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PRELIMINARY
TECHNICAL INFORMATION REPORT
FOR
Day Short Plat
CITY OF RENTON, WASHINGTON
Prepared by: Christian R. Vanderhoeven, E.I.T
Approved by: Sheri H. Murata, P.E.
Date: February 24, 2023
Core No.: 19003
2/24/23
Exhibit 4
RECEIVED
05/26/2023
AMorganroth
PLANNING DIVISION
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
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Day Short Plat
Table of Contents
1. PROJECT OVERVIEW ............................................................................................................................. 1
Figure 1-1 Vicinity Map ........................................................................................................................ 1
2. CONDITIONS AND REQUIREMENTS SUMMARY ................................................................................... 2
2.1 Core Requirements ............................................................................................................................. 3
2.1.1 Core Requirement #1: Discharge at the Natural Location ......................................................... 3
2.1.2 Core Requirement #2: Offsite Analysis ....................................................................................... 3
2.1.3 Core Requirement #3: Flow Control............................................................................................ 3
2.1.4 Core Requirement #4: Conveyance System ................................................................................ 3
2.1.5 Core Requirements #5: Erosion and Sediment Control .............................................................. 3
2.1.6 Core Requirement #6: Maintenance and Operations ................................................................. 3
2.1.7 Core Requirement #7: Financial Guarantees and Liability .......................................................... 3
2.1.8 Core Requirement #8: Water Quality ......................................................................................... 3
2.1.9 Core Requirement #9: On-Site BMPs .......................................................................................... 3
2.2 Special Requirements ......................................................................................................................... 3
2.2.1 Special Requirement #1: Other Adopted Area Specific Requirements ....................................... 3
2.2.2 Special Requirement #2: Flood Hazard Area Delineation ........................................................... 3
2.2.3 Special Requirement #3: Flood Protection Facilities ................................................................... 3
2.2.4 Special Requirement #4: Source Control .................................................................................... 3
2.2.5 Special Requirement #5: Oil Control ........................................................................................... 4
2.2.6 Special Requirement #6: Aquifer Protection Area ...................................................................... 4
3. OFFSITE ANALYSIS ................................................................................................................................ 5
Task 1 Study Area Definition and Maps .................................................................................................... 5
Task 2 Resource Review ........................................................................................................................... 5
Sensitive Areas Folio............................................................................................................................. 5
Task 3 Field Investigation ......................................................................................................................... 5
Task 4 Drainage System Description and Problem Description ............................................................... 8
4. FLOW CONTROL AND WATER QUALITY ANALYSIS ............................................................................... 9
4.1 Existing Conditions.............................................................................................................................. 9
Table 4.1 Existing Conditions ............................................................................................................... 9
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4.2 Developed Conditions ...................................................................................................................... 10
4.3 Flow Control System ......................................................................................................................... 12
4.4 BMP Feasibility ................................................................................................................................. 15
4.5 Water Quality Treatment Analysis and Design ................................................................................. 18
5. CONVEYANCE SYSTEM ANALYSIS AND DESIGN .................................................................................. 19
6. SPECIAL REPORTS AND STUDIES ........................................................................................................ 20
7. OTHER PERMITS ................................................................................................................................. 21
8. ESC ANALYSIS AND DESIGN ................................................................................................................ 22
9. BOND QUANTITIES, FACILITY SUMMARIES, AND DECLARATION OF COVENANT .............................. 23
10. OPERATIONS AND MAINTENANCE ..................................................................................................... 24
Appendix A .................................................................................................................................................. 25
Appendix B .................................................................................................................................................. 26
• WWHM Report
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1. PROJECT OVERVIEW
The proposed Day Short Plat project is located at 18600 116th Avenue SE, Renton, WA. See Vicinity Map
below. The site is composed of parcels #6197800080, 6197800263, and 6197800265 with an area of
approximately 2.88 acres. The site is bordered by 116th Avenue SE to the west and residential homes to
the north, east, and south.
Figure 1-1 Vicinity Map
Proposed development of the property will include the demolition of all existing structures and the
construction of 7 single family residences, access road, detention facilities, utilities, and frontage
improvements.
The subject project’s drainage facilities were designed using the guidelines and requirements
established in the 2019 City of Renton Surface Water Design Manual (RSWDM). Flow Control Duration
Standard (Forested Site Conditions) and Basic Water Quality Treatment are required for this project.
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2. CONDITIONS AND REQUIREMENTS SUMMARY
The proposed project is classified as requiring “Directed Drainage Review” per the 2022 RSWDM.
Therefore, all nine core requirements and six special requirements will be addressed per Section 1.2 and
1.3 of the 2022 RSWDM.
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2.1 Core Requirements
2.1.1 Core Requirement #1: Discharge at the Natural Location
The project preserves the site natural discharge location.
2.1.2 Core Requirement #2: Offsite Analysis
See Section 3 of this Report for the downstream drainage path.
2.1.3 Core Requirement #3: Flow Control
The site falls within the City’s Flow Control Duration Standard (Forested Site Conditions). See City’s Flow
Control Application Map in Appendix A. This flow control standard requires matching forested
conditions for 50% of the 2-year and up to the full 50-year flow. And matches the 2, 10-year peak rate
runoffs. See Section 4 of this report for a detailed Flow Control Analysis.
2.1.4 Core Requirement #4: Conveyance System
This core requirement will be addressed during final design.
