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6070CITYOFRENTON,WASHINGTONORDINANCENO.6070ANORDINANCEOFTHECITYOFRENTON,WASHINGTON,AMENDINGSUBSECTIONS4-6-030.CAND4-6-030.1ANDSECTION4-11-190OFTHERENTONMUNICIPALCODE,ADOPTINGBYREFERENCETHE2022CITYOFRENTONSURFACEWATERDESIGNMANUAL,ANDADOPTINGBYREFERENCETHEJULY2021KINGCOUNTYSTORMWATERPOLLUTIONPREVENTIONMANUAL,AUTHORIZINGCORRECTIONS,PROVIDINGFORSEVERABILITY,ANDESTABLISHINGANEFFECTIVEDATE.WHEREAS,theSurfaceWaterUtilityisupdatingtheCityofRentonSurfaceWaterDesignManualfRSWDM);andWHEREAS,theRSWDMisusedbyprivatedevelopersandtheCitywhendesigningnewstormsystems,upgradingexistingstormwaterinfrastructure,orperformingdrainagereviewonprojects;andWHEREAS,theCity’scurrent(2019-2024)MunicipalStormwaterPermit,issuedbytheDepartmentofEcology,requirestheadoptionofamanualequivalenttothe2019EcologyStormwaterManagementManualforWesternWashington(SWMMWW);andWHEREAS,withthisOrdinancetheCityprimarilyseekstoupdatetheCity’sSurfaceWaterDesignManualforconsistencywiththeSWMMWW,andreviseRMC4-6-030,Drainage(SurfaceWater)Standards,andRMC4-11-190,Definitions5,forconsistencywiththeRSWDMupdate;andWHEREAS,thismatterwasdulyreferredtothePlanningCommissionforinvestigationandstudy,andthematterwasconsideredbythePlanningCommission;andWHEREAS,pursuanttoRCW36.70A.106,onMay13,2022,theCitynotifiedtheStateofWashingtonofitsintenttoadoptamendmentstoitsdevelopmentregulations;and1 ORDINANCENO.6070WHEREAS,thePlanningCommissionheldapublichearingonMay18,2022,consideredallrelevantmatters,andheardallpartiesinsupportoropposition,andsubsequentlyforwardedarecommendationtotheCityCouncil;NOW,THEREFORE,THECITYCOUNCILOFTHECITYOFRENTON,WASHINGTON,DOORDAINASFOLLOWS:SECTIONI.AllportionsoftheRentonMunicipalCodeinthisordinancenotshowninstrikethroughandunderlineeditsremainineffectandunchanged.SECTIONII.Subsections4-6-030.Cand4-6-030.1oftheRentonMunicipalCodeareamendedasshownbelow.Allotherprovisionsin4-6-030remainineffectandunchanged.C.ADOPTIONOFSURFACEWATERDESIGNMANUAL:The2016KingCounty,Washington,SurfaceWaterDesignManualasitexistsormaybeamendedbytheCityofRentonAmendmentstotheKingCountySurfaceWaterDesignManual,dated-‘r12,2016,‘-onDecember13,2021,isadoptedbyreferenceandreferredtohereafterasytheCityofRentonforconsistencywiththecurrentversionoftheKingCountySurfaceWaterDesignManual.TheSurfaceWaterDesignManualshallbefiledwiththeCityClerkandavailableforviewingontheCity’swebsite.I.ADOPTIONOFSTORMWATERPOLLUTIONPREVENTIONMANUAL:TheApril2016July2021KingCountyStormwaterPollutionPreventionManual,herebyreferredtoastheStormwaterPollutionPreventionManual,isherebyadoptedbyreference.OnecopyofthemanualshallbefiledwiththeCityClerk.2 ORDINANCENO.6070SECTIONIII.Thedefinitionof“SurfaceWaterDesignManual”insection4-11-190oftheRentonMunicipalCodeisamendedasshownbelow.Allotherdefinitionsin4-11-190remainineffectandunchanged.SURFACEWATERDESIGNMANUALAmanual,asitexistsormaybeamended,adoptedbyreferencebytheCityofRenton,whichprovidesstormwaterpermitimplementationandmanagementguidanceconsistentwiththecurrentversionoftheKingCountySurfaceWaterDesignManual.Shallbcthe2016KingCounty,Washington,SurfaceWaterDc5ignManual,amendedbytheCityofRcntonAmcndmenttotheKingCountySurfaceWaterDesignManual,datedD...........i.....12,2016.SECTIONIV.The2022CityofRentonSurfaceWaterDesignManual,attachedheretoasAttachmentAandincorporatedbythisreference,isherebyadopted.SECTIONV.TheJuly2021KingCountyStormwaterPollutionPreventionManual,attachedheretoasAttachmentBandincorporatedbythisreference,isherebyadopted.SECTIONVI.UponapprovaloftheCityAttorney,theCityClerkisauthorizedtodirectthecodifiertomakenecessarycorrectionstothisordinance,includingthecorrectionsofscrivenersorclericalerrors;referencestootherlocal,state,orfederallaws,codes,rules,orregulations;orordinancenumberingandsection/subsectionnumberingandreferences.SECTIONVII.Ifanysection,subsection,sentence,clause,phrase,orwordofthisordinanceshouldbeheldtobeinvalidorunconstitutionalbyacourtorcompetentjurisdiction,suchinvalidityorunconstitutionalitythereofshallnotaffecttheconstitutionalityofanyothersection,subsection,sentence,clause,phrase,orwordofthisordinance.3 ORDINANCENO.6070SECTIONVIII.Thisordinanceshallbeinfullforceandeffectfive(5)daysafterpublicationofasummaryofthisordinanceintheCity’sofficialnewspaper.Thesummaryshallconsistofthisordinance’stitle.PASSEDBYTHECITYCOUNCILthis13thdayof2022.ia/onA.SerfCityClerkAPPROVEDBYTHEMAYORthisl3thdayofiune,20/2.Approvedastoform:ShaneMoloney,CityAttorneyDateofPublication:June16,2022(Summary)ORD-CED:2218:6/7/224             ATTACHMENT A  2022 CITY OF RENTON SURFACE WATER  DESIGN MANUAL  2022 CITY OF RENTON SURFACE WATER DESIGN MANUAL CITY OF RENTON PUBLIC WORKS DEPARTMENT SURFACE WATER UTILITY Adopted on June 22, 2022 Note: Some pages in this document have been purposely skipped or blank pages inserted so that this document will copy correctly when duplexed. 2022 City of Renton Surface Water Design Manual 6/22/2022 INTRODUCTION TABLE OF CONTENTS AND OVERVIEW CITY OF RENTON SURFACE WATER DESIGN MANUAL TABLE OF CONTENTS AND OVERVIEW 6/22/2022 2022 City of Renton Surface Water Design Manual (T his page intentionally left blan k. ) CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 TABLE OF CONTENTS CHAPTER 1 DRAINAGE REVIEW AND REQUIREMENTS Section Page 1.4 Adjustment Process 1-99 1.4.1 Adjustment Authority 1-99 1.4.2 Criteria for Granting Adjustments 1-99 1.4.3 Adjustment Application Process 1-100 1.4.4 Adjustment Review Process 1-101 1.4.5 Appeals 1-101 1.1 Drainage Review 1-11 1.1.1 Projects Requiring Drainage Review 1-12 1.1.2 Drainage Review Types and Requirements 1-12 1.1.3 Drainage Review Required By Other Agencies 1-22 1.1.4 Drainage Design Beyond Minimum Compliance 1-22 1.2 Core Requirements 1-23 1.2.1 Core Requirement #1: Discharge at the Natural Location 1-23 1.2.2 Core Requirement #2: Offsite Analysis 1-24 1.2.3 Core Requirement #3: Flow Control Facilities 1-35 1.2.4 Core Requirement #4: Conveyance System 1-50 1.2.5 Core Requirement #5: Construction Stormwater Pollution Prevention 1-54 1.2.6 Core Requirement #6: Maintenance and Operations 1-59 1.2.7 Core Requirement #7: Financial Guarantees and Liability 1-61 1.2.8 Core Requirement #8: Water Quality Facilities 1-63 1.2.9 Core Requirement #9: On-Site BMPs 1-73 1.3 Special Requirements 1-89 1.3.1 Special Requirement #1: Other Adopted Area-Specific Requirements 1-89 1.3.2 Special Requirement #2: Flood Hazard Area Delineation 1-90 1.3.3 Special Requirement #3: Flood Protection Facilities 1-91 1.3.4 Special Requirement #4: Source Controls 1-92 1.3.5 Special Requirement #5: Oil Control 1-94 1.3.6 Special Requirement #6: Aquifer Protection Area 1-97 TABLE OF CONTENTS AND OVERVIEW 6/22/2022 2022 City of Renton Surface Water Design Manual TABLE OF CONTENTS CHAPTER 2 DRAINAGE PLAN SUBMITTAL CHAPTER 3 HYDROLOGIC ANALYSIS & DESIGN Section Page Section Page 2.1 Plans for Permits and Drainage Review 2-3 2.1.1 Plans Required for Pre-Application Submittal 2-3 2.1.2 Site Plans Required for Drainage Review 2-3 2.2 Plans Required with Construction Permit Application 2-5 2.2.1 Subdivision, PUD, and Binding Site Plans 2-6 2.2.2 Short Subdivisions 2-7 2.2.3 Commercial Site Development 2-7 2.2.4 Single-Family Residential 2-7 2.2.5 Other Permits 2-7 2.3 Drainage Review Plan Specifications 2-9 2.3.1 Engineering Plan Specifications 2-10 2.3.2 Projects in Targeted Drainage Review (TDR) 2-35 2.4 Plans Required After Drainage Review 2-37 2.4.1 Plan Changes After Permit Issuance 2-37 2.4.2 Final Corrected Plan Submittal 2-37 2.4.3 Final Plat, Short Plat, and Binding Site Plan Submittals 2-38 3.1 Hydrologic Design Standards and Principles 3-3 3.1.1 Hydrologic Impacts and Mitigation 3-3 3.1.2 Flow Control Standards 3-5 3.1.3 Hydrologic Analysis Using Continuous Models 3-5 3.2 Runoff Computation and Analysis Methods 3-9 3.2.1 Rational Method 3-11 3.2.2 Continuous Models and the Runoff Files Method 3-19 3.2.3 The Approved Model 3-30 3.2.4 The HSPF Model 3-30 3.3 Hydrologic Design Procedures and Considerations 3-33 3.3.1 General Hydrologic Design Process 3-33 3.3.2 Flow Control Design Using the Runoff Files Method 3-34 3.3.3 Conveyance System Design with the Runoff Files Method 3-37 3.3.4 Safety Factors in Hydrologic Design 3-38 3.3.5 Design Options for Addressing Downstream Drainage Problems 3-38 3.3.6 Point of Compliance Analysis 3-38 3.3.7 Onsite Closed Depressions and Ponding Areas 3-41 TABLE OF CONTENTS AND OVERVIEW 2022 City of Renton Surface Water Design Manual 6/22/2022 TABLE OF CONTENTS CHAPTER 4 CONVEYANCE SYSTEM ANALYSIS & DESIGN CHAPTER 5 FLOW CONTROL DESIGN Section Page Section Page 4.1 Route Design and Easement Requirements 4-3 4.1.1 Route Design 4-3 4.1.2 Easement and Setback Requirements 4-3 4.2 Pipes, Outfalls, and Pumps 4-7 4.2.1 Pipe Systems 4-7 4.2.2 Outfall Systems 4-30 4.2.3 Pump Systems 4-36 4.3 Culverts and Bridges 4-37 4.3.1 Culverts 4-37 4.3.2 Culverts Providing for Fish Passage/Migration 4-50 4.3.3 Bridges 4-52 4.4 Open Channels, Floodplains, and Floodways 4-55 4.4.1 Open Channels 4-55 4.4.2 Floodplain/Floodway Analysis 4-71 5.1 Detention Facilities 5-3 5.1.1 Detention Ponds 5-3 5.1.2 Detention Tanks 5-17 5.1.3 Detention Vaults 5-21 5.1.4 Control Structures 5-25 5.1.5 Parking Lot Detention 5-35 5.1.6 Roof Detention 5-35 5.1.7 Simple Detention Pond for Cleared Areas 5-35 5.1.8 Alternative Detention Systems 5-42 5.2 Infiltration Facilities 5-45 5.2.1 General Requirements for Infiltration Facilities 5-45 5.2.2 Infiltration Ponds 5-56 5.2.3 Infiltration Tanks 5-59 5.2.4 Infiltration Vaults 5-62 5.2.5 Infiltration Trenches 5-64 5.2.6 Alternative Infiltration Systems 5-65 5.2.7 Small Infiltration Basins 5-66 TABLE OF CONTENTS AND OVERVIEW 6/22/2022 2022 City of Renton Surface Water Design Manual TABLE OF CONTENTS CHAPTER 6 WATER QUALITY DESIGN Section Page Section Page 6.1 Water Quality Menus 6-3 6.1.1 Basic Water Quality Menu 6-5 6.1.2 Enhanced Basic Water Quality Menu 6-8 6.1.3 Sensitive Lake Protection Menu 6-10 6.1.4 Sphagnum Bog Protection Menu 6-14 6.1.5 High-Use Menu 6-16 6.1.6 Pretreatment Facilities 6-18 6.2 General Requirements for WQ Facilities 6-19 6.2.1 Water Quality Design Flows and Treatment Volumes 6-19 6.2.2 Sequence of Facilities 6-22 6.2.3 Setbacks, Slopes, and Embankments 6-24 6.2.4 Facility Liners 6-28 6.2.5 Flow Splitter Designs 6-32 6.2.6 Flow Spreading Options 6-36 6.3 Vegetated Flowpath Facility Designs 6-41 6.3.1 Basic Bioswales 6-41 6.3.2 Wet Bioswales 6-57 6.3.3 Lateral Inflow Bioswales 6-59 6.3.4 Standard Filter Strips 6-60 6.3.5 Narrow Area Filter Strips 6-68 6.4 Wetpool Facility Designs 6-69 6.4.1 Wetponds — Basic and Large 6-69 6.4.2 Wetvaults 6-84 6.4.3 Stormwater Wetlands 6-90 6.4.4 Combined Detention and Wetpool Facilities 6-96 6.5 Filtration Facility Designs 6-101 6.5.1 General Requirements For Filtration Facilities 6-101 6.5.2 Sand Filters — Basic and Large 6-102 6.5.3 Sand Filter Vaults 6-118 6.5.4 Linear Sand Filters 6-123 6.6 Oil Control Facility Designs 6-127 6.6.1 Catch Basin Inserts 6-127 6.6.2 Oil/Water Separators 6-127 6.7 Proprietary Facility Designs 6-141 6.7.1 Ecology Requirements 6-141 6.7.2 City of Renton Requirements 6-141 6.8 Bioretention Facility Designs 6-145 6.8.1 Bioretention 6-145 6.9 WSDOT WQ Facility Designs 6-159 6.9.1 Media Filter Drain 6-159 6.9.2 Compost-Amended Filter Strips 6-169 6.9.3 Compost-Amended Biofiltration Swales 6-170 TABLE OF CONTENTS AND OVERVIEW TABLE OF CONTENTS 2022 City of Renton Surface Water Design Manual 6/22/2022 DEFINITIONS APPENDICES APPENDIX A Maintenance Requirements for Stormwater Facilities and On-site BMPs APPENDIX B Master Drainage Plan Objective, Criteria, Components and Review Process APPENDIX C Simplified Drainage Requirements APPENDIX D Construction Stormwater Pollution Prevention Standards REFERENCE 1. Surface Water Runoff Policy 2. Adopted Critical Drainage Areas 3. Other Adopted Area Specific Drainage Requirements 4. Other Drainage Related Regulations and Guidelines A Grading Code Soil Amendment Standard B Clearing & Grading Seasonal Limitations C Landscape Management Plan Guidelines D Shared Facility Maintenance Responsibility Guidance 5. Wetland Hydrology Protection Guidelines 6. Hydrologic/Hydraulic Design Methods A Infiltration Rate Test Methods B Pond Geometry Equations C Introduction to Level Pool Routing D Supplemental Modeling Guidelines REFERENCE (continued) 7. Engineering Plan Support A King County Standard Map Symbols B Standard Plan Notes and Example Construction Sequence C Stormfilter Facility Access and Cartridge Configuration 8. Forms and Worksheets A Technical Information Report (TIR) Worksheet B Offsite Analysis Drainage System Table C Water Quality Facility Sizing Worksheets D Flow Control and Water Quality Facility Summary Sheet and Sketch E CSWPP Worksheet Forms F Adjustment Application Form and Process Guidelines G Dedication and Indemnification Clause – Final Recording H Bond Quantities Worksheet I Maintenance and Defect Agreement J Drainage Facility Covenant K Drainage Release Covenant L Drainage Easement M On-Site BMP Covenant and Maintenance Instructions (recordable format) N Impervious Surface Limit Covenant O Clearing Limit Covenant P River Protection Easement Q Leachable Metals Covenant R Agreement to Construct Improvements 9. Interim Changes to Requirements A Blanket Adjustments B Administrative Changes 10. King County-Identified Water Quality Problems 11. Materials A (VACANT) B (VACANT) C Bioretention Soil Media Standard Specifications D (VACANT) E Roofing Erodible or Leachable Materials TABLE OF CONTENTS AND OVERVIEW TABLE OF CONTENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 12. (VACANT) 13. (VACANT) 14. Supplemental Approved Facilities A City of Renton Approved Proprietary Facilities for Use on Private Development Projects B City of Renton Approved Proprietary Facilities for Use in Public Projects CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 1 INTRODUCTION OVERVIEW The intent of this document is to provide requirements and guidance for the design, construction, and maintenance of on-site best management practices (BMPs), flow control facilities and water quality treatment facilities that are approved through the development permit process. This document is based on the 2021 King County Surface Water Design Manual with some modifications. Revisions have been made to the 2021 King County Surface Water Manual to reflect City of Renton-specific requirements. PURPOSE OF AND NEED FOR THIS DOCUMENT The City’s adoption this manual is required to comply with federal stormwater regulations. Specifically, the City’s Phase II National Pollutant Discharge Elimination System (NPDES) municipal stormwater permit establishes regulations for jurisdictions that: 1. Own and operate a storm drain system; 2. Discharge to surface waters; 3. Are located in urbanized areas; and 4. Have a population greater than 1,000. Washington State’s Department of Ecology (Ecology), who oversees stormwater requirements in the state, has developed the 2019 Stormwater Management Manual for Western Washington, which complies with the NPDES requirements. In addition, Ecology has approved the 2021 King County Surface Water Design Manual as equivalent to the 2019 Stormwater Management Manual for Western Washington. ORGANIZATION The chapters of this manual are organized as follows: Chapter 1 – DRAINAGE REVIEW AND REQUIREMENTS Describes the basic drainage requirements that implement the City of Renton’s adopted surface water runoff policies and explains how these requirements are applied to proposed projects through the drainage review process. Chapter 2 – DRAINAGE PLAN SUBMITTAL Describes the requirements and specifications for submittal of design plans for drainage review, including report and plan formats, and scopes. Chapter 3 – HYDROLOGIC ANALYSIS AND DESIGN INTRODUCTION 6/22/2022 2022 City of Renton Surface Water Design Manual 2 Presents the acceptable methods of hydrologic analysis used to estimate runoff and design flow control, conveyance, and water quality facilities. Chapter 4 – CONVEYANCE SYSTEM ANALYSIS AND DESIGN Presents the acceptable methods, details, and criteria for analysis and design of conveyance systems. Chapter 5 – FLOW CONTROL DESIGN Presents the acceptable methods, details, and criteria for analysis and design of flow control facilities. Chapter 6 – WATER QUALITY DESIGN Presents the acceptable methods, details, and criteria for analysis and design of water quality facilities. DEFINITIONS – A comprehensive list of the words, terms, and abbreviations accompanied by their meaning as applied in this manual. APPENDICES:  APPENDIX A – MAINTENANCE REQUIREMENTS FOR STORMWATER FACILITIES AND ON-SITE BMPs Contains the thresholds and standards for maintenance of all flow control facilities, on-site BMPs, conveyance systems, and water quality facilities required in this manual.  APPENDIX B – MASTER DRAINAGE PLAN OBJECTIVES, CRITERIA AND COMPONENTS, AND REVIEW PROCESS Describes in a general outline, the objectives, criteria, components and review process for Master Drainage Plans prepared for Urban Planned Developments and very large projects.  APPENDIX C – SIMPLIFIED DRAINAGE REQUIREMENTS Describes, the simplified drainage requirements for smaller projects that qualify for Simplified Drainage Review.  APPENDIX D – CONSTRUCTION STORMWATER POLLUTION PREVENTION STANDARDS Describes, the required measures to be implemented during construction to prevent discharges of sediment-laden runoff from the project site. It also describes effective management practices for spill control and chemical pollutants used during construction that may be needed to supplement the required erosion and sedimentation control measures. REFERENCE – Includes materials that are strictly for reference only and have not been adopted by the public rule adopting this manual. The applicant is responsible to ensure that the most current materials are used in preparing a permit application. KEY REVISIONS 2022 City of Renton Surface Water Design Manual 6/22/2022 3 KEY REVISIONS This section identifies the key revisions that the City has made to the 2021 King County Surface Water Design Manual. These revisions were necessary to address specific City of Renton requirements and to address deficiencies within the 2021 King County Surface Water Design Manual. 1. Aquifer Protection Area – The City of Renton Surface Water Design Manual includes an additional special requirement (Special Requirement #6) related to the Aquifer Protection Area. Certain facilities are restricted in Zone 1 of the Aquifer Protection Area or may require a liner in Zone 1 Modified or Zone 2 of the Aquifer Protection Area. 2. On-site BMPs – Core Requirement #9 was renamed to On-site BMPs instead of Flow Control BMPs to avoid confusion with Flow Control Facilities (Core Requirement #3). 3. Additional On-site BMP Options for Core Requirement #9 – a) Rain Gardens b) Soil Amendment (included in King County Code, but details were not included in the King County Surface Water Design Manual) c) Tree Retention Credit d) Vegetated Roofs (optional) 4. LID Performance Standard – The LID performance standard is optional, but is not required for any projects located within the City of Renton. 5. Flow Control Standards – The City developed specific terminology for flow control standards in Core Requirement #3: a) Flow Control Duration Standard – Matching Forested Site Conditions b) Flow Control Duration Standard – Matching Existing Site Conditions c) Peak Rate Flow Control Standard – Matching Existing Site Conditions d) Flood Problem Flow Control Standard 6. Additional Water Quality Facility Options for Core Requirement #8 – a) The following facilities are available as options on the Basic WQ Menu: i. Bioretention ii. WSDOT WQ Facilities – Media Filter Drain, Compost-amended Vegetated Filter Strips, and Compost-amended Biofiltration Swales b) The following facilities are available as options on the Enhanced Basic WQ Menu: i. Bioretention ii. Proprietary Facilities iii. WSDOT WQ Facilities – Media Filter Drain, Compost-amended Vegetated Filter Strips, and Compost-amended Biofiltration Swales c) The following facilities are available as options on the Sensitive Lake Protection Menu: i. Proprietary Facilities 7. Proprietary Facilities – Added specific proprietary facilities to Chapter 6, Appendix A, and Reference Section 14-A. 8. Alternative Detention and Infiltration Systems – Revised Section 5.2.6 Alternative Infiltration Systems and added new Section 5.1.8 Alternative Detention Systems for proprietary flow control facilities that deviate from the standard design requirements for other flow control facilities. INTRODUCTION 6/22/2022 2022 City of Renton Surface Water Design Manual 4 9. Element #11 of Core Requirement #2 – This Construction Stormwater Pollution Prevention element was modified to be more inclusive of other infiltration facilities that are not considered to be On-site BMPs. 10. Bioretention Design and Construction – The City incorporated the following modifications to the bioretention design site suitability factors and design criteria: a) Specific information added regarding perpendicular utility crossings b) Minimum bottom with is 18 inches c) Minimum ponding depth is 2 inches d) Maximum side slopes are 2.5 to 1 e) Minimum berm top width is 6 inches f) Minimum shoulder between road edge and bioretention side slope is 6 inches g) Water tolerant plant list has been revised h) Underdrains are allowed for bioretention facilities designed to meet Core Requirement #8 i) Added a section on construction sequencing 11. Permeable Pavement Design and Construction – The City incorporated the following modifications to the permeable pavement design criteria: a) Run-on is not allowed from pervious surfaces b) Underdrains are allowed in specific settings c) Added design criteria regarding the overflow d) Added figures depicting permeable pavement cross sections and permeable pavement with check dams e) Added a reference to ASTM C1701 and ASTM C1781 for infiltration rate verification f) Added a section on construction sequencing 12. Core Requirement #6 and #7 – Language specific to the City of Renton has been incorporated into Core Requirement #6 (Maintenance and Operations) and Core Requirement #7 (Financial Guarantees and Liabilities). 13. Basic WQ Thresholds – The thresholds for basic WQ treatment in Core Requirement #8 have been adjusted for consistency with the Amendments to the 2009 King County Surface Water Design Manual and the 2012 Stormwater Management Manual for Western Washington, as amended in 2014. 14. Target impervious surface – The City of Renton does not require including existing impervious surfaces added on or after January 8, 2001 in the definition of target impervious surfaces. 15. Continuous modeling timestep – Precipitation with a 15-minute precipitation is available for the entire City of Renton, so references to the 1-hour timestep were removed. 16. Continuous modeling precipitation series – The City of Renton allows either rain gage data (Sea- Tac Airport) or the 158-year extended precipitation timeseries (Puget East) to be used for modeling. 17. Allowable pipe materials – The City of Renton added a table of allowable pipe materials and minimum cover to Chapter 4. 18. Conveyance requirements – The City of Renton added requirements to Chapter 4 for changes in pipe size, structures, pipe cover, pipe clearances, pipe system connections, and pump systems. 19. Fencing requirements – The City of Renton added specific fencing requirements to Chapter 5 and Chapter 6 for detention ponds and wet ponds related to a City policy decision. KEY REVISIONS 2022 City of Renton Surface Water Design Manual 6/22/2022 5 20. Seeding – The City of Renton revised seeding rates, timing, and mixes in Chapter 5, Chapter 6, and Appendix C, and Appendix D. 21. Removing terminology that does not apply – Terminology related to the Urban Growth Area, Urban Planned Development (UPD), Agricultural Projects, Critical Drainage Areas, Large Rural Lots, Stormwater Compliance Plans (SWCPs), Experimental Design Adjustments, recreational tracts, and Landscape Management Plan was removed. 22. Removed BMPs that do not apply – Catch Basin Inserts (Chapter 6), Narrow Area Filter Strips (Chapter 6), and Farmland Dispersion (Appendix C). 23. References a) Provided links to the City’s website for covenants, easements, agreements, and worksheets OTHER APPLICABLE REFERENCES The City also adopts, by reference, the 2021 King County Stormwater Pollution Prevention Manual with amendments, for determining source control requirements. INTRODUCTION 6/22/2022 2022 City of Renton Surface Water Design Manual 6 (T his page intentionally left blan k. ) 2022 City of Renton Surface Water Design Manual 6/22/2022 CHAPTER 1 DRAINAGE REVIEW AND REQUIREMENTS CITY OF RENTON SURFACE WATER DESIGN MANUAL Section Page 1.1 Drainage Review 1-11 1.1.1 Projects Requiring Drainage Review 1-12 1.1.2 Drainage Review Types and Requirements 1-12 1.1.3 Drainage Review Required By Other Agencies 1-22 1.1.4 Drainage Design Beyond Minimum Compliance 1-22 1.2 Core Requirements 1-23 1.2.1 Core Requirement #1: Discharge at the Natural Location 1-23 1.2.2 Core Requirement #2: Offsite Analysis 1-24 1.2.3 Core Requirement #3: Flow Control Facilities 1-35 1.2.4 Core Requirement #4: Conveyance System 1-50 1.2.5 Core Requirement #5: Construction Stormwater Pollution Prevention 1-54 1.2.6 Core Requirement #6: Maintenance and Operations 1-59 1.2.7 Core Requirement #7: Financial Guarantees and Liability 1-61 1.2.8 Core Requirement #8: Water Quality Facilities 1-63 1.2.9 Core Requirement #9: On-Site BMPs 1-73 1.3 Special Requirements 1-89 1.3.1 Special Requirement #1: Other Adopted Area-Specific Requirements 1-89 1.3.2 Special Requirement #2: Flood Hazard Area Delineation 1-90 1.3.3 Special Requirement #3: Flood Protection Facilities 1-91 1.3.4 Special Requirement #4: Source Controls 1-92 1.3.5 Special Requirement #5: Oil Control 1-94 1.3.6 Special Requirement #6: Aquifer Protection Area 1-97 1.4 Adjustment Process 1-99 1.4.1 Adjustment Authority 1-99 1.4.2 Criteria for Granting Adjustments 1-99 1.4.3 Adjustment Application Process 1-100 1.4.4 Adjustment Review Process 1-101 1.4.5 Appeals 1-101 CITY OF RENTON SURFACE WATER DESIGN MANUAL 6/22/2022 2022 City of Renton Surface Water Design Manual (T his page intentionally left blan k. ) CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 1-1 CHAPTER 1 DRAINAGE REVIEW AND REQUIREMENTS This chapter describes the drainage review procedures and types, the drainage requirements, and the adjustment procedures necessary to implement surface water runoff policies codified in Chapter 4-6-030 of the City of Renton Municipal Code (RMC). It also provides direction for implementing the more detailed procedures and design criteria found in subsequent chapters of this manual. Chapter Organization The information presented in Chapter 1 is organized into four main sections as follows:  Section 1.1, “Drainage Review”  Section 1.2, “Core Requirements”  Section 1.3, “Special Requirements”  Section 1.4, “Adjustment Process” Each of these sections begins on an odd page so the user can insert tabs if desired for quicker reference. Formatting of Chapter Text The text of Chapter 1 and subsequent chapters has been formatted using the following conventions to aid the user in finding, understanding, and properly applying the thresholds, requirements, and procedures contained in this manual:  Italic is used to highlight the following: (a) terms when they are first introduced and defined within the same paragraph; (b) special notes that supplement or clarify thresholds, requirements, and procedures; (c) sentences considered important for purposes of understanding thresholds, requirements, and procedures; and (d) titles of publications.  Bold italic is used to highlight terms considered key to understanding and applying drainage review thresholds, requirements, and procedures. These are called “key terms” and are defined below. This convention applies after the key term is defined and does not necessarily apply to tables and figures.  Bold is used to highlight words and phrases that are not key terms but are considered important to emphasize for purposes of finding and properly applying thresholds, requirements, and procedures. Key Terms and Definitions (a complete list of definitions follows Chapter 6) Proper application of the drainage review and requirements in this chapter requires an understanding of the following key terms and their definitions. Other key terms may be defined in subsequent chapters. All such key terms are highlighted in bold italic throughout the manual. Other important terms that are not key terms are defined in the text when they are first introduced. These are highlighted in italic when they are first introduced but are not highlighted throughout the manual. All terms defined in this chapter are also found in the “Definitions” section of this manual as are other important terms defined throughout the Manual. CHAPTER 1 DRAINAGE REVIEW AND REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-2 Aquifer Protection Area (APA) means the portion of an aquifer within the zone of capture and recharge area for a well or well field owned or operated by the City of Renton, as depicted in the Wellhead Protection Area Zones layer of COR Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>). Arterial – A high traffic-volume road or street primarily for through traffic. The term generally includes roads or streets considered collectors. It does not include local access roads which are generally limited to providing access to abutting property. Arterial streets are depicted in the Arterials layer of COR Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>). Bioretention – An on-site and water quality treatment best management practice consisting of a shallow landscaped depression designed to temporarily store and promote infiltration of stormwater runoff. Standards for bioretention design, including soil mix, plants, storage volume and feasibility criteria, are specified in Appendix C of this manual. Bioretention can be used to meet Core Requirement #3, #8 and/or 9. CED means the Community and Economic Development Department. Certified Erosion and Sediment Control Lead (CESCL) means an individual who has current certification through an approved erosion and sediment control training program that meets the minimum training standards established by the Washington State Department of Ecology (Ecology). A CESCL is knowledgeable in the principles and practices of erosion and sediment control. The CESCL must have the skills to assess site conditions and construction activities that could impact the quality of stormwater and, the effectiveness of erosion and sediment control measures used to control the quality of stormwater discharges. Certification is obtained through an Ecology approved erosion and sediment control course. Civil engineer means a person licensed by the state of Washington as a professional engineer in civil engineering. Construct or modify means to install a new drainage pipe or ditch or make improvements to an existing drainage pipe or ditch, for purposes other than maintenance, that either serves to concentrate previously unconcentrated surface water or stormwater runoff or serves to increase, decrease or redirect the conveyance of surface water or stormwater runoff. Conveyance system nuisance problem means a flooding or erosion problem that does not constitute a severe flooding problem or severe erosion problem and that results from the overflow of a constructed conveyance system for runoff events less than or equal to a 10-year event. Examples include inundation of a shoulder or lane of a roadway, overflows collecting in yards or pastures, shallow flows across driveways, minor flooding of crawl spaces or unheated garages/outbuildings, and minor erosion. Development The division of a parcel of land into two (2) or more parcels; the construction, reconstruction, conversion, structural alteration, relocation or enlargement of any structure; any mining, excavation, landfill or land disturbance and any use or extension of the use of land. Development review engineer – The City of Renton employee responsible for the conditioning, review, inspection, and approval of right-of-way use permits, and road and drainage improvements constructed as part of development permits administered by CED. Effective impervious surface – Those impervious surfaces that are connected via sheet flow or discrete conveyance to a drainage system. Impervious surfaces are considered ineffective if: 1) the runoff is fully dispersed as described in Appendix C of this manual; 2) residential roof runoff is infiltrated in accordance with the full infiltration BMP described in Appendix C of this manual; or 3) approved continuous runoff modeling methods indicate that the entire runoff file is infiltrated. Erodible or leachable materials, wastes, or chemicals are those materials or substances that, when exposed to rainfall, measurably alter the physical or chemical characteristics of the rainfall runoff (Examples include but are not limited to erodible soil, uncovered process wastes, manure, fertilizers, CHAPTER 1—KEY TERMS AND DEFINITIONS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-3 oily substances, ashes, kiln dust, garbage dumpster leakage, commercial-scale vehicle and animal wash waste, galvanized structural, architectural, cabinet, and utility steel, architectural copper, bronze, brass, and lead, treated lumber, etc.). Erosion hazard area is the critical area designation, defined and regulated in RMC 4-3-050, that is applied to areas underlain by soils that are subject to severe erosion when disturbed. Erosion hazard areas are depicted in the Erosion Hazard - High layer of COR Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>). Existing site conditions means those that existed prior to May 1979 as determined from aerial photographs and, if necessary, knowledge of individuals familiar with the area, unless a drainage plan for land cover changes has been approved by the City of Renton since May 1979 as part of a City permit or approval (or County-approved permit if in an area that has been annexed by the City). If so, existing site conditions are those created by the site improvements and drainage facilities constructed per the approved drainage plan. Exposed means subject to direct or blown-in precipitation and/or direct or blown in runoff. Not fully covered. Exposed area or exposed material means not covered sufficiently to shield from rainfall and stormwater runoff. At a minimum, full coverage to not be considered exposed requires a roof with enough overhang in conjunction with walls of sufficient height to prevent rainfall blow-in; and the walls must extend into the ground or to a berm or footing to prevent runoff from being blown in or from running onto the covered area. Flood hazard area is the critical area designation, defined and regulated in RMC 4-3-050, that is applied to areas subject to flooding. Flood hazard areas are depicted in the Flood layer of COR Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>). Flow control facility means a drainage facility designed in accordance with the drainage requirements in this manual to mitigate the impacts of increased stormwater runoff generated by site development. A “flow control facility” is designed either to hold water for a considerable length of time and then release it by evaporation, plant transpiration or infiltration into the ground or to hold runoff for a short period of time and then release it to the conveyance system. Forested site conditions means those that existed on the site prior to any development in the Puget Sound region, assumed to be forest cover (see “historical site conditions”). Fully covered means covered sufficiently to shield from rainfall and stormwater runoff. At a minimum, full coverage requires a roof with enough overhang in conjunction with walls of sufficient height to prevent rainfall blow-in; and the walls must extend into the ground or to a berm or footing to prevent runoff from being blown in or from running onto the covered area. Not exposed. Fully dispersed means the runoff from an impervious surface or nonnative pervious surface has dispersed per the criteria for fully dispersed surface in Section 1.2.3.2 of this manual. Groundwater protection areas include the Cedar Valley Sole Source Aquifer Project Review Area designated by the federal Environmental Protection Agency, Wellfield Capture Zones as mapped by the Washington State Department of Health, and the Aquifer Protection Areas as mapped by the City. High-use site means a commercial or industrial site that (1) has an expected average daily traffic (ADT) count equal to or greater than 100 vehicles per 1,000 square feet of gross building area; (2) is subject to petroleum storage or transfer in excess of 1,500 gallons per year, not including delivered heating oil; or (3) is subject to use, storage, or maintenance of a fleet of 25 or more vehicles that are over 10 tons net weight (trucks, buses, trains, heavy equipment, etc.). Also included is any road intersection with a measured ADT count of 25,000 vehicles or more on the main roadway and 15,000 vehicles or more on any intersecting roadway, excluding projects proposing primarily pedestrian or bicycle use improvements. For the purposes of this definition, commercial and industrial site means that portion CHAPTER 1 DRAINAGE REVIEW AND REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-4 of a site’s developed area associated with an individual commercial or industrial business (e.g., the area occupied by the business’s buildings and required parking). Historical site conditions means those that existed on the site prior to any development in the Puget Sound region. For lands not currently submerged (i.e., outside the ordinary high water mark of a lake, wetland, or stream), historical site conditions shall be assumed to be forest cover unless reasonable, historical, site-specific information is provided to demonstrate a different vegetation cover. The historical site conditions exception in the King County Surface Water Design Manual does not apply to the City. Impaired waterbody or impaired receiving water means where the receiving waterbody is either (1) listed as impaired according to Ecology's Water Quality Assessment categories 2, 4, or 5 for water or sediment, as documented in the state’s Water Quality Assessment 303(d)/305(b) Integrated Report and as displayed in WA Ecology’s electronic database and map viewer of these waterbodies, and/or (2) where subject to any other local, state, or federal cleanup plan or contaminated site designation. Impervious surface means a non-vegetated surface area that either prevents or retards the entry of water into the soil mantle as under natural conditions before development; or that causes water to run off the surface in greater quantities or at an increased rate of flow compared to the flow present under natural conditions prior to development (see also new impervious surface). Common impervious surfaces include, but are not limited to, roof, walkways, patios, driveways, parking lots, or storage areas, areas that are paved, graveled or made of packed or oiled earthen materials or other surfaces that similarly impede the natural infiltration of surface water or stormwater. For the purposes of flow control and water quality treatment modeling and applying the impervious surface thresholds and exemptions contained in this manual, permeable pavement, vegetated roofs, and pervious surfaces with underdrains designed to collect stormwater runoff are considered impervious surface. An open uncovered flow control or water quality facility is not considered impervious surface for the purposes of applying impervious surface thresholds and exemptions but shall be modeled as impervious surface for the purposes of computing runoff. Land disturbing activity means any activity that results in a change in the existing soil cover, both vegetative and non-vegetative, or the existing soil topography. Land disturbing activities include, but are not limited to demolition, construction, clearing, grading, filling, excavation, and compaction. Land disturbing activity does not include tilling conducted as part of agricultural practices, landscape maintenance, or gardening. Landslide hazard is the critical area designation, defined and regulated in RMC 4-3-050, that is applied to areas subject to severe risk of landslide due to topography, soil conditions, and geology. Landslide hazard areas are depicted in the Landslide layer of COR Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>). Landslide hazard drainage area means an area where overland flows from a project may pose a significant threat to health and safety because of its close proximity to a landslide hazard. Local drainage system means any natural or constructed drainage feature that collects and concentrates runoff from the site and discharges it downstream. Low Impact Development (LID) – A stormwater and land use management strategy that strives to mimic pre-disturbance hydrologic processes of infiltration, filtration, storage, evaporation and transpiration by emphasizing conservation, use of onsite natural features, site planning, and distributed stormwater management practices that are integrated into a project design. LID Best Management Practices – Distributed stormwater management practices, integrated into a project design, that emphasize pre-disturbance hydrologic processes of infiltration, filtration, storage, evaporation and transpiration. LID BMPs are referred to as on-site BMPs in this manual and include, but are not limited to, bioretention, permeable pavements, limited infiltration systems, roof downspout controls, dispersion, soil quality and depth, and minimum disturbance foundations. CHAPTER 1—KEY TERMS AND DEFINITIONS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-5 LID Principles – Land use management strategies that emphasize conservation, use of onsite natural features, and site planning to minimize impervious surfaces, native vegetation loss, and stormwater runoff. Maintenance means those usual activities taken to prevent a decline, lapse, or cessation in the use of currently serviceable structures, facilities, BMPs, equipment, or systems if there is no expansion of any of these, and there are no significant hydrologic impacts. Maintenance includes the repair or replacement of non-functional facilities and BMPs, and the replacement of existing structures (e.g., catch basins, manholes, culverts) with different types of structures, if the repair or replacement is required to meet current engineering standards or is required by one or more environmental permits and the functioning characteristics of the original facility or structure are not changed. For the purposes of applying this definition to the thresholds and requirements of this manual, CED will determine whether the functioning characteristics of the original facility, structure, or BMP will remain sufficiently unchanged to consider replacement as maintenance. Note: The following pavement maintenance practices are exempt from drainage review:  Pothole and square cut patching  Overlaying existing non-permeable asphalt or non-permeable concrete pavement with asphalt or concrete without expanding the area of coverage  Shoulder grading  Reshaping/regrading drainage systems  Crack sealing  Resurfacing with in-kind material without expanding the road prism, pavement preservation activities that do not expand the paved prism  Vegetation maintenance. The following pavement maintenance practices are not categorically exempt from drainage review:  Removing and replacing a paved surface to base course or lower, or repairing the pavement base (i.e., “replaced impervious surfaces”).  Extending the edge of pavement without increasing the size of the paved area  Resurfacing that meets the definition of new impervious surface in this manual. Major receiving water means a large receiving water that has been determined by the City of Renton to be safe for the direct discharge of increased runoff from a proposed project without a flow control facility, subject to the restrictions on such discharges set forth in Core Requirement #3, Section 1.2.3. A list of major receiving waters is provided in Section 1.2.3.1. Major receiving waters are also considered safe for application of Basic WQ treatment in place of otherwise required Enhanced Basic WQ treatment (see Section 1.2.8.1), except where the receiving water meets the definition of impaired waterbody or impaired receiving water, specifically with regard to heavy metals. Multifamily project (or land use) means any project or land use that requires or would require a commercial building permit or commercial site development permit for development of residential dwelling units that are not detached single family dwelling units or attached two-unit (duplex) buildings. Native vegetated surface means a surface in which the soil conditions, ground cover, and species of vegetation are like those of the original native condition for the site. More specifically, this means (1) the soil is either undisturbed or has been treated according to the “native vegetated landscape” specifications in Appendix C, Section C.2.1.8; (2) the ground is either naturally covered with vegetation litter or has been top-dressed between plants with 4 inches of mulch consistent with the native vegetated landscape specifications in Appendix C; and (3) the vegetation is either (a) comprised predominantly of plant species, other than noxious weeds, that are indigenous to the coastal region of the Pacific Northwest and that reasonably could have been expected to occur naturally on the site or (b) comprised of plant species specified for a native vegetated landscape in Appendix C. Examples of CHAPTER 1 DRAINAGE REVIEW AND REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-6 these plant species include trees such as Douglas fir, western hemlock, western red cedar, alder, big- leaf maple and vine maple; shrubs such as willow, elderberry, salmonberry and salal; and herbaceous plants such as sword fern, foam flower, and fireweed. Natural discharge area means an onsite area tributary to a single natural discharge location. Natural discharge location means the location where surface and storm water runoff leaves (or would leave if not infiltrated or retained) the site or project site under existing site conditions. New impervious surface means the conversion of a pervious surface to an impervious surface; or the addition of a more compacted surface, such as resurfacing by upgrading from dirt to gravel, asphalt, or concrete; upgrading from gravel to asphalt, or concrete; or upgrading from a bituminous surface treatment (“chip seal”) to asphalt or concrete. New pervious surface means the conversion of a native vegetated surface or other native surface to a nonnative pervious surface (e.g., conversion of forest or meadow to pasture land, grass land, cultivated land, lawn, landscaping, bare soil, etc.), or any alteration of existing nonnative pervious surface that significantly increases surface and storm water runoff (e.g., conversion of pasture land, grass land, or cultivated land to lawn, landscaping, or bare soil; or alteration of soil characteristics). New PGIS means new impervious surface that is pollution-generating impervious surface or any alteration of existing pollution-generating impervious surface that changes the type of pollutants or results in increased pollution loads and/or concentrations. New PGPS means new pervious surface that is pollution-generating pervious surface or any alteration of existing pollution-generating pervious surface that changes the type of pollutants or results in increased pollution loads and/or concentrations. Offsite means any area lying upstream of the site that drains onto the site and any area lying downstream of the site to which the site drains including frontage improvements. Onsite means the entire site that includes the proposed development. On-site BMP means a small scale drainage facility or feature that is part of a development site strategy to use processes such as infiltration, dispersion, storage, evaporation, transpiration, forest retention, and reduced impervious surface footprint to mimic pre-developed hydrology and minimize stormwater runoff. Permeable pavement means pervious concrete, porous asphalt, permeable pavers or other forms of pervious or porous paving material intended to allow passage of water through the pavement section. It often includes an aggregate base that provides structural support and acts as a stormwater reservoir. Pervious Surface – Any surface material that allows stormwater to infiltrate into the ground. Examples include lawn, landscape, pasture, and native vegetation areas. This designation excludes permeable pavement, vegetated roofs, and pervious surfaces with underdrains designed to collect stormwater runoff (see “impervious surface”). Pollution-generating impervious surface (PGIS) means an impervious surface considered to be a significant source of pollutants in stormwater runoff. Such surfaces include those that are subject to vehicular use, industrial activities, or storage of erodible or leachable materials, wastes, or chemicals and that receive direct rainfall or the run-on or blow-in of rainfall. A covered parking area would be considered PGIS if runoff from uphill could regularly run through it or if rainfall could regularly blow in and wet the pavement surface. Metal roofs are also considered PGIS unless they are coated with an inert, non-leachable material (see Reference Section 11-E); or roofs that are exposed to the venting of significant amounts of dusts, mists, or fumes from manufacturing, commercial, or other indoor activities. PGIS includes vegetated roofs exposed to pesticides, fertilizers, or loss of soil. Other roofing types that may pose risk but are not currently regulated are listed Reference Section 11-E. Lawns, landscaping, sports fields, golf courses, and other areas that have modified runoff characteristics resulting from the addition of underdrains that have the pollution generating CHAPTER 1—KEY TERMS AND DEFINITIONS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-7 characteristics described under the “pollution-generating pervious surface” definition are also considered PGIS. Pollution-generating pervious surface (PGPS) means a non-impervious surface considered to be a significant source of pollutants in surface and storm water runoff. Such surfaces include those that are subject to vehicular use, industrial activities, storage of erodible or leachable materials, wastes, or chemicals, and that receive direct rainfall or the run-on or blow-in of rainfall; or subject to use of pesticides and fertilizers, or loss of soil. Such surfaces include, but are not limited to, the lawn and landscaped areas of residential, commercial, and industrial sites or land uses, golf courses, parks, sports fields (natural and artificial turf), cemeteries, and grassed modular grid pavement. Project means any proposed action to alter or develop a site. The proposed action of a permit application or an approval, which requires drainage review. Project site means that portion of a site and any offsite areas subject to proposed project activities, alterations, and improvements including those required by this manual. Offsite areas subject to proposed project activities, include, but are not limited to, frontage improvements required by the City. Rain Garden means a shallow, landscaped depression with compost-amended native soils and adapted plants. The depression is designed to pond and temporarily store stormwater runoff from adjacent areas, and to allow stormwater to pass through the amended soil profile. Rain gardens can only be used to meet Core Requirement #9. Receiving waters means bodies of water, surface water systems, or groundwater receiving water from upstream man-made or natural systems. Redevelopment project means a project that proposes to add, replace, or modify impervious surfaces (e.g., building, parking lot) for purposes other than a residential subdivision or maintenance on a site that is already substantially developed in a manner consistent with its current zoning or with a legal non- conforming use, or has an existing impervious surface coverage of 35% or more. The following examples illustrate how this definition may apply to residential and commercial sites. Redevelopment Project that Adds New Impervious Surface Redevelopment Project that Replaces Impervious Surface Redevelopment Project that Adds and Replaces Impervious Surface Replaced impervious surface means any existing impervious surface on the project site that is proposed to be removed and re-established as impervious surface, excluding impervious surface removed for the sole purpose of installing utilities or performing maintenance on underground infrastructure. For structures, removed means the removal of buildings down to the foundation. For other impervious surfaces, removed means the removal down to base course or bare soil. For purposes of this definition, base course is the layer of crushed rock that typically underlies an asphalt or concrete pavement. It does not include the removal of pavement material through grinding or other surface modification unless the entire layer of PCC or AC is removed. Replaced impervious surface also includes impervious surface that is moved from one location to another on the project site where the Existing House Residential Site New Bldg. Existing Parking New Bldg. Existing Pervious Area (65%) Existing Impervious Area (35%) Commercial Site Existing Bldg. Existing Parking New Parking Existing Bldg. Existing Impervious Area (35%) Commercial Site New Bldg. CHAPTER 1 DRAINAGE REVIEW AND REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-8 following two conditions are met: (A) runoff characteristics and volumes remain the same or are improved in the area where the existing impervious surface is removed , and (B) impervious surface at the new location is either designated as non-pollution generating or the pollution generating characteristics remain unchanged compared to that of the original location. Replaced PGIS means replaced impervious surface that is pollution-generating impervious surface. Sensitive lake means a designation applied by the City to lakes that are particularly prone to eutrophication from development-induced increases in phosphorus loading. Severe building flooding problem means there is flooding of the finished floor area1 of a habitable building,2 or the electrical/heating system of a habitable building for runoff events less than or equal to a 100-year event. Examples include flooding of finished floors of homes and commercial or industrial buildings, or flooding of electrical/heating system components in the crawl space or garage of a home. Severe erosion problem means there is an open drainage feature with evidence of or potential for erosion/incision sufficient to pose a sedimentation hazard to downstream conveyance systems or pose a landslide hazard by undercutting adjacent slopes. Severe erosion problems do not include roadway shoulder rilling or minor ditch erosion. Severe flooding problem means a severe building flooding problem or a severe roadway flooding problem. Severe roadway flooding problem means there is flooding over all lanes of a roadway,3 or a sole access driveway4 is severely impacted, for runoff events less than or equal to the 100-year event. A severely impacted sole access driveway is one in which flooding overtops a culverted section of the driveway, posing a threat of washout or unsafe access conditions due to indiscernible driveway edges, or flooding is deeper than 6 inches on the driveway, posing a severe impediment to emergency access. Single family residential project means any project that (a) constructs or modifies a single family dwelling unit or attached two-unit (duplex) building, (b) makes improvements (e.g., driveways, roads, outbuildings, play courts, etc.) or clears native vegetation on a lot that contains or will contain a single family dwelling unit or attached two-unit (duplex) building, or (c) is a plat, short plat, or boundary line adjustment that creates or adjusts lots that will contain single family dwelling units or attached two- unit (duplex) buildings. Site (a.k.a development site) means a single parcel; or, two or more contiguous parcels that are under common ownership or documented legal control; or a portion of a single parcel under documented legal control separate from the remaining parcel, used as a single parcel for a proposed project for purposes of applying for authority from the City to carry out a proposed project. For projects located primarily within dedicated rights-of-way, the length of the project site and the right-of-way boundaries define the site. Steep slope hazard area is the critical area designation, defined and regulated in RMC 4-3-050, that is applied to areas where extra protection of sensitive slopes is required. Steep slope hazard areas are depicted in the Regulated Slopes layer of COR Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>). Structure means a catch basin or manhole in reference to a storm drainage system. 