HomeMy WebLinkAboutRS_TIR_251105
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 Fax: (425) 391-3055 Phone: (509) 674-7433 Fax: (509) 674-7419
www.EncompassES.net
PRELIMINARY TECHNICAL INFORMATION REPORT
For
Meadow Ave Short Plat
3804 Meadow Avenue N
Renton, WA 98056
January 29, 2021
Revised March 7, 2022
Revised August 12, 2025
Revised October 31, 2025
Prepared by:
Ian Dahl
Encompass Engineering Job No. 20591
Prepared For:
Huy Nguyen
15400 SE 155th Place, Unit 99
Renton, WA 98058
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Table of Contents
List of Figures ...................................................................................................................................ii
I. PROJECT OVERVIEW ................................................................................................................ 1
II. CONDITIONS AND REQUIREMENTS SUMMARY ...................................................................... 8
III. DOWNSTREAM ANALYSIS ..................................................................................................... 12
IV. FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN ........................... 16
V. CONVEYANCE SYSTEM ANALYSIS AND DESIGN ..................................................................... 19
VII. OTHER PERMITS ..................................................................................................................... 21
VIII. CSWPP ANALYSIS AND DESIGN .............................................................................................. 21
IX. BOND QUANTITIES, FACILITY SUMMARIES AND DECLARATION of COVENANT .................. 21
X. OPERATION AND MAINTENANCE MANUAL .......................................................................... 21
List of Figures
Figure 1 – TIR Worksheet
Figure 2 – Vicinity Map
Figure 3 – Soils Map and Legend
Figure 4 – Existing Conditions Map
Figure 5 – Developed Conditions Map
Figure 6 – Drainage Review Flow Chart
Figure 7 – Downstream Map
Appendix A
Geotechnical Engineering Study by Earth Solutions NW dated March 12, 2020
Geotechnical Review Letter by Earth Solutions NW dated March 7, 2022
Appendix B
WWHM Output
Appendix C
Arborist Report/Tree Plan by Layton Tree Consulting, LLC dated February 12, 2020
Appendix D
CSWWP Analysis And Design
Appendix E
Bond Quantities, Facility Summaries And Declaration Of Covenant
Appendix F
Operation And Maintenance Manual
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I. PROJECT OVERVIEW
Project: Meadow Ave Short Plat
Site Address: 3804 Meadow Avenue N, Renton, WA 98056 (See Vicinity Map)
Tax Parcel #: 334270-0570
Zoning District: R-6, Single Family Residential
Site Area: 25,536 SF (0.59 Acres)
Site Location: The site is in the City of Renton within the NW quarter of Section 32,
Township 24 North, Range 5 East, W.M, King County, Washington. The
site is located on the northeast corner of the intersection of Meadow Ave
N and N 38th St.
Figure 2: Vicinity Map
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Pre-developed Site Conditions
The project site is located in the City of Renton on a 25,536 SF (0.59 Acres) lot that is zoned R-6 (single-
family residential). The site is accessed from Meadow Ave N, north of the intersection of N 38th St and
Meadow Ave N. The site is bordered to the north and south by single family residences, to the east by I-
405, and to the west by Meadow Ave N right-of way (ROW).
The site is currently developed with a single-family residence accessed from Meadow Ave N via two gravel
driveways on the western side of the site. The site slopes gently to the northwest at approximately 2%.
The property is located within the May Creek drainage basin. Stormwater runoff currently leaves the site
at a single natural discharge location at the northwest corner, creating one Threshold Discharge Area for
the site. The majority of the site gently slopes at approximately 2%, allowing stormwater to sheet flow
northwest into Meadow Ave. A cleared area on the adjacent parcel 334270-0630 is tributary to the site.
This off-site area sheet flows to the north over approximately 10-15% slopes onto the existing gravel
driveway located in the southern portion of the site. See full downstream analysis in Section III of this
Technical Information Report (TIR).
An Existing Conditions Map is included as Figure 4 at the end of this Section.