2.1.5 Core Requirements #5: Erosion and Sediment Control
This core requirement will be addressed during final design.
2.1.6 Core Requirement #6: Maintenance and Operations
This core requirement will be addressed during final design.
2.1.7 Core Requirement #7: Financial Guarantees and Liability
This core requirement will be addressed at the time the permit is issued.
2.1.8 Core Requirement #8: Water Quality
The project is required to provide basic water quality treatment as the new plus replaced pollution
generating impervious surface exceeds 5,000 SF. See section 4.5 for details.
2.1.9 Core Requirement #9: On-Site BMPs
See Section 4.4 of this Report for discussion on how this Core Requirement is addressed.
2.2 Special Requirements
2.2.1 Special Requirement #1: Other Adopted Area Specific Requirements
There are no known additional requirements for the subject project.
2.2.2 Special Requirement #2: Flood Hazard Area Delineation
Not applicable since the project does not contain nor is adjacent to a flood hazard area.
2.2.3 Special Requirement #3: Flood Protection Facilities
Not applicable since the project does not rely on an existing flood protection facility and doesn’t plan to
modify or construct a new flood protection facility.
2.2.4 Special Requirement #4: Source Control
This special requirement will be addressed during final design.
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2.2.5 Special Requirement #5: Oil Control
Not applicable since the project is not a high use site. The expected average daily traffic is less than 100
vehicles per 1,000 square feet of gross building area.
2.2.6 Special Requirement #6: Aquifer Protection Area
Not applicable since the project is not in an Aquifer Protection Area.
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3. OFFSITE ANALYSIS
Task 1 Study Area Definition and Maps
The proposed project is located within the Lower Cedar River drainage basin.
Task 2 Resource Review
Sensitive Areas Folio
Renton GIS was reviewed for sensitive areas. The proposed project site does not fall within the
following sensitive area, coal mines, erosion hazard, flood hazard, floodway, channel migration zone,
landslide, seismic hazard, regulated stream, wetland, or wellhead protection.
Task 3 Field Investigation
Upstream
The location of the project site is east of the border between two major drainage basins. All flow west of
the site will be directed west towards the Lower Green River drainage basin, while all on site flow is
directed east towards the Lower Cedar River drainage basin. 116th Avenue SE is the local highpoint.
Upstream flow along 116th Avenue SE is collected in the conveyance system which is found along the
eastern side of the road. This portion of runoff is diverted away from the site. Therefore, the only source
of upstream runoff onsite is found from neighboring parcels to the north and south of the site. This is
due to variation in eastbound slopes which range in direction from northeast to southeast.
Downstream
Date of Field Inspection: Tuesday, July 20, 2021
Weather Conditions: Cloudy, approximately 60 degrees Fahrenheit
The site consists of three parcels of approximately 2.87 acres. The site is almost entirely undeveloped
except for a mobile home on the westernmost parcel. Pervious area consists of thick forest cover
throughout all three parcels. The entire site is located within the Lower Cedar River drainage basin.
There are consistent moderate to steep eastern slopes on site. Site runoff sheet flows over pervious
forest cover and moves towards the eastern end of the project site. The center of the project site
contains steep southeastern slopes greater than 30%. At this location, a portion of site runoff is directed
onto SE 186th Street which does not contain stormwater structures, however mild slopes on the street
convey runoff east via sheet flow towards 120th avenue SE. The remainder of site runoff will exit the site
near the northeast corner of the site and be directed into a shallow ditch which is found on the eastern
adjacent property. This ditch collects upstream runoff from the site and directs it towards the drainage
ditch along the western edge of 120th Avenue SE. All project runoff collects in this ditch and flows into
the open culvert crossing underneath 120th Avenue SE.
After crossing 120th Avenue SE, runoff outlets the exposed culvert into a densely forested ravine.
Moderate to steep banks line both the north and south side walls of the ravine. Runoff continues east
meandering southeast around the northern boundary of the neighborhood on SE 186th Street. This
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runoff then exits the tree line at the eastern end of the neighborhood and outlets into an open field. The
flow continues east reaching the ¼ mile extent of the analysis. See downstream map below for details.
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Figure 3.1 Downstream Map
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Task 4 Drainage System Description and Problem Description
Drainage complaints were researched within a quarter mile of the project site. City of Renton does not
list any current complaints along the project’s downstream route.
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4. FLOW CONTROL AND WATER QUALITY ANALYSIS
Per Section 1.2.3 of the RSWDM the flow control standard requires matching forested conditions for
50% of the 2-year and up to the full 50-year flow, and matches the 2, 10-year peak rate runoffs. Basic
water quality treatment is required as the new plus replaced pollution generating impervious surface
exceeds 5,000 SF as outlined in Section 1.2.8 of RSWDM. Therefore, a flow control facility is proposed
for the project. See section 4.3 below for details. A water quality treatment facility is proposed as the
project is not exempt from water quality treatment as delineated in Section 4.5 of this Report. The
drainage analysis was modeled using WWHM software.
4.1 Existing Conditions
There are ditches and culverts on the east side of 116th Avenue SE that flow from north to south and do
not enter the project site which is largely undeveloped. There was an existing mobile home with access
off of 116th Avenue SE that has been removed. The site contains variable slopes ranging from 2% to
greater than 30% from west to east. As per the flow control standard designated for the site, the entire
project area will be modeled as Till Forest in WWHM. The predeveloped areas are shown in the table
below.
