bld20090795-Lowenthal_3.pdf
City of Edmonds
TH
121 5 AVENUE NORTH • EDMONDS, WA 98020 • (425) 771-0220 FAX(425) 771-0221
Website: www.ci.edmonds.wa.us
PUBLIC WORKS DEPARTMENT
Engineering Division
Plan Review Corrections
Plan Check :Date:
#BLD20090720 March 12, 2010
Project Name/Address:
Lowenthal – 18720 Soundview PL
Contact Person/Address/Fax:
Marci Bryant –
vmarcibryant@msn.com
Jennifer LambertDivision:Engineering
Reviewer:
During review of the subject submittal, it was found that the following information, corrections,
or clarifications would need to be addressed. Our handouts can be found online at the City of
Edmonds web site www.ci.edmonds.wa.us under City Government/Development
Services/Engineering Division.
st
1 Review - 12/11/09
nd
2 Review – 2/3/10
rd
3 Review – 3/12/10
CA.1 – Site Plan
1.OK
2.OK
3.OK
4.OK
5.OK
6.OK
Sheet C1 – General Notes
1.OK
2.OK
3.OK
4.OK
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5.OK
6.OK
Sheet C2 – TESC Plan and Details
1.OK
2.OK
3.OK
4.OK
Sheet C3 – Grading Paving & Utility Plan
1.OK
2.OK
3.OK
4.OK
5.OK
6.OK
7.OK
8.OK
9.OK
10.OK
11.OK – Please refer to Jerry Shuster’s comments # 12
12.OK
13.OK
14.OK
15.OK
16.OK
17.OK
18.OK
19.OK
Drainage/Detention Review comments from Storm Water Engineer, Jerry Shuster, P.E.:
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Items reviewed:
Outline Drainage Report dated January 8, 2010
Letter Report from GeoTech Consultants, Inc., dated December 28, 2009
Memorandum from John W. Rundall, P.E., dated January 6, 2010
Plan sheet C1 and C3 dated January 6, 2010
Preface
The comments contained in this document are contingent on a satisfactory resolution of the issue of
stormwater runoff from this site being directed to Fruitdale Creek as describe in an e-mail sent to the
project proponents on February 5, 2010 (attached).
Comments on “Outline Drainage Report” (WR Consulting, Inc., November 16, 2009)
collect water and allow for the water to soak in the ground
1. OK
2. OK
Since the characteristics of rain gardens is to infiltrate the city is classifing rain gardens as a type
3. OK
of infiltration system. However, since they city does not have a standard design for rain gardens we
4. The City of Edmonds code only allows detention and/or infiltration as an acceptable means to meet
certain code requirements. The rain gardens, however, will be accepted as infiltration for
streambank erosion control as long as the following standards can be met. The City will require
verification that the rain gardens will infiltrate the 100-yr storm event within 48-hours using the 1
inch/hr design infiltration rate. This is our current standard per the 1992 Dept. of Ecology manual
(see modeling comments below). From page 2, of the Drainage Report please remove reference to
City staff approving the use of the King County standard.
2/3/10 Comment: Comment was partially addressed. The response states that the rain gardens are
designed using the guidance from the 2009 King County Surface Water Design Manual (KCSWDM) that
assumes a design infiltration rate of 0.5 inches/hour. The report from GeoTech Consultants, Inc., dated
December 28, 2009 states that the site soils are capable of an infiltration rate of at least 1 inch per hour.
Aside from a inference from the soil boring log, no rationale for this infiltration rate is presented in the
submittals. Please convert the USCS classification for the soil boring to USDA soil textural classification
found on page 3-76, Volume III, 2005 Ecology Stormwater Manual for Western Washington. A table on
this page can be used to get defensible infiltration rates. Note the difference between the short-term
infiltration rate and the estimated long term infiltration that will be used in design. Both values should be
reported. If here is a change in the design infiltration rate, it should also be reflected in rain garden
freeboard requirements and the calculations on page 5 regarding the infiltration capacity.
11/16/2009 Comment: The rain gardens have been designed using criteria from the 2009 King County
Surface Water Design Manual that include a sizing factor of 0.25 inches of rain garden volume per square
feet of impervious surface area served. The City will may accept use of these design criteria under the
following conditions:
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The 0.25 inches/sf impervious standard in the King County manual must assume an infiltration rate for
the underlying soils. This infiltration rate should be presented in the drainage report. The actual
infiltration rate for the site in the areas where the rain gardens will be located (design infiltration rate not
a field infiltration rate) needs to be verified by a geotechnical engineer. This design infiltration rate needs
to be equal to or greater than the one used for the standard in the King County manual.
The drainage analysis needs to demonstrate that the rain gardens designed to the King County standard
have release rates from the site for the 10-yr and 100-yr, 24-hour storm events under developed conditions
less that or equal to the release rate for the pre-developed conditions for the 10-yr and 100-yr, 24-hour
storm events. The pre-developed condition should be modeled as secondary growth forest.
5. OK
6. In addition to the declaration of covenant for the rain gardens, the use of pumping system will require
the owner to have a covenant protecting the City from liability from any damages caused by pump
failure due to a mechanical problem, lack of power, or other circumstance. The covenant will
obligate the owner to regularly maintain and repair, when necessary, the pump system. It will also
allow (but not require) the City to enter the property to maintain or repair the pump system, should
it ever become a threat to public health and safety or threaten City-owned or operated infrastructure.
