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1191 SECOND AVE, SUITE 1100, SEATTLE WA 98101
DRAINAGE REPORT
Technical Information Report
Longacres Master Plan
Renton, WA
March 2026
PREPARED BY:
COUGHLIN PORTER LUNDEEN
1191 Second Avenue, Suite 1100
Seattle, WA 98101
P 206.343.0460
CONTACT / Tim Brockway, PE, LEED AP BD+C
Drainage Report
Longacres Master Plan
Coughlin Porter Lundeen
Project Number: C22054
March 2026
TABLE OF CONTENTS
I. PROJECT OVERVIEW ............................................................................................................................. 1
General Description .................................................................................................................................. 1
Existing Conditions ................................................................................................................................... 2
Proposed Drainage System ...................................................................................................................... 2
II. CONDITIONS AND REQUIREMENTS.................................................................................................... 5
Special Requirements:.............................................................................................................................. 5
Project Specific Requirements: ................................................................................................................ 7
III. OFF-SITE ANALYSIS ............................................................................................................................ 8
Task 1 – Study Area Definition and Maps ................................................................................................ 8
Task 2 - Resource Review ....................................................................................................................... 8
Task 3 - Field Inspection .......................................................................................................................... 8
Task 4 - Drainage System Description and Problem Descriptions .......................................................... 8
Upstream Analysis ................................................................................................................................ 9
Downstream Analysis ............................................................................................................................ 9
Task 5 – Mitigation of Existing or Potential Problems ........................................................................... 9
IV. FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN ............................ 10
Existing Site Hydrology (Part A) ............................................................................................................. 10
Developed Site Hydrology (Part B) ......................................................................................................... 11
Performance Standards (Parts C) .......................................................................................................... 13
ON-site Best Management Practices (on-site BMPs) Part D ................................................................. 13
Water Quality System (Part E) ............................................................................................................... 15
V. CONVEYANCE SYSTEM ANALYSIS AND DESIGN .......................................................................... 16
Standard Requirements (based on 2022 RSWDM and SAO): .............................................................. 16
On-site Conveyance ............................................................................................................................... 17
Existing Conditions: ............................................................................................................................. 17
Developed Storm System Description: ............................................................................................... 17
VI. SPECIAL REPORTS AND STUDIES .................................................................................................. 18
VII. OTHER PERMITS ............................................................................................................................... 19
VIII. CSWPPP ANALYSIS AND DESIGN ................................................................................................. 20
Standard Requirements .......................................................................................................................... 20
ESC Plan Analysis and Design (Part A) ................................................................................................. 21
SWPPS Plan Design (Part B) ................................................................................................................. 21
IX. BOND QUANTITY, FACILITY SUMMARIES, AND DECLARATION OF COVENANT ...................... 22
X. OPERATION AND MAINTENANCE MANUAL .................................................................................... 23
Appendix A – Figures .............................................................................................................................. 24
Appendix B – Engineering Calculations ................................................................................................ 25
Appendix C – Supplemental Materials ................................................................................................... 26
Appendix D – Supplemental Material Specifications ........................................................................... 27
Appendix E – BioPod and Bioscape Supplemental Materials ............................................................. 28
I. PROJECT OVERVIEW
GENERAL DESCRIPTION
The following Technical Information Report (TIR) provides the technical information and design analysis
required for developing the Drainage Plan for the Longacres Master Plan (LAMP) project. This document
will also be used in the future for developing the Temporary Erosion and Sedimentation Control Plan
(TESC). The stormwater design for the project was based on the requirements set forth in the 2022
Renton Surface Water Design Manual (RSWDM) (See Figure 1 – Technical Information Report
Worksheet).
The Longacres Master Development Project is located within the City of Renton, majorly situated at the
west of Oakesdale Ave SW and south of SW 16th St. A portion of the project will also be east of
Oakesdale Ave SW. (See Figure 2 – Vicinity Map). The site is in the NE ¼ of NW ¼ and NW ¼ of NE ¼
of Section 24, Township 23 North, Range 4 East, Willamette Meridian. The site encompasses parcel lots:
088670-0010, -0020, -0030, -0040, -0050, -0060, -0070, -0080, -0140, -0200, -0250 and 2423049-055, -
050 for a total disturbed area of approximately 63 acres.
Overall, the project will include 1,478,522 square-feet of mixed-use buildings, including but not limited to,
multifamily residential, retail, and mixed-use medical office buildings. Approximately 1,463,346 cubic-feet
includes floodplain mitigation along with the installation of storm conveyance infrastructure. Each parcel
in the LAMP project will be responsible for mitigation of their own floodplain impacts. At this stage of the
design process, we are providing one option for floodplain mitigation at each parcel with final design
being provided under future permits. The entire Longacres site is designed to provide floodplain fill
mitigation in compliance with the City of Renton code.
Soils for the area were mapped using the King County Soil Survey maps (See Figure 3A – USGS Soil
Survey Map), and a Geotechnical exploration has been performed to address slope and soil
characteristics. A total of 15 buildings is proposed across the property and connected via a main access
road along the western edge of Wetland A and Pond B and a secondary maintenance road on the west
side of the buildings. The drives connect as they run south along the property and eventually converge
with Oakesdale Ave SW.
