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23-106056-SU-TIR-01-19-24
SAGHALIE HEIGHTS PLAT TIR & Downstream Analysis January 3, 2024 Prepared for Prospect Development, LLC 22913 5th Ave NE, Ste. 201 Puyallup, WA 98372 (206) 276-7526 Submitted by ESM Consulting Engineers, LLC 33400 8th Avenue S, Suite 205 Federal Way, WA 98003 253.838.6113 tel 253.838.7104 fax www.esmcivil.com 01/05/2024 Saghalie Heights Plat Technical Information Report 2 01/03/2024 Table of Contents Section 1. Project Overview Section 2. Conditions and Requirements Summary Section 3. Off-Site Analysis Section 4. Flow Control & Water Quality Facility Analysis and Design Section 5. Conveyance System Analysis and Design Section 6. Special Reports and Studies Section 7. Other Permits Section 8. ESC Analysis and Design Section 9. Bond Quantities, Facility Summaries, & Declaration of Covenant Section 10. Operations and Maintenance Manual List of Figures 1.1 TIR Worksheet 1.2 Vicinity Map 1.3A Existing Drainage Basin 1.3B Developed Drainage Basin 1.4 Soils Map 3.1 303d Polluted Waters 3.2 Downstream Study Area 3.3A Downstream Corridor 3.3B Downstream Corridor Appendix Appendix A Hydrology Model Output Appendix B Geotechnical Study \\esm8\engr\esm-jobs\1316\004\023\stormreport\rprt - stormreport 2024-01-03.docx Saghalie Heights Plat Technical Information Report Page 3 01/03/2024 1. Project Overview The purpose of this report is to encapsulate the documents and analysis required by the Drainage Review as defined by FWRC 16.25.020 in accordance with the 2021 King County Surface Water Design Manual (KCSWDM) and the Federal Way Addendum to the King County Washington Surface Water Design Manual (Federal Way Addendum), the King County Stormwater Prevention Manual (KCSPPM), and the latest edition of the LID Technical Guidance Manual for the Puget Sound (LID Manual), for the proposed 26 Lot single family plat. Existing Site: The proposed Saghalie Heights Subdivision project site is in Section 19, Township 21 N, Range 04 E, W.M., situated on four separate parcels, Parcel #192104-9024, -9026, -9018, -9019. The site is east of SW 341st Pl and north of SW 344th St, a public Right-of-Way. The parcels are within the Residential Comprehensive Plan Designation and zoned Single Family Residential (RS-7.2). Parcel 1921049019 is 196,919 sf (4.52 ac). The parcel is ‘flag lot’ that is approximately 262’ x 683’. Across the property is a dense scattering of evergreen and deciduous trees with a vertical relief of approximately 20 feet over slopes averaging 3.5 percent (ranging from 1 to up 15 percent in isolated areas) from the southeast down to the northwest. Adjacent to this parcel is Orchard Lane Subdivision to the east, future Twin Trails Subdivision to the west, an undeveloped parcel to the north, and SW 344th Street along the south. Parcel 1921049018 is 170,752 sf (3.92 ac) and located north and east of parcel 1921049019. The parcel is rectangular in shape, approximately 263’ x 651’. Adjacent to this parcel is Orchard Lane Subdivision to the east, and three undeveloped parcels along with a portion of Orchard Land Subdivision to the north. Parcel 1921049024 is 45,370 sf (1.04 ac) and located north of parcel 1921049018 and east of parcel 1921049019. The parcel is rectangular in shape, approximately 130’ x 349’. Adjacent to this parcel is 1921049026 to the east, and an undeveloped parcel to the north. Parcel 1921049026 is 39,088 sf (0.90 ac) and located north of parcel 1921049018 and east of parcel 1921049024. The parcel is rectangular in shape, approximately 112’ x 349’. Adjacent to this parcel is an undeveloped tract to the east, and an undeveloped parcel to the north. This project site is currently undeveloped where the land cover is predominately forested. Three wetlands have been delineated onsite by the project biologist. This includes a larger wetland at the northern end of the site and two small wetlands in the middle region of the property. Grades across the project site range from 1 to 58 percent down from the southeast to the northwest corner and a vertical relief of approximately 50 feet. Refer to Figures 1.2 and 1.3A for a vicinity map and a visual representation of the existing site conditions. Proposed improvements: The plat infrastructure improvements include new public internal roadways, private alleys and tracts, existing public right-of-way improvements, associated utilities, and landscaping. The new internal roadways will be constructed to a Type W standard section. The new SW 343rd St extension SW 344th St will also be constructed to a Type ‘W’ standard section. Additionally, half street improvements will be provided along the adjacent existing rights-of-way. Saghalie Heights Plat Technical Information Report Page 4 01/03/2024 This includes the north half of SW 344th St to a Type ‘K’ road standard. Additionally, on the north end of the project, it is also proposed to construct a portion of a permanent cul-de-sac at the end of the currently improved SW 241st Pl. Stormwater will be conveyed and collected by a series of pipes and catch basins. Two stormwater facilities are proposed which includes an above ground retention (infiltration) pond on the west side of the property and a detention pond located on the southern side of the property along SW 344th St. in order to meet flow control requirements conditioned with this project. The storm facilities will mitigate most runoff generated from the project site. A biofiltration swale is proposed to treat stormwater prior to infiltration in the western retention (infiltration) pond at a minimum to Basic treatment standard. The southern detention pond will contain two wetpool cells to treat stormwater prior to being discharged to the wetland located north of the pond. See section 4 of this report for a further discussion of the existing and proposed hydrology and design details. Critical Areas Wetlands: A Wetland and Fish and Wildlife Habitat Assessment Report for Saghalie Heights, dated December 2023, was prepared by Soundview Consultants. The report describes the three wetlands located within the project limits for Saghalie Heights Subdivision. In summary, using current wetland delineation methodology, three wetlands were identified and delineated. This includes two category III depressional wetlands (Wetlands A and B) which are subject to an 80- foot buffer and a category IV depressional wetland (Wetland C) which is subject to a 50-foot buffer. Refer to Sound View Consultants assessment for further information included in the application submittal package. Geotechnical: A Geotechnical and Infiltration Report has been completed by Terra Associates dated May 15, 2008. A total of three exploration pits were dug across the project site by Terra Associates, including one at the location of the proposed infiltration pond. See Figure 2 in the Geotechnical and Infiltration Report (Appendix B) provided by Terra Associates for test pit locations. In summary, topsoil (where encountered) generally extended to depths ranging from about six inches below ground surface. Underlying the topsoil, native soils were encountered consisting primarily of advanced outwash and glacial till soil types. Advanced outwash soils consist of uniform sand and/or gravel deposits while the glacial till soils consist of dense silty sand with gravel. Test Pits exploration depths ranged between 10.5 to 18 feet below existing surface grades. The outwash soils were found in test pits TP-202 and TP-204 below a transition of color and increasing grain size at the base of the glacial till horizon at depths of 12 and 13 feet, respectively. The underlying soils encountered in test pit TP-202 are near the proposed infiltration pond and consist of a fairly clean gravel layer that begins at around 12 feet below existing grade and is consistent with outwash deposits. The outwash soils may be suitable for infiltration pending in-situ infiltration testing. For feasibility considerations, an infiltration rate of 4 inches per hour is recommended for preliminary sizing of the infiltration facilities. KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL TECHNICAL INFORMATION REPORT (TIR) WORKSHEET Part 1 PROJECT OWNER AND PROJECT ENGINEER Project Owner Phone _____ Address Project Engineer Company ____ Phone _______ Part 2 PROJECT LOCATION AND DESCRIPTION Project Name _ DLS-Permitting Permit #_____ Location Township Range _ Section _ Site Address _______ Part 3 TYPE OF PERMIT APPLICATION □ Land use (e.g.,Subdivision / Short Subd. / UPD) □ Building (e.g.,M/F / Commercial / SFR) □ Clearing and Grading □ Right-of-Way Use □ Other ___________________________ Part 4 OTHER REVIEWS AND PERMITS1 □ DFW HPA □ COE CWA 404 □ ECY Dam Safety □ FEMA Floodplain □ COE Wetlands □ Other _________ □ Shoreline Management □ Structural Rockery/Vault/______ □ ESA Section 7 Part 5 PLAN AND REPORT INFORMATION Technical Information Report Type of Drainage Review (check one): Date (include revision dates): Date of Final: □ □ □ □ □ Full Targeted Simplified Large Project Directed Site Improvement Plan (Engr. Plans) 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: __________________________ 1 DFW: WA State Dept. of Fish and Wildlife. HPA: hydraulic project approval. COE: (Army) Corps of Engineers. CWA: Clean Water Act. ECY: WA State Dept. of Ecology. FEMA: Federal Emergency Management Agency. ESA: Endangered Species Act. 2021 Surface Water Design Manual 1 Last revised 7/23/2021 22913 5th Ave NE, Suite #201 Puyallup, WA 98372 Brandon Loucks, PE ESM Consulting Engineers 253-838-6113 Saghalie Heights Subdivision 21 N 4 E 19 1525 SW 341st PL Federal Way, WA 98023 X X X Prospect Development (206) 276-7526 STRV18-0009 KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL TECHNICAL INFORMATION REPORT (TIR) WORKSHEET Part 7 MONITORING REQUIREMENTS Monitoring Required: Yes / No Start Date: Describe: Completion Date: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 □Steep Slope □ Lake □Erosion Hazard □ Wetlands □Landslide Hazard □ Closed Depression □Coal Mine Hazard □ Floodplain □Seismic Hazard □ Other □Habitat Protection □ Part 10 SOILS Soil Type Slopes Erosion Potential □ High Groundwater Table (within 5 feet) □ Sole Source Aquifer □ Other ______________ _______________________ □ Seeps/Springs □ Additional Sheets Attached 2021 Surface Water Design Manual 2 Last revised 7/23/2021 Hylebos Creek Conservation Flow Control Alderwood GravellySandy Loam (AgC)8% to 15%Moderate X 2 category III wetlands, 1 category IV wetland KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL TECHNICAL INFORMATION REPORT (TIR) WORKSHEET Part 11 DRAINAGE DESIGN LIMITATIONS REFERENCE LIMITATION / SITE CONSTRAINT □ Core 2 - Offsite Analysis____________________ _________________________________ □ Sensitive/Critical Areas _____________________ _________________________________ □ SEPA _____________________________________ _________________________________ □ LID Infeasibility____________________________ _________________________________ □ Other _____________________________________ _________________________________ □ □ 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 Level: 1 / 2 / 3 or Exemption Number ______________ summary sheet)Flow Control BMPs Conveyance System Spill containment located at: Erosion and Sediment Control /CSWPP/CESCL/ESC Site Supervisor: Construction Stormwater Pollution Prevention 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 Type (circle one): Basic / Sens. Lake / Enhanced Basic / Bog summary sheet)or Exemption No. Landscape Management Plan: Yes / No For Entire Project:Total Replaced Impervious surfaces on the site % of Target Impervious that had a feasible FCBMP Total New Pervious Surfaces on the site Repl. Imp. on site mitigated w/flow control facility implemented Repl. Imp. on site mitigated w/water quality facility Repl. Imp. on site mitigated with FCBMP 2021 Surface Water Design Manual 3 Last revised 7/23/2021 1 1 11 / 16 / 2023 TBD TBD X 3 onsite wetlands 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) 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: Source Control (commercial / 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 2021 Surface Water Design Manual 4 Last revised 7/23/2021 x x x x x x x x x Catch basins x x x KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL TECHNICAL INFORMATION REPORT (TIR) WORKSHEET Part 14 STORMWATER FACILITY DESCRIPTIONS (Note: Include Facility Summary and Sketch) Flow Control Type/Description Water Quality Type/Description □ Detention □Vegetated Flowpath □ Infiltration □Wetpool □ Regional Facility □Filtration □ Shared Facility □Oil Control □ Flow Control BMPs □Spill Control □ Other □Flow Control BMPs □Other Part 15 EASEMENTS/TRACTS Part 16 STRUCTURAL ANALYSIS □ Drainage Easement □ Cast in Place Vault □ Covenant □ Retaining Wall □ Native Growth Protection Covenant □ Rockery > 4’ High □ Tract □ Structural on Steep Slope □ Other □ 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 2021 Surface Water Design Manual 5 Last revised 7/23/2021 x x x Detention Pond Infiltration Pond Bioinfiltration Swalex 01/05/2024 Saghalie Heights Plat Technical Information Report Page 10 01/03/2024 FIGURE 1.2 – VICINITY MAP n SCALE: 1" = 100 SW 344TH ST 13TH PL SWSW 31ST ST SW 342ND PL SW 343RD ST n SCALE: 1" = 100 SW 344TH ST 13TH PL SWSW 342ND PL SW 343RD ST 6 Custom Soil Resource Report Soil Map 52379605238000523804052380805238120523816052382005238240523828052383205237960523800052380405238080523812052381605238200523824052382805238320548680 548720 548760 548800 548840 548880 548920 548960 548680 548720 548760 548800 548840 548880 548920 548960 47° 17' 48'' N 122° 21' 22'' W47° 17' 48'' N122° 21' 8'' W47° 17' 35'' N 122° 21' 22'' W47° 17' 35'' N 122° 21' 8'' WN Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 10N WGS84 0 50 100 200 300 Feet 0 25 50 100 150 Meters Map Scale: 1:1,910 if printed on A portrait (8.5" x 11") sheet. Soil Map may not be valid at this scale. FIGURE 1.4 - SOILS MAP MAP LEGEND MAP INFORMATION Area of Interest (AOI) Area of Interest (AOI) Soils Soil Map Unit Polygons Soil Map Unit Lines Soil Map Unit Points Special Point Features Blowout Borrow Pit Clay Spot Closed Depression Gravel Pit Gravelly Spot Landfill Lava Flow Marsh or swamp Mine or Quarry Miscellaneous Water Perennial Water Rock Outcrop Saline Spot Sandy Spot Severely Eroded Spot Sinkhole Slide or Slip Sodic Spot Spoil Area Stony Spot Very Stony Spot Wet Spot Other Special Line Features Water Features Streams and Canals Transportation Rails Interstate Highways US Routes Major Roads Local Roads Background Aerial Photography The soil surveys that comprise your AOI were mapped at 1:24,000. Warning: Soil Map may not be valid at this scale. Enlargement of maps beyond the scale of mapping can cause misunderstanding of the detail of mapping and accuracy of soil line placement. The maps do not show the small areas of contrasting soils that could have been shown at a more detailed scale. Please rely on the bar scale on each map sheet for map measurements. Source of Map: Natural Resources Conservation Service Web Soil Survey URL: Coordinate System: Web Mercator (EPSG:3857) Maps from the Web Soil Survey are based on the Web Mercator projection, which preserves direction and shape but distorts distance and area. A projection that preserves area, such as the Albers equal-area conic projection, should be used if more accurate calculations of distance or area are required. This product is generated from the USDA-NRCS certified data as of the version date(s) listed below. Soil Survey Area: King County Area, Washington Survey Area Data: Version 19, Aug 29, 2023 Soil map units are labeled (as space allows) for map scales 1:50,000 or larger. Date(s) aerial images were photographed: Jul 31, 2022—Aug 8, 2022 The orthophoto or other base map on which the soil lines were compiled and digitized probably differs from the background imagery displayed on these maps. As a result, some minor shifting of map unit boundaries may be evident. Custom Soil Resource Report 7 Map Unit Legend Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI AgC Alderwood gravelly sandy loam, 8 to 15 percent slopes 12.1 100.0% Totals for Area of Interest 12.1 100.0% Map Unit Descriptions The map units delineated on the detailed soil maps in a soil survey represent the soils or miscellaneous areas in the survey area. The map unit descriptions, along with the maps, can be used to determine the composition and properties of a unit. A map unit delineation on a soil map represents an area dominated by one or more major kinds of soil or miscellaneous areas. A map unit is identified and named according to the taxonomic classification of the dominant soils. Within a taxonomic class there are precisely defined limits for the properties of the soils. On the landscape, however, the soils are natural phenomena, and they have the characteristic variability of all natural phenomena. Thus, the range of some observed properties may extend beyond the limits defined for a taxonomic class. Areas of soils of a single taxonomic class rarely, if ever, can be mapped without including areas of other taxonomic classes. Consequently, every map unit is made up of the soils or miscellaneous areas for which it is named and some minor components that belong to taxonomic classes other than those of the major soils. Most minor soils have properties similar to those of the dominant soil or soils in the map unit, and thus they do not affect use and management. These are called noncontrasting, or similar, components. They may or may not be mentioned in a particular map unit description. Other minor components, however, have properties and behavioral characteristics divergent enough to affect use or to require different management. These are called contrasting, or dissimilar, components. They generally are in small areas and could not be mapped separately because of the scale used. Some small areas of strongly contrasting soils or miscellaneous areas are identified by a special symbol on the maps. If included in the database for a given area, the contrasting minor components are identified in the map unit descriptions along with some characteristics of each. A few areas of minor components may not have been observed, and consequently they are not mentioned in the descriptions, especially where the pattern was so complex that it was impractical to make enough observations to identify all the soils and miscellaneous