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Home My WebLink AboutRS_ Prelim TIR Drainage Report PRELIMINARY TECHNICAL INFORMATION REPORT FOR Hardy Short Plat CITY OF RENTON, WASHINGTON Prepared by: Andrew Oh, E.I.T Date: March 10, 2021 Revised: Core No.: 21019 3/10/21 Core Design, Inc. HARDY SHORT PLAT i Hardy Short Plat Table of Contents 1. PROJECT OVERVIEW .......................................................................................................................... 1-1 Figure 1-1 Vicinity Map ..................................................................................................................... 1-1 2. CONDITIONS AND REQUIREMENTS SUMMARY ................................................................................ 2-1 2.1 Core Requirements .......................................................................................................................... 2-2 2.1.1 Core Requirement #1: Discharge at the Natural Location ...................................................... 2-2 2.1.2 Core Requirement #2: Offsite Analysis .................................................................................... 2-2 2.1.3 Core Requirement #3: Flow Control......................................................................................... 2-2 2.1.4 Core Requirement #4: Conveyance System ............................................................................. 2-2 2.1.5 Core Requirements #5: Erosion and Sediment Control ........................................................... 2-2 2.1.6 Core Requirement #6: Maintenance and Operations .............................................................. 2-2 2.1.7 Core Requirement #7: Financial Guarantees and Liability ....................................................... 2-2 2.1.8 Core Requirement #8: Water Quality ...................................................................................... 2-2 2.1.9 Core Requirement #9: On-Site BMPs ....................................................................................... 2-2 2.2 Special Requirements ...................................................................................................................... 2-2 2.2.1 Special Requirement #1: Other Adopted Area Specific Requirements .................................... 2-2 2.2.2 Special Requirement #2: Flood Hazard Area Delineation ........................................................ 2-2 2.2.3 Special Requirement #3: Flood Protection Facilities ................................................................ 2-2 2.2.4 Special Requirement #4: Source Control ................................................................................. 2-3 2.2.5 Special Requirement #5: Oil Control ........................................................................................ 2-3 2.2.6 Special Requirement #6: Aquifer Protection Area ................................................................... 2-3 3. OFFSITE ANALYSIS ............................................................................................................................. 3-1 Task 1 Study Area Definition and Maps ................................................................................................. 3-1 Task 2 Resource Review ........................................................................................................................ 3-1 Sensitive Areas Folio.......................................................................................................................... 3-1 Task 3 Field Investigation ...................................................................................................................... 3-1 Task 4 Drainage System Description and Problem Description ............................................................ 3-1 4. FLOW CONTROL AND WATER QUALITY ANALYSIS ............................................................................ 4-1 4.1 Predeveloped Site Hydrology .......................................................................................................... 4-1 Table 4-1 Predeveloped Conditions .................................................................................................. 4-1 Core Design, Inc. HARDY SHORT PLAT ii 4.2 Developed Site Hydrology ............................................................................................................... 4-1 Table 4-2 Developed Conditions ....................................................................................................... 