HomeMy WebLinkAboutRS_Cyprus Lane Plat_Preliminary TIR Drainage Report_210628_v1
PRELIMINARY
TECHNICAL INFORMATION REPORT
FOR
Cyprus Lane Plat
CITY OF RENTON, WASHINGTON
Prepared by: Andrew Oh, E.I.T
Date: June 28, 2021
Revised:
Core No.: 20134
6/28/21
Core Design, Inc. CYPRUS LANE PLAT i
Cyprus Lane Plat Table of Contents
1. PROJECT OVERVIEW ............................................................................................................................. 1
Figure 1-1 Vicinity Map ........................................................................................................................ 1
2. CONDITIONS AND REQUIREMENTS SUMMARY ................................................................................... 3
2.1 Core Requirements ............................................................................................................................. 4
2.1.1 Core Requirement #1: Discharge at the Natural Location ......................................................... 4
2.1.2 Core Requirement #2: Offsite Analysis ....................................................................................... 4
2.1.3 Core Requirement #3: Flow Control............................................................................................ 4
2.1.4 Core Requirement #4: Conveyance System ................................................................................ 4
2.1.5 Core Requirements #5: Erosion and Sediment Control .............................................................. 4
2.1.6 Core Requirement #6: Maintenance and Operations ................................................................. 4
2.1.7 Core Requirement #7: Financial Guarantees and Liability .......................................................... 4
2.1.8 Core Requirement #8: Water Quality ......................................................................................... 4
2.1.9 Core Requirement #9: On-Site BMPs .......................................................................................... 4
2.2 Special Requirements ......................................................................................................................... 4
2.2.1 Special Requirement #1: Other Adopted Area Specific Requirements ....................................... 4
2.2.2 Special Requirement #2: Flood Hazard Area Delineation ........................................................... 4
2.2.3 Special Requirement #3: Flood Protection Facilities ................................................................... 4
2.2.4 Special Requirement #4: Source Control .................................................................................... 5
2.2.5 Special Requirement #5: Oil Control ........................................................................................... 5
2.2.6 Special Requirement #6: Aquifer Protection Area ...................................................................... 5
3. OFFSITE ANALYSIS ................................................................................................................................ 6
Task 1 Study Area Definition and Maps .................................................................................................... 6
Task 2 Resource Review ........................................................................................................................... 6
Sensitive Areas Folio............................................................................................................................. 6
Task 3 Field Investigation ......................................................................................................................... 6
Task 4 Drainage System Description and Problem Description ............................................................... 6
4. FLOW CONTROL AND WATER QUALITY ANALYSIS ............................................................................... 8
4.1 Predeveloped Site Hydrology ............................................................................................................. 8
West Basin ............................................................................................................................................ 8
Core Design, Inc. CYPRUS LANE PLAT ii
Table 4.1-1 West Basin Predeveloped Conditions ............................................................................... 8
South Basin ........................................................................................................................................... 8
Table 4.1-2 South Basin Predeveloped Conditions .............................................................................. 8
4.2 Developed Site Hydrology .................................................................................................................. 9
West Basin ............................................................................................................................................ 9
Table 4.2-1 Developed Conditions ....................................................................................................... 9
South Basin ......................................................................................................................................... 10
Table 4.2-2 Developed Conditions ..................................................................................................... 10
4.3 Flow Control System ......................................................................................................................... 12
West Basin .......................................................................................................................................... 12
South Basin ......................................................................................................................................... 14
4.4 BMP Feasibility ................................................................................................................................. 16
4.5 Water Quality Treatment Analysis and Design ................................................................................. 19
West Basin .......................................................................................................................................... 19
South Basin ......................................................................................................................................... 19
5. CONVEYANCE SYSTEM ANALYSIS AND DESIGN .................................................................................. 20
6. SPECIAL REPORTS AND STUDIES ........................................................................................................ 21
7. OTHER PERMITS ................................................................................................................................. 22
8. ESC ANALYSIS AND DESIGN ................................................................................................................ 23
9. BOND QUANTITIES, FACILITY SUMMARIES, AND DECLARATION OF COVENANT .............................. 24
10. OPERATIONS AND MAINTENANCE ..................................................................................................... 25
Appendix A .................................................................................................................................................. 26
Appendix B .................................................................................................................................................. 27
Core Design, Inc. CYPRUS LANE PLAT Page 1
1. PROJECT OVERVIEW
The proposed Cyprus Lane project is located at 5816 NE 4th Place, Renton, WA and 510 Nile Avenue NE,
Renton, WA. See Vicinity Map below.
Figure 1-1 Vicinity Map
The site is composed of parcels #1123059023, #1123059021, and #1123059031 with an area of
approximately 5 acres. The site is bordered by NE 4th Place to the south and residential homes to the
north, east, and west.
The south two parcels were previously developed with a single story residential home and detached
garage. The north parcel was previously developed with a single story residential home. The rest of the
site was left as forest. The site generally slopes at 5-10% from northeast to southwest.
Proposed development of the property will include the demolition of all existing structures and the
construction of 15 single family residences, access road, public alley, utilities, and frontage
improvements.
Core Design, Inc. CYPRUS LANE PLAT Page 2
The subject project’s drainage facilities were designed using the guidelines and requirements
established in the 2017 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.
Core Design, Inc. CYPRUS LANE PLAT Page 3
2. CONDITIONS AND REQUIREMENTS SUMMARY
The proposed project is classified as requiring “Full Drainage Review” per the 2017 RSWDM. Therefore,
all nine core requirements and six special requirements will be addressed per Section 1.2 and 1.3 of the
2017 RSWDM.
Core Design, Inc. CYPRUS LANE PLAT Page 4
2.1 Core Requirements
2.1.1 Core Requirement #1: Discharge at the Natural Location
The project has two natural discharge locations. Both are to the City’s conveyance system. The north
discharge location is along Nile Avenue NE and the south discharge location is on NE 4th Place.
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 the 2, 10, and 100-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 if 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 4.3 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 or plans to modify
or construct a new flood protection facility.
Core Design, Inc. CYPRUS LANE PLAT Page 5
2.2.4 Special Requirement #4: Source Control
This special requirement will be addressed during final design.
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.
Core Design, Inc. CYPRUS LANE PLAT Page 6
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
There is negligible upstream tributary area.
Downstream Drainage Path
West Basin
This downstream route description begins at the catch basin located along the east side of Nile Avenue
NE right at the southwest corner of the project site. This catch basin captures runoff from the site and
directs flow south until the intersection with NE 4th Street. At this point the runoff is redirected west
along the north side of NE 4th Street. At a point in between Lyons Avenue NE and Jericho Avenue NE, the
conveyance system crosses to the south side of NE 4th Street. This concludes the ¼ mile analysis. The
drainage will continue to follow this path until it is discharged into a stream tributary to Lake
Washington. See the next page for a downstream route exhibit.
South Basin
This downstream route description begins at the two catch basins located along the south side of NE 4th
Place, one at the west side and the other at the east side of the site. The two catch basins combine at
the next manhole and flow south along Orcas Place NE until it meets NE 4th Street. The storm system
directs runoff to the west along NE 4th Street for approximately 800 ft at which point the system flows
south through a residential neighborhood. The storm system emerges again on NE 3rd Street and
continues to flow south along Kitsap Ave NE. This is the end of the ¼ mile analysis. The drainage will
continue to follow this path until it is discharged into a stream tributary to Lake Washington. See the
next page for a downstream route exhibit.
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.
4,514
376
WGS_1984_Web_Mercator_Auxiliary_Sphere
Downstream Analysis
This map is a user generated static output from an Internet mapping site and
is for reference only. Data layers that appear on this map may or may not be
accurate, current, or otherwise reliable.
