21-101296 -Preliminary TIR-01-04-02-2021-V1
255 S. King Street, Suite #800, Seattle, WA 98104 | 206.426.2600 | JACOBSONENGINEERS.COM
PRELIMINARY
TECHNICAL INFORMATION REPORT
for Storm Water Management
Olympic View K8 School
2626 SW 327th ST, Federal Way, WA 98003
March 26, 2021
_______________________________________________
Prepared for
Federal Way Public Schools
33330 8th Avenue South
Federal Way, WA 98003
_______________________________________________
Prepared through
McGranahan Architects
2111 Pacific Ave #100
Tacoma, WA 98402
_______________________________________________
Prepared by
Jacobson Consulting Engineers
Sascha Eastman
(206) 426-2600
sascha@jacobsonengineers.com
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Preliminary Technical Information Report for
OLYMPIC VIEW K8 SCHOOL
Project No. C200004-0081
March 26, 2021
TABLE OF CONTENTS
1. PROJECT OVERVIEW.................................................................................................................... 3
GENERAL DESCRIPTION .............................................................................................................................. 3
EXISTING CONDITIONS ................................................................................................................................. 3
PROPOSED DRAINAGE SYSTEM ................................................................................................................. 4
2. CONDITIONS AND REQUIREMENTS SUMMARY .............................................................................. 5
CORE REQUIREMENT #1: DISCHARGE AT THE NATURAL LOCATION .................................................. 5
CORE REQUIREMENT #2: OFFSITE ANALYSIS ......................................................................................... 5
CORE REQUIREMENT #3: FLOW CONTROL .............................................................................................. 5
CORE REQUIREMENT #4: CONVEYANCE SYSTEM .................................................................................. 6
CORE REQUIREMENT #5: EROSION AND SEDIMENT CONTROL ........................................................... 6
CORE REQUIREMENT #6: MAINTENANCE AND OPERATIONS ............................................................... 6
CORE REQUIREMENT #7: FINANCIAL GUARANTEES .............................................................................. 7
CORE REQUIREMENT #8: WATER QUALITY .............................................................................................. 7
CORE REQUIREMENT #9: FLOW CONTROL BMP’S .................................................................................. 7
3. OFFSITE ANALYSIS ..................................................................................................................... 8
FIELD INSPECTION ........................................................................................................................................ 8
DRAINAGE SYSTEM PROBLEM DESCRIPTIONS ....................................................................................... 8
UPSTREAM ANALYSIS ................................................................................................................................... 8
DOWNSTREAM ANALYSIS ............................................................................................................................ 8
MITIGATION OF EXISTING OR POTENTIAL PROBLEMS ........................................................................... 9
4. FLOW CONTROL, LOW IMPACT DEVELOPMENT (LID) AND WATER QUALITY FACILITY ANALYSIS AND
DESIGN ........................................................................................................................................... 10
EXISTING SITE HYDROLOGY (PART A) ....................................................................................................10
DEVELOPED SITE HYDROLOGY (PART B) ...............................................................................................10
PERFORMANCE STANDARDS (PART C) ...................................................................................................11
FLOW CONTROL SYSTEM (PART D) .........................................................................................................11
WATER QUALITY SYSTEM (PART E) .........................................................................................................11
5. CONVEYANCE SYSTEM ANALYSIS AND DESIGN ........................................................................... 12
DEVELOPED STORM SYSTEM DESCRIPTION .........................................................................................12
OUTFALLS .....................................................................................................................................................12
CONVEYANCE CALCULATIONS DICUSSION ...........................................................................................12
6. SPECIAL REPORTS AND SUMMARY ............................................................................................ 13
7. OTHER PERMITS ...................................................................................................................... 14
8. CSWPP ANALYSIS AND DESIGN ................................................................................................. 15
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ESC PLAN ANALYSIS AND DESIGN (PART A) ..........................................................................................15
SWPPPS PLAN DESIGN (PART B) ..............................................................................................................15
9. BOND QUANTITIES, FACILTY SUMMARIES, AND DECLARATION OF COVENANT .............................. 16
BOND QUANTITIES WORKSHEET..............................................................................................................16
FLOW CONROL AND WATER QUALITY FACILITY SUMMARY SHEET AND SKETCH .........................16
DECLARATION OF COVENANT FOR PRIVATELY MAINTAINED FLOW CONTROL AND WQ
FACILITIES .....................................................................................................................................................16
DECLARATION OF COVENANT FOR PRIVATELY MAINTAINED FLOW CONTROL BMP’S .................16
10. OPERATIONS AND MAINTENANCE MANUAL ................................................................................ 17
11. FIGURES .................................................................................................................................. 18
12. APPENDICES ............................................................................................................................ 19
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1. PROJECT OVERVIEW
GENERAL DESCRIPTION
The following Drainage Report provides preliminary design analysis for the Olympic View Elementary School – K8
stormwater plan. The stormwater design for the project is based on the requirements set forth in the 2016 King
County Surface Water Design Manual (2016 KCSWDM) as adopted by City of Federal Way, with addendums per
City of Federal Way Addendum to the King County Surface Water Design Manual (January 8, 2017).
