HomeMy WebLinkAboutC_Response_Letter_VEK_On_Aberdeen_200508_v2
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
May 8, 2020
City of Renton
Attn: Angelea Weihs, Associate Planner
14616 SE 192nd Street
Renton, WA 98058
Subject: VEK on Aberdeen Townhomes Plat, LUA19-000280, ECF, MOD, PP, SA-H
Dear Ms. Weihs,
In response to your revision request dated January 10, 2020, we have revised the submittal documents as
requested. The city comments in Italics with a detailed response below in bold are indicated below.
“On Hold” Notice
Utility and Transportation
1. The following stormwater improvements are required and shall be discussed within the TIR:
a. There are regulated slope, erosion hazard, and landslide areas found within the western side of
the property. Due to the steep slopes downstream of the project, the project lies within a flood problem
flow control standard area (see section 1.2.3.1 (4) of the 2017 RSWDM). Therefore, unless a flow
control exemption is identified, and if stormwater will be discharged to the steep slope area, then the
project must match the Flow Duration of pre-developed rates for Existing Site Conditions from 50% of
the 2-year up to the full 50-year flow, and match Existing Site Conditions for the 2-, 10-, and 100-year
peaks. Based on the preliminary design, the facility meets this standard. Update Section 4 of the TIR
to include discussion of how this standard is being met (currently states that only the peak flow rate
standard is being met). Additionally, update the TIR and detention tank design to include a minimum
0.5-foot sediment storage and minimum 0.5-foot freeboard.
The outlet has been revised to discharge to the flat area adjacent to the wetland buffer. The tank
has been redesigned to provide the sediment storage and freeboard as requested.
b. Continuous modeling did not include flows from the steep forested area on the western portion of
the site to the natural discharge location. In order for these flows to be excluded from the point of
compliance analysis, per Section 1.2(A) of the 2017 RSWDM (Peak Rate Flow Control Target Surfaces
part 2): “…the extent of new pervious surface shall be assumed to be the entire lot area, except the
assumed impervious portion and any portion in which native conditions are preserved by covenant,
tract, or easement”. The final plat shall place the forested area in permanent and protected open
space/critical areas tract or easement or the facility redesigned to attenuate these flows at the point
of compliance. Update the preliminary plans and report indicating if the forested area will be placed
in permanent open space or provide updated calculations including this area.
City of Renton
Renton LUA19-000280
May 8, 2020
Page 2 of 5
The site area in question is included within the 0.66 Acre “Tract F Native Growth Protection Area”.
A note has been added to the preliminary plats indicating that Tract F-Native Growth Protection
Area, incorporating the forested area identified above, shall be placed in a permanent open space
tract at final plat.
c. Appropriate Flow control BMPs are required to help mitigate the new runoff created by this
development. The preliminary drainage plan indicates that the proposed development is considered a
“small subdivision”, however, due to the nature of the development being multi-family with individual
lots being created out of multi-family buildings sharing common walls the project is more in order with
the “large lot” BMP requirements as specified in Section 1.2.9.2.2 of the 2017 RSWDM. Due to steep
slopes, landslide and erosion hazard areas being located onsite, full dispersion, full infiltration, limited
dispersion, limited infiltration, bioretention and permeable pavement are all infeasible. Specifically,
Section 1.2.9.2.2 (5) of the 2017 RSWDM states: “The buildable portion of the site/lot is the total area
of the site/lot minus any critical areas and minus 200 ft. buffer areas from a steep slope hazard,
landslide hazard, or erosion hazard are”. For the applicability of BMP’s, the 200 ft. buffer encompasses
the entire site, therefore, per definition, there are no onsite buildable areas subject to BMP’s and the
project is exempt from meeting Core Requirement #9. Update the preliminary TIR to include a
discussion of how this project is not subject to the BMP requirement as described above.
Core Requirement #9 and Section IV of the report have been updated with the City’s comment.
d. Optional BMP’s maybe utilized to reduce the size of the flow control facility, however, any BMP’s
utilizing dispersion or infiltration techniques (i.e. permeable pavement, limited dispersion, or
bioretention) will require the use of underdrains and/or interceptor trenches to collect the runoff and
route it to the detention system. Update preliminary TIR if any BMP’s are being included for facility
sizing credits.