This project proposes to construct a drainage system with a new network of underground pipes and
catch basins to collect surface water runoff throughout the site and direct it to multiple water quality
treatment facilities. All disturbed areas will be provided with modern water quality treatment facilities as
required by code. Treated water from that facility will flow into both Wetland A and Pond B, along with
higher storm events that are not subject to the requirements for water quality treatment. Wetland A and
Pond B were designed for the Boeing Master Plan and was constructed to accommodate the full
detention and water quality treatment requirements for the overall Longacres Campus (See Appendix C
for the 1998 Sverdrup Drainage Report). Stormwater from Pond B flows into Wetland A via a flow control
facility (See Figure 12A and 12B) which in turn flows out though several human-made open channels to
the CTSC Wetland Delta, into Springbrook Creek, and ultimately the Duwamish River. This system is
consistent with the conditions of the original Longacres Development Agreement with the City which was
the basis for the existing development on the property
EXISTING CONDITIONS
The existing site consists of open grassy areas, two large existing office buildings with associated plaza,
walkways and parking lots. See Table 1 for site surface cover information. The site was previously home
to the Longacres Racetrack, outbuilding stable and support services area, which was demolished during
the development of the Longacres Office Park. Minimal conveyance systems exist within the project site
area and runoff now leaves the site via surface flow which makes its way ultimately into Wetland A and
Pond B through a network of catch basins and conveyance pipes . There is a total elevation change of
approximately 3 feet across the site, from 22 feet at SW 16th St to 19 feet at the south property line. The
site is generally otherwise considered “flat” terrain. Wetland A and Pond B have a dead storage elevation
of 11.7 and a live storage high water elevation of 14.7 feet. This was determined from the Pond’s flow
control facility (See Figure 12A and 12B, The highwater line of 11.5’ is converted to 14.7’ using the
NAVD 88 datum).
The majority of the Longacres Office Park sits within the 100-year flood zone of the Springbook Creek
basin. The area is in the FEMA Zone AE with a Based Flood Elevation of 20.0 feet. Any fill that is added
to this project site, within the mapped flood zone, between existing grade and elevation 20.0 feet will be
mitigated withing the project site.
The site also includes the Seattle Sounders FC Training Facility, originally permitted in May 2023 with
major construction activities concluded in May 2024, which developed four soccer fields and associated
areas. The project required 583,800 cubic feet of required floodplain mitigation and installed 591,000
cubic feet of floodplain mitigation.
PROPOSED DRAINAGE SYSTEM
This project proposes to construct a drainage system with a new network of underground conveyance
pipes and catch basins, to collect the drainage from the roofs and roadways as well as the surface water
runoff throughout the walkways and other areas adjacent to the buildings within the project site. This
system will direct runoff from the roadways to the water quality treatment system. Flows from this facility
will then be directed into the existing Pond B. See Table 2 for site surface cover information. According
to Figure 1.1.2.A of the 2022 Renton Surface Water Design Manual (RSWDM), this project meets the
criteria for a Full Drainage Review, including water quality requirements.
All water quality on site will be provided in treatment systems located in coordination with the site design,
and complying with, designed according to Chapter 6 of the 2022 RSWDM and State Ecology GULD
designation requirements. In order to provide the required enhanced basic water quality treatment for the
pollution generating roadways, the project plans to implement a series of BioPod Biofilter system units
(BioPods) and possibly Bioscape open systems using Filterra media, prior to discharging to Pond B or
Wetland A, depending on location and current basin patterns. These systems have General Use Level
Designation (GULD) approval for enhanced basic treatment , therefore allowing their use per the
RSWDM. See Appendix E for GULD approval.
The BioPod units will either have a curb inlet configuration that will accept runoff directly from the surface
or will be an underground vault with runoff piped into them. Underground vaults will be sized according to
off-line design discharge rates and curb inlet BioPod units will be sized according to the on-line design
discharge rates. An off-line configuration is when the unit is constructed with a physical divider wall
between the inlet/ outlet chamber and the media chamber, per the manufacturer’s reco mmendation.
BioPod Biofilter System
The Bioscape facility will be a scalable open-topped version of the Filterra vaults, and a representative
installation was approved and is functioning across Pond B below the soccer fields to the east . See the
image below for representative images of the Bioscape system. This installation allows for a more site -
integrated design on a larger scale. BioPod and Bioscape sizing for this project will be provided during
permitting.
Filterra Bioscape System
Conveyance for the Longacres site will be a combination of closed pipes and some open channels.
These conveyances are designed to carry stormwater runoff to the water quality systems and then the
receiving pond or wetland on site, in a consistent manner with their current discharges to these existing
systems Stormwater runoff will discharge into the same portions of the site drainage systems as they
currently do, with some areas discharging to the Pond/Wetland A, and some discharging to South Pond
B as they currently do. New or existing discharge pipes will outfall to their respective ponds with quarry
spalls for erosion control and energy dissipation which will be reused or added as appropriate. All
conveyance systems on site will be designed according to Chapter 4 of the 2022 RSWDM.
II. CONDITIONS AND REQUIREMENTS
This section will address the requirements set forth by the Core and Special Requirements listed in
Chapter 1 of the City of Renton Surface Water Design Manual.
City of Renton Surface Water Design Manual Core Requirements:
1. Discharge at the Natural Location (1.2.1): This project proposes all runoff to be conveyed to
the natural discharge location which is the existing south Pond B.
2. Off-site Analysis (1.2.2): Refer to Sections III and IV. A Level 1 downstream analysis has been
performed.
3. Flow Control (1.2.3.1): Refer to Section IV. The project is in a Peak Flow Control Area. Flow
control is not proposed for this project, as there is no increase in flow during the 100-year event,
versus the original Pond B design, based on coverage assumptions listed with that project’s
permit documentation from Sverdrup to the City.
4. Conveyance System (1.2.4): Conveyance analysis for all storm systems will be performed in
accordance with the current version of the City of Renton Surface Water Design manual and will
be provided during permitting.
5. Erosion and Sedimentation Control (1.2.5): Sediment controls to address the specific
conditions at the site will be provided during future permits.