Olympic View Elementary School is located at 2626 SW 327th ST, Federal Way, WA 98003 (See Figure 2 Vicinity
Map). The site is bounded by SW 327th Street to the south, condominiums, golf course, and single-family
residences to the west, 26th Ave SW to the east, and single-family residences to the north (see Figure 2, Vicinity
Map). The site is in the northwest quarter of Section 13, Township 21 North, Range 3 East, Willamette Meridian.
The area within the property boundary (parcel 132103-9008) is approximately 9.42 acres, of which approximately
7.10 acres will be redeveloped. The project will also be required to dedicate 30-feet of right-of-way along SW 327th
ST, which will decrease the property area by approximately 0.24 acres to be 9.18 acres.
The proposed project consists of redeveloping the entire school campus except for approximately 2.30 acres of
forested and vegetated land located along the west and north sides of the property, which will remain undisturbed.
The associated site redevelopment will consist of a new school building, including an outdoor natural learning
environment, an asphalt parking lot (staff and visitor) with a queuing and drop off lane, an asphalt bus loading
area, asphalt playground (soft and hard play), and grass play field. Concrete and asphalt sidewalks are also
proposed to be constructed to provide pedestrian access through the site and will be ADA Accessible as required to
meet City of Federal Way standards and Federal Guidelines.
Frontage Improvements will also be implemented along both SW 327th ST to the south and 26th Ave SW to the
east. According to City of Federal Way guidelines provided in the March 11, 2021 Pre-Application Conference
Summary, SW 327th ST and 26th Ave SW and SW 327 are both classified as Minor Collector streets. Per City
Standards, the project will only be required to improve half the street frontages with both streets, measured from
the right-of-way (ROW) centerline, having a 10-ft drive lane, an 8-ft parking lane, a vertical curb, a 4-ft planter
strip, and a 5-ft concrete sidewalk.
EXISTING CONDITIONS
The existing Olympic View Elementary School campus is currently occupied with a 43,160 gross square foot school
building, a 42,200 square foot asphalt and wood chip play area with roughly 2,500 square feet of it covered, an
asphalt parent pickup-drop off area containing 17 parking stalls, an asphalt bus pickup-drop off, and approximately
27,000 square feet of usable grass space (See Figure 3 Existing Conditions). The parent pickup-drop off and
parking area located on the south side of the site has driveway access from both 26th Ave SW and SW 327th ST.
The bus pickup-drop off and delivery lane is on the east side of the existing school and is accessible via 26th Ave
SW with one-way circulation and (2) two driveways.
The stormwater for the existing building and south and east sides of the school generally drains to the SE corner of
the site via a series of downspout tightlines and catch basin and conveyance pipes, where it discharges into the
public storm system in 26th Ave SW, where it continues to flow to the north. The remainder of the site on the west
and north sides of the existing school campus sheet flows as overland flow across the property and onto adjacent
residential properties with some of the stormwater possibly draining to either cul-de-sac in 29th Ave SW, SW 324th
PL, or 26th PL SW streets. There are no offsite flows coming on the property.
