22-102032-UP Prelim Land Use TIR with Downstream-5-4-22
Civil Engineers ● Structural Engineers ● Landscape Architects ● Community Planners ● Land Surveyors
Preliminary Technical
Information Report
PREPARED FOR:
Federal Way Public Schools
33330 8th Avenue South
Federal Way, WA 98003
PROJECT:
Illahee Middle School
36001 1st Avenue South
Federal Way, WA 98023
2210536.10
2210134.10PREPARED BY:
Steve Nickison, EIT
Project Engineer
REVIEWED BY:
David Nason, PE
Principal
DATE:
May 2022
Preliminary Technical
Information Report
PREPARED FOR:
Federal Way Public Schools
33330 8th Avenue South
Federal Way, WA 98003
PROJECT:
Illahee Middle School
36001 1st Avenue South
Federal Way, WA 98023
2210536.10
2210134.10PREPARED BY:
Steve Nickison, EIT
Project Engineer
REVIEWED BY:
David Nason, PE
Principal
DATE:
May 2022
I hereby state that this Technical
Information Report for the Illahee Middle
School project was prepared by me or
under my supervision, and meets the
standard of care and expertise that is
usual and customary in this community for
professional engineers. I understand that
the City of Federal Way does not and will
not assume liability for the sufficiency,
suitability, or performance of drainage
facilities prepared by me.
05/02/2022
0
Preliminary Technical Information Report
Illahee Middle School i
2210536.10
Table of Contents
Section Page
1.0 Project Overview ......................................................................................................................... 1-1
Purpose and Scope.......................................................................................................... 1-1
Existing Conditions........................................................................................................... 1-1
Post-Development Conditions ......................................................................................... 1-2
2.0 Conditions and Requirements Summary ................................................................................. 2-1
Core Requirements .......................................................................................................... 2-1
2.1.1 CR 1 - Discharge at the Natural Location ........................................................... 2-1
2.1.2 CR 2 - Offsite Analysis ........................................................................................ 2-1
2.1.3 CR 3 - Flow Control ............................................................................................ 2-1
2.1.4 CR 4 - Conveyance System ................................................................................ 2-1
2.1.5 CR 5 - Erosion and Sediment Control................................................................. 2-1
2.1.6 CR 6 - Maintenance and Operations .................................................................. 2-1
2.1.7 CR 7 - Financial Guarantees and Liability .......................................................... 2-1
2.1.8 CR 8 - Water Quality ........................................................................................... 2-2
2.1.9 CR 9 – Flow Control Best Management Practices (BMPs) ................................ 2-2
Special Requirements ...................................................................................................... 2-3
2.2.1 SR 1 - Other Adopted Area-Specific Requirements ........................................... 2-3
2.2.2 SR 2 - Floodplain/Floodway Delineation ............................................................. 2-3
2.2.3 SR 3 - Flood Protection Facilities........................................................................ 2-3
2.2.4 SR 4 - Source Control ......................................................................................... 2-3
2.2.5 SR 5 - Oil Control ................................................................................................ 2-3
3.0 Conclusion ................................................................................................................................... 3-1
Preliminary Technical Information Report
Illahee Middle School 1-1
2210536.10
1.0 Project Overview
Purpose and Scope
This report accompanies the civil engineering plans and documents for the proposed Illahee
Middle School project. Illahee Middle School is located at 36001 1st Ave S, Federal Way,
Washington. The site is to be accessed via 1st Avenue South. The site is located on Tax Parcel
No. 3021049013 in Section 30, Township 21 North, Range 4 East of the Willamette Meridian, city
of Federal Way, King County, Washington.
The existing of the property consists of an existing middle school, with associated parking,
outdoor student spaces, sports fields, and a running track. The site contains many large trees
throughout the school campus and a significant portion of the site exists as a dense forest. To the
north of the site is SW 360th Street, which is currently a private road. To the north of this road is
an existing single-family residential development. To the east of the site is 1st Avenue South. To
the east of this road are multiple single-family residential developments. To the south of the site is
a single-family residential development. To the west of the site is also single-family
developments, although separated by the existing forest on-site.