THIS MAP IS NOT TO BE USED FOR NAVIGATION
Notes
None
Legend
256 0 128 256 Feet
Information Technology - GIS
RentonMapSupport@Rentonwa.gov
11/10/2020
City and County Labels
City and County Boundary
Addresses
Parcels
Network Structures
Access Riser
Inlet
Manhole
Utility Vault
Clean Out
Unknown
Control Structures
Pump Stations
Discharge Points
Water Quality
Detention Facilities
Pond
Tank
Vault
Bioswale
Wetland
Other
Stormwater Mains
Culverts
Open Drain
Facility Outlines
Private Network Structures
Access Riser
Inlet
Manhole
Clean Out
Utility Vault
Unknown
Private Control Structures
Private Pump Stations
Private Discharge Points
Private Water Quality
Private Detention Facilities
Tank
Wetland
Filter Strip
Infiltration Trench
Vault
Pond
Core Design, Inc. CYPRUS LANE PLAT Page 8
4. FLOW CONTROL AND WATER QUALITY ANALYSIS
A flow control facility is proposed for the project. See section 4.1 below for details. A water quality
treatment facility is proposed as the project is not exempt from water quality treatment as delineated in
Section 4.2 of this Report. The drainage analysis was modeled using MGS Flood software.
4.1 Predeveloped Site Hydrology
The site was previously developed with two single story residential homes and detached garage. The
rest of the site was left as forest. The total project area is approximately 5 acres and includes the site as
well as frontage improvements.
As per the flow control standard designated for the site, the entire project area will be modeled as Till
Forest in MGS Flood. The predeveloped areas are shown in the table below.
West Basin
The existing impervious in the west basin is from the existing Nile Avenue NE that will be repaved for
frontage improvements as it will drain into the vault in the developed conditions.
Table 4.1-1 West Basin Predeveloped Conditions
Ground Cover Area (acres)
Till Forest 0.83
Ex. Impervious 0.06
Total Basin 0.89
South Basin
Table 4.1-2 South Basin Predeveloped Conditions
Ground Cover Area (acres)
Till Forest 4.08
Total Basin 4.08
Core Design, Inc. CYPRUS LANE PLAT Page 9
4.2 Developed Site Hydrology
Proposed development of the property will include the demolition of all existing structures and the
construction of 15 single family residences, access road, utilities, and frontage improvements along NE
4th Place and Nile Avenue NE. Flow control BMP credit has been taken into account for modeling for
future flow control BMPs. A max impervious area per lot of 4,000 SF was found per the guideline of
4,000 SF or max impervious based on zoning, whichever is less. The project is split into two basins to
capture flows from different areas of the site (West/South). The developed conditions are shown below.
West Basin
Table 4.2-1 Developed Conditions
Total Area (SF) Impervious (SF) Pervious (SF)
Lot 9 4,662 2,000 2,662
Lot 10 4,778 2,000 2,778
Lot 11 9,265 4,000 5,265
Lot 12 9,201 4,000 5,201
Right of Way 5,967 4,690 1,277
Storm Tract 4,750 0 4,750
Total (SF) 38,623 16,690 21,933
Total (Acres) 0.89 0.38 0.51
*The upper pads of lot 9 and 10 will discharge to the south basin and the lower pads will discharge to
the west basin.
Core Design, Inc. CYPRUS LANE PLAT Page 10
South Basin
Table 4.2-2 Developed Conditions
Total Area (SF) Impervious (SF) Pervious (SF)
Lot 1 10,866 4,000 6,866
Lot 2 9,612 4,000 5,612
Lot 3 9,611 4,000 5,611
Lot 4 9,611 4,000 5,611
Lot 5 10,427 4,000 6,427
Lot 6 12,524 4,000 8,524
Lot 7 9,071 4,000 5,071
Lot 8 9,650 4,000 5,650
Lot 9 4,662 2,000 2,662
Lot 10 4,778 2,000 2,778
Lot 13 12,586 4,000 8,586
Lot 14 9,614 4,000 5,614
Lot 15 9,614 4,000 5,614
Right of Way 37,761 27,188 10,573
Alley 6,381 5,946 435
Open Space Tract 10,846 0 10,846
Total (SF) 177,614 81,134 96,480
Total (Acres) 4.08 1.86 2.21
*The upper pads of lot 9 and 10 will discharge to the south basin and the lower pads will discharge to
the west basin.
DESIGNE N G I N E E R I N G P L A N N I N G S U R V E Y I N G12100 NE 195th St, Suite 300Bothell, Washington 98011425.885.7877 Fax 425.885.796320134
DESIGNE N G I N E E R I N G P L A N N I N G S U R V E Y I N G12100 NE 195th St, Suite 300Bothell, Washington 98011425.885.7877 Fax 425.885.796320134
Core Design, Inc. CYPRUS LANE PLAT Page 12
4.3 Flow Control System
West Basin
Using the predeveloped and developed areas above, a 20’ x 86’ with 5.1’ of live storage was sized. The
vault will be providing 8,772 CF of storage at riser crest. Below are the vault details and the flow rates.
The full MGS Flood report can be found in Appendix B.
Predevelopment Runoff Postdevelopment Runoff
Tr (Years) Discharge (cfs) Tr (Years) Discharge (cfs)
----------------------------------------------------------------------------------------------------------------------
2-Year 3.688E-02 2-Year 1.767E-02
5-Year 5.513E-02 5-Year 2.648E-02
10-Year 7.162E-02 10-Year 3.345E-02
25-Year 8.845E-02 25-Year 3.721E-02
50-Year 0.114 50-Year 8.210E-02
100-Year 0.124 100-Year 0.112
200-Year 0.177 200-Year 0.133
500-Year 0.248 500-Year 0.161
** Record too Short to Compute Peak Discharge for These Recurrence Intervals
**** Flow Duration Performance ****
Excursion at Predeveloped 50%Q2 (Must be Less Than or Equal to 0%): -0.9% PASS
Maximum Excursion from 50%Q2 to Q2 (Must be Less Than or Equal to 0%): -0.9% PASS
Maximum Excursion from Q2 to Q50 (Must be less than 10%): 8.1% PASS
Percent Excursion from Q2 to Q50 (Must be less than 50%): 3.9% PASS
-------------------------------------------------------------------------------------------------
MEETS ALL FLOW DURATION DESIGN CRITERIA: PASS
Core Design, Inc. CYPRUS LANE PLAT Page 13
Link Name: New Structure Lnk1
Link Type: Structure
Downstream Link: None
Prismatic Pond Option Used
Pond Floor Elevation (ft) : 0.00
Riser Crest Elevation (ft) : 5.10
Max Pond Elevation (ft) : 6.10
Storage Depth (ft) : 5.10
Pond Bottom Length (ft) : 86.0
Pond Bottom Width (ft) : 20.0
Pond Side Slopes (ft/ft) : L1= 0.00 L2= 0.00 W1= 0.00 W2= 0.00
Bottom Area (sq-ft) : 1720.
Area at Riser Crest El (sq-ft) : 1,720.
(acres) : 0.039
Volume at Riser Crest (cu-ft) : 8,772.
(ac-ft) : 0.201
Area at Max Elevation (sq-ft) : 1720.
(acres) : 0.039
Vol at Max Elevation (cu-ft) : 10,492.
(ac-ft) : 0.241
Hydraulic Conductivity (in/hr) : 0.00
Massmann Regression Used to Estimate Hydralic Gradient
Depth to Water Table (ft) : 100.00
Bio-Fouling Potential : Low
Maintenance : Average or Better
Riser Geometry
Riser Structure Type : Circular
Riser Diameter (in) : 12.00
Common Length (ft) : 0.000
Riser Crest Elevation : 5.10 ft
Hydraulic Structure Geometry
Number of Devices: 3
---Device Number 1 ---
Device Type : Circular Orifice
Control Elevation (ft) : 0.00
Diameter (in) : 0.56
Orientation : Horizontal
Elbow : No
---Device Number 2 ---
Device Type : Circular Orifice
Control Elevation (ft) : 2.19
Diameter (in) : 0.50
Orientation : Horizontal
Elbow : Yes
---Device Number 3 ---
Device Type : Circular Orifice
Control Elevation (ft) : 3.00
Diameter (in) : 0.50
Orientation : Horizontal
Elbow : Yes
Core Design, Inc. CYPRUS LANE PLAT Page 14
South Basin
Using the predeveloped and developed areas above, a 60’ x 86’ vault with 10’ of live storage was sized.
