's Engineering Division to schedule these inspections.
Noted.
1. Minimum Requirement #5: Onsite Stormwater Management: The City is pleased that the runoff from
the site will use a rain garden and infiltration chambers to manage the runoff.
a. Rain garden for Runoff Treatment (MR#6) and Flow Control (MR#a. Rain gardens can be used
to meet both minimum requirements if they are designed per the 2005 Ecology Stormwater
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Management Manual for Western WA, Volume V, page 3-8. It appears that the modeling in
the drainage report meets these requirements, although it is unclear since the infiltration
portion of the modeling is not included. The text states that the rain garden sizing is based on
an infiltration rate of 1 inch per hour with a safety factor of 0.25. Please clarify the modeling
and justify the selection of infiltration rate and safety factor.
Rain garden was updated using the amended soil infiltration rate of 1°' per hour.
b. In addition, ensure that at the maximum designed water level, the edge of the rain garden is at
least 10 feet way from the south parking area, since the water level looks like it will be higher
than the lower level of the building. Also ensure the rain garden is at least 5 feet from the
sidewalk (it appears to be).
Noted. MWS is at least 10 from south parking area and 5' from sidewalk.
2. Minimum Requirement 6: Runoff Treatment. page 4-2:
a. The report does not include the amount of pollution generating impervious surface (PGIS) and
the amount of pollution generation pervious surface (PGPS) created by the project. These
numbers are important to verify the specific requirements for treatment.
b. It appears from the information in the appendices that the CDS water quality device was
designed for a flow rate of 0.17 cfs. This is the value for water quality treatment flow for an
on-line facility for 1.32 acres (the entire site) of PGIS. Please verify that this is the correct PGIS
value. Only include parking and driving areas. Roof runoff does not meet to be treated. Add a
value for PGPS.
Since all runoff is being combined and run through the treatment facility, we are required to treat the
full 1.23 acres of roof, walkway, and driveway runoff. The previous 0.15 acres included in this
calculation was a part of the rain garden and is now being discharged directly to the storm system
located in Edmonds Way.
3. Minimum Requirement #7: Flow Control, page 4-2:
Please revise the first paragraph of MR#7 to say that since the site is in a direct discharge
basin, flow control is not required under the Phase II Permit requirements. However, the site
needs to meet the Edmonds -specific flow control standards for a site in the Edmonds Way
basin for the 10 -yr and 100 -yr recurrence events. For infiltration BMPs, these are 0.25
cfs/acre impervious for the 10 -yr event and 0.45 cfs/acre impervious for the 100 -yr event. If
100% of the runoff is infiltrated during these events, these standards are met.
4. Permanent Stormwater Control Plan page 5-1 and Infiltration Facility Performance, page 5-3:
a. The infiltration chambers are modeled using a precipitation gauge form Everett with a 0.8
scale factor, porosity of the surrounding rock of 0.4, and a design infiltration rate of 4 inches
/hr. Edmonds Stormwater Codes Supplement (ESCS) requires using the WDOT "Puget Sound
36" precipitation file. This needs to be selected in WWHM for all modeling including for the
rain garden and the CDS water quality device.
The system has been redesigned using the "Puget Sound 36" precipitation file.
Response to City of Edmonds 7/14/2011
Page 2
b. A specification for the rock with a porosity of 0.4 needs to be submitted and special
inspections during installation of the rock will be required to ensure this critical element of the
infiltration system iubuilt tospecifications.
c. Toobtain adesign infiltration rate, Appendix C of the ESCS provides 2methods that are
acceptable: the USDA Textural Classification (found in the 2005 Ecology Manual that includes
the %aond,silt, & clay) and the Modified Pilot Infiltration test. The City will also accept the
'D10" test found in the 2005 Ecology Manual, provided a proper soil gradation test was
performed. The geotechnical evaluation inthe appendices does not present adesign
infiltration rate in the context of any of these three methods and is, therefore, not acceptable.
The design infiltration rate must be presented in the geotechnical report using one of the three
acceptable methods. This should include the recommended design infiltration atand 3feet
below the design depth ofthe proposed infiltration facility.
d. Modeling the infiltration ohamberaysteminWWHMnhouldonnsio of3"layme." Layorsland
3should be8inches rock the porosity of0.4. The middle layer will bethe height ofthe
chamber/rock combination (2.53ft) and the area weighted porosity. Assume the cross
sectional area of the chamber has a porosity of 1 and the rock has a porosity of 0.4 and
calculate a porosity for this layer. The overall length and width entered into the models should
be that of the rock/chamber system.
System has been revised using the gravel trench design with 0.4 porosity for the rock layers and a
value of :1 for the porosity of the chamber.
e. The text states: "Our site isshowing two bottomless arch pipes with 5"minimum spacing
between arch pipe footinga." Sheet C5O1shows two rows ofchambers approximately 7feet
apart. Please reconcile.
The 5" outside dimension is the minimum, any area greater than 5" is just an added safety factor and
will be excluded from the calculations to be conservative.
5. The final submittal must include
0&K4 information for all stormwater control devices including the rain garden, the water quality
treatment devices, and the chamber system. In addition, ensure that these maintenance material are
provided to site's maintenance supervisor once construction is completed
6. The drawing inthe appendix tothe
Downstream Analysis appear tobefrom a previous design (4'diameterdetention pipe |ashmwn).
Please remove ormark aafrom aprevious design.
1. The plan shows the roof leaders and footing drains from the north building connecting into the side of
the infiltration chambers. Please show adetail for this connection and provide anexplanation that
the infiltration system can handle the flow from the footing drains.
Response toCity ofEdmonds 7/14/2011
The geotechnical report did not encounter any groundwater in any of the test pits and the soils are
fairly well drained therefore we feel the design is conservative enough to consider any flows from the
footing drains to be negligible and therefore can be ignored.
2. The note onC0#2(CDS unit) says detail isonSheet SU -1' The detail isnot onsheet SD-01nor any
other sheet included in the civil drawings. Please revise accordingly.
3. Audesigned, all of the site's runoff that isnot going to the rain garden must flow though the C[Swater
quality unit before being infiltrated. If there isafailure with this device, then all of this runoff could be
completely bypassed to the Edmonds Way storm system without treatment or infiltration. A better
approach may be to have the CDS system where CB#3 is currently feeding the two rows of infiltration
chambers. In this situation the high flow bypass that is not treated by the CDS would go into the
infiltration chambers rather than out to the Edmonds Way system. With this configuration, a system
overflow can be installed at the north end of the chamber system, connected to the current CB#1
proposed onEdmonds Way.
Per our conversation onJuly 1.2,2O11 reconfiguring the storm system k;not necessary.
4. Please indicate the location of the inspection ports to be installed with the ChamberMaxx system.
Inspection ports should bospaced nomore than 5Ofeet apart. The first one should bowithin 20feet
of an inlet to a row where clogging due to sedimentation would occur the most, then spaced 50 feet
apart.
Locations and of the additional inspection ports have been shown on the revised plans.
CIVIL SHEET C504 - DETENTIONMATER QUALITY DETAILS
1- Detail 2ofthe ChambedNaxxprovides aninspection port detail. Please add note for the installation to
be typical (typ).
Note has been called out to be typical.
Please call or email me with any questions orconcerns at (425) 216-4051 x225 or
8tamb|e@theb|ue|inegmup.00m.
SJnooxa|y,
Enclosures
Response to City of Edmonds 7/14/2011