The Illahee Middle School project proposes the construction of a new 108,962-square foot middle
school building, along with associated parking areas, service yard, outdoor play areas, outdoor
classroom spaces, garden program areas, a rain garden, and gravel access paths to existing
sports fields. Stormwater generated from the building and landscape areas will be collected via
downspouts, roof drains, and area drains and conveyed to a bioretention facility (rain garden).
Stormwater generated by other impervious surfaces will flow into catch basins to a underground
concrete stormwater vault located in the vicinity of the bus drop off loop and staff parking lot.
The Illahee Middle School property is completely developed within the school and field footprint
and includes paved roadways, parking lots, landscaped areas, and sport/recreation fields and
facilities. The project proposes approximately 332,000 square feet of new impervious surfaces,
including approximately 109,000 square feet of new building area. Existing and proposed
pollution generating surfaces, including fire lanes, will be collected, and treated to meet water
quality standards as explained further below.
The project site is located in the Lower Puget Sound Drainage Basin and drains to the Hylebos
Creek. Approximately 0.9 acres of the forest along the western property line is delineated as
erosion hazard area per the City of Federal Way critical areas map.
All construction proposed has been designed to meet the 2021 King County Surface Water
Design Manual (KCSWDM), as adopted by City Federal Way per the Addendum to the King
County Surface Water Manual (Addendum), January 8, 2017.
Existing Conditions
The property is currently fully developed and consists of the Illahee Middle School campus,
sport/recreational fields and facilities with sidewalks, paved roadways and parking lots, and
landscaped areas. In the existing condition, the site and immediate adjacent areas generally
slope down from north to south, with elevations ranging from 250 to 200.
Stormwater on the existing site sheet flows to existing catch basins and yard drains which route
to a 24” concrete storm pipe which outlets to the city owned stormwater main located in 1 st
Avenue South.
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Illahee Middle School 1-2
2210536.10
Post-Development Conditions
The project proposes to construct a new middle school building, along with associated parking
areas, service yard, outdoor play areas, outdoor classroom spaces, garden program areas, a rain
garden, and gravel access paths to existing sports fields. The new middle school will be located
in the footprint of the existing middle school campus. Stormwater runoff from the roof of the new
building will be collected via downspouts and roof drains and conveyed to a new bioretention / rain
garden facility. Stormwater runoff from the parking lots, fire lane, hardscape play areas, and
sidewalks will sheet flow to the catch basins and yard drains which will route to a proposed
underground concrete stormwater vault.
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Illahee Middle School 2-1
2210536.10
2.0 Conditions and Requirements Summary
Core Requirements
2.1.1 CR 1 - Discharge at the Natural Location
The project site currently slopes from north to south. Stormwater drains by means of overland
flow sheet flow to existing catch basins. The catch basins are connected to an enclosed pipe
network that conveys stormwater to 1st Avenue South.
In the developed condition, stormwater from the building roof will be conveyed to a bioretention
facility (rain garden) that will route to the new stormwater detention system. Runoff generated by
other hard surfaces will be collected via catch basins and also route to the proposed detention
system. Stormwater release will be delayed through a control structure and discharge to the
existing 24” RCP pipe which drains to 1st Avenue South.
Stormwater discharge created by the proposed improvements will not adversely affect
downstream properties or drainage systems.
2.1.2 CR 2 - Offsite Analysis
An offsite analysis is required for this project and is discussed in Section Error! Reference
source not found..
2.1.3 CR 3 - Flow Control
This project proposes approximately 223,000 square feet of new impervious surfaces. The
proposed underground concrete stormwater detention vault is provided 140,000 CF of storage to
detain stormwater to the required level 2 performance standard.
2.1.4 CR 4 - Conveyance System
The proposed conveyance systems for this project are composed of underground piped. The new
conveyance system will be designed to convey stormwater in conformance with section 1.2.4.1
KCSWDM.