Critical Areas
According to the City of Renton online mapping system, there is a High Erosion Hazard Area in the
northeast corner of the project site. There are no required buffers or building setbacks associated with
the erosion hazard area. No other critical areas have been identified on the project site.
Soils
Per the US Department of Agriculture (USDA), Natural Resources Conservation Service (NRCS) Web Soil
Survey (WSS) information, the majority of the project site is underlain with Indianola loamy sand, 5 to 15
percent slopes with a small portion of the site in the northeast corner underlain with Norma sandy loam
(See Figure 3 on the following page). Soil explorations detailed in the Geotechnical Engineering Study by
Earth Solutions NW dated March 12, 2020 (Appendix A) encountered primarily medium dense silty sand
and poorly graded sand. These findings are consistent with geologic and soil mapping resources. An in-
situ infiltration test was performed and a design infiltration rate of 5 inches per hour was determined by
the Geotech, which is suitable for stormwater infiltration.
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Figure 3: Soil Map and Legend
Developed Site Conditions
The project proposes the development of three (3) single-family lots within the 25,536 SF (0.59 Acres)
parcel, as well as a 7.5-foot right-of-way dedication with frontage improvements. Lot 1 is 8,230 SF (0.19
AC) and is located in the north portion of the site. Lot 2 is 8,043 SF (0.18 Acres) and is located in the central
portion of the site. Lot 3 is 7,868 SF (0.18 AC) and is located in the southern portion of the site. All three
(3) lots will have driveway access off of Meadow Ave N. Frontage improvements will include the addition
of a 5-foot sidewalk, 8-foot planter strip, as well as curb and gutter along the edge of asphalt roadway.
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The total disturbed area for this project, including the construction of frontage improvements, is 27,320
SF (0.627 AC).
The parcel is zoned R-6, which allows for a maximum building coverage of 40% and a maximum impervious
surface coverage of 55%. The maximum allowable building/impervious surface for each lot is discussed
below:
· Lot 1: The maximum allowable building coverage for Lot 1 is 8,230 SF*0.4 = 3,292 SF, and the
maximum impervious surface coverage is 8,230 SF*0.55 = 4,527 SF. The final site layout of Lot
1 has not been determined at this stage; therefore, this report assumes that the future
impervious surfaces required to construct the residence on Lot 1 will use the maximum
coverages stated above.
· Lot 2: The maximum allowable building coverage for Lot 2 is 8,043 SF*0.4 = 3,217 SF, and the
maximum impervious surface coverage is 8,043 SF*0.55 = 4,424 SF. The final site layout of Lot
2 has not been determined at this stage; therefore, this report assumes that the future
impervious surfaces required to construct the residence on Lot 2 will use the maximum
coverages stated above.
· Lot 3: The maximum allowable building coverage for Lot 2 is 7,868 SF*0.4 = 3,147 SF, and the
maximum impervious surface coverage is 7,868 SF*0.55 = 4,327 SF. The final site layout of Lot
3 has not been determined at this stage; therefore, this report assumes that the future
impervious surfaces required to construct the residence on Lot 3 will use the maximum
coverages stated above.
Stormwater runoff from the proposed development of Lots 1-3 will be managed as follows:
· Lot 1: Stormwater BMP’s satisfying Core Requirement #9 will be used to the maximum extent
feasible to mitigate new runoff created by the proposed single-family residence and other
impervious surfaces on-site. A 30’x9’ gravel filled full infiltration trench is proposed on Lot 1
to the west of the proposed structure to mitigate stormwater from impervious areas. An
emergency overflow to the public storm system from the infiltration trench will be provided.
This report assumes that the maximum impervious surface per zoning (4,527 SF) will be
conveyed to this infiltration trench.
· Lot 2: Stormwater BMP’s satisfying Core Requirement #9 will be used to the maximum extent
feasible to mitigate new runoff created by the proposed single-family residence and other
impervious surfaces on-site. A 30’x9’ gravel filled full infiltration trench is proposed on Lot 2
to the west of the proposed structure to mitigate stormwater from impervious areas. An
emergency overflow to the public storm system from the infiltration trench will be provided.