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PROPOSED DRAINAGE SYSTEM
The stormwater design for the project is based on the requirements set forth in the 2016 King County Surface
Water Design Manual (2016 KCSWDM) as adopted by City of Federal Way, with addendums per City of Federal
Way Addendum to the King County Surface Water Design Manual (January 8, 2017). Based on projected
disturbances the development can expect to provide flow control for rainwater falling on new or replaced
impervious surfaces as well as redeveloped pervious surfaces. The amount of disturbed area for the site is
approximately 6.9 acres with roughly 2.3 acres of forested land being left undisturbed. Of the 6.9 acres of the site
that will be redeveloped, approximately 5.0 acres of new plus replaced impervious surface will be created, while
1.9 acres will be redeveloped as pervious surfacing (landscaping). Approximately 1.0 acre of field space will be
redeveloped that will be under-drained and by code definition, is considered 100% impervious. The field space is
accounted for in the 5.0 acres of redeveloped impervious area.
Preliminary Geotechnical Engineering findings indicate there is some deeper (not at the surface) outwash soils that
may be suitable for infiltration of on-site stormwater. Infiltration provides an opportunity for the project to
introduce stormwater runoff that would normally be collected and discharged off-site, back into the ground that will
help to contribute to recharging local aquifers and reduce possible impacts with any localized downstream flooding.
Currently for design, as recommended in section 16.0 of the current Geotechnical Engineering Report, the project
is assuming an infiltration rate of 2-inches per hour. However, the geotechnical engineer has recently performed
on-site infiltration field testing and measured infiltration rates range from 4-inches per hour to 40-inches per hour.
The geotechnical engineer has added some preliminary correction factors to the field-tested rates to determine
design infiltration rates in the range of 1.5-inches per hour and 14.8-inches per hour. The current preliminary
design does not account for the updated infiltration numbers, but the system will be finalized and appropriately
sized prior to engineering permit submittal. The project will also issue an additional infiltration testing report, once
prepared by the geotechnical engineer, as part of the engineering permit submittal.
The project is required to provide Level 2 flow control and proposes to manage all required stormwater
management flows on-site and install a perforated CMP detention pipe with gravel finger drains (trenches) below
the pipe to provide a better hydraulic connection and infiltrate the stormwater into the native outwash soils. There
may also be an opportunity to install a shallower Stormtech Chamber or equal stormwater detention system but
that has not be finalized yet. An emergency overflow drain will be required to be installed and connected to the
existing storm drainage system in 26th Ave SW. Initial calculations, using an infiltration rate of 2-inches per hour,
indicate that approximately 45,600 cubic feet of stormwater detention is required to be collected and infiltrated
on-site. The storm detention/infiltration system is currently evenly split into three separate systems with Detention
System #1 draining the north portion of site (field and asphalt play and a portion of building roof area), Detention
System #2 collecting runoff from the east portion of the site (parking lot, plaza, and building roof area), and
Detention System #3 collects the southern portion of the site and the fire lane. The design for the redevelopment
is ongoing, so the detention systems have not been sized for each drainage basin but will be finalized and
appropriately sized prior to the permit submittal.
Water quality treatment will also need to be addressed for the pollution generating impervious asphalt parking lot
and parent pickup-drop off and bus loading driveway located on the east and south sides of the site, respectively.
The project site will be required to provide enhanced basic water quality treatment of the stormwater runoff before
releasing the “clean” treated stormwater into the ground. The project proposes to install two (2) modular wetland
vault systems (MWS), Filterra units, or other equal water quality treatment facility, that would be installed
upstream of detention prior to infiltrating the stormwater runoff on-site. The MWS or equal vaults would meet the
City’s stormwater code requirement for water quality treatment. Upstream of the infiltration flow control facility,
the project also proposes to install two (2) Contech CDS Separators, which would be used to separate out any
solids (debris, trash, etc.) prior to stormwater entering the detention or water quality treatment system, which will
help to maintain and increase the life of the storm system.
Perimeter perforated footing drains will also be provided around the new building, to pick up any surface water or
incidental ground water from being trapped adjacent to the building foundation.
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2. CONDITIONS AND REQUIREMENTS SUMMARY
This section will address the Core Requirements set forth by the 2016 King County Surface Water Design Manual
as adopted by the City of Federal Way.
CORE REQUIREMENT #1: DISCHARGE AT THE NATURAL LOCATION
All storm water runoff and surface water from a project must be discharged at the natural location so as not to be
diverted onto or away from downstream properties. The manner in which stormwater runoff and surface water are
discharged from the project site must not create a significant adverse impact to downhill properties or drainage
facilities (see "Discharge Requirements" below). Drainage facilities as described above means a constructed or
engineered feature that collects, conveys, stores, treats, or otherwise manages surface water or stormwater runoff.