Table 4.1 Existing Conditions
Ground Cover Area (acres)
Till Forest 2.83
Ex. Impervious 0.03
Total Basin 2.86
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4.2 Developed Conditions
Proposed development of the property will include the construction of 7 single family residences, access
road, utilities, and a detention vault. The impervious area per lot was determined using the criteria in
the Table 3.2.2.C of the 2022 RSWDM. Bypass area was determined to be onsite area which will not flow
downstream to the proposed vault. This area includes land cover outside of the wetland buffer zone.
Existing impervious area on 116th Avenue SE was used as a mitigation trade for bypassed area. These
areas were modeled for the sizing of the vault The site is zoned in an R-4 area. Therefore, maximum
impervious coverage is found to be either 4,000 square feet per lot or 55% of the lot area, whichever is
less. The developed conditions are shown below.
Table 4.2 Developed Conditions
Lot /Tract Total Area Impervious Pervious
1 9,503 4,000 5,503
2 9,001 4,000 5,001
3 9,009 4,000 5,009
4 9,011 4,000 5,011
5 9,085 4,000 5,085
6 9,054 4,000 5,054
7 10,216 4,000 6,216
Tract A 4,111 0 4,111
Tract C 1,071 0 1,071
Tract D 6,540 300 6,240
Tract E 525 0 525
ROW 38,848 27,114 11,734
Ex ROW 1,498 1,168 330
Total to Vault (sf) 117,472 56,582 60,890
Total to Vault (ac) 2.70 1.30 1.40
Bypass 8,048 6,735 1,313
Upstream Offsite 1,135 1,135 0
Bypass Modeled (Sf) 6,913 5,600 1,313
Bypass Modeled (sf) 0.16 0.13 0.03
TOTAL Project (SF) 124,385 62,182 62,203
TOTAL Project(ac) 2.86 1.43 1.43
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4.3 Flow Control System
Using the existing and developed areas above, a vault with dimensions of 86 feet by 48 feet with 7.5 feet
of live storage was sized. The vault will be providing 32,250 CF of storage at riser crest. Below are the
vault details and the flow rates. The second subbasin in the post-developed condition refers to the
bypass area displayed in the above exhibit. The full WWHM report can be found in Appendix A.
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4.4 BMP Feasibility
Per Section 1.2.9.1 in the RSWDM, projects subject to Core Requirement #9 must apply flow control
BMPs to either supplement the flow mitigation provided by required flow control facilities or provide
flow mitigation where flow control facilities are not required. Flow control BMPs must be implemented
per the requirements and approach detailed in Sections 1.2.9.2 and 1.2.9.3 for individual lots and
subdivisions or road improvement projects, respectively. Section 1.2.9.2. of the RSWDM is applicable to
this project.
Per Section 1.2.9.2, projects on individual sites/lots, flow control BMPs must be selected and applied
according to the individual lot BMP requirements. Section 1.2.9.2.1 Small Lot BMP Requirements were
implemented as the requirements applicable to the subject project (sites/lots <22,000 square feet).
1. The feasibility and applicability of full dispersion as detailed in Appendix C, Section C.2.1 must be
evaluated for all target impervious surfaces. If feasible and applicable, full dispersion must be
implemented as part of the proposed project. Typically, small lot full dispersion will be applicable
only in subdivisions where enough forest was preserved by tract, easement, or covenant to meet
the minimum requirements for full dispersion in Appendix C, Section C.2.1.1
Response: Full dispersion is not feasible as the minimum vegetated flow path of 100 feet is not
attainable.
2. Where full dispersion of target impervious roof areas is not feasible or applicable, or will cause
flooding or erosion impacts, the feasibility and applicability of full infiltration as detailed in Appendix
C, Section C.2.2 must be evaluated (note, this will require a soils report for the site/lot). If feasible
and applicable, full infiltration of roof runoff must be implemented as part of the proposed project.
Response: Per the Geotechnical report by Cobalt Geosciences, dated February 4, 2022, “Widespread
infiltration is not feasible due to the presence of a dense restrictive layer approximately 4 feet below site
elevations”.
3. All target impervious surfaces not mitigated by Requirements 1 and 2 above, must be mitigated to
the maximum extent feasible using one or more BMPs from the following list. Use of a given BMP is
subject to evaluation of its feasibility and applicability as detailed in Appendix C. Feasible BMPs are
required to be implemented. The BMPs listed below may be located anywhere on the site/lot
subject to the limitations and design specifications for each BMP. These BMPs must be implemented
as part of the proposed project.
• Full Infiltration per Appendix C, Section C.2.2, or per Section 5.2, whichever is applicable
Response: Per the Geotechnical report by Cobalt Geosciences, dated February 4, 2022, “Widespread
infiltration is not feasible due to the presence of a dense restrictive layer approximately 4 feet below site
elevations”.
• Limited Infiltration per Appendix C, Section C.2.3,
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Response: Per the Geotechnical report by Cobalt Geosciences, dated February 4, 2022, “Widespread
infiltration is not feasible due to the presence of a dense restrictive layer approximately 4 feet below site
elevations”.
• Rain Gardens per Appendix C, Section C.2.12, sized as follows:
Response: Per the Geotechnical report by Cobalt Geosciences, dated February 4, 2022, “Widespread
infiltration is not feasible due to the presence of a dense restrictive layer approximately 4 feet below site
elevations”.