The vault will be providing 51,600 CF of storage at riser crest. Below are the tank details and the flow
rates. The full MGS Flood report can be found in Appendix C.
Predevelopment Runoff Postdevelopment Runoff
Tr (Years) Discharge (cfs) Tr (Years) Discharge (cfs)
----------------------------------------------------------------------------------------------------------------------
2-Year 0.105 2-Year 5.396E-02
5-Year 0.168 5-Year 9.455E-02
10-Year 0.219 10-Year 0.126
25-Year 0.301 25-Year 0.153
50-Year 0.379 50-Year 0.162
100-Year 0.402 100-Year 0.302
200-Year 0.642 200-Year 0.357
500-Year 0.966 500-Year 0.426
** Record too Short to Compute Peak Discharge for These Recurrence Intervals
**** Flow Duration Performance ****
Excursion at Predeveloped 50%Q2 (Must be Less Than or Equal to 0%): -15.9% PASS
Maximum Excursion from 50%Q2 to Q2 (Must be Less Than or Equal to 0%): -2.2% PASS
Maximum Excursion from Q2 to Q50 (Must be less than 10%): -16.0% PASS
Percent Excursion from Q2 to Q50 (Must be less than 50%): 0.0% PASS
-------------------------------------------------------------------------------------------------
MEETS ALL FLOW DURATION DESIGN CRITERIA: PASS
Core Design, Inc. CYPRUS LANE PLAT Page 15
Link Name: New Structure Lnk1
Link Type: Structure
Downstream Link: None
Prismatic Pond Option Used
Pond Floor Elevation (ft) : 470.50
Riser Crest Elevation (ft) : 480.50
Max Pond Elevation (ft) : 481.50
Storage Depth (ft) : 10.00
Pond Bottom Length (ft) : 84.0
Pond Bottom Width (ft) : 58.0
Pond Side Slopes (ft/ft) : L1= 0.00 L2= 0.00 W1= 0.00 W2= 0.00
Bottom Area (sq-ft) : 4872.
Area at Riser Crest El (sq-ft) : 4,872.
(acres) : 0.112
Volume at Riser Crest (cu-ft) : 48,720.
(ac-ft) : 1.118
Area at Max Elevation (sq-ft) : 4872.
(acres) : 0.112
Vol at Max Elevation (cu-ft) : 53,592.
(ac-ft) : 1.230
Massmann Infiltration Option Used
Hydraulic Conductivity (in/hr) : 0.00
Massmann Regression Used to Estimate Hydralic Gradient
Depth to Water Table (ft) : 100.00
Bio-Fouling Potential : Low
Maintenance : Average or Better
Riser Geometry
Riser Structure Type : Circular
Riser Diameter (in) : 12.00
Common Length (ft) : 0.000
Riser Crest Elevation : 480.50 ft
Hydraulic Structure Geometry
Number of Devices: 3
---Device Number 1 ---
Device Type : Circular Orifice
Control Elevation (ft) : 470.50
Diameter (in) : 0.94
Orientation : Horizontal
Elbow : No
---Device Number 2 ---
Device Type : Circular Orifice
Control Elevation (ft) : 475.87
Diameter (in) : 1.00
Orientation : Horizontal
Elbow : Yes
---Device Number 3 ---
Device Type : Circular Orifice
Control Elevation (ft) : 477.50
Diameter (in) : 1.25
Orientation : Horizontal
Elbow : Yes
Core Design, Inc. CYPRUS LANE PLAT Page 16
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. The Section applicable to this project is Section
1.2.9.2.
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. The category of requirements applicable to the
subject project is the Small Lot BMP Requirements (for 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
Full dispersion is not feasible since there is no forested area.
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.
As per the Geotechnical report by Cobalt Geosciences dated February 2, 2021, “Infiltration is not
feasible due to the presence of dense fine-grained soil deposits, which act as an aquitard.”
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
As per the Geotechnical report by Cobalt Geosciences dated February 2, 2021, “Infiltration is not
feasible due to the presence of dense fine-grained soil deposits, which act as an aquitard.”
• Limited Infiltration per Appendix C, Section C.2.3,
As per the Geotechnical report by Cobalt Geosciences dated February 2, 2021, “Infiltration is not
feasible due to the presence of dense fine-grained soil deposits, which act as an aquitard.”
Core Design, Inc. CYPRUS LANE PLAT Page 17
• Rain Gardens per Appendix C, Section C.2.12, sized as follows:
As per the Geotechnical report by Cobalt Geosciences dated February 2, 2021, “Infiltration is not
feasible due to the presence of dense fine-grained soil deposits, which act as an aquitard.”
• 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,
As per the Geotechnical report by Cobalt Geosciences dated February 2, 2021, “Infiltration is not
feasible due to the presence of dense fine-grained soil deposits, which act as an aquitard.”
• Permeable Pavement per Appendix C, Section C.2.7
As per the Geotechnical report by Cobalt Geosciences dated February 2, 2021, “Infiltration is not
feasible due to the presence of dense fine-grained soil deposits, which act as an aquitard.”
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,
Basic Dispersion will be implemented for walkways. The minimum required BMP area will be met with
basic dispersion through sheetflow dispersion, splash blocks, or dispersion trenches. These BMPs will
be evaluated with the building permits.
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
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,
Core Design, Inc. CYPRUS LANE PLAT Page 18
• Native Growth Retention Credit per Appendix C, Section C.2.10.
• Tree Retention Credit per Appendix C, Section C.2.14
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.
The native grown retention credit is not feasible because there is no native vegetation on this site to
maintain.
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.
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.
Perforated pipe connections will be provided for any proposed connection of roof downspouts to the
local drainage system.
Core Design, Inc. CYPRUS LANE PLAT Page 19
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.
West Basin
Water quality will be provided through dead storage in the detention vault. The snip below from MGS
Flood shows that the vault will need a minimum of 2,537 CF of dead storage to achieve basic water
quality treatment. The proposed vault will provide 6,880 CF of dead storage (20’x86’x4’). Therefore,
basic water quality treatment has been met.
South Basin
Water quality will be provided through dead storage in the detention vault. The snip below from MGS
Flood shows that the vault will need a minimum of 11,959 CF of dead storage to achieve basic water
quality treatment. The proposed vault will provide 20,640 CF of dead storage (60’x86’x4’). Therefore,
basic water quality treatment has been met.
Core Design, Inc. CYPRUS LANE PLAT Page 20
5. CONVEYANCE SYSTEM ANALYSIS AND DESIGN
Conveyance system analysis and design will be addressed during final design.
Core Design, Inc. CYPRUS LANE PLAT Page 21
6. SPECIAL REPORTS AND STUDIES
The following reports and 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.
Geotechnical Evaluation
February 2, 2021
Cobalt Geosciences, LLC
P.O Box 82243
Kenmore, WA 98028
206-331-1097
Arborist Report
June 21, 2021
Greenforest Inc
4547 S. Lucile Street
Seattle, WA 98118
206-723-0656
Core Design, Inc. CYPRUS LANE PLAT Page 22
7. OTHER PERMITS
• Civil Construction Permit
• NPDES Permit
• ROW Use Permit
• Building Permit (Vaults)
Core Design, Inc. CYPRUS LANE PLAT Page 23
8. ESC ANALYSIS AND DESIGN
The ESC analysis and design will be addressed during final design.
Core Design, Inc. CYPRUS LANE PLAT Page 24
9. BOND QUANTITIES, FACILITY SUMMARIES, AND DECLARATION OF
COVENANT
The bond quantities, facility summary, and declaration of covenant will be addressed during final design.
Core Design, Inc. CYPRUS LANE PLAT Page 25
10. OPERATIONS AND MAINTENANCE
The operations and maintenance manual will be addressed during final design.