This report assumes that the maximum impervious surface per zoning (4,424 SF) will be
conveyed to this infiltration trench.
· Lot 3: Stormwater BMP’s satisfying Core Requirement #9 will be used to the maximum extent
feasible to mitigate new runoff created by the proposed single-family residence and other
impervious surfaces on-site. A 30’x9’ gravel filled full infiltration trench is proposed on Lot 3
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to the west of the proposed structure to mitigate stormwater from impervious areas. An
emergency overflow to the public storm system from the infiltration trench will be provided.
This report assumes that the maximum impervious surface per zoning (4,327 SF) will be
conveyed to this infiltration trench.
Please refer to Core Requirement # 9 in Section II and Section IV of this TIR for additional discussion on
stormwater BMPs. A Developed Conditions Map is provided as Figure 5 at this end of this Section.
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Figure 4 – Existing Conditions Map
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Figure 5 – Developed Conditions Map
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II. CONDITIONS AND REQUIREMENTS SUMMARY
The 2017 City of Renton Surface Water Design Manual (RSWDM) was utilized to determine and address
all core and special requirements. Based on the criteria specified in Figure 1.1.2.A of the RSWDM, the
project falls under Full Drainage Review. Per Section 1.1.2.4 of the RSWDM, the project must meet all nine
(9) core and all six (6) special requirements. See Figure 6 below for more information on how the type of
drainage review was determined.
Figure 6: Drainage Review Flow Chart
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Core Requirements
Core Requirement #1: Discharge at the Natural Location
The proposed development runoff will follow existing drainage patterns toward the Meadow Ave
N storm system or will be fully infiltrated on-site. Refer to the Level 1 Downstream Analysis in
Section III of this TIR for a complete description of the existing drainage paths.
Core Requirement #2: Downstream Analysis
A Level 1 Downstream analysis has been completed for the site and no existing or potential
drainage problems have been identified. This analysis is included in Section III of this TIR.
Core Requirement #3: Flow Control Facilities
Based on the City of Renton’s flow control application map, the project site is located within the
Peak Rate Flow Control Standard (Existing Site Conditions). Flow control facilities are required to
match the developed peak discharge rates to existing site conditions peak discharge rates for 2-,
10- and 100-year return periods.
The proposed stormwater BMPs result in a 0.2289 CFS decrease in the 100-year flow using 15-
minutes time steps. This is below the allowable threshold of a 0.15 CFS maximum increase per
Section 1.2.3.1.B of the RSWDM; therefore, the project meets flow control exemption 2. No
formal flow control facilities are proposed at this time; however, flow control BMPs will be
implemented as described in Core Requirement #9. Please refer to Section IV of this TIR for
additional discussion.
Core Requirement #4: Conveyance System
Conveyance in compliance with the requirements detailed in Section 1.2.4.1 of the City of Renton
2017 SWDM will be provided with final engineering.
Core Requirement #5: Construction Stormwater Pollution Prevention
The limits of construction for the project are less than 1 acre. A temporary erosion and sediment
control (TESC) plan providing details on best management practices (BMPs) to be implemented
during construction is included in the engineering plan set. A Construction Stormwater Pollution
Prevention Plan (CSWPPP) will be provided with final engineering. Please refer to Section VIII of
this TIR for additional discussion.
Core Requirement #6: Maintenance and Operations
An Operation and Maintenance Manual will be provided with final engineering.
Core Requirement #7: Financial Guarantees and Liability
The owner will arrange for any financial guarantees and liabilities required by the permit.
Core Requirement #8: Water Quality Facilities
In accordance with Section 1.2.8.1.A of the RSWDM, Basic Water Quality Treatment is not
required for this project as new plus replaced pollution generating impervious surfaces (PGIS)
does not exceed 5,000 SF. The development results in 768 SF of concrete driveway within the
ROW dedication area, 485 of pavement replacement within the existing ROW, and 3,700 SF of on-
site driveway is estimated for the future build-out of Lots 1-3. These quantities total to 4,953 SF
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of PGIS that will be created or replaced as a result of this project; therefore, the project meets
surface area exemption 1.