6. Maintenance and Operations (1.2.6): An Operations and Maintenance Manual will be provided
during permitting.
7. Financial Guarantees and Liability (1.2.7): UNICO Sounders and their contractor will obtain all
necessary permits and bonds prior to the beginning of construction.
8. Water Quality (1.2.8): Refer to Section IV. Water quality treatment for runoff from target pollution
generating surfaces will be provided by a combination of BioPods and/or Filterra Bioscape open
filtration system. See Section IV for more information.
9. Onsite BMP (1.2.9): Refer to Section IV. Onsite BMP’s have been addressed to the maximum
extent possible. See Section IV for more information.
SPECIAL REQUIREMENTS:
Special Requirement #1. Other Adopted Area-Specific Requirements Section 1.3.1
• Critical Drainage Areas (CDAs): Not Applicable
• Master Drainage Plans (MDPs): The Development Agreement for the Boeing Longacres
Masterplan provides a basis for the original drainage requirements for this property. While the
Development Agreement is no longer active, this document is provided to provide context on the
existing drainage system, its original compliance with the Peak Flow runoff requirement on this
campus and showing that the proposed improvements do not warrant the need for additional
detention.
• Basin Plans (BPs): The project is located within Black River Drainage Basin.
• Salmon Conservation Plans (SCPs): Not Applicable
• Lake Management Plans (LMPs): Not Applicable
• Shared Facility Drainage Plans (SFDPs): Not Applicable
Special Requirement #2. Flood Hazard Area Delineation, Section 1.3.2: See Figure 8 for 100-yr flood
zone and the table below. Filling is planned for within the 100-yr flood zone, with a total 1,471,979 cubic
feet of fill planned between existing grade and the 20 foot 100-yr flood elevation. Compensatory volume
is being proposed generally within the footprint of the proposed buildings from elevation 11.7, which is
the dead storage elevation line of the ponds, to elevation 20.0. In order to ensure non-floodwaters do not
enter the floodplain mitigation chambers several methods may be used. One method may be to set the
minimum depth of the floodplain mitigation chambers at an elevation of 14.7 feet, another may be to
utilize control gates or valves allowing for the chambers to have a base elevation below an elevation of
14.7 feet. We conservatively assume for floodplain analysis that Pond B would be at full detention when
a flood occurs, thus we have set the base elevation for the flood to start in our basin as 14.7.
The proposed compensatory flood mitigation systems via underground storage, should not result in net
loss of ecological function in this property. Please also see “Response to City Comments SW Impacts 4-
25-23” in Appendix C for a memo from Talasaea confirming no anticipated net loss of ecological
function related to the land use process that was approved for the soccer fields with the similar approach
of underground storage facilities functioning as flood volume.
The Longacres site experiences backwater floods, meaning as the creek level rises the floodwater
backs-up on to our property. The site is protected at several locations with salmonid screens downstream
of our location, and we will provide fine mesh screens on all discharge pipes if desired by the City, as
was done on the recent Sounders project. This will help to ensure no fish are trapped following the flood
in any of the facilities existing or proposed on site. The floodplain mitigation we are providing, following
standard practices, provides more storage of floodwaters at lower elevations than the site currently
contains. This lessens the impacts on the environment in the earlier stages of the flood while allowing for
our site to maintain the same floodplain retention at the 100-year flood as it historically has.
Additionally, this project meets the requirements of the Code of Federal Regulations:
CFR 60.3(b): Is not applicable as the area (Zone AE) has a published identified flood elevation of 20.0 ft.
CFR 60.3(c)(3): The project meets this requirement as all FFE’s are 1.0 feet above the base flood
elevation.
CFR 60.3(c)(10): We are providing floodplain mitigation to fully offset any proposed fill inside the
floodplain by this development.
Flood Mitigation Table Summary Results
Special Requirement #3. Flood Protection Facilities, Section 1.3.3: Compensatory storage to offset the
proposed fill within the floodplain, is proposed as described in SR#2 above.
Special Requirement #4. Source Controls, Section 1.3.4: See attached Activity Worksheet and
Required BMP’s.
Special Requirement #5. Oil Control, Section 6.1.5. Minimal traffic is anticipated in this area. No oil
control is required.
Special Requirement #6. Aquifer Protection Area. Section 1.3.6: Not included as a special requirement
in Section 1.3 of the 2009 King County Surface Water Design Manual.
PROJECT SPECIFIC REQUIREMENTS:
There are no applicable project specific requirements that we are award of outside of those proposed
under this EIS and subsequent findings listed as conditions, if any . Design and construction will abide by
the requirements set forth in these documents.
III. OFF-SITE ANALYSIS
TASK 1 – STUDY AREA DEFINITION AND MAPS
The Renton drainage basin map was used to verify that the site was fully in the Black River Drainage
Basin (See Figure 5 – Drainage Basin Map).
TASK 2 - RESOURCE REVIEW
a) Adopted Basin Plans: Black River Drainage Basin
b) Floodplain/floodway (FEMA) Maps: Site is located in the floodplain (See Figure 8)
c) Off-site Analysis Drainage System Table: See Figure 11, Reference 8-B
d) Sensitive Areas Folio: Sensitive Areas are present on the site. See
Figures 3A-3D and the Wetland Analysis
Report by Talasaea Consultants, provided separately
e) Drainage Complaints and Studies: No Drainage Complaints. See Figure 10.
f) Road Drainage Problems: No Current Road Drainage Problems.
g) King County Soils Survey: See Figure 3A – USGS Soil Map
h) Wetland Inventory Maps: See Wetland Analysis Report by
Talasaea Consultants, provided separately
i) Migrating Rivers Study: Not Applicable.
j) DOE’s Section 303d List of Polluted Waters: The Black River Drainage Basin is not identified
in Section 303d List of Polluted Waters.
k) KC Designated Water Quality Problems: Not Applicable.
l) City of Renton Critical Maps: See Figures 3A-3D.