Core Design, Inc. CYPRUS LANE PLAT Page 26
Appendix A
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, Washington 98028
www.cobaltgeo.com (206) 331-1097
February 2, 2021
Mr. Bob Wenzl
Tuscany Homes
RE: Geotechnical Evaluation
Proposed Residence
5816 NE 4th Place
Renton, Washington
Dear Mr. Wenzl,
In accordance with your authorization, Cobalt Geosciences, LLC has prepared this letter to
discuss the results of our geotechnical evaluation at the referenced site.
The purpose of our evaluation was to determine the feasibility of utilizing infiltration devices for
stormwater runoff management as well as relevant earthwork information.
Site and Project Description
The site is located at 5816 NE 4th Place in Renton, Washington. The site consists of two
rectangular parcels (No. 1123059021 and 1123059023) with a total area of about 111,000 square
feet.
The property is developed with a single-family residence, driveway, and detached shop. The
property is vegetated with grasses, bushes/shrubs, blackberry vines, ivy, and variable diameter
evergreen and deciduous trees.
The site slope gently downward from northeast to southwest at magnitudes of 5 to 20 percent and
total relief of about 25 feet.
The property is bordered to the east, west, and north by residential parcels and to the south by NE
4th Place.
The project includes construction of multiple new residential structures in the central portion of
the property with at least one access roadway. Stormwater management may include dispersion,
detention, or infiltration facilities depending on feasibility.
Area Geology
The Geologic Map of King County indicates that the site is underlain by Vashon Glacial Till.
Vashon Glacial Till is typically characterized by an unsorted, non-stratified mixture of clay, silt,
sand, gravel, cobbles and boulders in variable quantities. These materials are typically dense and
relatively impermeable. The poor sorting reflects the mixing of the materials as these sediments
were overridden and incorporated by the glacial ice.
February 2, 2021
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Soil & Groundwater Conditions
As part of our evaluation, we excavated two test pits to determine the shallow soil and
groundwater conditions, where accessible.
The test pit encountered approximately 6 inches of topsoil and vegetation underlain by
approximately 2.25 to 2.75 feet of loose to medium dense, silty-fine to medium grained sand with
gravel (Weathered Glacial Till). These materials were underlain by dense to very dense, silty-fine
to medium grained sand with gravel (Glacial Till).
Groundwater was not encountered in the test pits. Groundwater could be present seasonally
perched on the underlying glacial till.
Erosion Hazard
The Natural Resources Conservation Services (NRCS) maps for King County indicate that the site
is underlain by Alderwood gravelly sandy loam (8 to 15 percent slopes). These soils would have a
slight to moderate erosion potential in a disturbed state depending on the slope magnitude.
It is our opinion that soil erosion potential at this project site can be reduced through landscaping
and surface water runoff control. Typically, erosion of exposed soils will be most noticeable
during periods of rainfall and may be controlled by the use of normal temporary erosion control
measures, such as silt fences, hay bales, mulching, control ditches and diversion trenches. The
typical wet weather season, with regard to site grading, is from October 31st to April 1st. Erosion
control measures should be in place before the onset of wet weather.
Seismic Hazard
The overall subsurface profile corresponds to a Site Class D as defined by Table 1613.5.2 of the
International Building Code (IBC). A Site Class D applies to an overall profile consisting of
stiff/medium dense soils within the upper 100 feet.
We referenced the U.S. Geological Survey (USGS) Earthquake Hazards Program Website to
obtain values for SS, S1, Fa, and Fv. The USGS website includes the most updated published data
on seismic conditions. The following tables provide seismic parameters from the USGS web site
with referenced parameters from ASCE 7-10 and 7-16.
Seismic Design Parameters (ASCE 7-10)
Site
Class
Spectral
Acceleration
at 0.2 sec. (g)
Spectral
Acceleration
at 1.0 sec. (g)
Site
Coefficients
Design Spectral
Response Parameters
Design
PGA
Fa Fv SDS SD1
D 1.384 0.519 1.0 1.5 0.923 0.519 0.567
February 2, 2021
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Seismic Design Parameters (ASCE 7-16)
Site
Class
Spectral
Acceleration
at 0.2 sec. (g)
Spectral
Acceleration
at 1.0 sec. (g)
Site
Coefficients
Design Spectral
Response Parameters
Design
PGA
Fa Fv SDS SD1
D 1.384 0.473 1.2 Null 1.107 Null 0.589
Additional seismic considerations include liquefaction potential and amplification of ground
motions by soft/loose soil deposits. The liquefaction potential is highest for loose sand with a
high groundwater table. The site has a low likelihood of liquefaction.
Conclusions and Recommendations
General
The site is underlain by weathered and unweathered glacial till. There are likely areas of fill
around the existing residence in yard areas. The proposed residences may be supported on
shallow foundation systems bearing on medium dense or firmer native soils or on structural fill
placed on the native soils. Local overexcavation of loose fill and weathered native soils may be
necessary depending on the proposed elevations and locations of the new residences.
Infiltration is not feasible due to the presence of dense fine-grained soil deposits, which act as an
aquitard. We recommend utilizing dispersion systems, detention, or direct connection to City
infrastructure. We should be provided with the final plans to verify suitability of the system
locations and elevations.
Site Preparation
Trees, shrubs and other vegetation should be removed prior to stripping of surficial organic-rich
soil and fill. Based on observations from the site investigation program, it is anticipated that the
stripping depth will be 6 to 18 inches. Deeper excavations will be necessary below large trees
where root systems can extend to greater depths, in areas of existing foundation systems, and in
any areas underlain by undocumented fill.
The native soils consist of silty-sand with gravel. Most of the native soils may be used as
structural fill provided they achieve compaction requirements and are within 3 percent of the
optimum moisture. Some of these soils may only be suitable for use as fill during the summer
months, as they will be above the optimum moisture levels in their current state. These soils are
variably moisture sensitive and may degrade during periods of wet weather and under equipment
traffic.
Imported structural fill should consist of a sand and gravel mixture with a maximum grain size of
3 inches and less than 5 percent fines (material passing the U.S. Standard No. 200 Sieve).
Structural fill should be placed in maximum lift thicknesses of 12 inches and should be compacted
to a minimum of 95 percent of the modified proctor maximum dry density, as determined by the
ASTM D 1557 test method.
February 2, 2021
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Temporary Excavations
Based on our understanding of the project, we anticipate that the grading could include local cuts
on the order of approximately 4 feet or less for foundation and utility placement. Any deeper
temporary excavations should be sloped no steeper than 1.5H:1V (Horizontal:Vertical) in loose
native soils and fill, 1H:1V in medium dense native soils, and 3/4H:1V in dense to very dense
native soils. If an excavation is subject to heavy vibration or surcharge loads, we recommend that
the excavations be sloped no steeper than 2H:1V, where room permits.
Temporary cuts should be in accordance with the Washington Administrative Code (WAC) Part
N, Excavation, Trenching, and Shoring. Temporary slopes should be visually inspected daily by a
qualified person during construction activities and the inspections should be documented in daily
reports. The contractor is responsible for maintaining the stability of the temporary cut slopes
and reducing slope erosion during construction.
Temporary cut slopes should be covered with visqueen to help reduce erosion during wet weather,
and the slopes should be closely monitored until the permanent retaining systems or slope
configurations are complete. Materials should not be stored or equipment operated within 10 feet
of the top of any temporary cut slope.
Soil conditions may not be completely known from the geotechnical investigation. In the case of
temporary cuts, the existing soil conditions may not be completely revealed until the excavation
work exposes the soil. Typically, as excavation work progresses the maximum inclination of
temporary slopes will need to be re-evaluated by the geotechnical engineer so that supplemental
recommendations can be made. Soil and groundwater conditions can be highly variable.
Scheduling for soil work will need to be adjustable, to deal with unanticipated conditions, so that
the project can proceed and required deadlines can be met.