1 Finished floor area, for the purposes of defining severe building flooding problem, means any enclosed area of a building that is designed to be served by the building’s permanent heating or cooling system. 2 Habitable building means any residential, commercial, or industrial building that is equipped with a permanent heating or cooling system and an electrical system. 3 Roadway, for the purposes of this definition, means the traveled portion of any public or private road or street classified as such in the City of Renton Standard Details and City of Renton Transportation Department guidelines. 4 Sole access driveway means there is no other unobstructed, flood-free route for emergency access to a habitable building. CHAPTER 1—KEY TERMS AND DEFINITIONS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-9 Subject to vehicular use means the surface is regularly used by motor vehicles including but not limited to motorcycles, cars, trucks, busses, aircraft, tractors, and heavy equipment. The following surfaces are considered regularly used by motor vehicles: roads, un-vegetated road shoulders, bike lanes within the traveled lane of a roadway, driveways, parking lots, unrestricted access fire lanes, vehicular equipment storage yards, and airport taxiways and runways. The following surfaces are not considered regularly used by motor vehicles: paved bicycle pathways separated from and not subject to drainage from roads for motor vehicles, fenced or restricted access fire lanes, and maintenance access roads with a recurring use of no more than one routine vehicle access per week. Target impervious surface means that portion of a site’s new and/or replaced impervious surface from which runoff impacts are required to be mitigated by a particular set of drainage requirements (flow control facility, water quality facility, and/or on-site BMP). Type of Development Target Impervious Surface New development New plus replaced impervious surface Redevelopment with < 5,000 sf impervious or improvements < 50% of the assessed value of the existing site improvements New impervious surface Redevelopment with ≥ 5,000 sf impervious and improvements ≥ 50% of the assessed value of the existing site improvements New plus replaced impervious surface Target pervious surface means all areas subject to clearing and grading that have not been covered by an impervious surface, incorporated into a drainage facility, or engineered as structural fill or slope. Threshold discharge area means an onsite area draining to a single natural discharge location, or multiple natural discharge locations that combine within one-quarter-mile downstream (as determined by the shortest flowpath). The examples below illustrate this definition. This term is used to clarify how the thresholds, exemptions, and exceptions of this manual are applied to project sites with multiple discharge locations. CHAPTER 1 DRAINAGE REVIEW AND REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-10 Example of a Project Site with a Single Natural Discharge and a Single Threshold Discharge Area Example of a Project Site with Multiple Natural Discharges and a Single Threshold Discharge Area Example of a Project Site with Multiple Natural Discharges and Multiple Threshold Discharge Areas Transportation redevelopment project means a stand-alone transportation improvement project that proposes to add, replace, or modify impervious surface, for purposes other than maintenance, within a length of dedicated public or private road right-of-way that has an existing impervious surface coverage of thirty-five percent or more. Road right-of-way improvements required as part of a subdivision, commercial, industrial, or multifamily project may not be defined as a separate transportation redevelopment project. Zone 1 of the Aquifer Protection Area means the land area situated between a well or well field owned by the City of Renton and the one-year groundwater travel time contour and not otherwise designated as Zone 1 Modified, as depicted in the Wellhead Protection Area Zones layer of COR Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>). Zone 1 Modified of the Aquifer Protection Area means the land area situated between a well or well field owned by the City of Renton and the one-year groundwater travel time contour and designated as Zone 1 Modified, as depicted in the Wellhead Protection Area Zones layer of COR Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>). Zone 2 of the Aquifer Protection Area means the land area situated between the one-year groundwater travel time contour and the boundary of the zone of potential capture for a well or well field owned or operated by the City, as depicted in the Wellhead Protection Area Zones layer of COR Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>). Natural Discharge Area THRESHOLD DISCHARGE AREA (Shaded) Natural Discharge Location Natural Discharge Locations Natural Discharge Area 1 Natural Discharge Area 2 THRESHOLD DISCHARGE AREA (Shaded) Natural Discharge Area 1 Natural Discharge Area 2 THRESHOLD DISCHARGE AREA 1 (Shaded) THRESHOLD DISCHARGE AREA 2 ¼ Mile Downstream (shortest flow path) Natural Discharge Locations CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 1-11 1.1 DRAINAGE REVIEW Drainage review is the evaluation by City of Renton staff of a proposed project’s compliance with the drainage requirements of this manual. The City of Renton department responsible for drainage review is the Community and Economic Development (CED) Department unless otherwise specified in RMC 4-6-060. Drainage review by CED is an integral part of its permit review process for development projects. This section describes when and what type of drainage review is required for a proposed project and how to determine which drainage requirements apply. The section covers the following topics related to drainage review:  “Projects Requiring Drainage Review,” Section 1.1.1  “Drainage Review Types and Requirements,” Section 1.1.2  “Drainage Review Required By Other Agencies,” Section 1.1.3  “Drainage Design Beyond Minimum Compliance,” Section 1.1.4 Guide to Using Section 1.1 The following steps are recommended for efficient use of Section 1.1: 1. Determine whether your proposed project is subject to the requirements of this manual by seeing if it meets any of the thresholds for drainage review specified in Section 1.1.1. Making this determination requires an understanding of the key terms defined at the beginning of this chapter. 2. If drainage review is required per Section 1.1.1, use the flow chart in Figure 1.1.2.A to determine what type of drainage review will be conducted by the City. The type of drainage review defines the scope of drainage requirements that will apply to your project as summarized in Table 1.1.2.A. 3. Check the more detailed threshold information in Section 1.1.2 to verify that you have determined the correct type of drainage review. 4. After verifying the type of drainage review, use the information in Section 1.1.2 to determine which core requirements (found in Section 1.2) and which special requirements (found in Section 1.3) must be evaluated for compliance by your project. To determine how to comply with each applicable core and special requirement, see the more detailed information on these requirements contained in Sections 1.2 and 1.3 of this chapter. SECTION 1.1 DRAINAGE REVIEW 6/22/2022 2022 City of Renton Surface Water Design Manual 1-12 1.1.1 PROJECTS REQUIRING DRAINAGE REVIEW Drainage review is required for any proposed project (except those proposing only maintenance) that is subject to a City of Renton development permit or approval, including but not limited to those listed at right, AND that meets any one of the following conditions: 1. The project adds or will result in 2,000 square feet5 or more of new impervious surface, replaced impervious surface, or new plus replaced impervious surface, OR 2. The project proposes 7,000 square feet5 or more of land disturbing activity, OR 3. The project proposes to construct or modify a drainage pipe/ditch that is 12 inches or more in size/depth, or receives storm water runoff or surface water from a drainage pipe/ditch that is 12 inches or more in size/depth, OR 4. The project contains or is adjacent to a flood, erosion, or steep slope hazard area as defined in RMC 4-3-050, or projects located within a Landslide Hazard Drainage Area or Aquifer Protection Area, OR 5. Condition #5 does not apply to the City,6 OR 6. The project is a redevelopment project proposing $100,0007 or more of improvements to an existing high- use site. If drainage review is required for the proposed project, the type of drainage review must be determined based on project and site characteristics as described in Section 1.1.2. The type of drainage review defines the scope of drainage requirements that must be evaluated for compliance with this manual. 1.1.2 DRAINAGE REVIEW TYPES AND REQUIREMENTS For most projects resulting in 2,000 square feet or more of new plus replaced impervious surface, the full range of core and special requirements contained in Sections 1.2 and 1.3 must be evaluated for compliance through the drainage review process. However, for some types of projects, the scope of requirements applied is narrowed to allow more efficient, customized review. Each of the following five drainage review types tailors the review process and application of drainage requirements to a project’s size, location, type of development, and anticipated impacts to the local and regional surface water system:  “Simplified Drainage Review,” Section 1.1.2.1  “Targeted Drainage Review,” Section 1.1.2.2  “Directed Drainage Review,” Section 1.1.2.3  “Full Drainage Review,” Section 1.1.2.4  “Large Project Drainage Review,” Section 1.1.2.5. 5 The thresholds for new impervious surface, replaced impervious surface, and land disturbing activity shall be applied by project site and in accordance with the definitions of these surfaces and activities. 6 Footnote 6 is not used. 7 This is the “project valuation” as declared on the permit application submitted to CED. The dollar amount of this threshold may be adjusted on an annual basis using the local consumer price index (CPI). City of Renton Permits and Approvals Building Permits/Combination Building Permits Construction Permits Demolition Permits Flood Control Zone Permits Grading/Filling Permit Land Use Permit Mining, Excavation or Grading Permit or License Planned Urban Development Rezones Right-of-Way Permits Right-of-Way Use Application Site Plan Approvals Shoreline Permits Short Subdivision Developments (Short Plat) Special Permits Subdivision Developments (Plats) Temporary Permits when involving land disturbance Other City of Renton permits as required 1.1.2 DRAINAGE REVIEW TYPES AND REQUIREMENTS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-13 Each project requires only one of the above drainage review types, with the single exception that a project that qualifies for Simplified Drainage Review may also require Targeted Drainage Review. Figure 1.1.2.A can be used to determine which drainage review type is required. However, this may entail consulting the more detailed thresholds for each review type specified in the above-referenced sections. Table 1.1.2.A can be used to quickly identify which requirements are applied in each type of drainage review. The applicant must evaluate the requirements “checked” for a particular drainage review type to determine what is necessary for compliance. SECTION 1.1 DRAINAGE REVIEW 6/22/2022 2022 City of Renton Surface Water Design Manual 1-14 FIGURE 1.1.2.A FLOW CHART FOR DETERMINING TYPE OF DRAINAGE REVIEW REQUIRED 1.1.2 DRAINAGE REVIEW TYPES AND REQUIREMENTS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-15 TABLE 1.1.2.A REQUIREMENTS APPLIED UNDER EACH DRAINAGE REVIEW TYPE Simplified Single family residential projects that result in 2,000 sf of new plus replaced impervious surface or 7,000 sf of land disturbing activity but do not exceed the new plus replaced PGIS, new PGPS, and new pervious surface thresholds specified in Sec. 1.1.2.1. Note: The project may also be subject to Targeted Drainage Review. Directed Single family residential projects that result in 2,000 sf of new plus replaced impervious surface or 7,000 sf of land disturbing activity that are not subject to Simplified Drainage Review or Large Project Drainage Review. The project may also be subject to Targeted Drainage Review. Targeted New and redevelopment projects that are not subject to Directed, Full or Large Project Drainage Review, AND have characteristics of one or more of the following categories of projects: 1. Projects containing or adjacent to a flood, erosion, or steep slope hazard area; or projects within a Landslide Hazard Drainage Area or Aquifer Protection Area . 2. Projects that construct or modify a drainage pipe/ditch that is 12″ or larger or receive runoff from a 12″ or larger drainage pipe/ditch. 3. Redevelopment projects with $100,000 in improvements to a high-use site.(1) Full All projects that result in 2,000 sf of new plus replaced impervious surface or 7,000 sf of land disturbing activity but are not subject to Simplified Drainage Review, Directed Drainage Review, OR Large Project Drainage Review. Large Project Projects that result in 50 acres of new impervious surface within a subbasin or multiple subbasins that are hydraulically connected. DRAINAGE REVIEW TYPE Simplified Directed Targeted Full Large Project Categ 1 Categ 2 Categ 3 SIMPLIFIED DRAINAGE REQUIREMENTS SEE NOTE 4 CORE REQUIREMENT #1 Discharge at Natural Location (4)  (2,3) *(2)    CORE REQUIREMENT #2 Offsite Analysis (4)  (2,3) *(2) (3) (3) (3) CORE REQUIREMENT #3 Flow Control Facilities (4)  (2,3) *(2) (3) (3) CORE REQUIREMENT #4 Conveyance System (4)  (2,3) *(2)    CORE REQUIREMENT #5 Construction Stormwater Pollution Prevention (4)  (2,3)      CORE REQUIREMENT #6 Maintenance & Operations (4)  (2,3) *(2)     CORE REQUIREMENT #7 Financial Guarantees & Liability (4)  (2,3) *(2) (3) (3) (3) (3) CORE REQUIREMENT #8 Water Quality Facilities (4)  (2,3) *(2) (3) (3) CORE REQUIREMENT #9 On-site BMPs (4)    SECTION 1.1 DRAINAGE REVIEW 6/22/2022 2022 City of Renton Surface Water Design Manual 1-16 TABLE 1.1.2.A REQUIREMENTS APPLIED UNDER EACH DRAINAGE REVIEW TYPE DRAINAGE REVIEW TYPE Simplified Directed Targeted Full Large Project Categ 1 Categ 2 Categ 3 SPECIAL REQUIREMENT #1 Other Adopted Area-Specific Requirements (4)  (2,3) (3) (3) (3) SPECIAL REQUIREMENT #2 Flood Hazard Area Delineation (4)  (2,3) (3) (3) (3) SPECIAL REQUIREMENT #3 Flood Protection Facilities (4)  (2,3) (3) (3) (3) SPECIAL REQUIREMENT #4 Source Control (4)  (2,3) (3) (3) (3) (3) (3) SPECIAL REQUIREMENT #5 Oil Control (4)  (2,3) (3) (3) (3) SPECIAL REQUIREMENT #6 Aquifer Protection Areas  (2,3) (3) (3) (3) (3) (3) (1) Category 3 projects installing oil controls that construct or modify a 12-inch pipe/ditch are also Category 2 projects. (2) May be applied by CED based on project or site-specific conditions. Documentation of compliance required. (3) These requirements have exemptions or thresholds that may preclude or limit their application to a specific project. (4) A proposed project subject to Simplified Drainage Review that complies with the Simplified drainage requirements detailed in Appendix C is presumed to comply with all the core and special requirements in Sections 1.2 and 1.3 except those requirements that would apply to the project if it is subject to Targeted Drainage Review as specified in Section 1.1.2.2. 1.1.2.1 SIMPLIFIED DRAINAGE REVIEW Simplified Drainage Review is for small residential building projects or clearing projects that meet the threshold requirements below. The core and special requirements applied under Full Drainage Review are replaced with simplified drainage requirements that can be applied by a non-engineer. These requirements include simple stormwater dispersion, infiltration, and site design techniques called flow control Best Management Practices (BMPs), which provide the necessary mitigation of flow and water quality impacts for small projects. Also included are simple measures for erosion and sediment control (ESC). This simplified form of drainage review acknowledges that drainage impacts for many small project proposals can be effectively mitigated without construction of costly flow control and water quality facilities. The Simplified Drainage Review process minimizes the time and effort required to design, submit, review, and approve drainage facilities for these proposals. In most cases, the requirements can be met with submittals prepared by contractors, architects, or homeowners without the involvement of a civil engineer. Note: some projects subject to Simplified Drainage Review may also require Targeted Drainage Review if they meet any of the threshold criteria in Section 1.1.2.2. Threshold Simplified Drainage Review is required for any single family residential project that will result in 2,000 square feet8 or more of new impervious surface, replaced impervious surface, or new plus replaced 8 The thresholds of 2,000 and 7,000 square feet shall be applied by project site. All other thresholds specified in terms of square feet of impervious or pervious surface shall be applied by threshold discharge area and in accordance with the definitions of these surfaces in Section 1.1. Note: the calculation of total impervious surface may exclude any such added impervious surface that is confirmed by CED staff to be already mitigated by a City approved and inspected flow control facility or on-site BMP. 1.1.2 DRAINAGE REVIEW TYPES AND REQUIREMENTS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-17 impervious surface, or 7,000 square feet8 or more of land disturbing activity, AND that meets the following criteria: The project will result in less than 5,000 square feet of new plus replaced pollution generating impervious surface, will result in less than ¾ acre of new pollution generating pervious surfaces, AND meets one of the following three additional criteria: 1. The project meets the Basic Exemption from flow control in Core Requirement #3 : a) the project results in less than 5,000 square feet of new plus replaced impervious surface, AND b) less than ¾ acres of new pervious surface will be added. Note the Basic Exemption thresholds are applied by project site. 2. For projects on predominately till soils: The project results in no more than 7,947 square feet of target impervious surfaces as defined below AND proposed pervious area is equal to or less than 14,941 – 1.88 x (total target impervious surfaces). 3. For projects on predominately outwash soils: The project results in no more than 6,872 square feet of target impervious surfaces as defined below AND proposed pervious area is equal to or less than 20,343 – 2.96 x (total target impervious surfaces). Determination of Target Impervious Surface  If the project is a New Development project, then target impervious surfaces include new plus proposed replaced impervious surface.  If the project is a Redevelopment project where o New impervious surface is less than 5,000 square feet or o Valuation of improvements is less than 50% of the assessed value of the existing site improvements, then target impervious surfaces include new impervious surface.  If the project is a Redevelopment project where o New impervious surface is greater than or equal to 5,000 square feet and o Valuation of improvements is greater than or equal to 50% of the assessed value of the existing site improvements, then target impervious surfaces include new plus proposed replaced impervious surface. Note: for the purposes applying this threshold to a proposed single family residential subdivision (i.e., plat or short plat project), the impervious surface coverage assumed on each created lot shall be 4,000 square feet or the maximum allowed per RMC 4-2-110A, whichever is less. A lower impervious surface coverage may be assumed for any lot in which the lower impervious surface coverage is set as the maximum through a declaration of covenant recorded for the lot. Also, the new pervious surface assumed on each created lot shall be the entire lot area, except the assumed impervious portion and any portion in which native conditions are preserved by a clearing limit per RMC IV, a covenant or easement recorded for the lot, or a tract dedicated by the proposed subdivision. Scope of Requirements IF Simplified Drainage Review is required, THEN the proposed project must comply with the simplified project submittal and drainage design requirements detailed in Simplified Drainage Requirements adopted as Appendix C to this manual. These requirements include simplified BMPs/measures for flow control and erosion and sediment control. Presumption of Compliance with Core and Special Requirements The simplified drainage requirements applied under Simplified Drainage Review are considered sufficient to meet the overall intent of the core and special requirements in Sections 1.2 and 1.3, except under certain SECTION 1.1 DRAINAGE REVIEW 6/22/2022 2022 City of Renton Surface Water Design Manual 1-18 conditions when a proposed project has characteristics that trigger Targeted Drainage Review (see the threshold for Targeted Drainage Review in Section 1.1.2.2) and may require the involvement of a civil engineer. Therefore, any proposed project that is subject to Simplified Drainage Review as determined above and complies with the Simplified drainage requirements detailed in Appendix C is presumed to comply with all the core and special requirements in Sections 1.2 and 1.3 except those requirements that would apply to the project if it is subject to Targeted Drainage Review as specified in Section 1.1.2.2. 1.1.2.2 TARGETED DRAINAGE REVIEW Targeted Drainage Review (TDR) is an abbreviated evaluation by CED permit review staff of a proposed project’s compliance with selected core and special requirements. Projects subject to this type of drainage review are typically Simplified Drainage Review proposals or other small projects that have site-specific or project-specific drainage concerns that must be addressed by a civil engineer or CED engineering review staff. Under Targeted Drainage Review, engineering costs associated with drainage design and review are kept to a minimum because the review includes only those requirements that would apply to the particular project. Threshold Targeted Drainage Review is required for any proposed project that is subject to drainage review as determined in Section 1.1.1, but is not subject to Directed, Full or Large Project Drainage Review as determined in Sections 1.1.2.3, 1.1.2.4 and 1.1.2.5, AND that has the characteristics of one or more of the following project categories:  TDR Project Category #1: Projects that contain or are adjacent to a flood hazard, erosion hazard area, or steep slope hazard area as defined in RMC 4-3-050; OR projects located within a Landslide Hazard Drainage Area or Aquifer Protection Area. Note: at the discretion of CED, this category may also include any project in Simplified Drainage Review that has a design or site-specific issue that must be addressed by a civil engineer. A project is considered adjacent to a flood, erosion, or steep slope hazard area if any portion of the project site is within 50 feet.  TDR Project Category #2: Projects that propose to construct or modify a drainage pipe/ditch that is 12 inches or more in size/depth or receives surface and storm water runoff from a drainage pipe/ditch that is 12 inches or more in size/depth.  TDR Project Category #3: Redevelopment projects that propose $100,000 or more of improvements to an existing high-use site. Scope of Requirements IF Targeted Drainage Review is required, THEN the applicant must demonstrate that the proposed project complies with the selected core and special requirements corresponding to the project category or categories that best match the proposed project. The project categories and applicable requirements for each are described below and summarized in Table 1.1.2.A. Note: If the proposed project has the characteristics of more than one project category, the requirements of each applicable category shall apply. Compliance with these requirements requires the submittal of engineering plans and calculations stamped by a civil engineer, unless deemed unnecessary by CED and the City of Renton. The engineer need only demonstrate compliance with those core and special requirements that have been predetermined to be applicable based on specific project characteristics as detailed below. The procedures and requirements for submitting engineering plans and calculations can be found in Section 2.3. TDR Project Category #1 This category includes projects that are too small to trigger application of most core requirements, but may be subject to site-specific floodplain or drainage requirements related to certain critical areas, or other area-specific drainage requirements adopted by the City. Such projects primarily include single family residential projects in Simplified Drainage Review. 1.1.2 DRAINAGE REVIEW TYPES AND REQUIREMENTS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-19 IF the proposed project meets the characteristics of TDR Project Category #1, THEN the applicant must demonstrate that the project complies with the following requirements:  “Core Requirement #5: Construction Stormwater Pollution Prevention,” Section 1.2.5  “Special Requirement #1: Other Adopted Area-Specific Requirements,” Section 1.3.1  “Special Requirement #2: Floodplain/Floodway Analysis,” Section 1.3.2  “Special Requirement #3: Flood Protection Facilities,” Section 1.3.3  “Special Requirement #4: Source Control,” Section 1.3.4  “Special Requirement #6: Aquifer Protection Area,” Section 1.3.6. In addition, CED may require the applicant to demonstrate compliance with any one or more of the remaining seven core requirements in Section 1.2 based on project or site-specific conditions. For example, if the proposed project discharges to an erosion or steep slope hazard area as defined in RMC 4-3-050, CED may require compliance with “Core Requirement #1: Discharge at the Natural Location” (Section 1.2.1). This may in turn require compliance with “Core Requirement #2: Offsite Analysis” (Section 1.2.2) if a tightline is required by Core Requirement #1. If a tightline is found to be infeasible, CED may instead require a flow control facility per “Core Requirement #3: Flow Control” (Section 1.2.3). If a tightline is feasible, “Core Requirement #4: Conveyance System” (Section 1.2.4) would be required to ensure proper size and design. Any required flow control facility or tightline system may also trigger compliance with “Core Requirement #6: Maintenance and Operations” (Section 1.2.6), “Core Requirement #7: Financial Guarantees and Liability” (Section 1.2.7), and possibly “Core Requirement #8, Water Quality” (Section 1.2.8) if runoff from pollution-generating impervious surfaces is collected. The applicant may also need to address compliance with any applicable critical areas requirements in RMC 4-3-050 as determined by CED. TDR Project Category #2 This category is intended to apply selected core and special requirements to those projects that propose to construct or modify a drainage system of specified size, but are not adding sufficient impervious surface to trigger Full Drainage Review or Large Project Drainage Review. IF the proposed project meets the characteristics of TDR Project Category #2, THEN the applicant must demonstrate that the proposed project complies with the following requirements:  “Core Requirement #1: Discharge at the Natural Location,” Section 1.2.1  “Core Requirement #2: Offsite Analysis,” Section 1.2.2  “Core Requirement #4: Conveyance System,” Section 1.2.4  “Core Requirement #5: Construction Stormwater Pollution Prevention,” Section 1.2.5  “Core Requirement #6: Maintenance and Operations,” Section 1.2.6  “Core Requirement #7: Financial Guarantees and Liability,” Section 1.2.7  “Special Requirement #4: Source Control,” Section 1.3.4. TDR Project Category #3 This category is intended to improve water quality by applying source control and oil control requirements to redevelopment projects located on the most intensively used sites developed prior to current water quality requirements. These are referred to as high-use sites. IF the proposed project meets the characteristics of TDR Project Category #3, THEN the applicant must demonstrate that the proposed project complies with the following requirements:  “Core Requirement #5: Construction Stormwater Pollution Prevention,” Section 1.2.5  “Core Requirement #6: Maintenance and Operations,” Section 1.2.6  “Core Requirement #7: Financial Guarantees and Liability,” Section 1.2.7  “Special Requirement #4: Source Control,” Section 1.3.4 SECTION 1.1 DRAINAGE REVIEW 6/22/2022 2022 City of Renton Surface Water Design Manual 1-20  “Special Requirement #5: Oil Control,” Section 1.3.5. Note: In some cases, CED may determine that application of these requirements does not require submittal of engineering plans and calculations stamped by a civil engineer. 1.1.2.3 DIRECTED DRAINAGE REVIEW Directed Drainage Review (DDR) is an evaluation of a proposed single family residential project by CED permit review staff to determine a specialized list of submittal (plans, technical reports, etc.) and engineering requirements that ensures compliance with all core and special requirements in this chapter. Projects subject to this type of drainage review are single family residential projects that do not qualify for Simplified Drainage Review. CED staff will review proposals and determine the following: whether the project is exempt from a given core or special requirement based on exemptions and exceptions listed in this Manual; whether a pre- engineered solution is available and feasible for meeting a given core or special requirement; whether a licensed civil engineer is required to comply with a given core or special requirement; and the type of technical report and plan submittal required to document compliance with the core and special requirements. Depending upon a project’s site specific conditions, DDR may result in requirements for engineering or documentation that range from following the requirements of Appendix C to those required for full drainage review. CED will provide and/or require documentation of the DDR process and decision making to be included in the project file that demonstrates how compliance with all core and special requirements in this Manual are achieved. Under Directed Drainage Review, engineering costs associated with drainage design and review are minimized because the review is tailored to the particular project. Threshold Directed Drainage Review is required for any single family residential project that results in 2,000 square feet or more of new plus replaced impervious surface or 7,000 square feet or more of land disturbing activity (refer to Section 1.1.1) but is not subject to Simplified Drainage Review or Large Project Drainage Review as determined in Sections 1.1.2.1 and Section 1.1.2.5. Scope of Requirements IF Directed Review is required, THEN the proposed project must comply with the following requirements:  All nine core requirements in Section 1.2  All six special requirements in Section 1.3 CED may require submission of engineering plans and calculations stamped by a civil engineer to demonstrate compliance with these requirements. The procedures and requirements for submittal of engineering plans and calculations are as directed by CED in the DDR process. 1.1.2.4 FULL DRAINAGE REVIEW Full Drainage Review is the evaluation by City staff (CED unless otherwise specified in RMC 4-6-060) of a proposed project’s compliance with the full range of core and special requirements in this chapter. This review addresses the impacts associated with changing land cover on typical sites. Threshold Full Drainage Review is required for any proposed project, including a redevelopment project, that is subject to drainage review as determined in Section 1.1.1, OR that meets one or more of the following criteria: 1.1.2 DRAINAGE REVIEW TYPES AND REQUIREMENTS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-21  The project will result in 2,000 square feet9 or more of new impervious surface, replaced impervious surface, or new plus replaced impervious surface but is not subject to Simplified Drainage Review or Directed Drainage Review as determined in Sections 1.1.2.1 and 1.1.2.3, OR  The project will result in 7,000 square feet9 or more of land disturbing activity but is not subject to Simplified Drainage Review or Directed Drainage Review as determined in Sections 1.1.2.1 and 1.1.2.3. Scope of Requirements IF Full Drainage Review is required, THEN the applicant must demonstrate that the proposed project complies with the following requirements:  All nine core requirements in Section 1.2  All six special requirements in Section 1.3 Engineering plans and calculations stamped by a civil engineer must be submitted to demonstrate compliance with these requirements. The procedures and requirements for submittal of engineering plans and calculations are found in Section 2.3. 1.1.2.5 LARGE PROJECT DRAINAGE REVIEW Large Project Drainage Review is applied to development proposals that are large and/or involve resources or problems of special sensitivity or complexity. Because of the large size and complexities involved, there is usually a greater risk of significant impact or irreparable damage to sensitive resources. Such proposals often require a more definitive approach to drainage requirements than that prescribed by the core and special requirements in Sections 1.2 and 1.3; it may be appropriate to collect additional information about site resources, use more sophisticated models, and prepare special studies not specified in this manual. Large Project Drainage Review entails preparation of a master drainage plan (MDP) or limited scope MDP that is reviewed and approved by CED. Threshold Large Project Drainage Review is required for any proposed project that is subject to drainage review as determined in Section 1.1.1, AND that would, at full buildout, result in 50 acres or more of new impervious surface within a single subbasin or multiple subbasins that are hydraulically connected10 across subbasin boundaries. Hydraulically connected means connected through surface flow or water features such as wetlands or lakes. Scope of Requirements IF Large Project Drainage Review is required, THEN the applicant must do the following: 1. Prepare a MDP, limited scope MDP, or special study in accordance with the process and requirements described in the MDP guidelines, Master Drainage Planning for Large or Complex Site Developments, available from King County Department of Natural Resources and Parks (DNRP) or CED. The MDP or special study shall be completed, or a schedule for completion identified and agreed to by CED, prior to permit approval. Note: Generally, it is most efficient for the MDP process to parallel the State Environmental Policy Act (SEPA) process. 2. Demonstrate that the proposed project complies with all the core and special requirements in Sections 1.2 and 1.3, with some potential modifications as follows:  Core Requirement #2, Offsite Analysis, is typically modified during MDP scoping.  Core Requirement #3, Flow Control, may be modified to require more sophisticated hydrologic modeling.  Core Requirement #5, ESC, may be modified to require enhanced construction monitoring. 9 The thresholds of 2,000, 5,000, and 7,000 square feet shall be applied by project site. 10 Hydraulically connected means connected through surface flow or water features such as wetlands or lakes. SECTION 1.1 DRAINAGE REVIEW 6/22/2022 2022 City of Renton Surface Water Design Manual 1-22  Core Requirement #7, Financial Guarantees and Liability, may be modified to implement a monitoring fund.  Special pre- and post-development monitoring may also be required if deemed necessary by CED to adequately characterize sensitive site and downstream resources, and to ensure that onsite drainage controls and mitigation measures are effective in protecting sensitive or critical resources. Detailed guidelines for monitoring are appended to the MDP guidelines referenced above. 1.1.3 DRAINAGE REVIEW REQUIRED BY OTHER AGENCIES Drainage review for a proposed project’s impact on surface and storm waters may be addressed by processes or requirements apart from the City’s. Agencies such as those listed below may require some form of drainage review and impose drainage requirements that are separate from and in addition to the City’s drainage requirements. The applicant is responsible for coordinating with these agencies and resolving any conflicts in drainage requirements. Agency Permit/Approval Seattle/King County Department of Public Health Onsite Sewage Disposal and Well permits Washington State Department of Transportation Developer/Local Agency Agreement Department of Fish and Wildlife Hydraulic Project Approval Department of Ecology Short Term Water Quality Modification Approval Dam Safety permit UIC Well Registration NPDES Stormwater permit Department of Natural Resources Forest Practices Class IV permit United States Army Corps of Engineers Sections 10, 401, and 404 permits 1.1.4 DRAINAGE DESIGN BEYOND MINIMUM COMPLIANCE This manual presents the City of Renton’s minimum standards for engineering and design of drainage facilities. While the City believes these standards are appropriate for a wide range of development proposals, compliance solely with these requirements does not relieve the professional engineer submitting designs of his or her responsibility to ensure drainage facilities are engineered to provide adequate protection for natural resources and public and private property. Compliance with the standards in this manual does not necessarily mitigate all probable and significant environmental impacts to aquatic biota. Fishery resources and other living components of aquatic systems are affected by a complex set of factors. While employing a specific flow control standard may prevent stream channel erosion or instability, other factors affecting fish and other biotic resources (e.g., increases in stream flow velocities) are not directly addressed by this manual. Likewise, some wetlands, including bogs, are adapted to a very constant hydrologic regime. Even the most stringent flow control standard employed by this manual does not prevent increases in runoff volume, which can adversely affect wetland plant communities by increasing the duration and magnitude of water level fluctuations. Thus, compliance with this manual should not be construed as mitigating all probable and significant stormwater impacts to aquatic biota in streams and wetlands; additional mitigation may be required. CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 1-23 1.2 CORE REQUIREMENTS This section details the following nine core requirements:  “Core Requirement #1: Discharge at the Natural Location,” Section 1.2.1  “Core Requirement #2: Offsite Analysis,” Section 1.2.2  “Core Requirement #3: Flow Control,” Section 1.2.3  “Core Requirement #4: Conveyance System,” Section 1.2.4  “Core Requirement #5: Construction Stormwater Pollution Prevention,” Section 1.2.5  “Core Requirement #6: Maintenance and Operations,” Section 1.2.6  “Core Requirement #7: Financial Guarantees and Liability,” Section 1.2.7  “Core Requirement #8: Water Quality,” Section 1.2.8  “Core Requirement #9: On-site BMPs,” Section 1.2.9 1.2.1 CORE REQUIREMENT #1: DISCHARGE AT THE NATURAL LOCATION All storm water runoff and surface water from a project must be discharged at the natural location so as not to be diverted onto or away from downstream properties. The manner in which stormwater runoff and surface water are discharged from the project site must not create a significant adverse impact to downhill properties or drainage facilities (see “Discharge Requirements” below). Drainage facilities as described above means a constructed or engineered feature that collects, conveys, stores, treats, or otherwise manages surface water or stormwater runoff. “Drainage facility” includes, but is not limited to, a constructed or engineered stream, lake, wetland, or closed depression, or a pipe, channel, ditch, gutter, flow control facility, on-site BMP, water quality facility, erosion and sediment control facility, and any other structure and appurtenance that provides for drainage. Note: Projects that do not discharge all project site runoff at the natural location will require an approved adjustment of this requirement (see Section 1.4). CED may waive this adjustment, however, for projects in which only a small portion of the project site does not discharge runoff at the natural location and the runoff from that portion is unconcentrated and poses no significant adverse impact to downstream properties. Intent: To prevent adverse impacts to downstream properties caused by diversion of flow from one flowpath to another, and to discharge in a manner that does not significantly impact downhill properties or drainage systems. Diversions can cause greater impacts (from greater runoff volumes) than would otherwise occur from new development discharging runoff at the natural location. Diversions can also impact properties that rely on runoff water to replenish wells and ornamental or fish ponds.  DISCHARGE REQUIREMENTS Proposed projects must comply with the following discharge requirements (1, 2, and 3) as applicable: 1. Where no conveyance system exists at the abutting downstream property line and the natural (existing) discharge is unconcentrated, any runoff concentrated by the proposed project must be discharged as follows: a) IF the 100-year peak discharge11 is less than or equal to 0.2 cfs under existing conditions and will remain less than or equal to 0.2 cfs under developed conditions, THEN the concentrated runoff may be discharged onto a rock pad or to any other system that serves to disperse flows. 11 Peak discharges for applying this requirement are determined using the approved runoff model with 15-minute time steps as detailed in Chapter 3. SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-24 b) IF the 100-year peak discharge is less than or equal to 0.5 cfs under existing conditions and will remain less than or equal to 0.5 cfs under developed conditions, THEN the concentrated runoff may be discharged through a dispersal trench or other dispersal system provided the applicant can demonstrate that there will be no significant adverse impact to downhill properties or drainage systems. c) IF the 100-year peak discharge is greater than 0.5 cfs for either existing or developed conditions, or if a significant adverse impact to downhill properties or drainage systems is likely, THEN a conveyance system must be provided to convey the concentrated runoff across the downstream properties to an acceptable discharge point.12 Drainage easements for this conveyance system must be secured from downstream property owners and recorded prior to engineering plan approval. 2. IF a proposed project, or any natural discharge area within a project, is located within a Landslide Hazard Drainage Area and drains over the erodible soils of a landslide hazard with slopes steeper than 15%, THEN a tightline system must be provided through the landslide hazard to an acceptable discharge point unless one of the following exceptions applies. The tightline system must comply with the design requirements in Core Requirement #4 and in Section 4.2.2 unless otherwise approved by CED. Drainage easements for this system must be secured from downstream property owners and recorded prior to engineering plan approval. Exceptions: A tightline is not required for any natural discharge location where CED approves an alternative system based on a geotechnical evaluation/recommendation from a licensed geotechnical engineer that considers cumulative impacts on the hazard area under built out conditions AND one of the following conditions can be met: a) Less than 2,000 square feet of new impervious surface will be added within the natural discharge area, OR b) The developed conditions runoff from the natural discharge area is less than 0.1 cfs for the 100-year runoff event and will be infiltrated for runoff events up to and including the 100-year event, OR c) The developed conditions runoff volume13 from the natural discharge area is less than 50% of the existing conditions runoff volume from other areas draining to the location where runoff from the natural discharge area enters the landslide hazard onto slopes steeper than 15%, AND the provisions of Discharge Requirement 1 are met, OR d) CED determines that a tightline system is not physically feasible or will create a significant adverse impact based on a soils report by a geotechnical engineer. 3. For projects adjacent to or containing a landslide hazard, steep slope hazard area, or erosion hazard area as defined in RMC 4-3-050, the applicant must demonstrate that onsite drainage facilities and/or on-site BMPs will not create a significant adverse impact to downhill properties or drainage systems. 1.2.2 CORE REQUIREMENT #2: OFFSITE ANALYSIS All proposed projects must submit an offsite analysis report that assesses potential offsite drainage and water quality impacts associated with development of the project site, and that proposes appropriate 12 Acceptable discharge point means an enclosed drainage system (i.e., pipe system, culvert, or tightline) or open drainage feature (e.g., ditch, channel, swale, stream, river, pond, lake, or wetland) where concentrated runoff can be discharged without creating a significant adverse impact. 13 For the purposes of applying this exception, the developed conditions runoff volume is the average annual runoff volume as computed per Chapter 3. The analysis is performed using the entire period of record. The total volume is divided by the number of full water years being analyzed to determine the annual average runoff volume. Any areas assumed not to be cleared when computing the developed conditions runoff volume must be set aside in an open space tract or covenant in order for the proposed project to qualify for this exception. Preservation of existing forested areas in Landslide Hazard Drainage Areas is encouraged. 1.2.2 CORE REQUIREMENT #2: OFFSITE ANALYSIS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-25 mitigation of those impacts. The initial permit submittal shall include, at minimum, a Level 1 downstream analysis as described in Section 1.2.2.1 below. If impacts are identified, the proposed projects shall meet any applicable problem-specific requirements specified in Section 1.2.2.2 for mitigation of impacts to drainage problems and Section 1.2.2.3 for mitigation of impacts to water quality problems. Intent: To identify and evaluate offsite flooding, erosion, and water quality problems that may be created or aggravated by the proposed project, and to ensure appropriate measures are provided for preventing creation or aggravation of those problems. In addition, this requirement is intended to ensure appropriate provisions are made, as needed, to mitigate other identified impacts associated with the quantity and quality of surface and storm water runoff from the project site (e.g., impacts to the hydrology of a wetland as may be identified by a “critical area report” per RMC 4-3-050). The primary component of an offsite analysis report is the downstream analysis, which examines the drainage system within one-quarter mile downstream of the project site or farther as described in Section 1.2.2.1 below. It is intended to identify existing or potential/predictable downstream flooding, erosion, and water quality problems so that appropriate mitigation, as specified in Sections 1.2.2.2 and 1.2.2.3, can be provided to prevent aggravation of these problems. A secondary component of the offsite analysis report is an evaluation of the upstream drainage system to verify and document that significant flooding and erosion impacts will not occur as a result of the proposed project. The evaluation must extend upstream to a point where any backwater effects created by the project cease.  EXEMPTION FROM CORE REQUIREMENT #2 With the exception of:  Projects that trigger Core Requirement #3 (Flow Control Facilities) which must at minimum perform offsite analysis sufficient to identify and address “Downstream Drainage Problems Requiring Special Attention (Section 1.2.2.1.1), Problem Type 4 (Potential Impacts to Wetland Hydrology problem),” and  Projects that trigger Core Requirement # 8 (Water Quality Facilities) which must at minimum perform offsite analysis sufficient to identify and address “Downstream Water Quality Problems Requiring Special Attention (Section 1.2.2.1.2),” a proposed project is exempt from Core Requirement #2 if any one of the following is true: 1. The City of Renton determines there is sufficient information for them to conclude that the project will not have a significant adverse impact on the downstream and/or upstream drainage system, OR 2. The project adds less than 2,000 square feet of new impervious surface, AND less than ¾ acre of new pervious surface, AND does not construct or modify a drainage pipe/ditch that is 12 inches or more in size/depth or that receives runoff from a drainage pipe/ditch that is 12 inches or more in size/depth, AND does not contain or lie adjacent to a landslide hazard, steep slope hazard area, or erosion hazard area as defined in RMC 4-3-050, OR 3. The project does not change the rate, volume, duration, or location of discharges to and from the project site (e.g., where existing impervious surface is replaced with other impervious surface having similar runoff-generating characteristics, or where pipe/ditch modifications do not change existing discharge characteristics). 1.2.2.1 DOWNSTREAM ANALYSIS The level of downstream analysis required depends on specific site and downstream conditions. Each project submittal must include at least a Level 1 downstream analysis. Upon review of the Level 1 analysis, CED may require a Level 2 or Level 3 analysis. If conditions warrant, additional, more detailed analysis may be required. SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-26 The Level 1 downstream analysis is a qualitative survey of each downstream system and is the first step in identifying flooding problems, erosion problems, or potential impacts to wetland hydrology problems as described below under “Downstream Drainage Problems Requiring Special Attention.” The Level 1 analysis also identifies water quality problems as described below under “Downstream Water Quality Problems Requiring Special Attention.” Each Level 1 analysis is composed of four tasks at a minimum:  Task 1: Define and map the study area  Task 2: Review all available information on the study area  Task 3: Field inspect the study area  Task 4: Describe the drainage system, and its existing and predicted drainage and water quality problems. Upon review of the Level 1 analysis, CED may require a Level 2 or 3 downstream analysis, depending on the presence of existing or predicted flooding, erosion, or nuisance problems identified in the Level 1 analysis. Levels 2 and 3 downstream analysis quantify downstream flooding, erosion, or nuisance problems by providing information on the severity and frequency of an existing problem or the likelihood of creating a new problem. A Level 2 analysis is a rough quantitative analysis (non-survey field data, uniform flow analysis). Level 3 is a more precise analysis (e.g., survey field data, backwater analysis) of significant problems. If conditions warrant, additional, more detailed analysis may be required beyond Level 3. For Levels 2 and 3 downstream analyses, an additional Task 5, addressing mitigation of existing and potential flooding, erosion, or nuisance problems, will be required. Extent of Downstream Analysis The downstream analysis must consider the existing conveyance system(s) for a minimum flowpath distance downstream of one-quarter mile and beyond that, as needed, to reach a point where the project site area constitutes less than 15% of the tributary area. This minimum distance may be increased as follows:  Task 2 of a Level 1 downstream analysis (described in detail in Section 2.3.1.1) is a review of all available information on the downstream area and is intended to identify existing drainage and water quality problems. In all cases, this information review shall extend one mile downstream of the project site. The existence of flooding or erosion problems further downstream may extend the one-quarter- mile/15% minimum distance for other tasks to allow evaluation of impacts from the proposed development upon the identified flooding or erosion problems. The existence of documented water quality problems beyond the one-quarter-mile/15% distance may in some cases require additional mitigation of impacts as determined necessary by CED based on the type and severity of problem.  If a project’s impacts to flooding or erosion problems are mitigated by improvements to the downstream conveyance system, the downstream analysis will extend a minimum of one-quarter mile beyond the improvement. This is necessary because many such improvements result in a reduction of stormwater storage or an increase in peak flows from the problem location.  At their discretion, CED may extend the downstream analysis beyond the minimum distance specified above on the reasonable expectation of drainage or water quality impacts. A detailed description of the scope of offsite analysis and submittal requirements is provided in Section 2.3.1.1. Hydrologic analysis methods and requirements for Levels 2 and 3 downstream analyses are contained in Chapter 3; hydraulic analysis methods are contained in Chapter 4. 1.2.2.1.1 DOWNSTREAM DRAINAGE PROBLEMS REQUIRING SPECIAL ATTENTION While the area-specific flow control facility requirement in Core Requirement #3 (Section 1.2.3.1) serves to minimize the creation and aggravation of many types of downstream drainage problems, there are some types that are more sensitive to creation/aggravation than others depending on the nature or severity of the problem and which flow control facility standard is being applied. In particular, there are four types of downstream drainage problems for which the City has determined that the nature and/or severity of the 1.2.2 CORE REQUIREMENT #2: OFFSITE ANALYSIS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-27 problem warrants additional attention through the downstream analysis and possibly additional mitigation to ensure no creation/aggravation: 1. Conveyance system nuisance problem. 2. Severe erosion problem. 3. Severe flooding problem. 