TASK 3 - FIELD INSPECTION
Coughlin Porter Lundeen conducted site visits in preparing the project plans. Please refer to Task 4,
Downstream Analysis below for more information.
TASK 4 - DRAINAGE SYSTEM DESCRIPTION AND PROBLEM
DESCRIPTIONS
Runoff from the site will be conveyed through new and existing storm elements that discharge to the
existing on-site Pond B. A series of storm conveyance pipes and structures are proposed to collect and
convey runoff in the pollution generating impervious roadways and will discharge to Pond B at two
locations. See Appendix A, Figure 9 – Downstream Analysis.
Upstream Analysis
The site is within the Black River and specifically the Springbrook Creek drainage basin. This site is
located generally at the high point of the area topography, with a limited area of train track embankment
to the west that drains onto this site perimeter. Runoff from this area will be directed north or south as it
currently does into offsite areas. To the east is Oakesdale Avenue, which is improved and provides no
runoff to the site. To the south is Kaiser Permanente campus property which drains north through this
property via a pond and ditch that connect to the existing Pond B as this project will also do. See Figure
5 – Drainage Basin Map, for reference.
Downstream Analysis
As delineated in red on Figure 9 – Downstream Analysis, runoff from the site will be conveyed to a
series of BioPods or Bioscapes, then into the existing ponds on site. The site discharges to the managed
stormwater system within the Longacres property which was sized and constructed in anticipation of
receiving un-detained and un-treated runoff from this site. That system drains to Springbrook Creek in
accordance with the Development Agreement and associated Master Drainage Plan for the Longacres
Office Park (previously permitted by the City and State). Springbrook Creek, in turn, drains to the Black
River and, ultimately, the Duwamish River and Puget Sound. No known drainage problems have been
reported with the existing conveyance system beyond routine maintenance such as at the City’s
Oakesdale Avenue crossing culvert which frequently becomes clogged with debris based on field
reconnaissance and anecdotal conversations in the past with Boeing maintenance personnel.
Task 5 – Mitigation of Existing or Potential Problems
The proposed project will slightly increase the originally permitted amount of impervious area, which was
anticipated in the design for the current on-site ponds, A and B. This increase of impervious area will be
mitigated by level 1 detention. The historic ground cover of this site, since before 1929 when an aerial
photograph shows the presence of the original Longacres racetrack, with paddocks, maintenance yards,
etc, has consisted of primarily asphalt, gravel, heavily compacted till soils, and sparse areas of
compacted “lawn” as described by the RSWDM, adopted from King County SWDM. While the existing
ponds and drainage systems on site were intended to be the sole water quality and detention systems
needed for a fully developed site, the current Sounders facility project will be installing water quality
systems in accordance with the 2022 RSWDM. A series of BioPods and Bioscapes are proposed to treat
any pollution generation surfaces prior to discharging to Pond B. These systems will be sized to meet
Ecology GULD requirements for treatment associated with the Enhanced Basic level as re quired for this
project.
IV. FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS
AND DESIGN
EXISTING SITE HYDROLOGY (PART A)
The 138-acre site currently consists of a mixture of ponds and wetlands, grassed, and paved areas. The
site slopes generally south-southeast to north-northwest with about a 3-foot elevation change throughout
most of the area. Larger grade differences exist in localized areas within the site, especially along the
eastern and southern edges of the proposed roadway adjacent to Wetland A and Pond B’s side-slopes.
The preexisting site area conditions, Boeing 1995, constructed prior to the Sounders Practice Fields is
shown in the top half of Table 1 and Figure 4A, with the current Post Sounders Practice Field
Development shown in the bottom half of Table 1 and Figure 4B. These tables illustrate
1. The City, Ecology and USCOE approval from 1995 for the allowable coverage related to the
constructed stormwater detention facilities (Wetland A and Pond B) that were used to comply
with the EIS and Development Agreement that has governed the developed Boeing property.
2. The Unico/Sounders development of the soccer fields, which followed the foundational 1995
Level 1 Peak Flow matching compliance. As shown
The two historical developments dictate the existing conditions, which are shown in agreement with the
Sounders development project. Additional flow control was not required by the development of the
Sounders project as the project remained in excess the 29.2% Pervious Area set by the B oeing 1995
project that created the campus-wide detention and flood storage systems as required for Level 1
Detention, which is still the standard required for the site.
Table 1 - Existing Site Conditions Area Breakdown
BOEING 1995 CONDITIONS
(SF) (AC) PERCENTAGE OF SITE
TOTAL PERVIOUS 3,208,998 73.67 59.62
TOTAL IMPERVIOUS 2,173,502 49.90 40.38
TOTAL PONDS/WETLANDS 654,235 15.02 -
TOTAL AREA 6,036,735 138.59 -
CURRENT LONGACRES SITE CONDITIONS
(FOLLOWING SOUNDERS PRACTICE FIELD DEVELOPMENT)
(SF) (AC) PERCENTAGE OF SITE
TOTAL PERVIOUS 2,830,907 64.99 52.59
TOTAL IMPERVIOUS 2,551,593 58.58 47.41
TOTAL PONDS/WETLANDS 654,235 15.02 -
TOTAL AREA 6,036,735 138.59 -
DEVELOPED SITE HYDROLOGY (PART B)
The developed site will closely resemble the original land use coverage allowed under the original
Boeing Development Agreement and permit approval by the listed agencies. As described above, the
historic land cover pre-Boeing of the former Longacres Horse Racing site was mostly impervious, with
patches of grass and a smaller wetland area in the center of the infield. See Figure 4C–Historical
Conditions and Figure 4D–Historical Conditions Surface Area Correlation.