areas on the landscape. The presence of minor components in a map unit in no way diminishes the usefulness or accuracy of the data. The objective of mapping is not to delineate pure taxonomic classes but rather to separate the landscape into landforms or landform segments that have similar use and management requirements. The delineation of such segments on the map provides sufficient information for the development of resource plans. If intensive use of small areas is planned, however, onsite investigation is needed to define and locate the soils and miscellaneous areas. Custom Soil Resource Report 8 Saghalie Heights Plat Technical Information Report Page 14 01/03/2024 2. Conditions and Requirements Summary The proposed project is subject to Stormwater Review as a result of being subject to a City of Federal way development permit and results in more than 2,000 sf of new impervious surface according to FWRC 16.15.010. This review includes KCSWDM’s Full Drainage Requirements, Core Requirements 1-9, and Special Requirements 1-5. Review of the 9 Core Requirements and 5 Special Requirements This section describes how the project will meet the KCSWDM Core and Special Requirements. Core Requirement No. 1 Discharge at the Natural Location The project site, located on 4 parcels, is within one Threshold Discharge Area with a single natural discharge location. The site naturally drains from the southeast to the northwest. Similarly, all runoff generated from the developed basin will continue to be directed to the northwest end of the site. The proposed infiltration pond on the west side of the project will reduce the burden on the downstream stormwater network and reduce potential downstream flooding. Additionally, the detention pond located at the south end of the site will release stormwater at a controlled rate that is below the historic release rate from the site and also result in a reduction of potential downstream flooding. If the infiltration pond were to fail, overflow will be captured within a new overflow structure and conveyed to a 12-inch diameter bypass main. Core Requirement No. 2 Off-site Analysis An off-site analysis has been documented in Section 3 of this report. Core Requirement No. 3 Flow Control The site is located within a Conservation Flow Control Area requiring at minimum Level 2 flow control standards per Section 1.2.3.1.B of the KCSWDM and the Federal Way Addendum Flow Control Application map. Additionally, the project has been conditioned to meet Level 3 flow control requirements as directed by the city due to documented capacity/flooding issues downstream. The flow control standard will be met using an above ground infiltration pond and detention pond. By retaining most all stormwater from the site’s target surfaces the developed (discharge) flow rates are greatly reduced where the flow control facility requirement can be waved for the threshold discharge area according to Section 1.2.3.1.C of the manual. Refer to Section 4.C of this report for further discussion of the flow control performance standards. Core Requirement No. 4 Conveyance System The new conveyance network will be analyzed and designed with sufficient capacity to convey and contain the 25-year peak flow. Events that exceed the 25-year event that may over top the system structure will be contained within the public right-of-way. The conveyance analysis will be included with the final TIR. Core Requirement No. 5 Erosion and Sediment Control Erosion and sediment controls to prevent the transport of sediment from the project site to downstream drainage facilities, water resources, and adjacent properties will be provided on the construction plans and discussed in the Erosion and Sediment Control Report. Core Requirement No. 6 Maintenance and Operations The Operations and Maintenance manual will be included with the final TIR if requested by the city. Saghalie Heights Plat Technical Information Report Page 15 01/03/2024 Core Requirement No. 7 Financial Guarantees and Liability All drainage facilities constructed or modified for projects will comply with the financial guarantee requirements as provided in the City of Federal Way Bond Quantities Worksheet. Bond Quantities will be provided in the final TIR as requested by the City. Core Requirement No. 8 Water Quality All projects with the city are initially subject to the Enhanced-Basic treatment standard unless otherwise exempt according to Section 1.2.8.1 of the SWDM upon which the treatment standard may be reduced to Basic. Basic treatment may be allowed for lower density developments resulting in less than 8 du/acre. The Saghalie Heights Subdivision is zoned RS—7.2 resulting in approximately 6 dwelling units per acres. As a result, at a minimum, the site is subject to Basic Treatment. An on-line biofiltration swale is proposed upstream of the infiltration (retention) pond. The swale will also provide a pretreatment/presettling benefit for stormwater before reaching the infiltration pond. The detention pond will contain two internal wetpools in order to provide stormwater treatment within the pond. Refer to Section 4 of this report for further detail. Core Requirement No. 9 Flow Control BMP’s All applicable Flow Control BMP’s requirements are listed and discussed in Section 4.F of this report. Special Requirement No. 1 Other Adopted Area-Specific Requirements There is a known off-site, downstream stormwater conveyance restriction. Prior guidance from the City on this topic is that the applicant may choose to fix the conveyance restriction OR install flow control measures to meet the Level 3 (Flood Problem) standard. To assist with addressing the downstream conveyance restriction, a stormwater retention pond and detention pond are proposed to store and infiltrate stormwater onsite to ultimately meet the Level 3 flow control standard. Other than the flow control area-specific requirements, there are no known master drainage plans, basin plans, salmon conservation plans, stormwater compliance plans, flood hazard reduction plan updates, or shared facility drainage plans for this project. Special Requirement No. 2 Flood Hazard Area Delineation The site falls outside of the 500-year flood plain and located in an area of minimal flood hazard according to FEMA FIRM Map Number 53033C1250 G. Special Requirement No. 3 Flood Protection Facilities The developed project site is not protected by an existing flood protection facility. The proposed site improvements do not include the modification of an existing flood protection facility. Special Requirement No. 3 does not apply. Special Requirement No. 4 Source Control The site is a residential development and is not subject to this requirement. Special Requirement No. 4 does not apply. Special Requirement No. 5 Oil Control The project does not have a “high-use site” characteristic and is not a redevelopment of a high- use site. Special Requirement No. 5 does not apply. A “high-use site” is a commercial or industrial site that typically generates or is subject to runoff containing high concentrations of oil due to high traffic turnover, on-site vehicle or heavy or Saghalie Heights Plat Technical Information Report Page 16 01/03/2024 stationary equipment use, or the frequent transfer of liquid petroleum or coal derivative products. Saghalie Heights Plat Technical Information Report Page 17 01/03/2024 3. Level 1 Off-Site Analysis This narrative is to provide a Level 1 Downstream Analysis for the proposed Twin Trails development per Core Requirement #2, Section 1.2.2 of the 2021 King County Surface Water Design Manual (SWDM). The analysis is to identify and evaluate offsite flooding, erosion, and water quality problems that may be created or aggravated by the proposed project. The primary component of this offsite analysis is the downstream corridor. The second component is to evaluate the upstream drainage system to verify any offsite run-on that may impact the project. The following Level 1 downstream analysis is a qualitative survey of the downstream system of the project site and is composed of the following four tasks: Task 1 – Define and map the study area Task 2 – Downstream Resource Review for 1-mile downstream Task 3 – Field Inspection Task 4 – Drainage System Description and Problem Descriptions Task 1: Study Area Definition and Maps The project site is located within the Hylebos Drainage basin, within the Central Puget Sound watershed which is a part of the Duwamish-Green water resource inventory area (WRIA #9). The presented drainage study area is approximately a one-mile-long path encompassing the site’s downstream corridor. Refer to Figure 3.2 for a map of the basic study area. The study area also includes a 1/4 mile downstream field investigation of stormwater released from the project site. See Figure 1.3A, Figure 3.3A and Figure 3.3B for the Existing Site Conditions and maps of the downstream corridor. Task 2: Resource Review Flow Control Map According to the Federal Way Flow Control Applications Map, the project is located within the Conservation Flow Control Area and required to comply with Level 2 Flow Control Standards and has been additionally conditioned to meet Level 3 Flow Control due to downstream conveyance