HomeMy WebLinkAboutC_Response_Letter_Sierra_Homes_SP_230306_v2.pdf
Western Washington Division Eastern Washington Division
165 NE Juniper St., Ste 201, Issaquah, WA 98027 108 East 2nd Street, Cle Elum, WA 98922
Phone: (425) 392-0250 Fax: (425) 391-3055 Phone: (509) 674-7433 Fax: (509) 674-7419
www.EncompassES.net
March 1, 2023
City of Renton – Department of Community and Economic Development
Attn: Nathan Janders, Development Engineer
1055 S Grady Way
Renton, WA 98057
Subject: Sierra Homes Short Plat Comment Response Letter
Dear Nathan,
In response to your comment letter dated January 20, 2023, we have revised the plans and report as
requested. The city comments in Italics with a detailed response below in bold are indicated below.
1. In the TIR a flow control facility exemption is claimed for core requirement # 3. However, the modeling
used to support the claim must be revised for the following items:
a. The WWHM model used an hourly timestep, a 15 minute timestep shall be used.
The WWHM model has been revised to use 15-minute timesteps.
b. The WWHM model has a target surface area of 0.195 AC which is equal to the new impervious
surface only. However, per section 1.2.3.1.B – Target Surfaces – 2 the new pervious surface shall
be assumed to be the entire lot area. The total lot area is 0.735 AC and thus both the predeveloped
and developed conditions should have an area equal to 0.735 AC.
The WWHM model has been revised model the entire site for the proposed flow control facility,
with the exception of 7,096 SF of lawn runoff that was found to be tributary to the off-site
wetland to the southeast. An equivalent amount of lawn area in the proposed condition will be
graded towards the wetland to maintain existing drainage patterns and is therefore not
tributary to the on-site storm system.
c. The WWHM model appears to use flow control facility sizing credits, however, per table 1.2.9.A
privately maintained BMP’ s are not allowed to receive facility sizing credits unless using full
dispersion or full infiltration (neither of which are indicated as feasible).
The storm design has been revised to use limited infiltration to meet the minimum BMP
requirements, however these BMPS have not been used to receive facility sizing credits for
modeling.
City of Renton – Department of Community and Economic Development
Sierra Homes Short Plat Comment Response Letter
03/01/2023
Page | 2
d. If a flow control facility is found to be required, then the applicant shall demonstrate that there
is no downstream drainage problem to the wetland; See section 1.2.2.1.1 – Potential Impacts to
Wetlands Hydrology Problem of the RSWDM for further information.
After a close examination of the topographic survey, it was found that approximately 7,096 SF
of existing lawn area sheet flows to the southeast towards the off-site wetland. To preserve this
existing drainage pattern, an equivalent area of lawn in the proposed condition will be graded
towards the wetland, primarily on Lots 1 & 2. A detention system was found to be required
after updating the WWHM model. Proposed surfaces including rooftop areas, driveways and
sidewalks and the remainder of lawn areas will be tributary to this system. As the existing
drainage condition to the wetland will be maintained, no downstream drainage problems will
be created as a part of this storm system.
2. The TIR appears to be based on the 2022 Renton Surface Water Design Manual (RSWDM) however there
are references to the 2017 RSWDM. The report shall be updated for compliance with the 2022 RSWDM. If
you have any questions, please email or call me, thank you.
The reference to the 2017 RSWDM has been removed. All references are now to the 2022 manual.
3. All existing and proposed utility lines along the property frontage must be relocated underground per
RMC 4-6-090. There is an existing utility pole along the properties Nile Ave NE frontage that shall be
removed and those services spanning to the north and south shall be installed underground to the next
utility pole. Those utilities that service houses on the west side of Nile Ave NE from this span may remain
overhead, however, only up to the serviced properties connection with the ROW; a new service pole may
be installed to retain overhead service and that portion of the service within the ROW shall be installed
underground. Further the subject utility pole contains an existing streetlight, a new streetlight conforming
to COR std. plan 117.1 shall be provided.
This has been noted, and called out on Sheet 5 of the Plan Set.
Sincerely,
Encompass Engineering and Surveying, Inc.
Chad Allen, PE
Senior Engineer