4-1 4.3 Flow Control System ........................................................................................................................ 4-4 4.4 BMP Feasibility ................................................................................................................................ 4-6 4.5 Water Quality Treatment Analysis and Design ................................................................................ 4-9 Table 4-3 PGIS Areas ......................................................................................................................... 4-9 5. CONVEYANCE SYSTEM ANALYSIS AND DESIGN ................................................................................. 5-1 6. SPECIAL REPORTS AND STUDIES ....................................................................................................... 6-1 7. OTHER PERMITS ................................................................................................................................ 7-1 8. ESC ANALYSIS AND DESIGN ............................................................................................................... 8-1 9. BOND QUANTITIES, FACILITY SUMMARIES, AND DECLARATION OF COVENANT ............................. 9-1 10. OPERATIONS AND MAINTENANCE .................................................................................................. 10-1 Appendix A ............................................................................................................................................... 10-2 Appendix B ............................................................................................................................................... 10-3 Core Design, Inc. HARDY SHORT PLAT Page 1-1 1. PROJECT OVERVIEW The proposed Hardy Short Plat project is located at 857 Field Avenue NE, Renton, WA 98059. See Vicinity Map below. Figure 1-1 Vicinity Map The site is composed of a single parcel number 1437650040 with an area of approximately 0.35 acres. The total site area, including frontage, is approximately 0.41 acres. The site is bordered by Field Avenue NE to the east and residential homes to the north, south, and west. The site was previously developed with a single story residential home. The site generally slopes at 5- 10% from east to west. Proposed development of the property will include the demolition of all existing structures and the construction of two single family residences, shared access road, utilities, and frontage improvements. 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) are required for this project. KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL TECHNICAL INFORMATION REPORT (TIR) WORKSHEET Part 1 PROJECT OWNER AND PROJECT ENGINEER Part 2 PROJECT LOCATION AND DESCRIPTION Project Owner ___________________________ Phone _________________________________ Address _______________________________ _______________________________________ Project Engineer _________________________ Company ______________________________ Phone _________________________________ Project Name _________________________ DPER Permit # ________________________ Location Township ______________ Range ________________ Section ________________ Site Address __________________________ _____________________________________ Part 3 TYPE OF PERMIT APPLICATION Part 4 OTHER REVIEWS AND PERMITS Landuse (e.g.,Subdivision / Short Subd. / UPD) Building (e.g.,M/F / Commercial / SFR) Clearing and Grading Right-of-Way Use Other _______________________ DFW HPA COE 404 DOE Dam Safety FEMA Floodplain COE Wetlands Other ________ Shoreline Management Structural Rockery/Vault/_____ ESA Section 7 Part 5 PLAN AND REPORT INFORMATION Technical Information Report Site Improvement Plan (Engr. Plans) T ype of Drainage Review (check one): Date (include revision dates): Date of Final: Full Targeted Simplified Large Project Directed __________________ __________________ __________________ Plan Type (check one): Date (include revision dates): Date of Final: Full Modified Simplified __________________ __________________ __________________ Part 6 SWDM ADJUSTMENT APPROVALS Type (circle one): Standard / Experimental / Blanket Description: (include conditions in TIR Section 2) ____________________________________________________________________________________ ____________________________________________________________________________________ ____________________________________________________________________________________ Approved Adjustment No. ______________________ Date of Approval: ______________________ 2016 Surface Water Design