“Drainage facility” includes, but is not limited to, a constructed or engineered stream, lake, wetland, or closed
depression, or a pipe, channel, ditch, gutter, flow control facility, flow control BMP, water quality facility, erosion
and sediment control facility, and any other structure and appurtenance that provides for drainage. Note: Projects
that do not discharge all project site runoff at the natural location will require an approved adjustment of this
requirement (see Section 1.4). DPER may waive this adjustment, however, for projects in which only a small
portion of the project site does not discharge runoff at the natural location and the runoff from that portion is
unconcentrated and poses no significant adverse impact to downstream properties.
The project threshold area lies within a critical aquifer recharge area and proposes to manage all required
stormwater management flows on-site and install a perforated CMP detention pipe with gravel finger drains below
the pipe to infiltrate the stormwater into the native outwash soils. This will allow for the project to introduce
stormwater runoff back into the ground that will help to recharge local aquifers and reduce contributions to
localized downstream flooding.
The remainder of the site that will remain undisturbed, consisting of native vegetation and forest located along the
north and west sides of the property, will maintain existing natural drainage courses.
CORE REQUIREMENT #2: OFFSITE ANALYSIS
All proposed projects must submit an offsite analysis report that assesses potential offsite drainage and water
quality impacts associated with development of the project site, and that proposes appropriate mitigation of those
impacts. The initial permit submittal shall include, at minimum, a Level 1 downstream analysis as described in
Section 1.2.2.1 below. If impacts are identified, the proposed projects shall meet any applicable problem-specific
requirements specified in Section 1.2.2.2 (p. 1-32) for mitigation of impacts to drainage problems and Section
1.2.2.3 (p. 1-35) for mitigation of impacts to water quality problems.
A Level 1 downstream analysis will be provided, is discussed in additional detail in Section 3 below.
CORE REQUIREMENT #3: FLOW CONTROL
All proposed projects, including redevelopment projects, must provide onsite flow control facilities to mitigate the
impacts of storm and surface water runoff generated by new impervious surface, new pervious surface, and
replaced impervious surface targeted for flow mitigation as specified in the following sections. Flow control
facilities must be provided and designed to perform as specified by the area-specific flow control facility
requirement in Section 1.2.3.1 (p. 1-39) and in accordance with the applicable flow control facility
implementation requirements in Section 1.2.3.2 (p. 1-50).
The project is in a Conservation Flow Control Area and will be required to provide Level 2 flow control area, as
established from the City of Federal Way Flow Control Applications Map and conveyed to us in the pre-application
meeting.
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CORE REQUIREMENT #4: CONVEYANCE SYSTEM
All engineered conveyance system elements for proposed projects must be analyzed, designed, and constructed to
provide a minimum level of protection against overtopping, flooding, erosion, and structural failure as specified in
the following groups of requirements:
· "Conveyance Requirements for New Systems," Section 1.2.4.1 (below)
· "Conveyance Requirements for Existing Systems," Section 1.2.4.2 (p. 1-56)
· "Conveyance System Implementation Requirements," Section 1.2.4.3 (p. 1-57)
The new project will ensure that all stormwater drainage pipes are sized accordingly to convey the 100-year storm
event. A conveyance sizing analysis will be performed and presented as part of the engineering permit submittal
(Pending).
CORE REQUIREMENT #5: EROSION AND SEDIMENT CONTROL
All proposed projects that will clear, grade, or otherwise disturb the site must provide erosion and sediment
controls to prevent, to the maximum extent practicable, the transport of sediment from the project site to
downstream drainage facilities, water resources, and adjacent properties. All proposed projects that will conduct
construction activities onsite or offsite must provide stormwater pollution prevention and spill controls to prevent,
reduce, or eliminate the discharge of pollutants to onsite or adjacent stormwater systems or watercourses. To
prevent sediment transport and pollutant discharges as well as other impacts related to land-disturbing and
construction activities, Erosion and Sediment Control (ESC) measures and Stormwater Pollution Prevention and
Spill Control (SWPPS) measures that are appropriate to the project site must be applied through a comprehensive
Construction Stormwater Pollution Prevention (CSWPP) plan as described in Sections 1.2.5.1 and 1.2.5.3 and
shall perform as described in Section 1.2.5.2. In addition, these measures, both temporary and permanent, shall
be implemented consistent with the requirements in Section 1.2.5.3 that apply to the proposed project.