Core Requirement #9: Flow Control BMPs
Per section 1.2.9.3.1 of the RSWDM, this project is classified as a small subdivision; therefore, it is
subject to the Small Road Improvement and Urban Road Improvement Project BMP Requirements
detailed in Section 1.2.9.3.2 in the RSWDM. Although implementation of individual lot BMPs is
not required until building permit application, BMPs have been considered for the future
improvements on Lots 1-3 based on Section 1.2.9.2 of the RSWDM. See Section IV of this TIR for
further discussion and flow control analysis.
Impervious Surface BMPs
Full Dispersion: Infeasible for Lots 1-3.
· Lots 1-3: The space required for a 100-foot native vegetated flowpath segment is not available
on these sites.
Full Infiltration: Feasible for Lots 1-3.
· Lots 1-3: Soil explorations detailed in the Geotechnical Engineering Study by Earth Solutions
NW dated March 12, 2020 (Appendix A) encountered primarily medium dense silty sand and
poorly graded sand. An in-situ infiltration test was performed and a design infiltration rate of
5 inches per hour was determined which is suitable for stormwater infiltration. Per section
C.2.2.3 of the RSWDM, 30 liner feet of 2’ wide trench per 1,000 SF of tributary impervious
surface can be used to design full infiltration trenches in coarse sands. For Lots 1-3, three
infiltration trenches of 30’x9’ are proposed which provides an infiltration area of 270 SF each.
A 30’x9’ trench has the ability to mitigate up to (270 SF/60 SF)x1,000 SF= 4500 SF of tributary
impervious surface. This allows each trench to mitigate the entire maximum impervious
surface coverage for each lot. As an additional safety measure, an emergency overflow to the
public storm system from the infiltration trenches will be provided.
As all target impervious surfaces on the project site have been mitigated with flow control
BMPs, no further BMPs have been considered.
Pervious Surface BMPs
Soil Amendment: Feasible for Lots 1-3
· Lots 1-3: All disturbed, pervious areas of the project will meet soil amendment requirements
as detailed in Section C.2.13 of the RSWDM.
Special Requirements
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Special Requirement #1: Other Adopted Area-Specific Requirements
Critical Drainage Area – N/A
Master Drainage Plan – N/A
Basin Plan – N/A
Lake management Plan – N/A
Shared Facility Drainage Plan – N/A
Special Requirement #2: Flood Hazard Area Delineation
The limits of this project do not lie within a delineated FEMA 100-year floodplain.
Special Requirement #3: Flood Protection Facilities
This project does not rely on or propose to modify/construct a new flood protection facility.
Special Requirement #4: Source controls
The project is a single-family residential development; therefore, this requirement is not
applicable.
Special Requirement #5: Oil Control
This project is not considered high-use in need of oil control.
Special Requirement #6: Aquifer Protection Area
The site is not located within an Aquifer Protection Area Zone per the City of Renton Groundwater
Protection Area Map Reference 15-B.
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III. DOWNSTREAM ANALYSIS
A Level 1 Downstream analysis has been conducted per the requirements in Section 1.2.2.1 of the
RSWDM. Please see Tasks 1 through 4 below for a summary of the results.
Task 1: Define and Map the Study Area
The area of analysis extends from the site discharge points along Meadow Ave N and the eastern limit of
the site along the I-405 barrier wall to approximately a quarter-mile downstream where the two discharge
areas converge. A Downstream Map is provided in Figure 7 below.
Figure 7: Downstream Map
Task 2: Review All Available Information on the Study Area
Per King county resources, there have been no significant drainage complaints within a quarter-mile
downstream of the site.
Task 3: Field Inspect the Study Area
A field inspection was performed by Encompass Engineering & Surveying on January 26, 2021. Please refer
to Task 4 for a detailed description of the downstream drainage system and analysis.