Manual 4/24/2016 1 (NA) KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL TECHNICAL INFORMATION REPORT (TIR) WORKSHEET Part 7 MONITORING REQUIREMENTS Monitoring Required: Yes / No Start Date: _______________________ Completion Date: _______________________ Describe: _________________________________ _________________________________________ _________________________________________ Re: KCSWDM Adjustment No. ________________ Part 8 SITE COMMUNITY AND DRAINAGE BASIN Community Plan : ____________________________________________________________________ Special District Overlays: ______________________________________________________________ Drainage Basin: _____________________________________________________________________ Stormwater Requirements: ____________________________________________________________ Part 9 ONSITE AND ADJACENT SENSITIVE AREAS River/Stream ________________________ Lake ______________________________ Wetlands ____________________________ Closed Depression ____________________ Floodplain ___________________________ Other _______________________________ _______________________________ Steep Slope __________________________ Erosion Hazard _______________________ Landslide Hazard ______________________ Coal Mine Hazard ______________________ Seismic Hazard _______________________ Habitat Protection ______________________ _____________________________________ Part 10 SOILS Soil Type _________________ _________________ _________________ _________________ Slopes _________________ _________________ _________________ _________________ Erosion Potential _________________ _________________ _________________ _________________ High Groundwater Table (within 5 feet) Other ________________________________ Sole Source Aquifer Seeps/Springs Additional Sheets Attached 2016 Surface Water Design Manual 4/24/2016 2 KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL TECHNICAL INFORMATION REPORT (TIR) WORKSHEET Part 11 DRAINAGE DESIGN LIMITATIONS REFERENCE  Core 2 – Offsite Analysis_________________  Sensitive/Critical Areas__________________  SEPA________________________________  LID Infeasibility________________________  Other________________________________  _____________________________________ LIMITATION / SITE CONSTRAINT _______________________________________ _______________________________________ _______________________________________ _______________________________________ _______________________________________ _______________________________________  Additional Sheets Attached Part 12 TIR SUMMARY SHEET (provide one TIR Summary Sheet per Threshold Discharge Area) Threshold Discharge Area: (name or description) Core Requirements (all 8 apply): Discharge at Natural Location Number of Natural Discharge Locations: Offsite Analysis Level: 1 / 2 / 3 dated:__________________ Flow Control (include facility summary sheet) Level: 1 / 2 / 3 or Exemption Number ____________ Flow Control BMPs _______________________________ Conveyance System Spill containment located at: _________________________ Erosion and Sediment Control / Construction Stormwater Pollution Prevention CSWPP/CESCL/ESC Site Supervisor: _____________________ Contact Phone: _________________________ After Hours Phone: _________________________ Maintenance and Operation Responsibility (circle one): Private / Public If Private, Maintenance Log Required: Yes / No Financial Guarantees and Liability Provided: Yes / No Water Quality (include facility summary sheet) Type (circle one): Basic / Sens. Lake / Enhanced Basic / Bog or Exemption No. ______________________ Landscape Management Plan: Yes / No Special Requirements (as applicable): Area Specific Drainage Requirements Type: CDA / SDO / MDP / BP / LMP / Shared Fac. / None Name: ________________________ Floodplain/Floodway Delineation Type (circle one): Major / Minor / Exemption / None 100-year Base Flood Elevation (or range): ______________ Datum: Flood Protection Facilities Describe: 2016 Surface Water Design Manual 4/24/2016 3 KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL TECHNICAL INFORMATION REPORT (TIR) WORKSHEET Part 12 TIR SUMMARY SHEET (provide one TIR Summary Sheet per Threshold Discharge Area) Source Control (comm ercial / industrial land use) Describe land use: Describe any structural controls: Oil Control High-use Site: Yes / No Treatment BMP: ________________________________ Maintenance Agreement: Yes / No with whom? ____________________________________ Other Drainage Structures Describe: Part 13 EROSION AND SEDIMENT CONTROL REQUIREMENTS MINIMUM