A concept ESC plan with requirements for the Contractor to provide and implement a design for a system to treat
construction runoff to no more than 25 NTU’s over baseline is included with the submittal. A baseline will be
established at the start of construction and the contractor will be required to maintain levels no greater than 25
NTU’s above this level.
Our experience is that this is approach is much more realistic for both the bidding Contractor, development design
engineer, and the municipalities who are permitting and inspecting. We have found that if we take a “snapshot in
time” of what an ESC system may be and put that on the plans, it cannot consider all the phasing required to
construct the project and does not account for the contractors means and methods for how to manage a
construction site.
So often the bidding contractor submits a change order to the owner, saying we bid “the included ESC plan”, but
that design didn’t include any pump and filtration systems that may be necessary in conjunction with traditional
sediment settlement facilities, to treat the construction runoff to allowable discharge levels that work with the
contractors means and methods.
A SWPPP will be prepared and issued with the engineering permit submittal (Pending).
CORE REQUIREMENT #6: MAINTENANCE AND OPERATIONS
Maintenance and operation of all drainage facilities is the responsibility of the applicant or property
owner, except those facilities for which King County assumes maintenance and operation as described
below and in KCC 9.04.115 and KCC 9.04.120. Drainage facilities must be maintained and operated in
accordance with the maintenance standards in Appendix A of this manual, or other maintenance standards
as approved by King County.
A Maintenance and Operations Manual will be provided and issued with the engineering permit submittal
(Pending).
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CORE REQUIREMENT #7: FINANCIAL GUARANTEES
All drainage facilities constructed or modified for projects must comply with the financial guarantee requirements
in King County Ordinance 12020 and the liability requirements of King County Code 9.04.100, excepting those
privately maintained flow control BMPs not serving a private road designed for 2 or more lots. There are two types
of financial guarantees for projects constructing or modifying drainage facilities. These are as follows:
· The drainage facilities restoration and site stabilization guarantee
· The drainage defect and maintenance guarantee.
Since the owner is Federal Way Public Schools (FWPS) and a public agency, they are not subject to bonding
requirements. We are not aware if there is any other sort of Financial Guarantee that the City of Federal Way will
require, but FWPS is open to conversations to come up with a mutually agreeable solution.
CORE REQUIREMENT #8: WATER QUALITY
All proposed projects, including redevelopment projects, must provide water quality (WQ) facilities to treat the
runoff from those new and replaced pollution-generating impervious surfaces and new pollution-generating
pervious surfaces targeted for treatment as specified in the following sections. These facilities shall be selected
from a menu of water quality facility options specified by the area-specific facility requirements in Section 1.2.8.1
(p. 1-71) and implemented according to the applicable WQ implementation requirements in Section 1.2.8.2 (p. 1-
80).
Water Quality treatment is being provided for all of the targeted areas, which will provide Enhanced Basic Water
Quality Treatment via a Modular Wetland Vault Systems, Filterra units, or other equal water quality treatment
facility. See Section 4, Part E, for additional discussion.
CORE REQUIREMENT #9: FLOW CONTROL BMP’S
All proposed projects, including redevelopment projects, must provide onsite flow control BMPs to mitigate the
impacts of storm and surface water runoff generated by new impervious surface, new pervious surface, existing
impervious surfaces, and replaced impervious surface targeted for mitigation as specified in the following sections.
Flow control BMPs must be selected and applied according to the basic requirements, procedures, and provisions
detailed in this section and the design specifications for each BMP in Appendix C, Section C.2.
Flow control BMPs are methods and designs for dispersing, infiltrating, or otherwise reducing or preventing
development-related increases in runoff at or near the sources of those increases. Flow control BMPs include, but
are not limited to, preservation and use of native vegetated surfaces to fully disperse runoff; use of other pervious
surfaces to disperse runoff; roof downspout infiltration; permeable pavements; bioretention; limited infiltration
systems; and reduction of development footprint.
The project proposes to manage all required stormwater management flows on-site and install a perforated CMP
detention pipe with gravel finger drains below the pipe to infiltrate the stormwater into the native outwash soils.
The remainder of the site that will remain undisturbed, consisting of native vegetation and forest located along the
north and west sides of the property, will maintain existing natural drainage courses via overland sheet flow.