If any variations or undesirable conditions are encountered during construction, we should be
notified so that supplemental recommendations can be made. If room constraints or
groundwater conditions do not permit temporary slopes to be cut to the maximum angles allowed
by the WAC, temporary shoring systems may be required. The contractor should be responsible
for developing temporary shoring systems, if needed. We recommend that Cobalt Geosciences
and the project structural engineer review temporary shoring designs prior to installation, to
verify the suitability of the proposed systems.
Foundation Design
The proposed residential buildings may be supported on shallow spread footing foundation
systems bearing on undisturbed dense or firmer native soils or on properly compacted structural
fill placed on the suitable native soils. Any undocumented fill and/or loose native soils should be
removed and replaced with structural fill below foundation elements. Structural fill below
footings should consist of clean angular rock 5/8 to 4 inches in size. Please note that significant
overexcavation may be required in some areas.
For shallow foundation support, we recommend widths of at least 16 and 24 inches, respectively,
for continuous wall and isolated column footings supporting the proposed structure. Provided
that the footings are supported as recommended above, a net allowable bearing pressure of 2,500
pounds per square foot (psf) may be used for design. Detention vaults at least 5 feet below grade
may be designed using 5,000 psf bearing.
February 2, 2021
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A 1/3 increase in the above value may be used for short duration loads, such as those imposed by
wind and seismic events. Structural fill placed on bearing, native subgrade should be compacted
to at least 95 percent of the maximum dry density based on ASTM Test Method D1557. Footing
excavations should be inspected to verify that the foundations will bear on suitable material.
Exterior footings should have a minimum depth of 18 inches below pad subgrade (soil grade) or
adjacent exterior grade, whichever is lower. Interior footings should have a minimum depth of 12
inches below pad subgrade (soil grade) or adjacent exterior grade, whichever is lower.
If constructed as recommended, the total foundation settlement is not expected to exceed 1 inch.
Differential settlement, along a 25-foot exterior wall footing, or between adjoining column
footings, should be less than ½ inch. This translates to an angular distortion of 0.002. Most
settlement is expected to occur during construction, as the loads are applied. However, additional
post-construction settlement may occur if the foundation soils are flooded or saturated. All
footing excavations should be observed by a qualified geotechnical consultant.
Resistance to lateral footing displacement can be determined using an allowable friction factor of
0.35 acting between the base of foundations and the supporting subgrades. Lateral resistance for
footings can also be developed using an allowable equivalent fluid passive pressure of 225 pounds
per cubic foot (pcf) acting against the appropriate vertical footing faces (neglect the upper 12
inches below grade in exterior areas). The frictional and passive resistance of the soil may be
combined without reduction in determining the total lateral resistance.
Care should be taken to prevent wetting or drying of the bearing materials during construction.
Any extremely wet or dry materials, or any loose or disturbed materials at the bottom of the
footing excavations, should be removed prior to placing concrete. The potential for wetting or
drying of the bearing materials can be reduced by pouring concrete as soon as possible after
completing the footing excavation and evaluating the bearing surface by the geotechnical engineer
or his representative.
Concrete Retaining Walls
The following table, titled Wall Design Criteria, presents the recommended soil related design
parameters for retaining walls with a level backslope. Contact Cobalt if an alternate retaining wall
system is used. This has been included for detention vaults.
Wall Design Criteria
“At-rest” Conditions (Lateral Earth Pressure – EFD+) 55 pcf (Equivalent Fluid Density)
“Active” Conditions (Lateral Earth Pressure – EFD+) 35 pcf (Equivalent Fluid Density)
Seismic Increase for “At-rest” Conditions
(Lateral Earth Pressure)
21H* (Uniform Distribution) 1 in 2,500 year
event
Seismic Increase for “At-rest” Conditions
(Lateral Earth Pressure)
14H* (Uniform Distribution) 1 in 500 year event
Seismic Increase for “Active” Conditions
(Lateral Earth Pressure)
7H* (Uniform Distribution)
Passive Earth Pressure on Low Side of Wall Neglect upper 2 feet, then 300 pcf EFD+
February 2, 2021
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(Allowable, includes F.S. = 1.5)
Soil-Footing Coefficient of Sliding Friction (Allowable;
includes F.S. = 1.5)
0.35
*H is the height of the wall; Increase based on one in 500 year seismic event (10 percent probability of being exceeded in
50 years),
+EFD – Equivalent Fluid Density
The stated lateral earth pressures do not include the effects of hydrostatic pressure generated by
water accumulation behind the retaining walls. Uniform horizontal lateral active and at-rest
pressures on the retaining walls from vertical surcharges behind the wall may be calculated using
active and at-rest lateral earth pressure coefficients of 0.3 and 0.5, respectively. A soil unit weight
of 125 pcf may be used to calculate vertical earth surcharges.
To reduce the potential for the buildup of water pressure against the walls, continuous footing
drains (with cleanouts) should be provided at the bases of the walls. The footing drains should
consist of a minimum 4-inch diameter perforated pipe, sloped to drain, with perforations placed
down and enveloped by a minimum 6 inches of pea gravel in all directions.
The backfill adjacent to and extending a lateral distance behind the walls at least 2 feet should
consist of free-draining granular material. All free draining backfill should contain less than 3
percent fines (passing the U.S. Standard No. 200 Sieve) based upon the fraction passing the U.S.
Standard No. 4 Sieve with at least 30 percent of the material being retained on the U.S. Standard
No. 4 Sieve. The primary purpose of the free-draining material is the reduction of hydrostatic
pressure. Some potential for the moisture to contact the back face of the wall may exist, even with
treatment, which may require that more extensive waterproofing be specified for walls, which
require interior moisture sensitive finishes.
We recommend that the backfill be compacted to at least 90 percent of the maximum dry density
based on ASTM Test Method D1557. In place density tests should be performed to verify
adequate compaction. Soil compactors place transient surcharges on the backfill. Consequently,
only light hand operated equipment is recommended within 3 feet of walls so that excessive stress
is not imposed on the walls.
Slab-on-Grade
We recommend that the upper 18 inches of the existing fill and/or native soils within slab areas
be re-compacted to at least 95 percent of the modified proctor (ASTM D1557 Test Method).
Often, a vapor barrier is considered below concrete slab areas. However, the usage of a vapor
barrier could result in curling of the concrete slab at joints. Floor covers sensitive to moisture
typically requires the usage of a vapor barrier. A materials or structural engineer should be
consulted regarding the detailing of the vapor barrier below concrete slabs. Exterior slabs
typically do not utilize vapor barriers.
The American Concrete Institutes ACI 360R-06 Design of Slabs on Grade and ACI 302.1R-04
Guide for Concrete Floor and Slab Construction are recommended references for vapor barrier
selection and floor slab detailing.
Slabs on grade may be designed using a coefficient of subgrade reaction of 210 pounds per cubic
inch (pci) assuming the slab-on-grade base course is underlain by structural fill placed and
compacted as outlined in Section 8.1. A 4- to 6-inch-thick capillary break layer should be placed
February 2, 2021
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over the prepared subgrade. This material should consist of pea gravel or 5/8 inch clean angular
rock.
A perimeter drainage system is recommended unless interior slab areas are elevated a minimum
of 12 inches above adjacent exterior grades. If installed, a perimeter drainage system should
consist of a 4 inch diameter perforated drain pipe surrounded by a minimum 6 inches of drain
rock wrapped in a non-woven geosynthetic filter fabric to reduce migration of soil particles into
the drainage system. The perimeter drainage system should discharge by gravity flow to a
suitable stormwater system.
Exterior grades surrounding buildings should be sloped at a minimum of one percent to facilitate
surface water flow away from the building and preferably with a relatively impermeable surface
cover immediately adjacent to the building.
Stormwater Management Feasibility
The site is underlain by weathered and unweathered glacial till. Groundwater could be present at
3 to 4 feet below grade during the wet season.