ESC REQUIREMENTS DURING CONSTRUCTION  Clearing Limits  Cover Measures  Perimeter Protection  Traffic Area Stabilization  Sediment Retention  Surface Water Collection  Dewatering Control  Dust Control  Flow Control  Protection of Flow Control BMP Facilities (existing and proposed)  Maintain BMPs / Manage Project MINIMUM ESC REQUIREMENTS AFTER CONSTRUCTION  Stabilize exposed surfaces  Remove and restore Temporary ESC Facilities  Clean and remove all silt and debris, ensure operation of Permanent Facilities, restore operation of Flow Control BMP Facilities as necessary  Flag limits of SAO and open space preservation areas  Other ______________________ Part 14 STORMWATER FACILITY DESCRIPTIONS (Note: Include Facility Summary and Sketch) Flow Control Type/Description Water Quality Type/Description  Detention  Infiltration  Regional Facility  Shared Facility  Flow Control BMPs  Other ________________ ________________ ________________ ________________ ________________ ________________  Vegetated Flowpath  Wetpool  Filtration  Oil Control  Spill Control  Flow Control BMPs  Other ________________ ________________ ________________ ________________ ________________ ________________ ________________ 2016 Surface Water Design Manual 4/24/2016 4 KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL TECHNICAL INFORMATION REPORT (TIR) WORKSHEET Part 15 EASEMENTS/TRACTS Part 16 STRUCTURAL ANALYSIS  Drainage Easement  Covenant  Native Growth Protection Covenant  Tract  Other ___________________________  Cast in Place Vault  Retaining Wall  Rockery > 4’ High  Structural on Steep Slope  Other ______________________________ Part 17 SIGNATURE OF PROFESSIONAL ENGINEER I, or a civil engineer under my supervision, have visited the site. Actual site conditions as observed were incorporated into this worksheet and the attached Technical Information Report. To the best of my knowledge the information provided here is accurate. Signed/Date 2016 Surface Water Design Manual 4/24/2016 5 Core Design, Inc. HARDY SHORT PLAT Page 2-1 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. HARDY SHORT PLAT Page 2-2 2.1 Core Requirements 2.1.1 Core Requirement #1: Discharge at the Natural Location The existing site drainage exits the site via sheet flow over the west site boundary and passes through neighboring properties for 200 ft before entering the city’s conveyance system along Duvall Pl NE. The proposed project will not discharge flow across the west site boundary, but redirect flow north along the west site boundary to combine with the same existing city conveyance system. The existing and proposed drainage path will combine within 300 ft. 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). 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. HARDY SHORT PLAT Page 2-3 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. HARDY SHORT PLAT Page 3-1 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 Runoff from the site initially sheet flows across the western site boundary through the neighboring properties until it reaches Duvall Pl NE. At this point the runoff follows the grade of the road north where it enters an existing catch basin. The runoff is directed to the north side of NE 9th Street at which point it continues north for approximately 300 ft before entering an existing storm pond. The pond will detain the storm runoff and discharge at acceptable flow rates to the southwest at which point the ¼ mile analysis will end. The runoff will eventually enter the Maplewood Creek and into the Cedar River. See the following 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. 4,514752 City of Renton Print map Template 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 NAVIGATIONWGS_1984_Web_Mercator_Auxiliary_Sphere Notes None 03/10/2021 Legend 512 0 256 512 Feet Information Technology - GIS RentonMapSupport@Rentonwa.gov City and County Labels Addresses Parcels City and County Boundary <all other values> Renton Network Structures Access Riser Inlet Manhole Utility Vault Clean Out Unknown Control Structure Pump Station Discharge Point Water Quality Detention Facilities Pond Tank Vault Bioswale Wetland Other Surface Water Main Culvert Open Drains Facility Outline Private Network Structures Access Riser Inlet Manhole Clean Out Utility Vault Unknown Private Control Structure Private Pump Station Private Discharge Point Private Water Quality Private Detention Facilities Tank, No Stormwater Wetland, No; Natural Wetland, No Filter Strip, No Infiltration Trench, No Vault, No Pond, No; Pond, Unknown Bioswale, No Stormtech Chamber, No Other, No Private Pipe Private Culvert Private Open Drains Private Facility Outline Fence Stormwater Ponds Facility Transfer Streets SITE Core Design, Inc. HARDY SHORT PLAT Page 4-1 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 not proposed as the project does not trigger any requirements for water quality treatment as delineated in Section 4.5 of this Report. The drainage analysis was modeled using MGS Flood software. 