• Bioretention per Appendix C, Section C.2.6, sized as follows:
o SeaTac regional scale factor equals 1.0: In till soils, provide bioretention volume based on 0.6
inches of equivalent storage depth; in outwash soils provide bioretention volume based on 0.1
inches of equivalent storage depth,
o SeaTac regional scale factor greater than 1.0: In till soils, provide bioretention volume based on
0.8 inches of equivalent storage depth; in outwash soils, provide bioretention volume based on 0.4
inches of equivalent storage depth,
Response: Per the Geotechnical report by Cobalt Geosciences, dated February 4, 2022, “Widespread
infiltration is not feasible due to the presence of a dense restrictive layer approximately 4 feet below site
elevations”.
• Permeable Pavement per Appendix C, Section C.2.7
Response: Per the Geotechnical report by Cobalt Geosciences, dated February 4, 2022, “Widespread
infiltration is not feasible due to the presence of a dense restrictive layer approximately 4 feet below site
elevations”.
4. All target impervious surfaces not mitigated by Requirements 1, 2 and 3 above, must be mitigated to
the maximum extent feasible using the Basic Dispersion BMP described below. Use of Basic
Dispersion is subject to evaluation of its feasibility and applicability as detailed in Appendix C.
Feasible BMPs are required to be implemented. Basic Dispersion BMPs may be located anywhere on
the site/lot subject to the limitations and design specifications cited in Appendix C. The BMP must
be implemented as part of the proposed project.
• Basic Dispersion per Appendix C, Section C.2.4,
Response: This BMP will be evaluated with the building permits during final design.
5. BMPs must be implemented, at minimum, for an impervious area equal to at least 10% of the
site/lot for site/lot sizes up to 11,000 square feet and at least 20% of the site/lot for site/lot sizes
between 11,000 and 22,000 square feet. For projects located in Zone 1 of the Aquifer Protection
Area, these impervious area amounts must be doubled. Doubling of the minimum impervious area
required for BMP implementation in Zone 1 of the Aquifer Protection Area is not required for
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projects located within 200 feet of a steep slope hazard area, landslide hazard, or erosion hazard
area. If these minimum areas are not mitigated using feasible BMPs from Requirements 1, 2, 3, and
4 above, one or more BMPs from the following list are required to be implemented to achieve
compliance. These BMPs must be implemented as part of the proposed project.
• Reduced Impervious Surface Credit per Appendix C, Section C.2.9,
• Native Growth Retention Credit per Appendix C, Section C.2.10.
• Tree Retention Credit per Appendix C, Section C.2.14
Response: Reduced impervious surface credit is not feasible due to the insufficient lot area/space to
provide applicable compliance levels and maintain required setbacks while having reasonable use of the
lot. BMPs must be implemented for an impervious area equal to at least 10% of impervious surface on
each lot. The native growth retention credit is not feasible because this type of set aside area is not
proposed on site.
6. The soil moisture holding capacity of new pervious surfaces (target pervious surfaces) must be
protected in accordance with the soil amendment BMP as detailed in Appendix C, Section C.2.13.
Response: Soil amendment will be incorporated for disturbed areas not covered with hard surfaces.
7. Any proposed connection of roof downspouts to the local drainage system must be via a perforated
pipe connection as detailed in Appendix C, Section C.2.11.
Response: Perforated pipe connections will be implemented to the maximum extent feasible.
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4.5 Water Quality Treatment Analysis and Design
The subject project has more than 5,000 SF of PGIS, therefore basic water quality treatment will be
provided per basic water quality treatment menu in Section 1.2.8 of the 2022 RSWDM.
Water quality will be provided through dead storage in the detention vault. MGS Flood software was
used to calculate dead storage requirements. The snip below from MGS Flood shows that the vault will
need a minimum of 7,122 CF of dead storage to achieve basic water quality treatment. A single cell of
the vault storage will be used for water quality treatment. The vault will provide greater than the
minimum required dead storage calculated using MGS Flood.
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5. CONVEYANCE SYSTEM ANALYSIS AND DESIGN
Conveyance system analysis and design will be addressed during final design.
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6. SPECIAL REPORTS AND STUDIES
The following reports are submitted under separate cover. Assessments are provided for reference
and informational purposes only. Core Design takes no responsibility or liability for these reports,
assessments, or designs as they were not completed under the direct supervision of Core Design.
➢ Wetland Reconnaissance and Stream Study
April 9, 2021
Altmann Oliver Associates, LLC
PO Box 578
Carnation, WA 98014
425-333-4535
➢ Geotechnical Evaluation
February 4, 2022
Cobalt Geosciences, LLC
PO Box 82243
Kenmore, WA 98028
(206) 331-1097
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7. OTHER PERMITS
• Civil Construction Permit
• NPDES Permit
• ROW Use Permit
• Building Permit (Vault)
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8. ESC ANALYSIS AND DESIGN
The ESC analysis and design will be addressed during final design.
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9. BOND QUANTITIES, FACILITY SUMMARIES, AND DECLARATION OF
COVENANT
The bond quantities, facility summary, and declaration of covenant will be addressed during final design.
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10. OPERATIONS AND MAINTENANCE
The operations and maintenance manual will be addressed during final design.