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3. OFFSITE ANALYSIS
FIELD INSPECTION
Site visits were made in the fall of 2020 and winter of 2021 to gather information about the existing drainage
system, including a Level 1 Downstream Analysis. The project survey, existing record drawings and City GIS
information have also been utilized as a resource for the discussion below. Please refer to Downstream Analysis
below for more information.
DRAINAGE SYSTEM PROBLEM DESCRIPTIONS
The school district, or City of Federal Way to our knowledge, has not raised concerns about any existing drainage
problems. As such, no drainage problems are anticipated to be present in the redevelopment as a result of
previously existing problems.
UPSTREAM ANALYSIS
There are not upstream drainage areas that currently contribute runoff to the Olympic View K8 School property.
However, redevelopment of the frontages along both SW 327th ST and 26th Ave SW, will result in a portion of new
roadway to be constructed where either gravel parking or rolled curb exists, respectively. Both streets are proposed
to be reconditioned with a grind and overlay, which does not trigger stormwater detention or water quality treatment
measures to be implemented.
We are anticipating that the amount of new plus replaced impervious surfacing in the right-of-way, which consists
of concrete sidewalks and new plus replaced asphalt roadway will be less than 10,000 square feet and therefore
not triggering stormwater flow control requirements. The amount of new PGIS asphalt pavement is also anticipated
to be less than 5,000 square feet, which does not trigger then the threshold requirement for water quality
treatment. However, for budgetary purposes, the project is currently installing a Filterra Unit for water quality
treatment in both SW 327th ST and 26th Ave SW and routing the stormwater runoff to the project’s on-site
stormwater detention infiltration system. As the design is finalized, we anticipate the project will not include water
quality treatment or flow control for any portion of the redeveloped right-of-way and will be reflected in the permit
submittal (Pending).
DOWNSTREAM ANALYSIS
The existing on-site stormwater connection to the existing public storm system in the right-of-way, is located at the
SE corner of the property (See Figure 7 City OLV Outfall POC). The point of compliance for the site is at the
approximate intersection of 26th Ave SW and SW 323rd ST, where the stormwater runoff flows along 26th Ave SW
and a portion of SW 323rd ST to the west, including approximately half of the cul-de-sac for 26th PL SW, converge
and outfall to the existing private Lake Ponce de Leon. There are (3) drainage basins located on the existing
development. One drainage basin collects stormwater runoff from the roof and pavements on the south and east
sides of the site and is drained to a series of catch basins and piped conveyance system. The stormwater from this
basin enters the city’s storm drainage system in the SE corner of the site as described above. Another storm
drainage basin is located on the west side of the site and drains stormwater via overland flow across the existing
asphalt play surfacing, play field, and adjacent grass hillside to the west that generally drains from south to north
with (2) apparent flow paths, one to the west hillside and the other to the northwest corner. These flow paths are
approximate and not clearly defined as there are no defined conveyance ditches or channels are assumed to drain
onto the neighboring residential properties to the west. The other drainage basin is located between the existing
field and property line and consists of sloped vegetated and forest that slopes to the north and northeast corner.
(See Figure 5 Downstream Quarter Mile Map).
Main Site, South and East
Stormwater runoff that is collected and conveyed to the southeast corner of the property, is conveyed to the public
storm drainage system that varies in pipe size and material ranging from 12-inch diameter to 32-inch diameter and
concrete and corrugated metal pipe, respectively. The stormwater travels through the existing City storm drainage
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system to the north along 26th Ave SW until it reaches the intersection of 26th Ave SW and SW 325th ST, at
which point the stormwater is conveyed to the northeast direction through a 30-inch diameter concrete pipe that
outfalls into in a private lake listed on Google Maps as Lake Ponce de Leon. The lake outfall is located roughly
1,200 feet from the existing Olympic View Elementary School stormwater POC (See Figure 6 Downstream
Analysis).
North and West
The north and west sides of the existing school campus sheet flows stormwater across the site to the north,
northeast, northwest, and west directions as described above where it is believed, based on contour data, the
stormwater runoff makes its way to the cul-de-sacs of 26th PL SW, SW 324th PL, and 29th Ave SW through or
across neighboring residential properties. The stormwater runoff then sheet flows along existing rolled curb and
gutters until the stormwater is collected in catch basins in SW 323rd ST and conveyed either west or east.