No optional BMPs are proposed for the site development. Therefore, no sizing credits have been
applied to the project modeling.
e. The final design and Technical Information Report shall take into account all provisions of Core
Requirement #1 (Discharge at Natural Location). Specifically, Section 1.2.1 (2) states that: “IF a
proposed project, or any natural discharge area within a project, is located within a Landslide Hazard
Drainage Area and drains over the erodible soils of the landslide hazard with slopes steeper than 15%,
THEN a tight-line system must be provided through the landslide hazard to an acceptable discharge
point…”. The preliminary TIR identifies than an HDPE tight-line over the steep slope will discharge into
the onsite drainage way (which is still located on slopes above 15% within the erosion and landslide
area), however, additional analysis and mitigation is required to show that the decrease in runoff
volume and energy dissipation fully meets Core Requirement #1. At a minimum, the tight-line must
extend to John’s Creek where the slopes are less than 15% and the other nearby properties discharge
to. Alternatively, Section 1.2.1 (2) provides and exception where CED can approve an alternative
system based of geotechnical evaluation/recommendation where the developed conditions runoff
volume from the natural discharge area is less than 50% of the existing conditions runoff volume.
Update the preliminary plans and TIR to reflect how Core Requirement #1 is being met. If the
alternative exception is being proposed, provide a drainage modification request with all applicable
geotechnical and engineering justifications and recommendations provided.
City of Renton
Renton LUA19-000280
May 8, 2020
Page 3 of 5
The tight-line outlet has been extended to discharge flows at a location further to the west, located
adjacent to the wetland buffer and outside of the sloped areas. The TIR and plans have been
updated to reflect this change.
f. Per RMC 4-3-050 (G)(2) steep slope and landslide hazard buffers shall be established by the
geotechnical engineer. Section 4.6.6 of the Geotechnical Report states: “The proposed townhomes
have been designed to step down with the existing topography and the majority of the development
is on the least sensitive area of the site. Three townhomes are proposed on top of the slope outside of
the sensitive slopes and setback from the slope. Based on the results of our subsurface explorations
and slope stability analysis, the proposal will not increase the threat of the geological hazard to
adjacent or abutting properties beyond predevelopment conditions; and will not adversely impact
other critical areas; and the proposed development can be safely accommodated on the site”,
however, the report references a previous site plan which shows the proposed buildings set back
considerably further than the current site plan and does not take into account the drainage structures
and retaining wall which is located on and around the steep slope. Prior to land-use approval, the
geotechnical engineer shall provide a quantitative analysis of the steep slope and landslide hazard
area and provide set-back based on the recommendations for the updated site plan and facilities being
proposed closer than identified in the report. Additionally, a 15’ building set-back line (BSBL) will be
required from the edge of the buffer, including the case that the buffer is determined to be 0’. Building,
retaining walls and subsurface drainage structures shall not be located within the buffer or BSBL.
Update the preliminary plans, geotechnical report and TIR to include any changes resulting in the
buffer and BSBL and relocating facilities and/or buildings outside of this area.
Kristina Weller, PE of Riley Group, Inc (‘RGI’) conducted slope stability analysis. It was determined
that only a 15’ BSBL and limited slope buffer is needed from the top of slope as indicated on the
updated site plans. This has resulted in the storm facilities and site grading to be redesigned so that
no structures are located within the buffer or BSBL area as indicated by RGI. A portion of the
detention tank and control structure do encroach into the BSBL, however the detention tank does
not have a foundation and is not considered a structure or building. It only serves to collect and
convey stormwater to its natural discharge point in the western portion of the property. As such it
is not prohibited from the BSBL as it is not a building. Per 4-3-050(3) storm drainage piping is
exempt, and permitted within critical areas and associated buffers. The preliminary plans,
geotechnical report and TIR have been updated to include the changes resulting from the slope
stability analysis. RGI’s response letter is also included with this resubmittal.
g. The final technical information report shall include the “bypass” area in the point of compliance
analysis since the conveyance system located within Aberdeen Ave NE converges at the site’s point of
compliance at the southwest corner (John’s Creek) less than ¼ mile downstream. Based on the
preliminary design, the bypass area was included in the facility sizing calculation. See comments 1a
and 1b. Update the level one downstream analysis to include this downstream drainage system to the
point where the flows converge at the western portion of the site.
The updated plans include capturing the frontage area and have been included in the revised model
therefore, there will be no bypass.