This current new Longacres Master Plan project will consist of 84 acres of impervious area including
existing and proposed buildings, sidewalks, and roadways and 40 acres of pervious landscape area.
These totals also include the added hardscape and/or replaced on the site. See Figure 6– Proposed
Conditions, for reference. The proposed development maintains a Pervious Area greater than 29.2% of
the total area, thus no additional flow control will be required. The addition of the new buildings,
roadways, and related site improvements will result in similar site hydrology and coverage limits that the
existing Longacres (formerly Boeing) drainage systems were designed to accommodate, thus complying
with the Peak Flow matching code requirement.
These values will be tracked though the proposed development, as will be required in the Development
Agreement, and if future design and development reduces the pervious area below the 29.2% threshold
additional flow control will be analyzed and provided at that time, related to the specific site permit project
causing the increased coverage. Table 2 below, summarizes the Master Plan land cover proposed.
TABLE 2 - DEVELOPED SITE CONDITIONS AREA BREAKDOWN
UNICO PROPOSED DEVELOPMENT
(SF) (AC) PERCENTAGE OF SITE
TOTAL PERVIOUS 1,748,613 40.14 32.33
TOTAL IMPERVIOUS 3,659,638 84.01 67.67
TOTAL PONDS/WETLANDS 628,464 14.43 -
TOTAL AREA 6,036,715 138.58 -
Note the percentages of the site do not include the wetted areas of the various wetland/ponds in either
the existing or proposed conditions summary table. The main bodies of water, Wetland A and Pond B’s
footprint will change from the existing to proposed condition, mostly due to the elimination of the Kaiser
Pond and incorporating it into Pond B’s footprint. Before the existing pond is removed, the associated
stormwater detention and floodplain mitigation volumes will be accounted for and replaced within the
property and basin limits. This work will be detailed and permitted under the specific development project
that results in these changes, so that confirmation of continuity of coverage and function, can be
confirmed.
PERFORMANCE STANDARDS (PARTS C)
According to Section 1.2.3.1 (See Figure 7 - Flow Control Application Map) of the 2022 Renton Surface
Water Design Manual (RSWDM), this project is located within the Peak Flow Control area. This project
lies within the tributary area of an existing detention facility developed as part of the 1998 Boeing Site
Development Project. According to the Longacres Land Use Permits analysis conducted in 2009 by
Perkins Cloe and Affiliates, approximately 65 percent of the 158-acre Longacres site is allocated for
impervious improvement. See Appendix C for a copy of this analysis. An additional 35 percent of the
site is dedicated to the detention systems and associated landscaping and open space. Removing the
detention pond surface area, the remaining site has a 70.8% allocation for impervious area. The intention
of the LAMP developments is to remain below this impervious threshold. The current targeted impervious
percentage of 73% will be reduced to meet the 70.8% threshold by utilizing practices including but not
limited to additional green area, green roofs, and added detention measures. See Appendix B Figure
23 - Pond B Hydrology Analysis for the site coverage calculations.
The Longacres site is approximately 158 acres in size. The proposed Longacres site will be designed to
meet the 70.8%, threshold as designed by Sverdrup in 1998. However, since the proposed master plan
exceeds this allocation, any area over the 70.8% shall be detained by a level 1 detention system .
The Longacres site as proposed by the 1998 report can be considered one drainage basin with two sub -
basins generally draining to either Pond A or Pond B. See Figure 12C – Longacres Existing Drainage
Basin. These ponds are hydraulically linked and are both considered detention ponds. While the site
does have separate sub-basins, the 1998 report should be viewed as a generalized study of a 158-acre
site with future developments needing to look at the specifics of modifications to existing drainage. This
project does not modify the drainage patterns of the site.
To mitigate the impacts of storm and surface water runoff generated by new impervious surface onsite
BMP’s are proposed. Per Section 1.2.9 of the 2022 Renton Surface Water Design Manual (RSWDM) for
implementation of BMPs, an evaluation of the BMPs was done. Since the proposed roadways represent
Pollution Generating Impervious Surfaces (PGIS) as defined by the RSWDM, a combination of BioPods
and Filterra Bioscape filtration systems, which both conform to State Ecology GULD requirements for
Enhanced Basic criteria, is proposed.
ON-SITE BEST MANAGEMENT PRACTICES (ON -SITE BMPS) PART D
Onsite BMP’s have been assessed for this project to mitigate the impacts of storm and surface water
runoff generated by new impervious surfaces, existing impervious surfaces, and replaced impervious
surface targeted for mitigation. Due to the till soils on site, the LID performance standard does not need
to be met and bioretention is being implemented for the development as a BMP. However, all other
BMP’s have been assessed to the maximum extent possible per section 1.2.9 of the 2022 Renton
Surface Water Design Manual (RSWDM). See assessment below.
On-site BMPs Assessment
SECTION
REFERENCE BMP DESCRIPTION ACTION
C.2.1 Full Dispersion Full Dispersion is infeasible due to the lack of vegetated area
between project improvements and existing pond and wetland.
C.2.2 Full Infiltration Full Infiltration is infeasible because the soil types found on site are
not conducive to infiltration, per the Geotechnical Analysis.
C.2.3 Limited Infiltration To be conservative, no infiltration is included in any design
assumptions for stormwater design.
C.2.4 Basic Dispersion
Basic Dispersion is infeasible because there is not sufficient length
to install a dispersion trench, and there are no locations where a
“vegetated flow path segment” of at least 25-feet can be created.