2.1.5 CR 5 - Erosion and Sediment Control
An erosion and sediment control plan was developed for this site in accordance with the
KCSWDM and to conform to the requirements of the National Pollutant Discharge Elimination
System (NPDES) Construction Stormwater General Permit, issued by the Department of Ecology
(DOE).
2.1.6 CR 6 - Maintenance and Operations
The onsite drainage facilities will be privately maintained by Federal Way Public Schools. An
Operations and Maintenance Manual is provided in Section Error! Reference source not
found..
2.1.7 CR 7 - Financial Guarantees and Liability
The owner and contractor will obtain all necessary permits required by City of Federal Way prior
to the beginning of construction. Federal Way Public Schools is exempt from typical bonding
requirements because it is a public agency. Because of this, a Bond Quantity Worksheet is not
required and has not been prepared.
Preliminary Technical Information Report
Illahee Middle School 2-2
2210536.10
2.1.8 CR 8 - Water Quality
The proposed project includes the construction of a student drop off loop and visitor parking lot,
bus loop and staff parking lot, and fire lanes around the proposed building. Runoff from a ll
proposed PGIS surfacing will be collected in catch basins and routed to a stormwater water
quality treatment vault, providing treatment in compliance with the enhanced basic water quality
standard required per the KCSWDM.
The City of Federal Way requires that existing pollution generating surfacing be upgraded to meet
current water quality standards. Existing areas on-site that will be upgraded are the existing track
and field, existing baseball field, existing portables parking lot to the northwest of the proposed
school, and the existing staff parking lot at the northeast corner of the site. These areas will be
connected to new underground water quality treatment vaults providing treatment in compliance
with he enhanced basic water quality standard required per the KCSWDM.
2.1.9 CR 9 – Flow Control Best Management Practices (BMPs)
The Illahee Middle School site is classified as a Large Lot BMP per Section 1.2.9.2.2 of the
KCSWDM. Below is a summary of the flow control BMPs that were considered.
1. Full Dispersion – Full dispersion will be implemented where proposed impervious and
pervious areas can be feasibly drained into existing forested area on-site. This is from the
proposed courtyard area to the west of the proposed school to the existing forest to the
west of the proposed school.
2. Full Infiltration & Limited Infiltration – Onsite soils consist of glacial till, which is not suitable
for stormwater infiltration. Some small deposits of recessional outwash exist on-site,
however these areas are underlain with till soils. These infiltration BMPs are infeasible
because of the poorly infiltrating soils.
3. Basic Dispersion – Where feasible, basic dispersion will be implemented to drain roof areas
to adjacent native forest areas. These may be upstaged by full dispersion BMPs within the
same area.
4. Bioretention – A bioretention cell is proposed to manage stormwater from the building roof.
5. Permeable Pavement - Onsite soils consist of glacial till, which is not suitable for
stormwater infiltration. This deems permeable pavement infeasible for this project.
6. Rainwater Harvesting – This BMP may be used for in conjunction with the school’s
gardening program. This will be confirmed with the district.
7. Reduced Impervious Surface Credit – The sub-criteria to this BMP are not applicable to the
project and this credit will not be pursued.
8. Native Growth Retention Credit – Areas on non-pollution generating hardscape within the
play area and courtyard area which can be fully dispersed into native vegetated areas may
be considered for Native Growth Retention Credit.
9. Perforated Pipe Connection – Onsite soils consist of glacial till, which is not suitable for
stormwater infiltration. This deems perforated pipe connections infeasible for this project.
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Illahee Middle School 2-3
2210536.10
Special Requirements
2.2.1 SR 1 - Other Adopted Area-Specific Requirements
To our knowledge, no other adopted area-specific requirements are applicable to the project site.
2.2.2 SR 2 - Floodplain/Floodway Delineation
Federal Emergency Management Agency (FEMA) Flood Insurance Rate Map No. 53033C125G
was reviewed for this project. It indicates that the project site is within a Zone X area. This area
is determined to be outside of the 500-year floodplain. Refer to Figure 2-1 for the Flood
Insurance Rate Map.