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Appendix A
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vvddNNEEddmmoonnddssAAvveeNNEEAAbbeerrddeeeennAAvveeNNEEEEMM eerrcceerrWWaayyWWeessttVVaalllleeyyHHwwyyEast Valley RdEast Valley Rd,§-405
,§-405
,§-405
µ0 1 2Miles
Flow Control Application Map
Reference 15-A
Date: 01/09/2014
Flow Control Standards
Peak Rate Flow Control Standard (Existing Site Conditions)
Flow Control Duration Standard (Existing Site Conditions)
Flow Control Duration Standard (Forested Conditions)
Flood Problem Flow
Unincorporated King County Flow Control Standards
Renton City Limits
Potential Annexation Area
Project Site
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
Soil Map—King County Area, Washington
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
7/26/2021
Page 1 of 352538605253900525394052539805254020525406052541005253860525390052539405253980525402052540605254100561340561380561420561460561500561540561580561620561660561700
561340 561380 561420 561460 561500 561540 561580 561620 561660 561700
47° 26' 14'' N 122° 11' 11'' W47° 26' 14'' N122° 10' 53'' W47° 26' 6'' N
122° 11' 11'' W47° 26' 6'' N
122° 10' 53'' WN
Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 10N WGS84
0 50 100 200 300
Feet
0 25 50 100 150
Meters
Map Scale: 1:1,740 if printed on A landscape (11" x 8.5") sheet.
Soil Map may not be valid at this scale.
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
MAP LEGEND MAP INFORMATION
Area of Interest (AOI)
Area of Interest (AOI)
Soils
Soil Map Unit Polygons
Soil Map Unit Lines
Soil Map Unit Points
Special Point Features
Blowout
Borrow Pit
Clay Spot
Closed Depression
Gravel Pit
Gravelly Spot
Landfill
Lava Flow
Marsh or swamp
Mine or Quarry
Miscellaneous Water
Perennial Water
Rock Outcrop
Saline Spot
Sandy Spot
Severely Eroded Spot
Sinkhole
Slide or Slip
Sodic Spot
Spoil Area
Stony Spot
Very Stony Spot
Wet Spot
Other
Special Line Features
Water Features
Streams and Canals
Transportation
Rails
Interstate Highways
US Routes
Major Roads
Local Roads
Background
Aerial Photography
The soil surveys that comprise your AOI were mapped at
1:24,000.
Warning: Soil Map may not be valid at this scale.
Enlargement of maps beyond the scale of mapping can cause
misunderstanding of the detail of mapping and accuracy of soil
line placement. The maps do not show the small areas of
contrasting soils that could have been shown at a more detailed
scale.
Please rely on the bar scale on each map sheet for map
measurements.
Source of Map: Natural Resources Conservation Service
Web Soil Survey URL:
Coordinate System: Web Mercator (EPSG:3857)
Maps from the Web Soil Survey are based on the Web Mercator
projection, which preserves direction and shape but distorts
distance and area. A projection that preserves area, such as the
Albers equal-area conic projection, should be used if more
accurate calculations of distance or area are required.
This product is generated from the USDA-NRCS certified data as
of the version date(s) listed below.
Soil Survey Area: King County Area, Washington
Survey Area Data: Version 16, Jun 4, 2020
Soil map units are labeled (as space allows) for map scales
1:50,000 or larger.
Date(s) aerial images were photographed: Jul 25, 2020—Jul 27,
2020
The orthophoto or other base map on which the soil lines were
compiled and digitized probably differs from the background
imagery displayed on these maps. As a result, some minor
shifting of map unit boundaries may be evident.
Soil Map—King County Area, Washington
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
7/26/2021
Page 2 of 3
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
Map Unit Legend
Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI
AgB Alderwood gravelly sandy
loam, 0 to 8 percent slopes
0.6 22.3%
AgC Alderwood gravelly sandy
loam, 8 to 15 percent slopes
2.0 77.7%
Totals for Area of Interest 2.6 100.0%
Soil Map—King County Area, Washington
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
7/26/2021
Page 3 of 3
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
Core Design, Inc. DAY SHORT PLAT Page 26
Appendix B
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
WWHM2012
PROJECT REPORT
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
19003 - Vault Add Bypass 2/13/2023 9:32:46 AM Page 2
General Model Information
Project Name:19003 - Vault Add Bypass
Site Name:
Site Address:
City:
Report Date:2/13/2023
Gage:Seatac
Data Start:1948/10/01
Data End:2009/09/30
Timestep:15 Minute
Precip Scale:1.000
Version Date:2021/08/18
Version:4.2.18
POC Thresholds
Low Flow Threshold for POC1:50 Percent of the 2 Year
High Flow Threshold for POC1:50 Year
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
19003 - Vault Add Bypass 2/13/2023 9:32:46 AM Page 3
Landuse Basin Data
Predeveloped Land Use
Basin 1
Bypass:No
GroundWater:No
Pervious Land Use acre
C, Forest, Mod 2.83
Pervious Total 2.83
Impervious Land Use acre
ROADS MOD 0.03
Impervious Total 0.03
Basin Total 2.86
Element Flows To:
Surface Interflow Groundwater
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
19003 - Vault Add Bypass 2/13/2023 9:32:46 AM Page 4
Mitigated Land Use
Basin 1
Bypass:No
GroundWater:No
Pervious Land Use acre
C, Lawn, Mod 1.4
Pervious Total 1.4
Impervious Land Use acre
ROADS MOD 0.66
ROOF TOPS FLAT 0.64
Impervious Total 1.3
Basin Total 2.7
Element Flows To:
Surface Interflow Groundwater
Vault 1 Vault 1
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
19003 - Vault Add Bypass 2/13/2023 9:32:46 AM Page 5
Bypass
Bypass:Yes
GroundWater:No
Pervious Land Use acre
C, Lawn, Mod 0.03
Pervious Total 0.03
Impervious Land Use acre
ROADS MOD 0.13
Impervious Total 0.13
Basin Total 0.16
Element Flows To:
Surface Interflow Groundwater
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
19003 - Vault Add Bypass 2/13/2023 9:32:46 AM Page 7
Mitigated Routing
Vault 1
Width:50 ft.