4. Potential Impacts to Wetland Hydrology problem. These four types of downstream drainage problem are further described below and precisely defined at the beginning of Chapter 1. Conveyance System Nuisance Problem (Type 1) Conveyance system nuisance problems are minor but chronic flooding or erosion problems that result from the overflow of a constructed conveyance system that is substandard or has become too small as a result of upstream development. Such problems warrant additional attention because of their chronic nature and because they result from the failure of a conveyance system to provide a minimum acceptable level of protection. If a conveyance system nuisance problem is identified or predicted downstream, the need for additional mitigation must be evaluated as specified in Section 1.2.2.2 under “Drainage Problem-Specific Mitigation Requirements“. This may entail additional onsite flow control or other measures as needed to prevent creation or significant aggravation of the problem. For any other nuisance problem that may be identified downstream, this manual does not require mitigation beyond the area-specific flow control facility requirement applied in Core Requirement #3 (Section 1.2.3.1) because preventing aggravation of such problems (e.g., those caused by the elevated water surfaces of ponds, lakes, wetlands, and closed depressions or those involving downstream erosion) can require two to three times as much onsite detention volume, which is considered unwarranted for nuisance problems. However, if under some unusual circumstance, the aggravation of such a nuisance problem is determined by CED to be a significant adverse impact, additional mitigation may be required. Severe Erosion Problem (Type 2) Severe erosion problems can be caused by conveyance system overflows or the concentration of runoff into erosion-sensitive open drainage features. Severe erosion problems warrant additional attention because they pose a significant threat either to health and safety or to public or private property. If a severe erosion problem is identified or predicted downstream, additional mitigation must be considered as specified in Section 1.2.2.2 under “Drainage Problem-Specific Mitigation Requirements“. This may entail additional onsite flow control or other measures as needed to prevent creation or aggravation of the problem. Severe Flooding Problem (Type 3) Severe flooding problems (i.e., a severe building flooding problem or severe roadway flooding problem) can be caused by conveyance system overflows or the elevated water surfaces of ponds, lakes, wetlands, or closed depressions. Severe flooding problems warrant additional attention because they pose a significant threat either to health and safety or to public or private property. If a severe flooding problem is identified or predicted downstream, the need for additional mitigation must be evaluated as specified in Section 1.2.2.2 under “Drainage Problem-Specific Mitigation Requirements“. This may entail consideration of additional onsite flow control or other measures as needed to prevent creation or significant aggravation of the problem. Potential Impacts to Wetlands Hydrology Problem (Type 4) Potential impacts to wetlands hydrology can be caused by changes in the rate, duration, and quantity of stormwater discharged from the project site to a wetland. SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-28 Where wetlands are identified on the site, the applicant shall submit a critical area report at a level determined by CED to adequately evaluate the proposal and probable impacts. Where wetlands are identified off the site AND the project is not exempt from Core Requirement #3, the applicant shall submit a critical area report at a level determined by CED to adequately evaluate the proposal and probable impacts. Projects or threshold discharge areas within projects discharging to wetlands, unless exempt from providing a flow control facility per Core Requirement 3, must demonstrate that the existing wetland hydroperiod is maintained in accordance with the wetland hydrology protection guidelines in Reference Section 5. Based upon the critical area report and, if applicable, the analysis of project compliance with the wetland hydrology protection guidelines in Reference Section 5, CED will determine if changes in the rate, duration, and/or quantity of surface and storm water runoff from a proposed project or threshold discharge area within a proposed project could significantly alter the hydrology of a wetland-- in which case, CED will require (as described in Section 1.2.2.2 under “Drainage Problem-Specific Mitigation Requirements”), implementation of additional flow control or other measures to mitigate the adverse impacts of this alteration in accordance with the wetland hydrology protection guidelines in Reference Section 5. 1.2.2.1.2 DOWNSTREAM WATER QUALITY PROBLEMS REQUIRING SPECIAL ATTENTION A water quality problem, for the purposes of impact mitigation in this manual, is a situation in which a waterbody of the State is documented by the Federal Government, State, or City to be exceeding or at concern of exceeding the State’s numeric water quality standards, or is subject to a federal, state, or City cleanup program or action. Water quality problems and associated water quality standards encompass surface water, groundwater, and sediment quality. The goal of this manual is to prevent creation or significant aggravation of such problems to the maximum extent practicable. While the area-specific water quality facility requirement in Section 1.2.8.1, the source controls required in Section 1.3.4, and the oil controls required in Section 1.3.5 all serve to minimize the creation and aggravation of many types of downstream water quality problems, there are some types that are either not addressed by these requirements (e.g., temperature problems) or warrant additional measures/considerations to minimize the proposed project’s impacts to the maximum extent practicable. In particular, there are currently 7 types of downstream water quality problems for which the City has determined that additional attention needs to be given to preventing or minimizing increases in the pollutant or pollutants of concern discharging from the site. These are as follows: 1. Bacteria Problem 2. Dissolved Oxygen Problem 3. Temperature Problem 4. Metals Problem 5. Phosphorus Problem 6. Turbidity Problem 7. High pH Problem These problems are defined below and the mitigation of impacts to them is addressed in Section 1.2.2.3. Bacteria Problem (Type 1) A bacteria problem is defined as a stream reach, lake, or other waterbody of the state that is either (1) currently designated by the state as a Category 5, 4, or 2 Waterbody due to exceedance or concern for exceedance of the state’s numeric water quality standard for fecal coliform as documented in the state’s Water Quality Assessment 303(d)/305(b) Integrated Report and as displayed in WA Ecology’s 1.2.2 CORE REQUIREMENT #2: OFFSITE ANALYSIS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-29 electronic database and map viewer14 of these waterbodies, or (2) where subject to any other local, state, or federal cleanup plan or contaminated site designation for fecal coliform. Dissolved Oxygen (DO) Problem (Type 2) A dissolved oxygen problem is defined as a stream reach, lake, or other waterbody of the state that is either (1) currently designated by the state as a Category 5, 4, or 2 Waterbody due to exceedance or concern for exceedance of the state’s numeric water quality standard for dissolved oxygen as documented in the state’s Water Quality Assessment 303(d)/305(b) Integrated Report and as displayed in WA Ecology’s electronic database and map viewer14 of these waterbodies, or (2) where subject to any other local, state, or federal cleanup plan or contaminated site designation for DO. Temperature Problem (Type 3) A temperature problem is defined as a stream reach, lake, or other waterbody of the state that is either (1) currently designated by the state as a Category 5, 4, or 2 Waterbody due to exceedance or concern for exceedance of the state’s numeric water quality standard for temperature as documented in the state’s Water Quality Assessment 303(d)/305(b) Integrated Report and as displayed in WA Ecology’s electronic database and map viewer14 of these waterbodies, or (2) where subject to any other local, state, or federal cleanup plan or contaminated site designation for temperature. Metals Problem (Type 4) A metals problem is defined as a stream reach, lake, or other waterbody of the state that is either (1) currently designated by the state as a Category 5, 4, or 2 Waterbody due to exceedance or concern for exceedance of the state’s numeric water quality standards for metals (e.g., copper, zinc, lead, mercury, etc.) as documented in the state’s Water Quality Assessment 303(d)/305(b) Integrated Report and as displayed in WA Ecology’s electronic database and map viewer16 of these waterbodies, or (2) where subject to any other local, state, or federal cleanup plan or contaminated site designation for metals. Phosphorus Problem (Type 5) A phosphorus problem is defined as a stream reach, lake, or other waterbody of the state that is either (1) currently designated by the state as a Category 5, 4, or 2 Waterbody due to exceedance or concern for exceedance of the state’s numeric action standard for total phosphorus as documented in the state’s Water Quality Assessment 303(d)/305(b) Integrated Report and as displayed in WA Ecology’s electronic database and map viewer,14 of these waterbodies, or (2) where subject to any other local, state, or federal cleanup plan or contaminated site designation for total phosphorus. Turbidity Problem (Type 6) A turbidity problem is defined as a stream reach, lake, or other waterbody of the state that is either (1) currently designated by the state as a Category 5, 4, or 2 Waterbody due to exceedance or concern for exceedance of the state’s numeric water quality standard for turbidity as documented in the state’s Water Quality Assessment 303(d)/305(b) Integrated Report and as displayed in WA Ecology’s electronic database and map viewer14 of these waterbodies, or (2) where subject to any other local, state, or federal cleanup plan or contaminated site designation for turbidity. High pH Problem (Type 7) A High pH problem is defined as a stream reach, lake, or other waterbody of the state that is either (1) currently designated by the state as a Category 5, 4, or 2 Waterbody due to exceedance or concern for exceedance of the state’s numeric water quality standard for high pH as documented in the state’s Water Quality Assessment 303(d)/305(b) Integrated Report and as displayed in WA Ecology’s electronic database and map viewer16 of these waterbodies, or (2) where subject to any other local, state, or federal cleanup plan or contaminated site designation for pH. 14 The link to the Query Tool is <https://apps.ecology.wa.gov/ApprovedWQA/ApprovedPages/ApprovedSearch.aspx>; select all appropriate mediums. The Map Tool is at <https://apps.ecology.wa.gov/waterqualityatlas/wqa/map>. SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-30 1.2.2.2 DRAINAGE PROBLEM IMPACT MITIGATION A proposed project must not significantly aggravate existing downstream drainage problems or create new problems as a result of developing the site. This manual does not require development proposals to fix or otherwise reduce the severity of existing downstream drainage problems, although doing so may be an acceptable mitigation. Principles of Impact Mitigation for Drainage Problems Aggravation of an existing downstream drainage problem means increasing the frequency of occurrence and/or severity of the problem. Increasing peak flows at the location of a problem caused by conveyance system overflows can increase the frequency of the problem’s occurrence. Increasing durations of flows at or above the overflow return frequency can increase the severity of the problem by increasing the depth and duration of flooding. Controlling peaks and durations through onsite detention can prevent aggravation of such problems by releasing the increased volumes from development at return frequencies below the conveyance overflow return frequency, which limits their effect to just causing the conveyance system to flow full for a longer period of time. When a problem is caused by high water-surface elevations of a volume-sensitive water body, such as a lake, wetland, or closed depression, aggravation is the same as for problems caused by conveyance overflows. Increasing the volume of flows to a volume-sensitive water body can increase the frequency of the problem’s occurrence. Increasing the duration of flows for a range of return frequencies both above and below the problem return frequency can increase the severity of the problem; mitigating these impacts requires control of flow durations for a range of return frequencies both above and below the problem return frequency. The net effect of this duration control is to release the increased volumes from development only at water surface elevations below that causing the problem, which in turn can cause an increase in these lower, but more frequently occurring, water surface elevations. This underscores an unavoidable impact of development upstream of volume-sensitive water bodies: the increased volumes generated by the development will cause some range of increase in water surface elevations, no matter what detention standard is applied. Creating a new drainage problem means increasing peak flows and/or volumes so that after development, the frequency of conveyance overflows or water surface elevations exceeds the thresholds for the various problem types discussed in Section 1.2.2.1. For example, application of the Peak Rate Flow Control Standard requires matching the existing site conditions 2- and 10-year peak flows. The 100-year peak flow is only partially attenuated, and the flow increase may be enough to cause a severe flooding problem as described in Section 1.2.2.1.1. The potential for causing a new problem is often identified during the Level 1 downstream analysis, where the observation of a reduction in downstream pipe sizes, for example, may be enough to predict creation of a new problem. A Level 2 or 3 analysis will typically be required to verify the capacity of the system and determine whether 100-year flows can be safely conveyed. Significance of Impacts to Existing Drainage Problems The determination of whether additional onsite mitigation or other measures are needed to address an existing downstream drainage problem depends on the significance of the proposed project’s predicted impact on that problem. For some identified problems, CED will make the determination as to whether the project’s impact is significant enough to require additional mitigation. For Type 1, 2, and 3 downstream drainage problems described in Section 1.2.2.1.1, this threshold of significant impact or aggravation is defined below. For a Type 4, “Potential Impacts to Wetland Hydrology problem,” CED will make this determination based on required critical area report findings, whether the project is in compliance with the wetland hydrology protection guidelines found in Reference Section 5, the project’s relative contribution to the identified wetland’s hydrology, and the mitigation proposed in meeting other requirements (e.g., flow control facilities and on-site BMPs). For conveyance system nuisance problems, the problem is considered significantly aggravated if there is any increase in the project’s contribution to the frequency of occurrence and/or severity of the problem for runoff events less than or equal to the 10-year event. Note: Increases in the project’s contribution to 1.2.2 CORE REQUIREMENT #2: OFFSITE ANALYSIS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-31 this type of problem are considered to be prevented if sufficient onsite flow control and/or offsite improvements are provided as specified in Table 1.2.3.A. For severe erosion problems, the problem is considered significantly aggravated if there is any increase in the project’s existing contribution to the flow duration15 of peak flows ranging from 50% of the 2-year peak flow up to the full 50-year peak flow at the eroded area. Note: Increases in the project’s contribution to this type of problem are considered to be prevented if Flow Control Duration Standard or offsite improvements are provided as specified in Table 1.2.3.A. For severe building flooding problems, the problem is considered significantly aggravated if there is any increase in the project’s existing contribution16 to the frequency, depth, or duration of the problem for runoff events less than or equal to the 100-year event. For severe roadway flooding problems, the problem is considered significantly aggravated if any of the following thresholds are exceeded and there is any increase in the project’s existing contribution19 to the frequency, depth, or duration of the problem for runoff events less than or equal to the 100-year event:  The existing flooding17 over all lanes of a roadway or overtopping the culverted section of a sole access driveway is predicted to increase in depth more than a quarter-inch or 10% (whichever is greater) for the 100-year runoff event.  The existing flooding over all lanes of a roadway or severely impacting a sole access driveway is more than 6 inches deep or faster than 5 feet per second for runoff events less than or equal to the 100-year event. A severely impacted sole access driveway is one in which flooding overtops a culverted section of the driveway, posing a threat of washout or unsafe access conditions due to indiscernible driveway edges, or flooding is deeper than 6 inches on the driveway, posing a severe impediment to emergency access.  The existing flooding over all lanes of a sole access roadway18 is more than 3 inches deep or faster than 5 feet per second for runoff events less than or equal to the 100-year event, or is at any depth for runoff events less than or equal to the 10-year event.  DRAINAGE PROBLEM-SPECIFIC MITIGATION REQUIREMENTS 1. IF a proposed project or threshold discharge area within a project drains to one or more of Type 1, Type 2, or Type 3 downstream drainage problems described in Section 1.2.2.1 as identified through a downstream analysis, THEN the applicant must do one of the following: a) Submit a Level 2 or Level 3 downstream analysis per Section 2.3.1 demonstrating that the proposed project will not create or significantly aggravate the identified downstream drainage problem(s), OR b) Show that the natural discharge area or threshold discharge area draining to the identified problem(s) qualifies for an exemption from Core Requirement #3: Flow Control (Section 1.1.1) or an exception from the applicable area-specific flow control facility requirement per Section 1.2.3.1, OR 15 Flow duration means the aggregate time that peak flows are at or above a particular flow rate (e.g., the amount of time over the last 50 years that peak flows were at or above the 2-year flow rate). Note: flow duration is not considered to be increased if it is within the tolerances specified in Chapter 3. 16 Increases in the project’s contribution are considered to be prevented if sufficient onsite flow control and/or offsite improvements are provided as specified for severe flooding problems in Table 1.2.3.A. For severe flooding problems located within the mapped 100-year floodplain of a major receiving water (see Table 1.2.3.B) or the mapped 100-year floodplain of a major stream for which there is an adopted basin plan, increases in the project’s contribution are considered negligible (zero) regardless of the flow control standard being applied, unless CED determines there is a potential for increased flooding separate from that associated with the existing 100-year floodplain. 17 Existing flooding, for the purposes of this definition, means flooding over all lanes of the roadway or driveway has occurred in the past and can be verified by County records, County personnel, photographs, or other physical evidence. 18 Sole access roadway means there is no other flood-free route for emergency access to one or more dwelling units. SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-32 c) Document that the applicable area-specific flow control facility requirement specified in Core Requirement #3 is adequate to prevent creation or significant aggravation of the identified downstream drainage problem(s) as indicated in Table 1.2.3.A with the phrase, “No additional flow control needed,” OR d) Provide additional onsite flow control necessary to prevent creation or significant aggravation of the downstream drainage problem(s) as specified in Table 1.2.3.A and further detailed in Section 3.3.5, OR e) Provide offsite improvements necessary to prevent creation or significant aggravation of the identified downstream drainage problem(s) as detailed in Chapter 3 unless identified as not necessary in Table 1.2.3.A, OR f) Provide a combination of additional onsite flow control and offsite improvements sufficient to prevent creation or significant aggravation of the downstream drainage problem(s) as demonstrated by a Level 2 or Level 3 downstream analysis. 2. IF it is identified that the manner of discharge from a proposed project may create a significant adverse impact as described in Core Requirement #1, THEN CED may require the applicant to implement additional measures or demonstrate that the impact will not occur. 3. IF it is identified through a critical area review as described under “Potential Impacts to Wetlands Hydrology Problem (Type 4),” that changes in the rate, duration, and/or quantity of surface and storm water runoff from a proposed project or threshold discharge area within a proposed project could significantly alter the hydrology of a wetland (Type 4 problem), THEN CED shall require the applicant to implement additional flow control or other measures to mitigate the adverse impacts of this alteration in accordance with the wetland hydrology protection guidelines in Reference Section 5. Intent: To ensure provisions are made (if necessary) to prevent creation or significant aggravation of the four types of downstream drainage problems requiring special attention by this manual, and to ensure compliance with the discharge requirements of Core Requirement #1. In addressing downstream drainage problems per Problem-Specific Mitigation Requirement 1 above, additional onsite flow control will often be the easiest provision to implement. This involves designing the required onsite flow control facility to meet an additional set of performance criteria targeted to prevent significant aggravation of specific downstream drainage problems. To save time and analysis, a set of predetermined flow control performance criteria corresponding to each of the three types of downstream drainage problems is provided in Table 1.2.3.A and described in more detail in Chapter 3. Note that in some cases the area-specific flow control facility requirement applicable to the proposed project per Section 1.2.3.1 is already sufficient to prevent significant aggravation of many of the defined downstream drainage problem types. Such situations are noted in Table 1.2.3.A as not needing additional onsite flow control or offsite improvements. For example, if the project is located within a Flow Control Duration Standard Area subject to the Flow Control Duration Standard per Section 1.2.3.1.B, and a conveyance system nuisance problem is identified through offsite analysis per Core Requirement #2, no additional onsite flow control is needed, and no offsite improvements are necessary. 1.2.2.3 WATER QUALITY PROBLEM IMPACT MITIGATION As stated in Section 1.2.2.1, the goal of this manual is to prevent creation and/or significant aggravation of water quality problems to the maximum extent practicable. This is accomplished through a number of mitigation requirements, including (1) the area-specific water quality facility requirement in Section 1.2.8.1, (2) any mitigation required by other adopted area-specific requirements per Special Requirement #1, Section 1.2.9, (3) the source controls required in Special Requirement #4, Section 1.3.4, (4) the oil control required in Special Requirement #5, Section 1.3.5, and (5) the water quality problem- specific mitigation requirements presented in this section. Note that this manual does not require development proposals to fix or otherwise reduce the severity of existing downstream water quality problems, although doing so may be an acceptable mitigation. 1.2.2 CORE REQUIREMENT #2: OFFSITE ANALYSIS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-33  WATER QUALITY PROBLEM-SPECIFIC MITIGATION REQUIREMENTS IF a proposed project drains to one or more of the 7 types of downstream water quality problems defined in Section 1.2.2.1 as identified through a downstream analysis, THEN the applicant must comply with the following problem-specific mitigation requirements that apply. Note that CED may require additional measures if the opportunity exists to further mitigate the pollutants of concern associated with these types of problems. Bacteria Problem (Type 1) IF the proposed project drains to a bacteria problem located within the quarter mile/15% distance downstream (or beyond as deemed necessary by CED), THEN the following requirements must be met as applicable: 1. IF a water quality facility is required per Core Requirement #8, THEN a sand filter or stormwater wetland shall be used to meet the area-specific water quality facility requirement. Sand filters are the preferred option. Other treatment options for meeting the area-specific facility requirement may be used in lieu of a sand filter or stormwater wetland only if combined with an emerging technology treatment method that provides equivalent removal of fecal coliform as demonstrated through an experimental design adjustment per Section 1.4. 2. IF the proposed project is a residential subdivision, THEN signage shall be provided in the subdivision’s public areas (i.e., recreation/open space areas and right-of-way) requesting that pet waste be picked up in order to protect downstream water quality. The extent and location of this signage shall be reviewed and approved by CED. 3. IF the proposed project is a multifamily development with a recreation/open area or is a park improvement, THEN signage shall be provided requesting that pet waste be picked up in order to protect downstream water quality. The extent and location of this signage shall be reviewed and approved by CED. Dissolved Oxygen (DO) Problem (Type 2) IF the proposed project drains to a DO problem located within the quarter mile/15% distance downstream (or beyond as deemed necessary by CED), THEN the following requirements must be met as applicable: 1. IF the proposed project includes a wetpond or wetvault, THEN the wetpool depth shall not exceed 6 feet, AND the outflow system shall include a measure designed to promote aeration of the facility’s discharges for 2-year runoff events and smaller. One way to do this is to create a drop in flow elevation within a manhole by placing the outlet invert of the incoming pipe a minimum of 12 inches above the 2-year headwater elevation of the outgoing pipe. Alternatively, if the outflow system discharges to an open channel, the same drop in flow elevation could be achieved by placing the outlet invert a minimum of 12 inches above the 2-year tailwater elevation created by the channel. Other equivalent approaches may be used as approved by CED. 2. IF the proposed project includes a wetvault, THEN the required ventilation area specified in Chapter 6 shall be doubled. 3. IF the DO problem is documented to be caused by excessive phosphorus and a water quality facility is required per Core Requirement #8, THEN a water quality facility option from the Sensitive Lake Protection menu shall be a component of the required treatment system. Temperature Problem (Type 3) IF the proposed project drains to a temperature problem located within the quarter mile/15% distance downstream (or beyond as deemed necessary by CED), THEN the following requirements must be met as applicable: 1. IF a water quality facility is required per Core Requirement #8, THEN use of a wetpond is prohibited unless it will be at least 50% shaded at midday in the summer or its discharges will flow SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-34 through 200 feet or more of open channel that is at least 50% shaded at midday in the summer. CED shall review and approve the extent and location of this shading. 2. IF the proposed project includes open drainage features, THEN vegetation or other means shall be used where practicable to maximize shading of the drainage features, except bioswales and filter strips. The extent and location of this shading shall be reviewed and approved by CED. Metals Problem (Type 4) IF the proposed project drains to a metals problem located within the quarter mile/15% distance downstream (or beyond as deemed necessary by CED), THEN the following requirements must be met as applicable: 1. IF a water quality facility is required per Core Requirement #8, THEN a water quality facility option from the Enhanced Basic WQ menu shall be a component of the project’s required treatment system. 2. IF the proposed project is a residential subdivision, THEN a covenant shall be recorded for each lot and common area tract prohibiting use of leachable heavy metals (e.g., galvanized metals) that will be exposed to the weather (use the covenant in Reference Section 8-Q). 3. IF the proposed project includes road right-of-way improvements, THEN use of leachable heavy metals (e.g., galvanized metals) that will be exposed to the weather (e.g., guard rails, street lights, etc.) shall be avoided. Phosphorus Problem (Type 5) IF the proposed project drains to a phosphorus problem located within the quarter mile/15% distance downstream (or beyond as deemed necessary by CED), THEN the following requirements must be met as applicable: 1. IF a water quality facility is required per Core Requirement #8, THEN the project shall be assumed to be located within a designated Sensitive Lake WQ Treatment Area for the purposes of applying the area-specific water quality treatment requirement in Section 1.2.8.1. 2. For the purposes of applying the Erosion and Sediment Control Standards in Appendix D, the project shall be assumed to be located within a designated Sensitive Lake WQ Treatment Area. Turbidity Problem (Type 6) IF the proposed project drains to a turbidity problem located within the quarter mile/15% distance downstream (or beyond as deemed necessary by CED) AND the downstream flow path from the project site to the turbidity problem is through a landslide hazard, steep slope hazard area, erosion hazard area or any actively eroding area, THEN the project shall provide a tightline system through the area in accordance with the same criteria and exceptions specified in Core Requirement #1, Discharge Requirement 2 for projects located within a designated Landslide Hazard Drainage Area. Other means for safely conveying project site discharges through the area of concern for erosion may be proposed subject to approval by CED. High pH Problem (Type 7) IF the proposed project drains to a pH problem located within the quarter mile/15% distance downstream (or beyond as deemed necessary by CED) AND the proposed project includes a concrete vault structure for stormwater control purposes, THEN the vault’s submerged surfaces shall be coated or otherwise treated to prevent alteration of pH. 1.2.3 CORE REQUIREMENT #3: FLOW CONTROL FACILITIES 2022 City of Renton Surface Water Design Manual 6/22/2022 1-35 1.2.3 CORE REQUIREMENT #3: FLOW CONTROL FACILITIES All proposed projects, including redevelopment projects, must provide onsite flow control facilities to mitigate the impacts of storm and surface water runoff generated by new impervious surface, new pervious surface, and replaced impervious surface targeted for flow mitigation as specified in the following sections. Flow control facilities must be provided and designed to perform as specified by the area-specific flow control facility requirement in Section 1.2.3.1 and in accordance with the applicable flow control facility implementation requirements in Section 1.2.3.2. Intent: To ensure the minimum level of control needed to protect downstream properties and resources from increases in peak, duration, and volume of runoff generated by new development. The level of control varies depending on location and downstream conditions identified under Core Requirement #2.  EXEMPTION FROM CORE REQUIREMENT #3 There is a single exemption from the flow control provisions of Core Requirement #3: Basic Exemption A proposed project is exempt if it meets the following criteria: 1. Less than 5,000 square feet of new plus replaced impervious surface will be created, AND 2. Less than ¾ acres of new pervious surface will be added. 1.2.3.1 AREA-SPECIFIC FLOW CONTROL FACILITY REQUIREMENT Projects subject to Core Requirement #3 must provide flow control facilities as specified by the area- specific facility requirements and exceptions for the designated flow control area in which the proposed project or threshold discharge area of the proposed project is located as described in Subsections A, B, and C below. Guide to Applying the Area-Specific Flow Control Facility Requirement The flow control facility requirement varies across the City according to the flow control area within which the project or a threshold discharge area of the project is located. There are currently four such flow control areas, three of which are depicted in the Flow Control Application layer in COR Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>). These are referred to as follows. 1. Flow Control Duration Standard – Matching Forested site conditions for areas draining to streams and subject to flow-related water quality problems such as erosion or sedimentation. 2. Flow Control Duration Standard – Matching Existing site conditions in designated highly urbanized areas draining to streams that are currently stable or showing no impacts caused by high flows. 3. Peak Rate Flow Control Standard – Matching Existing site conditions 2, 10 and 100-year peak- rate runoff for areas draining to constructed (man-made) or highly modified drainage systems so as not to create a downstream flooding problem. 4. Flood Problem Flow Control Standard – The City may apply this standard where projects discharge to a severe flooding or erosion problems. The standard includes matching existing site conditions for 100-year peaks in addition to fulfilling requirements for the flow control duration standard, either matching forested or existing site conditions based on the downstream flow control area designation. SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-36 Guide to Applying the Area-Specific Flow Control Facility Requirement (cont.) Note that the minimum required performance of the facility as specified by this requirement may need to be increased to ensure that downstream drainage problems are not created or significantly aggravated as set forth in Section 1.2.2.2, “Drainage Problem-Specific Mitigation Requirements.” Table 1.2.3.A provides a quick guide for selecting the flow control performance criteria necessary to meet both the area-specific flow control facility requirement and the problem-specific mitigation requirement. This is further explained in Step 4 below. For efficient application of the flow control facility requirement, the following steps are recommended: 1. Check the Direct Discharge Exemption in Section 1.2.3.1 to determine if and/or which portions of your project are exempt from the flow control facility requirement. If exempt from the flow control facility requirement, proceed to Step 6. 2. Refer to the Flow Control Application layer in COR Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>) to determine the flow control area in which your project is located. 3. Consult the detailed requirement and exception language for the identified flow control area to determine if and how the flow control facility requirement applies to your project. This requirement and exception language is detailed on subsequent pages for each of the flow control areas. If a flow control facility is not applicable per the area-specific exceptions, proceed to Step 6. 4. If downstream drainage problems were identified through offsite analysis per Core Requirement #2 and are proposed to be addressed through onsite flow control, use Table 1.2.3.A to determine if and what additional flow control performance is necessary to mitigate impacts (i.e., to prevent creation or aggravation of the identified problems). 5. Use Section 1.2.3.2 to identify the applicable requirements for implementing the flow control facility requirement. These requirements cover facility siting, analysis and design, unusual situations, and other site-specific considerations. 6. Use Core Requirement #9 to identify the on-site BMPs that must be applied to your project site regardless of whether a flow control facility is required. 1.2.3 CORE REQUIREMENT #3: FLOW CONTROL FACILITIES 2022 City of Renton Surface Water Design Manual 6/22/2022 1-37 TABLE 1.2.3.A SUMMARY OF FLOW CONTROL PERFORMANCE CRITERIA ACCEPTABLE FOR IMPACT MITIGATION(1) IDENTIFIED PROBLEM DOWNSTREAM AREA-SPECIFIC FLOW CONTROL FACILITY REQUIREMENT Peak Rate Flow Control Standard Areas Flow Control Duration Standard Matching Existing Condition Areas Flow Control Duration Standard Matching Forested Condition Areas Flood Problem Flow Control Standard Areas No Problem Identified Apply the minimum area-specific flow control performance criteria. Apply the Peak Rate Flow Control Standard, which matches the 2-, 10-, and 100-year peaks Apply the Flow Control Duration Standard, which matches the flow duration of pre- developed rates for existing site conditions over the range of flows extending from 50% of 2-year up to the full 50-year flow AND matches peaks for the 2- and 10-year return periods. Apply the Flow Control Duration Standard which matches the flow duration of pre- developed rates for forested (historical) site conditions over the range of flows extending from 50% of 2-year up to the full 50- year flow AND matches peaks for the 2- and 10-year return periods Apply the existing or forested (historical) site conditions Flow Control Duration Standard (whichever is appropriate based on downstream flow control areas) AND match existing site conditions 100-year peaks Type 1 Drainage Problem Conveyance System Nuisance Problem Additional Flow Control Hold 10-year peak to overflow Tr peak(2)(3) Additional Flow Control The City may require design adjustments to meet the Flow Control Duration Standard matching forested (historical) conditions. No additional flow control or other mitigation is needed No additional flow control or other mitigation is needed Type 2 Drainage Problem Severe Erosion Problem Additional Flow Control Apply the Flow Control Duration Standard matching forested (historical) conditions (3)(4) Additional Flow Control Apply the Flow Control Duration Standard matching forested (historical) conditions.(3)(4) No additional flow control is needed, but other mitigation may be required(4) No additional flow control is needed, but other mitigation may be required(4) Type 3 Drainage Problem Severe Flooding Problem Additional Flow Control Apply the Flow Control Duration Standard matching forested (historical) conditions. If flooding is from a closed depression, make design adjustments as needed to meet the “special provision for closed depressions”(3)(5) Additional Flow Control Apply the Flow Control Duration Standard matching forested (historical) conditions. If flooding is from a closed depression, make design adjustments as needed to meet the “special provision for closed depressions”(3)(5) Additional Flow Control If flooding is from a closed depression, make design adjustments as needed to meet the “special provision for closed depressions”(3)(5) Additional Flow Control If flooding is from a closed depression, make design adjustments as needed to meet the “special provision for closed depressions” (3)(5) SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-38 TABLE 1.2.3.A SUMMARY OF FLOW CONTROL PERFORMANCE CRITERIA ACCEPTABLE FOR IMPACT MITIGATION(1) IDENTIFIED PROBLEM DOWNSTREAM AREA-SPECIFIC FLOW CONTROL FACILITY REQUIREMENT Peak Rate Flow Control Standard Areas Flow Control Duration Standard Matching Existing Condition Areas Flow Control Duration Standard Matching Forested Condition Areas Flood Problem Flow Control Standard Areas Type 4 Potential Impact to Wetland Hydrology as Determined through a Critical Area Review per RMC Title IV Additional Flow Control The City may require design adjustments per the wetland hydrology protection guidelines in Reference Section 5. Additional Flow Control The City may require design adjustments per the wetland hydrology protection guidelines in Reference Section 5. Additional Flow Control The City may require design adjustments per the wetland hydrology protection guidelines in Reference Section 5. Additional Flow Control The City may require design adjustments per the wetland hydrology protection guide-lines in Reference Section 5. Notes: (1) More than one set of problem-specific performance criteria may apply if two or more downstream drainage problems are identified through offsite analysis per Core Requirement #2. If this happens, the performance goals of each applicable problem-specific criterion must be met. This can require extensive, time-consuming analysis to implement multiple sets of outflow performance criteria if additional onsite flow control is the only viable option for mitigating impacts to these problems. In these cases, it may be easier and more prudent to implement the Flow Control Duration Standard matching forested conditions standard in place of the otherwise required area- specific standard. Use of the Flow Control Duration Standard matching forested conditions standard satisfies the specified performance criteria for all the area-specific and problem-specific requirements except if adjustments are required per the special provision for closed depressions described below in Note 5. (2) Overflow Tr is the return period of conveyance system overflow. To determine Tr requires a minimum Level 2 downstream analysis as detailed in Section 2.3.1.1. To avoid this analysis, a Tr of 2 years may be assumed. (3) Offsite improvements may be implemented in lieu of or in combination with additional flow control as allowed in Section 1.2.2.2 and detailed in Section 3.3.5. (4) A tightline system may be required regardless of the flow control standard being applied if needed to meet the discharge requirements of Core Requirement #1 or the outfall requirements of Core Requirement #4, or if deemed necessary by the City of Renton where the risk of severe damage is high. (5) Special Provision for Closed Depressions with a Severe Flooding Problem: IF the proposed project discharges by overland flow or conveyance system to a closed depression experiencing a severe flooding problem AND the amount of new impervious surface area proposed by the project is greater than or equal to 10% of the 100-year water surface area of the closed depression, THEN use the “point of compliance analysis technique” described in Section 3.3.6 to verify that water surface levels are not increasing for the return frequencies at which flooding occurs, up to and including the 100-year frequency. If necessary, iteratively adjust onsite flow control performance to prevent increases. Note: The point of compliance analysis relies on certain field measurements taken directly at the closed depression (e.g., soils tests, topography, etc.). If permission to enter private property for such measurements is denied, the City of Renton may waive this provision and apply the Flow Control Duration Standard matching forested conditions standard with a mandatory 20% safety factor on the storage volume. 1.2.3 CORE REQUIREMENT #3: FLOW CONTROL FACILITIES 2022 City of Renton Surface Water Design Manual 6/22/2022 1-39  DIRECT DISCHARGE EXEMPTION Any onsite natural drainage area is exempt from the flow control facility requirement if the area drains to one of the major receiving waters listed in Table 1.2.3.B, AND meets the following criteria for direct discharge20 to that receiving water: 1. The flowpath from the project site discharge point to the edge of the 100-year floodplain of the major receiving water will be no longer than a half mile, except for discharges to Lake Washington, AND 2. The conveyance system between the project site and the major receiving water will extend to the ordinary high water mark, and will be comprised of manmade conveyance elements (pipes, ditches, etc.) and will be within public right-of-way or a public or private drainage easement, AND 3. The conveyance system will have adequate capacity21 to convey the 25-year peak flow (per Core Requirement #4, Conveyance System), for the entire contributing drainage area, assuming build-out conditions to current zoning for the equivalent area portion (the area that is contained within an arc formed by the shortest, straight line distance from the conveyance system discharge point to the furthermost point of the proposed project) and existing conditions for the remaining area, AND 4. The conveyance system will be adequately stabilized to prevent erosion, assuming the same basin conditions as assumed in Criteria (c) above, AND 5. The direct discharge proposal will not divert flows from or increase flows to an existing wetland or stream sufficient to cause a significant adverse impact. A. PEAK RATE FLOW CONTROL STANDARD AREAS The Peak Rate Flow Control Standard is a peak-rate matching standard intended to prevent increases of peak flows for specific events rather than match flow-durations over a range of flows. The standard is appropriate for use in areas where the concern is flooding rather than stream bed erosion. Within the City of Renton, this standard is allowed for those areas that are highly urbanized prior to 1985 and that drain to pipes or non-fish bearing constructed conveyance systems leading to the lower Cedar River, Lake Washington or the portion of the Green River Valley floor located in Renton. Minimum Required Performance Facilities in Peak Rate Flow Control Standard Areas must comply with the following flow control performance standards and assumptions unless modified by offsite analysis per Core Requirement #2 (see Table 1.2.3.A): Peak Rate Flow Control Standard: Match the developed peak discharge rates to existing site conditions peak discharge rates for 2-, 10-, and 100-year return periods. Intent The Peak Rate Flow Control Standard is intended to protect flow-carrying capacity and limit increased erosion within the downstream conveyance system for runoff events less than or equal to the 100-year event. Matching the 2-, 10-, and 100-year peak flows is intended to prevent increases in return-frequency peak flows less than or equal to the 100-year peak flow down to the 2-year peak 19 Footnote 22 is not used. 20 Direct discharge means undetained discharge from a proposed project to a major receiving water. 21 Note: The City does not charge a special use fee. TABLE 1.2.3.B MAJOR RECEIVING WATERS19  Cedar River downstream of Taylor Creek confluence  Johns Creek downstream of Interstate-405 (I-405) east right-of- way  Lake Washington Note: The major receiving waters listed above do not include side adjacent or associated channels, spring- or groundwater-fed streams, or wetlands. SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-40 flow. This level of control is also intended to prevent creation of new conveyance system nuisance problems as described in Section 1.2.2.1. Effectiveness in Addressing Downstream Drainage Problems While the Peak Rate Flow Control Standard provides reasonable protection from many development-induced conveyance problems (up to the 100-year event), it does not prevent increases in runoff volumes or flow durations that tend to aggravate the three types of downstream drainage problems described in Section 1.2.2.1. Consequently, if one or more of these problems are identified through offsite analysis per Core Requirement #2, additional onsite flow control and/or offsite improvements will likely be required (see “Drainage Problem-Specific Mitigation Requirements” in Section 1.2.2.2). Target Surfaces Facilities in Peak Rate Flow Control Standard Areas must mitigate (either directly or in effect) the runoff from the following target surfaces within the threshold discharge area for which the facility is required: 1. New impervious surface that is not fully dispersed per the criteria in Section 1.2.3.2.C as specified in Appendix C. For individual lots within residential subdivision projects, the extent of new impervious surface shall be assumed as specified in Chapter 3. Note, any new impervious surface such as a bridge or boardwalk that spans the ordinary high water of a stream, pond, or lake may be excluded as a target surface if the runoff from such span is conveyed to the ordinary high water area in accordance with Criteria (b), (c), (d), and (e) of the “Direct Discharge Exemption“ (p 1-39). 2. New pervious surface that is not fully dispersed as specified in Appendix C. For individual lots within residential subdivision projects, the extent of new pervious surface shall be assumed to be the entire lot area, except the assumed impervious portion and any portion in which native conditions are preserved by covenant, tract, or easement. In addition, the new pervious surface on individual lots shall be assumed to be 100% grass. Exceptions The following exceptions apply only in Peak Rate Flow Control Standard Areas: 1. The facility requirement in Peak Rate Flow Control Standard Areas is waived for any threshold discharge area in which the target surfaces subject to this requirement will generate no more than a 0.15-cfs increase (when modeled using 15 minute time steps) in the existing site conditions 100 -year peak flow (modeled using same time step unit (e.g., 15 -minute) used to calculate the developed flow). Note: for the purposes of this calculation, target surfaces served by on-site BMPs per Appendix C may be modeled in accordance with the on-site BMP sizing credits in Core Requirement #9, Table 1.2.9.A. 2. The facility requirement in Peak Rate Flow Control Standard Areas may be waived for any threshold discharge area of a redevelopment project in which all of the following criteria are met: a) The target surfaces subject to the Peak Rate Flow Control Standard Areas facility requirement will generate no more than a 0.15-cfs increase (when modeled using 15 -minute time steps) in the existing site conditions 100 -year peak flow (modeled using same time step unit (e.g., 15 -minute) used to calculate the developed flow) at any natural discharge location from the project site (note: for the purposes of this calculation, target surfaces served by on-site BMPs per Appendix C may be modeled in accordance with the on-site BMP sizing credits in Core Requirement #9, Table 1.2.9.A, AND b) The increased runoff from target surfaces will not significantly impact a critical area, severe flooding problem, or severe erosion problem. 1.2.3 CORE REQUIREMENT #3: FLOW CONTROL FACILITIES 2022 City of Renton Surface Water Design Manual 6/22/2022 1-41 B. FLOW CONTROL DURATION STANDARD AREAS The flow control duration standard requires runoff from urban developments to be detained and released at a rate that matches the flow duration of predeveloped rates over the range of flows extending from ½ of the 2-year up to the 50-year flow. Also match developed peak discharge rates to predeveloped peak discharge rates for the 2- and 10-year return periods. Flow duration specifies the cumulative amount of time that various flows are equaled or exceeded during a long-term simulation using historical rainfall. The target flow duration may be the “historical” (i.e., fully forested condition) or in specific situations it may be the existing site or “pre-project” condition as described below. The Flow Control Application layer in COR Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>) shows the areas where the “forested” and “existing” conditions are allowed. Forested land cover – Runoff from the developed site will be controlled and released at a rate that matches the flow duration for a forested (“historical”) land cover. The “historical” land cover is the default standard required by the technical requirements of the NPDES permit. The standard is applicable to those areas draining to streams that have erodible channels where runoff from urban areas has the potential to destabilize the channel. Existing land cover – Runoff from the developed site will be controlled and released at a rate that matches the flow duration for the site conditions existing before the development. These are areas that have been developed for years and drain to stream channels that have become stabilized to a new hydrologic regime. Ecology has proposed that the existing land cover can be used in basins that have had at least 40% total impervious surface area for the 20 years preceding Ecology’s adoption of the 2005 Stormwater Management Manual for Western Washington (called the 40/20 rule) and the stream channels receiving the runoff are considered stable from the standpoint of excessive erosion or sedimentation. In developing the “40/20 rule” for highly urbanized basins, Ecology conducted a preliminary analysis and produced maps that identify those areas that may meet the criteria. Portions of Renton were included in the initial maps prepared by Ecology. These maps have been adjusted to better represent the areas that were 40% impervious in 1985 as well as drainage basin divides within the City. Flow control facilities designed to the “40/20 rule” will only have to mitigate for the added impervious surface. As a result, these flow control facilities will be smaller than those required to be designed to match runoff from a fully forested site. Minimum Required Performance Facilities in Flow Control Duration Standard Areas must comply with the following flow control performance standard and assumptions unless modified by offsite analysis per Core Requirement #2 (see Table 1.2.3.A): Flow Control Duration Standard Matching Forested Site Conditions: Developed discharge durations shall not exceed predeveloped durations for the range of predeveloped discharge rates from 50% of the 2-year peak flow up to the full 50-year peak flow. Developed peak discharge rates shall not exceed predeveloped peak discharge rates for the 2- and 10-year return periods. Assume forested (historical) site conditions as the predeveloped condition. Flow Control Duration Standard Matching Existing Site Conditions: Developed discharge durations shall not exceed predeveloped durations for the range of predeveloped discharge rates from 50% of the 2-year peak flow up to the full 50-year peak flow. Developed peak discharge rates shall not exceed predeveloped peak discharge rates for the 2- and 10-year return periods. Assume existing site conditions as the predeveloped condition. Intent The Flow Control Duration Standard flow control standard assuming historical site conditions is intended to limit the amount of time that erosive flows are at work generating erosion and sedimentation within natural and constructed drainage systems. Such control is effective in preventing development-induced increases in natural erosion rates and reducing existing erosion rates where they may have been increased by past development of the site. This is accomplished by maintaining at SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-42 historical predevelopment levels the aggregate time that developed flows exceed an erosion-causing threshold (i.e., 50% of the historical 2-year peak flow). Maintaining natural erosion rates within streams and their tributary areas is important for preventing increases in stream channel erosion and sediment loading that are detrimental to salmonid habitat and production. Effectiveness in Addressing Downstream Drainage Problems While the Flow Control Duration Standard flow control standard assuming historical site conditions provides a reasonable level of protection for preventing most development-induced problems, it does not necessarily prevent increases in existing site conditions 100-year peak flows that can aggravate severe flooding problems as described in Core Requirement #2, nor does it necessarily prevent aggravation of all severe erosion problems. Consequently, if one or more of these problems are identified through offsite analysis per Core Requirement #2, additional onsite flow control and/or offsite improvements will likely be required (see “Drainage Problem-Specific Mitigation Requirements” in Section 1.2.2.2). Target Surfaces Facilities in Flow Control Duration Standard Areas22 must mitigate (either directly or in effect) the runoff from the following target developed surfaces within the threshold discharge area for which the facility is required: 1. New impervious surface that is not fully dispersed per the criteria on Section 1.2.3.2.C as specified in Appendix C. For individual lots within residential subdivision projects, the extent of new impervious surface shall be assumed as specified in Chapter 3. Note, any new impervious surface such as a bridge or boardwalk that spans the ordinary high water of a stream, pond, or lake may be excluded as a target surface if the runoff from such span is conveyed to the ordinary high water area in accordance with Criteria (b), (c), (d), and (e) of the “Direct Discharge Exemption“ (p 1-39). 2. New pervious surface that is not fully dispersed as specified in Appendix C. For individual lots within residential subdivision projects, the extent of new pervious surface shall be assumed to be the entire lot area, except the assumed impervious portion and any portion in which native conditions are preserved by covenant, tract, or easement. In addition, the new pervious surface on individual lots shall be assumed to be 100% grass. 3. Replaced impervious surface that is not fully dispersed as specified in Appendix C on a non- redevelopment project in which the total of new plus replaced impervious surface is 5,000 square feet or more, OR new pervious surface is ¾ acre or more. 4. Replaced impervious surface that is not fully dispersed on a transportation redevelopment project in which new impervious surface is 5,000 square feet or more and totals 50% or more of the existing impervious surface within the project limits. 