Stormwater along the curb flowline travels west to roughly the intersection of SW 323rd ST and 32nd Ave SW,
where it is a quarter mile (0.25 mi) from where the stormwater runoff from the site enters the public street and
drainage system. A portion of stormwater runoff along 26th PL SW, travels north, then east along SW 323rd ST
until is eventually discharged to private Lake Ponce de Leon.
MITIGATION OF EXISTING OR POTENTIAL PROBLEMS
The school district, or City of Federal Way to our knowledge, has not raised concerns about any existing drainage
problems. As such, no drainage problems are anticipated to be present in the redevelopment as a result of
previously existing problems.
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4. FLOW CONTROL, LOW IMPACT DEVELOPMENT (LID) AND WATE R
QUALITY FACILITY ANALYSIS AND DESIGN
EXISTING SITE HYDROLOGY (PART A)
The existing site totals approximately 9.42 acres and consists of the existing school building, parking lots,
landscaping, sidewalks, hard and soft play areas, and sports field. The stormwater for the existing building and
south and east sides of the school generally drains to the SE corner of the site via a series of downspout tightlines
and catch basin and conveyance pipes, where it discharges into the public storm system in 26th Ave SW, where it
continues to flow to the north. The remainder of the site on the west and north sides of the existing school campus
sheet flows as overland flow across the property and onto adjacent residential properties with some of the
stormwater possibly draining to either cul-de-sac in 29th Ave SW, SW 324th PL, or 26th PL SW streets. There are no
offsite flows coming on the property.
Table 1 summarizes the land cover characteristics of the Threshold Discharge Areas of the existing site (See Figure
3 Existing Conditions).
TABLE 1 – EXISTING SITE CONDITIONS – AREA BREAKDOWN S
Land Cover Area (acres)
Impervious Area 3.26
Landscape Area 3.65
Forested Area (Undisturbed) 2.27
Total Site 9.18
% Impervious of Site 35.5%
DEVELOPED SITE HYDROLOGY (PART B)
Storm water mitigation will be required for construction of the new project redevelopment. This will include all
disturbed site areas. Table 2 summarizes the land cover characteristics of the proposed redevelopment (See Figure
4 New + Replaced Impervious Areas).
TABLE 2 –DEVELOPED SITE CONDITIONS AREA BREAKDOWN
Land Cover Area (acres)
Impervious Area 4.99
Landscape Area 1.92
Forested Area (Undisturbed) 2.27
Total Site 9.18
% Impervious of Site 54.4%
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PERFORMANCE STANDARDS (PART C)
The project is in a Conservation Flow Control Area and will be required to provide Level 2 flow control area, as
established from the City of Federal Way Flow Control Applications Map and conveyed to us in the pre-application
meeting. In accordance with the 2016 KCSWDM manual, MGSFlood Version 4 (an accepted continuous-modeling
software) will be used to model the runoff from existing and proposed site drainage basins. Existing site
development conditions were modeled as having forested surface cover. New and replaced impervious areas are
modeled as impervious surfaces.
FLOW CONTROL SYSTEM (PART D)
Flow control for the disturbed site areas of the redeveloped property, will be provided via a perforated CMP
detention pipe with gravel finger drains below the pipe to infiltrate the stormwater into the native outwash soils.
Initial calculations, using an infiltration rate of 2-inches per hour, indicate that approximately 45,600 cubic feet of
stormwater detention is required to be collected and infiltrated on-site. The CMP detention/infiltration system is
approximately 46,100 CF and includes 6-inches of sediment storage.
The storm detention/infiltration system is currently evenly split into three separate systems with Detention System
#1 draining the north portion of site (field and asphalt play and a portion of building roof area), Detention System
#2 collecting runoff from the east portion of the site (parking lot, plaza, and building roof area), and Detention
System #3 collects the southern portion of the site and the fire lane. The design for the redevelopment is ongoing,
so the detention systems have not been sized for each drainage basin but will be finalized and appropriately sized
prior to the permit submittal.
See Appendix A for preliminary Flow Control Calculations.
WATER QUALITY SYSTEM (PART E)
Water quality treatment will need to be provided for the pollution generating impervious asphalt parking lot and
parent pickup-drop off and bus loading driveway located on the east and south sides of the site, respectively.