Length:86 ft.
Depth:8.5 ft.
Discharge Structure
Riser Height:7.5 ft.
Riser Diameter:18 in.
Orifice 1 Diameter:0.8125 in.Elevation:0 ft.
Orifice 2 Diameter:0.6875 in.Elevation:4.4 ft.
Orifice 3 Diameter:1 in.Elevation:4.6 ft.
Element Flows To:
Outlet 1 Outlet 2
Vault Hydraulic Table
Stage(feet)Area(ac.)Volume(ac-ft.)Discharge(cfs)Infilt(cfs)
0.0000 0.098 0.000 0.000 0.000
0.0944 0.098 0.009 0.005 0.000
0.1889 0.098 0.018 0.007 0.000
0.2833 0.098 0.028 0.009 0.000
0.3778 0.098 0.037 0.011 0.000
0.4722 0.098 0.046 0.012 0.000
0.5667 0.098 0.055 0.013 0.000
0.6611 0.098 0.065 0.014 0.000
0.7556 0.098 0.074 0.015 0.000
0.8500 0.098 0.083 0.016 0.000
0.9444 0.098 0.093 0.017 0.000
1.0389 0.098 0.102 0.018 0.000
1.1333 0.098 0.111 0.019 0.000
1.2278 0.098 0.121 0.019 0.000
1.3222 0.098 0.130 0.020 0.000
1.4167 0.098 0.139 0.021 0.000
1.5111 0.098 0.149 0.022 0.000
1.6056 0.098 0.158 0.022 0.000
1.7000 0.098 0.167 0.023 0.000
1.7944 0.098 0.177 0.024 0.000
1.8889 0.098 0.186 0.024 0.000
1.9833 0.098 0.195 0.025 0.000
2.0778 0.098 0.205 0.025 0.000
2.1722 0.098 0.214 0.026 0.000
2.2667 0.098 0.223 0.027 0.000
2.3611 0.098 0.233 0.027 0.000
2.4556 0.098 0.242 0.028 0.000
2.5500 0.098 0.251 0.028 0.000
2.6444 0.098 0.261 0.029 0.000
2.7389 0.098 0.270 0.029 0.000
2.8333 0.098 0.279 0.030 0.000
2.9278 0.098 0.289 0.030 0.000
3.0222 0.098 0.298 0.031 0.000
3.1167 0.098 0.307 0.031 0.000
3.2111 0.098 0.317 0.032 0.000
3.3056 0.098 0.326 0.032 0.000
3.4000 0.098 0.335 0.033 0.000
3.4944 0.098 0.345 0.033 0.000
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
19003 - Vault Add Bypass 2/13/2023 9:32:46 AM Page 8
3.5889 0.098 0.354 0.033 0.000
3.6833 0.098 0.363 0.034 0.000
3.7778 0.098 0.372 0.034 0.000
3.8722 0.098 0.382 0.035 0.000
3.9667 0.098 0.391 0.035 0.000
4.0611 0.098 0.400 0.036 0.000
4.1556 0.098 0.410 0.036 0.000
4.2500 0.098 0.419 0.036 0.000
4.3444 0.098 0.428 0.037 0.000
4.4389 0.098 0.438 0.040 0.000
4.5333 0.098 0.447 0.042 0.000
4.6278 0.098 0.456 0.049 0.000
4.7222 0.098 0.466 0.055 0.000
4.8167 0.098 0.475 0.060 0.000
4.9111 0.098 0.484 0.064 0.000
5.0056 0.098 0.494 0.067 0.000
5.1000 0.098 0.503 0.070 0.000
5.1944 0.098 0.512 0.073 0.000
5.2889 0.098 0.522 0.075 0.000
5.3833 0.098 0.531 0.078 0.000
5.4778 0.098 0.540 0.080 0.000
5.5722 0.098 0.550 0.082 0.000
5.6667 0.098 0.559 0.085 0.000
5.7611 0.098 0.568 0.087 0.000
5.8556 0.098 0.578 0.089 0.000
5.9500 0.098 0.587 0.091 0.000
6.0444 0.098 0.596 0.093 0.000
6.1389 0.098 0.606 0.095 0.000
6.2333 0.098 0.615 0.096 0.000
6.3278 0.098 0.624 0.098 0.000
6.4222 0.098 0.634 0.100 0.000
6.5167 0.098 0.643 0.102 0.000
6.6111 0.098 0.652 0.103 0.000
6.7056 0.098 0.661 0.105 0.000
6.8000 0.098 0.671 0.106 0.000
6.8944 0.098 0.680 0.108 0.000
6.9889 0.098 0.689 0.109 0.000
7.0833 0.098 0.699 0.111 0.000
7.1778 0.098 0.708 0.112 0.000
7.2722 0.098 0.717 0.114 0.000
7.3667 0.098 0.727 0.115 0.000
7.4611 0.098 0.736 0.117 0.000
7.5556 0.098 0.745 0.327 0.000
7.6500 0.098 0.755 1.039 0.000
7.7444 0.098 0.764 1.998 0.000
7.8389 0.098 0.773 3.068 0.000
7.9333 0.098 0.783 4.112 0.000
8.0278 0.098 0.792 5.004 0.000
8.1222 0.098 0.801 5.658 0.000
8.2167 0.098 0.811 6.082 0.000
8.3111 0.098 0.820 6.511 0.000
8.4056 0.098 0.829 6.874 0.000
8.5000 0.098 0.839 7.218 0.000
8.5944 0.098 0.848 7.546 0.000
8.6889 0.000 0.000 7.861 0.000