2.2.3 SR 3 - Flood Protection Facilities
The project site does not contain, nor is it adjacent to, any existing flood protection faciliti es.
Project improvements do not include flood protection measures.
2.2.4 SR 4 - Source Control
The King County Stormwater Pollution Prevention Manual (KCSPPM) will be referenced for
source control measures, in addition to erosion and sediment control measures, during
construction.
2.2.5 SR 5 - Oil Control
The project site is not considered a high-use site. Therefore, it is not subject to oil control
requirements
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Illahee Middle School 3-1
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3.0 Conclusion
The Illahee Middle School site was designed to meet the 2021 King County Surface Water
Design Manual (KCSWDM), as adopted by the City Federal Way per the Addendum to the King
County Surface Water Manual (Addendum), January 8, 2017. The site incorporates detention,
conveyance, bioretention, and water quality devices to manage stormwater generated from the
site.
This analysis is based on data and records either supplied to or obtained by AHBL. These documents
are referenced within the text of the analysis. The analysis was prepared using procedures and practices
within the standard accepted practices of the industry. We conclude that this project, as schematically
represented, will not create any new problems within the downstream drainage system. This project will
not noticeably aggravate any existing downstream problems due to either water quality or quantity.
AHBL, Inc.
Steve Nickison, EIT
Project Engineer
SLN/lsk
May 2022
Q:\2021\2210134\WORDPROC\Reports\20210708 Rpt (TIR) 2210134.10.docx
Illahee Middle School
Downstream Analysis
April 28, 2022
Completed by Jason Hein
The public storm system was investigated to determine the onsite storm waters natural flow
path after leaving the site and to determine the location the storm water discharges. The
information in this analysis was gathered from a combination of the site survey, King County
iMap and visual observations while walking the private and public system.
Stormwater from Illahee M.S. is collected and conveyed via an enclosed storm system and
discharged to the public stormwater system located in 1st Avenue South. The catch basins
located in 1st Ave S convey the stormwater to the south in a 12” concrete pipe. At a catch basin
located between S 361st Place and S 363rd Place on the west side of 1st Ave S the public system
crosses the street and continues to flow to the south. At this location the storm system
increases to a 36” concrete pipe. The public system continues to flow to the south to a catch
basin located on the east side of 1st Ave S near the south east corner of the school property.
Survey shows the system turning to the east still in a 36” concrete pipe. From this location I
walked behind the property of 123 S 363rd Place, a private residence. This area is classified as
the Hylebos Wetland. Approximately 160 feet from 1 st Ave S a 7’ wide structure was located
with a locking manhole lid and 20 feet past this structure a 36” discharge from the public
system was located.
This downstream analysis confirmed that the storm water from the Illahee M.S. discharges to
the public system and ultimately drains into the Hylebos Wetlands. The Hylebos Wetland is
classified on the National Wetland Inventory as a Freshwater Forested/Shrub Wetland . This
wetland drains to the south into the West Hylebos Creek and ultimately into the Hylebos
Waterway in the Port of Tacoma.
1.0 KING COUNTY LEVEL 1 DOWNSTREAM ANALYSIS
Downstream Analysis
1.1.1 Task 1 – Study Area Definition and Maps
Task 1 of the offsite analysis involves mapping and defining the stormwater characteristics
around the subject property one-quarter mile downstream. According to the King County iMap
resource, the site is part of the Puyallup River – Hylebos Creek Basin. In general, the existing site
slopes down from north to south and has a topographic relief across the entire parcel of
approximately 32 feet.
The parcel is developed with the existing Illahee Middle School. Currently, runoff is captured
through a series of roof downspouts and catch basin structures and routed to the public
stormwater system located in 1st Avenue South.
Refer to Figure 1 for the Downstream Map.