4.1 Predeveloped Site Hydrology The site was previously developed with a single story residential home. The rest of the site was left as landscaping and forest. The total project area is approximately 0.41 acres and includes the site as well as frontage improvements. As per the flow control standard designated for the site, the project area will be modeled as Till Forest in MGS Flood. However, the developed site will collect runoff from half of the existing frontage road that will not be disturbed. Therefore, the existing frontage area will be modeled as impervious. The predeveloped areas are shown in the table below. See Figure 4-1 on the following page for the predeveloped conditions exhibit. Table 4-1 Predeveloped Conditions Ground Cover Area (acres) Till Forest 0.35 Till Grass 0.01 Impervious 0.05 Total Basin 0.41 4.2 Developed Site Hydrology Proposed development of the property will include the demolition of all existing structures and the construction of two single family residences, access road, utilities, and frontage improvements along Field Avenue NE. The impervious area per lot was determined using the existing zoning of the site (R-8) and the city’s municipal code. The maximum building coverage for a zoning of R-8 is 50% per lot. The impervious area per lot is based on max per zoning or 4,000 sf, whichever is less. The impervious area from zoning is less than 4,000 sf. The developed conditions are shown in the table below. See Figure 4-2 on the following page for the developed conditions exhibit. Table 4-2 Developed Conditions Total Area (SF) Impervious (SF) Pervious (SF) Lot 1 7,565 3,783 3,782 Lot 2 4,816 2,408 2,408 Tract A 2,494 1,376 1,118 Right of Way 2,805 2,401 404 Total (SF) 17,680 9,968 7,712 Total (Acres) 0.41 0.23 0.18 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 425.885.796321019 12ROWTRACT ADESIGNE 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 425.885.796321019 Core Design, Inc. HARDY SHORT PLAT Page 4-4 4.3 Flow Control System Using the predeveloped and developed areas above, a 40’ x 14’ vault with 6.25’ of live storage was sized. The vault requires 3,500 CF for sufficient capacity and will be designed to provide 3,500 CF. 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 2.601E-02 2-Year 1.123E-02 5-Year 3.547E-02 5-Year 1.326E-02 10-Year 4.231E-02 10-Year 2.277E-02 25-Year 6.117E-02 25-Year 3.618E-02 50-Year 6.949E-02 50-Year 3.832E-02 100-Year 7.434E-02 100-Year 6.266E-02 200-Year 9.864E-02 200-Year 7.643E-02 500-Year 0.131 500-Year 9.407E-02 ** 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%): -53.7% PASS Maximum Excursion from 50%Q2 to Q2 (Must be Less Than or Equal to 0%): -1.8% PASS Maximum Excursion from Q2 to Q50 (Must be less than 10%): 8.8% PASS Percent Excursion from Q2 to Q50 (Must be less than 50%): 7.6% PASS ------------------------------------------------------------------------------------------------- MEETS ALL FLOW DURATION DESIGN CRITERIA: PASS ------------------------------------------------------------------------------------------------- Core Design, Inc. HARDY SHORT PLAT Page 4-5 Link Name: Vault Link Type: Structure Downstream Link: None Prismatic Pond Option Used Pond Floor Elevation (ft) : 455.25 Riser Crest Elevation (ft) : 461.50 Max Pond Elevation (ft) : 462.00 Storage Depth (ft) : 6.25 Pond Bottom Length (ft) : 40.0 Pond Bottom Width (ft) : 14.0 Pond Side Slopes (ft/ft) : L1= 0.00 L2= 0.00 W1= 0.00 W2= 0.00 Bottom Area (sq-ft) : 560. Area at Riser Crest El (sq-ft) : 560. (acres) : 0.013 Volume at Riser Crest (cu-ft) : 3,500. (ac-ft) : 0.080 Area at Max Elevation (sq-ft) : 560. (acres) : 0.013 Vol at Max Elevation (cu-ft) : 3,780. (ac-ft) : 0.087 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 : 461.50 ft Hydraulic Structure Geometry Number of Devices: 4 ---Device Number 1 --- Device Type : Circular Orifice Control Elevation (ft) : 455.25 Diameter (in) : 0.50 Orientation : Horizontal Elbow : No ---Device Number 2 --- Device Type : Circular Orifice Control Elevation (ft) : 459.25 Diameter (in) : 0.62 Orientation : Horizontal Elbow : Yes ---Device Number 3 --- Device Type : Circular Orifice Control Elevation (ft) : 460.50 Diameter (in) : 0.75 Orientation : Horizontal Elbow : Yes --- Device Number 4 --- Device Type : Rectangular Weir that Intersects the Riser Top Invert Elevation (ft) : 461.00 Length (ft) : 0.170 Core Design, Inc. HARDY SHORT PLAT Page 4-6 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 due to non-existent native 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 Stormwater Feasibility Evaluation by Cobalt Geosciences dated January 6, 2021, “We do not recommend the use of infiltration systems at this site.” 