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Task 4: Describe the Drainage System
Stormwater runoff currently leaves the site at a single natural discharge location at the northwest corner
creating one Threshold Discharge Area for the site. The majority of the site gently slopes at approximately
2%, allowing stormwater to sheet flow northwest into Meadow Ave. A cleared area on the adjacent parcel
334270-0630 is tributary to the site. This off-site area and sheet flows to the north over approximately
10-15% slopes onto the existing gravel driveway in the southern portion of the site.
Runoff in the southeast portion of the site sheet flows on gravel driveway and grass towards a low point
along the I-405 wall on the eastern side of the site (A). From here, stormwater is assumed to infiltrate
directly into the soil as no outlet exists. This observation is consistent with type A/B outwash soils found
and detailed in the Geotechnical Engineering Study by Earth Solutions NW dated March 12, 2020
(Appendix A). Runoff from the southwestern portion of the site sheets flows towards Meadow Ave N
where it enters the City of Renton (COR) storm system in a Type 1 CB (B) on the edge of asphalt. Runoff
from the northern portion of the site sheet flows to the north and west, eventually entering Meadow
Ave N and entering a Type 1 CB (C) just northwest of the site at the edge of asphalt. The southern of
these two Type 1 CBs (B) outlets to the NW through a 10” DI storm pipe into a Type 2 CB approximately
12 feet away (D). From here, stormwater flows to the north through an 18” Poly storm pipe (E) into
another Type 2 CB (F). This Type 2 CB collects stormwater from the Type 1 CB (C) just northwest of the
site via an 8” DI storm pipe.
Stormwater continues north on Meadow Ave N through a series of Type 2 CBs and 18” Poly storm pipe.
It turns to the west when it reaches the intersection of Meadow Ave N and N 40th St, and runs westward
on N 40th St downhill towards lake Washington. Stormwater continues through a series of Type 2 CBs
and 18” poly pipe, through the intersection of N 40th St and Park Ave N, to the intersection of N 40th St
and Wells Ave N. The Type 2 CB in this location (G) is where the analysis was completed, just over ¼ mile
downstream of the site. Please refer to Figure 7 on the previous page, for the approximate location of
identified drainage features.
If conveyance system nuisance, severe erosion, severe flooding, or wetland hydrology problems are
identified downstream of the site under Core Requirement #2, additional impact analysis and/or
mitigation may be required. As discussed in Task 2, there have been no recent significant drainage
complaints within a quarter-mile downstream of the site. As no existing drainage or conveyance issues
have been recently identified, further impact analysis and mitigation of the downstream stormwater
system is not required. Please refer to the conveyance analysis in Section V of this TIR for additional
discussion and evaluation of the on-site conveyance system capacity. Photographs from the site visit are
included on the pages that follow.
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Element A- Low point on eastern side of site by I-405 Wall
Element B- Type 1 CB on Meadow Ave N collecting runoff from southern site frontage and yard
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Element C- Type 1 CB on Meadow Ave N collecting water from the northern site frontage and yard
Element G- Type 2 CB on N 40th St
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IV. FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN
Part A: Existing Site Hydrology
The 25,536 SF (0.59 AC) site is currently developed with a single-family residence accessed from Meadow
Ave N via two gravel driveways on the west side of the site. The property is located within the May Creek
drainage basin. Stormwater runoff currently leaves the site at a single natural discharge location at the
northwest corner creating one Threshold Discharge Area for the site. The majority of the site gently slopes
at approximately 2%, allowing stormwater to sheet flow northwest into Meadow Ave. A cleared area on
the adjacent parcel 334270-0630 is tributary to the site. This off-site area and sheet flows to the north
over approximately 10-15% slopes onto the existing gravel driveway in the southern portion of the site.
See full downstream analysis in Section III of this Technical Information Report (TIR).