1.1.2 Task 2 – Resource Review
Task 2 of the offsite analysis involves reviewing available references and records that describe
the site’s flow path and potential flooding, erosion, and water quality problems. The following
references were used to complete the offsite analysis:
• 2021 King County Surface Water Design Manual
• King County iMap Resource
• FEMA Flood Maps
• King County Soils Survey
• Site Survey
1.1.3 Task 3 – Field Inspection
Task 3 of the offsite analysis involves an inspection of the existing onsite and offsite drainage
system. The existing storm network was observed for deficiencies and drainage problems.
Downstream Analysis
A ¼-mile downstream analysis was performed per the requirements of the 2021 KCSWDM.
Drainage from the project site is currently captured by a network of underdrains and catch basin
structures. Runoff is conveyed through a closed-pipe network toward the north and east property
boundary. The onsite network connects with the existing County storm system at 1st Avenue
South. This location, near the access to the track field, serves as a junction point for runoff from
residential properties, 1st Avenue South, and the property. At this point, the closed-pipe network is
a 36-inch concrete pipe and continues east to a catch basin located on the east side of 1st
Avenue South. The 36-inch pipe turns to the south at a slope of around 5%. A storm structure in
1st Ave. S near the south east corner of the school property redirects the stormwater to the east
towards the Hylebos Wetlands. Approximately 160 LF to the east of 1st Ave. S is the final
structure in the system, a 7’ wide concrete structure with a manhole access locking lid.
Stormwater discharges from the structure through a 36” outlet and flows into the wetlands.
At the time of the Downstream Analysis there were no problems reported. While walking the
downstream path from the discharge from the concrete structure there was no evidence of any
downstream drainage problems or deposits of sedimentation. There are no known drainage
complaints associated from stormwater runoff from the site drainage. The weather was cloudy
and 52 degrees.
See Figure 1 for the Downstream Map and Table 1 for the associated Downstream Analysis
Table.
1.1.4 Task 4 – Drainage System Description and Problem Descriptions
As described above, the existing onsite drainage system consists of a closed-pipe system that
conveys runoff to 1st Avenue South. The project site drainage enters the public system at a catch
basin on the west side of 1st Avenue South near that access to the athletic track and field. At this
structure the system upsizes from a 12” concrete pipe to a 36” concrete pipe that crosses 1st
Avenue South to a structure located on the east side of the street. Below is a detailed description
of the system from the project site to the wetland.
Structure type* Pipe Size Pipe Length Pipe Slope Land Cover
CB 1764 36” 42 LF 1.4% Enclosed
CB 1542 36” 170 LF 3.1% Enclosed
CB 1555 36” 228 LF 6.8% Enclosed
CB 2513 36” 160 LF 1.0% Enclosed
7’ Structure with
36” outlet
36” Point of
Discharge
None Enclosed
Sheet Flow None None Surface Slope
5-10%
Vegetation
*Structure info from survey
There are no drainage complaints that are known to be open at this time.
1.1.5 Task 5 – Mitigation of Existing or Potential Problems
No outstanding or potential problems were determined in the downstream analysis. The proposed
project will not create a potential drainage or water quality problem.
Hylebos CreekPuyallup RiverWRIA 104/28/2236" Conc (heads E)0 feet212 feet600 feet1,320 feet36" Conc (heads S)36" Conc (heads E)36" Outlet (heads E)3.1%5-10%7' Structure with 36" outletSheet FlowVegetationClosed Pipe SystemSite discharge toPoint of ConnectionClosed Pipe SystemClosed Pipe SystemNo evidence of erosion1.4%±1.0%42 feet36" Conc (heads E)Closed Pipe System6.8%440 feet
2215 North 30th Street, Suite 300
Tacoma, WA 98403
253.383.2422 TEL 253.383.2572 FAX
Illahee Middle School
Downstream Map FIG. 1
DRAWN BY:DATE:JOB NO.:
Ex. Discharge Point
Downstream Flowpath
End Downstream
J. Hein 4-28-2022 2210536.10