HomeMy WebLinkAboutJ_Drainage_Adjustment_Justification_260422_v1Standard Adjustment Request: Justification
Lincoln Peak Short Plat
2022 City of Renton Surface Water Design Manual
Section 1.4.2
Western Washington Division Eastern Washington Division
165 NE Juniper St., Ste 201, Issaquah, WA 98027 407 Swiftwater Blvd, Cle Elum, WA 98922
Phone: (425) 392-0250 Phone: (509) 674-7433
www.EncompassES.net
April 22nd, 2026
City of Renton
Attn: Review Team
1055 S Grady Way
Renton, WA 98057
This application has been prepared to request an adjustment to Core Requirement #1 of the 2022
RSWDM. Core Requirement #1 states that “All storm water runoff and surface water from a project must
be discharged at the natural location so as not to be diverted onto or away from downstream properties…”.
The intent of this requirement is to prevent adverse impacts to downstream properties and natural
resources (i.e. streams, wetlands, and other natural elements of high ecological value). The addition of
flow to a non-natural discharge location could result in drainage issues for downstream properties if the
storm conveyance system is not adequately equipped to handle the additional flow. Please see below for
a description of how this project cannot meet the requirements listed in Core Requirement #1:
In the existing conditions, stormwater runoff produced from the western side of Lincoln Avenue NE (which
is a crowned road) flows in the western direction towards the natural discharge area for the site. In the
proposed conditions, stormwater runoff produced from Lincoln Avenue NE will flow west towards the
proposed curb & gutter which then conveys runoff in the northern direction along the flowline. To pick
up all of this runoff and convey it to the natural discharge location (instead of allowing it to continue to
flow north), a catch basin would need to be placed at the northernmost section of proposed curb & gutter;
however, a multitude of existing and proposed utilities are located in this area which do not allow enough
space to install a catch basin in this location. The closest location to the endpoint of curb and gutter that
a catch basin is feasible is approximately 35-feet south of the endpoint. A catch basin installed at this
location still presents issues in tying the system back towards the natural discharge location (into the
proposed detention vault) such as utility conflicts and topography limitations. Additionally, the detention
vault would no longer fit within its current constraints if the additional surfaces along Lincoln Avenue NE
were conveyed into it (as its size would significantly increase). Therefore, the solution that we are
proposing is to convey all runoff produced from Lincoln Avenue NE to the north within the existing storm
system installed beneath Lincoln Avenue NE.
City of Renton
Lincoln Peak Short Plat
04/22/2026
Page | 2
2022 City of Renton Surface Water Design Manual Section 1.4.2 - Criteria for Granting Adjustments:
Adjustments to the requirements in this manual may be granted provided that granting the adjustment
will achieve the following:
1. Produce a compensating or comparable result that is in the public interest.
The proposed adjustment to Core Requirement #1 would result in a comparable result
that is in the public’s interest. An additional 4,670 SF of new impervious/pervious areas
are proposed to be conveyed to the north within the existing storm system within Lincoln
Avenue NE (2,022 SF of area currently flows to the north / 6,692 SF of area will flow to the
north in the developed conditions) as opposed to being conveyed to our proposed
detention system. As this area is minimal, and no other options are feasible, this result is
in the public’s interest and will produce a comparable result to if these surfaces were
conveyed to our detention system. It should be noted that the additional surfaces along
Lincoln Avenue NE that will be conveyed north (into what we are labeling as Basin 2)
result in a 0.0942 CFS increase in the 100-year flow using 15-minute time steps, which is
below the flow control exemption threshold of 0.15 CFS.
2. Meet the objectives of safety, function, appearance, environmental protection, and
maintainability based on sound engineering judgment.
The objectives of safety, function, appearance, environmental protection and
maintainability will be met with the proposed stormwater conveyance route for the
surfaces along Lincoln Avenue NE. This proposal will result in fewer utility conflicts and
proposed storm infrastructure, which in turn results in better safety, function, and
maintainability.