restrictions. Site Soils Refer to the project overview in Section 1 of this report. Federal Way Critical Areas Map Properties and streams along the 1-mile downstream study area and portions of the upstream basin were researched such that offsite flooding, erosion, and water quality problems that may be created or aggravated by the proposed project can be identified and evaluated: The following items were investigated according to the available studies and the City’s 2016 Critical Areas Map. • Drainage Studies ......................... None Mapped • Streams ....................................... None Mapped • Lakes ........................................... None Mapped Saghalie Heights Plat Technical Information Report Page 18 01/03/2024 • Potential Steep Slope Hazard ...... None Mapped • Erosion Hazard Area .................... None Mapped • Landslide Hazard Area................. None Mapped • FEMA Floodway or Floodplain ..... None Mapped 100 Year Floodplain The Federal Emergency Management Agency prepared maps for all areas within the City of Federal Way. These maps can be found on the FEMA website. Panel #53033C1250G, effective 05/16/1995 depicts the areas, if any, subject to flooding in the vicinity of this project. By inspection of this map, the project is located in Zone X, which is designated as areas outside the 0.2 percent annual chance floodplain. The site falls outside of the 500-year flood plain and located in an area of minimal flood hazard according to FEMA FIRM Map Number 53033C1250 G. Wetlands A wetland and fish and wildlife habitat assessment report has been completed by Soundview Consultants, dated September 28, 2021, for the project site. The report describes the three wetlands located within the project limits for Saghalie Heights Plat. In summary, using current wetland delineation methodology, three wetlands were identified and previously delineated and assessed by Wetland Resources in 2023. This includes two category III depressional wetland subject to an 80-foot buffer and a category IV depressional wetland with a 50-foot buffer. Refer to Sound View Consultants assessment for further information included as a part of the plat application submittal package. Downstream Drainage Complaints Research of relevant drainage complaints within the downstream corridor is currently on going. When the data becomes available from the city, it will be added to this report as applicable. Water Quality Assessment The Department of Ecology Water Quality Assessment 303(d)/305(b) lists were reviewed to see if there are any known downstream water quality concerns. Waters whose beneficial uses are impaired by pollutants that require a water improvement project are placed in the polluted water category (Category 5) and put on the 303(d) list. The 305(b) lists all waters and all categories. Pollutants of concern could be Bacteria, Dissolved oxygen, temperature, metals, phosphorus, turbidity, or high pH. From the site, water is tributary to Panther Lake (tributary to Hylebos Creek). According to the assessment, the downstream path to Panther Lake, approximately 1.5 miles downstream, is not comprised of any assessed impairments. Refer to Figure 3.1 at the end of this section of the DOE 303(d) water quality assessment map. Designated water quality problems There are no designated water quality problems known at this time. Saghalie Heights Plat Technical Information Report Page 19 01/03/2024 Task 3: Field Inspection (Level 1 Inspection) A site visit was conducted in November 2023 by ESM Consulting Engineers for the purpose of analyzing the project site and its upstream and downstream corridors. The weather conditions were sunny with no prior rainfall for the previous week. The ground surface was dry. A description of the drainage path is provided below. Refer to Figures 3.3A and 3.3B at the end of the downstream analysis for maps of the downstream reach and point locations. Upstream and onsite runoff Based on upstream topography, it is assumed that the offsite portion of wetland A located north and east of the project site is tributary to the site. The upstream area is approximated to be 2.27 acres of both saturated forested (wetland), as unsaturated forest. Other areas upstream of the project site are developed residential subdivisions. These developments are assumed to have stormwater flow control facilities to manage onsite runoff from these areas. Natural Discharge #1 Downstream Corridor The natural discharge location is located at the Northwest corner of Parcel 1921049019. This is also the location of the proposed infiltration pond’s emergency overflow. Reach 1: Upon leaving the site, stormwater is intercepted by the stormwater network installed with the Twin Trails plat where runoff is collected in an interceptor gravel trench and then conveyed to a bypass storm system constructed as part of the the Twin Trails plat project. The stormwater is then conveyed to the Lakehaven Estates plat where stormwater is directed into the Lakehaven Estates infiltration pond. Reach 2: The Lakehaven Estates infiltration pond was installed with an emergency overflow riser. Emergency overflow is conveyed to the 30” bypass line at the intersection of SW 340th St and 19th Ave SW. Reach 3: Runoff at this point continues within 30-to-48-inch diameter pipes north in the 19th Ave SW right-of-way until reaching Campus Drive. Task 4: Drainage Description and Problem Descriptions The downstream drainage system is comprised of manmade conveyance pipes. The downstream system was studied extensively in the Saghalie Firs Technical Information Report prepared by ESM Consulting Engineers in 2005 for the installation of the bypass system through Saghalie Firs. An additional study of the downstream system was prepared with the Lakehaven Estates plat prepared by Barghausen Consulting Engineers in 2006. Noted in Lakehaven Estates TIR from studies previously completed was a history of capacity concerns in the SW 19th Street conveyance system. In addition, the same capacity concerns were passed on in the conditions of approval for the Saghalie Firs project, which was required to provide additional detention due to the downstream system. The Saghalie Heights project proposes to reduce the release rates of stormwater flows from onsite areas to the bypass network and the SW 341st Place Conveyance system by use of an infiltration pond and a detention pond with flow restriction. Figure 3.1 - 303d Polluted Waters Sources: Esri, HERE, Garmin, Intermap, increment P Corp., GEBCO, USGS,FAO, NPS, NRCAN, GeoBase, IGN, Kadaster NL, Ordnance Survey, EsriJapan, METI, Esri China (Hong Kong), (c) OpenStreetMap contributors, and October 1, 2021 0 0.25 0.50.125Miles K Assessed Water/SedimentWater Category 5 - 303d Category 4C Category 4B Category 4A Category 2 Category 1 Sediment Category 5 - 303d Category 4C Category 4B Category 4A Category 2 Category 1 DOWNSTREAM CORRIDOR PROJECT SITE FIGURE 3.2 - DOWNSTREAM STUDY AREA SITE DOWNSTREAM CORRIDOR 1.5 MILES DOWNSTREAM NATURAL DISCHARGE LOCATION #1 King C ou nty, Ea g le View Tech nolog i es, In c. Figure 3.3A - Dow nstre am Cor ridor Da t e: 11/20/20 2 3 No t es: Th e informa tion in clud e d on th is ma p h a s b e en comp ile d b y K in g C oun t y s ta ff from a va rie ty o f sou rc es a nd is su bje ct tochange w ith o ut n o tic e. King C oun ty ma k es n o re p res e nt ation s or w a rra n tie s, exp re ss o r imp lie d , a s to ac curacy,co mp let e ne s s, time lin ess, o r rig hts to t h e u se o f su ch inf o rma tio n . This do cu me n t is no t int end e d f or use as a surve yproduct. K in g Cou n ty sh a ll n o t be lia ble f or a ny g en era l, sp e cia l, ind ire ct , incid en ta l, or co n se q u e nt ia l d am a ge s in clu d in g ,bu t n ot limit e d to, lo st re ve nue s or lo st p rof its resultin g fro m th e u se or misu se of t he inf o rma tio n co n taine d on this ma p .An y sa le of th is ma p o r in formatio n o n t h is m ap is p ro h ibit ed e xce pt by writ te n p erm issio n o f K ing Co un ty.± REACH 1 12" STORM NATURAL DISCHARGE LOCATION #1 REACH 2 EXISTING CONVEYANCE CHANNEL REACH 3 30" STORM INFILTRATION POND 640 LF DOWNSTREAM REFER TO FIGURE 3.3B FOR DOWNSTREAM CONTINUATION PLAT OF SAGHALIE FIRS SITE PLAT OF LAKEHAVEN ESTATES FUTURE PLAT OF TWIN TRAILS King C ou n ty, Eag le Vie w Figu re 3.3 B - Downstrea m Corr idor Dat e: 9 /3 0 /2 021 Not es: Th e inform a tion in clud e d o n th is ma p h a s b e en comp ile d by K in g Count y s ta ff fr o m a varie ty of so u rc es a nd is sub je ct tochange w ith o ut n o tic e. King C ou n ty mak es n o re p res e nt a tion s or w a rra ntie s, express or imp lie d , a s to ac cu ra cy,comp let e ne s s, time lin ess, o r rig h ts to t he u se o f su ch inf o rma tio n . This d o cu men t is no t int ende d f or u se a s a su rve yproduct. K in g Cou n ty sh a ll n o t be lia ble f or a ny g en eral, spe cial, ind ire ct , incid en ta l, or co n se qu e nt ial d am a ge s in clu d ing ,but n ot limit e d to, lo st re ve nu e s or lo st p rof its re sultin g fro m th e use or misu se of t he inf o rma tio n co ntain e d on th is ma p.An y sa le of th is ma p o r in formation o n t h is m ap is p ro h ibit ed e xce pt by wr it te n p e r missio n o f K ing Co u n ty.