5. Replaced impervious surface that is not fully dispersed as specified in Appendix C, on a parcel redevelopment project in which the total of new plus replaced impervious surface is 5,000 square feet or more and whose valuation of proposed improvements (including interior improvements and excluding required mitigation improvements) exceeds 50% of the assessed value of: (a) the existing project site improvements on commercial or industrial projects, or (b) the existing site improvements on other projects. Exceptions The following exceptions apply only in Flow Control Duration Standard Areas: 1. The historical site conditions exception does not apply to the City. 22 Note: Any threshold discharge area that appears to be located within a Flow Control Duration Standard Area according to the Flow Control Application layer in COR Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>) but drains entirely by non-erodible manmade conveyance to a major receiving water (listed on page 1-51) is considered to be located within a Peak Rate Flow Control Standard Area. 1.2.3 CORE REQUIREMENT #3: FLOW CONTROL FACILITIES 2022 City of Renton Surface Water Design Manual 6/22/2022 1-43 2. The facility requirement in Flow Control Duration Standard Matching Existing Site Conditions Areas is waived for any threshold discharge area in which there is no more than a 0.15-cfs difference (when modeled using 15 minute time steps) in the sum of developed 100-year peak flows for those target surfaces subject to this requirement and the sum of historical site conditions 100-year peak flows (modeled using same time step unit (e.g., 15 minute) used to calculate the developed flow) for the same surface areas. Note: for the purposes of this calculation, target surfaces served by on-site BMPs per Appendix C may be modeled in accordance with the on-site BMP sizing credits in Core Requirement #9, Table 1.2.9.A. 3. The facility requirement in Flow Control Duration Standard Matching Forested Site Conditions Areas is waived for any threshold discharge area in which there is no more than a 0.15-cfs difference (when modeled using 15 minute time steps) in the sum of developed 100-year peak flows for those target surfaces subject to this requirement and the sum of forested (historical) site conditions 100-year peak flows (modeled using same time step unit (e.g., 15 minute) used to calculate the developed flow) for the same surface areas. Note: for the purposes of this calculation, target surfaces served by on-site BMPs per Appendix C may be modeled in accordance with the on-site BMP sizing credits in Core Requirement #9, Table 1.2.9.A. 4. The facility requirement in Flow Control Duration Standard Areas may be reduced or waived for any threshold discharge area where a plan or study approved by the City and Ecology shows that a lower standard (e.g., Peak Rate Control Standard or targeting existing site conditions instead of forested conditions) is sufficient or no facility is necessary to protect or allow for restoration of water body beneficial uses and habitat functions essential to salmonids. 5. The regional facilities plan exception does not apply to the City. 6. The facility requirement in Flow Control Duration Standard Areas as applied to replaced impervious surface may be reduced by the CED Manager/designee using the adjustment process detailed in Sections 1.4.3 and 1.4.4 of the adjustment process, if the cost of flow control facilities to mitigate all target surfaces exceeds that necessary to mitigate only for new impervious surface plus new pervious surface and also exceeds 1/3 of the valuation of proposed improvements (including interior improvements) or twice the cost of a facility to mitigate equivalent surfaces on a new development site, whichever is less. The amount of reduction shall be limited such that the cost of flow control facilities is at least equal to that necessary to mitigate only for new impervious surface plus new pervious surface, and beyond this amount, is no greater than 1/3 of the valuation of proposed improvements (including interior improvements) or twice the cost of a facility to mitigate equivalent surfaces on a new development site, whichever is less. C. FLOOD PROBLEM FLOW CONTROL STANDARD AREAS Flood Problem Flow Control Standard Areas are designated by the City of Renton where the City has determined that a higher average level of flow control is needed to prevent aggravation of existing documented flooding problems. At this time, the City has not mapped specific areas, but may apply this standard when a project discharges to a severe flooding or erosion problem. Within Flood Problem Flow Control Standard Areas, or where required by the City to protect aggravation of a downstream problem, required flow control facilities must comply with the following minimum requirements for facility performance and mitigation of targeted surfaces, except where such requirements or the facility requirement itself is waived or reduced by the area-specific exceptions at the end of this subsection. Minimum Required Performance Facilities in Flood Problem Flow Control Standard Areas must comply with the following flow control performance standard and assumptions unless modified by offsite analysis per Core Requirement #2 (see Table 1.2.3.A): SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-44 Flood Problem Flow Control Standard: Apply the Flow Control Duration Standard, AND match the developed 100-year peak discharge rate to the predeveloped 100-year peak discharge rate. If the Flood Problem Flow Control Area is located within a Flow Control Duration Standard Area and does not drain entirely by non-erodible manmade conveyance to a major receiving water (see Table 1.2.3.B), then historical site conditions shall be assumed as the predeveloped condition except for the purposes of matching 100-year peak discharge rates. For all other situations and for the purposes of matching 100-year peak discharge rates, existing site conditions may be assumed. Intent The Flood Problem Flow Control Standard is intended to prevent significant increases in existing water surface levels for 2-year through 100-year return frequencies. Such increases are expected to occur as the volume of runoff discharging to the water body is increased by upstream development. Because inflow rates to these water bodies are typically much higher than the outflow rates, increased runoff volumes from upstream development are, in effect, stacked on top of existing volumes in the water body, resulting in higher water surface levels. The duration-matching and 100-year peak- matching criteria of the Flood Problem Flow Control Standard counteract this stacking effect by slowing the arrival of additional runoff volumes. Because it can prevent significant aggravation of existing flooding, the Flood Problem Flow Control Standard is also applicable to other flow control areas where severe flooding problems have been identified per Core Requirement #2. Effectiveness in Addressing Downstream Drainage Problems If the Flood Problem Flow Control Standard is implemented onsite, no additional measures are required to prevent aggravation of the three types of downstream drainage problems described in Core Requirement #2. The one exception is for a wetland or lake that is a closed depression with a severe flooding problem, and the proposed project is adding impervious surface area amounting to more than 10% of the 100-year water surface area of the closed depression. In this case, additional onsite flow control or offsite improvements may be necessary as determined by a “point of compliance analysis” (see “Special Provision for Closed Depressions” in Table 1.2.3.A, and see Section 3.3.6, “Point of Compliance Analysis”). Target Surfaces Facilities in Flood Problem Flow Control Standard Areas must mitigate (either directly or in effect) the runoff from the following target developed surfaces within the threshold discharge area for which the facility is required: 1. If the Flood Problem Flow Control Standard Area is located within a Flow Control Duration Standard Area, then the target surfaces are the same as those required for facilities in Flow Control Duration Standard Areas (see Section 1.2.3.1.B) unless otherwise allowed by the area-specific exceptions for Flow Control Duration Standard Areas. Note: Any Flood Problem Flow Control Standard Area that appears to be located within a Flow Control Duration Standard Area according to the Flow Control Application layer in COR Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>), but drains entirely by non- erodible manmade conveyance to a major receiving water (see Table 1.2.3.B), is considered to be located within a Peak Rate Flow Control Standard Area. 2. If the Flood Problem Flow Control Standard Area is located within a Peak Rate Flow Control Standard Area or drains entirely by non-erodible manmade conveyance to a major receiving water, then the target surfaces are the same as those required for facilities in Peak Rate Flow Control Standard Areas (see Section 1.2.3.1.A). Exceptions The following exceptions apply only in Flood Problem Flow Control Standard Areas: 1. If the Flood Problem Flow Control Standard Area is located within a Flow Control Duration Standard Matching Existing Site Conditions Area or Peak Rate Flow Control Area, then the facility 1.2.3 CORE REQUIREMENT #3: FLOW CONTROL FACILITIES 2022 City of Renton Surface Water Design Manual 6/22/2022 1-45 requirement is waived for any threshold discharge area in which there is no more than a 0.15-cfs difference (when modeled using 15 minute time steps) in the sum of developed 100-year peak flows for the target surfaces subject to this requirement and the sum of historical site conditions 100-year peak flows (modeled using same time step unit (e.g., 15 minute) used to calculate the developed flow) for the same surface areas. Agricultural zoned projects in current agricultural use may use existing site conditions as the predeveloped condition for purposes of this exception calculation. Note: for the purposes of this calculation, target surfaces served by on-site BMPs per Appendix C may be modeled in accordance with the on-site BMP sizing credits in Core Requirement #9, Table 1.2.9.A. Also, any Flood Problem Flow Control Standard Area that appears to be located within a Flow Control Duration Standard Area according to the Flow Control Application layer in COR Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>), but drains entirely by non-erodible manmade conveyance to a major receiving water (see Table 1.2.3.B), is considered to be located within a Peak Rate Flow Control Standard Area. 2. If the Flood Problem Flow Control Standard Area is located within a Peak Rate Flow Control Standard Area, then the facility requirement is waived for any threshold discharge area in which the target surfaces subject to this requirement will generate no more than a 0.15-cfs increase (when modeled using 15-minute time steps) in the existing site conditions 100-year peak flow (modeled using same time step unit (e.g., 15-minute) used to calculate the developed flow. Note: for the purposes of this calculation, target surfaces served by on-site BMPs per Appendix C may be modeled in accordance with the on-site BMP sizing credits in Core Requirement #9, Table 1.2.9.A. 3. Any required application of the Flood Problem Flow Control Standard Areas facility requirement to replaced impervious surface may be waived if the City has adopted a plan and implementation schedule approved by the state Department of Ecology for fulfilling this requirement with regional facilities. 4. Any required application of the Flood Problem Flow Control Standard Areas facility requirement to replaced impervious surface may be reduced by CED using the procedures detailed in Sections 1.4.3 and 1.4.4 of the adjustment process, if the cost of flow control facilities to mitigate all target surfaces exceeds that necessary to mitigate only for new impervious surface plus new pervious surface and also exceeds 1/3 of the valuation of proposed improvements (including interior improvements) or twice the cost of a facility to mitigate the same surfaces on a new development site, whichever is less. The amount of reduction allowed by this exception shall be limited such that the cost of flow control facilities is at least equal to that necessary to mitigate only for new impervious surface plus new pervious surface, and beyond this amount, is no greater than 1/3 of the valuation of proposed improvements (including interior improvements) or twice the cost of a facility to mitigate equivalent surfaces on a new development site, whichever is less. 1.2.3.2 FLOW CONTROL FACILITY IMPLEMENTATION REQUIREMENTS Flow control facilities shall be designed and implemented in accordance with the following requirements, allowances, and flexible compliance provisions: A. ONSITE VS. OFFSITE IMPLEMENTATION All required flow control facilities must be implemented onsite except where the requirements below can be met by direct discharge to a regional or shared facility constructed to provide flow control for the proposed project. Regional facilities are typically constructed as part of a City-approved plan or study (e.g., basin plan, stormwater compliance plan, or master drainage plan). Shared facilities may be constructed under a City-developed shared facility drainage plan or under an agreement between two or more private developers. 1. The regional or shared facility must be of adequate size and design to meet the current flow control requirements for the proposed project. Note: the current flow control requirements are those specified SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-46 by Core Requirement #3 of this manual unless superseded by other adopted area-specific flow control requirements per Special Requirement #1 (see Section 1.3.1). In some cases where the current flow control requirements differ from those used to originally design the regional or shared facility, additional analysis and possible retrofitting of the facility may be required to ensure adequate size and design. In other cases where the current flow control requirements are not significantly different or are less stringent, adequate size and design may already be documented by an adopted City basin plan or master drainage plan, an approved shared facility drainage plan, or a detailed drainage analysis approved by the City for a separate permitted development. 2. The regional or shared facility must be fully operational at the time of construction of the proposed project. In the case of a shared facility, the proposed project must comply with the terms and conditions of all contracts, agreements, and permits associated with the shared facility. If the offsite facility is an existing City-owned facility, the City may charge a special use fee equal to or based on the property value of the detention capacity being used. 3. The conveyance system between the project site and the regional facility must meet the same criteria specified for direct discharge to a major receiving water except for Criterion (a) (see “Direct Discharge Exemption” in Section 1.2.3.1). In the case of a shared facility, the criteria are the same, except the conveyance system need only have adequate capacity and erosion protection for buildout of the participating portion23 of the contributing drainage area. B. METHODS OF ANALYSIS AND DESIGN Flow control facilities must be analyzed and designed using a continuous flow simulation method such as HSPF (Hydrologic Simulation Program FORTRAN) or the simplified HSPF-based runoff files method. An overview of the runoff files method is found in Chapter 3. Specifications for use of the approved modeling software is provided in the software documentation and augmented with limited SWDM-specific guidance in Reference Section 6-D. Detailed design specifications for flow control facilities are found in Chapter 5. C. SIZING CREDITS FOR FULLY DISPERSED SURFACES A fully dispersed surface (either impervious or nonnative pervious) is one that conforms to the BMP strategy for “full dispersion” detailed in Appendix C, Section C.2.1. This strategy calls for minimizing the area of onsite developed surface relative to native vegetated surface, together with the application of dispersion techniques that utilize the natural retention/detention capacity of the native vegetated surface to mitigate the runoff effects of the developed surfaces. Developed surfaces conforming to this strategy are considered to have a negligible impact downstream, and therefore, may be modeled as forest and are not subject to the area-specific flow control facility requirement (Section 1.2.3.1) or the area-specific water quality facility requirement (Section 1.2.8.1). In order for developed surfaces to qualify as fully dispersed, they must meet the basic criteria listed below and further detailed in Appendix C, Section C.2.1. Criteria for Fully Dispersed Surfaces 1. The total area of impervious surface being fully dispersed must be no more than 15% of the total area of native vegetated surface being preserved by a clearing limit by a City-approved recorded tract, easement, or covenant within the same threshold discharge area. The total area of impervious surface plus nonnative pervious surface24 being fully dispersed must be no more than 35% of a threshold discharge area. 2. The runoff from a fully dispersed surface must be discharged using one of the following dispersion devices in accordance with the design specifications and maximum area of fully dispersed surface for each device set forth in Appendix C, Section C.2.1: a) Splash blocks b) Rock pads c) Gravel filled trenches 23 The participating portion includes those properties that have agreements for use of the shared facility. 24 Nonnative pervious surface means a pervious surface that does not meet the definition of a native vegetated surface. 1.2.3 CORE REQUIREMENT #3: FLOW CONTROL FACILITIES 2022 City of Renton Surface Water Design Manual 6/22/2022 1-47 d) Sheet flow Note: The dispersion device must be situated so as to discharge within the same threshold discharge area of the surface it serves. 3. A native vegetated flowpath segment of at least 100 feet in length (25 feet for sheet flow from a nonnative pervious surface) must be available along the flowpath that runoff would follow upon discharge from a dispersion device listed in Minimum Requirement 2 above. The native vegetated flowpath segment must meet all of the following criteria: a) The flowpath segment must be over native vegetated surface. b) The flowpath segment must be onsite or an offsite tract or easement area reserved for such dispersion. c) The average slope of the flowpath segment must be no steeper than 15% for any 20-foot reach of the flowpath segment. d) The flowpath segment must be located between the dispersion device and any downstream drainage feature such as a pipe, ditch, stream, river, pond, lake, or wetland. e) The flowpath segments for adjacent dispersion devices must comply with the minimum spacing requirements in Appendix C, Section C.2.1. These requirements do not allow overlap of flowpath segments, except in the case where sheet flow from a nonnative pervious surface overlaps with the flowpath of any dispersion device listed in Minimum Requirement 2 above. In this case, the longer of the two overlapping flowpath segments must be extended at least 1 foot for every 3 feet of distance along the most representative path that runoff would travel from the upstream end to the discharge end of the nonnative pervious surface. 4. On sites with septic systems, the discharge of runoff from dispersion devices must not be upgradient of the drainfield. This requirement may be waived by CED if site topography clearly prohibits flows from intersecting the drainfield. 5. The dispersion of runoff must not create flooding or erosion impacts as determined by CED. If runoff is proposed to be discharged toward a landslide hazard, erosion hazard area, or steep slope hazard area (i.e., slopes steeper than 20%), CED may require the applicant to have the proposal evaluated by a geotechnical engineer, engineering geologist, or CED. D. SIZING CREDITS FOR USE OF ON-SITE BMPS Projects that implement on-site BMPs as detailed in Core Requirement #9 and Appendix C, whether required or optional, may use the on-site BMP sizing credits as described and allowed in Section 1.2.9.4 and Table 1.2.9.A. E. MITIGATION OF TARGET SURFACES THAT BYPASS FACILITY On some sites, topography may make it difficult or costly to collect all target surface runoff for discharge to the onsite flow control facility. Therefore, some project runoff subject to flow control may bypass required onsite flow control facilities provided that all of the following conditions are met: 1. The point of convergence for runoff discharged from the bypassed target surfaces and from the project’s flow control facility must be within a quarter-mile downstream25 of the facility’s project site discharge point, AND 2. The increase in the existing site conditions 100-year peak discharge from the area of bypassed target surfaces must not exceed 0.4 cfs, AND 25 Note: CED may allow this distance to be extended beyond a half mile to the point where the project site area constitutes less than 15% of the tributary area. SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-48 3. Runoff from the bypassed target surfaces must not create a significant adverse impact to downstream drainage systems, salmonid habitat, or properties as determined by CED, AND 4. Water quality requirements applicable to the bypassed target surfaces must be met, AND 5. Compensatory mitigation by a flow control facility must be provided so that the net effect at the point of convergence downstream is the same with or without the bypass. This mitigation may be waived if the existing site conditions 100-year peak discharge from the area of bypassed target surfaces is increased by no more than 0.15 cfs (modeled using 15 minute time steps) and on-site BMPs as detailed in Appendix C are applied to all impervious surfaces within the area of bypassed target surfaces. One or combination of the following methods may be used to provide compensatory mitigation by a flow control facility subject to permission/approvals from other parties as deemed necessary by CED: a) Design the project’s flow control facility or retrofit an existing offsite flow control facility as needed to achieve the desired effect at the point of convergence, OR b) Design the project’s flow control facility or provide/retrofit an offsite flow control facility to mitigate an existing developed area (either onsite or offsite) that has runoff characteristics (i.e., peak flow and volume) equivalent to those of the bypassed target surfaces but is currently not mitigated or required to be mitigated to the same flow control performance requirement as the bypassed target surfaces. Consideration of an offsite area to be mitigated for must take into account the likelihood of that area redeveloping in the future. Those areas determined by the City to have a high likelihood of future redevelopment that will provide its own mitigation may not be used as compensatory mitigation. F. BYPASS OF RUNOFF FROM NON-TARGET SURFACES The performance of flow control facilities can be compromised if the contributing area, beyond that which must be mitigated by the facility, is too large. Therefore, IF the existing 100-year peak flow rate from any upstream area (not targeted for mitigation) is greater than 50% of the 100-year developed peak flow rate (undetained) for the area that must be mitigated, THEN the runoff from the upstream area must bypass the facility. Offsite areas that naturally drain onto the project site must be intercepted at the natural drainage course within the project site and conveyed in a separate conveyance system and must bypass onsite stormwater facilities. The bypass of upstream runoff must be designed so that all of the following conditions are met: 1. Any existing contribution of flows to an onsite wetland must be maintained, AND 2. Upstream flows that are naturally attenuated by natural detention on the project site under predeveloped conditions must remain attenuated, either by natural means or by providing additional onsite detention so that peak flows do not increase, AND 3. Upstream flows that are dispersed or unconcentrated on the project site under predeveloped conditions must be discharged in a safe manner as described in Core Requirement #1 under “Discharge Requirements“. 4. Bypasses shall be designed in accordance with standards of Core Requirement #4, Conveyance System G. MITIGATION TRADES A project’s flow control facility may be designed to mitigate an existing developed non-target surface area (either onsite or offsite) in trade for not mitigating part or all of the project’s target surface area, provided that all of the following conditions are met: 1. The existing developed non-target surface area (i.e., an area of existing impervious surface and/or nonnative pervious surface) must have runoff discharge characteristics (i.e., peak flow and volume) equivalent to those of the target surface area for which mitigation is being traded and must not be currently mitigated to the same flow control performance requirement as the target surface area, AND 1.2.3 CORE REQUIREMENT #3: FLOW CONTROL FACILITIES 2022 City of Renton Surface Water Design Manual 6/22/2022 1-49 2. Runoff from both the target surface area being traded and the flow control facility must converge prior to discharge of the runoff from the target surface area being traded onto private property without an easement or through any area subject to erosion, AND 3. The net effect in terms of flow control at the point of convergence downstream must be the same with or without the mitigation trade, AND 4. The undetained runoff from the target surface area being traded must not create a significant adverse impact to downstream drainage systems, salmonid habitat, or properties prior to convergence with runoff from the flow control facility. 5. Mitigation trade proposals must be reviewed and approved with input from the City of Renton. 6. The existing non-targeted surface area that is mitigated for purposes of the required flow control must be documented and tracked by CED. Documentation should clarify that future redevelopment of the existing non-targeted area used for the mitigation trade will incur additional flow control mitigation requirements if the redevelopment exceeds Core Requirement #3 thresholds. This additional flow control mitigation must be met in addition to that previously required and provided for the mitigation trade. Applicants must consider sizing flow control facilities sufficient for both the mitigation trade area and future development of the existing non-targeted area, if feasible. H. MANIFOLD DETENTION FACILITIES A manifold detention facility is a single detention facility designed to take the place of two or more otherwise required detention facilities. It combines the runoff from two or more onsite drainage areas having separate natural discharge locations, and redistributes the runoff back to the natural discharge locations following detention. Because manifold detention facilities divert flows from one natural discharge location to another and then back, they are not allowed except by an approved adjustment (see Section 1.4). I. FACILITY REQUIREMENT IN LANDSLIDE HAZARD DRAINAGE AREAS Proposed projects subject to Discharge Requirement 2 in Core Requirement #1 must provide a tightline system unless the 100-year runoff from the project site can be feasibly infiltrated or one of the other exceptions listed in Section 1.1.2.2. For infiltration to be used as an alternative to the tightline requirement, it must be feasible per the facility design requirements and limitations specified in Section 5.2. When evaluating the feasibility of infiltration, multiple facility locations scattered throughout the project site shall be considered and used where feasible and practical to avoid concentrating infiltrated water in one location. If multiple facilities are not feasible or practical, then a single infiltration facility meeting the minimum setback requirements in Section 5.2 may be used where feasible. Where infiltration is not feasible, it is still possible for a proposed project to qualify for one of the other exceptions to the tightline requirement specified in Core Requirement #1. If such a project is subject to the flow control facility requirement in Core Requirement #3, the required facility must be a detention pond sized to meet, at minimum, the Flow Control Duration Standard Matching Forested site conditions flow control facility standard with a safety factor of 20% applied to the storage volume. The detention pond must be sited and designed so as to maximize the opportunity for infiltration in the pond. To accomplish this, all of the following design requirements must be met: 1. The detention pond must be preceded by either a water quality treatment facility per Core Requirement #8 or a presettling basin per Section 5.2, AND 2. All detention pond side slopes must be 3H:1V or flatter and must be earthen, AND 3. Detention pond liners that impede infiltration shall not be used, AND 4. The pond bottom shall be at or above the seasonal high groundwater table, AND 5. The detention pond outflow must meet the discharge dispersal requirements specified in Discharge Requirement 1 of Core Requirement #1. SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-50 1.2.4 CORE REQUIREMENT #4: CONVEYANCE SYSTEM All engineered conveyance system elements for proposed projects must be analyzed, designed, and constructed to provide a minimum level of protection against overtopping, flooding, erosion, and structural failure as specified in the following groups of requirements:  “Conveyance Requirements for New Systems,” Section 1.2.4.1  “Conveyance Requirements for Existing Systems,” Section 1.2.4.2  “Conveyance System Implementation Requirements,” Section 1.2.4.3 Intent: To ensure proper design and construction of engineered conveyance system elements. Conveyance systems are natural and engineered drainage facilities that provide for the collection and transport of surface water or stormwater runoff. This core requirement applies to the engineered elements of conveyance systems (primarily pipes, culverts, and ditches/channels). 1.2.4.1 CONVEYANCE REQUIREMENTS FOR NEW SYSTEMS All new conveyance system elements,26 both onsite and offsite, shall be analyzed, designed, and constructed according to the following requirements. Also see Section 4.1 for route design and easement requirements. Pipe Systems 1. New pipe systems shall be designed with sufficient capacity to convey and contain (at minimum) the 25-year peak flow, assuming developed conditions for onsite tributary areas and existing conditions for any offsite tributary areas. 2. Pipe system structures may overtop for runoff events that exceed the 25-year design capacity, provided the overflow from a 100-year runoff event does not create or aggravate a severe flooding problem or severe erosion problem as described in Core Requirement #2, Section 1.2.2. Any overflow occurring onsite for runoff events up to and including the 100-year event must discharge at the natural location for the project site. In residential subdivisions, this overflow must be contained within an onsite drainage easement, tract, covenant, or public right-of-way. 3. The upstream end of a pipe system that receives runoff from an open drainage feature (pond, ditch, etc.) shall be analyzed and sized as a culvert as described below. Culverts 1. New culverts shall be designed with sufficient capacity to meet the headwater requirements in Section 4.3.1 and convey (at minimum) the 25-year peak flow, assuming developed conditions for onsite tributary areas and existing conditions for any offsite tributary areas. 2. New culverts must also convey as much of the 100-year peak flow as is necessary to preclude creating or aggravating a severe flooding problem or severe erosion problem as described in Core Requirement #2, Section 1.2.2. Any overflow occurring onsite for runoff events up to and including the 100-year event must discharge at the natural location for the project site. In residential subdivisions, this overflow must be contained within an onsite drainage easement, tract, covenant, or public right-of-way. 3. New culverts proposed in streams with salmonids shall be designed to provide for fish passage as detailed in Section 4.3.2. Note: The City’s critical areas regulations (RMC 4-3-050) or the state Department of Fish and Wildlife may require a bridge to facilitate fish passage. 26 New conveyance system elements are those that are proposed to be constructed where there are no existing constructed conveyance elements. 1.2.4 CORE REQUIREMENT #4: CONVEYANCE SYSTEM 2022 City of Renton Surface Water Design Manual 6/22/2022 1-51 Ditches/Channels 1. New ditches/channels shall be designed with sufficient capacity to convey and contain, at minimum, the 25-year peak flow, assuming developed conditions for onsite tributary areas and existing conditions for any offsite tributary areas. 2. New ditches/channels must also convey as much of the 100-year peak flow as is necessary to preclude creating or aggravating a severe flooding problem or severe erosion problem as described in Core Requirement #2, Section 1.2.2. Any overflow occurring onsite for runoff events up to and including the 100-year event must discharge at the natural location for the project site. In residential subdivisions, such overflow must be contained within an onsite drainage easement, tract, covenant, or public right-of-way. 3. In both conditions listed above, ditches must be designed with a 6-inch minimum freeboard. Tightline Systems Traversing Steep Slopes New tightline conveyance systems traversing slopes that are steeper than 15% and greater than 20 feet in height, or are within a steep slope hazard area as defined in RMC 4-3-050, shall be designed with sufficient capacity to convey and contain (at minimum) the 100-year peak flow, assuming full build-out conditions27 for all tributary areas, both onsite and offsite. Tightline systems shall be designed as detailed in Section 4.2.2. Bridges New bridges shall be designed to accommodate the 100-year peak flow as specified in Section 4.3.3 and in accordance with the floodplain development standards in RMC 4-3-050. 1.2.4.2 CONVEYANCE REQUIREMENTS FOR EXISTING SYSTEMS The following conveyance requirements for existing systems are less rigorous than those for new systems to allow some salvaging of existing systems that are in useable condition. Existing systems may be utilized if they are capable of providing a minimum level of protection as-is or with minor modifications. Existing Onsite Conveyance Systems No Change in Flow Characteristics: Existing onsite conveyance systems that will not experience a change in flow characteristics (e.g., peak flows or volume of flows) as a result of the proposed project need not be analyzed for conveyance capacity. Change in Flow Characteristics: Existing onsite conveyance systems that will experience a change in flow characteristics as a result of the proposed project must comply with the following conveyance requirements: 1. The existing system must be analyzed and shown to have sufficient capacity to convey and contain (at minimum) the 25-year peak flow assuming developed conditions for onsite tributary areas and existing conditions for any offsite tributary areas. 2. The applicant must demonstrate that the 100-year peak flow to the existing system will not create or aggravate a severe flooding problem or severe erosion problem as described in Core Requirement #2, Section 1.2.2. 3. Minor modifications may be made to the conveyance system to achieve the required capacity stated above. Examples of minor modifications include raising a catch-basin rim, replacing or relaying a section of pipe to match the capacity of other pipes in the system, improving a pipe inlet, or enlarging a short, constricted reach of ditch or channel. 4. Modifications to an existing conveyance system or element that acts to attenuate peak flows, due to the presence of detention storage upstream, shall be made in a manner that does not significantly 27 Full build-out conditions means the tributary area is developed to its full zoning potential except where there are existing sensitive areas, open space tracts, and/or native growth protection easements/covenants. SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-52 increase peak flows downstream. For example, if water is detained in a pond upstream of a restrictive road culvert, then installing an overflow system for the culvert should prevent overtopping of the road without significantly reducing existing detention storage. Existing Offsite Conveyance Systems 1. Existing offsite conveyance systems need not be analyzed for conveyance capacity except as required by Core Requirement #2, or if offsite improvements or direct discharge are proposed per Core Requirement #3. 2. Improvements made to existing offsite conveyance systems to address the drainage problem-specific mitigation requirements in Section 1.2.2.2 need only change existing conveyance capacity sufficient to prevent aggravation of the drainage problem(s) being addressed. 3. Existing offsite conveyance systems proposed to be used for direct discharge to a major receiving water per Core Requirement #3 shall meet the same conveyance requirements specified in Section 1.2.4.1 for new systems. 1.2.4.3 CONVEYANCE SYSTEM IMPLEMENTATION REQUIREMENTS Conveyance systems shall be designed and implemented in accordance with the following requirements, allowances, and flexible compliance provisions: A. METHODS OF ANALYSIS AND DESIGN Properly sized conveyance elements provide sufficient hydraulic capacity to convey peak flows of the return frequencies indicated in Sections 1.2.4.1 and 1.2.4.2. Conveyance capacity shall be demonstrated using the methods of analysis detailed in Chapter 4. Design flows for sizing conveyance systems shall be determined using the appropriate runoff computation method specified in Section 3.2. B. COMPOSITION Where feasible, conveyance systems shall be constructed of vegetation-lined channels, as opposed to pipe systems, except in Zone 1 of the Aquifer Protection Area where pipe systems are required. Vegetative channels shall generally be considered feasible if all of the following conditions are present: 1. The channel gradient generally does not exceed 5 percent, AND 2. Ditches/roadway section must be approved by the City, AND 3. The channel will be accessible for maintenance (see Section 1.2.6), AND 4. The channel will not be subject to erosion. Exceptions: The following are exceptions to the requirement for vegetative channels:  Conveyance systems proposed under roadways, driveways, or parking areas  Conveyance systems proposed between houses in urban-zoned plats and short plats  Conveyance systems conveying roof runoff only.  Conveyance systems in Zone 1 of the Aquifer Protection Area. C. INTERFLOW AND INTERCEPTION Interflow is near-surface groundwater that moves laterally through the soil horizon following the hydraulic gradient of underlying relatively impermeable soils. When interflow is expressed on the surface, it is termed a spring or seepage. Any significant springs or seepage areas that impact a roadway or structure proposed by the project must be intercepted and directed into a conveyance system. Where roadways may impede the passage of interflow to downstream wetlands or streams, provision for passage of unconcentrated flows must be made. 1.2.4 CORE REQUIREMENT #4: CONVEYANCE SYSTEM 2022 City of Renton Surface Water Design Manual 6/22/2022 1-53 D. PROVISION FOR LOT DRAINAGE WITHIN SUBDIVISIONS Within subdivision projects,28 provision must be made for the safe conveyance of runoff from the discharge location of each lot to the subdivision’s main conveyance system or road drainage system. This may include, but is not limited to, provisional stub-outs from an enclosed roadway drainage system to the edge of the road right-of-way at each created lot, or lot-line pipes or ditches that collect lot drainage and convey it to the subdivision’s main conveyance system or road drainage system. E. OUTFALLS An outfall is defined as a point where collected and concentrated surface and storm water runoff is discharged from a pipe system or culvert. Energy Dissipation: At a minimum, rock erosion protection is required at outfalls from all drainage systems and elements except where CED determines that erosion protection is being provided by other means or is not needed. Details on outfall structures are included in Section 4.2.2. New Point Discharges Over Steep Slopes: Proposed outfalls that will discharge runoff in a location where the natural (existing) discharge is unconcentrated over a slope steeper than 15% and greater than 20 feet in height, or over a steep slope hazard area (as defined in RMC 4-3-050), must meet the following criteria:  A tightline conveyance system must be constructed to convey the runoff to the bottom of the slope unless other measures are approved by CED based on an evaluation/report by a licensed geotechnical engineer.  The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built-out conditions.  Tightline systems must be designed so that existing baseflow conditions are not significantly changed and adequate energy dissipation is provided at the bottom of the slope.  Where alternative measures (e.g., dispersal trench) to the tightline system are approved upstream of a landslide hazard or steep slope hazard area, they may be placed no closer than 50 feet from the top of the hazard area slope based on an evaluation/report by a licensed geotechnical engineer. F. OUTFALLS TO THE GREEN RIVER New stormwater outfalls or modifications to existing stormwater outfalls discharging to the Green River between River Mile 6 (South Boeing Access Road) and SR 18 are allowed only through the adjustment process. These outfalls must comply with requirements of the Green River Pump Operations Procedure Plan, which establishes storage volumes and release rate criteria for developments proposing to construct or modify outfalls. Copies of the plan are available from DNRP. G. SPILL CONTROL Projects proposing to construct or replace onsite conveyance system elements that receive runoff from non-roof-top pollution-generating impervious surface must provide a spill control device as detailed in Section 4.2.1.1 prior to discharge from the site or into a natural onsite drainage feature.29 More specifically, this requirement applies whenever a proposed project does either of the following:  Constructs a new onsite conveyance system that receives runoff from non-roof-top pollution- generating impervious surface, OR  Removes and replaces an existing onsite conveyance system element that receives runoff from 5,000 square feet or more of non-roof-top pollution-generating impervious surface onsite. 28 For purposes of this requirement, the term subdivision project refers to any project that creates a short plat, plat, or binding site plan. 29 Natural onsite drainage feature means a natural swale, channel, stream, closed depression, wetland, or lake. SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-54 The intent of this device is to temporarily detain oil or other floatable pollutants before they enter the downstream drainage system in the event of an accidental spill or illegal dumping. It may consist of a tee section in a manhole or catch basin, or an equivalent alternative as specified in Section 4.2.1.1. Note that in addition to this spill control requirement to protect offsite and natural drainage systems, there are other spill control requirements in this manual for discharges to certain water quality facilities and all infiltration facilities (see the design criteria for water quality facilities in Chapter 6 and the general requirements for infiltration facilities in Section 5.2). The application of these requirements must be such that all stated intents are satisfied. H. GROUNDWATER PROTECTION Any reach of new ditch or channel proposed by a project in which the untreated runoff from 5,000 square feet or more of pollution-generating impervious surface or ¾ acre or more of pollution-generating pervious surface comes into direct contact with an outwash soil must be lined with either a low permeability liner or a treatment liner consistent with the specifications for such liners in Section 6.2.4, except where it can be demonstrated that the soil meets the soil suitability criteria listed in Section 5.2.1. The intent of this requirement is to reduce the likelihood that pollutants will be discharged to groundwater when untreated runoff is conveyed in ditches or channels constructed in soils with high infiltration rates. I. PUMP SYSTEMS Pump systems may be used to convey water from one location or elevation to another within the project site provided they meet the design criteria specified for such systems in Section 4.2.3 and will be privately owned and maintained. Pump systems discharging flows from the project site that would not have discharged by gravity flow under existing site conditions will require an approved adjustment to Core Requirement #1 (see Section 1.4, “Adjustment Process“). These pump systems will be considered only when there is no other physical gravity alternative and they are necessary to prevent creation or aggravation of a flooding or erosion problem as specified in Section 1.2.2. 1.2.5 CORE REQUIREMENT #5: CONSTRUCTION STORMWATER POLLUTION PREVENTION All proposed projects that will clear, grade, or otherwise disturb the site must provide erosion and sediment controls to prevent, to the maximum extent practicable, the transport of sediment from the project site to downstream drainage facilities, water resources, and adjacent properties. All proposed projects that will conduct construction activities onsite or offsite must provide stormwater pollution prevention and spill controls to prevent, reduce, or eliminate the discharge of pollutants to onsite or adjacent stormwater systems or watercourses. To prevent sediment transport and pollutant discharges as well as other impacts related to land-disturbing and construction activities, Erosion and Sediment Control (ESC) measures and Stormwater Pollution Prevention and Spill Control (SWPPS) measures that are appropriate to the project site must be applied through a comprehensive Construction Stormwater Pollution Prevention (CSWPP) plan as described in Sections 1.2.5.1 and 1.2.5.3 and shall perform as described in Section 1.2.5.2. In addition, these measures, both temporary and permanent, shall be implemented consistent with the requirements in Section 1.2.5.3 that apply to the proposed project. Intent:  To prevent the transport of sediment and other impacts, like increased runoff, related to land disturbing activities. Erosion of disturbed areas on construction sites can result in excessive sediment transport to adjacent properties and to surface waters. This sediment can result in major adverse impacts, such as flooding from obstructed drainage ways, smothering of salmonid spawning beds, algal blooms in lakes, and exceedances of state water quality standards for turbidity. These impacts can also result from the increased runoff generated by land disturbing activities on construction sites. 1.2.5 CORE REQUIREMENT #5: CONSTRUCTION STORMWATER POLLUTION PREVENTION 2022 City of Renton Surface Water Design Manual 6/22/2022 1-55  To prevent, reduce, or eliminate the discharge of pollutants to onsite or adjacent stormwater systems or watercourses from construction-related activities such as materials delivery and storage, onsite equipment fueling and maintenance, demolition of existing buildings and disposition of demolition materials and other waste, and concrete handling, washout and disposal. 1.2.5.1 CSWPP MEASURES Construction Stormwater Pollution Prevention (CSWPP) measures include Erosion and Sediment Control (ESC) measures and Stormwater Pollution Prevention and Spill (SWPPS) measures. ESC Measures Each of the following categories of ESC measures must be considered for application to the project site as detailed in the Erosion and Sediment Control (ESC) Standards located in the Construction Stormwater Pollution Prevention Standards adopted as Appendix D of this manual: 1. Clearing Limits 2. Cover Measures 3. Perimeter Protection 4. Traffic Area Stabilization 5. Sediment Retention 6. Surface Water Collection 7. Dewatering Control 8. Dust Control 9. Flow Control 10. Control Pollutants (also see SWPPS Measures below) 11. Protect Existing and Proposed Stormwater Facilities and On-site BMPs 12. Maintain Protective BMPs 13. Manage the Project SWPPS Measures Each of the following categories of SWPPS measures must be considered for application to the project site as detailed in the Stormwater Pollution Prevention and Spill Control (SWPPS) Standards located in the CSWPP Standards adopted as Appendix D of this manual:  Follow effective pollutant handling and disposal procedures.  Provide cover and containment for materials, fuel and other pollutants.  Manage the project site to maximize pollutant control and minimize pollutant sources.  Protect from spills and drips of petroleum products and other pollutants.  Avoid overapplication or untimely application of chemicals and fertilizers.  Prevent or treat contamination of stormwater runoff by pH modifying sources. 1.2.5.2 CSWPP PERFORMANCE AND COMPLIANCE PROVISIONS The changing conditions typical of construction sites call for frequent field adjustments of existing ESC and SWPPS measures or additional ESC and SWPPS measures in order to meet required performance. In some cases, strict adherence to specified measures may not be necessary or practicable based on site conditions or project type. In other cases, immediate action may be needed to avoid severe impacts. Therefore, careful attention must be paid to ESC and SWPPS performance and compliance in accordance with the following provisions: SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-56 A. CSWPP SUPERVISOR For projects in Targeted, Full or Large Project Drainage Review, or projects in Directed Drainage Review as determined by the CED permit reviewer, the applicant must designate a CSWPP supervisor who shall be responsible for the performance, maintenance, and review of ESC and SWPPS measures and for compliance with all permit conditions relating to CSWPP as described in the CSWPP Standards. The applicant’s selection of a CSWPP supervisor must be approved by the City. This approval may be rescinded for non-compliance, requiring the applicant to select another CSWPP supervisor and obtain City approval prior to continuing work on the project site. For projects that disturb one acre or more of land, the CSWPP supervisor must be a Certified Professional in Erosion and Sediment Control (see <www.cpesc.net> for more information) or a Certified Erosion and Sediment Control Lead whose certification is recognized by the Department of Ecology or King County.30 The City may also require a certified ESC professional for sites smaller than one acre of disturbance if CED determines that onsite ESC measures are inadequately installed, located, or maintained. For larger, more sensitive sites, the City may require a certified ESC professional with several years of experience in construction supervision/inspection and a background in geology, soil science, or agronomy (See Appendix D, Section D.2.3.1 for more information). B. MONITORING OF DISCHARGES The CSWPP supervisor shall have a turbidity meter onsite and shall use it to monitor surface and storm water discharges from the project site and into onsite wetlands, streams, or lakes whenever runoff occurs from onsite activities and during storm events. If the project site is subject to a NPDES general permit for construction issued by the Washington State Department of Ecology (Ecology), then the project must comply with the monitoring requirements of that permit. The CSWPP supervisor shall also use the specific SWPPS control BMP procedures for monitoring surface and stormwater discharge for pollutants and acceptable discharge levels. The CSWPP supervisor shall keep logs as required by the procedures of all measurements taken onsite and make them available to CED on request. C. ESC PERFORMANCE ESC measures shall be applied/installed and maintained to prevent, to the maximum extent practicable, the transport of sediment from the project site to downstream drainage systems or surface waters or into onsite wetlands, streams, or lakes or onto adjacent properties. This performance is intended to be achieved through proper selection, installation, and operation of the above ESC measures as detailed in the CSWPP Standards (Appendix D) and approved by the City. However, the CSWPP supervisor or the City may determine at any time during construction that the approved measures are not sufficient and that additional action is required based on one of the following criteria: 1. If a turbidity test of surface and storm water discharges leaving the project site is greater than the benchmark value of 25 NTU (nephelometric turbidity units) set by the Washington State Department of Ecology, but less than 250 NTU, the CSWPP Supervisor shall do all of the following: a) Review the ESC plan for compliance and make appropriate revisions within 7 days of the discharge that exceeded the benchmark of 25 NTU, AND b) Fully implement and maintain appropriate ESC measures as soon as possible but no later than 10 days after the discharge that exceeded the benchmark, AND c) Document ESC implementation and maintenance in the site log book. 2. If a turbidity test of surface or storm water entering onsite wetlands, streams, or lakes indicates a turbidity level greater than 5 NTU above background when the background turbidity is 50 NTU or 30 King County recognition of certification means that the individual has taken a King County-approved third party training program and has passed the King County-approved test for that training program. 1.2.5 CORE REQUIREMENT #5: CONSTRUCTION STORMWATER POLLUTION PREVENTION 2022 City of Renton Surface Water Design Manual 6/22/2022 1-57 less, or 10% above background when the background turbidity is greater than 50 NTU, then corrective actions and/or additional measures beyond those specified in Section 1.2.5.1 shall be implemented as deemed necessary by the City inspector or onsite CSWPP supervisor. 3. If discharge turbidity is 250 NTU or greater, the CSWPP Supervisor shall do all of the following: a) Notify the City by telephone, AND b) Review the ESC plan for compliance and make appropriate revisions within 7 days of the discharge that exceeded the benchmark of 25 NTU, AND c) Fully implement and maintain appropriate ESC measures as soon as possible but no later than 10 days after the discharge that exceeded the benchmark, AND d) Document ESC implementation and maintenance in the site log book. AND e) Continue to sample discharges until turbidity is 25 NTU or lower, or the turbidity is no more than 10% over background turbidity. 4. If the City determines that the condition of the construction site poses a hazard to adjacent property or may adversely impact drainage facilities or water resources, THEN additional measures beyond those specified in Section 1.2.5.1 may be required by the City. D. SWPPS PERFORMANCE SWPPS measures shall be applied/installed and maintained so as to prevent, reduce, or eliminate the discharge of pollutants to onsite or adjacent stormwater systems or watercourses or onto adjacent properties. This performance is intended to be achieved through proper selection, installation, and operation of the above SWPPS measures as detailed in the CSWPP Standards (Appendix D) and approved by the City. However, the CSWPP supervisor designated per Section 1.2.5.2.A or the City may determine at any time during construction that such approved measures are not sufficient and additional action is required based on the criteria described in the specific SWPPS BMP standard and/or conditions of an approved adjustment: E. FLEXIBLE COMPLIANCE Some projects may meet the intent of Core Requirement #5 while varying from specific CSWPP requirements contained here and in the CSWPP Standards. If a project is designed and constructed to meet the intent of this core requirement, the City may determine that strict adherence to a specific ESC requirement is unnecessary; an approved adjustment (see Section 1.4) is not required in these circumstances. Certain types of projects are particularly suited to this greater level of flexibility, for instance, projects on relatively flat, well drained soils, projects that are constructed in closed depressions, or projects that only disturb a small percentage of a forested site may meet the intent of this requirement with very few ESC measures. However, SWPPS requirements may actually be emphasized on well- drained soils, particularly in groundwater or well protection areas, or in close proximity to water bodies. More information on intent and general ESC and SWPPS principles is contained in the CSWPP Standards in Appendix D. F. ROADS AND UTILITIES Road and utility projects often pose difficult erosion control challenges because they frequently cross surface waters and are long and narrow with limited area available to treat and store sediment-laden water. Because of these factors, road and utility projects are allowed greater flexibility in meeting the intent of Core Requirement #5 as described in the CSWPP Standards. G. ALTERNATIVE AND EXPERIMENTAL MEASURES All measures proposed for erosion and sediment control shall conform to the details and specifications in the CSWPP Standards unless an alternative is approved by the City, and if the alternative is a new SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-58 technology, it must also be approved through Ecology’s CTAPE program (see “Alternative and Experimental Measures” in the CSWPP Standards, Appendix D). 