C.2.5 Farmland Dispersion This BMP does not apply to the City of Renton.
C.2.6 Bioretention Bioretention is infeasible because the soil types found on site are
not conducive to infiltration, per the Geotechnical Analysis.
C.2.7 Permeable Pavement Permeable Pavement is infeasible because the soil types found on
site are not conducive to infiltration, per the Geotechnical Analysis.
C.2.8 Rainwater Harvesting Rainwater Harvesting is a potential future consideration when
building design is permitted.
C.2.9
Reduced Impervious
Surface Credit
Reduced Impervious Surface Credit requirements will not be met
with this project. Due to the nature of most of the over 70%
impervious surfaces this credit is infeasible.
C.2.10
Native Growth
Retention Credit
Native Growth Retention Credit is infeasible as we are unable to
provide 3.5 sf of native vegetated pervious surface for every 1 sf of
new impervious surface on the site.
C.2.11 Perforated Pipe
Perforated Pipe has been deemed infeasible as native soil types on
site are not conducive to infiltration and close proximity to the
wetland buffer.
C.2.12 Rain Gardens Rain Gardens infeasible because the soil types found on site are
not conducive to infiltration, per the Geotechnical Analysis.
C.2.13 Soil Amendment Soil Amendment is feasible and has been proposed for the site. All
soil within the wetland buffer will remain wherever possible.
SECTION
REFERENCE BMP DESCRIPTION ACTION
C.2.14 Tree Retention Credit All trees on site outside the extents of construction are to be
retained or not retained per the Arborist report.
C.2.15 Vegetated Roofs Vegetated Roofs are a potential future consideration when
building design is permitted.
WATER QUALITY SYSTEM (PART E )
Section 1.2.8.1(A) of the 2022 RSWDM outlines the specific land uses within Basic Water Quality
Treatment areas which are subject to providing Enhanced Basic Water Quality treatment. Because the
project is a commercial site, it is subject to Enhanced Basic Treatment for all new and replaced pollution -
generating impervious surfaces (PGIS). This treatment will be provided by the combination of BioPods
and Filterra Bioscape systems, both of which hold a General Use Level Designation (GULD) from the
Washington Department of Ecology for enhanced basic treatment and are deemed suitable for this
application. See Appendix E for BioPod and Filterra Bioscape GULD Approvals. The water quality
systems will be installed to treat runoff prior to constructing the entire newly developed site, and then
discharge into the existing Large Wetpond, that is Pond B. A total of 307,913 square-feet (7.07 acres) of
pollution generating impervious surface will require treatment.
All water quality facilities will be designed and sized in accordance with the standards outlined in the
2022 RSWDM and submitted for permitting.
V. CONVEYANCE SYSTEM ANALYSIS AND DESIGN
This section discusses the criteria that will be used to analyze and design the proposed storm
conveyance system.
STANDARD REQUIREMENTS (BASED ON 2022 RSWDM AND SAO):
1. Facilities must convey the 100-year flow without overtopping the crown of the roadway,
flooding buildings, and if sheet flow occurs it must pass through a drainage easement . All
stormwater conveyance will be designed such that the 100-year flow is conveyed without
flooding any nearby buildings.
2. New pipe systems and culverts must convey the 25-year flow with at least 0.5 feet of
freeboard. (1.2.4.1). The new pipe system proposed for this site will be designed to convey the
25-year flow. See Conveyance Calculations under Appendix B. These new 12” pipes will meet
the conveyance requirement per Section 1.2.4.1. of the 2022 RSWDM.
3. Bridges must convey the 100-year flow and provide a minimum of two feet, varying up to
six feet, of clearance based on 25% of the mean channel width . (1.2.4-2)(4.3.5-6). This
project does not propose a bridge.
4. Drainage ditches must convey the 25-year flow with 0.5 feet of freeboard and the 100-year
flow without overtopping. (1.2.4-2). This project does not propose open channel drainage
ditches.
5. Floodplain Crossings must not increase the base flood elevation by more than 0.01 feet
[41(83.C)] and shall not reduce the flood storage volume [37(82.A)]. Piers shall not be
constructed in the FEMA floodway. [41(83.F.1)]. The project will place 1,463,346 cubic feet of
fill inside the floodplain. Floodplain mitigation, as is required, will be provided through the use of
concrete vaults under proposed buildings, in arched storage chambers, or other industry
standard methods. The Sounders project, built in 2023, also provided floodplain mitigation and
has 7,200 CF that can provide future construction inside the floodplain by the Longacres
Masterplan project, if needed and with the agreement of the Seattle Sounders organization.
Per the Federal Emergency Management Agency Flood Insurance Study Number 53033CV001B
Appendix C, the 100 year flood flow at Springbook Creek/Black River and Green River
confluence is 1,230 CFS at the pump station. This location is downstream of our site but will be
used for analysis as conservative. The basin that floods contributing to this point covers 21.9
square miles (14,016 acres). Each floodplain mitigation chamber shall be designed with no fewer
than two points of connection to the floodplain. The points of connection will be designed to
provide a sufficient flow between the ponds and the mitigation chambers. The flow rate will be
calculated proportionate to the volume being mitigated with a conservative factor of safety
applied.
6. Stream Crossings shall require a bridge for class 1 streams that does not disturb or
banks. For type 2 and type 3 streams, open bottom culverts or other method may be used
that will not harm the stream or inhibit fish passage. [60(95.B)]. The project does not
propose a stream crossing.
7. Discharge at natural location is required and must produce no significant impacts to the
downstream property (1.2.1-1). The project will discharge to the existing Pond B and Wetland
A in the existing Longacres site. As originally designed under the permit for those improvements
that exist on-site discharge will continue into these areas and convey down to Springbrook
Creek.