± 640 LF DOWNSTREAM REFER TO FIGURE 3.3A FOR UPSTREAM CONTINUATION REACH 3 30" to 48" STORM 1,825 LF DOWNSTREAM Saghalie Heights Plat Technical Information Report Page 24 01/03/2024 4. Flow Control & Water Quality Facility Analysis and Design 4.1 Existing Site Hydrology Much of the existing project site is vegetated with either short grasses, or dense vegetation and ranges in grades between 1 and 58 percent. Onsite stormwater runoff sheet flows across the property through native vegetation toward the northwest region of the property. Based on upstream topography, it is assumed that the offsite portion of wetland A located north of the site is tributary to the project site. 4.2 Developed Site Hydrology In summary, the project proposes to construct a series of catch basins and pipe that will collect and convey stormwater runoff to either a new infiltration pond located in the northwest corner of the project or to a new detention pond located at the southern limit of the site. The infiltration pond will infiltrate 100% of tributary runoff and the detention pond will release runoff at controlled release rates to the onsite wetlands. A biofiltration swale is proposed upstream of the proposed infiltration pond to provide at a minimum basic treatment for polluted runoff. Additionally, the proposed detention pond will include a wetpool for water quality treatment. As discussed in Section 4.1, there is anticipated to be upstream run-on from the three delineated wetlands and surrounding wooded areas. To accommodate potential run on, a bypass storm drain system is proposed to collect this runon. The water will be routed to a bypass storm system located in SW 341st Pl. This bypass system was initially installed with the Saghalie Firs Plat and is set to be extended to the east end of SW 341st Pl as part of the future Twin Trails Plat improvements. The following tables represent the pre-developed and developed conditions for the project. Table 4.1 – Historic Conditions (POC #1) Threshold Discharge Area #1 Area Forest, C, Flat, sf (ac) Total* 318,203 (7.305) *Area is equal to the limits of construction under developed conditions within the ROW for surfaces considered ‘New’ or ‘Replaced’. Areas of asphalt overlay are not included. Saghalie Heights Plat Technical Information Report Page 25 01/03/2024 Table 4.2 – Developed Conditions POC #1 Area Impervious sf (ac) Amended Soil sf (ac) Impervious Bypass sf (ac) Amended Soil Bypass sf (ac) Total sf (ac) East Basin (Detention Pond) Lot 1-14* 65,912 (1.513) 43,941 (1.009) 0 0 109,853 (2.522) ROW/Tracts+ 26,160 (0.601) 4,617 (0.106) 0 0 30,777 (0.707) South Detention Pond Tract 6,634.5 (0.152) 6,634.5 (0.152) 0 0 13,269 (0.478) Total** 98,707 (2.266) 55,192 (1.267) 0 0 153,899 (3.533) West Basin (Infiltration Pond) ROW/Tracts+ 36,521 (0.838) 6,445 (0.148) 0 0 42,966 (0.986) Lot 15-26* 58,589 (1.345) 39,059 (0.897) 0 0 97,648 (2.242) West Infiltration Pond Tract 11,845 (0.272) 11,845 (0.272) 0 0 23,690 (0.544) Total** 106,955 (2.455) 57,349 (1.317) 0 0 164,304 (3.772) Total (POC #1) 205,662 (4.721) 112,541 (2.584) 0 0 318,203 (7.305) *60% max impervious surface per lot per FWRC 19.200.010 **Approximately 40% of the east basin runoff is conveyed to the infiltration pond. The remaining 60% of the east basin is conveyed to the detention pond. +Assumed to be 85% impervious. To determine the increase in flowrate between predeveloped and developed conditions, WWHM2012 was used to analyze and compare the pre-developed and developed flow rates. The WWHM model input is shown in Table 4.3 below. 4.3 Performance Standards Flow Control (Sec 1.2.3.2 SWDM) Stormwater systems onsite are to be designed to mitigate runoff generated from the project per the requirements of the KCSWDM. The Federal Way flow Control Applications Map provides the area-specific flow control facility standard. The project site falls within the Conservation Flow Control Area. Conservation Flow Control areas are to apply the Level 2 flow control standard. Additionally, the project will meet Level 3 flow control requirements as there is documented capacity/flooding issues downstream. The Level 3 flow control standard is to match developed discharge durations to predeveloped durations for the range of predeveloped discharge rates from 50% of the 2-year peak flow up to the full 100-year peak flow. Also match developed peak Saghalie Heights Plat Technical Information Report Page 26 01/03/2024 discharge rates to predeveloped peak discharge rates for the 2- and 10- year return periods. This also assumes historic site conditions as the predeveloped condition. Water Quality (Sec 1.2.8.1 SWDM & 1.2.8.2.D) Stormwater treatment is required for this project as a result of exceeding the 5,000 sf PGIS site area threshold to provide a water quality benefit to stormwater coming in contact with pollution generating impervious surfaces (PGIS) prior to discharging from the project site. All projects with the city are initially subject to the Enhanced-Basic treatment standard unless otherwise exempt according to Section 1.2.8.1 of the SWDM upon which the treatment standard may be reduced to Basic. The goal for this level of treatment is 80% removal of total suspended solids (TSS) for flows or volumes up to and including the Water Quality design flow or volume for a typical rainfall year. Basic treatment may be allowed for lower density developments resulting in less than 8 du/acre. The Twin Trails plat is zoned RS—7.2 resulting in approximately 6 dwelling units per acres. As a result, the site, at a minimum, the site is subject to Basic Treatment. Preceding retention/detention, the design water quality flow rate is the flow rate from the contributing drainage basin at or below which 91% of the total runoff volume will be treated. per Section 6.2.1 of the SWDM. These flows are determined using WWHM. 4.4 Flow Control System West Basin (POC #1) The flow control system proposed is a combination of two stormwater mitigation facilities. The first of which is an infiltration pond located at a low point on the west side of the project. This facility will provide flow control for the entire west basin and a portion of the east basin. The facility is designed to mitigate runoff generated from the project per the requirements of the Manual as discussed in Section 1 of this report, Minimum Requirement #3. The proposed infiltration pond was designed in accordance with the consideration procedures found in Section 5.2 of the SWDM and the Design Criteria in Section 5.2.2. A discussion of the considerations for the infiltration pond design is provided below. Infiltration Pond Consideration Procedures Step 1 – Site Reconnaissance: Review of the site topography helped determine the best location of the infiltration pond being the northwest corner of the project site. Step 2 – Minimum requirement for infiltration facilities Minimum requirements were evaluated to confirm the infiltration facility would be feasible in the selected location. a. The base of the infiltration basin shall be a minimum of 3 feet above seasonal high groundwater levels, bedrock, or other low permeability layers. Infiltration basins may not be constructed within a floodplain area. After review of the geotechnical study for the infiltration pond, 3 feet of separation between a restrictive layer and the bottom of the infiltration pond is achievable. Also, available maps indicate that the site is not within a floodplain area. Saghalie Heights Plat Technical Information Report Page 27 01/03/2024 b. The location of the infiltration pond will be down gradient from the rest of the site. c. Infiltration facilities shall not be placed closer than 100 feet from drinking water wells and Septic systems and 200 feet from springs used for drinking water supplies. To the best of our knowledge, there are no known water wells, springs, or septic fields within 200 feet of the proposed pond. d. The pond shall be setback 20 feet from property lines, tracts, or easements but may be reduced to 5 feet with support from the geotechnical engineer. The pond surface water is setback approximately 10 to 20 feet from the tract property line. e. Building setback lines for adjacent internal lots shall be 20 feet. This may be reduced to the minimum allowed by zoning with support from the geotechnical engineer. f. If the depth of the infiltration facility being considered is greater than the largest surface dimensions, it is considered an injection well. This proposed infiltration basin is not considered an injection well due to the shallow depth when compared to the length and width. g. The proposed depth of the infiltration facility is approximately 6 feet below existing grade, below the 20-foot maximum. Step 3 – Method of Analysis The size of the pond is determined using the hydrologic analysis and routing methods approved by the SWDM and Department of Ecology. The storage volume in the pond is used to retain runoff prior to infiltration. To determine compliance with the flow control requirements, WWHMv2012 was utilized. The pond facility was represented by using the predetermined infiltration rate. The pond was then sized to achieve 100 percent infiltration. Step 4 – Geotechnical