We conducted infiltration testing in TP-1 at a depth of 3 feet below grade. Following saturation,
testing and application factors for site variability (0.8), influent control (0.9), and testing (0.5),
the infiltration rate was 0.11 inches per hour. We do not recommend utilizing infiltration devices
at this site. Runoff will migrate to the unweathered till and then laterally across that soil layer.
Ultimately, this runoff would migrate onto adjacent properties.
Depending on the site grading and layout, local dispersion trenches could be utilized. We
anticipate that detention systems will be required for most of the new runoff from impervious
surfaces.
We should be provided with final plans for review to determine if the intent of our
recommendations has been incorporated or if additional modifications are needed.
Erosion and Sediment Control
Erosion and sediment control (ESC) is used to reduce the transportation of eroded sediment to
wetlands, streams, lakes, drainage systems, and adjacent properties. Erosion and sediment
control measures should be implemented, and these measures should be in general accordance
with local regulations. At a minimum, the following basic recommendations should be
incorporated into the design of the erosion and sediment control features for the site:
Schedule the soil, foundation, utility, and other work requiring excavation or the disturbance
of the site soils, to take place during the dry season (generally May through September).
However, provided precautions are taken using Best Management Practices (BMP’s), grading
activities can be completed during the wet season (generally October through April).
All site work should be completed and stabilized as quickly as possible.
Additional perimeter erosion and sediment control features may be required to reduce the
possibility of sediment entering the surface water. This may include additional silt fences, silt
fences with a higher Apparent Opening Size (AOS), construction of a berm, or other filtration
systems.
February 2, 2021
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Any runoff generated by dewatering discharge should be treated through construction of a
sediment trap if there is sufficient space. If space is limited other filtration methods will need
to be incorporated.
Closure
The information presented herein is based upon professional interpretation utilizing standard
practices and a degree of conservatism deemed proper for this project. We emphasize that this
report is valid for this project as outlined above and for the current site conditions and should not
be used for any other site.
Sincerely,
Cobalt Geosciences, LLC
2/2/2021
Phil Haberman, PE, LG, LEG
Principal
PH/sc
Proposed Development
5816 NE 4th Place
Renton, Washington
N
SITE PLAN
FIGURE 1
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
TP-1
TP-2
PT
Well-graded gravels, gravels, gravel-sand mixtures, little or no fines
Poorly graded gravels, gravel-sand mixtures, little or no fines
Silty gravels, gravel-sand-silt mixtures
Clayey gravels, gravel-sand-clay mixtures
Well-graded sands, gravelly sands, little or no fines
COARSE
GRAINED
SOILS
(more than 50%
retained on
No. 200 sieve)
Primarily organic matter, dark in color,
and organic odor Peat, humus, swamp soils with high organic content (ASTM D4427)HIGHLY ORGANIC
SOILS
FINE GRAINED
SOILS
(50% or more
passes the
No. 200 sieve)
MAJOR DIVISIONS SYMBOL TYPICAL DESCRIPTION
Gravels
(more than 50%
of coarse fraction
retained on No. 4
sieve)
Sands
(50% or more
of coarse fraction
passes the No. 4
sieve)
Silts and Clays
(liquid limit less
than 50)
Silts and Clays
(liquid limit 50 or
more)
Organic
Inorganic
Organic
Inorganic
Sands with
Fines
(more than 12%
fines)
Clean Sands
(less than 5%
fines)
Gravels with
Fines
(more than 12%
fines)
Clean Gravels
(less than 5%
fines)
Unified Soil Classification System (USCS)
Poorly graded sand, gravelly sands, little or no fines
Silty sands, sand-silt mixtures
Clayey sands, sand-clay mixtures
Inorganic silts of low to medium plasticity, sandy silts, gravelly silts,
or clayey silts with slight plasticity
Inorganic clays of low to medium plasticity, gravelly clays, sandy clays,
silty clays, lean clays
Organic silts and organic silty clays of low plasticity
Inorganic silts, micaceous or diatomaceous fine sands or silty soils,
elastic silt
Inorganic clays of medium to high plasticity, sandy fat clay,
or gravelly fat clay
Organic clays of medium to high plasticity, organic silts
Moisture Content Definitions
Grain Size Definitions
Dry Absence of moisture, dusty, dry to the touch
Moist Damp but no visible water
Wet Visible free water, from below water table
Grain Size Definitions
Description Sieve Number and/or Size
Fines <#200 (0.08 mm)
Sand
-Fine
-Medium
-Coarse
Gravel
-Fine
-Coarse
Cobbles
Boulders
#200 to #40 (0.08 to 0.4 mm)
#40 to #10 (0.4 to 2 mm)
#10 to #4 (2 to 5 mm)
#4 to 3/4 inch (5 to 19 mm)
3/4 to 3 inches (19 to 76 mm)
3 to 12 inches (75 to 305 mm)
>12 inches (305 mm)
Classification of Soil Constituents
MAJOR constituents compose more than 50 percent,
by weight, of the soil. Major constituents are capitalized
(i.e., SAND).
Minor constituents compose 12 to 50 percent of the soil
and precede the major constituents (i.e., silty SAND).
Minor constituents preceded by “slightly” compose
5 to 12 percent of the soil (i.e., slightly silty SAND).
Trace constituents compose 0 to 5 percent of the soil
(i.e., slightly silty SAND, trace gravel).
Relative Density Consistency
(Coarse Grained Soils) (Fine Grained Soils)
N, SPT, Relative
Blows/FT Density
0 - 4 Very loose
4 - 10 Loose
10 - 30 Medium dense
30 - 50 Dense
Over 50 Very dense
N, SPT, Relative
Blows/FT Consistency
Under 2 Very soft
2 - 4 Soft
4 - 8 Medium stiff
8 - 15 Stiff
15 - 30 Very stiff
Over 30 Hard
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
Soil Classification Chart Figure C1
Test Pit
Logs
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
Proposed Residences
5816 NE 4th Place
Renton, Washington
Test Pit TP-1
Date: December 29, 2021
Contractor: Client provided
Depth: 7’
Elevation: Logged By: PH Checked By: SC
Groundwater: None
Material Description
Moisture Content (%)Plastic
Limit
Liquid
Limit
10 20 30 400 50
1
2
3
4
5
6
DCP Equivalent N-Value
7
8
9
10
Loose to medium dense, silty-fine to medium grained sand with gravel,
reddish brown to yellowish brown, moist.
(Weathered Glacial Till)
SM
End of Test Pit 7’
Dense, silty-fine to medium grained sand with gravel,
grayish brown, moist. (Glacial Till) -Locally cemented
SM
Topsoil/Grass
Test Pit TP-2
Date: December 29, 2021
Contractor: Client provided
Depth: 7’
Elevation: Logged By: PH Checked By: SC
Groundwater: None
Material Description
Moisture Content (%)Plastic
Limit
Liquid
Limit
10 20 30 400 50
1
2
3
4
5
6
DCP Equivalent N-Value
7
8
9
10
Loose to medium dense, silty-fine to medium grained sand with gravel,
reddish brown to yellowish brown, moist.