ON-SITE CONVEYANCE
Existing Conditions:
Stormwater runoff currently flows to the existing ponds that are central to the site. This runoff then flows
north and northeast through the existing Longacres pond and channel system, under Oakesdale Avenue
in a City culvert, to Springbrook Creek, then ultimately to the Green River and Puget Sound.
Developed Storm System Description:
The project will redevelop approximately 42 acres of the existing site, which is currently impervious
parking and gravel areas and pervious landscape areas with meandering sidewalk paths, will include 15
new building structures that are connected by roadways. These new roads and their adjacent walkways
will drain into a system of catch basins, which will be routed to conveyance piping sized to accommodate
the 25-year storm as required by the RSWDM. These conveyance pipes will flow to the proposed
BioPods and Bioscapes for water quality treatment and will then discharge into the existing Pond B and
Wetland A combination Detention and Water Quality Ponds, as shown in the documents.
Conveyance analysis for all storm systems will be performed in accordance with the current version of
the City of Renton Surface Water Design manual and will be provided under future permits.
VI. SPECIAL REPORTS AND STUDIES
DRAFT TO BE ADDED UPON RECIEPT
VII. OTHER PERMITS
This project will require a City of Renton Civil Construction permit, City of Renton Conditional Use or SPE
approval, and an NPDES permit for each portion of work. An associated Lot Line Adjustment has been
prepared and recorded to correspond with the project work extents.
Building permits are required for fences over 6 feet, maintenance building, and possibly other features.
VIII. CSWPPP ANALYSIS AND DESIGN
This section lists the requirements that will be used when designing the TESC plan for this site. The
CSWPPP will be provided for permitting.
STANDARD REQUIREMENTS
Erosion/Sedimentation Plan shall include the following:
1. Clearing Limits
a. Clearing limits are noted on plans and will be implemented prior to any offsite impacts or
damage due to construction.
2. Cover Measures
a. During construction, temporary cover BMPs will be implemented to prevent erosion. The
project will meet wet season cover requirements.
3. Perimeter Protection
a. Silt Fence will be implemented around construction limits to preserve undisturbed areas.
4. Traffic Area Stabilization
a. The Contractor, per the plans, shall implement necessary BMP measures to ensure
sediment does not leave the site onto streets or adjacent properties.
5. Sediment Retention
a. Inlet protection will be implemented to prevent sediment from entering the existing
drainage system.
6. Surface Water Collection
a. Existing stormwater catch basins on-site that are proposed to remain will continue to
operate during construction to collect runoff.
7. Dewatering Control
a. Dewatering needs shall be monitored throughout construction.
8. Dust Control
a. Dust control BMPs will be implemented throughout construction.
9. Flow Control
a. Flow control is not required for the project as there will be no increase in impervious
surface coverage beyond the existing approved system’s basis of design.
10. Control Pollutants
a. BMPs will be implemented to prevent or treat contamination of stormwater runoff by pH
modifying sources. In addition, all waste materials from the site will be removed in a
manner that does not cause contamination of water.
11. Protect Existing and Proposed Stormwater facilities and On-Site BMPs
a. Inlet protection BMPs will be implemented on the site for existing and proposed catch
basins affected by construction.
12. Maintain Protective BMPs
a. BMPs listed in the SWPPP shall be maintained as needed through the project. As
portions of the project get completed, portions of the established BMPs shall be adjusted
to other areas of the project site until their completion.
13. Manage the Project
a. Proposed erosion and sediment control measures will be implemented throughout
construction.
ESC PLAN ANALYSIS AND DESIGN (PART A)
This will be provided in future permit submittals related to construction work phase.
SWPPS PLAN DESIGN (PART B)
The CSWPPP will be included in future permit submittals related to construction work phase.
IX. BOND QUANTITY, FACILITY SUMMARIES, AND DECLARATION
OF COVENANT
A Bond Quantity Worksheet will be provided for permitting.
X. OPERATION AND MAINTENANCE MANUAL
An Operations and Maintenance Manual will be provided for permitting.