City of Renton
Renton LUA19-000280
May 8, 2020
Page 4 of 5
2. The development is required to provide enhanced water quality treatment prior to discharge.
Project water quality treatment will consist of conveyance to a Bio-pod system prior to connection to the
proposed detention tank.
a. Pre-settling shall be being provided per Section 6.1.1 and 6.5.1 of the 2017 RSWDM. Update the
plans and TIR to include all relevant design and calculations for pre-settling prior to entering Bio-pod
system. If the manufacturer’s system provides pre-settling, indicate how the requirement is being met
using the manufacturer’s design criteria and any applicable calculations in sizing of the facility.
The bio-pod is now located downstream of the detention tank which has been designed with 0.5’
of sediment storage.
b. Per Section 6.2.1 (Water Quality Design Flows and Treatment Volumes) 2017 RSWDM the on-line
facility will have a modified water quality flow modeling rate (ratio k) of 1.88 based on the 2-yr
isopluvial of 1.95” (linear interpolation of Table 6.2.1.A). Based on the WWHM on-line facility target
flow of 0.0369 cfs, Qwq = kQ or 0.0694 cfs. This will need to be the number that is submitted to the
manufacturer for sizing of the facility. Update the plans and TIR with the revised water quality facility
design, including manufacturer’s data sheets based on the updated water quality flow. Ensure to note
to the manufacturer during the sizing that the enhanced water quality treatment menu is being met
as their calculations will take this into account for a decrease in media filtration rate.
The bio-pod selection parameters have been updated since it is now downstream of detention and
will provide water quality for the full 2-year release rate from the tank (0.518 CFS). Oldcastle
Infrastructure’s “Standard Sizes for the Pacific Northwest” biopod sizing chart shows that a “X-46Y-
Z S/P/T Internal Bypass” model possesses a maximum treatment flowrate of 0.074 cfs and therefore
meets this requirement. This model was previously proposed for the site’s enhanced water quality
treatment facility and has not been changed. Section IV of the TIR has been updated to include the
sizing chart, correct flowrate, and updated facility discussion.
c. A maintenance access road is required to the stormwater facilities in the proposed storm
tract/easement and shall be in accordance with the design requirements outlined in the RSWDM.
Update the plans and TIR to include this access road.
Not required with new design
Critical Areas
3. The Critical Area Report submitted with the project application identified a wetland on the project site.
The Critical Area Report does not delineate or classify the on-site wetland. Please provide a Wetland
Assessment consistent with the requirements of RMC 4-8-120D.23.
The Critical Area Report was updated with the delineation of the on-site wetland by Mark Heckert
of Beaver Creek Environmental Services. A Wetland Assessment consistent with Renton’s
requirements was included in the update CAR. The updated report is included with this resubmittal.
City of Renton
Renton LUA19-000280
May 8, 2020
Page 5 of 5
4. The Critical Area Report submitted with the project application identified an unclassified stream on
the project site. Per RMC 4-8-120D.19, if the site contains an unclassified stream, a qualified biologist
shall provide a proposed classification of the stream(s) based on RMC 4-3-050G7, Streams and Lakes,
and a rational for the proposed rating. Please revise the stream study to comply with all Standard
Stream Study requirements per RMC 4-8-120D.19, including evaluation of impacts from proposed
improvements.
Mark Heckert, a qualified biologist, updated the Critical Area Report with a Stream and Lake
Assessment. Mr. Heckert determined that the drainages located on site do meet the criteria for
definition as a “stream” as they are not physically connected by an above ground channel system
to a Type S, F, or Np Waters. The updated report, dated Feb. 20, 2020 is included with this
resubmittal.
5. Include critical area buffers, and buffer setbacks on plan sets.
All critical area buffers and BSBLs have been included in the updated plan sets.
Planning
6. Per RMC 4-2-110D.20, an additional ten feet of max wall plate height and an additional story for a
residential dwelling structure may be obtained through the provision of additional amenities such as
additional recreation facilities, underground parking, and additional landscaped open space areas, as
determined through the site development plan review process and depending on the compatibility of
the proposed buildings with adjacent or abutting existing residential development. Please provide a
full description of proposed additional amenities. As part of your description, differentiate between
code-required open space and amenities, and the additional amenities you are proposing for this
project.
Please find a revised narrative including exhibits describing how the project is exceeding code-
required open space and amenities in order to obtain the additional 10’ of max wall plate height is
included with this resubmittal.
If you have any questions, please email or call me, thank you!
Sincerely,
Encompass Engineering and Surveying, Inc.
Nicole Mecum, PE
Senior Engineer