Analysis A Geotechnical and Infiltration Report has been completed by Terra Associates dated May 15, 2008. A total of three exploration pits were dug across the project site by Terra Associates, including one at the location of the proposed infiltration pond. See Figure 2 in the Geotechnical and Infiltration Report (Appendix B) provided by Terra Associates for test pit locations. In summary, topsoil (where encountered) generally extended to depths ranging from about six inches below ground surface. Underlying the topsoil, native soils were encountered consisting primarily of advanced outwash and glacial till soil types. Advanced outwash soils consist of uniform sand and/or gravel deposits while the glacial till soils consist of dense silty sand with gravel. Test Pits exploration depths ranged between 10.5 to 18 feet below existing surface grades. The outwash soils were found in test pits TP-202 and TP-204 below a transition of color and increasing grain size at the base of the glacial till horizon at depths of 12 and 13 feet, respectively. The underlying soils encountered in test pit TP-202 are near the proposed infiltration pond and consist of a fairly clean gravel layer that begins at around 12 feet below existing grade and is consistent with outwash deposits. The outwash soils may be suitable for infiltration pending in-situ Saghalie Heights Plat Technical Information Report Page 28 01/03/2024 infiltration testing. For feasibility considerations, an infiltration rate of 4 inches per hour is recommended for preliminary sizing of the infiltration facilities. East Basin (POC #1) The flow control system proposed is a combination of two stormwater mitigation facilities. The first is an infiltration pond which has been discussed above. The second of which is a detention pond located at a low point on the south end of the project. The pond is designed to mitigate runoff generated from the project per the requirements of the Manual as discussed in Section 1 of this report, Minimum Requirement #3. The proposed detention pond has been designed based on the design criteria and methods of analysis found in Section 5.1.1 of the SWDM. Design Criteria The size of the total site basin was determined by routing the influent runoff file generated by the continuous runoff model through the two proposed pond elements. The infiltration facility has two discharge methods. The primary mode of discharge is into the ground. The second mode of discharge is any overflow that may occur if the infiltration capacity was reached. The detention facility also has two discharge methods through which water is released gradually through a combination of 3 flow restrictors. The second mode of discharge is any overflow that may occur if the detention capacity of the pond up to the 100-year storm event is reached. Tables 4.4A & 4.4B below summarize the input values used to evaluate each of the proposed ponds. Table 4.4A Infiltration Pond Parameters Parameter WWHM input Proposed Bottom Square footage 4,000 sf 4,352 sf Storage Depth 5 ft 5 ft Effective Depth 6 ft 6 ft Side Slopes 2:1 2:1 Total Live Storage 27,667 cf 0.635 ac-ft 30,910 cf 0.710 ac-ft Design Infiltration Rate 4 in/hr 4 in/hr Percent Infiltrated 100% 100% Table 4.4B Detention Pond Parameters Parameter WWHM input Proposed Bottom Square footage 4,800 sf 3,834 sf Storage Depth 4 ft 4 ft Effective Depth 5 ft 5 ft Side Slopes 2:1 2:1 Total Live Storage 24,001 cf 0.551 ac-ft 25,140 cf 0.577 ac-ft Saghalie Heights Plat Technical Information Report Page 29 01/03/2024 Pretreatment A basic treatment facility per Section 6.5.1 of the KCSWDM is proposed upstream of the infiltration facility to filter out suspended solids from the influent. The detention pond will include wetpool cells to provide water quality treatment. Further discussion and analysis of the treatment facilities are provided in the Water Quality analysis in this section. Emergency Overflow An overflow route will be provided for stormwater flow that overtops the ponds when capacity is exceeded, or the facility becomes plugged and fails. Overflow from the infiltration pond is proposed to be directed to the 36" bypass line installed with the Saghlie Firs Plat within the SW 341st Pl Right-of-Way and is set to be extended to the west end of SW 341 st Pl as a part of the Twin Trails Plat improvements. The overflow system proposed with the detention pond will direct water toward the central wetlands on site. Further overflow analysis will be provided as a part of the engineering review. 4.5 Water Quality System: West Basin (POC #1) As discussed in Section 4.3, Basic water quality treatment standards apply to this project. A biofiltration swale is proposed upstream of the retention pond to provide a pretreatment/presettling benefit for stormwater before reaching the infiltration pond. The swale was designed by following the bioswale standards in Section 6.3.1 of the KCSWDM. Water quality calculations will be provided with the future engineering application. East Basin (POC #1) As discussed in Section 4.3, Basic water quality treatment standards apply to this project. The detention pond will incorporate two wetpools to provide treatment/settling benefit for the stormwater prior to being discharged. Water quality calculations will be provided with the future engineering application. 4.6 Flow Control BMPs (Core Requirement #9): On-Site BMPs were evaluated for the project site as outlined in the KCSWDM under Section 1.2.9.3.1. If the proposed project is a subdivision project that is within the Urban Growth Area (UGA), then flow control BMPs for the plat infrastructure improvements (e.g. road and sidewalk etc.) are to meet the requirements described in Section 1.2.9.3.2 for "Small Road Improvement and Urban Road Improvements Project BMP Requirements". Implementation of flow control BMPs required for/on the individual lots of the subdivision may be deferred until a permit is obtained for construction on each lot. Target surfaces for application of Core Requirement #9 include new impervious surfaces, new pervious surfaces, and replaced impervious surfaces. Implementation of flow control BMPs will be deferred until a permit is obtained for construction on each lot. The BMP’s required for the plat infrastructure per KCSWDM Section 1.2.9.4 (e.g. road and sidewalk etc.) are described in order of precedence below with feasibility determined. Requirement #1 Full Dispersion (Section C.2.1) has been evaluated for the project site. There is insufficient onsite native vegetated flow path to which target impervious surfaces may be dispersed. Therefore, minimum design requirement #1 (specified in Section C.2.1.1) cannot be met; hence, this BMP is infeasible for the project site. Saghalie Heights Plat Technical Information Report Page 30 01/03/2024 Requirement #2 Full Infiltration (Section C.2.2 or Section 5.2, whichever is applicable) has been evaluated. A Geotechnical and Infiltration Report has been completed by Terra Associates dated May 15, 2008. Underlying the weathered till in the northwest portion of the site are outwash soils suitable for infiltration. An infiltration pond has been proposed as one of the two methods of stormwater management for the project. A detention pond will also be provided along the southern limit of the site for flow control of remaining site areas. Runoff from frontage improvements on SW 344th Street will be routed to both ponds. Requirement #3 Basic Dispersion has been evaluated for the project site. There is insufficient area for onsite vegetated flowpath, less than 25 feet, to which target impervious surfaces may be dispersed. Hence, this BMP is infeasible for the project site. Requirement #6 All new pervious surfaces will be amended in accordance with KCC 16.82.100(F) and (G) to satisfy the requirements specified therein (notes will be included on the final landscape plans). Saghalie Heights Plat Technical Information Report Page 31 01/03/2024 5. Conveyance System Analysis and Design Onsite Conveyance System: Runoff from the developed project site will be collected from the developed lot lawns, roofs, and drive aisles by the proposed conveyance system located throughout the project. The proposed stormwater drainage system is composed of catch basin structures with 6-inch and 12-inch diameter pipes. A conveyance and backwater analysis will be completed to verify the capacity of the critical pipes in the system. The proposed stormwater drainage system will be designed to convey the 25-year peak flow rate generated by the developed tributary basin as required in the SWDM. Conveyance calculations will be provided with the future engineering application. Saghalie Heights Plat Technical Information Report Page 32 01/03/2024 6. Special Reports and Studies Geotechnical Study A Geotechnical and Infiltration Report has been completed by Terra Associates dated May 15, 2008. This study has been provided in Appendix B of this report. Environmental Study Wetland and Fish and Wildlife Habitat Assessment Report, Saghalie Heights, December 2023; prepared by Soundview Consultants. This study has been included as a part of the project submittal package. Saghalie Heights Plat Technical Information Report Page 33 01/03/2024 