Any costs associated with repairs made by the city will be passed on to the owner. It should be clear
that the City is not advocating or requiring a pump system and it is solely the choice of the owner to
have this system.
2/3/10 Comment: OK. The City is still working on working a declaration of covenant for the rain gardens.
7. OK
8. OK
Additional Comments on Revised Outline Drainage Report (dated January 8, 2010).
9. OK
Comments on Plan sheet C3 (dated January 6, 2010)
10.Comment #10: The “turfstone” is accepted as 50% pervious and 50% impervious at the currently
designed slope and subgrade configuration.
2/3/10 Comment: The Plan sheet calls out a “Grasscrete driveway” and detail 2 shows a the cross section
for this item. The Outline Drainage Report discusses a “Grasspave driveway.” Grasscrete and Grasspave
are two different modular pavement grid systems with different sub-base requirements for proper function.
Also, Grasspave literature does recommend the product for slopes greater than 8%. The driveway, as
designed, has segments with a slope of 10%. Decide on the type of modular pavement grid system for the
project and include the manufacturers specification for sub-base in order to qualify for the 50% impervious
treatment of these areas.
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11.These comments have not been fully addressed. It is still unclear why the pumping system is designed
to pump up to 25 gpm. The maximum discharge should be 15 gpm for streambank erosion
protection. A second pump should only be used if the first fails or to cycle off the first one.
a)Please provide the rationale for using the pumps in parallel and almost doubling the
discharge rate.
2/3/10 Comment: The City prefers that the pump system does not discharge directly into the City’s storm
system for maintenance reasons. Gravity flow from private property is the preferred approach. Also, the
release from the detention system should be controlled by the orifice and not the pumps (especially if the
orifice system is designed to release approximately 14 gallons per minute (gpm) and the pumps are capable
of 25 gpm.) To this end, it is recommended that a wet well be located at lower elevation of the site
connected to the pump system. This pump system would discharge into the upstream end of a detention
system located at the northeast part of the site. This dentition system would have a control manhole that
may be able to be discharge by gravity into the City catch basin on Soundview Place.
12. Comment not addressed. Provide geotechnical engineer approval of location and design of footing
drain system.
The following comments on the modeling in the Drainage Report are new, since the modeling is new.
The following comments apply to Appendix B:
2/3/10 Comment: The planned configuration of the footing drains to outlet via area drains may not be
appropriate so close to the steep slope. Documentation from the geotechnical consultant regarding the
potential impact of these footing drain discharges should be included. (Note that the City does allow the
connection of footing drain discharges to stormwater systems but only downstream of detention system
discharges.)
13. Modeling using the 1 inch per hour infiltration rate is sufficient.
a)Remove the modeling for the 0.5 in/hr scenario.
14. Both modeling scenarios use a curve number for “Woods/grass...” using hydrologic soil group (HSG)
B for basin 3S. The site has Everett soils that are HSG A soils.
a)Please re-run the models using the appropriate curve number for Type A soils (and the
other changes described below)
15. The rain gardens are modeled as one “pond” with a volume of 468 cf (17.2 ft wide by 20 ft long by 1ft
high, with 3:1 side slopes). Page 3 of the Drainage Report shows a total rain garden volume of 558 cf
with a design depth of 0.75 ft (9 inches).
a)Please modify either the model or the design to be equivalent.
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16. All modeling runs use antecedent moisture content (AMC) of 3.
a)Please re-run the models using AMC of 2 (it is our standard practice for site runoff
modeling).
17. It is unclear what times of concentration were used in the modeling. For example, I could not
duplicate the inflow to the rain gardens for the 100-yr event (0.07 cfs).
a)Please re-run and report the times of concentration for each basin so the modeling can be
verified.
Comments below refer to issues raised in a February 5, 2010 e-mail from Jennifer Lambert, City of
Edmonds to Marcy Bryant and have been assigned a comment number.
18.Use of the E72 handout for sizing a detention system is accepted since less than 5,000 square feet of
impervious area is tributary to this system (per ECDC 18.30.060.A.1). The pumped discharge rate
from the proposed system of approximately 15 gallons per minute (0.033 cfs) is equal to the
maximum outflow through an orifice in a gravity system specified in the E72 handout for 4,500
square feet of impervious area, therefore it is acceptable.
a)From page 1, please remove reference to City staff approving the use the 2005 Ecology
stormwater manual threshold of 0.1 cfs. This is not our current standard. By using the
E72 handout, as specified by our current code, with a discharge rate of 0.03 cfs, the project
has met current standards.
19.The downstream analysis on pages 10-11 of the Drainage Report is adequate to demonstrate a de
minimus effect on Fruitdale Creek from the outflow from this developed property.
Additional comments on plan sheet C3 – Grading Paving & Utility Plan
20.A catch basin should be placed at the connection point of the roof drains and the detention tank.
This is required to access the tank for maintenance. The downstream manhole that houses the
pumps is not sufficient to provide maintenance access.
21.Detail 2 should be renamed “Turfstone Detail” to match the chosen paving product.
Please resubmit 3 copies of the revised plans/documents to a Development Services Coordinator. Please
contact me at 425-771-0220 if you have specific questions regarding these plan corrections.
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