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
19003 - Vault Add Bypass 2/13/2023 9:32:46 AM Page 9
Analysis Results
POC 1
+ Predeveloped x Mitigated
Predeveloped Landuse Totals for POC #1
Total Pervious Area:2.83
Total Impervious Area:0.03
Mitigated Landuse Totals for POC #1
Total Pervious Area:1.43
Total Impervious Area:1.43
Flow Frequency Method:Log Pearson Type III 17B
Flow Frequency Return Periods for Predeveloped. POC #1
Return Period Flow(cfs)
2 year 0.089702
5 year 0.146547
10 year 0.187321
25 year 0.24137
50 year 0.283049
100 year 0.325682
Flow Frequency Return Periods for Mitigated. POC #1
Return Period Flow(cfs)
2 year 0.088424
5 year 0.116069
10 year 0.135817
25 year 0.162472
50 year 0.183604
100 year 0.20586
Annual Peaks
Annual Peaks for Predeveloped and Mitigated. POC #1
Year Predeveloped Mitigated
1949 0.114 0.100
1950 0.128 0.094
1951 0.190 0.137
1952 0.063 0.058
1953 0.050 0.064
1954 0.074 0.069
1955 0.120 0.076
1956 0.100 0.106
1957 0.087 0.084
1958 0.086 0.070
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
19003 - Vault Add Bypass 2/13/2023 9:33:36 AM Page 10
1959 0.073 0.065
1960 0.138 0.133
1961 0.072 0.097
1962 0.047 0.059
1963 0.063 0.074
1964 0.094 0.074
1965 0.066 0.092
1966 0.059 0.071
1967 0.144 0.097
1968 0.084 0.108
1969 0.081 0.075
1970 0.063 0.075
1971 0.075 0.084
1972 0.147 0.123
1973 0.065 0.083
1974 0.078 0.085
1975 0.108 0.085
1976 0.079 0.076
1977 0.018 0.072
1978 0.065 0.078
1979 0.037 0.096
1980 0.185 0.124
1981 0.054 0.080
1982 0.123 0.146
1983 0.099 0.084
1984 0.060 0.064
1985 0.034 0.074
1986 0.150 0.081
1987 0.137 0.118
1988 0.054 0.062
1989 0.039 0.084
1990 0.326 0.156
1991 0.179 0.125
1992 0.076 0.070
1993 0.067 0.058
1994 0.027 0.053
1995 0.094 0.072
1996 0.227 0.155
1997 0.172 0.155
1998 0.053 0.082
1999 0.198 0.138
2000 0.069 0.079
2001 0.017 0.087
2002 0.085 0.103
2003 0.127 0.097
2004 0.131 0.145
2005 0.098 0.072
2006 0.106 0.101
2007 0.250 0.216
2008 0.298 0.170
2009 0.144 0.098
Ranked Annual Peaks
Ranked Annual Peaks for Predeveloped and Mitigated. POC #1
Rank Predeveloped Mitigated
1 0.3262 0.2159
2 0.2978 0.1697
3 0.2501 0.1562
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
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4 0.2275 0.1550
5 0.1981 0.1547
6 0.1905 0.1459
7 0.1849 0.1446
8 0.1792 0.1381
9 0.1717 0.1365
10 0.1503 0.1330
11 0.1466 0.1246
12 0.1444 0.1240
13 0.1437 0.1235
14 0.1385 0.1178
15 0.1368 0.1080
16 0.1312 0.1059
17 0.1284 0.1026
18 0.1270 0.1010
19 0.1233 0.1000
20 0.1200 0.0984
21 0.1138 0.0971
22 0.1080 0.0970
23 0.1064 0.0968
24 0.1003 0.0964
25 0.0986 0.0942
26 0.0977 0.0921
27 0.0943 0.0873
28 0.0940 0.0851
29 0.0869 0.0850
30 0.0864 0.0843
31 0.0852 0.0840
32 0.0842 0.0839
33 0.0806 0.0836
34 0.0794 0.0831
35 0.0777 0.0821
36 0.0759 0.0811
37 0.0750 0.0802
38 0.0743 0.0787
39 0.0732 0.0780
40 0.0724 0.0758
41 0.0692 0.0756
42 0.0672 0.0753
43 0.0662 0.0753
44 0.0653 0.0743
45 0.0653 0.0742
46 0.0632 0.0736
47 0.0631 0.0719
48 0.0630 0.0718
49 0.0595 0.0717
50 0.0588 0.0708
51 0.0543 0.0700
52 0.0536 0.0697
53 0.0525 0.0693
54 0.0505 0.0655
55 0.0466 0.0641
56 0.0387 0.0639
57 0.0370 0.0618
58 0.0344 0.0590
59 0.0275 0.0585
60 0.0184 0.0581
61 0.0172 0.0535
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
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Duration Flows
The Facility PASSED