(Weathered Glacial Till)
SM
End of Test Pit 7’
Dense, silty-fine to medium grained sand with gravel,
grayish brown, moist. (Glacial Till) -Locally cemented
SM
Topsoil/Grass
Core Design, Inc. CYPRUS LANE PLAT Page 27
Appendix B
—————————————————————————————————
MGS FLOOD
PROJECT REPORT
Program Version: MGSFlood 4.52
Program License Number: 200210008
Project Simulation Performed on: 05/05/2021 4:14 PM
Report Generation Date: 05/05/2021 4:28 PM
—————————————————————————————————
Input File Name: West Vault.fld
Project Name: West Vault
Analysis Title:
Comments:
———————————————— PRECIPITATION INPUT ————————————————
Computational Time Step (Minutes): 15
Extended Precipitation Time Series Selected
Climatic Region Number: 16
Full Period of Record Available used for Routing
Precipitation Station : 96004405 Puget East 44 in_5min 10/01/1939-10/01/2097
Evaporation Station : 961044 Puget East 44 in MAP
Evaporation Scale Factor : 0.750
HSPF Parameter Region Number: 1
HSPF Parameter Region Name : USGS Default
********** Default HSPF Parameters Used (Not Modified by User) ***************
********************** WATERSHED DEFINITION ***********************
Predevelopment/Post Development Tributary Area Summary
Predeveloped Post Developed
Total Subbasin Area (acres) 0.890 0.890
Area of Links that Include Precip/Evap (acres) 0.000 0.000
Total (acres) 0.890 0.890
----------------------SCENARIO: PREDEVELOPED
Number of Subbasins: 1
---------- Subbasin : Subbasin 1 ----------
-------Area (Acres) --------
Till Forest 0.830
Impervious 0.060
----------------------------------------------
Subbasin Total 0.890
----------------------SCENARIO: POSTDEVELOPED
Core Design, Inc. CYPRUS LANE PLAT Page 28
Number of Subbasins: 1
---------- Subbasin : Subbasin 1 ----------
-------Area (Acres) --------
Till Grass 0.510
Impervious 0.380
----------------------------------------------
Subbasin Total 0.890
************************* LINK DATA *******************************
----------------------SCENARIO: PREDEVELOPED
Number of Links: 0
************************* LINK DATA *******************************
----------------------SCENARIO: POSTDEVELOPED
Number of Links: 1
------------------------------------------
Link Name: New Structure Lnk1
Link Type: Structure
Downstream Link: None
Prismatic Pond Option Used
Pond Floor Elevation (ft) : 0.00
Riser Crest Elevation (ft) : 5.10
Max Pond Elevation (ft) : 6.10
Storage Depth (ft) : 5.10
Pond Bottom Length (ft) : 86.0
Pond Bottom Width (ft) : 20.0
Pond Side Slopes (ft/ft) : L1= 0.00 L2= 0.00 W1= 0.00 W2= 0.00
Bottom Area (sq-ft) : 1720.
Area at Riser Crest El (sq-ft) : 1,720.
(acres) : 0.039
Volume at Riser Crest (cu-ft) : 8,772.
(ac-ft) : 0.201
Area at Max Elevation (sq-ft) : 1720.
(acres) : 0.039
Vol at Max Elevation (cu-ft) : 10,492.
(ac-ft) : 0.241
Hydraulic Conductivity (in/hr) : 0.00
Massmann Regression Used to Estimate Hydralic Gradient
Depth to Water Table (ft) : 100.00
Bio-Fouling Potential : Low
Maintenance : Average or Better
Riser Geometry
Riser Structure Type : Circular
Riser Diameter (in) : 12.00
Core Design, Inc. CYPRUS LANE PLAT Page 29
Common Length (ft) : 0.000
Riser Crest Elevation : 5.10 ft
Hydraulic Structure Geometry
Number of Devices: 3
---Device Number 1 ---
Device Type : Circular Orifice
Control Elevation (ft) : 0.00
Diameter (in) : 0.56
Orientation : Horizontal
Elbow : No
---Device Number 2 ---
Device Type : Circular Orifice
Control Elevation (ft) : 2.19
Diameter (in) : 0.50
Orientation : Horizontal
Elbow : Yes
---Device Number 3 ---
Device Type : Circular Orifice
Control Elevation (ft) : 3.00
Diameter (in) : 0.50
Orientation : Horizontal
Elbow : Yes
**********************FLOOD FREQUENCY AND DURATION STATISTICS*******************
----------------------SCENARIO: PREDEVELOPED
Number of Subbasins: 1
Number of Links: 0
----------------------SCENARIO: POSTDEVELOPED
Number of Subbasins: 1
Number of Links: 1
********** Link: New Structure Lnk1 ********** Link WSEL Stats
WSEL Frequency Data(ft)
(Recurrence Interval Computed Using Gringorten Plotting Position)
Tr (yrs) WSEL Peak (ft)
======================================
1.05-Year 1.469
1.11-Year 1.615
1.25-Year 2.013
2.00-Year 2.598
3.33-Year 3.026
5-Year 3.391
10-Year 4.226
25-Year 4.792
50-Year 5.124
100-Year 5.135
Core Design, Inc. CYPRUS LANE PLAT Page 30
***********Groundwater Recharge Summary *************
Recharge is computed as input to Perlnd Groundwater Plus Infiltration in Structures
Total Predeveloped Recharge During Simulation
Model Element Recharge Amount (ac-ft)
-----------------------------------------------------------------------------------------------
Subbasin: Subbasin 1 157.899
_____________________________________
Total: 157.899
Total Post Developed Recharge During Simulation
Model Element Recharge Amount (ac-ft)
-----------------------------------------------------------------------------------------------
Subbasin: Subbasin 1 65.686
Link: New Structure Lnk1 0.000
_____________________________________
Total: 65.686
Total Predevelopment Recharge is Greater than Post Developed
Average Recharge Per Year, (Number of Years= 158)
Predeveloped: 0.999 ac-ft/year, Post Developed: 0.416 ac-ft/year
***********Water Quality Facility Data *************
----------------------SCENARIO: PREDEVELOPED
Number of Links: 0
----------------------SCENARIO: POSTDEVELOPED
Number of Links: 1
********** Link: New Structure Lnk1 **********
Basic Wet Pond Volume (91% Exceedance): 2537. cu-ft
Computed Large Wet Pond Volume, 1.5*Basic Volume: 3806. cu-ft
Infiltration/Filtration Statistics--------------------
Inflow Volume (ac-ft): 315.99
Inflow Volume Including PPT-Evap (ac-ft): 315.99
Total Runoff Infiltrated (ac-ft): 0.00, 0.00%
Total Runoff Filtered (ac-ft): 0.00, 0.00%
Primary Outflow To Downstream System (ac-ft): 315.96
Secondary Outflow To Downstream System (ac-ft): 0.00
Percent Treated (Infiltrated+Filtered)/Total Volume: 0.00%
***********Compliance Point Results *************
Scenario Predeveloped Compliance Subbasin: Subbasin 1
Core Design, Inc. CYPRUS LANE PLAT Page 31
Scenario Postdeveloped Compliance Link: New Structure Lnk1
*** Point of Compliance Flow Frequency Data ***
Recurrence Interval Computed Using Gringorten Plotting Position
Predevelopment Runoff Postdevelopment Runoff
Tr (Years) Discharge (cfs) Tr (Years) Discharge (cfs)
----------------------------------------------------------------------------------------------------------------------
2-Year 3.688E-02 2-Year 1.767E-02
5-Year 5.513E-02 5-Year 2.648E-02
10-Year 7.162E-02 10-Year 3.345E-02
25-Year 8.845E-02 25-Year 3.721E-02
50-Year 0.114 50-Year 8.210E-02
100-Year 0.124 100-Year 0.112
200-Year 0.177 200-Year 0.133
500-Year 0.248 500-Year 0.161
** Record too Short to Compute Peak Discharge for These Recurrence Intervals
**** Flow Duration Performance ****
Excursion at Predeveloped 50%Q2 (Must be Less Than or Equal to 0%): -0.9% PASS
Maximum Excursion from 50%Q2 to Q2 (Must be Less Than or Equal to 0%): -0.9% PASS
Maximum Excursion from Q2 to Q50 (Must be less than 10%): 8.1% PASS
Percent Excursion from Q2 to Q50 (Must be less than 50%): 3.9% PASS
-------------------------------------------------------------------------------------------------
MEETS ALL FLOW DURATION DESIGN CRITERIA: PASS
-------------------------------------------------------------------------------------------------
Core Design, Inc. CYPRUS LANE PLAT Page 32
—————————————————————————————————
MGS FLOOD
PROJECT REPORT
Program Version: MGSFlood 4.50
Program License Number: 200210008
Project Simulation Performed on: 06/24/2021 1:18 PM
Report Generation Date: 06/24/2021 1:28 PM —————————————————————————————————
Input File Name: Vault.fld
Project Name: South Vault
Analysis Title:
Comments:
———————————————— PRECIPITATION INPUT ————————————————
Computational Time Step (Minutes): 15
Extended Precipitation Time Series Selected
Climatic Region Number: 16
Full Period of Record Available used for Routing
Precipitation Station : 96004405 Puget East 44 in_5min 10/01/1939-10/01/2097
Evaporation Station : 961044 Puget East 44 in MAP
Evaporation Scale Factor : 0.750
HSPF Parameter Region Number: 1
HSPF Parameter Region Name : USGS Default
********** Default HSPF Parameters Used (Not Modified by User) ***************
********************** WATERSHED DEFINITION ***********************
Predevelopment/Post Development Tributary Area Summary
Predeveloped Post Developed
Total Subbasin Area (acres) 4.080 4.080
Area of Links that Include Precip/Evap (acres) 0.000 0.000
Total (acres) 4.080 4.080
----------------------SCENARIO: PREDEVELOPED
Number of Subbasins: 1
---------- Subbasin : Subbasin 1 ----------
-------Area (Acres) --------
Till Forest 4.080
----------------------------------------------
Subbasin Total 4.080
----------------------SCENARIO: POSTDEVELOPED
Number of Subbasins: 1
Core Design, Inc. CYPRUS LANE PLAT Page 33
---------- Subbasin : Subbasin 1 ----------
-------Area (Acres) --------
Till Grass 2.220
Impervious 1.860
----------------------------------------------
Subbasin Total 4.080
************************* LINK DATA *******************************
----------------------SCENARIO: PREDEVELOPED
Number of Links: 0
************************* LINK DATA *******************************
----------------------SCENARIO: POSTDEVELOPED
Number of Links: 1
------------------------------------------
Link Name: New Structure Lnk1
Link Type: Structure
Downstream Link: None
Prismatic Pond Option Used
Pond Floor Elevation (ft) : 470.50
Riser Crest Elevation (ft) : 480.50
Max Pond Elevation (ft) : 481.50
Storage Depth (ft) : 10.00
Pond Bottom Length (ft) : 84.0
Pond Bottom Width (ft) : 58.0
Pond Side Slopes (ft/ft) : L1= 0.00 L2= 0.00 W1= 0.00 W2= 0.00
Bottom Area (sq-ft) : 4872.