7. Other Permits Building permits will be required for this project, together with permits for utility connections. Saghalie Heights Plat Technical Information Report Page 34 01/03/2024 8. ESC Analysis and Design The Erosion and Sedimentation Control will be provided with the final TIR. Saghalie Heights Plat Technical Information Report Page 35 01/03/2024 9. Bond Quantities, Facility Summaries, and Declaration of Covenant The Bond Quantities worksheet will be provided with the final TIR. Saghalie Heights Plat Technical Information Report Page 36 01/03/2024 10. Operations and Maintenance The Operations and Maintenance manual will be provided with the final TIR. Appendix A – Hydrology Model Output WWHM2012 PROJECT REPORT Saghalie Heights 12-07-2023 12/8/2023 4:31:57 PM Page 2 General Model Information Project Name:Saghalie Heights 12-07-2023 Site Name:Saghalie Heights Site Address: City: Report Date:12/8/2023 Gage:Seatac Data Start:1948/10/01 Data End:2009/09/30 Timestep:15 Minute Precip Scale:1.000 Version Date:2021/08/18 Version:4.2.18 POC Thresholds Low Flow Threshold for POC1:50 Percent of the 2 Year High Flow Threshold for POC1:50 Year Low Flow Threshold for POC2:50 Percent of the 2 Year High Flow Threshold for POC2:50 Year Low Flow Threshold for POC3:50 Percent of the 2 Year High Flow Threshold for POC3:50 Year Saghalie Heights 12-07-2023 12/8/2023 4:31:57 PM Page 3 Landuse Basin Data Predeveloped Land Use Pre-Dev Basin Bypass:No GroundWater:No Pervious Land Use acre C, Forest, Flat 7.305 Pervious Total 7.305 Impervious Land Use acre Impervious Total 0 Basin Total 7.305 Element Flows To: Surface Interflow Groundwater Saghalie Heights 12-07-2023 12/8/2023 4:31:57 PM Page 4 Predeveloped (Infiltration Pond) Bypass:No GroundWater:No Pervious Land Use acre C, Forest, Flat 5.1852 Pervious Total 5.1852 Impervious Land Use acre Impervious Total 0 Basin Total 5.1852 Element Flows To: Surface Interflow Groundwater Saghalie Heights 12-07-2023 12/8/2023 4:31:57 PM Page 5 Predeveloped (Detention Pond) Bypass:No GroundWater:No Pervious Land Use acre C, Forest, Flat 2.1198 Pervious Total 2.1198 Impervious Land Use acre Impervious Total 0 Basin Total 2.1198 Element Flows To: Surface Interflow Groundwater Saghalie Heights 12-07-2023 12/8/2023 4:31:57 PM Page 6 Mitigated Land Use Dev East Basin Bypass:No GroundWater:No Pervious Land Use acre C, Lawn, Flat 1.267 Pervious Total 1.267 Impervious Land Use acre ROADS FLAT 2.266 Impervious Total 2.266 Basin Total 3.533 Element Flows To: Surface Interflow Groundwater Flow Splitter 1 Flow Splitter 1 Saghalie Heights 12-07-2023 12/8/2023 4:31:57 PM Page 7 Dev West Basin Bypass:No GroundWater:No Pervious Land Use acre C, Lawn, Flat 1.317 Pervious Total 1.317 Impervious Land Use acre ROADS FLAT 2.455 Impervious Total 2.455 Basin Total 3.772 Element Flows To: Surface Interflow Groundwater Infiltration Pond -West Infiltration Pond -West Saghalie Heights 12-07-2023 12/8/2023 4:31:57 PM Page 8 Unmitigated Flows (Infiltration Pond) Bypass:No GroundWater:No Pervious Land Use acre C, Lawn, Flat 1.8238 Pervious Total 1.8238 Impervious Land Use acre ROADS FLAT 3.3614 Impervious Total 3.3614 Basin Total 5.1852 Element Flows To: Surface Interflow Groundwater Saghalie Heights 12-07-2023 12/8/2023 4:31:57 PM Page 9 Unmitigated Flows (Detention Pond) Bypass:No GroundWater:No Pervious Land Use acre C, Lawn, Flat 0.7602 Pervious Total 0.7602 Impervious Land Use acre ROADS FLAT 1.3596 Impervious Total 1.3596 Basin Total 2.1198 Element Flows To: Surface Interflow Groundwater Saghalie Heights 12-07-2023 12/8/2023 4:31:57 PM Page 10 Routing Elements Predeveloped Routing Saghalie Heights 12-07-2023 12/8/2023 4:31:57 PM Page 11 Mitigated Routing Detention Pond - South Bottom Length:80.00 ft. Bottom Width:60.00 ft. Depth:5 ft. Volume at riser head:0.5608 acre-feet. Side slope 1:2 To 1 Side slope 2:2 To 1 Side slope 3:2 To 1 Side slope 4:2 To 1 Discharge Structure Riser Height:4 ft. Riser Diameter:18 in. Orifice 1 Diameter:2 in.Elevation:0 ft. Orifice 2 Diameter:3 in.Elevation:2.5 ft. Element Flows To: Outlet 1 Outlet 2 Pond Hydraulic Table Stage(feet)Area(ac.)Volume(ac-ft.)Discharge(cfs)Infilt(cfs) 0.0000 0.110 0.000 0.000 0.000 0.0556 0.110 0.006 0.025 0.000 0.1111 0.111 0.012 0.036 0.000 0.1667 0.112 0.018 0.044 0.000 0.2222 0.113 0.024 0.051 0.000 0.2778 0.113 0.031 0.057 0.000 0.3333 0.114 0.037 0.062 0.000 0.3889 0.115 0.043 0.067 0.000 0.4444 0.116 0.050 0.072 0.000 0.5000 0.116 0.056 0.076 0.000 0.5556 0.117 0.063 0.080 0.000 0.6111 0.118 0.069 0.084 0.000 0.6667 0.118 0.076 0.088 0.000 0.7222 0.119 0.083 0.092 0.000 0.7778 0.120 0.089 0.095 0.000 0.8333 0.121 0.096 0.099 0.000 0.8889 0.121 0.103 0.102 0.000 0.9444 0.122 0.109 0.105 0.000 1.0000 0.123 0.116 0.108 0.000 1.0556 0.124 0.123 0.111 0.000 1.1111 0.124 0.130 0.114 0.000 1.1667 0.125 0.137 0.117 0.000 1.2222 0.126 0.144 0.120 0.000 1.2778 0.127 0.151 0.122 0.000 1.3333 0.128 0.158 0.125 0.000 1.3889 0.128 0.165 0.127 0.000 1.4444 0.129 0.172 0.130 0.000 1.5000 0.130 0.180 0.132 0.000 1.5556 0.131 0.187 0.135 0.000 1.6111 0.131 0.194 0.137 0.000 1.6667 0.132 0.202 0.140 0.000 1.7222 0.133 0.209 0.142 0.000 1.7778 0.134 0.216 0.144 0.000 1.8333 0.135 0.224 0.147 0.000 Saghalie Heights 12-07-2023 12/8/2023 4:31:57 PM Page 12 1.8889 0.135 0.231 0.149 0.000 1.9444 0.136 0.239 0.151 0.000 2.0000 0.137 0.247 0.153 0.000 2.0556 0.138 0.254 0.155 0.000 2.1111 0.139 0.262 0.157 0.000 2.1667 0.139 0.270 0.159 0.000 2.2222 0.140 0.278 0.161 0.000 2.2778 0.141 0.285 0.163 0.000 2.3333 0.142 0.293 0.165 0.000 2.3889 0.143 0.301 0.167 0.000 2.4444 0.143 0.309 0.169 0.000 2.5000 0.144 0.317 0.171 0.000 2.5556 0.145 0.325 0.231 0.000 2.6111 0.146 0.333 0.256 0.000 2.6667 0.147 0.341 0.277 0.000 2.7222 0.147 0.350 0.294 0.000 2.7778 0.148 0.358 0.309 0.000 2.8333 0.149 0.366 0.323 0.000 2.8889 0.150 0.374 0.336 0.000 2.9444 0.151 0.383 0.349 0.000 3.0000 0.152 0.391 0.360 0.000 3.0556 0.152 0.400 0.371 0.000 3.1111 0.153 0.408 0.382 0.000 3.1667 0.154 0.417 0.392 0.000 3.2222 0.155 0.425 0.402 0.000 3.2778 0.156 0.434 0.411 0.000 3.3333 0.157 0.443 0.421 0.000 3.3889 0.158 0.452 0.430 0.000 3.4444 0.158 0.460 0.438 0.000 3.5000 0.159 0.469 0.447 0.000 3.5556 0.160 0.478 0.455 0.000 3.6111 0.161 0.487 0.463 0.000 3.6667 0.162 0.496 0.471 0.000 3.7222 0.163 0.505 0.479 0.000 3.7778 0.164 0.514 0.487 0.000 3.8333 0.164 0.523 0.494 0.000 3.8889 0.165 0.532 0.501 0.000 3.9444 0.166 0.542 0.509 0.000 4.0000 0.167 0.551 0.516 0.000 4.0556 0.168 0.560 0.731 0.000 4.1111 0.169 0.570 1.117 0.000 4.1667 0.170 0.579 1.611 0.000 4.2222 0.171 0.589 2.180 0.000 4.2778 0.171 0.598 2.799 0.000 4.3333 0.172 0.608 3.439 0.000 4.3889 0.173 0.617 4.073 0.000 4.4444 0.174 0.627 4.673 0.000 4.5000 0.175 0.637 5.214 0.000 4.5556 0.176 0.647 5.679 0.000 4.6111 0.177 0.656 6.056 0.000 4.6667 0.178 0.666 6.348 0.000 4.7222 0.179 0.676 6.574 0.000 4.7778 0.180 0.686 6.855 0.000 4.8333 0.180 0.696 7.080 0.000 4.8889 0.181 0.706 7.298 0.000 4.9444 0.182 0.716 7.510 0.000 5.0000 0.183 0.727 7.715 0.000 5.0556 0.184 0.737 7.915 0.000 Saghalie Heights 12-07-2023 12/8/2023 4:31:57 PM Page 13 Saghalie Heights 12-07-2023 12/8/2023 4:31:57 PM Page 14 Infiltration Pond -West Bottom Length:100.00 ft. Bottom Width:40.00 ft. Depth:6 ft. Volume at riser head:0.6351 acre-feet. Infiltration On Infiltration rate:4 Infiltration safety factor:1 Wetted surface area On Total Volume Infiltrated (ac-ft.):627.255 Total Volume Through Riser (ac-ft.):0 Total Volume Through Facility (ac-ft.):627.255 Percent Infiltrated:100 Total Precip Applied to Facility:0 Total Evap From Facility:0 Side slope 1:2 To 1 Side slope 2:2 To 1 Side slope 3:2 To 1 Side slope 4:2 To 1 Discharge Structure Riser Height:5 ft. Riser Diameter:18 in. Element Flows To: Outlet 1 Outlet 2 Pond Hydraulic Table Stage(feet)Area(ac.)Volume(ac-ft.)Discharge(cfs)Infilt(cfs) 0.0000 0.091 0.000 0.000 0.000 0.0667 0.092 0.006 0.000 0.373 0.1333 0.093 0.012 0.000 0.377 0.2000 0.094 0.018 0.000 0.380 0.2667 0.095 0.024 0.000 0.384 0.3333 0.096 0.031 0.000 0.387 0.4000 0.097 0.037 0.000 0.391 0.4667 0.097 0.044 0.000 0.394 0.5333 0.098 0.050 0.000 0.398 0.6000 0.099 0.057 0.000 0.402 0.6667 0.100 0.064 0.000 0.405 0.7333 0.101 0.070 0.000 0.409 0.8000 0.102 0.077 0.000 0.412 0.8667 0.103 0.084 0.000 0.416 0.9333 0.104 0.091 0.000 0.420 1.0000 0.105 0.098 0.000 0.423 1.0667 0.106 0.105 0.000 0.427 1.1333 0.106 0.112 0.000 0.431 1.2000 0.107 0.119 0.000 0.434 1.2667 0.108 0.126 0.000 0.438 1.3333 0.109 0.134 0.000 0.442 1.4000 0.110 0.141 0.000 0.445 1.4667 0.111 0.148 0.000 0.449 1.5333 0.112 0.156 0.000 0.453 1.6000 0.113 0.163 0.000 0.457 1.6667 0.114 0.171 0.000 0.460 1.7333 0.115 0.179 0.000 0.464 1.8000 0.116 0.186 0.000 0.468 Saghalie Heights 12-07-2023 12/8/2023 4:31:57 PM Page 15 1.8667 0.117 0.194 0.000 0.472 1.9333 0.118 0.202 0.000 0.476 2.0000 0.119 0.210 0.000 0.480 2.0667 0.120 0.218 0.000 0.483 2.1333 0.120 0.226 0.000 0.487 2.2000 0.121 0.234 0.000 0.491 2.2667 0.122 0.242 0.000 0.495 2.3333 0.123 0.250 0.000 0.499 2.4000 0.124 0.259 0.000 0.503 2.4667 0.125 0.267 0.000 0.507 2.5333 0.126 0.275 0.000 0.511 2.6000 0.127 0.284 0.000 0.515 2.6667 0.128 0.292 0.000 0.519 2.7333 0.129 0.301 0.000 0.523 2.8000 0.130 0.310 0.000 0.527 2.8667 