The project proposes to install two (2) modular wetland vault systems (MWS), Filterra units, or other equal water
quality treatment facility, that would be installed upstream of detention prior to infiltrating the stormwater runoff
on-site. The MWS or equal vaults would meet the City’s stormwater code requirement for water quality treatment.
Water Quality Calculations for the sizing of the water quality treatment facilities will be provided in the engineering
permit submittal (Pending).
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5. CONVEYANCE SYSTEM ANALYSIS AND DESIGN
This section discusses the criteria that will be used to analyze and design the proposed storm conveyance system
(Pending).
DEVELOPED STORM SYSTEM DESCRIPTION
Section will be updated for the engineering permit submittal.
OUTFALLS
Section will be updated for the engineering permit submittal.
CONVEYANCE CALCULATIONS DICUSSION
Section will be updated for the engineering permit submittal.
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6. SPECIAL REPORTS AND SUMMARY
Please refer to Geotech Report by Associated Earth Sciences Inc. - "SUBSURFACE EXPLORATION, GEOLOGIC
HAZARD, AND PRELIMINARY GEOTECHNICAL ENGINEERING REPORT" - NOVEMBER 4, 2020.
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7. OTHER PERMITS
Land Use Process III Submittal – PENDING
Building Permit – PENDING
Engineering (EN) Permit – PENDING
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8. CSWPP ANALYSIS AND DESIGN
ESC PLAN ANALYSIS AND DESIGN (PART A)
Section will be updated for the engineering permit submittal.
SWPPPS PLAN DESIGN (PART B)
A SWPPP will be prepared for this project and included with the engineering permit submittal.
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9. BOND QUANTITIES, FACILTY SUMMARIES, AND DECLARATION OF
COVENANT
BOND QUANTITIES WORKSHEET
A Bond Quantity Worksheet will not be required, unless requested by the City of Federal Way.
FLOW CONROL AND WATER QUALITY FACILITY SUMMARY SHEET AND SKETCH
Flow Control and Water Quality Facility Summary Sheets and sketches will be included with the engineering permit
submittal (Pending).
DECLARATION OF COVENANT FOR PRIVATELY MAINTAINED FLOW CONTROL AND WQ
FACILITIES
King County’s standard Declaration of Covenant for Privately Maintained Flow Control and WQ Facilities will be
included with the permit submittal for City of Federal Way review and comment prior to recording. It is anticipated
that if this document is required, that it would be just one document that would be utilized for all of the new
facilities installed as part of this project; 1) Perforated CMP Pipes, and 2) MWS Vaults, Filterra Units, or other
Water Quality Treatment facilities draining to Perforated CMP Pipes.
DECLARATION OF COVENANT FOR PRIVATELY MAINTAINED FLOW CONTROL BMP’S
King County’s standard Declaration of Covenant for Maintenance and Inspection of Flow Control BMP’s will be
included with the permit submittal for City of Federal Way review and comment prior to recording.
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10. OPERATIONS AND MAINTENANCE MANUAL
An Operations and Maintenance Manual will be included with the engineering permit submittal.
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11. FIGURES
Figure 1 – TIR Worksheet
Figure 2 – Vicinity Map
Figure 3 – Existing Conditions
Figure 4 – New + Replaced Impervious Areas
Figure 5 – Downstream Quarter Mile Map
Figure 6 – Downstream Analysis Exhibit
Figure 7 – City OLV Outfall POC
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12. APPENDICES
Appendix A – Preliminary Calculations
Appendix B – AESI “SUBSURFACE EXPLORATION, GEOLOGIC HAZARD, AND GEOTECHNICAL
ENGINEERING REPORT”
Appendix C – Operation & Maintenance Manual (PENDING)
Appendix D – Conveyance Calculations and Exhibits (PENDING)
Appendix E – Bond Quantities, Facility Summaries, and Declaration of Covenant (PENDING)
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Appendix A
Preliminary Calculations
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Appendix B
AESI “SUBSURFACE EXPLORATION, GEOLOGIC HAZARD, AND
GEOTECHNICAL ENGINEERING REPORT”
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Appendix C
Operation & Maintenance Manual
(PENDING)
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Appendix D
Conveyance Calculations and Exhibits
(PENDING)
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Appendix E
Bond Quantities, Facility Summaries, and Declaration of Covenant
(PENDING)