Soil explorations detailed in the Geotechnical Engineering Study by Earth Solutions NW dated March 12,
2020 (Appendix A) encountered primarily medium dense silty sand and poorly graded sand. An in-situ
infiltration test was performed and a design infiltration rate of 5 inches per hour was determined by the
Geotech, which is suitable for stormwater infiltration.
WWHM 2012 was used to model the existing condition within the 27,320 SF (0.627 AC) limits of
disturbance using existing surfaces found on site including rooftop, driveway, lawn and walkways. See the
summary of existing and developed analyses on the following pages, as well as the existing conditions
map provided as Figure 4 for more details.
Part B: Developed Site Hydrology
The project proposes the development of three (3) single-family lots within the 25,536 SF (0.59 Acres)
parcel, as well as a 7.5-foot right-of-way dedication with frontage improvements. Lot 1 is 8,230 SF (0.19
AC) and is located in the north portion of the site. Lot 2 is 8,043SF (0.18 Acres) and is located in the central
portion of the site. Lot 3 is 7,868 SF (0.18 AC) and is located in the southern portion of the site. All three
(3) lots will have driveway access off of Meadow Ave N. Frontage improvements will include the addition
of a 5-foot sidewalk, 8-foot planter strip, as well as curb and gutter along the edge of asphalt roadway.
Stormwater runoff from the proposed development of Lots 1-3 will be managed as follows:
· Lot 1: Stormwater BMP’s satisfying Core Requirement #9 will be used to the maximum extent
feasible to mitigate new runoff created by the proposed single-family residence and other
impervious surfaces on-site. A 30’x9’ gravel filled full infiltration trench is proposed on Lot 1
to the west of the proposed structure to mitigate stormwater from impervious areas. This
report assumes that the maximum impervious surface per zoning (4,527 SF) will be conveyed
to this infiltration trench. Additionally, an emergency overflow to the public storm system
from the infiltration trench will be provided.
· Lot 2: Stormwater BMP’s satisfying Core Requirement #9 will be used to the maximum extent
feasible to mitigate new runoff created by the proposed single-family residence and other
impervious surfaces on-site. A 30’x9’ gravel filled full infiltration trench is proposed on Lot 2
to the west of the proposed structure to mitigate stormwater from impervious areas. This
report assumes that the maximum impervious surface per zoning (4,424 SF) will be conveyed
to this infiltration trench. Additionally, an emergency overflow to the public storm system
from the infiltration trench will be provided.
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· Lot 3: Stormwater BMP’s satisfying Core Requirement #9 will be used to the maximum extent
feasible to mitigate new runoff created by the proposed single-family residence and other
impervious surfaces on-site. A 30’x9’ gravel filled full infiltration trench is proposed on Lot 3
to the west of the proposed structure to mitigate stormwater from impervious areas. This
report assumes that the maximum impervious surface per zoning (4,327 SF) will be conveyed
to this infiltration trench. Additionally, an emergency overflow to the public storm system
from the infiltration trench will be provided.
WWHM 2012 was used to model the proposed condition as follows:
· Lot 1: The modeled limits of disturbance for Lot 1 is 8,230 SF. The maximum impervious
surface per R-6 zoning of 4,527 SF was assumed, with the entirety of this area being conveyed
to the full infiltration trench. Per Table 1.2.9.A in the RSWDM, the area going to full infiltration
can be subtracted from the model. The remaining 3,703 SF of pervious surface on Lot 1 will
meet soil amendment requirement as detailed in Section C.2.13 of the RSWDM; therefore, it
has been modeled as 100% pasture with flat slopes in type A/B soils.
· Lot 2: The modeled limits of disturbance for Lot 2 is 8,043 SF. The maximum impervious
surface per R-6 zoning of 4,424 SF was assumed, with the entirety of this area being conveyed
to the full infiltration trench. Per Table 1.2.9.A in the RSWDM, the area going to full infiltration
can be subtracted from the model. The remaining 3,619 SF of pervious surface on Lot 1 will
meet soil amendment requirement as detailed in Section C.2.13 of the RSWDM; therefore, it
has been modeled as 100% pasture with flat slopes in type A/B soils.