1.2.5.3 CSWPP IMPLEMENTATION REQUIREMENTS Proposed projects must identify, install, and maintain required erosion and sediment control and stormwater pollution prevention and spill control measures consistent with the following requirements: A. CSWPP PLAN As specified in Chapter 2, all proposed projects must submit a CSWPP plan for implementing CSWPP measures. The CSWPP plan is comprised of the ESC plan and the SWPPS plan. The ESC plan must show the location and details of all ESC measures as specified in Chapter 2 and the CSWPP Standards and shall include a CSWPP report, which contains additional directions and supporting information like a detailed construction sequence as proposed by the design engineer and any calculations or information necessary to size ESC measures and demonstrate compliance with Core Requirement #5. The CSWPP plan shall also contain plan notes that outline specific permit conditions as outlined in Appendix D Section D.4.2 Standard ESC and SWPPS Plan Notes. The City may require large, complex projects to phase construction and to submit multiple ESC plans for the different stages of construction. New CSWPP plans are not required for changes that are necessary during construction, unless required by the City inspector. B. WET SEASON CONSTRUCTION During the wet season (October 1 to April 30) any site with exposed soils shall be subject to the “Wet Season Requirements” contained in the ESC Standards. In addition to the ESC cover measures, these provisions include covering any newly-seeded areas with mulch and seeding as much disturbed area as possible during the first week of October to provide grass cover for the wet season. Other ESC measures such as baker tanks and portable sand filters may be required for use during the wet season. A separate “Wet Season” ESC plan shall be submitted and approved by the City before continuing work on any site during the wet season. C. CONSTRUCTION WITHIN CRITICAL AREAS AND BUFFERS Any construction that will result in disturbed areas on or within a stream or associated buffer, within a wetland or associated buffer, or within 50 feet of a lake shall be subject to the “Critical Area Restrictions” contained in the CSWPP Standards. These provisions include phasing the project whenever possible so that construction in these areas is limited to the dry season. D. MAINTENANCE All ESC and SWPPS measures shall be maintained and reviewed on a regular basis as prescribed in the CSWPP Standards. E. FINAL STABILIZATION Prior to obtaining final construction approval, the site shall be stabilized, structural ESC and SWPPS measures (such as silt fences, sediment traps and concrete waste collection pits) shall be removed, and drainage facilities shall be cleaned as specified in the CSWPP Standards. A separate ESC plan describing final stabilization may be required by the City prior to implementation. F. CONSIDERATION OF OTHER REQUIRED PERMITS Consideration should be given to the requirements and conditions that may be applied by other agencies as part of other permits required for land-disturbing activities. In particular, the following permits may be required and should be considered when implementing CSWPP measures:  A Class IV Special Forest Practices Permit is required by the Washington State Department of Natural Resources for projects that will clear more than two acres of forest or 5,000 board feet of 1.2.6 CORE REQUIREMENT #6: MAINTENANCE AND OPERATIONS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-59 timber. All such clearing is also subject to the State Environmental Policy Act (RCW 43.21C) and will require SEPA review.  A NPDES General Permit for Construction (pursuant to the Washington State Department of Ecology’s Construction Stormwater General Permit) is required for projects that will disturb one or more acres for purposes of constructing or allowing for construction of a development, or projects disturbing less than one acre that are part of a larger common plan of sale31 that will ultimately disturb one or more acres. 1.2.6 CORE REQUIREMENT #6: MAINTENANCE AND OPERATIONS Maintenance and operation of all drainage facilities is the responsibility of the applicant or property owner, except those facilities for which the City assumes maintenance and operation as described below and in RMC 4-6-030.M. Drainage facilities must be maintained and operated in accordance with the maintenance standards in Appendix A of this manual, or other maintenance standards as approved by the City. Intent: To ensure that the maintenance responsibility for drainage facilities is clearly assigned and that these facilities will be properly maintained and operated in perpetuity. Drainage facilities serving private improvements are not allowed in public right-of-way. On-site BMPs serving private improvements are also not allowed in the public right-of-way. Under certain situations, drainage facilities for single family residential subdivisions with public roads may be allowed in the public right-of-way through the City adjustment/variance process. Examples of conditions in which facilities may be considered for placement in the public way are:  Dead end streets or cul-de-sacs where future extensions of the road is unlikely and where drainage facilities will not conflict with existing utility improvements.  Unimproved right-of-way where future improvements are not anticipated and would not conflict with existing or future utility improvements. Drainage Facilities to be Maintained by the City of Renton The City will assume maintenance and operation of the following drainage facilities 32 for any residential subdivision with public streets, except where the City grants an adjustment per Section 1.4, allowing the facilities to be maintained by the homeowners association:  Flow control and water quality treatment facilities within a stormwater tractor right-of-way dedicated to the City.  On-site BMPs serving more than one lot, and serving public improvements within a stormwater tract.  Bioretention facilities in City right-of-way, mitigating for public improvements.  Where serving public improvements, on-site BMP vegetated flow paths for full dispersion within an easement that includes provisions for access and maintenance. The City maintenance of these vegetated flow paths will be limited to their functionality. All other maintenance shall remain the responsibility of the owner(s).  The conveyance system within a drainage easement, tract or improved public road right-of-way granted to the City. 31 Common plan of development or sale means a site where multiple separate and distinct construction activities may take place at different times or on different schedules, but still under a single plan. Examples include: 1) phased projects and projects with multiple filings or lots, even if the separate phases or filings/lots will be constructed under separate contract or by separate owners (e.g., a development where lots are sold to separate builders); 2) a development plan that may be phased over multiple years, but is still under a consistent plan for long-term development; and 3) projects in a contiguous area that may be unrelated but still under the same contract, such as construction of a building extension and a new parking lot at the same facility. 32 Note: the City of Renton does not assume maintenance of individual lot drainage systems or drainage stub-outs serving single family residential lot downspout, footing, or yard drains. SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-60 Note: The City may assume maintenance of facilities serving any mix of developments through an adjustment. The City will assume maintenance and operation of these facilities two years after final construction approval by CED and an inspection by the City to ensure the facilities have been properly maintained and are operating as designed. Flow control facilities, water quality treatment facilities, and on-site BMPs to be maintained and operated by the City, along with the required perimeter landscaping (as required per RMC 4-9-150), must be located in a stormwater tract. For drainage facilities requiring perimeter landscaping, the stormwater tract shall be granted and conveyed with all ownership and maintenance obligations (excluding maintenance of the drainage facilities) to the subdivision’s lot owners. An easement under and upon said tract shall be dedicated to the City for the purpose of operating, maintaining, and repairing the drainage facilities contained in the stormwater tract. If perimeter landscaping is not required, then the stormwater tract shall be dedicated to the City along with the maintenance of the drainage facility contained therein. Required vegetated flow paths for full dispersion and basic dispersion BMPs require a recorded declaration of covenant that stipulates restrictions on use AND shall be located in an easement that includes provisions for access and maintenance. City maintenance of these vegetated flow paths will be limited to their functionality. All other maintenance shall remain the responsibility of the owner(s). Access roads serving these facilities must also be located in the tract or right-of-way and must be connected to an improved public road right-of-way. Conveyance systems to be maintained and operated by the City must be located in a drainage easement, tract, or right-of-way granted to the City. Note: the City does not normally assume maintenance responsibility for conveyance systems that are outside of improved public road right-of-way. Drainage Facilities to be Maintained by Private Parties For residential subdivisions of nine lots or less with private streets, planned unit developments, and commercial and industrial sites, maintenance and operation of flow control and water quality treatment facilities including on-site BMPs are the responsibility of the property owner (s) and must be located in a tract or easement that identifies each property owner as having equal and undivided interest. Shared facilities shall be maintained jointly by the property owners or users of the facility. Shared facilities must have a City approved maintenance plan or agreement regarding assignment of maintenance and operation. All drainage facilities maintained privately, by the City or by other public agencies must be maintained as specified in Appendix A, “Maintenance Requirements for Stormwater Facilities and On-Site BMPs,” and as further prescribed in Chapter 6 for water quality facilities, unless otherwise approved by the City. A copy of the Operation and Maintenance Manual submitted as part of the permit application for flow control and water quality treatment facilities (see Section 2.3.1) shall be retained on site and shall be transferred with the property to the new owner. A log of maintenance activity indicating when cleaning occurred and where waste was disposed of shall also be kept by the owner and be available for inspection by the City. All privately maintained on-site BMPs must be maintained as specified in the site/lot’s declaration of covenant and grant of easement per Section 1.2.9. The City shall annually inspect all privately maintained drainage facilities for compliance with these requirements. The City may reduce the inspection frequency based on maintenance records of double the length of time of the proposed inspection frequency. If the property owner(s) fails to maintain their facilities to the acceptable standards, the City shall issue a written notice specifying the required remedial actions and requiring a schedule for timely completion of the actions. If these actions are not performed in a timely manner, the City shall enter the property to perform the actions needed and bill the property owner(s) for the cost of the actions. If a hazard to public safety exists, the City shall perform remedial actions without written notice. 1.2.7 CORE REQUIREMENT #7: FINANCIAL GUARANTEES AND LIABILITY 2022 City of Renton Surface Water Design Manual 6/22/2022 1-61 If the proposed project is a commercial, industrial, or multifamily development or redevelopment, or a single family residential building permit, a drainage facility declaration of covenant and grant of easement must be recorded at the King County Office of Records and Elections. Whenever a flow control facility, water quality treatment facility, or on-site BMP is proposed to be located on a parcel separate from the parcel or parcels containing the target surfaces mitigated by the facility or BMP, provisions must be made to ensure that the owner or owners of the target surfaces have a perpetual right to operate and maintain the facility. This may be done either by recording an easement granting this right to the owner(s) of the target surfaces, or by conveying the land on which the facility sits (or an interest therein) to the owner(s) of target surfaces. If the proposed project is a residential subdivision development, all privately maintained conveyance systems or other drainage facilities that convey flows through private property must be located in a drainage easement dedicated to convey surface and stormwater . Individual owners of the properties containing these easements must maintain the drainage facilities through their property. The legal instrument creating drainage easements on private property must contain language that requires a private property owner to obtain written approval from the City prior to removing vegetation (except by routine mowing) from any drainage easement containing open, vegetated drainage facilities (such as swales, channels, ditches, ponds, etc.). Maintenance of On-Site BMPs Maintenance and operation of all on-site BMPs are the responsibility of the property owner unless specified above in Section 1.2.6 (Drainage Facilities to be Maintained by the City of Renton). On-site BMPs are not allowed in City right-of-way unless constructed to mitigate for public improvements. Maintenance and operation of on-site BMPs constructed in the right-of-way is the responsibility of the adjacent property owner in accordance with RMC 4-6-060. 1.2.7 CORE REQUIREMENT #7: FINANCIAL GUARANTEES AND LIABILITY In accordance with RMC 4-6-030, CED shall require all persons constructing any surface water facilities (including flow control/water quality facilities, conveyance systems, erosion control, and road drainage), to post with the City of Renton a bond, assignment of funds or certified check. The applicant must also maintain liability insurance as described in this Core Requirement #7. Intent: To ensure financial guarantees are posted to sufficiently cover the cost of correcting, if necessary, incomplete or substandard drainage facility construction work, and to warrant for two years the satisfactory performance and maintenance of those newly-constructed drainage facilities. Core Requirement #7 is also intended to ensure that a liability policy is provided that protects the proponent and the City from any damages relating to the construction or maintenance of required drainage facilities by private parties. Construction Bond for Required Improvements Before a permit, pursuant to the provisions of RMC 4-6-030, may be issued, the applicant may be required to execute to the City a construction bond. In some instances, and at the sole option of the City, a certificate of occupancy, final inspection, or final approval may be issued prior to completion of required public or site improvements if an acceptable form of guarantee is provided by the applicant. Amount of Required Construction Bond: The construction bond shall be for not less than 100 percent of the amount calculated in the bond quantity worksheet (as provided in Reference Section 8-H) of all required drainage improvements associated with the proposed project. The bond quantity worksheet shall be provided by the applicant and is subject to review and acceptance by the City. SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-62 Utilization of Funds Provided by the Construction Bond: If the required improvements associated with the proposed project are not completed by the termination date of the construction bond, the City shall use the bond to construct the improvements in accordance with the City’s standards. Release of Construction Bond: The construction bond will be released when the applicant completes the following:  Correct any defects noted in the final inspection.  Address, to the satisfaction of the City, all deficiencies noted in the final inspection by the City.  Provide to the City as-built drawings, final recorded plat, recorded easements, bill of sale, cost data inventory of public storm system improvements to be owned and maintained by the City, and recorded restricted covenant and grant of easement.  Receive a City Final inspection to ensure the drainage facilities have been properly installed and are operated as desired.  Submission of maintenance bond to the City. Maintenance Bond Prior to acceptance by the City of any newly constructed public improvements to be deeded to the City, or any onsite or offsite private storm drainage improvements, the applicant shall file with the City a construction maintenance bond. The maintenance bond is to be held by the City for a period of two years. Amount of Maintenance Bond: The maintenance bond shall be for 20 percent of the amount calculated in the bond quantity worksheet. Utilization of Funds Provided by the Maintenance Bond: In the event that required improvements are not properly maintained during the required maintenance guarantee period, the City shall notify the developer/owner. If the developer/owner fails to correct the problem within a period of 15 days, the City shall use the maintenance guarantee to perform the maintenance work. Should any failures occur in regard to required improvements associated with a development project within the warranty period, the City shall require the developer/owner to correct all failures. Should the developer/owner fail to perform within a period of 15 days, the City shall use the maintenance and warranty bond to correct any failures. Release of Maintenance Bond: Maintenance bond will be released upon completion of the two-year maintenance bond period following final inspection and correction of any maintenance defects identified in the final inspection by the City. Hold Harmless The permittee shall protect, defend, indemnify, and save harmless the City, its officers, employees, and agents from any and all costs, claims, judgments, or awards of damages, arising out of or in any way resulting from the negligent acts or omissions of the permittee. The permittee agrees that its obligations under this Section extend to any claim, demand, and/or cause of action brought by, or on behalf of, any of its employees or agents. Insurance Required Before a permit shall be issued for any construction, insurance will be required as follows: 1. The applicant shall secure and maintain in force throughout the duration of the permit: Commercial General Liability insurance written on an occurrence basis with limits no less than one million dollars ($1,000,000) per occurrence/two million dollars ($2,000,000) aggregate. 2. Copies of such insurance policy or policies shall be furnished unto the City with a special endorsement in favor of the City with the City named as a primary and noncontributory additional insured on the insurance policy and an endorsement stating such shall be provided to the City. 3. The policy shall provide that it will not be canceled or reduced without 30 days’ advanced written notice to the City. 1.2.8 CORE REQUIREMENT #8: WATER QUALITY FACILITIES 2022 City of Renton Surface Water Design Manual 6/22/2022 1-63 4. Upon showing of a hardship and at the discretion of the Administrator or his/her designee, the insurance requirements may be reduced or waived for single-family or two-family residential applications. Other Important Information about Core Requirement #7 Other requirements include the following:  Cash Bond Returned: The cash bond will be returned to applicant when work is accepted by the City, less any sums due to the City under the terms of this Core Requirement #7.  Reimbursement of City’s Costs Incurred to Obtain Funds Provided by Guarantees: If the City finds it necessary to utilize funds provided for any guarantee, and incurs expenses in obtaining and administering such funds, a portion of these monies shall also be used to reimburse the City for such recovery costs. If the guarantee is not adequate to cover all necessary costs, the developer/owner is required to make up the deficit in cash within 30 days of receipt of written notice from the City. 1.2.8 CORE REQUIREMENT #8: WATER QUALITY FACILITIES All proposed projects, including redevelopment projects, must provide water quality (WQ) facilities to treat the runoff from those new and replaced pollution-generating impervious surfaces and new pollution-generating pervious surfaces targeted for treatment as specified in the following sections. These facilities shall be selected from a menu of water quality facility options specified by the area-specific facility requirements in Section 1.2.8.1 and implemented according to the applicable WQ implementation requirements in Section 1.2.8.2. Intent: To require an efficient, cost-effective level of water quality treatment tailored to the sensitivities and resource protection needs of the downstream receiving water to which the project site drains, or, in the case of infiltration, protection of the receiving groundwater system. Guide to Applying Core Requirement #8 Core Requirement #8 requires that WQ facilities be provided to remove pollutants from runoff discharging from a project site in accordance with land use-specific WQ facility requirements found in Section 1.2.8.1. For efficient application of Core Requirement #8, the following steps are recommended: 1. Check the exemption language in Section 1.2.8 to determine if or which threshold discharge areas of the project site must provide WQ facilities per Core Requirement #8. 2. Use the Basic WQ treatment areas section (Section 1.2.8.1.A) to determine if basic or enhanced treatment is required. 3. Consult Section 1.2.8.2 for other design requirements, allowances, and flexible compliance provisions related to implementing water quality treatment. 4. Consult Sections 1.2.2, Core Requirement #2: Offsite Analysis, 1.2.2.1, Downstream Analysis, and 1.2.2.1.2, Downstream Water Quality Problems Requiring Special Attention. Other Important Information about Core Requirement #8 Core Requirement #8 is the primary component of an overall water quality protection strategy required by this manual. Other requirements include the following:  Core Requirement #4: Conveyance System, Spill Control Provisions, Section 1.2.4 — This provision generally applies whenever a project constructs or replaces onsite conveyance system elements that receive runoff from pollution-generating impervious surfaces. The provision requires that runoff from such impervious surfaces be routed through a spill control device prior to discharge from the project site or into a natural onsite drainage feature.  Core Requirement #4: Conveyance System, Groundwater Protection, Section 1.2.4 —This provision requires that ditches/channels be lined as needed to reduce the risk of groundwater contamination when they convey runoff from pollution-generating impervious surfaces that comes SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-64 into direct contact with an outwash soil. Facilities that allow runoff to have direct contact with the soil and open channel conveyance systems that are note concrete lined are not allowed in Zone 1 of the Aquifer Protection Area.  Special Requirement #4: Source Control, Section 1.3.4 — This requirement applies water quality source controls from the King County Stormwater Pollution Prevention Manual to commercial, industrial, and multifamily projects.  Special Requirement #5: Oil Control, Section 1.3.5 — This requirement applies special oil controls to those projects proposing to develop or redevelop a high-use site.  EXEMPTIONS FROM CORE REQUIREMENT #8 There are four possible exemptions from the requirement to provide a water quality facility per Core Requirement #8: 1. Surface Area Exemption A proposed project or any threshold discharge area within the project site is exempt if it meets all of the following criteria: a) Less than 5,000 square feet of new plus replaced PGIS will be created, AND b) Less than ¾ acre of new PGPS will be added. 2. Surface Exemption for Transportation Redevelopment Projects A proposed transportation redevelopment project or any threshold discharge area within the project site is exempt if it meets all of the following criteria: a) The total new impervious surface within the project limits is less than 50% of the existing impervious surface, AND b) Less than 5,000 square feet of new PGIS will be added, AND c) Less than ¾ acre of new PGPS will be added. 3. Cost Exemption for Parcel Redevelopment Projects A proposed redevelopment project on a single or multiple parcel site or any threshold discharge area within the project site is exempt if it meets all of the following criteria: a) The total valuation of the project’s proposed improvements (including interior improvements and excluding required mitigation improvements) is less than 50% of the assessed value of: (a) the existing project site improvements on commercial or industrial projects, or (b) the existing site improvements on other projects, AND b) Less than 5,000 square feet of new PGIS will be added, AND c) Less than ¾ acre of new PGPS will be added. 4. Soil Treatment Exemption A proposed project or any drainage area within a project is exempt if the runoff from pollution- generating impervious surfaces is infiltrated in soils that meet the “groundwater protection criteria” outlined below. These soil properties must be met by the undisturbed native soils onsite (i.e. in situ). Soil may not be imported in order to meet groundwater protection criteria. Groundwater Protection Criteria: The first 2 feet or more of the soil beneath an infiltration facility must have a cation exchange capacity greater than 5 (tested using EPA Laboratory Method 9081) and an organic content of 1.0% or greater (measured on a dry weight basis using ASTM D 2974), AND must meet one of the following specifications for general protection of groundwater: a) The soil must have a measured infiltration rate37 of less than or equal to 9 inches per hour, except in groundwater protection areas where the measured rate must be less than or equal to 2.4 inches per hour, OR 1.2.8 CORE REQUIREMENT #8: WATER QUALITY FACILITIES 2022 City of Renton Surface Water Design Manual 6/22/2022 1-65 b) The soil must be composed of less than 25% gravel by weight with at least 75% of the soil passing the #4 sieve, and the portion passing the #4 sieve must meet one of the following gradations:  At least 50% must pass the #40 sieve and at least 2% must pass the #100 sieve, OR  At least 25% must pass the #40 sieve and at least 5% must pass the #200 sieve.33 This exemption is not allowed for areas that are infiltrated (1) within one-quarter-mile of a sensitive lake, or (2) within one-quarter-mile of fresh water with existing or designated aquatic life use whose land use would otherwise trigger application of a facility from the enhanced basic treatment menu, or (3) within one-quarter-mile of a phosphorous or metals problem as described in Section 1.2.2.1.2. 1.2.8.1 LAND USE-SPECIFIC WATER QUALITY FACILITY REQUIREMENT Projects subject to Core Requirement #8 must provide a water quality facility selected from a menu of water quality facility options identified in the area-specific facility requirements and exceptions for the WQ treatment area in which the proposed project or threshold discharge area of the proposed project is located. These WQ treatment areas are listed below and their requirements and exceptions are detailed in the following subsections: A. Basic WQ Treatment Areas B. Sensitive Lake WQ Treatment Areas C. Sphagnum Bog WQ Treatment Areas. Intent: To apply an appropriate level of water quality treatment based on the sensitivities of receiving waters for the drainage area in which the project lies. These drainage areas are identified as WQ treatment areas on the WQ Applications Map adopted with this manual. In addition to a minimum basic standard, which applies broadly to most geographic areas, special menus are provided for land uses that generate the highest concentrations of metals in stormwater and for sites within the watersheds of sensitive lakes, and sphagnum bog wetlands. A. BASIC WQ TREATMENT AREAS Basic WQ Treatment Areas are designated by the City of Renton where a general, cost-effective level of treatment is sufficient for most land uses. Most direct discharges only require Basic WQ Treatment. Some land uses, however, will need an increased level of treatment (Enhanced Basic WQ Treatment) because they generate high concentrations of metals in stormwater runoff and acute concentrations of metals in streams are toxic to fish. Required Treatment Menu Within Basic WQ Treatment Areas, a water quality facility option from the Basic WQ menu shall be used to treat runoff from the surfaces listed under “Target Surfaces” below, except where such treatment is waived or reduced by the area-specific exceptions at the end of this subsection and except where the Enhanced Basic WQ menu is applicable as follows. If 50% or more of the runoff that drains to any proposed water quality facility is from one or more of the following land uses, then the Enhanced Basic WQ menu shall be used in place of the Basic WQ menu for the design of this facility, except if such treatment is waived or reduced by the area-specific exceptions at the end of this subsection: 1. Commercial, industrial, or multifamily land use. 2. A road with an expected average daily traffic (ADT) count of 7,500 or more vehicles. 33 Measured infiltration rate shall be as measured by the EPA method or the Double Ring Infiltrometer Method (ASTM D3385). For some soils, an infiltration rate of less than 9 inches per hour may be assumed based on a soil texture determination rather than a rate measurement. For more details, see the “Groundwater Protection” requirements in Section 5.2.1. SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-66 Treatment Goal and Options The treatment goal for facility options in the Basic WQ menu is 80% removal of total suspended solids (TSS) for flows or volumes up to and including the WQ design flow or volume for a typical rainfall year, assuming typical pollutant concentrations in urban runoff.34 TSS is the general performance indicator for basic water quality protection because it is the most obvious pollutant of concern. TSS is not a single pollutant; it is a general term for a highly variable mixture of solid pollutants with variable particle size and particle density distributions, and to one degree or another containing a variety of sorbed dissolvable pollutants. The Basic WQ menu includes facilities such as wetponds, combined detention/wetponds, bioswales, vegetated filter strips, and sand filters. See Chapter 6 for specific facility choices and design details. Additional facility designs may appear in Reference Section 14 in the future. The treatment goal for facility options in the Enhanced Basic WQ menu is to accomplish better removal of heavy metals and potentially other toxic materials than can be achieved by basic treatment, while still meeting the basic treatment goal of 80% TSS removal. The specific target performance is > 30% reduction of dissolved copper and > 60% removal of dissolved zinc. Dissolved copper and zinc are indicators of a wider range of metals typically found in urban runoff that are potentially toxic to fish and other aquatic life. The Enhanced Basic WQ menu includes options for use of a basic-sized stormwater wetland, a large sand filter, or a combination of two facilities in series. See Chapter 6 for specific facility options and designs. Additional facility designs may appear in Reference S14 in the future. Intent The Basic WQ menu is intended to be applied to both stormwater discharges draining to surface waters and those infiltrating into soils that do not provide adequate groundwater protection (see Exemption 4 from Core Requirement #8). Overall, the 80% TSS removal objective, in conjunction with special requirements for source control and high-use site controls, should result in good stormwater quality for all but the most sensitive water bodies. Increased water quality treatment is necessary for developments that generate the highest concentrations of metals and for developments that drain to sensitive lakes and sphagnum bog wetlands. Facility options in the Enhanced Basic WQ menu are intended to remove more metals than expected from those in the Basic WQ menu. Lower metal concentrations reduce the risk to fish from exposure to both chronic and acute toxic concentrations of metals such as copper and zinc, and very low concentration copper deleterious olfactory effects. As the toxicity of metals depends on their concentration, this standard is most effective for project sites with a larger proportion of pollution-generating impervious surface like roadways and medium to high density subdivisions. The Enhanced Basic WQ menu is intended to apply to all such project sites that drain by surface flows to a fish-bearing stream. However, projects that drain entirely by pipe to the major receiving waters listed Table 1.2.3.B may be excused from the increased treatment and may revert to the Basic WQ menu because concentration effects are of less concern as the overall flow volume increases; however, this exception is not applicable for metals impaired segments per Section 1.2.2.1: Downstream Analysis, and 1.2.2.1.2: Downstream Water Quality Problems Requiring Special Attention, Metals Problem (Type 4). Target Surfaces Facilities in Basic WQ Treatment Areas must treat (either directly or in effect) the runoff from the following target surfaces within the threshold discharge area for which the facility is required: 1. New PGIS that is not fully dispersed per the Criteria for Fully Dispersed Surfaces (see Section 1.2.3.2.C) in Core Requirement #3. For individual lots within residential subdivision projects, the extent of new PGIS shall be assumed based on expected driveway size as approved by CED. 2. New PGPS that is not fully dispersed and from which there will be a concentrated surface discharge in a natural channel or man-made conveyance system from the site. For individual lots within residential 34 The influent concentration range for demonstrated pollutant removal is 100 to 200 mg/L. For influent concentrations lower than 100 mg/l the effluent goal is equal to or less than 20 mg/l. For influent concentrations greater than 200 mg/l, the goal is greater than 80% TSS removal. 1.2.8 CORE REQUIREMENT #8: WATER QUALITY FACILITIES 2022 City of Renton Surface Water Design Manual 6/22/2022 1-67 subdivision projects, the extent of new pervious surface shall be assumed to be the entire lot area, except the assumed impervious portion as specified in Chapter 3 and any portion in which native conditions are preserved by covenant, tract, or easement. 3. Replaced PGIS that is not fully dispersed on a non-redevelopment project. 4. Replaced PGIS that is not fully dispersed on a transportation redevelopment project in which new impervious surface is 5,000 square feet or more and totals 50% or more of the existing impervious surface within the project limits. 5. Replaced PGIS that is not fully dispersed on a parcel redevelopment project in which the total of new plus replaced impervious surface is 5,000 square feet or more and whose valuation of proposed improvements (including interior improvements and excluding required mitigation improvements) exceeds 50% of the assessed value of: (a) the existing project site improvements on commercial or industrial projects, or (b) the existing site improvements on other projects. Exceptions The following exceptions apply only in Basic WQ Treatment Areas: 1. Exception #1 does not apply to the City 2. The Enhanced Basic WQ menu as specified above for certain land uses may be reduced to the Basic WQ menu for treatment of any runoff that is infiltrated per the standards of Section 5.2. This exception is not allowed where infiltrating into soils that do not meet the groundwater protection standards described in Section 5.2.1, if within one-quarter-mile of a fresh water designated for aquatic life use or that has an existing aquatic life use. 3. The Enhanced Basic WQ menu as specified above for certain land uses may be reduced to the Basic WQ menu for treatment of any runoff that is discharged directly, via a non-fish-bearing conveyance system, all the way to the ordinary high water mark of a stream with a mean annual flow of 1,000 cfs or more (at the discharge point of the conveyance system), a lake that is 300 acres or larger, or a waterbody that is listed as a major receiving water per Table 1.2.3.B. This exception does not apply where the receiving water is impaired for metals per Section 1.2.2.1: Downstream Analysis, and 1.2.2.1.2: Downstream Water Quality Problems Requiring Special Attention, Metals Problem (Type 4). 4. The Enhanced Basic WQ menu as specified above for treating runoff from a commercial land use may be reduced to the Basic WQ menu if all of the following criteria are met: a) A facility from the Enhanced Basic WQ menu is not feasible, AND b) No leachable heavy metals are currently used or proposed to be used in areas of the site, exposed to the weather, AND c) A covenant is recorded that prohibits future such use of leachable, heavy metals on the site (use the covenant in Reference Section 8-Q), AND d) Less than 50% of the runoff draining to the proposed water quality facility is from any area of the site comprised of one or both of the following land uses:  Commercial land use with an expected ADT of 100 or more vehicles per 1,000 square feet of gross building area.  Commercial land use involved with vehicle repair, maintenance, or sales. 5. The facility requirement as applied to replaced PGIS may be waived if the City has adopted a plan and implementation schedule for fulfilling this requirement using regional facilities. SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-68 B. SENSITIVE LAKE WQ TREATMENT AREAS There are no Sensitive Lake WQ Treatment Areas in the City at the time this manual was adopted; however, this section has been retained in case of future changes in lake status. Required Treatment Menu Within Sensitive Lake WQ Treatment Areas, a water quality facility option from the Sensitive Lake Protection menu shall be used to treat runoff from the surfaces listed under “Target Surfaces” below, except where such treatment is waived or reduced by the area-specific exceptions at the end of this subsection and except where the Enhanced Basic WQ menu is applicable as follows. If 50% or more of the runoff that drains to any proposed water quality facility is from one or more of the following land uses, then a water quality facility option common to both the Sensitive Lake Protection menu and Enhanced Basic WQ menu shall be used for the design of this facility, except if such treatment is waived or reduced by the area-specific exceptions at the end of this subsection: 1. Commercial, industrial, or multifamily land use. 2. A road with an expected average daily traffic (ADT) count of 7,500 or more vehicles. Treatment Goal and Options The treatment goal for facility options in the Sensitive Lake Protection menu is 50% annual average total phosphorus (TP) removal assuming typical pollutant concentrations in urban runoff.35 This goal was chosen as a realistic and cost-effective level of phosphorus removal. The Sensitive Lake Protection menu includes options for using either Basic WQ facilities of larger size, combinations of two facilities in series,36 or a single facility in combination with land use planning elements that reduce phosphorus. See Chapter 6 for specific facility options and design details. On some developments or portions thereof that have surface uses that generate the highest concentrations of metals in stormwater runoff, the treatment goal is expanded to include > 30% reduction of dissolved copper and > 60% removal of dissolved zinc. This expanded goal requires use of a water quality facility option that is common to both the Sensitive Lake Protection menu and the Enhanced Basic menu. Intent A project discharging runoff via surface flow contributes phosphorus loading to a sensitive lake regardless of distance from the lake. If discharge is via infiltration through coarse soils, it is also possible that phosphorus would be transported through the ground for some distance without attenuation. This groundwater transport distance is considered to be typically no more than one-quarter mile. Therefore, onsite treatment using the Sensitive Lake Protection menu is required prior to infiltration within one- quarter mile of a sensitive lake. Infiltration through finer soils is expected to provide significant attenuation of TP, so the general groundwater protection criteria specified in Section 1.2.8 under “Soil Treatment Exemption” are considered sufficient for infiltration through finer soils. Where the treatment goal is expanded to include > 30% reduction of dissolved copper and > 60% removal of dissolved zinc, the facility options common to both the Sensitive Lake Protection menu and the Enhanced Basic WQ menu should meet this goal as well as the lake protection goal of 50% removal of annual average total phosphorous. The intent behind the enhanced heavy metals removal goal and why it is applied is described in Section 1.2.8.1. 35 Phosphorus concentrations of between 0.10 and 0.50 mg/L are considered typical of Seattle area runoff (Table 1, “Water Quality Thresholds Decision paper,” King County Surface Water Management Division, April 1994). 36 In series means that the entire treatment water volume flows from one facility to the other in turn. 1.2.8 CORE REQUIREMENT #8: WATER QUALITY FACILITIES 2022 City of Renton Surface Water Design Manual 6/22/2022 1-69 Target Surfaces Facilities in Sensitive Lake WQ Treatment Areas must mitigate (either directly or in effect) the runoff from the following target surfaces within the threshold discharge area for which the facility is required: 1. New PGIS that is not fully dispersed per the Criteria for Fully Dispersed Surfaces (see Section 1.2.3.2.C) in Core Requirement #3. For individual lots within residential subdivision projects, the extent of new PGIS shall be assumed based on expected driveway size as approved by CED. 2. New PGPS that is not fully dispersed and from which there will be a concentrated surface discharge in a natural channel or man-made conveyance system from the site. For individual lots within residential subdivision projects, the extent of new pervious surface shall be assumed to be the entire lot area, except the assumed impervious portion as specified in Chapter 3 and any portion in which native conditions are preserved by covenant, tract, or easement. Note: where the runoff from target PGPS is separated from the runoff from target PGIS, the Basic WQ menu may be used in place of the Sensitive Lake Protection menu for treatment of runoff from the target PGPS (see the area-specific exceptions at the end of this subsection). 3. Replaced PGIS that is not fully dispersed, on a non-redevelopment project. 4. Replaced PGIS that is not fully dispersed on a transportation redevelopment project in which new impervious surface is 5,000 square feet or more and totals 50% or more of the existing impervious surface within the project limits. 5. Replaced PGIS that is not fully dispersed, on a parcel redevelopment project in which the total of new plus replaced impervious surface is 5,000 square feet or more and whose valuation of proposed improvements (including interior improvements and excluding required mitigation improvements) exceeds 50% of the assessed value of: (a) the existing project site improvements on commercial or industrial projects, or (b) the existing site improvements on other projects. Exceptions The following exceptions apply only in Sensitive Lake WQ Treatment Areas: 1. The Basic WQ menu may be used in place of the Sensitive Lake Protection menu for treatment of any runoff that is infiltrated according to the standards in Section 5.2. This exception is not allowed where infiltrating into soils that do not meet the groundwater protection standards described in Section 5.2.1, if within one-quarter-mile of a phosphorous sensitive receiving water or a tributary to that receiving water. 2. Application of the Enhanced Basic WQ menu as specified above for certain land uses may be waived for treatment of any runoff that is infiltrated according to the standards in Section 5.2 (A facility from the Sensitive Lake Protection menu is still required unless that requirement has been reduced to the Basic WQ Menu by another exception). This exception is not allowed where infiltrating into soils that do not meet the groundwater protection standards described in Section 5.2.1, if within one-quarter-mile of a fresh water designated for aquatic life use or that has an existing aquatic life use. 3. Application of the Enhanced Basic WQ menu as specified above for certain land uses may be waived for treatment of any runoff that is discharged, via a non-fish-bearing conveyance system, all the way to the ordinary high water mark of a stream with a mean annual flow of 1,000 cfs or more (at the discharge point of the conveyance system), a lake that is 300 acres or larger, or a waterbody that is listed as a major receiving water per Table 1.2.3.B (A facility from the Sensitive Lake Protection menu is still required unless that requirement has been reduced to the Basic WQ Menu by another exception). This exception is not applicable for WQ impaired segments per Section 1.2.2.1: Downstream Analysis, and 1.2.2.1.2: Downstream Water Quality Problems Requiring Special Attention, Metals Problem (Type 4). 4. The Enhanced Basic WQ menu as specified above for treating runoff from a commercial land use may be waived (A facility from the Sensitive Lake Protection menu is still required unless that SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-70 requirement has been reduced to the Basic WQ Menu by another exception) if the all of the following criteria are met: a) No leachable metals (e.g., galvanized metals) are currently used or proposed to be used in areas of the site, exposed to the weather, AND b) A covenant is recorded that prohibits future such use of leachable metals on the site, exposed to the weather (use the covenant in Reference Section 8-Q), AND c) Less than 50% of the runoff draining to the proposed water quality facility is from any area of the site comprised of one or both of the following land uses:  Commercial land use with an expected ADT of 100 or more vehicles per 1,000 square feet of gross building area.  Commercial land use involved with vehicle repair, maintenance, or sales. 5. The Basic WQ menu may be used for treatment of any runoff from target PGPS that is treated separately from the runoff from target PGIS. 6. Exception #6 does not apply in the City. 7. The facility requirement as applied to replaced PGIS may be waived if the City has adopted a plan and implementation schedule for fulfilling this requirement using regional facilities. Note: If a lake management plan has been prepared and adopted by the City, additional treatment and/or other water quality measures may be required as specified in the plan and pursuant to Special Requirement #1, Section 1.2.9. C. SPHAGNUM BOG WQ TREATMENT AREAS There are no Sphagnum Bog WQ Treatment Areas in the City at the time this manual was adopted; however, this section has been retained in case of future changes. Sphagnum Bog WQ Treatment Areas are areas of King County from which runoff drains to or otherwise comes into contact with the vegetation of a sphagnum bog wetland37 larger than 0.25 acres in size.38 These wetlands support unique vegetation communities, and they tend to develop in areas where water movement is minimized. Although sphagnum bog wetlands are typically isolated from significant sources of surface and ground water and receive their main water supply from rainfall, there are instances where they are components of larger wetlands and may be subject to inundation by those wetlands during high intensity or long duration runoff events. Sphagnum bog wetlands are generally uncommon in the Puget Sound area; of all the inventoried wetlands in King County, only a small percentage have sphagnum bog wetland components.39 Only a portion of all sphagnum bog wetlands have been identified and mapped by King County. Consequently, many of these wetlands and their contributing drainage areas must be identified during the wetland identification and delineation for a project site and during offsite analysis as required in Core Requirement #2. A list of identified sphagnum bog wetlands is included on the WQ Applications Map and in the 1997 King County Bog Inventory, updated November 2002, available at http://your.kingcounty.gov/dnrp/library/2002/kcr249-2002.pdf ; however, if a wetland that meets the definition of a sphagnum bog wetland is found downstream of a project site and runoff from the project site drains to or otherwise comes into contact with the wetland’s vegetation, the project site is considered to be within a Sphagnum Bog WQ Treatment Area whether the wetland is listed or not. 37 A sphagnum bog wetland is defined as a wetland dominated by sphagnum moss and which has an associated acid-loving plant community. See the "Definitions" section for more details on how King County defines a sphagnum bog wetland. 38 The size of a sphagnum bog wetland is defined by the boundaries of the sphagnum bog plant community. 39 Approximately 3% of wetlands in the 1990 sensitive areas inventory are either sphagnum bog wetlands or include portions of a lake or wetland with sphagnum bog wetland characteristics. 1.2.8 CORE REQUIREMENT #8: WATER QUALITY FACILITIES 2022 City of Renton Surface Water Design Manual 6/22/2022 1-71 Note: Any threshold discharge area from which runoff drains to or comes into contact with the vegetation of a sphagnum bog wetland larger than 0.25 acres in size is considered to be within a Sphagnum Bog WQ Treatment Area regardless of the WQ treatment area indicated by the WQ Applications Map. Required Treatment Menu A treatment option from the Sphagnum Bog Protection menu shall be used to treat runoff from the target surfaces specified below, except where this mitigation is waived or reduced by the area-specific exceptions at the end of this subsection. Treatment Goals and Options The treatment goals for protection of sphagnum bog wetlands include the control of nutrients, alkalinity, and pH. Although these goals may change as additional information about these wetlands becomes available, target pollutant removals for sphagnum bog protection are currently as follows:  Total phosphorus reduction of 50%  Nitrate + nitrite reduction of 40%  pH below 6.5  Alkalinity below 10 mg CaCO3/L. Facility options to meet these goals are limited; therefore, the City discourages developments from discharging runoff to sphagnum bog wetlands. Where infiltration of developed area runoff is not feasible or applicable per Section 5.2, water quality facility options include a treatment train 40 of two or three facilities in series. One of the facilities in the train must be a sand filter. The order of facilities in the treatment train is important; see Chapter 6 for specific facility options and design details. Intent Sphagnum bog wetlands support unique vegetation communities that are extremely sensitive to changes in alkalinity and nutrients from surface water inputs. The most effective way to prevent these changes is to infiltrate or redirect developed area runoff so it does not come into contact with the vegetation of a sphagnum bog wetland. However, this is not practicable for most development projects due to soil constraints precluding infiltration (see Section 5.2) and the onerous nature of bypassing runoff around a wetland. Therefore, where runoff contact with sphagnum bog vegetation cannot be avoided, the bog protection menu seeks to minimize certain changes in the chemistry of developed area runoff to protect this unique vegetation. This menu applies not only to runoff that drains directly to a sphagnum bog wetland but to runoff that otherwise comes into contact with the bog’s vegetation, such as through inundation of the bog by an adjacent water body during high intensity or long duration runoff events. While water quality facility options emphasize reduction of mineral elements (alkalinity) and nutrients in the runoff, little is known about their ability to reduce alkalinity or to actually protect sphagnum-based plant communities. In addition, the effect of frequent water level changes on the sphagnum plant community is also unknown but could be damaging. Hence, it is best to avoid discharge to sphagnum bog wetlands whenever possible. Permeable pavements that are tributary to sphagnum bog wetlands should be types other than Portland cement (PCC) permeable pavement, if feasible. Target Surfaces Facilities in Sphagnum Bog WQ Treatment Areas must mitigate (either directly or in effect) the runoff from the following target surfaces within the threshold discharge area for which the facility is required: 1. New PGIS that is not fully dispersed per the Criteria for Fully Dispersed Surfaces (p. 1-46) in Core Requirement #3. For individual lots within residential subdivision projects, the extent of new PGIS shall be assumed based on expected driveway size as approved by CED. 40 A treatment train is a combination of two or more treatment BMPs connected in series (i.e., the design water volume passes through each facility in turn). SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-72 2. New PGPS that is not fully dispersed and from which there will be a concentrated surface discharge in a natural channel or man-made conveyance system from the site. For individual lots within residential subdivision projects, the extent of new pervious surface shall be assumed to be the entire lot area, except the assumed impervious portion as specified in Chapter 3 and any portion in which native conditions are preserved by covenant, tract, or easement. 3. Replaced PGIS that is not fully dispersed, on a non-redevelopment project. 4. Replaced PGIS that is not fully dispersed on a transportation redevelopment project in which new impervious surface is 5,000 square feet or more and totals 50% or more of the existing impervious surface within the project limits. 