HomeMy WebLinkAboutRS_JacksonShortPlat_Drainage_Assessment_020523February 5, 2023
Renton City Hall
Mike Sippo, Civil Engineer 3
Development Review Division
1055 South Grady Way
Renton, WA 98057
RE: Written Drainage Assessment for the CJ Short Plat Located on Tax Parcel # 034570-0180; Located at
2223 NE 23rd Street, Renton WA 98056
Dear Mr. Sippo:
This Written Drainage Assessment was prepared for a proposed single-family home associated with an ongoing 2-
lot short plat located at the above-noted tax property in accordance with Appendix C (Simplified Drainage
Requirements) of the 2021 King County Surface Water Design Manual (2021 SWDM). The City of Renton (City)
adopted their own 2017 Renton Surface Water Design Standards, which compliments the 2016 KC SWDM. But the
2021 KC SWDM has been forthcoming and was anticipated to be adopted by Renton City Council in 2022. Several
years ago, City staff hosted a Pre-Application Meeting for this project which was numbered PRE19-000170.
The 0.24-acre (10,556 square foot) subject property (site) owned by Don Tsai is located in a suburban single-family
residential enclave within Renton Highlands. An existing home built in 1981 is located on the site’s north side that
is accessed from the west via NE 23rd Street (public ROW). The existing home will not be modified as part of this
project. The site’s existing impervious surface area is approximately 1,535 sf. In the existing condition, only the
site’s west side contains impervious surface area.
The site does not contain any sensitive or critical areas (wetlands, streams, steep slopes, et c.). The site’s topography
slopes down from southeast to northwest. The site’s high point is at the southwest property corner at elevation 309
and the low point is along the northwest property line at elevation 293. There is 16 feet of grade drop across the site.
It is assumed that the existing home is connected to Renton public water and Renton public sewer and the proposed
home will be connected to public water and public sewer.
Attached to this Written Drainage Assessment is an Infiltration Evaluation / Soils Study dated 7/15/2016 authored
by Kyle Campbell, P.E. at Earth Solutions NW that describes the site’s soils. As part of that work, three formal test
pits were dug and soil samples were evaluated. The site’s soils were classified as outwash, which means they are
moderately to very permeable, conducive to infiltration and don’t produce significant stormwater runoff. To my
knowledge, since those soils samples were conducted in 2016, there haven’t been any substantial changes to the site
that would affect this storm drainage design. I concur with Earth Solutions NW conclusion that the soils should be
conducive for infiltration.
Following upcoming final short plat approval by the City, a home is proposed to be built on the site’s south side
with access from the east via NE 22nd Street (public ROW). Proposed project disturbance includes some tree
clearing and grading, construction of a driveway, utility connections and some minor offsite improvements. The
total proposed onsite impervious surface area (ISA) equals 2,249 sf, which includes the new driveway (810 sf), new
home (1,069 sf), and new patio (370).
As required by the SWDM, projects proposing more than 2,000 square feet of ISA trigger Drainage Review. The
ISA needs application of a flow control BMP per Appendix C. The flow control BMP feasibility criteria is shown
on Appendix C pages C-18 and C-19. To address Simplified Drainage Requirements, the mitigation of target
impervious surface was followed as described below.
The first stormwater BMP that was considered was the applicability and feasibility of Full Dispersion. Full
Dispersion was determined infeasible for this project, because there was nowhere onsite to meet the required 100’
minimum length of native vegetated flowpath segment (NVFS). The site is just too small. Thus, Full Dispersion
is infeasible.
As shown on Appendix page C-19, the next stormwater BMP considered was Full Infiltration. As noted above,
there is a July 15, 2016 geotechnical report prepared by Earth Solutions NW stating the soils are conducive to
infiltration. The soils were classified as ‘coarse sand’ per the Earth Solutions report (page 3), thus Full Infiltration
can be utilized.
Per Appendix C, there are setback requirements when using infiltration. The required setback (minimum) distance
for an infiltration trench (or a dry well) to a home is 15 feet. The site is too small to meet this requirement for the
infiltration trench, because an infiltration trench is too linear. There is also a 5-foot minimum setback from a
property line to an infiltration trench or a dry well. Typically, a dry well will be circular and be 4 or 5-foot in
diameter, however a dry well could not meet the 15-foot home setback and 5-foot property line setback. However, a
triangular dry well is able to fit in between the proposed home’s 15-foot setback and proposed internal property
line’s 5-foot setback, while barely encroaching into the 10-foot wide public utility easement that parallels the north
property line. The area of the triangular dry well of 8 foot x 8 foot x 12.8 foot dry well is 31 square feet. In
contrast, the area of a 4-foot diameter dry well is only 13 sf.
The proposed driveway extension sheet flows into an existing catch basin. The very small patio will disperse into
adjacent vegetation, thus only the home downspouts are being accounted for by the dry well. Total home square
footage is 1,049 sf.
Per Appendix C, the required infiltration dry well volume of gravel is 60 cubic feet per 1,000 sf of impervious
surface area served for a ‘coarse sand’. Thus, for 1,049 sf, the required volume is 63 cubic feet. The provided dry
well volume shown on the 2/2023 civil plans for the 4-foot deep triangular dry well is (4 feet deep) * 31 sf equals
124 cubic feet. As a result, the dry well has adequate storage. If required by the City, the new driveway area can
also be directed to the dry well, but since the driveway extension area of 810 square feet is very small and there’s
already an existing catch basin present, I thought any drainage impact to the downstream system would be de
minimis. The dry well has a factor of safety as well.
Soil amendment will be provided in any graded areas to mitigate for lost moisture holding capacity where
compaction or removal of underlying topsoil occurs. During construction, in order to prevent erosion and trap
sediments within the project site, the following BMPs will be used as shown in the TESC Plan:
• Clearing limits will be marked by fencing or other means on the ground.
• Runoff will not be allowed to concentrate.
• Silt fencing will be placed along the downslope limit of clearing as approximately shown on the plans.
• Mulch will be spread over any disturbed areas of the site when they are not being worked. Mulch will
consist of air-dried straw.
• The adjacent road wi ll be monitored for sediment. If sediment is tracked out, then it needs to be
immediately cleaned up.
If you have any questions or concerns regarding this Written Drainage Assessment, please do not hesitate to contact
me at markrigos@hotmail.com. Thank you for your kind attention.
Sincerely,
Mark Rigos, P.E.
440 SE Darst Street
Issaquah, WA 98027
(425) 652-6013
Encl: Updated Storm Drainage and TESC Plans (2/2023) on 5 Full Size Sheets
Earth Solutions Geotechnical Report (2016)
CC: Jason Beagle, Architect via jasonb@mlarch.com
Charles Jackson, Applicant via charles@vista-dc.com
MJR:mjr