· Lot 3: The modeled limits of disturbance for Lot 3 is 7,868 SF. The maximum impervious
surface per R-6 zoning of 4,327 SF was assumed, with the entirety of this area being conveyed
to the full infiltration trench. Per Table 1.2.9.A in the RSWDM, the area going to full infiltration
can be subtracted from the model. The remaining 3,541 SF of pervious surface on Lot 1 will
meet soil amendment requirement as detailed in Section C.2.13 of the RSWDM; therefore, it
has been modeled as 100% pasture with flat slopes in type A/B soils.
· ROW Dedication & Road Widening: The modeled limits of disturbance for the ROW
dedication area and road widening is 2,995 SF. This will include 838 SF of sidewalk, 768 SF of
driveway entrance as well as 485 SF of asphalt road repair. The remaining 904 SF of pervious
surface will meet soil amendment requirement as detailed in Section C.2.13 of the RSWDM;
therefore, it has been modeled as 100% pasture with moderate slopes in type C soils.
Please refer to Core Requirement #9 in Section II of the TIR for additional discussion. A Developed
Conditions Map is provided as Figure 5. A summary of the existing and developed analyses is provided in
the table on the following page.
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On-site + Off-Site Existing Proposed
Condition Measured Modeled Measured Modeled
Lawn, Mod: 14,050 SF
(0.322 AC)
14,050 SF
(0.322 AC)
11,850
(0.272 AC)
11,850
(0.272 AC)
Pasture, Mod:
Roof, Flat: 1,995 SF
(0.046 AC)
1,995 SF
(0.046 AC)
9,729 SF
(0.223 AC)
Driveway, Flat: 10,122 SF
(0.232 AC)
10,122 SF
(0.232 AC)
4,418 SF
(0.101 AC)
768 SF
(0.018 AC)
Sidewalk, Flat: 719
(0.017 AC)
719
(0.017 AC)
838 SF
(0.019 AC)
838 SF
(0.019 AC)
Road, Flat: 434 SF
(0.010 AC)
434 SF
(0.010 AC)
485 SF
(0.011 AC)
485 SF
(0.011 AC)
Total Area: 27,320 SF
(0.627 AC)
27,320 SF
(0.627 AC)
27,320 SF
(0.627 AC)
13,941 SF
(0.320 AC)
Part C: Performance Standards
Based on the City of Renton’s flow control map, the project site is located within the Peak Rate Flow
Control Standard (Existing Site Conditions). Flow control facilities are required to match the developed
peak discharge rates to existing site conditions peak discharge rates for 2-, 10- and 100-year return
periods. However, as determined in Part D on the following page, the site meets flow control exemption
2 per Section 1.2.3.1.B of the RSWDM, and flow control facilities are not required.
Per section 1.2.9.3.1 of the RSWDM, this project is classified as a small subdivision; therefore, it is
subject to the Small Road Improvement and Urban Road Improvement Project BMP Requirements
detailed in Section 1.2.9.3.2 in the RSWDM.
The site falls within a Basic Water Quality treatment area in accordance with Section 1.2.8.1.A of the
RSWDM, however new plus replaced impervious pollution generating areas within the project site is
under 5,000 SF. Therefore, water quality treatment is not required.
Part D: Flow Control System
As shown in the table below, the proposed stormwater BMPs result in a 0.1956 CFS decrease in the 100-
year flow using 15-minutes time steps. This is below the allowable threshold of a 0.15 CFS maximum
increase per Section 1.2.3.1.B of the RSWDM; therefore, the project meets flow control exemption 2. No
formal flow control facilities are proposed at this time; however, flow control BMPs will be implemented
as described in Core Requirement #9 in Section II of this TIR. Please refer to Appendix B for a copy of the
full WWHM data output.