Flow(cfs)Predev Mit Percentage Pass/Fail
0.0449 15631 14354 91 Pass
0.0473 13980 12570 89 Pass
0.0497 12521 11178 89 Pass
0.0521 11133 10119 90 Pass
0.0545 9965 9291 93 Pass
0.0569 8992 8570 95 Pass
0.0593 8128 7790 95 Pass
0.0617 7321 7112 97 Pass
0.0641 6590 6412 97 Pass
0.0665 6000 5852 97 Pass
0.0689 5482 5339 97 Pass
0.0713 5005 4817 96 Pass
0.0737 4601 4372 95 Pass
0.0761 4194 3970 94 Pass
0.0785 3846 3615 93 Pass
0.0809 3516 3313 94 Pass
0.0833 3240 3061 94 Pass
0.0858 2984 2787 93 Pass
0.0882 2725 2517 92 Pass
0.0906 2483 2222 89 Pass
0.0930 2274 1989 87 Pass
0.0954 2082 1748 83 Pass
0.0978 1879 1498 79 Pass
0.1002 1728 1296 75 Pass
0.1026 1593 1125 70 Pass
0.1050 1439 905 62 Pass
0.1074 1306 704 53 Pass
0.1098 1192 509 42 Pass
0.1122 1110 398 35 Pass
0.1146 1017 322 31 Pass
0.1170 947 273 28 Pass
0.1194 877 235 26 Pass
0.1218 809 196 24 Pass
0.1243 751 144 19 Pass
0.1267 699 115 16 Pass
0.1291 643 97 15 Pass
0.1315 596 86 14 Pass
0.1339 541 77 14 Pass
0.1363 494 71 14 Pass
0.1387 442 62 14 Pass
0.1411 398 60 15 Pass
0.1435 357 56 15 Pass
0.1459 316 48 15 Pass
0.1483 288 40 13 Pass
0.1507 258 38 14 Pass
0.1531 233 35 15 Pass
0.1555 205 31 15 Pass
0.1579 177 28 15 Pass
0.1603 154 27 17 Pass
0.1627 134 25 18 Pass
0.1652 117 24 20 Pass
0.1676 107 21 19 Pass
0.1700 94 19 20 Pass
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
19003 - Vault Add Bypass 2/13/2023 9:33:36 AM Page 14
0.1724 80 17 21 Pass
0.1748 68 16 23 Pass
0.1772 61 13 21 Pass
0.1796 50 13 26 Pass
0.1820 44 11 25 Pass
0.1844 36 10 27 Pass
0.1868 28 10 35 Pass
0.1892 25 10 40 Pass
0.1916 22 9 40 Pass
0.1940 18 9 50 Pass
0.1964 14 8 57 Pass
0.1988 13 7 53 Pass
0.2012 10 4 40 Pass
0.2036 8 3 37 Pass
0.2061 7 3 42 Pass
0.2085 7 2 28 Pass
0.2109 7 2 28 Pass
0.2133 7 2 28 Pass
0.2157 7 1 14 Pass
0.2181 7 0 0 Pass
0.2205 6 0 0 Pass
0.2229 6 0 0 Pass
0.2253 6 0 0 Pass
0.2277 5 0 0 Pass
0.2301 5 0 0 Pass
0.2325 5 0 0 Pass
0.2349 5 0 0 Pass
0.2373 5 0 0 Pass
0.2397 5 0 0 Pass
0.2421 5 0 0 Pass
0.2446 5 0 0 Pass
0.2470 4 0 0 Pass
0.2494 4 0 0 Pass
0.2518 3 0 0 Pass
0.2542 3 0 0 Pass
0.2566 3 0 0 Pass
0.2590 3 0 0 Pass
0.2614 3 0 0 Pass
0.2638 3 0 0 Pass
0.2662 3 0 0 Pass
0.2686 3 0 0 Pass
0.2710 3 0 0 Pass
0.2734 3 0 0 Pass
0.2758 3 0 0 Pass
0.2782 3 0 0 Pass
0.2806 3 0 0 Pass
0.2830 3 0 0 Pass
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
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Water Quality
Water Quality BMP Flow and Volume for POC #1
On-line facility volume:0 acre-feet
On-line facility target flow:0 cfs.
Adjusted for 15 min:0 cfs.
Off-line facility target flow:0 cfs.
Adjusted for 15 min:0 cfs.
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
19003 - Vault Add Bypass 2/13/2023 9:33:36 AM Page 16
LID Report
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
19003 - Vault Add Bypass 2/13/2023 9:34:08 AM Page 17
Model Default Modifications
Total of 0 changes have been made.
PERLND Changes
No PERLND changes have been made.
IMPLND Changes
No IMPLND changes have been made.
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
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Appendix
Predeveloped Schematic
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
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Mitigated Schematic
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26