HomeMy WebLinkAboutZK Short Plat Storm Filter Adjustment.pdfDenis Law -
Mayor Clty Of 1�Y
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Community & Economic Development Department
C.E."Chip"Vincent, Administrator
December 22, 2015
Mr. Chris Pressey
PO Box 40173
Bellevue, WA 98015
RE: ZK Short Plat (U15002516) — StormFilter° without pre -settling - Adjustment
2015-03
Dear Mr. Pressey:
The City of Renton has completed review of the adjustment request for the ZK Short Plat
Development (LUA13-001564, U15002516) in accordance with City adopted 2009 King
County Surface Water Design Manual (KCSWDM) and associated City Amendments. Our
review of the information leads us to understand that the applicant's engineer is
requesting an adjustment for the implementation of the StormFilter System for Basic
Water Quality Treatment without pre -settling requirements.
Findings:
1. Section 6.1.1 of the 2009 King County Surface Water Design Manual requires a pre -
settling facility to be provided when the filtration facility is not preceded by a
detention facility. However, under section 6.5.5 an adjustment without pre -settling
may be considered on an experimental basis for drainage basins less than 1/2 acre
of impervious area.
2. The Stormwater Management StormFilter° (StormFilter) is a flow-through
stormwater filtration system comprised of a vault that houses media -filled
cartridges. As stormwater fills the treatment chamber, stormwater filters through
the media to the center of the cartridge, and treated stormwater is collected and
discharged through underdrain collection pipes.
3. The proposed ZPG media StormFilter systems for basic treatment is located at the
downstream point of the private access tract; and is collecting and treating runoff
from the site. The proposed impervious area is 0.27 acres.
Based on these understandings, findings, and section 1.4 of the 2009 King County
Surface Water Design Manual as amended by the City of Renton, the adjustment for the
implementation of the StormFilter System for Basic Water Quality Treatment without
pre -settling requirements is approved under the following conditions:
Renton City Hall . 1055 South Grady Way . Renton, Washington 98057 . rentonwa.gov
Mr. Chris Pressey
Page 2 of 3
December 22, 2015
Conditions:
1. The Homeowner's Association or Facility Owner(s) shall enter into a service
contract with an approved contractor or StormFilter® representative for the long
term maintenance of the system located on private property to be maintained
by the Homeowners Association or Facility Owners.
2. To account for the fact that there will be no pre-treatment provided upstream of
the proposed filtration system, the ZPG media StormFilter shall be oversized to
account for the total impervious area (0.27 acres) to be constructed on-site
(pavement area plus building roof area).
3. The ZPG media StormFilter systems are designed to treat 35 percent of the
developed two-year peak flow rate for all the impervious areas to be constructed
(including the building).
4. The StormFilter shall be sized per the manufacturer to comply with all the
criteria listed in section 6.5.5 of the 2009 KCSWDM as amended by the City of
Renton.
5. Further analysis and design calculations for the StormFilter® system shall be
included in the TIR for final approval.
6. Facility inspection, maintenance, and reporting are required by the City of
Renton Surface Water Utility for the Property Owner maintained StormFilter° in
perpetuity. The facility inspection, maintenance and reporting is required per
the Ecology National Pollutant Discharge Elimination System Phase II Municipal
Stormwater Permit (Section S5.C.4.c.iii). Facility owners are responsible for
ensuring that stormwater facilities are properly maintained and functioning as
designed and permitted. The StormFilter° facility shall be inspected every six
months by the supplier during the first year of operation as offered with the
purchase of the StormFilter® systems. Inspections will be used to determine the
site-specific maintenance schedules and requirements. Maintenance procedures
should follow those given in the most recent version of the StormFilter®
Operation and Maintenance Manual. During the first year of operation of the
facility, written records of the inspections and maintenance shall be submitted
to the City of Renton Surface Water Utility Private Stormwater Inspection
Program.
7. Following the first year of operation, the facility owner(s) shall annually have the
StormFilter® inspected and maintained per the procedures in the most recent
version of the StormFilter° Operation and Maintenance Manual. Written
records of the inspection and maintenance shall be submitted to the City of
Renton Surface Water Utility, to the attention of Surface Water Utility Private
Stormwater Facility Inspection Program. If more frequent inspection and
maintenance of the StormFilter° is required by the manufacturer or is needed to
Mr. Chris Pressey
Page 3 of 3
December 22, 2015
ensure performance of the facility, then the additional inspection and
maintenance reports completed within the year shall be provided with the
annual report.
8. Written records of the inspection and maintenance shall be submitted to the City
of Renton Surface Water Utility, to the attention of Surface Water Utility Private
Stormwater Facility Inspection Program. Inspection reports will be used to
determine future site-specific maintenance schedules and requirements.
9. The applicant will need to submit a Drainage Facility Covenant all lots within the
ZK Short Plat for inspection and maintenance of the private StormFilterl
treatment facility that will be privately maintained. The Drainage Facility
Covenant can be found in Reference J of the City of Renton Amendments to the
2009 King County Surface Water Design Manual. A site plan showing the
location of the treatment facility must be included as Exhibit A with the
declaration of covenant.
10. A letter signed by the Homeowner's Association or property owner(s) agreeing
to the required inspection and maintenance schedule is required as a condition
of granting this adjustment. The letter is to make the property owner(s) aware
of the possible increase in the cost of maintenance, inspection, and replacement
of cartridges that may result from placing the filtration system without
presetting. The letter shall be submitted to the City prior to construction permit
approval.
11. The approval of this adjustment does not relieve the applicant from other city,
state, or federal requirements.
If you have any questions about this adjustment, please contact Ron Straka, Surface
Water Utility Engineering Manager, at (425) 430-7248.
Singerely,
Prianne Bannwar �.E.y
Development Engineering Manager
cc: Lys Hornsby, P.E., Utility Systems Director
Mike Stenhouse, Maintenance Services Director
Rohini Nair, Plan Reviewer
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Hebe Bernardo, Surface Water Utility Engineer
Ron Straka, P. .
Surface Water Utility Engineering Manager
\\RvFPS-02\Depts\PW\File Sys\SWP - Surface Water Projects\SWP-27 - Surface Water Projects (CIP)\27-3129 Renton
Stormwater Manual\ADJUSTMENTS\2015\2015-3 ZK Short PPlat - StormFilter no presettling.doc