Appendix A – Figures
Figure 1 Technical Information Report Worksheet
Figure 2 Vicinity Map
Figure 3A USGS Soil Survey
Figure 3B Wetlands and Water Classifications
Figure 3C Erosion Hazard
Figure 3D Flood Hazard
Figure 4A Existing Conditions – BOEING 1995
Figure 4B Existing Conditions – Post Sounders Practice Field Development
Figure 4C Historical Conditions
Figure 4D Historical Conditions Surface Area Correlation
Figure 5 Drainage Basin map
Figure 6 Proposed Conditions
Figure 7 Flow Control Application Map
Figure 8 FEMA FIRM Map
Figure 9 Downstream Analysis
Figure 10 Drainage Complaints
Figure 11 RSWDM Reference 8-B
Figure 12A Pond B Flow Control Facility
Figure 12B Pond B 24 Inch Outfall
Figure 12C Longacres Existing Drainage Basins
King County Department of Development and Environmental Services
TECHNICAL INFORMATION REPORT (TIR) WORKSHEET
Part 1 PROJECT OWNER AND
PROJECT ENGINEER
Part 2 PROJECT LOCATION
AND
DESCRIPTION
Project Owner:
UNICO
Project Name:
Longacres Master Plan
Address:
2nd + Seneca Building
1191 Second Avenue Suite 1100
Seattle, WA 98101
Location
Phone:
206.343.0460
Township: 23N
Range: 4E
Section: 24 Project Engineer:
Tim Brockway, PE
Company: Coughlin Porter Lundeen
Address/Phone:
801 Second Avenue, Ste 900
Seattle, WA 98104
(206) 343-0460
Part 3 TYPE OF PERMIT
APPLICATION Part 4 OTHER REVIEWS AND PERMITS
Subdivison
Short Subdivision
Grading
Commercial
Other:
DFW HPA Shoreline Management
COE 404 Rockery
DOE Dam Safety Structural Vaults
FEMA Floodplain Other
COE Wetlands
Part 5 SITE COMMUNITY AND DRAINAGE BASIN
Community:
Drainage Basin:
Black River Drainage Basin
Part 6 SITE CHARACTERISTICS
River
Stream
Critical Stream Reach
Depressions/Swales
Floodplain Limited
Wetlands
Seeps/Springs
High Groundwater Table
FIGURE 1
Technical Information Report Worksheet
Lake
Steep Slopes
Groundwater Recharge
Other
Part 7 SOILS
Soil Type Slopes Erosion Potential Erosive Velcoties
Ur <15% Low
Additional Sheets Attached
Part 8 DEVELOPMENT LIMITATIONS
REFERENCE LIMITATION/SITE CONSTRAINT
Ch.4 – Downstream Analysis None
Additional Sheets Attached
Part 9 ESC REQUIREMENTS
MINIMUM ESC REQUIREMENTS
DURING CONSTRUCTION
MINIMUM ESC REQUIREMENTS
AFTER CONSTRUCTION
Sedimentation Facilities
Stabilized Construction Entrance
Perimeter Runoff Control
Clearing and Graing Restrictions
Cover Practices
Construction Sequence
Other
Stabilize Exposed Surface
Remove and Restore Temporary ESC
Facilities
Clean and Remove All Silt and Debris
Ensure Operation of Permanent Facilities
Flag Limits of SAO and open space
preservation areas
Other
FIGURE 1
Technical Information Report Worksheet
Part 10 SURFACE WATER SYSTEM
Grass Lined
Channel
Pipe System
Open Channel
Dry Pond
Wet Pond
Tank
Vault
Energy Dissapator
Wetland
Stream
Infiltration
Depression
Flow Dispersal
Waiver
Regional
Detention
Method of Analysis
Compensation/Mitigati
on of Eliminated Site
Storage
Brief Description of System Operation
The project proposes to construct a drainage system with a new network of underground pipes,
catch basins, curbs and gutter, to collect surface water runoff throughout the site and direct it to a
combined detention wetpond located on Boeing's Tract B for both water quality treatment and
detention.
Facility Related Site Limitations
Reference Facility Limitation
N/A
Part 11 STRUCTURAL ANALYSIS Part 12 EASEMENTS/TRACTS
Cast in Place Vault
Retaining Wall
Rockery > 4’ High
Structural on Steep Slope
Other
Drainage Easement
Access Easement
Native Growth Protection Easement
Tract
Other
Part 13 SIGNATURE OF PROFESSIONAL ENGINEER
I or a civil engineer under my supervision my supervision have visited the site. Actual site
conditions as observed were incorporated into this worksheet and the attachments. To the best of
my knowledge the information provided here is accurate.
Signed/Date
FIGURE 1
Technical Information Report Worksheet
20
1
4
Project Site
FIGURE 2 - Vicinity Map
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Reference 15-C
Renton City Limits
Potential Annexation Area
Groundwater Protection Area Boundary
Aquifer Protection Area Zone 1
Aquifer Protection Area Zone 1 Modified
Soil Type
AgB
AgC
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Date: 01/09/2014
0 1 2MilesµSoil Survey
Project Location
Figure 3A - Soil Survey Map
15,8402640
Figure 4B - Wetlands and Water Classifications
All data, information, and maps are provided "as is" without warranty or any representation of accuracy, timeliness of
completeness. The burden for determining accuracy, completeness, timeliness, merchantability and fitness for or the
appropriateness for use rests solely on the user.
WGS_1984_Web_Mercator_Auxiliary_Sphere
Notes Legend
1795 898 Feet17950
City and County Labels
City and County Boundary
Renton
<all other values>
Environment Designations
Natural
Shoreline High Intensity
Shoreline Isolated High Intensity
Shoreline Residential
Urban Conservancy
Jurisdictions
Streams (Classified)
S - Shoreline
F - Fish
Np - Non-Fish
Ns - Non-Fish Seasonal
Unclassfied
Wetlands
Streets
Parks
Waterbodies
Designated Neighborhoods
2021.sid
Red: Band_1
Green: Band_2
Blue: Band_3
Figure 3B - Wetlands and Water Classifications
57,6189603
Figure 4D - Erosion Hazard
All data, information, and maps are provided "as is" without warranty or any representation of accuracy, timeliness of
completeness. The burden for determining accuracy, completeness, timeliness, merchantability and fitness for or the
appropriateness for use rests solely on the user.
WGS_1984_Web_Mercator_Auxiliary_Sphere
Notes Legend
6530 3265 Feet65300
City and County Labels
City and County Boundary
Renton
<all other values>
Erosion Hazard - High
Environment Designations
Natural
Shoreline High Intensity
Shoreline Isolated High Intensity
Shoreline Residential
Urban Conservancy
Jurisdictions
Citations
Figure 3C - Erosion Hazard
36,1126019
Figure 4E - Flood Hazard
All data, information, and maps are provided "as is" without warranty or any representation of accuracy, timeliness of
completeness. The burden for determining accuracy, completeness, timeliness, merchantability and fitness for or the
appropriateness for use rests solely on the user.
WGS_1984_Web_Mercator_Auxiliary_Sphere
Notes Legend
4093 2046 Feet40930
City and County Labels
City and County Boundary
Renton
<all other values>
Floodway
Special Flood Hazard Areas (100
year flood)
Citations
Figure 3D - Flood Hazard