5. Replaced PGIS that is not fully dispersed on a parcel redevelopment project in which the total of new plus replaced impervious surface is 5,000 square feet or more and whose valuation of proposed improvements (including interior improvements and excluding required mitigation improvements) exceeds 50% of the assessed value of: (a) the existing project site improvements on commercial or industrial projects, or (b) the existing site improvements on other projects. Exceptions The following exceptions apply only in Sphagnum Bog WQ Treatment Areas: 1. The Basic WQ menu may be used in place of the Sphagnum Bog Protection menu for treatment of any runoff that is infiltrated in a facility per Section 5.2. This exception is not allowed where infiltrating into soils that do not meet the groundwater protection standards described in Section 5.2.1, if within one-quarter-mile of a phosphorous sensitive receiving water or a tributary to that receiving water. If the infiltration facility is located in soils not meeting the groundwater protection standards described in Section 5.2.1, and within the prescribed distance of a sensitive lake, then the Sensitive Lake Protection menu shall be used. 2. The facility requirement for Sphagnum Bog WQ Treatment Areas may be reduced to that of the surrounding WQ treatment area (i.e., either the Basic WQ Treatment Area or Sensitive Lake Treatment Area, whichever contains the Sphagnum Bog WQ Treatment Area) for treatment of any replaced PGIS runoff. 1.2.8.2 WATER QUALITY IMPLEMENTATION REQUIREMENTS Water quality facilities shall be designed and implemented in accordance with the following requirements, allowances, and flexible compliance provisions: A. METHODS OF ANALYSIS AND DESIGN Water quality facilities shall be analyzed and designed as detailed in Chapter 6. B. SITING OF WATER QUALITY FACILITIES Required water quality facilities shall be located so as to treat the runoff from all target surfaces, except as allowed below under “Treatment Trades” and “Untreated Discharges.” Any other onsite or offsite runoff draining to a proposed water quality facility must be treated whether it is from a target pollution-generating surface or not and regardless of whether the runoff has already been treated by another facility. The facility must be sized for all flows/volumes entering the facility. This is because treatment effectiveness is determined in part by the total volume of runoff entering the facility. C. TREATMENT TRADES The runoff from target pollution-generating surfaces may be released untreated if an existing non- targeted pollution-generating surface of equivalent size and pollutant characteristics lying within the same watershed or stream reach tributary area is treated on the project site. Such substitution is subject to all of the following restrictions: 1.2.9 CORE REQUIREMENT #9: ON-SITE BMPS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-73 1. The existing non-targeted pollution-generating surface is not currently being treated, is not required to be treated by any phase of the proposed project, is not subject to NPDES or other permit requirements, and is not under a compliance order or other regulatory action. 2. The existing non-targeted pollution-generating surface that is treated for purposes of the treatment trade must be documented and tracked by CED. Documentation should clarify that future redevelopment of the existing non-targeted, treated area used for the treatment trade will incur additional water quality treatment requirements if the redevelopment exceeds Core Requirement #8 thresholds. Any additional water quality treatment triggered by redevelopment of the non-targeted, treated area must be achieved by implementing an additional treatment trade. 3. The proposal is reviewed and approved by CED. D. UNTREATED DISCHARGES If site topographic constraints are such that runoff from a target pollution-generating surface must be pumped to be treated by the required water quality facility, then CED may allow the area’s runoff to be released untreated provided that all of the following conditions are met: 1. Treatment of the constrained area by filter strip, bioswale, or a linear sand filter is not feasible, and a treatment trade as described above is not possible, AND 2. The untreated target surface is less than 5,000 square feet of new plus replaced PGIS. E. USE OF PROPRIETARY FACILITIES Water quality facilities other than those identified in Chapter 6, Reference Section 14-A, or Reference Section 14-B may be allowed if it can be demonstrated that they are likely to meet the pollutant removal goal for the applicable receiving water. Use of such facilities requires an adjustment, which requires approval by the City according to Section 1.4, “Adjustment Process,” and Section 6.7, “Alternative Facilities.” Any new treatment technologies must be approved through the state Department of Ecology’s TAPE41 program before the technology can be considered by the City. Monitoring will be required, the nature of which will depend on the pre-existing Ecology use-level designation, the number of existing facilities of this design for which monitoring data already exists, and review of the monitoring results from those facilities. When sufficient data on performance and maintenance requirements have been collected and if both are acceptable, the new facility may be added to the appropriate water quality menu for common use through a blanket adjustment or update of this manual. Criteria may be set, which if not met, may require replacement of the facility with a standard facility from Chapter 6. F. OWNER RESPONSIBILITY FOR WATER QUALITY Regardless of the means by which a property owner chooses to meet the water quality requirements of this manual – whether a water quality facility, a train of facilities, or a treatment trade – it is the responsibility of the property owner to ensure that runoff from their site does not create water quality problems or degrade beneficial uses downstream. It is also the responsibility of the property owner to ensure that the discharge from their property is not in violation of state and federal laws. 1.2.9 CORE REQUIREMENT #9: ON-SITE BMPS All proposed projects, including redevelopment projects, must provide on-site BMPs to mitigate the impacts of storm and surface water runoff generated by new impervious surface, new pervious surface, existing impervious surfaces, and replaced impervious surface targeted for mitigation as specified in the following sections. On-site BMPs must be selected and applied according to the basic requirements, 41 Ecology W, 2011. Technical Guidance Manual for Evaluating Emerging Stormwater Treatment Technologies: Technology Assessment Protocol – Ecology (TAPE), Publication No. 11-10-061, 2011 ed. Washington State Department of Ecology, Lacey, WA, pp. 1–73. <https://fortress.wa.gov/ecy/publications/summarypages/1110061.html>. SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-74 procedures, and provisions detailed in this section and the design specifications for each BMP in Appendix C, Section C.2. On-site BMPs are methods and designs for dispersing, infiltrating, or otherwise reducing or preventing development-related increases in runoff at or near the sources of those increases. On-site BMPs include, but are not limited to, preservation and use of native vegetated surfaces to fully disperse runoff; use of other pervious surfaces to disperse runoff; roof downspout infiltration; permeable pavements; bioretention; limited infiltration systems; and reduction of development footprint. Intent: To provide mitigation of hydrologic impacts that are not possible/practical to mitigate with a flow control facility. Such impacts include increases in runoff volumes and peak discharges and decreases in groundwater recharge. Increased runoff volume and peak discharges leads to higher and more variable stream velocities at low flows and more frequent water level fluctuations in streams and wetlands. This causes wash-out and stranding of aquatic species, algal scour and washout of organic matter, loss of vegetation diversity and habitat quality, and disruption of cues for spawning, egg hatching, and migration. Decreased groundwater recharge reduces water supply for human use and summer base flows in streams, which is critical to water temperature, salmonid use of smaller streams, and the habitat quality of mainstem side channels and wetlands used for spawning, rearing, and flood refuge. On-site BMPs seek to reduce runoff volumes and flashiness and increase groundwater recharge by reducing imperviousness and making use of the pervious portions of development sites to maximize infiltration and retention of stormwater onsite. Thus, the goal is to apply on-site BMPs to new impervious surfaces, new pervious surfaces, and replaced impervious surfaces, to the maximum extent feasible without causing flooding or erosion impacts.  EXEMPTIONS FROM CORE REQUIREMENT #9 There are two exemptions from the on-site BMP provisions of Core Requirement #9: 1. Basic Exemption A proposed project is exempt if it meets the following criteria: a) Less than 2,000 square feet of new plus replaced impervious surface will be created, AND b) Less than 7,000 square feet of land disturbing activity will occur. 2. Infiltration Flow Control Facility Exemption Any impervious surface served by an infiltration facility designed in accordance with the flow control facility requirement (Section 1.2.3.1), the facility implementation requirements (Section 1.2.3.2), and the design criteria for infiltration facilities (Section 5.2) is exempt from the on-site BMP requirement. 1.2.9.1 ON-SITE BMP REQUIREMENTS OVERVIEW Projects that are subject to Core Requirement #9 must apply on-site BMPs to either supplement the flow mitigation provided by required flow control facilities or provide flow mitigation where flow control facilities are not required. All such on-site BMPs are detailed in Appendix C of this manual. On-site BMPs must be implemented per the requirements and approach detailed in Sections 1.2.9.2 and 1.2.9.3 below for individual lots and subdivisions or road improvement projects, respectively. As described within Sections 1.2.9.2 and 1.2.9.3, there are two methods of satisfying the on-site BMP requirement: (1) application of BMPs to the maximum extent feasible using lists specific to the project location, size, and impervious coverage; or (2) using a continuous runoff model to demonstrate compliance with the Low Impact Development (LID) Performance Standard, described below. Demonstrating compliance with the LID Performance Standard using modeling is an optional method for all projects. A. TARGET SURFACES Target surfaces for application of Core Requirement #9 (On-site BMPs) include new impervious surfaces, new pervious surfaces, and replaced impervious surfaces. 1.2.9 CORE REQUIREMENT #9: ON-SITE BMPS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-75 Projects that trigger Core Requirement #9 by disturbing 7,000 square feet or more of land, but where new plus replaced impervious surface is less than 2,000 square feet, may consider basic dispersion as an equal choice for treating the target impervious surfaces alongside full infiltration, limited infiltration, bioretention, and permeable pavement. These projects are not required to meet the minimum BMP implementation requirements described in “Small Lot BMP Requirements” and “Large Lot BMP Requirements,” (Requirement #5 on both lists), and are not required to comply with Core Requirement #6 . Target pervious surfaces must be protected in accordance with the soil amendment BMP as detailed in Appendix C, Section C.2.13. Projects or threshold discharge areas of projects qualifying as exempt from the flow control facility requirement using the Direct Discharge Exemption in accordance with Section 1.2.3.1 do not have to achieve the LID Performance Standard (described below), nor consider bioretention, permeable pavement, and full dispersion. However, target pervious surfaces must be protected in accordance with the soil amendment BMP as detailed in Appendix C, Section C.2.13; and target impervious surfaces must implement full infiltration as detailed in Appendix C, Section C.2.2, Basic Dispersion per Appendix C, Section C.2.4; perforated pipe connection as detailed in Appendix C, Section C.2.11 for roofs, if feasible; and Basic Dispersion per Appendix C, Section C.2.4 for other impervious surfaces, if feasible. B. LOW IMPACT DEVELOPMENT PERFORMANCE STANDARD The LID Performance Standard is defined as follows: For the target surfaces subject to Core Requirement #9, Stormwater discharges shall match developed discharge durations to pre-developed durations for the range of pre-developed discharge rates from 8% of the 2-year peak flow to 50% of the 2-year peak flow. Assume historical site conditions as the predeveloped condition. Projects that opt to demonstrate compliance with the LID Performance Standard using a continuous runoff model must protect the soil moisture capacity of new pervious in accordance with the soil amendment BMP as detailed in Appendix C, Section C.2.13. Additionally, 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. Projects that are required or opt to model compliance with the LID Performance Standard are still subject to meeting applicable area specific flow control requirements as determined in Core Requirement #3 (Section 1.2.3). Note that when demonstrating compliance with the LID Performance Standard, on-site BMPs are modeled explicitly, utilizing design infiltration rates as determined and selected per Section 5.2.1. However, when modeling flow control facility sizing, water quality facility sizing, and the peak flow exceptions from the area-specific flow control facility requirement in Sections 1.2.3.1.A, B, and C, these BMPs are not modeled explicitly, but may use modeling credits as allowed and subject to the limitations described in Section 1.2.9.4 and Table 1.2.9.A. On-site BMPs used to demonstrate compliance with the LID Performance Standard must meet the implementation requirements described in Section 1.2.9.4. C. DEMONSTRATING COMPLIANCE WITH THE LID PERFORMANCE STANDARD Project applicants may opt to use the LID Performance Standard in lieu of the BMP selection and application requirements described in Sections 1.2.9.2 and 1.2.9.3 below. D. IMPLEMENTATION Four kinds of implementation for the on-site BMP requirement are described in this section as follows: 1. For non-subdivision projects making improvements on an individual site/lot, implementation of this requirement shall be in accordance with the “Individual Lot BMP Requirements” in Section 1.2.9.2, which specify the selection of BMPs and the extent of their application on the site/lot. This required implementation of on-site BMPs must occur as part of the proposed project and provisions SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-76 must be made for their future maintenance as specified in Section 1.2.9.2. As allowed in Sections 1.2.3 and 1.2.8, credits for the application of on-site BMPs per Table 1.2.9.A may be used to reduce the size of a required flow control facility, reduce the size of a water quality facility, qualify for a flow control facility exception or bypass of target surfaces, or reduce the target surfaces subject to flow control or water quality facility requirements. 2. Subdivision projects and road improvement projects on sites that are 5 acres or larger AND located outside the Urban Growth Area (UGA). This requirement does not apply to the City of Renton. 3. For subdivision projects, implementation of on-site BMPs for associated plat infrastructure improvements (e.g., roads, sidewalks) shall be done per Section 1.2.9.3 and must occur concurrently and as part of the proposed project, while BMPs associated with the individual lot improvements may be delayed until construction on the lots. As allowed in Sections 1.2.3 and 1.2.8, credits for the application of on-site BMPs per Table 1.2.9.A. may be used to reduce the size of a required flow control facility, reduce the size of a water quality facility, qualify for a flow control facility exception or bypass of target surfaces, or reduce the target surfaces subject to flow control or water quality facility requirements. To use these credits, on-site BMPs must be implemented as part of the proposed project and provisions must be made for their future maintenance as specified in Section 1.2.9.4. For subdivision projects proposing to take credit for future implementation of BMPs on individual lots, provisions must be made to ensure their implementation as specified in Section 1.2.9.4. 4. For road improvement projects, implementation of on-site BMPs must occur as part of the proposed project. As allowed in Sections 1.2.3 and 1.2.8, credits for the application of on-site BMPs per Table 1.2.9.A may be used to reduce the size of a required flow control facility, reduce the size of a water quality facility, qualify for a flow control facility exception or bypass of target surfaces, or reduce the target surfaces subject to flow control or water quality facility requirements. To use these credits, on-site BMPs must be implemented as part of the proposed project and provisions must be made for their future maintenance as specified in Section 1.2.9.4. The information presented in this section is organized as follows:  Section 1.2.9.2, “Individual Lot BMP Requirements” “Small Lot BMP Requirements,” Section 1.2.9.2.1 “Large Lot BMP Requirements,” Section 1.2.9.2.2 “Large Rural Lot BMP Requirements,” Section 1.2.9.2.3 “Implementation Requirements for Individual Lot BMPs,” Section 1.2.9.2.4  Section 1.2.9.3, “Subdivision and Road Improvement Projects BMP Requirements” “Small Subdivision Project BMP Requirements,” Section 1.2.9.3.1 “Small Road Improvement and Urban Road Improvement Projects BMP Requirements,” Section 1.2.9.3.2 “Large Rural Subdivision and Large Rural Road Improvement Projects BMP Requirements,” Section 1.2.9.3.3  Section 1.2.9.4, “Requirements for Use of BMP Credits” “Use of Credits by Subdivision Projects,” Section 1.2.9.4.1 “Use of Credits by Projects within Rights-of-Way,” Section 1.2.9.4.2 1.2.9.2 INDIVIDUAL LOT BMP REQUIREMENTS For projects on individual sites/lots, on-site BMPs must be selected and applied according to the individual lot BMP requirements in this section. For purposes of applying on-site BMPs to individual 1.2.9 CORE REQUIREMENT #9: ON-SITE BMPS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-77 sites/lots, three categories of requirements have been established based on the size of site/lot subject to improvements by the project, and the extent of impervious surface coverage resulting from the project on the site/lot. These categories of requirements are as follows:  Small Lot BMP Requirements (for sites/lots <22,000 square feet)  Large Lot BMP Requirements (for sites/lots 22,000 square feet) On-site BMPs must be applied in the order of preference and to the extent specified for the category of individual lot requirements applicable to the proposed project as described in the following subsections. Note: for lots created by a previous subdivision, some or all of these requirements may have been addressed by on-site BMPs installed on the lots or within common areas, tracts, or road right-of-way. In some cases, the type of BMPs required for a subdivision lot have already been established by a recorded covenant on the lot. See Section 1.2.9.4 for more information on pre-installed or pre-determined BMPs in subdivisions. 1.2.9.2.1 SMALL LOT BMP REQUIREMENTS IF the proposed project is on a site/lot smaller than 22,000 square feet, THEN on-site BMPs must be applied as specified in the requirements below OR the project must demonstrate compliance with the LID Performance Standard (described in Section 1.2.9.1.B) using an approved continuous runoff model. Projects on small lots are typically single family residential improvements (e.g., homes, outbuildings, etc.) but could be a small commercial development. 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 design requirements for full dispersion in Appendix C, Section C.2.1.1 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. 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  Limited Infiltration per Appendix C, Section C.2.3,  Rain Gardens per Appendix C, Section C.2.12, sized as follows: o Rain gardens have a maximum contributing area of 5,000 square feet. o Rain gardens must have a minimum horizontal projected surface area below the overflow that is at least 5% of the area draining to it.  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,  Permeable Pavement per Appendix C, Section C.2.7 SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-78 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, 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.  Tree Retention Credit per Appendix C, Section C.2.14 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. 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. 1.2.9.2.2 LARGE LOT BMP REQUIREMENTS IF the proposed project is on a site/lot that is 22,000 square feet or larger, THEN on-site BMPs must be applied as specified in the requirements below OR the project must demonstrate compliance with the LID Performance Standard (described in Section 1.2.9.1.B) using an approved continuous runoff model. 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 for any such surface, then full dispersion must be applied to that surface and implemented as part of the proposed project. Typically, full dispersion will be applicable only on the largest sites/lots where there may be enough forest area available within a threshold discharge area to meet the 15% ratio of fully dispersed impervious area to native vegetated surface. 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 of roof runoff must be evaluated in accordance with Appendix C, Section C.2.2, or Section 5.2, whichever is applicable based on the type of project.42 If feasible and applicable, full infiltration of roof runoff must be implemented as part of the proposed project. 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. 42 For projects subject to Simplified Drainage Review, and for any single family residential project subject to Full or Large Project Drainage Review, the design requirements and specifications in Appendix C, Section C.2.2 may be used for evaluation and design of full infiltration on individual lots. For all other projects, full infiltration must be evaluated and designed in accordance with the infiltration facility standards in Section 5.2. 1.2.9 CORE REQUIREMENT #9: ON-SITE BMPS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-79  Full Infiltration per Section C.2.2, or per Section 5.2, whichever is applicable  Limited Infiltration per Appendix C, Section C.2.3  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,  Permeable Pavement per Appendix C, Section C.2.7 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 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 5. BMPs must be implemented, at minimum, for impervious area amounts defined as follows.  For projects that will result in an impervious surface coverage on the buildable portion of the site/lot of less than 45%, on-site BMPs must be applied to 50% of target impervious surfaces.  For projects that will result in an impervious surface coverage 45-65% on the buildable portion of the site/lot, on-site BMPs must be applied to 50% of target impervious surfaces reduced by 1.5% for each 1% of impervious surface coverage above 45% (e.g., impervious coverage of 55% results in a requirement of on-site BMPs applied to 35% of target impervious surfaces).  For projects that will result in an impervious surface coverage greater than 65% on the buildable portion of the site/lot, on-site BMPs must be applied to 20% of the target impervious surfaces or to an impervious area equal to at least 10% of the site/lot, whichever is less. The buildable portion of the site/lot is the total area of the site/lot minus any critical areas and minus 200 ft. buffer areas from a steep slope hazard, landslide hazard, or erosion hazard area. If these minimum areas are not mitigated using feasible BMPs from Requirements 1, 2, 3, and 4 above, one or more BMPs from the following list are required to be implemented to achieve compliance. These BMPs must be implemented as part of the proposed project.  Reduced Impervious Surface Credit per Appendix C, Section C.2.9,  Native Growth Retention Credit per Appendix C, Section C.2.10,  Tree Retention Credit per Appendix C, Section C.2.14. 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. 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. 1.2.9.2.3 LARGE RURAL LOT BMP REQUIREMENTS This requirement does not apply in the City of Renton. SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-80 1.2.9.2.4 43IMPLEMENTATION REQUIREMENTS FOR INDIVIDUAL LOT BMPS The on-site BMPs required in Sections 1.2.9.2 above must be implemented in accordance with the following requirements: 1. Implementation Responsibility. All on-site BMPs required for the site/lot must be implemented (installed) by the applicant as part of the proposed project unless they have already implemented as part of a subdivision project that created the lot per Section 1.2.9.4. 2. Maintenance Responsibility. Maintenance of all required on-site BMPs is the responsibility of the owner of the site/lot served by these BMPs. The responsibility for such maintenance must be clearly assigned to the current and future owners of the site/lot through a “declaration of covenant and grant of easement” as described in Requirement 3 below. 3. Declaration of Covenant and Grant of Easement. To ensure future maintenance of on-site BMPs and allow for City inspection of BMPs, a declaration of covenant and grant of easement must be recorded for each site/lot that contains on-site BMPs. A draft of the proposed covenant must be reviewed and approved by CED prior to recording. All required covenants must be recorded prior to final construction approval for the proposed project. If the individual site/lot contains or will contain flow control or water quality facilities, then the drainage facility covenant in Reference Section 8-J (or equivalent) must be used, and is designed to achieve the following: a) Provide notice to future owners of the presence of on-site BMPs on the lot and the responsibility of the owner to retain, uphold, and protect the on-site BMPs, features, pathways, limits, and restrictions. b) Include as an exhibit, a recordable version44 of the following drainage plan information:  The site plan showing all developed surfaces (impervious and pervious) and the location and dimensions of on-site BMPs, features, flowpaths (if applicable), limits of native growth retention areas (if applicable), and limits of tree retention areas (if applicable). This plan(s) must be to scale and include site topography in accordance with the specifications for such plans in Appendix C, Section C.4.2. Also indicate any areas where City access is excluded (see paragraph 3.d below). Note: CED may waive this element if, for example, the only on- site BMP proposed is a limit on impervious surface (reduced footprint).  The on-site BMP design and maintenance details for each on-site BMP per Appendix C, Section C.4.3. This includes a diagram (if applicable) of each on-site BMP and written maintenance and operation instructions and restrictions for each device, feature, flowpath (if applicable), native growth retention area (if applicable) and impervious surface coverage (if applicable). See Reference Section 8-M for prepared 8-1/2″ x 11″ maintenance instruction sheets. See City of Renton’s Surface Water Design Standards web site: <www.rentonwa.gov/swdm> for downloadable BMP details. Ensure the exhibits are correctly cross-referenced in the declaration of covenant (the site plan is typically Exhibit A and the design/maintenance details are typically Exhibit B). c) Require that each on-site BMP be operated and maintained at the owner’s expense in accordance with the above exhibit. d) Grant the City the right to enter the property at reasonable times for purposes of inspecting the on-site BMPs and to perform any corrective maintenance, repair, restoration, or mitigation work on the on-site BMPs that has not been performed by the property owner within a reasonable time 43 Footnote 48 is not used. 44 Recordable version means one that meets King County’s “Standard Formatting Requirements for Recording Documents” pursuant to RCW 36.18.010 and 65.04.045, available online at < https://kingcounty.gov/~/media/depts/records- licensing/recorders-office/documents/Requirements_WAState_Formatting.ashx?la=en> or from the King County Recorder’s Office. These requirements include specifications for such things as page size (81/2″ x 14″ or smaller), font size (at least 8- point), and margin width (1″ on all sides of every page if there is a standard cover sheet). 1.2.9 CORE REQUIREMENT #9: ON-SITE BMPS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-81 set by CED, and to charge the property owner for the cost of any maintenance, repair, restoration, or mitigation work performed by the City. e) The right to enter typically applies to the entire property, but occasionally accepts areas on the property agreed upon by the City to be excluded from access. Such areas are to be shown on the site plan described above. f) Prohibit any modification or removal of on-site BMPs without written approval from the City. The approval must be obtained from CED and a covenant must be recorded to reflect the changes. Approval will be granted only if equivalent protection in terms of hydrologic performance is provided by other means. 4. Timing of Implementation. All required on-site BMPs must be installed prior to final inspection approval of constructed improvements. For BMPs that rely on vegetation, the vegetation must be planted and starting to grow prior to final construction approval. 5. Acceptance standards. On-site BMPs may be inspected during and/or following construction. Approval of the constructed BMPs will be based on verification that the materials and placement appear to meet the specifications and that the BMPs appear to function as designed. Onsite observations may be used to verify that materials are as specified and material receipts checked. Performance may be evaluated by a site visit while it is raining or by testing with a bucket of water or garden hose to check pavement permeability or proper connection to BMP devices/features, etc. 6. Drainage concerns. If CED determines that there is a potential for drainage impacts to a neighboring property, then additional measures may be required. Some on-site BMPs may not be appropriate in certain situations, and will not be allowed by CED where they may cause drainage problems. 7. Geotechnical concerns. A geotechnical engineer, engineering geologist, or CED must evaluate and approve on-site BMPs that are proposed: (A) on slopes steeper than 15%; (B) within a setback from the top of slope equal to the total vertical height of the slope area that is steeper than 15%; or (C) within 200 feet of a steep slope hazard area, erosion hazard area, or landslide hazard. In addition, CED may require review by a geotechnical engineer or engineering geologist of any proposed BMP that infiltrates, disperses, or directs overflow adjacent to or towards a steep slope hazard area, erosion hazard area, or landslide hazard. CED may also require some projects to route flows down or around such slopes using non-perforated pipes. Some on-site BMPs may not be appropriate for these locations, and will not be allowed by CED where flows may cause erosion problems. 8. Sewage system concerns. If CED determines that there is a potential conflict between onsite sewage systems and on-site BMPs, additional measures may be required. Some projects may need to route flows past onsite sewage systems using non-perforated pipes. Also, some on-site BMPs may not be appropriate for these sites, and will not be allowed where sewage systems may be impacted. 9. Engineering Concerns. While most of the on-site BMPs in Appendix C can be implemented by a non-engineer, there are some that have structural components that must be designed or evaluated by a civil engineer or structural engineer. When a BMP is proposed that has such components as identified in Section C.2 in Appendix C, CED may require submittal of engineering plans for that component signed and stamped by a civil engineer or structural engineer. 10. Connection to Subsurface Drains. On-site BMPs should not be connected to subsurface drains (e.g., footing drains) as these connections may adversely affect the performance of the BMPs, and in some cases may cause reverse flow into the footing drains during storm events. 11. Simplified Drainage Plan. The type, size, and placement of proposed on-site BMPs are to be shown on the site plan submitted for the proposed project. This plan must be in accordance with the specifications for such plans outlined in Section C.4 in Appendix C unless otherwise directed by CED. SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-82 1.2.9.3 SUBDIVISION AND ROAD IMPROVEMENT PROJECTS BMP REQUIREMENTS For subdivision and road improvement projects, on-site BMPs must be selected and applied according to the subdivision and road improvement projects BMP requirements in this section. For purposes of applying on-site BMPs to these projects, two categories of requirements have been established based on the size of site/lot subject to improvements by the project. These categories of requirements are as follows:  Small Subdivision Project BMP Requirements  Small Road Improvement and Urban Road Improvement Projects BMP Requirements On-site BMPs must be applied in the order of preference and to the extent specified for the category of requirements applicable to the proposed project as described in the following subsections. 1.2.9.3.1 SMALL SUBDIVISION PROJECT BMP REQUIREMENTS On-site BMPs for plat infrastructure improvements (e.g., road and sidewalk etc.) of these projects shall meet the requirements described in Section 1.2.9.3.2 below for “Small Road Improvement and Urban Road Improvement Project BMP Requirements.” Implementation of on-site BMPs required for/on the individual lots of the subdivision may be deferred until a permit is obtained for construction on each lot and is therefore optional. However, if the applicant wishes to implement or make provision for implementation of BMPs for the lot improvements as part of the subdivision project for purposes of receiving BMP modeling credits, the individual lot BMP requirements described in Section 1.2.9.2 and implementation requirements for subdivision projects described Section 1.2.9.4.1 must be met. 1.2.9.3.2 SMALL ROAD IMPROVEMENT AND URBAN ROAD IMPROVEMENT PROJECTS BMP REQUIREMENTS IF the proposed project is a road improvement project that is on a site/parcel less than 5 acres in size, THEN on-site BMPs must be applied as specified in the requirements below. 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 design requirements for full dispersion in Appendix C, Section C.2.1.1. 2. All target impervious surfaces not mitigated by Requirement 1 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. Infeasible BMPs are not 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 Section C.2.2, or per Section 5.2, whichever is applicable  Limited Infiltration per Appendix C, Section C.2.3,  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,  Permeable Pavement per Appendix C, Section C.2.7, 3. All target impervious surfaces not mitigated by Requirements 1 and 2 above, must be mitigated to the maximum extent feasible using the Basic Dispersion BMP described below. Use of Basic Dispersion 1.2.9 CORE REQUIREMENT #9: ON-SITE BMPS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-83 is subject to evaluation of its feasibility and applicability as detailed in Appendix C. Infeasible BMPs are not 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 BMPs must be implemented as part of the proposed project.  Basic Dispersion per Appendix C, Section C.2.4, 4. 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. 1.2.9.3.3 LARGE RURAL SUBDIVISION AND LARGE RURAL ROAD IMPROVEMENT PROJECTS BMP REQUIREMENTS This requirement does not apply in the City of Renton. 1.2.9.4 REQUIREMENTS FOR USE OF BMP CREDITS Projects that implement on-site BMPs, whether required or optional, may use the on-site BMP credits described in this section subject to the implementation requirements in Section 1.2.9.2.4 (for Individual Lots), Section 1.2.9.4.1 (for Subdivision Projects), Section 1.2.9.4.2 (for Right-of-Way Projects), and any restrictions noted in this section or Table 1.2.9.A. For all project types, modeling credits cannot be used for on-site BMPs that will be privately maintained, with the exception of the full dispersion and full infiltration BMPs. An alternative approach is to perform continuous runoff modeling per Chapter 3 instead of applying the on-site BMP credits in Table 1.2.9.A. Two kinds of credits are available. First, any impervious surface served by an on-site BMP that meets the design specifications for that BMP in Appendix C may be modeled as indicated and allowed in Table 1.2.9.A. Such credits may be used in the following situations: 1. To compute post-development runoff time series when sizing required flow control facilities. 2. To compute post-development 100-year peak flows when assessing any of the peak flow exceptions from the area-specific flow control facility requirement in Sections 1.2.3.1.A, B, and C. 3. To compute post-development runoff time series when sizing required flow rate based water quality facilities (e.g., bioswales) and to re-characterize post developed land types when sizing volume based water quality facilities (e.g., wetponds, wetvaults). Use of credits for water quality facility sizing as described above is limited to BMPs that are treating flows downstream from the BMP and tributary to a required water quality facility. Second, any impervious or nonnative pervious surface that is fully dispersed per the full dispersion criteria in Section 1.2.3.2.C is not considered a target surface of the area-specific flow control facility requirement (Section 1.2.3.1) or the area-specific water quality facility requirement (Section 1.2.8.1). SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-84 TABLE 1.2.9.A ON-SITE BMP SIZING CREDITS(1) On-Site BMP Type Sizing Credit Privately Maintained On-Site BMP Publicly Maintained On-Site BMP Full dispersion Model fully dispersed surface as forest(2) Model fully dispersed surface as forest(2) Full infiltration(3) Subtract impervious area that is fully infiltrated Subtract impervious area that is fully infiltrated Limited infiltration None Model tributary impervious surface as 90% impervious, 10% grass Basic dispersion None Model dispersed impervious surface as 90% impervious, 10% grass Rain garden None None Bioretention None Model tributary impervious surface as 90% impervious, 10% grass Permeable pavement (unlined with no underdrain) None Model permeable pavement area as 50% impervious, 50% grass. Run-on from other impervious surfaces does not receive a credit. Grassed modular grid pavement None Model modular grid pavement as all grass Rainwater harvesting None Credit only allowed via, and as specified in, an approved drainage adjustment that details conditions of use Restricted footprint None Model footprint as restricted (Appendix Section C.2.9.2) Wheel strip driveways None Model credited area as 50% impervious, 50% grass Minimum disturbance foundation None Model foundation area as 50% impervious, 50% grass Open grid decking over pervious area None Model deck area as 50% impervious, 50% grass Native growth retention credit None Model mitigated impervious area as 50% impervious, 50% grass Perforated pipe connection None None Notes: (1) These credits do not apply when determining eligibility for exemptions from Core Requirement #3, Core Requirement #8, or exceptions from the flow control or water quality facility requirements unless otherwise noted in the exemption or exception. Modeling credits cannot be used for on-site BMPs that will be privately maintained, with the exception of full dispersion and full infiltration BMPs. Explicit modeling of BMP infiltration for facility sizing is not allowed. When applying modeling credits for flow control facility sizing, infiltrative BMPs tributary to the facility that are included in the modeling scenario (including the permeable pavement element with area reduced to 50% impervious area fraction, or other BMPs (e.g., bioretention, trenches, drywells) treating upstream runoff) must have the infiltration option turned off during the flow routing analysis for facility sizing to avoid double-counting the BMP infiltration benefit. Alternatively, permeable pavement with infiltration turned off may be represented by an impervious area land use element of equivalent area. (2) Surface shall be modeled using the soil type found at that location on the site. (3) For any project subject to Simplified Drainage Review, and for any single family residential project subject to Directed, Full or Large Project Drainage Review, the design requirements and specifications in Appendix C, Section C.2.2 may be used for design of full infiltration on individual lots. For all other projects, including any project where full infiltration is proposed to serve more than one lot, full infiltration must be designed in accordance with infiltration facility standards in Section 5.2. 1.2.9.4.1 USE OF CREDITS BY SUBDIVISION PROJECTS If a proposed project is a subdivision project,45 implementation of on-site BMPs for plat infrastructure improvements (e.g., road, sidewalk, or other non-lot improvements) is required concurrent with the subdivision improvements. Implementation of on-site BMPs on the individual lots of the subdivision may be deferred until a permit is obtained for construction on each lot and is therefore optional as part of the subdivision project. In order to receive the modeling credits (noted above) for on-site BMPs required for plat infrastructure improvements (e.g., road, sidewalk, or other non-lot improvements), and/or for individual lot BMPs where the applicant elects to implement or make provision for implementation of individual lot BMPs as part of 45 For purposes of applying on-site BMPs, the term subdivision or subdivision project refers to any project that is a short plat, plat, or binding site plan. 1.2.9 CORE REQUIREMENT #9: ON-SITE BMPS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-85 the subdivision project, the following requirements must be met depending on where the BMPs are located on the site. These requirements are in addition to any restrictions for use of modeling credits noted in Section 1.2.9.4 and/or Table 1.2.9.A. A. SUBDIVISION IMPLEMENTATION OF BMPS WITHIN ROAD RIGHT-OF-WAY These are on-site BMPs installed within public or private road right-of-way as part of the construction of street and drainage improvements for the subdivision. To receive credit for these BMPs, the subdivision project must meet all of the following requirements: 1. The BMPs must serve impervious surface located only within the road right-of-way. 2. The BMPs must be shown on the site improvement plans submitted with the engineering plans for the proposed project as specified in Section 2.3.1.2. 3. If the road right-of-way will be privately maintained, provision must be made for future maintenance of the BMPs in accordance with Core Requirement #6, Section 1.2.6. As specified in Core Requirement #6, the City will assume maintenance of such BMPs in certain cases. 4. If the City will be assuming maintenance of the BMPs, the BMPs must comply with the drainage facility financial guarantee and liability requirements in Core Requirement #7, Section 1.2.7. B. SUBDIVISION IMPLEMENTATION OF BMPS WITHIN DEDICATED TRACTS These are on-site BMPs installed on or associated with the features (e.g., forest) of common area tracts dedicated by the subdivision. Such BMPs may serve future improvements on lots, common area improvements, or road right-of-way improvements. To receive credit for these BMPs, the subdivision project must meet all of the following requirements: 1. The BMPs must be shown on the site improvement plans submitted with the engineering plans for the proposed project as specified in Section 2.3.1.2. 2. Provision must be made for future maintenance of the BMPs in accordance with Core Requirement #6, Section 1.2.6. When maintenance by the City is specified by Core Requirement #6, the City will assume maintenance of BMP devices (e.g., dispersion trenches) that are within a tract dedicated to the City for drainage purposes. The City will not assume maintenance of BMPs located in common areas dedicated for purposes other than just drainage (e.g., play areas, parks, etc.). Where City maintenance is specified by Core Requirement #6, the City will assume maintenance for on-site BMP vegetated flow paths that are within an easement that allows for inspection and maintenance by the City. The City maintenance of these vegetated flow paths will be limited to their on-site BMP functionality. All other maintenance shall remain the responsibility of the owner(s). 3. BMPs to be maintained by the City in accordance with Core Requirement #6 must comply with the drainage facility financial guarantee and liability requirements in Core Requirement #7, Section 1.2.7. 4. If the BMPs installed within a dedicated tract satisfy some or all of the BMP requirements for individual lots per Section 1.2.9.2, then a note must be placed on the recorded documents for the subdivision indicating those lots for which BMPs have been provided. C. SUBDIVISION IMPLEMENTATION OF BMPS ON INDIVIDUAL LOTS These are on-site BMPs installed on a subdivision’s proposed lots as part of the subdivision project. For example, the subdivision developer may elect to pre-install some or all of the on-site BMPs required by the individual lot BMP requirements in Section 1.2.9.2. To receive credits for these BMPs, the subdivision project must meet all of the following requirements: 1. The on-site BMPs must be installed and implemented in accordance with the individual lot BMP requirements in Section 1.2.9.2. This includes recording a declaration of covenant and grant of easement for each lot with BMPs as specified in Implementation Requirement 3 of Section 1.2.9.2.4. SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-86 If not all of the required BMPs are installed on a lot as part of the subdivision project, language must be included in the covenant notifying the future lot owner of additional required BMPs. 2. BMPs to be installed on individual lots as part of the subdivision project must be shown on the site improvement plans submitted with the engineering plans for the proposed project as specified in Section 2.3.1.2. D. SUBDIVISION FUTURE IMPLEMENTATION OF BMPS ON INDIVIDUAL LOTS These are on-site BMPs stipulated to be installed on some or all of a subdivision’s proposed lots by a declaration of covenant recorded for each such lot. To receive credits for these BMPs, the subdivision project must meet all of the following requirements: 1. Demonstrate through a lot-specific assessment that the on-site BMPs stipulated for each lot are feasible and applicable according to the individual lot BMP requirements in Section 1.2.9.2 and the BMP design specifications in Appendix C. This lot-specific assessment must be included in the TIR submitted with engineering plans for the subdivision. The assessment shall include any soils reports, calculations, or other information necessary to select and properly apply BMPs. 2. Record a declaration of covenant and grant of easement for each lot stipulating the type or types of BMP being proposed for credit. This covenant must be as specified in Implementation Requirement 3 of Section 1.2.9.2.4, except as follows: a) The site plan requirement may be waived depending on the BMPs proposed or may be conceptual, showing only the information necessary to stipulate the type or types of BMP being proposed for credit. For example, if the BMP is full dispersion, the approximate location of future impervious surface and the limits of the “native vegetated flowpath segment” (see Appendix C, Section C.2.1) must be shown. If the BMP is full infiltration, the approximate location of future impervious surface, septic drain field (if applicable), and infiltration devices must be shown. For all other BMPs, the “design and maintenance details” (see Item b below) for each proposed BMP per Appendix C may be sufficient as determined by CED. b) The on-site BMP design and maintenance details must include the dimensions of all proposed devices, features, and flowpaths, expressed as unit amounts per square foot of impervious surface served or as a percentage of the lot size or impervious surface created. c) The notice to future lot owners must indicate that they are responsible to install the on-site BMP or BMPs stipulated for the lot prior to final inspection approval of constructed lot improvements. Alternative BMPs that provide equivalent performance may be proposed at the time of permit application for proposed lot improvements. In any case, a revised covenant will need to be recorded to reflect the final approved BMPs and site improvement plan(s). 3. If single family residential lots are being created, a note must be placed on the recorded documents for the subdivision indicating the following: “Single family residences and other improvements constructed on the lots created by this subdivision must implement the flow control best management practices (BMPs) stipulated in the drainage plan declaration of covenant and grant of easement recorded for each lot. Compliance with this stipulation must be addressed in the small project drainage plan submitted for drainage review when application is made for a single family residential building permit for the lot.” 4. If commercial lots are being created, a note must be placed on the recorded documents for the subdivision indicating the following: “Improvements constructed on the lots created by this subdivision must implement the flow control best management practices (BMPs) stipulated in the drainage plan declaration of covenant and grant of easement recorded for each lot. Compliance with this stipulation must be addressed in the engineering plans submitted for drainage review when application is made for a permit to make improvements to the lot.” 1.2.9 CORE REQUIREMENT #9: ON-SITE BMPS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-87 5. If a binding site plan is being created, a note must be placed on the recorded documents for the subdivision indicating the following: “Improvements constructed on the lots created by this binding site plan must implement the flow control best management practices (BMPs) stipulated in the drainage plan declaration of covenant and grant of easement recorded for each lot. Compliance with this stipulation must be addressed in the engineering plans submitted for drainage review when application is made for a permit to make improvements to the lot.” 1.2.9.4.2 USE OF CREDITS BY PROJECTS WITHIN RIGHTS-OF-WAY If a proposed project is located primarily within an established public or private right-of-way, implementation of on-site BMPs is as required per Section 1.2.9.3. To receive credit for these BMPs, the project must meet all of the following requirements in addition to any restrictions for use of modeling credits noted in Section 1.2.9.4 and/or Table 1.2.9.A.: 1. The BMPs must serve impervious surface located only within the right-of-way. 2. If the right-of-way will be privately maintained, provision must be made for future maintenance of the BMPs in accordance with Core Requirement #6, Section 1.2.6. 3. If the City will be assuming maintenance of the BMPs, the BMPs must comply with the drainage facility financial guarantee and liability requirements in Core Requirement #7, Section 1.2.7. SECTION 1.2 CORE REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-88 (T his page intentionally left blan k. ) CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 1-89 1.3 SPECIAL REQUIREMENTS This section details the following six special drainage requirements that may apply to the proposed project depending on its location or site-specific characteristics:  “Special Requirement #1: Other Adopted Area-Specific Requirements,” Section 1.3.1  “Special Requirement #2: Flood Hazard Area Delineation,” Section 1.3.2  “Special Requirement #3: Flood Protection Facilities,” Section 1.3.3  “Special Requirement #4: Source Control,” Section 1.3.4  “Special Requirement #5: Oil Control,” Section 1.3.5  “Special Requirement #6: Aquifer Protection Area,” Section 1.3.6 1.3.1 SPECIAL REQUIREMENT #1: OTHER ADOPTED AREA-SPECIFIC REQUIREMENTS This manual is one of several adopted regulations in the City of Renton that apply requirements for controlling drainage on an area-specific basis. Other adopted area-specific regulations include requirements that have a more direct bearing on the drainage design of a proposed project. These regulations include the following:  Master Drainage Plans (MDPs): MDPs are comprehensive drainage plans prepared for urban planned developments (UPDs) or other large, complex projects (described in Section 1.1.2.5). Projects covered by a MDP must meet any adopted requirements specific to that plan.  Basin Plans (BPs): The City of Renton adopts basin plans to provide for the comprehensive assessment of resources and to accommodate growth while controlling adverse impacts to the environment. A basin plan may recommend specific land uses, regional capital projects, and special drainage requirements for future development within the basin area it covers.  Salmon Conservation Plans (SCPs): Salmon conservation plans are comprehensive, ecosystem- based plans intended to identify and assess the means to protect and restore salmon habitat through mechanisms such as habitat improvements, regulations, incentives, BMPs, land acquisition, and public education activities. These plans are developed in collaboration with other jurisdictions within a water resource inventory area (WRIA) designated by the state under WAC 173-500-040 and spanning several basins or subbasins.  Lake Management Plans: The City of Renton may adopt lake management plans to provide for comprehensive assessment of resources and to accommodate growth while controlling adverse impacts from nutrient loading to selected lakes. A lake management plan may recommend nutrient control through special drainage and source control requirements for proposed projects within the area it covers.  Hazard Mitigation Plan: The City’s Hazard Mitigation Plan prepared in accordance with RCW 86.12.200 includes information on reducing flood risks.  Shared Facility Drainage Plans (SFDPs): SFDPs are approved by the City of Renton to allow two or more projects to share drainage facilities required by this manual. Projects covered by a SFDP must meet any specific requirements of that plan. SECTION 1.3 SPECIAL REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-90 Threshold Requirement IF a proposed project is in an area included in an adopted master drainage plan, basin plan, salmon conservation plan, stormwater compliance plan, hazard mitigation plan, lake management plan, or shared facility drainage plan … THEN the proposed project shall comply with the drainage requirements of the master drainage plan, basin plan, salmon conservation plan, stormwater compliance plan, hazard mitigation plan, lake management plan, or shared facility drainage plan, respectively. Application of this Requirement The drainage requirements of adopted MDPs, BPs, SCPs, Hazard Mitigation Plan, lake management plans, and SFDPs shall be applied in addition to the drainage requirements of this manual unless otherwise specified in the adopted regulation. Where conflicts occur between the two, the drainage requirements of the adopted area-specific regulation shall supersede those in this manual. Examples of drainage requirements found in other adopted area-specific regulations include the following:  More or less stringent flow control  More extensive water quality controls  Forest retention requirements  Infiltration restrictions  Groundwater recharge provisions  Discharge to a constructed regional flow control or conveyance facility. Adjustments to vary from the specific drainage requirements mandated by BPs, SCPs, FHMPs, and lake management plans may be pursued through the adjustment process described in Section 1.4 of this manual. Copies of all adopted basin plans, SCPs, Hazard Mitigation Plan, and lake management plans are available from the City of Renton. Projects covered by SFDPs shall demonstrate that the shared facility will be available by the time the project is constructed and that all onsite requirements are met. Projects covered by a SFDP are still required to provide any onsite controls necessary to comply with drainage requirements not addressed by the shared facility. 1.3.2 SPECIAL REQUIREMENT #2: FLOOD HAZARD AREA DELINEATION Flood hazard areas are composed of the 100-year floodplain, zero-rise flood fringe, zero-rise floodway, and FEMA floodway. If a proposed project contains or is adjacent to a flood hazard area as determined by CED, this special requirement requires the project to determine those components that are applicable and delineate them on the project’s site improvement plans and recorded maps. Floodplains are subject to inundation during extreme events. The 100-year floodplain, and floodway if applicable, is delineated in order to minimize flooding impacts to new development and to prevent aggravation of existing flooding problems by new development. Regulations and restrictions concerning development within a 100-year floodplain are found in the critical areas code, RMC 4-3-050. 