0.131 0.318 0.000 0.531 2.9333 0.132 0.327 0.000 0.535 3.0000 0.133 0.336 0.000 0.539 3.0667 0.134 0.345 0.000 0.543 3.1333 0.135 0.354 0.000 0.547 3.2000 0.136 0.363 0.000 0.551 3.2667 0.137 0.372 0.000 0.555 3.3333 0.138 0.382 0.000 0.559 3.4000 0.139 0.391 0.000 0.563 3.4667 0.140 0.400 0.000 0.567 3.5333 0.141 0.410 0.000 0.572 3.6000 0.142 0.419 0.000 0.576 3.6667 0.143 0.429 0.000 0.580 3.7333 0.144 0.438 0.000 0.584 3.8000 0.146 0.448 0.000 0.588 3.8667 0.147 0.458 0.000 0.593 3.9333 0.148 0.468 0.000 0.597 4.0000 0.149 0.478 0.000 0.601 4.0667 0.150 0.488 0.000 0.605 4.1333 0.151 0.498 0.000 0.610 4.2000 0.152 0.508 0.000 0.614 4.2667 0.153 0.518 0.000 0.618 4.3333 0.154 0.528 0.000 0.622 4.4000 0.155 0.538 0.000 0.627 4.4667 0.156 0.549 0.000 0.631 4.5333 0.157 0.559 0.000 0.635 4.6000 0.158 0.570 0.000 0.640 4.6667 0.159 0.581 0.000 0.644 4.7333 0.160 0.591 0.000 0.649 4.8000 0.162 0.602 0.000 0.653 4.8667 0.163 0.613 0.000 0.657 4.9333 0.164 0.624 0.000 0.662 5.0000 0.165 0.635 0.000 0.666 5.0667 0.166 0.646 0.273 0.671 5.1333 0.167 0.657 0.771 0.675 5.2000 0.168 0.668 1.404 0.680 5.2667 0.169 0.679 2.123 0.684 5.3333 0.170 0.691 2.882 0.689 5.4000 0.172 0.702 3.632 0.693 5.4667 0.173 0.714 4.326 0.698 5.5333 0.174 0.725 4.924 0.702 5.6000 0.175 0.737 5.401 0.707 5.6667 0.176 0.749 5.754 0.711 Saghalie Heights 12-07-2023 12/8/2023 4:31:57 PM Page 16 5.7333 0.177 0.760 6.014 0.716 5.8000 0.178 0.772 6.338 0.720 5.8667 0.179 0.784 6.597 0.725 5.9333 0.181 0.796 6.846 0.730 6.0000 0.182 0.808 7.086 0.734 6.0667 0.183 0.821 7.319 0.739 Saghalie Heights 12-07-2023 12/8/2023 4:31:57 PM Page 17 Flow Splitter 1 Bottom Length:10.00 ft. Bottom Length:10.00 ft. Depth:10 ft. Side slope 1:0 To 1 Side slope 2:0 To 1 Side slope 3:0 To 1 Side slope 4:0 To 1 Control Structure Splitter Hydraulic Table Stage(feet)Area(ac.)Volume(ac-ft.)Primary(cfs)Secondary(cfs) 0.000 0.002 0.000 0.000 0.000 0.111 0.002 0.000 1.302 2.930 0.222 0.002 0.000 1.842 4.144 0.333 0.002 0.000 2.256 5.076 0.444 0.002 0.001 2.605 5.861 0.555 0.002 0.001 2.912 6.553 0.666 0.002 0.001 3.190 7.178 0.777 0.002 0.001 3.446 7.754 0.888 0.002 0.002 3.684 8.289 1.000 0.002 0.002 3.907 8.792 1.111 0.002 0.002 4.119 9.267 1.222 0.002 0.002 4.320 9.720 1.333 0.002 0.003 4.512 10.15 1.444 0.002 0.003 4.696 10.56 1.555 0.002 0.003 4.873 10.96 1.666 0.002 0.003 5.044 11.35 1.777 0.002 0.004 5.210 11.72 1.888 0.002 0.004 5.370 12.08 2.000 0.002 0.004 5.526 12.43 2.111 0.002 0.004 5.677 12.77 2.222 0.002 0.005 5.825 13.10 2.333 0.002 0.005 5.969 13.43 2.444 0.002 0.005 6.109 13.74 2.555 0.002 0.005 6.246 14.05 2.666 0.002 0.006 6.381 14.35 2.777 0.002 0.006 6.512 14.65 2.888 0.002 0.006 6.641 14.94 3.000 0.002 0.006 6.768 15.22 3.111 0.002 0.007 6.892 15.50 3.222 0.002 0.007 7.014 15.78 3.333 0.002 0.007 7.134 16.05 3.444 0.002 0.007 7.252 16.31 3.555 0.002 0.008 7.368 16.57 3.666 0.002 0.008 7.482 16.83 3.777 0.002 0.008 7.595 17.08 3.888 0.002 0.008 7.706 17.33 4.000 0.002 0.009 7.815 17.58 4.111 0.002 0.009 7.923 17.82 4.222 0.002 0.009 8.029 18.06 4.333 0.002 0.009 8.134 18.30 4.444 0.002 0.010 8.238 18.53 4.555 0.002 0.010 8.340 18.76 4.666 0.002 0.010 8.441 18.99 4.777 0.002 0.011 8.541 19.21 4.888 0.002 0.011 8.640 19.44 5.000 0.002 0.011 8.737 19.66 5.111 0.002 0.011 9.224 20.46 Saghalie Heights 12-07-2023 12/8/2023 4:31:57 PM Page 18 5.222 0.002 0.012 9.976 21.72 5.333 0.002 0.012 10.70 23.18 5.444 0.002 0.012 11.20 24.61 5.555 0.002 0.012 11.55 25.82 5.666 0.002 0.013 11.87 26.68 5.777 0.002 0.013 12.17 27.38 5.888 0.002 0.013 12.45 28.01 6.000 0.002 0.013 12.72 28.62 6.111 0.002 0.014 12.98 29.20 6.222 0.002 0.014 13.23 29.76 6.333 0.002 0.014 13.47 30.31 6.444 0.002 0.014 13.70 30.83 6.555 0.002 0.015 13.93 31.35 6.666 0.002 0.015 14.15 31.85 6.777 0.002 0.015 14.37 32.33 6.888 0.002 0.015 14.58 32.81 7.000 0.002 0.016 14.79 33.28 7.111 0.002 0.016 14.99 33.74 7.222 0.002 0.016 15.19 34.19 7.333 0.002 0.016 15.39 34.63 7.444 0.002 0.017 15.58 35.06 7.555 0.002 0.017 15.77 35.49 7.666 0.002 0.017 15.96 35.91 7.777 0.002 0.017 16.14 36.33 7.888 0.002 0.018 16.32 36.74 8.000 0.002 0.018 16.50 37.14 8.111 0.002 0.018 16.68 37.54 8.222 0.002 0.018 16.85 37.93 8.333 0.002 0.019 17.03 38.32 8.444 0.002 0.019 17.20 38.70 8.555 0.002 0.019 17.36 39.08 8.666 0.002 0.019 17.53 39.45 8.777 0.002 0.020 17.69 39.82 8.888 0.002 0.020 17.86 40.18 9.000 0.002 0.020 18.02 40.55 9.111 0.002 0.020 18.18 40.90 9.222 0.002 0.021 18.33 41.26 9.333 0.002 0.021 18.49 41.61 9.444 0.002 0.021 18.64 41.96 9.555 0.002 0.021 18.80 42.30 9.666 0.002 0.022 18.95 42.64 9.777 0.002 0.022 19.10 42.98 9.888 0.002 0.022 19.25 43.31 10.00 0.002 0.023 19.40 43.65 10.11 0.002 0.023 19.54 43.97 Discharge Structure Riser Height:5 ft. Riser Diameter:12 in. Orifice 1 Diameter:12 in.Elevation:0 ft. Element Flows To: Outlet 1 Outlet 2 Infiltration Pond -West Detention Pond - South Saghalie Heights 12-07-2023 12/8/2023 4:31:57 PM Page 19 Analysis Results POC 1 + Predeveloped x Mitigated Predeveloped Landuse Totals for POC #1 Total Pervious Area:7.305 Total Impervious Area:0 Mitigated Landuse Totals for POC #1 Total Pervious Area:2.584 Total Impervious Area:4.721 Flow Frequency Method:Log Pearson Type III 17B Flow Frequency Return Periods for Predeveloped. POC #1 Return Period Flow(cfs) 2 year 0.214773 5 year 0.33731 10 year 0.406753 25 year 0.480322 50 year 0.525847 100 year 0.564568 Flow Frequency Return Periods for Mitigated. POC #1 Return Period Flow(cfs) 2 year 0.137243 5 year 0.1926 10 year 0.235975 25 year 0.299101 50 year 0.352632 100 year 0.412149 Annual Peaks Annual Peaks for Predeveloped and Mitigated. POC #1 Year Predeveloped Mitigated 1949 0.211 0.125 1950 0.263 0.133 1951 0.473 0.267 1952 0.149 0.109 1953 0.121 0.116 1954 0.185 0.112 1955 0.295 0.151 1956 0.235 0.134 1957 0.190 0.135 1958 0.214 0.127 Saghalie Heights 12-07-2023 12/8/2023 4:32:30 PM Page 20 1959 0.183 0.121 1960 0.320 0.223 1961 0.180 0.127 1962 0.112 0.098 1963 0.154 0.122 1964 0.203 0.117 1965 0.145 0.131 1966 0.140 0.103 1967 0.292 0.144 1968 0.182 0.111 1969 0.178 0.126 1970 0.147 0.118 1971 0.157 0.125 1972 0.352 0.153 1973 0.160 0.112 1974 0.174 0.104 1975 0.235 0.147 1976 0.170 0.119 1977 0.020 0.101 1978 0.149 0.126 1979 0.090 0.102 1980 0.334 0.155 1981 0.133 0.116 1982 0.257 0.278 1983 0.230 0.132 1984 0.142 0.109 1985 0.084 0.125 1986 0.372 0.170 1987 0.329 0.216 1988 0.130 0.111 1989 0.085 0.089 1990 0.688 0.256 1991 0.415 0.273 1992 0.160 0.114 1993 0.167 0.119 1994 0.056 0.084 1995 0.239 0.132 1996 0.503 0.259 1997 0.421 0.305 1998 0.095 0.116 1999 0.394 0.158 2000 0.166 0.122 2001 0.030 0.101 2002 0.182 0.183 2003 0.232 0.103 2004 0.301 0.429 2005 0.216 0.140 2006 0.254 0.132 2007 0.511 0.352 2008 0.659 0.403 2009 0.324 0.158 Ranked Annual Peaks Ranked Annual Peaks for Predeveloped and Mitigated. POC #1 Rank Predeveloped Mitigated 1 0.6885 0.4290 2 0.6592 0.4032 3 0.5110 0.3518 Saghalie Heights 12-07-2023 12/8/2023 4:32:30 PM Page 21 4 0.5032 0.3048 5 0.4729 0.2782 6 0.4207 0.2733 7 0.4145 0.2669 8 0.3945 0.2595 9 0.3723 0.2558 10 0.3522 0.2233 11 0.3338 0.2161 12 0.3293 0.1834 13 0.3237 0.1700 14 0.3198 0.1578 15 0.3006 0.1577 16 0.2952 0.1552 17 0.2920 0.1533 18 0.2631 0.1510 19 0.2566 0.1468 20 0.2543 0.1445 21 0.2388 0.1403 22 0.2353 0.1347 23 0.2349 0.1343 24 0.2321 0.1334 25 0.2300 0.1324 26 0.2156 0.1324 27 0.2135 0.1315 28 0.2110 0.1310 29 0.2029 0.1267 30 0.1895 0.1265 31 0.1851 0.1256 32 0.1832 0.1255 33 0.1822 0.1252 34 0.1820 0.1248 35 0.1804 0.1245 36 0.1782 0.1224 37 0.1736 0.1215 38 0.1701 0.1214 39 0.1665 0.1193 40 0.1661 0.1186 41 0.1600 0.1184 42 0.1599 0.1167 43 0.1572 0.1164 44 0.1541 0.1158 45 0.1492 0.1156 46 0.1490 0.1137 47 0.1470 0.1125 48 0.1453 0.1117 49 0.1420 0.1109 50 0.1397 0.1107 51 0.1334 0.1094 52 0.1299 0.1093 53 0.1205 0.1039 54 0.1123 0.1027 55 0.0952 0.1026 56 0.0902 0.1016 57 0.0848 0.1010 58 0.0843 0.1010 59 0.0560 0.0982 60 0.0298 0.0886 61 0.0202 0.0839 Saghalie Heights 12-07-2023 12/8/2023 4:35:21 PM Page 44 Appendix Predeveloped Schematic Saghalie Heights 12-07-2023 12/8/2023 4:35:22 PM Page 45 Mitigated Schematic Saghalie Heights 12-07-2023 12/8/2023 4:35:23 PM Page 62 Disclaimer Legal Notice This program and accompanying documentation are provided 'as-is' without warranty of any kind. The entire risk regarding the performance and results of this program is assumed by End User. Clear Creek Solutions Inc. and the governmental licensee or sublicensees disclaim all warranties, either expressed or implied, including but not limited to implied warranties of program and accompanying documentation. In no event shall Clear Creek Solutions Inc. be liable for any damages whatsoever (including without limitation to damages for loss of business profits, loss of business information, business interruption, and the like) arising out of the use of, or inability to use this program even if Clear Creek Solutions Inc. or their authorized representatives have been advised of the possibility of such damages. Software Copyright © by : Clear Creek Solutions, Inc. 2005-2023; All Rights Reserved. Clear Creek Solutions, Inc. 6200 Capitol Blvd. Ste F Olympia, WA. 98501 Toll Free 1(866)943-0304 Local (360)943-0304 www.clearcreeksolutions.com Appendix B – Geotechnical Report