Area at Riser Crest El (sq-ft) : 4,872.
(acres) : 0.112
Volume at Riser Crest (cu-ft) : 48,720.
(ac-ft) : 1.118
Area at Max Elevation (sq-ft) : 4872.
(acres) : 0.112
Vol at Max Elevation (cu-ft) : 53,592.
(ac-ft) : 1.230
Massmann Infiltration Option Used
Hydraulic Conductivity (in/hr) : 0.00
Massmann Regression Used to Estimate Hydralic Gradient
Depth to Water Table (ft) : 100.00
Bio-Fouling Potential : Low
Maintenance : Average or Better
Riser Geometry
Riser Structure Type : Circular
Riser Diameter (in) : 12.00
Common Length (ft) : 0.000
Core Design, Inc. CYPRUS LANE PLAT Page 34
Riser Crest Elevation : 480.50 ft
Hydraulic Structure Geometry
Number of Devices: 3
---Device Number 1 ---
Device Type : Circular Orifice
Control Elevation (ft) : 470.50
Diameter (in) : 0.94
Orientation : Horizontal
Elbow : No
---Device Number 2 ---
Device Type : Circular Orifice
Control Elevation (ft) : 475.87
Diameter (in) : 1.00
Orientation : Horizontal
Elbow : Yes
---Device Number 3 ---
Device Type : Circular Orifice
Control Elevation (ft) : 477.50
Diameter (in) : 1.25
Orientation : Horizontal
Elbow : Yes
**********************FLOOD FREQUENCY AND DURATION STATISTICS*******************
----------------------SCENARIO: PREDEVELOPED
Number of Subbasins: 1
Number of Links: 0
----------------------SCENARIO: POSTDEVELOPED
Number of Subbasins: 1
Number of Links: 1
********** Link: New Structure Lnk1 ********** Link WSEL Stats
WSEL Frequency Data(ft)
(Recurrence Interval Computed Using Gringorten Plotting Position)
Tr (yrs) WSEL Peak (ft)
======================================
1.05-Year 473.520
1.11-Year 473.813
1.25-Year 474.409
2.00-Year 475.968
3.33-Year 476.720
5-Year 477.535
10-Year 478.531
25-Year 479.800
50-Year 480.205
100-Year 480.532
Core Design, Inc. CYPRUS LANE PLAT Page 35
***********Groundwater Recharge Summary *************
Recharge is computed as input to Perlnd Groundwater Plus Infiltration in Structures
Total Predeveloped Recharge During Simulation
Model Element Recharge Amount (ac-ft)
-----------------------------------------------------------------------------------------------
Subbasin: Subbasin 1 776.179
_____________________________________
Total: 776.179
Total Post Developed Recharge During Simulation
Model Element Recharge Amount (ac-ft)
-----------------------------------------------------------------------------------------------
Subbasin: Subbasin 1 285.927
Link: New Structure Lnk1 0.000
_____________________________________
Total: 285.927
Total Predevelopment Recharge is Greater than Post Developed
Average Recharge Per Year, (Number of Years= 158)
Predeveloped: 4.913 ac-ft/year, Post Developed: 1.810 ac-ft/year
***********Water Quality Facility Data *************
----------------------SCENARIO: PREDEVELOPED
Number of Links: 0
----------------------SCENARIO: POSTDEVELOPED
Number of Links: 1
********** Link: New Structure Lnk1 **********
Basic Wet Pond Volume (91% Exceedance): 11959. cu-ft
Computed Large Wet Pond Volume, 1.5*Basic Volume: 17939. cu-ft
Infiltration/Filtration Statistics--------------------
Inflow Volume (ac-ft): 1478.83
Inflow Volume Including PPT-Evap (ac-ft): 1478.83
Total Runoff Infiltrated (ac-ft): 0.00, 0.00%
Total Runoff Filtered (ac-ft): 0.00, 0.00%
Primary Outflow To Downstream System (ac-ft): 1478.63
Secondary Outflow To Downstream System (ac-ft): 0.00
Percent Treated (Infiltrated+Filtered)/Total Volume: 0.00%
***********Compliance Point Results *************
Scenario Predeveloped Compliance Subbasin: Subbasin 1
Core Design, Inc. CYPRUS LANE PLAT Page 36
Scenario Postdeveloped Compliance Link: New Structure Lnk1
*** Point of Compliance Flow Frequency Data ***
Recurrence Interval Computed Using Gringorten Plotting Position
Predevelopment Runoff Postdevelopment Runoff
Tr (Years) Discharge (cfs) Tr (Years) Discharge (cfs)
----------------------------------------------------------------------------------------------------------------------
2-Year 0.105 2-Year 6.280E-02
5-Year 0.168 5-Year 0.102
10-Year 0.219 10-Year 0.148
25-Year 0.301 25-Year 0.182
50-Year 0.379 50-Year 0.191
100-Year 0.402 100-Year 0.281
200-Year 0.642 200-Year 0.347
500-Year 0.966 500-Year 0.433
** Record too Short to Compute Peak Discharge for These Recurrence Intervals
**** Flow Duration Performance ****
Excursion at Predeveloped 50%Q2 (Must be Less Than or Equal to 0%): -7.1% PASS
Maximum Excursion from 50%Q2 to Q2 (Must be Less Than or Equal to 0%): -0.2% PASS
Maximum Excursion from Q2 to Q50 (Must be less than 10%): 5.3% PASS
Percent Excursion from Q2 to Q50 (Must be less than 50%): 8.2% PASS
-------------------------------------------------------------------------------------------------
MEETS ALL FLOW DURATION DESIGN CRITERIA: PASS
-------------------------------------------------------------------------------------------------