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 Stormwater Feasibility Evaluation by Cobalt Geosciences dated January 6, 2021, “We do not recommend the use of infiltration systems at this site.” • Limited Infiltration per Appendix C, Section C.2.3, As per the Stormwater Feasibility Evaluation by Cobalt Geosciences dated January 6, 2021, “We do not recommend the use of infiltration systems at this site.” Core Design, Inc. HARDY SHORT PLAT Page 4-7 • Rain Gardens per Appendix C, Section C.2.12, sized as follows: As per the Stormwater Feasibility Evaluation by Cobalt Geosciences dated January 6, 2021, “We do not recommend the use of infiltration systems at this site.” • 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 Stormwater Feasibility Evaluation by Cobalt Geosciences dated January 6, 2021, “We do not recommend the use of infiltration systems at this site.” • Permeable Pavement per Appendix C, Section C.2.7 As per the Stormwater Feasibility Evaluation by Cobalt Geosciences dated January 6, 2021, “We do not recommend the use of infiltration systems at this site.” 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 evaluated with the building permits and therefore no credits have been taken in sizing the vault. 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, • Native Growth Retention Credit per Appendix C, Section C.2.10. Core Design, Inc. HARDY SHORT PLAT Page 4-8 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 growth 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. A perforated pipe connection will be placed upstream of the vault. Core Design, Inc. HARDY SHORT PLAT Page 4-9 4.5 Water Quality Treatment Analysis and Design The building footprints for the lots have not yet been determined, therefore the driveway area may vary. However, the project will assume a driveway area of 800 SF each to be conservative. The subject project does not exceed 5,000 SF of PGIS, therefore basic water quality treatment is not required. See Table 4-3 below for an area breakdown. Table 4-3 PGIS Areas Ground Cover Area (SF) Shared Access Road 1,601 Driveway 1,600 Total Area 3,201 Core Design, Inc. HARDY SHORT PLAT Page 5-1 5. CONVEYANCE SYSTEM ANALYSIS AND DESIGN Conveyance system analysis and design will be addressed during final design. Core Design, Inc. HARDY SHORT PLAT Page 6-1 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.  Stormwater Feasibility Evaluation January 6, 2021 Cobalt Geosciences, LLC P.O Box 82243 Kenmore, WA 98028 206-331-1097 Core Design, Inc. HARDY SHORT PLAT Page 7-1 7. OTHER PERMITS Core Design, Inc. HARDY SHORT PLAT Page 8-1 8. ESC ANALYSIS AND DESIGN The ESC analysis and design will be addressed during final design. Core Design, Inc. HARDY SHORT PLAT Page 9-1 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. HARDY SHORT PLAT Page 10-1 10. OPERATIONS AND MAINTENANCE The operations and maintenance manual will be addressed during final design. Core Design, Inc. HARDY SHORT PLAT Page 10-2 Appendix A Core Design, Inc. HARDY SHORT PLAT Page 10-3 Appendix B ————————————————————————————————— MGS FLOOD PROJECT REPORT Program Version: MGSFlood 4.52 Program License Number: 200210008 Project Simulation Performed on: 03/10/2021 12:15 PM Report Generation Date: 03/10/2021 12:53 PM ————————————————————————————————— Input File Name: Vault Sizing.fld Project Name: Hardy Short Plat Analysis Title: Vault Sizing 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) 0.410 0.410 Area of Links that Include Precip/Evap (acres) 0.000 0.000 Total (acres) 0.410 0.410 ----------------------SCENARIO: PREDEVELOPED Number of Subbasins: 1 ---------- Subbasin : Subbasin 1 ---------- -------Area (Acres) -------- Till Forest 0.350 Till Grass 0.010 Impervious 0.050 ---------------------------------------------- Subbasin Total 0.410 Core Design, Inc. HARDY SHORT PLAT Page 10-4 ----------------------SCENARIO: POSTDEVELOPED Number of Subbasins: 1 ---------- Subbasin : Subbasin 1 ---------- -------Area (Acres) -------- Till Grass 0.180 Impervious 0.230 ---------------------------------------------- Subbasin Total 0.410 ************************* LINK DATA ******************************* ----------------------SCENARIO: PREDEVELOPED Number of Links: 0 ************************* LINK DATA ******************************* ----------------------SCENARIO: POSTDEVELOPED Number of Links: 1 ------------------------------------------ Link Name: Vault Link Type: Structure Downstream Link: None Prismatic Pond Option Used Pond Floor Elevation (ft) : 455.25 Riser Crest Elevation (ft) : 461.50 Max Pond Elevation (ft) : 462.00 Storage Depth (ft) : 6.25 Pond Bottom Length (ft) : 40.0 Pond Bottom Width (ft) : 14.0 Pond Side Slopes (ft/ft) : L1= 0.00 L2= 0.00 W1= 0.00 W2= 0.00 Bottom Area (sq-ft) : 560. Area at Riser Crest El (sq-ft) : 560. (acres) : 0.013 Volume at Riser Crest (cu-ft) : 3,500. (ac-ft) : 0.080 Area at Max Elevation (sq-ft) : 560. (acres) : 0.013 Vol at Max Elevation (cu-ft) : 3,780. (ac-ft) : 0.087 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 Core Design, Inc. HARDY SHORT PLAT Page 10-5 Riser Diameter (in) : 12.00 Common Length (ft) : 0.000 Riser Crest Elevation : 461.50 ft Hydraulic Structure Geometry Number of Devices: 4 ---Device Number 1 --- Device Type : Circular Orifice Control Elevation (ft) : 455.25 Diameter (in) : 0.50 Orientation : Horizontal Elbow : No ---Device Number 2 --- Device Type : Circular Orifice Control Elevation (ft) : 459.25 Diameter (in) : 0.62 Orientation : Horizontal Elbow : Yes ---Device Number 3 --- Device Type : Circular Orifice Control Elevation (ft) : 460.50 Diameter (in) : 0.75 Orientation : Horizontal Elbow : Yes --- Device Number 4 --- Device Type : Rectangular Weir that Intersects the Riser Top Invert Elevation (ft) : 461.00 Length (ft) : 0.170 **********************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: Vault ********** Link WSEL Stats WSEL Frequency Data(ft) (Recurrence Interval Computed Using Gringorten Plotting Position) Tr (yrs) WSEL Peak (ft) ====================================== 1.05-Year 456.760 1.11-Year 457.023 1.25-Year 457.330 2.00-Year 458.082 Core Design, Inc. HARDY SHORT PLAT Page 10-6 3.33-Year 458.581 5-Year 459.199 10-Year 459.954 25-Year 460.819 50-Year 460.945 100-Year 461.122 ***********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 67.872 _____________________________________ Total: 67.872 Total Post Developed Recharge During Simulation Model Element Recharge Amount (ac-ft) ----------------------------------------------------------------------------------------------- Subbasin: Subbasin 1 23.183 Link: Vault 0.000 _____________________________________ Total: 23.183 Total Predevelopment Recharge is Greater than Post Developed Average Recharge Per Year, (Number of Years= 158) Predeveloped: 0.430 ac-ft/year, Post Developed: 0.147 ac-ft/year ***********Water Quality Facility Data ************* ----------------------SCENARIO: PREDEVELOPED Number of Links: 0 ----------------------SCENARIO: POSTDEVELOPED Number of Links: 1 ********** Link: Vault ********** Basic Wet Pond Volume (91% Exceedance): 1341. cu-ft Computed Large Wet Pond Volume, 1.5*Basic Volume: 2011. cu-ft Infiltration/Filtration Statistics-------------------- Inflow Volume (ac-ft): 159.66 Inflow Volume Including PPT-Evap (ac-ft): 159.66 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): 159.65 Secondary Outflow To Downstream System (ac-ft): 0.00 Core Design, Inc. HARDY SHORT PLAT Page 10-7 Percent Treated (Infiltrated+Filtered)/Total Volume: 0.00% ***********Compliance Point Results ************* Scenario Predeveloped Compliance Subbasin: Subbasin 1 Scenario Postdeveloped Compliance Link: Vault *** 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 2.601E-02 2-Year 1.123E-02 5-Year 3.547E-02 5-Year 1.326E-02 10-Year 4.231E-02 10-Year 2.277E-02 25-Year 6.117E-02 25-Year 3.618E-02 50-Year 6.949E-02 50-Year 3.832E-02 100-Year 7.434E-02 100-Year 6.266E-02 200-Year 9.864E-02 200-Year 7.643E-02 500-Year 0.131 500-Year 9.407E-02 ** 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%): -53.7% PASS Maximum Excursion from 50%Q2 to Q2 (Must be Less Than or Equal to 0%): -1.8% PASS Maximum Excursion from Q2 to Q50 (Must be less than 10%): 8.8% PASS Percent Excursion from Q2 to Q50 (Must be less than 50%): 7.6% PASS ------------------------------------------------------------------------------------------------- MEETS ALL FLOW DURATION DESIGN CRITERIA: PASS -------------------------------------------------------------------------------------------------