1.3.3 SPECIAL REQUIREMENT #3: FLOOD PROTECTION FACILITIES 2022 City of Renton Surface Water Design Manual 6/22/2022 1-91 Threshold Requirement IF a proposed project contains or is adjacent to a flood hazard area for a river, stream, lake, wetland, closed depression, , or if other City of Renton regulations require study of flood hazards related to the proposed project … THEN the 100-year floodplain, and applicable floodway, shall be determined and their boundaries, together with the boundaries of the severe and moderate channel migration hazard area (if applicable), shall be delineated on the site improvement plans and profiles, and on any final subdivision maps prepared for the proposed project. Application of this Requirement The applicant is required to use the best available floodplain/floodway data when delineating the 100-year floodplain and floodway boundaries on site improvement plans and profiles, and on any final subdivision maps. The floodplain/floodway delineation used by the applicant shall be in accordance with RMC 4-3-050 and associated public rules. If floodplain/floodway data and delineation does not exist, then a floodplain/floodway analysis shall be prepared by the applicant as described in Section 4.4.2, “Floodplain/Floodway Analysis.” 1.3.3 SPECIAL REQUIREMENT #3: FLOOD PROTECTION FACILITIES Flood protection facilities, such as levees and revetments require a high level of confidence in their structural integrity and performance. Proper analysis, design, and construction are necessary to protect against the potentially catastrophic consequences if such facilities should fail. Threshold Requirement IF a proposed project will:  Rely on an existing flood protection facility (such as a levee or revetment) for protection against hazards posed by erosion or inundation, OR  Modify or construct a new flood protection facility … THEN the applicant shall demonstrate that the flood protection facility, as determined by a licensed professional engineer, conforms with siting, structural stability, environmental, and all other relevant standards cited in the following regulations and documents:  Washington State Integrated Streambank Protection Guidelines,  Corps of Engineers Manual for Design and Construction of Levees (EM 1110-2-1913),  RMC 4-3-050 and  Special Requirement #1 (specifically the City Hazard Mitigation Plan) AND, flood containment levees shall meet or exceed the professional engineering standards summarized in FEMA National Flood Insurance mapping regulations … (44 CFR, subsection 65.10) or FEMA’s Analysis and Mapping Procedures for non-Accredited Levee Systems. SECTION 1.3 SPECIAL REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-92 Application of this Requirement Conformance with the requirements listed above shall be addressed in the Technical Information Report submitted with the project’s engineering plans (see Section 2.3.1.1). Conformance also requires that certain easement requirements (outlined in Section 4.1) be met in order to allow County access to the facility. If the proposed project contains an existing City of Renton flood protection facility or proposes to rely on a City of Renton flood protection facility, the applicant shall provide an easement to the City of Renton consistent with the river protection easement requirements outlined in Section 4.1. 1.3.4 SPECIAL REQUIREMENT #4: SOURCE CONTROLS Water quality source controls prevent rainfall and runoff water from coming into contact with pollutants, thereby reducing the likelihood that pollutants will enter public waterways and violate water quality standards or City stormwater discharge permit limits. A Stormwater Pollution Prevention Manual was prepared for citizens, businesses, and industries to identify and implement source controls for activities that often pollute water bodies. The City of Renton provides education about source control implementation upon request. The City will implement a progressive enforcement policy to require mandatory source controls through education and outreach, technical assistance, and enforcement. Threshold Requirement IF a proposed project requires a commercial building or commercial site development permit … THEN water quality source controls applicable to the proposed project shall be applied as described below in accordance with the King County Stormwater Pollution Prevention Manual and Renton Municipal Code, Title IV. Application of this Requirement When applicable per the Stormwater Pollution Prevention Manual, structural source control measures, such as car wash pads or dumpster area roofing, shall be applied to the entire site containing the proposed project, not just the project site. If the applicant is a tenant or lessee for only a portion of the site, CED may limit the entire site application of structural source controls to only that portion of the site occupied or leased by the applicant. All applicable structural source control measures shall be shown on the site improvement plans submitted for engineering review and approval. Other, nonstructural source control measures, such as covering storage piles with plastic or isolating areas where pollutants are used or stored, are to be implemented after occupancy and need not be addressed during the plan review process. All commercial, industrial, and multifamily projects (irrespective of size) undergoing drainage review are required to implement applicable source controls. Activities That May Result In Structural Improvements There are a number of activities that may require structures and/or specific drainage configurations in order to protect stormwater and maintain compliance with county code. Roof structure s, wheel washes, cement pads, shutoff valves, containment berms and indoor mop sinks are all examples of things that need to be in place prior to commencing the activity. These may require building permits and other approvals prior to construction. Below are some highlighted activities and the numbered BMP activity sheets in the Stormwater Pollution Prevention Manual that provide more detail: 1.3.4 SPECIAL REQUIREMENT #4: SOURCE CONTROLS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-93 Commercial Composting Structural improvements: paved composting and storage pads, leachate collection system, lined collection ponds, wheel wash system  A-4 Outdoor Storage of Soil, Sand, and Other Erodible Materials  A-24 Commercial Composting Food and Beverage Manufacturing and Storage Structural improvements: roofed enclosures, containment, wastewater collection, storage, and disposal system  A-7 Food and Beverage Manufacturing and Storage Fueling of Equipment and Vehicles Structural improvements: Portland cement pads, roofs, spill control devices, trench drains, oil/water separators  A-17 Stationary Fueling Operations  A-48 Older Stationary Fueling Operations Greenhouses and Plant Nurseries Structural improvements: berms, covering, and erosion control measures  A-4 Outdoor Storage of Soil, Sand, and Other Erodible Materials Horse Stables Structural improvements: Wash racks connected to sanitary sewer or separate infiltration area, manure containment areas  A-35 Keeping Livestock in Stables, Pens, Pastures, or Fields Mining of Sand or Gravel Structural improvements: Wheel wash system and track-out control, catch basin inserts  A-41 Wheel Wash and Tire Bath Track Out Control Painting, Finishing, and Coating of Vehicles and Equipment Structural improvements: Permitted, enclosed paint booths  A-22 Painting, Finishing, & Coating of Vehicles, Products, & Equipment Restaurants and Food Trucks Structural improvements: Indoor sinks format and rack washing and mop and wastewater disposal.  A-8 Storage of Solid and Food Wastes (Including Cooking Grease)  A-12 Cleaning or Washing of Food Services Areas and Equipment Outdoor Storage of Erodible Materials (e.g., compost, bark, sand, etc.) Structural improvements: Wheel wash system and track-out control, berms, containment areas, covering, catch basin inserts  A-41 Wheel Wash and Tire Bath Track-Out Control Outdoor Storage or Processing of Galvanized Materials Structural improvements: Roofs or other covering, stormwater collection and treatment system  A-21 Manufacturing and Post-Processing of Metal Products SECTION 1.3 SPECIAL REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-94 Storage of Liquid Materials Structural improvements: Secondary containment, roofed structures, spill control devices  A-2 Outdoor Storage of Liquid Materials in Stationary Tanks  A-3 Storage of Liquid Materials in Portable Containers Utility Corridor Maintenance Structural improvements: Road stabilization  A-45 Maintenance of Public and Private Utility Corridors and Facilities Washing of Cars, Trucks, and Equipment (not just commercial car washes) Structural improvements: Dedicated wash pads, sewer connection, holding tanks, catch basin inserts  A-13 Vehicle Washing and Steam Cleaning Wood Treatment and Preserving Structural improvements: Paved, contained and covered storage and processing areas  A-4 Outdoor Storage of Soil, Sand, and Other Erodible Materials  A-23 Wood Treatment & Preserving 1.3.5 SPECIAL REQUIREMENT #5: OIL CONTROL Projects proposing to develop or redevelop a high-use site must provide oil controls in addition to any other water quality controls required by this manual. Such sites typically generate high concentrations of oil due to high traffic turnover, onsite vehicle or heavy or stationary equipment use, some business operations, e.g., automotive recycling, or the frequent transfer of liquid petroleum or coal derivative products. The traffic threshold in the definition above focuses on vehicle turnover per square foot of building area (trip generation) rather than ADT alone because oil leakage is greatest when engines are idling or cooling. In general, all-day parking areas are not intended to be captured by these thresholds except those for diesel vehicles, which tend to leak oil more than non-diesel vehicles. The petroleum storage and transfer stipulation is intended to address regular transfer operations like service stations, not occasional filling of heating oil tanks. Threshold Requirement IF a proposed project:  Develops a site that will have high- use site characteristics, OR  Is a redevelopment project proposing $100,000 or more of improvements to an existing high-use site, OR  Is a redevelopment project that results in new plus replaced pollution generating impervious surfaces of 5,000 square feet or more or new pollution generating pervious surface of ¾ acre or more improvements to an existing high-use site … THEN the project must treat runoff from the high-use portion of the site using oil control treatment options from the High- Use menu (described below and detailed in Chapter 6). 1.3.4 SPECIAL REQUIREMENT #4: SOURCE CONTROLS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-95 High-Use Menu High-use oil control options are selected to capture and detain oil and associated pollutants. The goal of this treatment is no visible sheen on runoff leaving the facility, or less than 10 mg/L total petroleum hydrocarbons (TPH) in the runoff, depending on the facility option used. Oil control options include facilities that are small, handle only a limited tributary area, and require frequent maintenance, as well as facilities that treat larger areas and generally have less frequent maintenance needs. Facility choices include linear sand filters and oil/water separators. See Chapter 6 for specific facility choices and design details. Application of this Requirement For high-use sites located within a larger commercial center, only the impervious surface associated with the high-use portion of the site is subject to treatment requirements. If common parking for multiple businesses is provided, treatment shall be applied to the number of parking stalls required for the high-use business only. However, if the treatment collection area also receives runoff from other areas, the water quality facility must be sized to treat all water passing through it. High-use roadway intersections shall treat the intersection itself, plus lanes where vehicles accumulate during the signal cycle, including all lanes, from the beginning of the left turn pocket (see Figure 1.3.5.A below). If no left turn pocket exists, the treatable area shall begin at a distance of 75 feet from the stop line. If runoff from the intersection drains to more than two collection areas that do not combine within the intersection, treatment may be limited to any two of the collection areas. Oil control facilities shall be designed for all flows tributary to the oil control facility including flow from otherwise exempt areas that are not bypassed around the facility. Note: For oil control facilities to be located in public road right-of-way and maintained by the City of Renton, only coalescing plate or baffle oil/water separators shall be used unless otherwise approved through an adjustment. Catch basin inserts are not allowed for oil control. Methods of Analysis The traffic threshold for the High-Use menu shall be estimated using information from Trip Generation, published by the Institute of Transportation Engineers, from a traffic study prepared by a professional engineer or transportation specialist with experience in traffic estimation, or from documented data from the City. SECTION 1.3 SPECIAL REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-96 FIGURE 1.3.5.A TREATABLE AREAS FOR HIGH-USE ROAD INTERSECTIONS 1.3.6 SPECIAL REQUIREMENT #6: AQUIFER PROTECTION AREA 2022 City of Renton Surface Water Design Manual 6/22/2022 1-97 1.3.6 SPECIAL REQUIREMENT #6: AQUIFER PROTECTION AREA Aquifer Protection Area(s) (APA) are identified in the RMC 4-3-050. If a proposed project is located within the APA, this special requirement requires the project to determine those components that are applicable and delineate them on the project’s site improvements plans. APA zones are depicted in the Wellhead Protection Area Zones layer of COR Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>). Threshold Requirement IF a proposed project is in Zone 1 of the APA… THEN the following are prohibited: a. Facilities that allow runoff to have direct contact with the soil, such as flow control and water quality treatment ponds, stormwater wetlands, filter strips, and infiltration facilities. b. On-site BMPs that rely on infiltration, such as bioretention and permeable pavement. c. Open channel conveyance systems that are not concrete lined, such as ditches and swales. IF a proposed project is in Zone 1 Modified or Zone 2 of the APA… THEN the following may require a liner in accordance with the design criteria in Section 6.2.4: a. Facilities that allow runoff to have direct contact with the soil, such as flow control and water quality treatment ponds, stormwater wetlands, filter strips, and infiltration facilities. b. Open channel conveyance systems that are not concrete lined, such as ditches as swales. SECTION 1.3 SPECIAL REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-98 (T his page intentionally left blan k. ) CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 1-99 1.4 ADJUSTMENT PROCESS For proposed projects subject to drainage review by the City of Renton this process is provided for the occasions when a project proponent desires to vary from one of the core or special requirements, or any other specific requirement or standard contained in this manual. Proposed adjustments should be approved prior to final permit approval, but they may be accepted up to the time the City approves final construction or accepts drainage facilities for maintenance. Types of Adjustments To facilitate the adjustment process and timely review of adjustment proposals, the following types of adjustments are provided:  Standard Adjustments: These are adjustments of the standards and requirements contained in the following chapters and sections of this manual: o Chapter 1, “Drainage Review and Requirements” o Chapter 2, “Drainage Plan Submittal” o Chapter 3, “Hydrologic Analysis and Design” o Chapter 4, “Conveyance System Analysis and Design” o Chapter 5, “Flow Control Design” o Chapter 6, “Water Quality Design” o Appendix A, “Maintenance Requirements for Stormwater Facilities and On-Site BMPs” o Appendix B, “Master Drainage Plans” o Appendix C, “Simplified Drainage Requirements” o Appendix D, “Construction Stormwater Pollution Prevention Standards” Requests for standard adjustments will be accepted only for permits pending approval or approved permits that have not yet expired.  Blanket Adjustments: This type of adjustment may be established by the City based on approval of any of the above-mentioned adjustments. Blanket adjustments are usually based on previously approved adjustments that can be applied routinely or globally to all projects where appropriate. Blanket adjustments are also used to effect minor changes or corrections to manual design requirements or to add new designs and methodologies to this manual. There is no application process for establishing blanket adjustments because they are initiated solely by the City. 1.4.1 ADJUSTMENT AUTHORITY CED shall have full authority to determine if and what type of adjustment is required for any proposed project subject to drainage review by CED. The authority to grant adjustments for such projects is distributed as follows:  CED shall have full authority to approve or deny adjustments, except those involving outfalls or pump discharges to the Green River between River Mile 6 and SR 18 per Section 1.2.4.2.F and 1.2.4.2.I. CED decisions on those adjustments are subject to approval by the King County Flood Control District. 1.4.2 CRITERIA FOR GRANTING ADJUSTMENTS Adjustments to the requirements in this manual may be granted provided that granting the adjustment will achieve the following: 1. Produce a compensating or comparable result that is in the public interest, AND 2. Meet the objectives of safety, function, appearance, environmental protection, and maintainability based on sound engineering judgment. Also, the granting of any adjustment that would be in conflict with the requirements of any other City department will require review and concurrence with that department. SECTION 1.4 ADJUSTMENT PROCESS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-100 Criteria Exception If it can be demonstrated that meeting the above criteria for producing a compensating or comparable result will deny reasonable use of a property, approval of the adjustment will require an adjustment criteria exception to be approved by the City. An adjustment that requires a criteria exception may be granted following legal public notice of the adjustment request, the proposed decision on the request, and a written finding of fact that documents the following: 1. There are special physical circumstances or conditions affecting the property such that strict application of the criteria for producing a compensating or comparable result would deprive the applicant of all reasonable use of the parcel of land in question, and every effort has been made to find creative ways to meet the intent of the requirement for which the adjustment is sought, AND 2. Granting the adjustment for the individual property in question will not create a significant adverse impact to public health, welfare, water quality, and properties downstream or nearby, AND 3. The adjustment requires the best practicable alternative for achieving the spirit and intent of the requirement in question. In addition, the written finding of fact must include the following information:  The current (pre-project) use of the site.  How application of the requirement for which an adjustment is being requested denies reasonable use of the site compared to the restrictions that existed under the 2009 King County Surface Water Design Manual and City of Renton Amendments to the 2009 King County Surface Water Design Manual.  The possible remaining uses of the site if the criteria exception were not granted.  The uses of the site that would have been allowed under the 2009 King County Surface Water Design Manual and City of Renton Amendments to the 2009 King County Surface Water Design Manual .  A comparison of the estimated amount and percentage of value loss as a result of the requirements of this manual versus the estimated amount and percentage of value loss as a result of requirements that existed under the 2009 King County Surface Water Design Manual and City of Renton Amendments to the 2009 King County Surface Water Design Manual.  The feasibility for the owner to alter the project to apply the requirements of this manual.46,47 1.4.3 ADJUSTMENT APPLICATION PROCESS Standard Adjustments The application process for standard adjustments is as follows:  Requests for standard adjustments will be accepted only for permits pending approval or approved permits that have not yet expired.  The completed adjustment request application forms must be submitted to CED along with sufficient engineering information (described in Chapter 2) to evaluate the request. The application shall note the specific requirement for which the adjustment is sought.  If the adjustment request involves use of a previously unapproved construction material or construction practice, the applicant should submit documentation that includes, but is not limited to, a record of successful use by other agencies and/or evidence of meeting criteria for quality and performance, such as that for the American Association of State Highway and Transportation Officials (AASHTO) and the American Society of Testing and Materials (ASTM).  If the adjustment requires a criteria exception, additional engineering or other information may be required by CED to document that denial of reasonable use would occur, that every effort was made to achieve compliance, and that the best practicable alternative will not cause significant adverse impact.  A fee reduction may be requested if it is demonstrated that the adjustment request requires little or no engineering review. 46 Footnote 51 is not used. 47 Footnote 52 is not used. 1.4.4 ADJUSTMENT REVIEW PROCESS 2022 City of Renton Surface Water Design Manual 6/22/2022 1-101 Blanket Adjustments There is no application process for establishing blanket adjustments because they are initiated and issued solely by the City. 1.4.4 ADJUSTMENT REVIEW PROCESS All adjustments are governed by the review procedures and time lines set forth by the City. Consistent with these procedures, the general steps of the review process for specific types of adjustments are presented as follows. Standard Adjustments  CED will review the adjustment request application forms and documentation for completeness and inform the applicant in writing as to whether additional information is required from the applicant in order to complete the review. The applicant will also be informed if CED determines that special technical support is required in cases where the adjustment involves a major policy issue or potentially impacts a City drainage facility.  The CED Development Review Engineer will review and either approve or deny the adjustment request following determination that all necessary information has been received from the applicant.  If a criteria exception is required for the adjustment, CED will issue a legal public notice of the adjustment request that indicates the director’s proposed decision on the request, including the written finding of fact specified in Section 1.4.2. The public notice will include a 15-working-day public comment period within which a request for reconsideration may be made to the CED director as described in Section 1.4.5. Absent a request for reconsideration, the director’s decision becomes final after the two week public comment period.  Approvals of standard adjustments will expire upon expiration of the permit to which they apply. Blanket Adjustments Blanket adjustments may be established at the discretion of CED. Blanket adjustments are established by memorandum based on: 1. Previously approved adjustments and supporting documentation, AND 2. Monitoring results in conjunction with any TAPE or CTAPE results AND 3. Information presenting the need for the blanket adjustment. Typically, blanket adjustments should apply globally to design or procedural requirements and be independent of site conditions. CED must approve creation of a blanket adjustment. Applicants may use any approved blanket adjustment listed in Reference Section 14, by submitting the form titled “Surface Water Design Manual Requirements/Standards Blanket Adjustment No. ____” to the CED plan reviewer currently reviewing the specific project proposal, but no further approval is required. 1.4.5 APPEALS Any appeals from administrative determinations for variances or adjustments related to the Storm Drainage regulations and codes shall be filed in writing to the Hearing Examiner by any person aggrieved, or by any officer, department, board or bureau of the City affected by such determination per RMC 4-8-110. SECTION 1.4 ADJUSTMENT PROCESS 6/22/2022 2022 City of Renton Surface Water Design Manual 1-102 (T his page intentionally left blan k. ) 2022 Surface Water Design Manual 6/22/2022 CHAPTER 2 DRAINAGE PLAN SUBMITTAL CITY OF RENTON SURFACE WATER DESIGN MANUAL Section Page 2.1 Plans for Permits and Drainage Review 2-3 2.1.1 Plans Required for Pre-Application Submittal 2-3 2.1.2 Site Plans Required for Drainage Review 2-3 2.2 Plans Required with Construction Permit Application 2-5 2.2.1 Subdivision, PUD, and Binding Site Plans 2-6 2.2.2 Short Subdivisions 2-7 2.2.3 Commercial Site Development 2-7 2.2.4 Single-Family Residential 2-7 2.2.5 Other Permits 2-7 2.3 Drainage Review Plan Specifications 2-9 2.3.1 Engineering Plan Specifications 2-10 2.3.2 Projects in Targeted Drainage Review (TDR) 2-35 2.4 Plans Required After Drainage Review 2-37 2.4.1 Plan Changes After Permit Issuance 2-37 2.4.2 Final Corrected Plan Submittal 2-37 2.4.3 Final Plat, Short Plat, and Binding Site Plan Submittals 2-38 CITY OF RENTON SURFACE WATER DESIGN MANUAL 6/22/2022 2022 City of Renton Surface Water Design Manual (T his page intentionally left blan k. ) CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 2-1 CHAPTER 2 DRAINAGE PLAN SUBMITTAL This chapter details the drainage related submittal requirements for engineering design plans as part of a permit application to the City of Renton Community and Economic Development (CED) Department. The intent of these requirements is to present consistent formats for design plans and the technical support data required to develop the plans. These conventions are necessary to review engineering designs for compliance with City of Renton ordinances and regulations, and to ensure the intent of the plan is easily understood and implemented in the field. Properly drafted design plans and supporting information also facilitate the construction, operation, and maintenance of the proposed system long after its review and approval. When plans comply with the formats and specifications contained herein, they facilitate review and approval with a minimum of time-consuming corrections and resubmittals. Note that this chapter primarily describes how to submit drainage plans for review—what must be submitted, in what formats, at what times and to what offices. The basic drainage requirements that these plans must address are contained in Chapter 1, “Drainage Review and Requirements.” The specific design methods and criteria to be used are contained in Chapters 3, 4, 5, and 6. Several key forms used in the plan review process are reproduced in Reference Section 8, “Forms and Worksheets.” The drainage submittal requirements for different types of developments are contained in this chapter with the exception of Master Drainage Plans, which if required, the scope of the requirements will be determined by the Surface Water Utility and will generally follow King County’s Master Drainage Planning for Large or Complex Site Development and requirements. For information on general requirements for any permit type, refer to the City of Renton website or customer information counter at CED. Chapter Organization The information presented in this chapter is organized into four main sections as follows:  Section 2.1, “Plans for Permits and Drainage Review”  Section 2.2, “Plans Required with Construction Permit Application”  Section 2.3, “Drainage Review Plan Specifications”  Section 2.4, “Plans Required After Drainage Review” These sections begin on odd pages so the user can insert tabs if desired for quicker reference. CHAPTER 2 DRAINAGE PLAN SUBMITTAL 6/22/2022 2022 City of Renton Surface Water Design Manual 2-2 (T his page intentionally left blan k) CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 2-3 2.1 PLANS FOR PERMITS AND DRAINAGE REVIEW CED is responsible for the review of all engineering aspects of private development proposals. Drainage review is a primary concern of engineering design. This section describes the types of engineered drainage plans required for engineering review at various permit review stages. Refer to the City website for other details or requirements, such as the submittal and expiration periods set for each type of permit application, review fees, right-of-way use requirements, and other code requirements. 2.1.1 PLANS REQUIRED FOR PRE-APPLICATION SUBMITTAL Most projects require some degree of drainage plans or analysis to be submitted for drainage review; the extent of which will depend upon the type of permit, size and type of project, and project vicinity to any critical areas. All proposed developments within the City of Renton have the option to request a free pre- application meeting to gain feedback on development requirements and feasibility prior to formal submittal of any required permit application materials. Subdivisions, PUDs, short plats and binding site plans require conceptual plans (may be engineered or non-engineered) be submitted with the pre- application meeting request. Preliminary plans provide general information on the proposal, including location of critical areas, road alignments and right-of-way, site topography, building locations, land use information, and lot dimensions. They are used to determine the appropriate drainage conditions and requirements to be applied to the proposal during the drainage review process. For more information refer to the permit submittal requirements documents that are applicable to the development proposal (available on the City’s website and from staff in the pre-application meeting, if held). 2.1.2 SITE PLANS REQUIRED FOR DRAINAGE REVIEW For drainage review purposes, engineering plans consist of the following: 1. Site improvement plans (see Section 2.3.1.2), which include all plans, profiles, details, notes, and specifications necessary to construct road, drainage, utilities, and off-street parking improvements. 2. A construction stormwater pollution prevention (CSWPP) plan, which identifies the measures and BMPs required to prevent the discharge of sediment-laden water and other pollutants associated with construction/land disturbing activities. The CSWPP plan includes two component plans: an erosion and sediment control (ESC) plan (see Section 2.3.1.3), which addresses prevention of sediment- laden discharges; and a stormwater pollution prevention and spill (SWPPS) plan (see Section 2.3.1.4), which addresses prevention of other pollutant discharges. 3. A technical information report (TIR) (see Section 2.3.1.1), which contains all the technical information and analysis necessary to develop the site improvement plan and CSWPP plan. Projects Under Targeted Drainage Review usually require engineering plans, except that only certain sections of the technical information report are required to be completed and the site improvement plan may have a limited scope depending upon the characteristics of the proposed project. The scope of these plans should be confirmed during the pre-application meeting with CED. For other permits, such as single-family residential permits, the scope of the targeted engineering analysis is usually determined during CED engineering review. SECTION 2.1 PLANS FOR PERMITS AND DRAINAGE REVIEW 6/22/2022 2022 City of Renton Surface Water Design Manual 2-4 Plans Required for Simplified Drainage Review Simplified drainage plans are a simplified form of site improvement and CSWPP plans (without a TIR or a separate SWPPS plan) that may be prepared by a non-engineer from a set of pre-engineered design details. Simplified drainage plans are only allowed for single family in Simplified Drainage Review but may be required for individual lots created by a subdivision project to show how required on-site BMPs, ESC and SWPPS measures will be applied to future lot construction. For single-family residential permits, the level and scope of drainage plan requirements are determined by CED during drainage review. Some projects subject to Simplified Drainage Review may also require Targeted Drainage Review. CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 2-5 2.2 PLANS REQUIRED WITH CONSTRUCTION PERMIT APPLICATION This section describes the submittal requirements for construction permit applications at the City of Renton. Required plans for each permit or project type described in Section 2.2.1 through 2.2.5 are summarized in Table 2.2.A. The timing for submittal of engineering plans will vary depending on permit type. For commercial, subdivisions, short plats, and other types of construction permits, this submittal usually follows the City’s approval of plans described in Section 2.2. For commercial building permits, engineering plans must be submitted as part of the construction permit application, unless otherwise approved by CED. For other permit types the drainage plan requirements are determined during the permit review process. Note: If engineering plans are required to be submitted with the construction permit application, they must be accompanied by the appropriate supporting documents (e.g., required application forms, reports, etc.). For more details, see the City’s website. Design Plan Certification All preliminary plans and engineering plans must be stamped by a civil engineer. All land boundary surveys and legal descriptions used for preliminary and engineering plans must be stamped by a land surveyor licensed in the State of Washington. Topographic survey data and mapping prepared specifically for a proposed project may be performed by the civil engineer stamping the engineering plans as allowed by the Washington State Board of Registration for Professional Engineers and Land Surveyors. TABLE 2.2.A DRAINAGE PLAN SUBMITTALS Type of Permit or Project Plans Required with Construction Permit Application Type of Drainage Review Plans Required for Drainage Review SUBDIVISIONS, PUDs, AND BINDING SITE PLANS Plat Map(5) Engineering Plans(1),(2) Full or Targeted Drainage Review(2)  Preliminary Plans(5)  Engineering Plans(1) Large Project Drainage Review  Preliminary Plans(5)  Master Drainage Plan(4) or Special Study  Engineering Plans(1) SHORT PLATS Site Plan(5) Simplified Drainage Review Simplified Drainage Plans(3) Site Plan(5) Preliminary Reports Simplified Drainage Review AND Targeted Drainage Review(2)  Simplified Drainage Plans(3)  Engineering Plans(1) Full or Targeted Drainage Review(2) Engineering Plans(1) COMMERCIAL PERMITS Engineering Plans(1),(2) Full or Targeted Drainage Review Engineering Plans(1) SINGLE- FAMILY Site Plan(5) for Single-Family Residential Building Permits Simplified Drainage Review Simplified Drainage Plans(3) SECTION 2.2 PLANS REQUIRED WITH CONSTRUCTION PERMIT APPLICATION 6/22/2022 2022 City of Renton Surface Water Design Manual 2-6 TABLE 2.2.A DRAINAGE PLAN SUBMITTALS Type of Permit or Project Plans Required with Construction Permit Application Type of Drainage Review Plans Required for Drainage Review RESIDENTIAL BUILDING PERMITS Simplified Drainage Review AND Targeted Drainage Review(2) AND Directed Drainage Review(6)  Simplified Drainage Plans(3)  Engineering Plans(1)(6) Full or Targeted Drainage Review(2) Engineering Plans(1) OTHER PROJECTS OR PERMITS Project-specific (contact CED or the City’s website) Full or Targeted Drainage Review(2) Engineering Plans(1) Notes: (1) Submittal specifications for engineering plans are detailed in Section 2.3.1. (2) Submittal specifications for Targeted Drainage Review are found in Section 2.3.2. (3) Specifications for submittal of Simplified drainage plans are found in Appendix C, Simplified Drainage Requirements. (4) Specifications for submittal of master drainage plans or special studies are found in the King County publication titled Master Drainage Planning for Large or Complex Site Developments. (5) Submittal specifications for these plans are found on the City’s website and/or from CED staff in the pre-application meeting. (6) Scope of submittals for Directed Drainage Review is determined by CED staff at the City’s Permit Counter and/or during the plan review process. Submittal specifications per Notes 1, 2, and 3. 2.2.1 SUBDIVISION, PUD, AND BINDING SITE PLANS Applications for proposed subdivision, PUD, and binding site plan projects must include engineered preliminary plans, which are used to help determine engineering plan requirements to recommend to the Hearing Examiner. Preliminary plans shall include the following: 1. A conceptual drainage plan prepared, stamped, and signed by a civil engineer. This plan must show the location and type of the following: a) Existing and proposed flow control facilities b) Existing and proposed water quality facilities c) Existing and proposed conveyance systems. The level of detail of the plan should correspond to the complexity of the project. 2. A Level 1 Downstream Analysis as required in Core Requirement #2 and outlined under “TIR Section 3, Offsite Analysis.” This offsite analysis shall be submitted in order to assess potential offsite drainage and water quality impacts associated with development of the project, and to help propose appropriate mitigation of those impacts. A higher level of offsite analysis may be requested by the City prior to preliminary approval, or as a condition of engineering plan submittal. The offsite analysis must be prepared, stamped, and signed by a civil engineer. 3. Survey/topographic information. The submitted site plan and conceptual drainage plan shall include the following: a) Field topographic base map to accompany application (aerial topography allowed with CED permission) b) Name and address of surveyor and surveyor’s seal and signature 2.2.5 OTHER PERMITS 2022 City of Renton Surface Water Design Manual 6/22/2022 2-7 c) Notation for field or aerial survey d) Datum and benchmark/location and basis of elevation e) Location of all critical areas f) Contour intervals per the following chart: Zoning Designation Contour Intervals Densities of developed area of over 4 DU per acre 2 feet at less than 15% slope 5 feet at 15% slope or more Densities of developed area of 4 DU or less per acre 5 feet 2.2.2 SHORT SUBDIVISIONS Applications for proposed short plats1 require a proposed site plan drawn to scale showing geographic features such as adjacent streets, existing buildings, and critical areas if any are known to be present; and a Level 1 Downstream Analysis. Site plans are usually engineered, except for projects exempt from drainage review or projects subject to Simplified Drainage Review for the entire project. The specifications for submittal of site plans are outlined on the City’s website. The Level 1 Downstream Analysis is required for all short plats except those meeting the exemptions outlined in Section 1.2.2 or those subject to Simplified Drainage Review for the entire project. A higher level of offsite analysis may be requested by CED prior to preliminary approval, or as a condition of engineering plan submittal. 2.2.3 COMMERCIAL SITE DEVELOPMENT Applications for commercial permits require that engineering plans be submitted as part of the building permit application, unless otherwise approved by CED. Most commercial projects will go through Full Drainage Review and require complete engineering plans. Projects that qualify for limited scope engineering design should request Targeted Drainage Review during the pre-application meeting with CED. 2.2.4 SINGLE-FAMILY RESIDENTIAL Applications for single-family residential permits1 require a non-engineered site plan to be submitted. The specifications for site plans are outlined on the City’s website. 2.2.5 OTHER PERMITS Other permit applications1 will require project-specific information. Submittal requirements can be obtained by contacting CED or consulting the City’s website. 1 The specific level of required drainage analysis and design is usually determined during the preliminary drainage review of the plans submitted with the application. The overall plan review process may be expedited if the project is submitted with the appropriate level of detail. SECTION 2.2 PLANS REQUIRED WITH CONSTRUCTION PERMIT APPLICATION 6/22/2022 2022 City of Renton Surface Water Design Manual 2-8 (T his page intentionally left blan k. ) CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 2-9 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS This section presents the specifications and contents required of plans to facilitate drainage review. Most projects subject to Full Drainage Review will require engineering plans that include a “technical information report (TIR),” “site improvement plans,” and a “construction stormwater pollution prevention (CSWPP) plan,” which includes an “erosion and sediment control (ESC) plan” and a “stormwater pollution prevention and spill (SWPPS) plan.” For more information on the types of projects subject to Full Drainage Review, see Section 1.1.2.4. Additional information is available at the City’s website and from the staff in the pre-application meeting, if held. Small projects with specific drainage concerns that are subject to Targeted Drainage Review also require engineering plans that include the same elements, except that the TIR may be of limited scope. The site improvement plans, ESC and SWPPS plans may also be of limited scope, but must meet all applicable specifications. For more information on the types of projects subject to Targeted Drainage Review, see Section 1.1.2.2. Projects subject to Simplified Drainage Review may be required to submit “Simplified drainage plans.” These are simplified drainage and erosion control plans that may be prepared by a non-engineer from a set of pre-engineered design details, and which do not require a TIR or a separate SWPPS plan. The Simplified Drainage Requirements booklet available at King County Department of Permitting and Environmental Review and appended to this manual (Appendix C) contains the specifications for Simplified drainage plans and details on the Simplified Drainage Review process. Note: Projects in Simplified Drainage Review may be required to submit engineering plans if they are also subject to Targeted Drainage Review as determined in Section 1.1.2.2 and Appendix C. Also, short plats in Simplified Drainage Review will be required to submit engineering plans if roadway construction is a condition of preliminary approval. Single-family residential projects that do not qualify for Simplified Drainage Review may qualify for Directed Drainage Review, which requires a specialized list of submittals (plans, technical reports, etc.) and engineering requirements determined by CED review staff that ensures compliance with all core and special requirements of the SWDM. The scope of the submittal requirement is determined during the initial review of the application. Specifications for the plans and TIR generally follow those described for the other review types but may be reduced in scope or complexity in accordance with CED’s determination. Design Plan Certification All preliminary plans and engineering plans must be stamped by a civil engineer. All land boundary surveys, and legal descriptions used for preliminary and engineering plans must be stamped by a land surveyor licensed in the State of Washington. Topographic survey data and mapping prepared specifically for a proposed project may be performed by the civil engineer stamping the engineering plans as allowed by the Washington State Board of Registration for Professional Engineers and Land Surveyors. SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS 6/22/2022 2022 City of Renton Surface Water Design Manual 2-10 2.3.1 ENGINEERING PLAN SPECIFICATIONS For drainage review purposes, engineering plans must consist of the following: 1. A TIR as detailed in Section 2.3.1.1, AND 2. Site improvement plans as detailed in Section 2.3.1.2, AND 3. A CSWPP plan, which includes an ESC plan as detailed in Section 2.3.1.3 and a SWPPS plan as detailed in Section 2.3.1.4. Projects in Targeted Drainage Review require a limited scope TIR with site improvement plans and a CSWPP plan, as detailed in Section 2.3.2. CED may allow a modified site improvement plan for some projects in Targeted Drainage Review (see Section 2.3.2) or where major improvements (e.g., detention facilities, conveyance systems, bridges, road right-of-way improvements, etc.) are not proposed. 2.3.1.1 TECHNICAL INFORMATION REPORT (TIR) The full TIR is a comprehensive supplemental report containing all technical information and analysis necessary to develop the site improvement plan. This report shall contain all calculations, conceptual design analysis, reports, and studies required and used to construct a complete site improvement plan based on sound engineering practices and careful geotechnical and hydrological design. The TIR must be stamped and dated by a civil engineer. The TIR shall contain the following ten sections, preceded by a table of contents: 1. Project Overview 2. Conditions and Requirements Summary 3. Offsite Analysis 4. Flow Control, Low Impact Development (LID) and Water Quality Facility Analysis and Design 5. Conveyance System Analysis and Design 6. Special Reports and Studies 7. Other Permits 8. CSWPP Analysis and Design 9. Bond Quantities, Facility Summaries, and Declaration of Covenant 10. Operations and Maintenance Manual. Every TIR must contain each of these sections; however, if a section does not apply, the applicant may simply mark “N/A” and a brief explanation shall be provided. This standardized format allows a quicker, more efficient review of information required to supplement the site improvement plan. The table of contents should include a list of the ten section headings and their respective page numbers, a list of tables with page numbers, and a list of numbered references, attachments, and appendices. When the TIR package requires revisions, the revisions must be submitted in a complete TIR package.  TIR SECTION 1 PROJECT OVERVIEW The project overview must provide a general description of the proposal, predeveloped and developed site conditions, site and project site area, size of the improvements, and the disposition of stormwater runoff before and after development. The overview shall identify and discuss difficult site parameters, the natural drainage system, and drainage to and from adjacent property, including bypass flows. The following figures are required: 2.3.1 ENGINEERING PLAN SPECIFICATIONS 2022 City of Renton Surface Water Design Manual 6/22/2022 2-11 Figure 1. TIR Worksheet Include a copy of the TIR Worksheet (see Reference Section 8-A). Figure 2. Site Location Provide a map that shows the general location of the site. Identify all roads that border the site and all significant geographic features and critical areas (lakes, streams, steep slopes, etc.). Figure 3. Drainage Basins, Subbasins, and Site Characteristics This figure shall display the following: 1. Show acreage of subbasins. 2. Identify all site characteristics. 3. Show all areas used for treatment trades or mitigation trades, if applicable. 4. Show all onsite and offsite bypass areas, if applicable. 5. Show all threshold discharge areas (TDAs). 6. Show existing discharge points to and from the site. 7. Show routes of existing, construction, and future flows at all discharge points and downstream hydraulic structures. 8. Use a minimum USGS 1:2400 topographic map as a base for the figure. 9. Show (and cite) the length of travel from the farthest upstream end of a proposed storm system in the development to any proposed flow control facility. Figure 4. Soils Show the soils within the following areas: 1. The project site. 2. The area draining to the site. 3. The drainage system downstream of the site for the distance of the downstream analysis (see Section 1.2.2). Copies of King County Soil Survey maps may be used; however, if the maps do not accurately represent the soils for a proposed project (including offsite areas of concern), it is the design engineer’s responsibility to ensure that the actual soil types are properly mapped. Soil classification symbols that conform to the SCS Soil Survey for King County shall be used; and the equivalent soil type (till, outwash, or wetlands) per the approved stormwater model shall be indicated (see Table 3.2.2.B). Projects will need to evaluate the soils on each lot for applicability of the full infiltration and other low impact on-site BMPs as specified in Core Requirement #9. This soils report, as well as geotechnical investigations necessary for proposed infiltration facilities, shall be referenced in the TIR Overview and submitted under Special Reports and Studies, TIR Section VI. A figure in the required geotechnical report that meets the above requirements may be referenced to satisfy 1, 2, and 3 above. Projects located in outwash soils may need to provide a low-permeability liner or a treatment liner for facilities that allow runoff to have direct contact with the soil and open channel conveyance systems that are not concrete lined, consistent with the specifications for such liners in Section 6.2.4.  TIR SECTION 2 CONDITIONS AND REQUIREMENTS SUMMARY The intent of this section is to ensure all preliminary approval conditions and applicable requirements pertaining to site engineering issues have been addressed in the site improvement plan. All conditions and requirements for the proposed project shall be included. SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS 6/22/2022 2022 City of Renton Surface Water Design Manual 2-12 In addition to the core requirements of this manual, adopted basin plans and other plans as listed in Special Requirement #1 should be reviewed and applicable requirements noted. Additionally, critical area requirements, conditions of plat approval, and conditions associated with development requirements (e.g., conditional use permits, rezones, variances and adjustments, SEPA mitigations, etc.) shall be included.  TIR SECTION 3 OFFSITE ANALYSIS All projects in engineering review shall complete, at a minimum, an Offsite Analysis, except for projects meeting the exemptions outlined in Section 1.2.2. The Offsite Analysis is usually completed as part of the initial permit application and review process, and is to be included in the TIR. Note: If offsite conditions have been altered since the initial submittal, a new offsite analysis may be required. The primary component of the offsite analysis is the downstream analysis described in detail below. Upstream areas are included in this component to the extent they are expected to be affected by backwater effects from the proposed project. Other components of the offsite analysis could include, but are not limited to, evaluation of impacts to fish habitat, groundwater levels, groundwater quality, or other environmental features expected to be significantly impacted by the proposed project due to its size or proximity to such features. Levels of Analysis The offsite analysis report requirements vary depending on the specific site and downstream conditions. Each project submittal shall include at least a Level 1 downstream analysis. Upon review of the Level 1 analysis, CED may require a Level 2 or Level 3 analysis. If conditions warrant, additional, more detailed analysis may be required. Note: Potential impacts upstream of the proposal shall also be evaluated. Level 1 Analysis The Level 1 analysis is a qualitative survey of each downstream system leaving a site. This analysis is required for all proposed projects and shall be submitted with the initial permit application. Depending on the findings of the Level 1 analysis, a Level 2 or 3 analysis may need to be completed or additional information may be required. If further analysis is required, the applicant may schedule a meeting with CED staff. Level 2 or 3 Analysis If drainage problems are identified in the Level 1 analysis, a Level 2 (rough quantitative) analysis or a Level 3 (more precise quantitative) analysis may be required to further evaluate proposed mitigation for the problem. CED staff will determine whether a Level 2 or 3 analysis is required based on the evidence of existing or potential drainage problems identified in the Level 1 analysis and on the proposed design of onsite drainage facilities. The Level 3 analysis is required when results need to be as accurate as possible: for example, if the site is flat; if the system is affected by downstream controls; if minor changes in the drainage system could flood roads or buildings; or if the proposed project will contribute more than 15 percent of the total peak flow to the drainage problem location. The Level 2 or 3 analysis may not be required if CED determines from the Level 1 analysis that adequate mitigation will be provided. Additional Analysis Additional, more detailed hydrologic analysis may be required if CED determines that the downstream analysis has not been sufficient to accurately determine the impacts of a proposed project on an existing or potential drainage problem. This more detailed analysis may include a point of compliance analysis as detailed in Section 3.3.6. 2.3.1 ENGINEERING PLAN SPECIFICATIONS 2022 City of Renton Surface Water Design Manual 6/22/2022 2-13 Scope of Analysis Regardless of the level of downstream analysis required, the applicant shall define and map the study area (Task 1), review resources (Task 2), inspect the study area (Task 3), describe the drainage system and problems (Task 4), and propose mitigation measures (Task 5) as described below. Task 1. Study Area Definition and Maps For the purposes of Task 2 below, the study area shall extend downstream one mile (minimum flowpath distance) from the proposed project discharge location and shall extend upstream as necessary to encompass the offsite drainage area tributary to the proposed project site. For the purposes of Tasks 3, 4, and 5, the study area shall extend downstream to a point on the drainage system where the proposed project site constitutes less than 15 percent of the total tributary drainage area, but not less than one-quarter mile (minimum flowpath distance). The study area shall also extend upstream of the project site a distance sufficient to preclude any backwater effects from the proposed project. The offsite analysis shall include a site map showing property lines, and the best available topographical map (e.g., from CED and Renton topographic map) with the study area boundaries, site boundaries, downstream flowpath for a distance of one mile, and potential/existing problems (Task 4) shown. Other maps, diagrams, photographs and aerial photos may be helpful in describing the study area. Task 2. Resource Review To assist the design engineer in preparing an offsite analysis, Renton has gathered information regarding existing and potential flooding, erosion, and water quality problems. For all levels of analysis, all of the resources described below shall be reviewed for existing/potential problems in the study area (upstream and one mile downstream of the project site):  Adopted basin plans available at King County DLS-Permitting, King County DNRP, and CED. For areas where there is no adopted basin plan, Basin Reconnaissance Summary Reports may be useful.  Floodplain/floodway (FEMA) maps available at CED and the library.  Other offsite analysis reports in the same subbasin, if available (check with CED staff).  Sensitive Areas Folio available at King County DLS-Permitting, King County DNRP, and COR Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>) must be used to document the distance downstream from the proposed project to the nearest critical areas.  2U.S. Department of Agriculture, King County Soils Survey available at King County DLS- Permitting and the library.  