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Part E: Water Quality System
In accordance with Section 1.2.8.1.A of the RSWDM, Basic Water Quality Treatment is not required for
this project as new plus replaced pollution generating impervious surfaces (PGIS) does not exceed 5,000
SF. The development results in 768 SF of concrete driveway within the ROW dedication area, 485 SF of
pavement replacement within the existing ROW, and 3,700 SF of on-site driveway is estimated for the
future build-out of Lots 1-3. These quantities total to 4,953 SF of PGIS that will be created or replaced as
a result of this project; therefore, the project meets surface area exemption 1.
V. CONVEYANCE SYSTEM ANALYSIS AND DESIGN
This project is using infiltration using the BMP prescriptive method. Rainwater infiltrates and the pipes
are provided for emergency overflow. Determining the flow value will be based on one of the six
infiltration trenches failing and triggering the overflow. There are two infiltration trenches per lot, so each
infiltration trench will take one half of the roof area, assumed to be 2,000 SF. Using the Rational Method
Q=CIA, we define the flow (Q) as being equivalent to:
C = Curve Number = 0.98
I = Rain Intensity (from WA 100 yr Isopluvials) = 3.9 in/hr
A = 2000 SF = .046 AC
Q = 0.98*3.9*.046 = 0.18 cfs
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Manning's formula is used to calculate the flow rate (Q) in open channels and pipes. The formula is:
Q = A × (1.486/n) × R^(2/3) × S^(1/2)
Where:
· Q = Discharge (cubic feet per second, cfs)
· A = Cross-sectional area of flow (square feet, ft²)
· n = Manning's roughness coefficient
· R = Hydraulic radius (feet)
· S = Slope of the pipe (feet/feet)
Parameters:
Diameter of Pipe = 6 inches (0.5 ft)
Slope (S)= 2% (0.02)
Manning's n (PVC)= 0.009 - 0.011
Cross-sectional Area (A)
For a circular pipe:
A = π × (d/2)²
· Diameter (d) = 0.5 ft
· A = π × (0.5/2)² = π × (0.25)² = π × 0.0625 ≈ 0.1963 ft²
Hydraulic Radius (R)
R = A / P
Where P is the wetted perimeter. For a circular pipe:
P = π × d = π × 0.5 ≈ 1.57 ft
· R = 0.1963 / 1.57 ≈ 0.125 ft
Flow Rate Calculation
Using the average Manning's n for PVC (0.010):
Q = 0.1963 × (1.486/0.010) × (0.125)^(2/3) × (0.02)^(1/2)
Calculating each part:
· Q ≈ 0.1963 × 148.6 × 0.0502 × 0.1414
· Q ≈ 0.1963 × 148.6 × 0.00707
· Q ≈ 0.1963 × 1.051 ≈ 0.207 cfs
Conclusion
The maximum flow rate for a 6-inch PVC pipe at a 2% slope is approximately 0.207 cfs which exceeds the
flow expected from a failed infiltration trench of 0.18 cfs. A scenario of more than one failed infiltration
trench is not expected as the Operations and Maintenance Manual for the BMPs would implement the
repair of the infiltration trench should any flow be observed in the overflow pipe.
VI. SPECIAL REPORTS AND STUDIES
· Geotechnical Engineering Study by Earth Solutions NW dated March 12, 2020
· Arborist Report/Tree Plan by Layton Tree Consulting, LLC dated February 12, 2020
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VII. OTHER PERMITS
· Civil Construction Permit
· Final Short Plat
· Building Permits
· Right-of-Way Use Permit
VIII. CSWPP ANALYSIS AND DESIGN
See Appendix D.
IX. BOND QUANTITIES, FACILITY SUMMARIES AND DECLARATION of
COVENANT
See Appendix E
X. OPERATION AND MAINTENANCE MANUAL
See Appendix F
Appendix A
Geotechnical Engineering Study by Earth Solutions NW dated March 12, 2020
Appendix B
WWHM Output
Appendix C
Arborist Report/Tree Plan by Layton Tree Consulting, LLC dated February 12, 2020
Appendix D
CSWPPP Analysis And Design
Appendix E
Bond Quantities, Facility Summaries And Declaration Of Covenant
Appendix F
Operation And Maintenance Manual