Wetlands Inventory maps available at CED.  Washington State Department of Ecology’s latest published Clean Water Act Section 303d list of polluted waters posted at the following website: <https://ecology.wa.gov/Water- Shorelines/Water-quality/Water-improvement/Assessment-of-state-waters-303d>.  City of Renton Erosion Maps and Landslide Maps. Potential/existing problems identified in the above documents shall be documented in the Drainage System Table (see Reference Section 8-B) as well as described in the text of the Level 1 Downstream Analysis Report. If a document is not available for the site, note in the report that the information was not available as of a particular date. If necessary, additional resources are available from King County, the Washington State Department of Fisheries and Wildlife (WDFW), the State Department of Ecology (Ecology), the United States Army Corps of Engineers (Corps), and the public works departments of other municipalities in the vicinity of the proposed project site. 2 Footnote 2 is not used. SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS 6/22/2022 2022 City of Renton Surface Water Design Manual 2-14 Task 3. Field Inspection The design engineer shall physically inspect the existing on- and offsite drainage systems of the study area for each discharge location. Specifically, he/she shall investigate any evidence of the following existing or potential problems and drainage features: Level 1 Inspection: 1. Investigate any problems reported or observed during the resource review. 2. Locate all existing/potential constrictions or lack of capacity in the existing drainage system. 3. Identify all existing/potential downstream drainage problems as defined in Section 1.2.2.1. 4. Identify existing/potential overtopping, scouring, bank sloughing, or sedimentation. 5. Identify significant destruction of aquatic habitat or organisms (e.g., severe siltation, bank erosion, or incision in a stream). 6. Collect qualitative data on features such as land use, impervious surfaces, topography, and soil types. 7. Collect information on pipe sizes, channel characteristics, drainage structures, and relevant critical areas (e.g., wetlands, streams, steep slopes). 8. Verify tributary basins delineated in Task 1. 9. Contact neighboring property owners or residents in the area about past or existing drainage problems, and describe these in the report (optional). 10. Note the date and weather conditions at the time of the inspection. Level 2 or 3 Inspection: 1. Perform a Level 1 Inspection. 2. Document existing site conditions (approved drainage systems or pre-1979 aerial photographs) as defined in Core Requirement #3. 3. Collect quantitative field data. For Level 2, conduct rough field survey using hand tape, hand level, and rod; for Level 3, collect field survey profile and cross-section topographic data prepared by an experienced surveyor. Task 4. Drainage System Description and Problem Descriptions Each drainage system component and problem shall be addressed in the offsite analysis report in three places: on a map (Task 1), in the narrative (Task 4), and in the Offsite Analysis Drainage System Table (see Reference Section 8-B). Drainage System Descriptions: The following information about drainage system components such as pipes, culverts, bridges, outfalls, ponds, tanks, and vaults shall be included in the report: 1. Location (corresponding map label and distance downstream/upstream from site discharge) 2. Physical description (type, size, length, slope, vegetation, and land cover) 3. Problems including copies of any relevant drainage complaints 4. Field observations. Problem Descriptions: All existing or potential drainage and water quality problems (e.g., ponding water, high/low flows, siltation, erosion, listed water bodies, etc.) identified in the resource review or field inspection shall be described in the offsite analysis. These descriptions will help in determining if such problems require special attention per Core Requirement #2 (see Section 1.2.2.1) because they are one of three defined drainage problem types or one of seven defined water quality problem types. Special attention may include more analysis, additional flow control, or other onsite or offsite 2.3.1 ENGINEERING PLAN SPECIFICATIONS 2022 City of Renton Surface Water Design Manual 6/22/2022 2-15 mitigation measures as specified by the problem-specific mitigation requirements set forth in Sections 1.2.2.2 and 1.2.2.3. The following information shall be provided for each existing or potential drainage problem: 1. Description of the problem (ponding water, high or low flows, siltation, erosion, slides, etc.). 2. Magnitude of or damage caused by the drainage problem (siltation of ponds, dried-up ornamental ponds, road inundation, flooded property, flooded building, flooded septic system, significant destruction of aquatic habitat or organisms). 3. General frequency and duration of drainage problem (dates and times the problem occurred, if available). 4. Return frequency of storm or flow (cfs) of the water when the problem occurs (optional for Level 1 and required for Levels 2 and 3). Note: A Level 2 or 3 analysis may be required to accurately identify the return frequency of a particular downstream problem; see Section 3.3.3. 5. Water surface elevation when the problem occurs (e.g., elevation of building foundation, crest of roadway, elevation of septic drainfields, or wetland/stream high water mark). 6. Names and concerns of involved parties (optional for all levels of analysis). 7. Current mitigation of the drainage problem. 8. Possible cause of the drainage problem. 9. Whether the proposed project is likely to aggravate (increase the frequency or severity of) the existing drainage problem or create a new one based on the above information. For example, an existing erosion problem should not be aggravated if Durational flow control is already required in the region for the design of onsite flow control facilities. Conversely, a downstream flooding problem inundating a home every 2 to 5 years will likely be aggravated if only Peak Flow Control is being applied in the region. See Section 1.2.3.1 for more details on the effectiveness of flow control standards in addressing downstream problems. The following information shall be provided for each existing or potential water quality problem: 1. Description of the problem as documented by the State, County, or City in the problem’s listing. This should include the pollutant or pollutants of concern, the nature or category of the listing, and any other background information provided in the listing. 2. Flow path distance downstream of the project site and percentage of area draining to the problem that the project site occupies. 3. Possible or probable cause of the water quality problem. 4. Any current mitigation of the water quality problem. Task 5. Mitigation of Existing or Potential Problems For any existing or potential offsite drainage problem determined to be one of the three defined problem types in Section 1.2.2.1, the design engineer must demonstrate that the proposed project neither aggravates (if existing) nor creates the problem as specified in the drainage problem-specific mitigation requirements set forth in Section 1.2.2.2. The engineer must review each relevant drainage complaint found and include a narrative explaining how each complaint problems is addressed or mitigated. Actual copies of the relevant complaints must be included in the Analysis. To meet these requirements, the proposed project may need to provide additional onsite flow control as specified in Table 1.2.3.A (see also Section 3.3.5), or other onsite or offsite mitigation measures as described in Section 3.3.5. For any existing or potential water quality problem determined to be one of the seven defined water quality problem types in Section 1.2.2.1, the design engineer must document how the applicable water quality problem-specific mitigation requirement in Section 1.2.2.3 will be met. SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS 6/22/2022 2022 City of Renton Surface Water Design Manual 2-16  TIR SECTION 4 FLOW CONTROL, LOW IMPACT DEVELOPMENT (LID) AND WATER QUALITY FACILITY ANALYSIS AND DESIGN Existing Site Hydrology (Part A) This section of the TIR shall include a discussion of assumptions and site parameters used in analyzing the existing site hydrology. The acreage, soil types, and land covers used to determine existing flow characteristics, along with basin maps, graphics, and exhibits for each subbasin affected by the development, shall be included. The following information must be provided on a topographical map: 1. Delineation and acreage of areas contributing runoff to the site 2. Flow control facility and BMP location(s) 3. Outfall(s) 4. Overflow route(s) 5. Area(s) used for treatment trade or mitigation trade, if applicable 6. Onsite and offsite bypass area(s), if applicable 7. Threshold discharge area(s) The scale of the map and the contour intervals must be sufficient to determine the basin and subbasin boundaries accurately. The direction of flow, the acreage of areas contributing drainage, and the limits of development shall all be indicated on the map. Each subbasin contained within or flowing through the site shall be individually labeled and parameters for the approved stormwater model referenced to that subbasin. All natural streams and drainage features, including wetlands and depressions, must be shown. Rivers, closed depressions, streams, lakes, and wetlands must have the 100-year floodplain (and floodway where applicable) delineated as required in Special Requirement #2 (see Section 1.3.2) and by the critical areas requirements in RMC 4-3-050. Developed Site Hydrology (Part B) This section shall provide narrative, mathematical, and graphical presentations of parameters selected and values used for the developed site conditions, including acreage, soil types and land covers, roadway layouts, and all constructed drainage facilities and any required on-site BMPs. Developed subbasin areas and flows shall be clearly depicted on a map and cross-referenced to computer printouts or calculation sheets. Relevant portions of the calculations shall be highlighted and tabulated in a listing of all developed subbasin flows. All maps, exhibits, graphics, and references used to determine developed site hydrology must be included, maintaining the same subbasin labeling as used for the existing site hydrology whenever possible. If the boundaries of the subbasin have been modified under the developed condition, the labeling should be modified accordingly (e.g., Subbasin “Am” is a modified version of existing Subbasin “A”). Performance Standards (Part C) The design engineer shall include brief discussions of the following:  The applicable area-specific flow control facility standard as depicted in the Flow Control Application layer in COR Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>) per Section 1.2.3.1, any modifications to the standard to address onsite or offsite drainage conditions, and applicable on-site BMP requirements determined from Section 1.2.3.3 and Core Requirement #9; and 2.3.1 ENGINEERING PLAN SPECIFICATIONS 2022 City of Renton Surface Water Design Manual 6/22/2022 2-17  The applicable area-specific water quality treatment menu determined from the Water Quality Applications Map per Section 1.2.8.1, and any applicable special requirements for source control or oil control determined from Sections 1.3.4 and 1.3.5. Flow Control System (Part D) This section requires:  An illustrative sketch of the flow control facility (or facilities), required on-site BMPs, and appurtenances. The facility sketch (or sketches) must show basic measurements necessary to calculate the storage volumes available from zero to the maximum head, all orifice/restrictor sizes and head relationships, control structure/restrictor orientation to the facility, and facility orientation on the site. The on-site BMP sketch (or sketches) must show basic measurements and dimensions, orientation on the site, flowpath lengths, etc.  The applicant shall include all supporting documentation such as computer printouts, calculations, equations, references, storage/volume tables, graphs, soils data, geotechnical reports and any other aides necessary to clearly show results and methodology used to determine the storage facility volumes and on-site BMP applications.  Facility documentation files, flow duration comparison files, peaks files, return frequency or duration curves, etc., developed with the approved model shall be included to verify the facility meets the performance standards indicated in Part C.  The volumetric safety factor used in the design shall be clearly identified, as well as the reasoning used by the design engineer in selecting the safety factor for this project.  If on-site BMP credits are used as allowed in Core Requirement #9, documentation must be provided, explaining how the credits will be used and how the criteria for use of credits will be met.  If the flow control system is an infiltration facility, the soils data, groundwater mounding analysis, and other calculations used to determine the design infiltration rate shall be provided.  On-site BMP infeasibility discussion and supporting documentation shall also be included in Part D. Water Quality System (Part E) This section requires an illustrative sketch of the proposed water quality facility (or facilities), source controls, oil controls, and appurtenances. This sketch (or sketches) of the facility, source controls, and oil controls must show basic measurements and dimensions, orientation on the site, location of inflow, bypass, and discharge systems, etc. The applicant shall also include all supporting documentation such as computer printouts, calculations, equations, references, and graphs necessary to show the facility was designed and sized in accordance with the specifications and requirements in Chapter 6. If the water quality credit option is used as allowed in Section 6.1.3, documentation must be provided, identifying the actions that will be taken to acquire the requisite credits.  TIR SECTION 5 CONVEYANCE SYSTEM ANALYSIS AND DESIGN This section shall present a detailed analysis of any existing conveyance systems, and the analysis and design of the proposed stormwater collection and conveyance system for the development. This section also includes any analysis required for the design of bridges to convey flows and pass sediments and debris per Section 4.4.3. Analysis information should be presented in a clear, concise manner that can be easily followed, checked, and verified. This section shall explain the applicable conveyance system capacity standards per Section 1.2.4. All pipes, culverts, catch basins, channels, swales, and other stormwater conveyance appurtenances must be clearly labeled and correspond directly to the engineering plans. The minimum information included shall be pipe flow tables, flow profile computation tables, nomographs, charts, graphs, detail drawings, and other tabular or graphic aides used to design and confirm performance of the conveyance system. SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS 6/22/2022 2022 City of Renton Surface Water Design Manual 2-18 Verification of capacity and performance must be provided for each element of the conveyance system. The analysis must show design velocities and flows for all drainage facilities within the development, as well as those offsite that are affected by the development. If the final design results are on a computer printout, a separate summary tabulation of conveyance system performance shall also be provided.  TIR SECTION 6 SPECIAL REPORTS AND STUDIES Some site characteristics, such as steep slopes or wetlands, pose unique road and drainage design problems that are particularly sensitive to stormwater runoff. As a result, CED may require the preparation of special reports and studies that further address the site characteristics, the potential for impacts associated with the development, and the measures that would be implemented to mitigate impacts. Special reports shall be prepared by people with expertise in the particular area of analysis. Topics of special reports may include any of the following:  Floodplain delineation in accordance with Section 1.3.2  Flood protection facility conformance in accordance with Section 1.3.3  Critical areas analysis and delineation  Geotechnical/soils (soils documentation supporting on-site BMP design, infiltration rate determination and infeasibility conclusions may also be located in TIR Section 6)  Groundwater, including groundwater mounding analyses required for infiltration design  Slope protection/stability  Erosion and deposition  Geology  Hydrology  Fluvial geomorphology  Anadromous fisheries impacts  Water quality  Structural design  Structural fill  Aquifer Protection Areas  TIR SECTION 7 OTHER PERMITS Construction of road and drainage facilities may require additional permits from other agencies for some projects. These additional permits may contain more restrictive drainage plan requirements. This section of the TIR should provide the titles of any other permits, the agencies requiring the other permits, and the permit requirements that affect the drainage plan. Examples of other permits are listed in Section 1.1.3. If a UIC well registration is required, a copy must be provided.  TIR SECTION 8 CSWPP PLAN ANALYSIS AND DESIGN This section of the TIR should include the analysis and design information used to prepare the required construction stormwater pollution prevention (CSWPP) plan . This information should be presented in two parts associated with the CSWPP plan’s two component plans, the erosion sediment control (ESC) plan (Part A) and the stormwater pollution prevention and spill control (SWPPS) plan (Part B). See Sections 2.3.1.3 and 2.3.1.4 for plan specifications and contents. ESC Plan Analysis and Design (Part A) This section must include all hydrologic and hydraulic information used to analyze and design the erosion and sediment control measures, including final site stabilization measures. The TIR shall explain how proposed ESC measures comply with the Erosion and Sediment Control Standards in Appendix D and 2.3.1 ENGINEERING PLAN SPECIFICATIONS 2022 City of Renton Surface Water Design Manual 6/22/2022 2-19 show compliance with the implementation requirements of Core Requirement #5, Section 1.2.5. Part A must include the following: 1. Provide sufficient information to justify the overall ESC plan and the choice of individual ESC measures. At a minimum, there shall be a discussion of each of the measures specified in Section 1.2.5 and their applicability to the proposed project. 2. Include all hydrologic and hydraulic information used to analyze and size the ESC facilities shown in the engineering plans. Describe the methodology, and attach any graphics or sketches used to size the facilities. 3. Identify areas with a particularly high susceptibility to erosion because of slopes or soils, as well as areas to be protected for existing and proposed on-site BMPs. Discuss any special measures taken to protect these areas as well as any special measures proposed to protect water resources on or near the site. 4. Identify any ESC recommendations in any of the special reports prepared for the project. In the project geotechnical report supporting on-site BMP design, provide recommendations to address mitigation of on-site BMP areas impacted by erosion and/or sedimentation during construction. If these special reports’ recommendations are not included in the ESC plan, provide justification. 5. If proposing exceptions or modifications to the standards detailed in the Erosion and Sediment Control Standards in Appendix D, clearly present the rationale. If proposing techniques or products different from those detailed in the ESC Standards, provide supporting documentation so the City can determine if the proposed alternatives provide similar protection. SWPPS Plan Design (Part B) The stormwater pollution prevention and spill control plan must identify all activities that could contribute pollutants to surface and storm water during construction. This section of the TIR must provide sufficient information to justify the selection of specific stormwater pollution prevention (SWPPS) BMPs proposed to be applied to the pollution-generating activities that will occur with construction of the proposed project. BMPs applicable to such activities are found in the Construction Stormwater Pollution Prevention and Spill Control (CSWPP) Standards (Appendix D) and the King County Stormwater Pollution Prevention Manual (viewable at <https://www.kingcounty.gov/sppm>) adopted pursuant to RMC 4-6-030. At a minimum, there shall be a discussion of each anticipated pollution-generating activity and the pollution prevention BMPs selected to address it. If there are any calculations required for the selected BMP, include those in the discussion. If an alternative BMP or major modification to one of the City’s standard BMPs will be used, a written request must be submitted for review and approval, detailing how the alternative will work. An “Alternative BMP Request Form” is available in the Stormwater Pollution Prevention Manual. Updates or revisions to the SWPPS plan may be requested by CED at any time during project construction if CED determines that pollutants generated on the construction site have the potential to contaminate surface, storm, or ground water. The SWPPS plan shall also discuss the receiving waters, especially if the receiving water body is listed on the 303d list. Information must be provided that shows the plan meets TMDL requirements. Discuss the 303(d) listed pollutant generated or used onsite and any special handling requirements or BMPs.  TIR SECTION 9 BOND QUANTITIES, FACILITY SUMMARIES, AND DECLARATION OF COVENANT Bond Quantities Worksheet Each plan submittal requires a construction quantity summary to establish appropriate bond amounts. Using the Site Improvement Bond Quantities Worksheet furnished by CED (see the City’s website), the SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS 6/22/2022 2022 City of Renton Surface Water Design Manual 2-20 design engineer shall separate existing right-of-way and erosion control quantities from other onsite improvements. In addition, the design engineer shall total the amounts based on the unit prices listed on the form. Drainage facilities for single-family residential building permits, which are normally not bonded, shall be constructed and approved prior to finalization of the permit. Flow Control and Water Quality Facility Summary Sheet and Sketch Following approval of the plans, a Flow Control and Water Quality Facility Summary Sheet and Sketch (see Reference Section 8-D) shall be submitted along with an 81/2″ x 11″ plan sketch for each facility proposed for construction. The plan shall show a north arrow, the tract, the facility access road, the extent of the facility, and the control structure location. The approximate street address shall be noted. At project completion, the Summary Sheet and Sketch shall be updated in the Final Corrected TIR to reflect the completed project (see Section 2.4.2). Declaration of Covenant for Privately Maintained Flow Control and WQ Facilities Any declaration of covenant and grant of easement required for proposed flow control and water quality facilities per Section 1.2.6 must be included here for review and approval before recording. The necessary covenant exhibits, and maintenance instructions associated with the facility type (see Reference Section 5), shall be included with the declaration of covenant. After approval by CED, the declaration of covenant and grant of easement must be signed and recorded at the office of King County Records and Elections before finalization of any permit. A copy of the recorded document shall be included in the Final Corrected TIR (see Section 2.4.2). Declaration of Covenant for Privately Maintained On-Site BMPs Any declarations of covenant and grant of easement required for proposed on-site BMPs per Core Requirement #9 must be included here for review and approval before recording. The necessary covenant exhibits, and maintenance instructions associated with the on-site BMP type (see Reference Section 5), shall be included with the declaration of covenant. After approval by CED, all such documents must be signed and recorded at the office of King County Records and Elections before finalization of any permit. A copy of the recorded document shall be included in the Final Corrected TIR (see Section 2.4.2) or otherwise provided to the CED if no TIR was required.  TIR SECTION 10 OPERATIONS AND MAINTENANCE MANUAL For each flow control and water quality facility and/or BMP that is to be privately maintained, and for those that have special non-standard features, the design engineer shall prepare an operations and maintenance manual. The manual should be simply written and should contain a brief description of the facility or BMP, what it does, and how it works. In addition, the manual shall include a copy of the Maintenance Requirements for Flow Control, Conveyance, and WQ Facilities (see Appendix A) and provide an outline of maintenance tasks and the recommended frequency each task should be performed. This is especially important for on-site BMP and water quality facilities where proper maintenance is critical to facility performance. For this reason, most of the flow control facility designs in Chapter 5 and the water quality facility designs in Chapter 6 include “maintenance considerations” important to the performance of each facility. BMP maintenance instructions by BMP type, prepared in 8-1/2″ x 11″ size for inclusion in TIRs and declarations of covenant, are also provided in Reference Section 5. 2.3.1.2 SITE IMPROVEMENT PLAN Site improvement plans shall portray design concepts in a clear and concise manner. The plans must present all the information necessary for persons trained in engineering to review the plans, as well as those persons skilled in construction work to build the project according to the design engineer’s intent. Supporting documentation for the site improvement plans must also be presented in an orderly and concise format that can be systematically reviewed and understood by others. 2.3.1 ENGINEERING PLAN SPECIFICATIONS 2022 City of Renton Surface Water Design Manual 6/22/2022 2-21 Survey Datum and Precision (RC) The horizontal component of all surveys shall have as its coordinate base: The North American Datum of 1983/91. All horizontal control for projects must be referenced to or in conjunction with a minimum of two of the City of Renton’s Survey Control Network monuments. The source of the coordinate values used will be shown on the survey drawing per RCW 58.09.070. The horizontal component of all surveys shall meet or exceed the closure requirements of WAC 332-130-060. The control base lines for all surveys shall meet or exceed the requirements for a Class A survey revealed in Table 2 of the Minimum Standard Detail Requirements for ALTA/ACSM Land Title Surveys jointly established and adopted by ALTA and ACSM in 1992 or comparable classification in future editions of said document. The angular and linear closure and precision ratio of traverses used for survey control shall be revealed on the face of the survey drawing, as shall the method of adjustment. The horizontal component of the control system for surveys using global positioning system methodology shall exhibit at least 1 part in 50,000 precision in line length dependent error analysis at a 95 percent confidence level and performed pursuant to Federal Geodetic Control Subcommittee Standards for GPS control surveys as defined in Geometric Geodetic Accuracy Standards & Specifications for Using GPS Relative Positioning Techniques dated August 1, 1989 or comparable classification in future editions of said document. The vertical component of all surveys shall be based on NAVD 1988, the North American Vertical Datum of 1988, and tied to at least one of the City of Renton Survey Control Network benchmarks. If there are two such benchmarks within 3000 feet of the project site a tie to both shall be made. The benchmark(s) used will be shown on the drawing. If a City of Renton benchmark does not exist within 3,000 feet of a project, one must be set on or near the project in a permanent manner that will remain intact throughout the duration of the project. Source of elevations (benchmark) will be shown on the drawing, as well as a description of any bench marks established. See the City of Renton Survey and Drafting Standards. The site improvement plans consist of all the plans, profiles, details, notes, and specifications necessary to construct road, drainage structure, and off-street parking improvements. Site improvement plans include the following:  A base map (described below), and  Site plan and profiles (described below). Note: Site improvement plans must also include grading plans if onsite grading extends beyond the roadway. Modified Site Improvement Plan CED may allow a modified site improvement plan for some projects in Targeted Drainage Review (see Section 2.3.2) or Directed Drainage Review, or where major improvements (e.g., detention facilities, conveyance systems, bridges, road right-of-way improvements, etc.) are not proposed. The modified site improvement plan must: 1. Be drawn on a 11″ x 17″ or larger sheet, 2. Accurately locate structure(s) and access, showing observance of the setback requirements given in this manual, the critical areas code (RMC 4-3-050), or other applicable documents, 3. Provide enough information (datum, topography, details, notes, etc.) to address issues as determined by CED.  GENERAL PLAN FORMAT Site improvement plans should use City of Renton Drafting Standards as appropriate, and must include Standard Plan Notes (see Reference Section 7). Each plan must follow the general format detailed below: 1. Plan sheets and profile sheets, or combined plan and profile sheets, specifications, and detail sheets as required shall be on 22-inch by 34-inch sheets (22″ x 34″). Right-of-way improvements must be on 22-inch by 34-inch sheets (22″ x 34″). Original sheets shall be archive quality reproducibles, electronic pdf format. SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS 6/22/2022 2022 City of Renton Surface Water Design Manual 2-22 2. Drafting details shall generally conform to City of Renton Survey and Drafting Standards (see <https://rentonwa.gov/city_hall/community_and_economic_development/permits/civil_construction >) with standard text height of 0.125″ (1/8″). Existing features shall be shown with dashed lines or as half-toned (screened) in order to clearly distinguish existing features from proposed improvements. Hatch patterns shall not be used. 3. Each submittal shall contain a project information/cover sheet with the following: a) Title: Project name and CED file number(s) b) Table of contents (if more than three pages) c) Vicinity map d) Name and phone number of utility field contacts (e.g., water, sanitary sewer, gas, power, telephone, and TV) and the One-Call number (811 or 1-800-424-5555) e) The City’s preconstruction/inspection notification requirements f) Name and phone number of the erosion control/CSWPP supervisor g) Name and phone number of the surveyor h) Name and phone number of the owner/agent i) Name and phone number of the applicant j) Legal description k) Plan approval signature block for CED l) Name and phone number of the engineering firm preparing the plans (company logos acceptable) m) Renton Regional Fire Authority’s’ approval stamp (if required) n) Statement that mailbox locations have been designated or approved by the U.S. Postal Service (where required) o) List of conditions of preliminary approval and conditions of approved adjustments and variances on all site improvements 4. An overall site plan shall be included if more than three plan sheets are used. The overall plan shall be indexed to the detail plan sheets and include the following: a) The complete property area development b) Right-of-way information c) Street names and road classification d) All project phasing and proposed division boundaries e) All natural and proposed drainage collection and conveyance systems with catch basin numbers shown 5. Each sheet of the plan set shall be stamped, signed, and dated by a civil engineer. At least one sheet showing all boundary survey information and tied to two City of Renton monuments must be provided and stamped by a land surveyor licensed in the State of Washington. 6. Detail sheets shall provide sufficient information to construct complex elements of the plan. Details may be provided on plan and profile sheets if space allows. 7. The City of Renton title block shall be provided on each plan sheet. Title block can be obtained at the City’s website. At a minimum, the title block shall list the following: a) Development title b) Name, address, and phone number of the firm or individual preparing the plan 2.3.1 ENGINEERING PLAN SPECIFICATIONS 2022 City of Renton Surface Water Design Manual 6/22/2022 2-23 c) A revision block d) Page (of pages) numbering e) Sheet title (e.g., road and drainage, grading, erosion and sediment control, stormwater pollution prevention and spill control) 8. A blank CED approval block (included on the City’s title block) shall be provided on each plan sheet. 9. The location and label for each section or other detail shall be provided. 10. Critical areas, critical area buffers, and critical area building setbacks as required by RMC 4-3-050 shall be delineated and labeled. 11. All match lines with matched sheet number shall be provided. 12. All division or phase lines and the proposed limits of construction under the permit application shall be indicated. 13. Wetlands shall be labeled with the number from the City’s wetland inventory, or shall be labeled as “uninventoried” if not listed on the wetland inventory. 14. The standard plan notes that apply to the project shall be provided on the plans (see Reference Section 7-B). 15. Commercial building permit applications shall include the designated zoning for all properties adjacent to the development site(s).  BASE MAP A site improvement plan base map provides a common base and reference in the development and design of any project. A base map helps ensure that the engineering plans, grading plans, and CSWPP plans are all developed from the same background information. This base map shall include the information listed in Table 2.3.1.A. TABLE 2.3.1.A BASE MAP REQUIREMENTS Feature Requirements Ground Surface Topography Provide topography within the site and extending beyond the property lines. Contour lines must be shown as described in “Plan View: Site Plan and Roadway Elements.” Surface Water Discharge Provide ground surface elevations for a reasonable “fan” around points of discharge extending at least 50 feet downstream of all point discharge outlets. Hydrologic Features Provide spot elevations in addition to contour lines to aid in delineating the boundaries and depth of all existing floodplains, wetlands, channels, swales, streams, storm drainage systems and BMPs, roads (low spots), bogs, depressions, springs, seeps, swales, ditches, pipes, groundwater, and seasonal standing water. Other Natural Features Show the location and relative sizes of other natural features such as rock outcroppings, existing vegetation, and trees 12 inches in diameter and greater that could be disturbed by the project improvements and construction activities (within tree canopy), noting species. Flows Provide arrows that indicate the direction of surface flow on all public and private property and for all existing conveyance systems. Floodplains/ Floodways Show the floodplain/floodways as required by the flood hazard portion of the critical areas code (RMC 4-3-050) and Section 4.4.2. SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS 6/22/2022 2022 City of Renton Surface Water Design Manual 2-24 General Background Information Show the location and limits of all existing:  Property boundaries  Structures  Easements (including dimensions)  Total property (including dimensions)  Roads and right-of-way  Sanitary sewers and water utilities  Common open space  Public dedications  Other manmade features affecting existing topography/proposed improvements. Development Limitations Delineate limitations to the development that may occur as identified on the TIR worksheet, Part 11 (see Reference Section 8-A).  SITE PLAN AND PROFILES The design engineer shall provide plans and profiles for all construction, including but not limited to the following information: Plan View: Site Plan and Roadway Elements 1. Provide property lines, right-of-way lines, and widths for proposed roads and intersecting roads. Note: the condition of all public right-of-way and the right to use it as proposed must be verified. 2. Provide all existing and proposed roadway features, such as centerlines, edges of pavement and shoulders, ditchlines, curbs, and sidewalks. In addition, show points of access to abutting properties and roadway continuations. 3. Show existing and proposed topography contours at 2-foot intervals (5-foot intervals for slopes greater than 15 percent, 10-foot intervals for slopes greater than 40 percent). Contours may be extrapolated from USGS mapping, aerial photos, or other topography map resources. However, contours shall be field verified for roadway and stream centerlines, steep slopes, floodplains, drainage tracts easements, and conveyance systems. Contours shall extend 20 feet beyond property lines to resolve questions of setback, cut and fill slopes, drainage swales, ditches, and access or drainage to adjacent property. 4. Show the location of all existing utilities and proposed utilities (except those designed by the utility and not currently available) to the extent that these will be affected by the proposed project. Clearly identify all existing utility poles. 5. Identify all roads and adjoining subdivisions. 6. Show right-of-way for all proposed roadways, using sufficient dimensioning to clearly show exact locations on all sections of existing and proposed dedicated public roadway. 7. Clearly differentiate areas of existing pavement and areas of new pavement. If the project is a redevelopment project, delineate areas of replaced impervious surface. 8. For subdivision projects, generally use drawing scales of 1″=20′; however, 1″=50′ is optional for development of lots one acre or larger. For commercial, multi-family, or other projects, generally use scales of 1″=20′; however, 1″=10′, 1″ = 30′, 1″=40′ and 1″=50′ are acceptable. Show details for clarification, including those for intersections and existing driveways, on a larger scale. Plan View: Drainage Conveyance 1. Sequentially number all catch basins and curb inlets starting with the structure farthest downstream. 2.3.1 ENGINEERING PLAN SPECIFICATIONS 2022 City of Renton Surface Water Design Manual 6/22/2022 2-25 2. Represent existing storm drainage facilities and BMPs in dashed lines and label with “Existing.” 3. Clearly label existing storm drainage facilities to be removed with “Existing to be removed.” 4. Show the length, diameter, and material for all pipes, culverts, and stub-outs. Include the slope if not provided on the profile view. Material may be noted in the plan notes. Where an open channel conveyance system that is not concrete lined is provided, and a low-permeability liner or treatment liner is required per Section 6.2.4, indicate the limits of such liner(s). 5. Clearly label catch basins as to size and type (or indicate in the plan notes). 6. Clearly label stub-out locations for footing drains and other lot-specific connections to the storm drainage system. Locate all stub-outs to allow gravity flow from the lowest corner of the lot to the connecting catch basin. 7. Show datum, benchmark locations, and elevations on each plan sheet. 8. Clearly label all stub-out locations for any future pipe connections. 9. Clearly show on the plans all drainage easements, tracts, access easements, Native Growth Retention Areas, Critical Area Tracts, Critical Area Setback Areas, and building setback lines. Show dimensions, type of restriction, and use. 10. Using arrows, indicate the drainage direction of hydraulic conveyance systems. 11. Clearly label storm drainage facilities, on-site BMPs, pipes, and structures as either privately or publicly maintained. Plan View: Other 1. Show the location, identification, and dimensions of all buildings, property lines, streets, alleys, and easements. 2. Show the locations of structures on abutting properties within 50 feet of the proposed project site. 3. Show the location of all proposed drainage facility fencing, together with a typical section view of each fencing type. 4. Provide section details of all retaining walls and rockeries, including sections through critical portions of the rockeries or retaining walls. 5. Show all existing and proposed buildings with projections and overhangs. 6. Show the location of all wells on site and within 100 feet of the site. Note wells to be abandoned. 7. Show the location and dimensions of proposed on-site BMPs, features, pathways, limits, and set- asides. 8. Show the location and dimensions of structural source control BMPs required by the SWPPS Standards in Appendix D and the King County Stormwater Pollution Prevention Manual. Profiles: Roadway and Drainage 1. Provide existing centerline ground profile at minimum of 50-foot stations and at significant ground breaks and topographic features, with average accuracy to within 0.1 feet on unpaved surface and 0.02 feet on paved surface. 2. For publicly maintained roadways, provide final road and storm drain profile with the same stationing as the horizontal plan, to show stationing of points of curve, tangent, and intersection of vertical curves, with elevation of 0.01 feet. Include tie-in with intersecting pipe runs. 3. On a grid of numbered lines, provide a continuous plot of vertical positioning against horizontal. 4. Show finished road grade and vertical curve data (road data measured at centerline or edge of pavement). Include stopping sight distance. SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS 6/22/2022 2022 City of Renton Surface Water Design Manual 2-26 5. Show all roadway drainage, including drainage facilities and BMPs that are within the right-of-way or easement. 6. On the profile, show slope, length, size, and type (in plan notes or on a detail sheet) for all pipes and detention tanks in public right-of-way. 7. Indicate the inverts of all pipes and culverts and the elevations of catch basin grates or lids. It is also desirable, but not required, to show invert elevations and grate elevations on plan sheets. 8. For pipes that are proposed to be within 2.0 feet of finished grade, indicate the minimum cover dimensions. 9. Indicate roadway stationing and offset for all catch basins. 10. Indicate vertical and horizontal scale. 11. Clearly label all profiles with respective street names and plan sheet reference numbers, and indicate all profile sheet reference numbers on plan sheets, if drawn on separate sheets. 12. Locate match points with existing pavements, and show elevations. 13. Show all property boundaries. 14. Label all match line locations. 15. Provide profiles for all 12-inch and larger pipes and for channels (that are not roadside ditches). 16. Show the location of all existing and proposed (if available or critical for clearance) gas, water, and sanitary sewer crossings. 17. Show energy dissipater locations. 18. Identify datum used and all benchmarks (may be shown on plan view instead). Datum and benchmarks must refer to established control when available. 19. Use a vertical scale of 1″=5′. As an exception, vertical scale shall be 1″=10′ if the optional 1″=50′ horizontal scale is used on projects with lots one acre or larger. Clarifying details, including those for intersections and existing driveways, should use a larger scale. 20. Split sheets, with the profile aligned underneath the plan view, are preferred but not required.  DETAILS The design engineer shall provide details for all construction, including but not limited to the following. Flow Control, Water Quality, and Infiltration Facility and BMP Details 1. Provide a scaled drawing and supporting details of each detention pond or vault, on-site BMP, and water quality facility, including the tract boundaries. 2. Show predeveloped and finished grade contours at 2-foot intervals. Show and label maximum design water elevation. 3. Dimension all berm widths. 4. Show and label at least two cross sections through a pond or water quality facility, or any BMP large enough to require design elements of ponds and/or water quality facilities. One cross section must include the restrictor when included in the design. 5. Specify soils and compaction requirements for pond construction and on-site BMP construction. Specify low-permeability liners or treatment liners as required for facilities that allow runoff to have direct contact with the soil and open channel conveyance systems that are not concrete lined per Section 6.2.4. 6. Show the location and detail of emergency overflows, spillways, and bypasses. 2.3.1 ENGINEERING PLAN SPECIFICATIONS 2022 City of Renton Surface Water Design Manual 6/22/2022 2-27 7. Specify rock protection/energy dissipation requirements and details. 8. Provide inverts of all pipes, grates, inlets, tanks, and vaults, and spot elevations of the pond bottom. 9. Show the location of access roads to control manholes and pond/forebay bottoms. 10. Provide plan and section views of all energy dissipaters, including rock splash pads. Specify the size of rock and thickness. 11. Show bollard locations on plans. Typically, bollards are located at the entrance to drainage facility access roads. 12. On the pond or water quality facility detail, show the size, type (or in plan notes), slope, and length of all pipes. 13. Show to scale the section and plan view of restrictor and control structures. The plan view must show the location and orientation of all inlet pipes, outlet pipes, and flow restrictors. 14. Draw details at one of the following scales: 1″=1′, 1″=2′, 1″=4′, 1″=5′, 1″=10′, or 1″=20′. Structural Plan Details Any submittal that proposes a structure (e.g., bridge crossing, reinforced concrete footings, walls, or vaults) shall include plan sheets that include complete working drawings showing dimensions, steel placement, and specifications for construction. Structures may require a design prepared and stamped by a professional structural engineer licensed in the State of Washington, and an application for a separate commercial building permit. 2.3.1.3 EROSION AND SEDIMENT CONTROL (ESC) PLAN This section details the specifications and contents for ESC plans. Note that an ESC plan includes the plan’s drawings plus an ESC report, which provides all supporting information and any additional direction necessary for implementing ESC measures and meeting ESC implementation requirements. The ESC plan’s drawings may be simplified by the use of the symbols and codes provided for each ESC measure in the Erosion and Sediment Control Standards in Appendix D. In general, the ESC plan’s drawings shall be submitted as a separate plan sheet(s). However, there may be some relatively simple projects where providing separate grading and ESC plan drawings is unnecessary.  GENERAL SPECIFICATIONS The site improvement plan shall be used as the base of the ESC plan. Certain detailed information that is not relevant (e.g., pipe/catch basin size, stub-out locations, etc.) may be omitted to make the ESC plan easier to read. At a minimum, the ESC plan shall include all of the information required for the base map (see Table 2.3.1.A), as well as existing and proposed roads, driveways, parking areas, buildings, drainage facilities and BMPs, utility corridors not associated with roadways, relevant critical areas3 and critical area buffers, and proposed final topography. A smaller scale may be used to provide better comprehension and understanding. The ESC plan shall generally be designed for proposed topography, not existing topography, since rough grading is usually the first step in site disturbance. The ESC plan shall address all phases of construction (e.g., clearing, grading, installation of utilities, surfacing, and final stabilization). If construction is being phased, separate ESC plans may need to be prepared to address the specific needs for each phase of construction. The ESC plan outlines the minimum requirements for anticipated site conditions. During construction, ESC plans shall be revised as necessary by the CSWPP supervisor or as directed by CED to address changing site conditions, unexpected storm events, or non-compliance with the ESC performance criteria in Core Requirement #5. 3 Relevant critical areas, for the purposes of drainage review, include aquatic areas, wetlands, flood hazard areas, erosion hazard areas, landslide hazards, steep slope hazard areas, and critical aquifer recharge areas. SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS 6/22/2022 2022 City of Renton Surface Water Design Manual 2-28 The ESC plan shall be consistent with the information provided in Section 8 of the TIR and shall address the following: 1. Identify areas with a high susceptibility to erosion. 2. Provide all details necessary to clearly illustrate the intent of the ESC design. 3. Include ESC measures for all on- and offsite utility construction included in the project. 4. Specify the construction sequence. The construction sequence shall be specifically written for the proposed project. An example construction sequence is provided in Appendix D. 5. Include ESC standard plan notes (see Reference Section 7-B). 6. Include an inspection and maintenance program for ESC measures, including designation of a CSWPP supervisor who is a certified ESC professional and identification of phone numbers for 24- hour contact. 7. Include the basis and calculations for selection and sizing of ESC measures.  MEASURE-SPECIFIC INFORMATION ESC plan drawings must include the following information specific to applicable ESC measures and implementation requirements. As noted above, this information may need to be updated or revised during the life of the project by the CSWPP supervisor or as directed by CED. Clearing Limits 1. Delineate clearing limits. 2. Provide details sufficient to install and maintain the clearing limits. Cover Measures 1. Specify the type and location of temporary cover measures to be used onsite. 2. If more than one type of cover measure is to be used onsite, indicate the areas where the different measures will be used, including steep cut and fill slopes. 3. If the type of cover measures to be used will vary depending on the time of year, soil type, gradient, or some other factor, specify the conditions that control the use of the different measures . 4. Specify the nature and location of permanent cover measures. If a landscaping plan is prepared, this may not be necessary. 5. Specify the approximate amount of cover measures necessary to cover all disturbed areas. 6. If netting, blankets, or plastic sheeting are specified, provide typical detail sufficient for installation and maintenance. 7. Specify the mulch types, seed mixes, fertilizers, and soil amendments to be used, as well as the application rate for each item. 8. For surface roughening, describe methods, equipment and areas where surface roughening will be use. 9. If PAM is used, show location(s) and describe application method. 10. When compost blankets are used, show location, application rates, and the name of the supplier to document that compost meets quality specifications per Reference Section 11-C. Perimeter Protection 1. Specify the location and type of perimeter protection to be used. 2. Provide typical details sufficient to install and maintain the perimeter protection. 3. If silt fence is to be used, specify the type of fabric to be used. 2.3.1 ENGINEERING PLAN SPECIFICATIONS 2022 City of Renton Surface Water Design Manual 6/22/2022 2-29 4. If compost berms or socks are used, documentation must be provided to ensure the supplier meets the criteria and compost meets quality standards per Reference Section 11-C. Traffic Area Stabilization 1. Locate the construction entrance(s). 2. Provide typical details sufficient to install and maintain the construction entrance. 3. Locate the construction roads and parking areas. 4. Specify the measure(s) that will be used to create stabilized construction roads and parking areas. Provide sufficient detail to install and maintain. 5. If a wheel wash or tire bath system will be installed, provide location, typical details for installation and maintenance. 6. Provide a list of dust control products that will be used onsite and the location of potential application areas. Sediment Retention 1. Show the locations of all sediment ponds and traps. 2. Dimension pond berm widths and all inside and outside pond slopes. 3. Indicate the trap/pond storage required and the depth, length, and width dimensions. 4. Provide typical section views through pond and outlet structures. 5. If chemical or electrocoagulation treatment of sediment-laden waters will be used, approval documentation from Ecology must be included (see SWPPS plan requirements for chemical storage). 6. Provide details for disposal of contaminated or chemically treated waters (e.g., where Chitosan or CO2 have been used) (see SWPPS plan requirements for chemical storage). 7. Include appropriate approval documentation from local sewer districts if contaminated or chemically treated water will be discharged to the sanitary sewer. 8. Provide typical details of the control structure and dewatering mechanism . 9. Detail stabilization techniques for outlet/inlet protection. 10. Provide details sufficient to install cell dividers. 11. Specify mulch or recommended cover of berms and slopes. 12. Indicate the required depth gage with a prominent mark at 1-foot depth for sediment removal. 13. Indicate catch basins that are to be protected. 14. Indicate existing and proposed on-site BMP areas that are to be protected. 15. Provide details of the catch basin and on-site BMP protection sufficient to install and maintain. 16. Provide sediment retention prior to any discharge to the City sewer or local sewer district system. Surface Water Control 1. Locate all pipes, ditches, interceptor ditches, dikes, and swales that will be used to convey stormwater. 2. Provide details sufficient to install and maintain all conveyances. 3. Indicate locations of outlet protection and provide detail of protections. 4. Indicate locations and outlets of any possible dewatering systems. Provide details of alternative discharge methods from dewatering systems if adequate infiltration rates cannot be achieved. Do not route dewatering water, clean or untreated, through stormwater sediment ponds. SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS 6/22/2022 2022 City of Renton Surface Water Design Manual 2-30 5. Indicate the location of any level spreaders and provide details sufficient to install and maintain. 6. Show all temporary pipe inverts. 7. Provide location and specifications for the interception of runoff from disturbed areas and the conveyance of the runoff to a non-erosive discharge point. 8. Provide locations of rock check dams. 9. Provide details, including front and side sections, of typical rock check dams. Protection of Existing and Proposed On-site BMP Areas 1. Provide perimeter protection at existing and proposed on-site BMP locations 2. Provide cautionary plan notes emphasizing avoidance of negative impacts to receptor soils and existing vegetation to remain. BMP Maintenance 1. Provide adequate plan notes for guidance of BMP maintenance methods and schedules. 2. Include an inspection and maintenance program for ESC measures. Management of the Project 1. Provide plan notes to clarify and emphasize the management responsibilities for the project. 2. Include an inspection and maintenance program for ESC measures, including designation of a CSWPP supervisor who is a certified ESC professional and identification of phone numbers for 24- hour contact. Wet Season Requirements 1. Provide a list of all applicable wet season requirements. 2. Clearly identify that from October 1st through April 30th, no soils shall be exposed for more than two consecutive working days. Also note that this two-day requirement may be applied at other times of the year if storm events warrant more conservative measures. 3. Clearly identify that exposed soils shall be stabilized at the end of the workday prior to a weekend, holiday, or predicted rain event. 4. Weekly maintenance report is required to be submitted to CED. Critical Areas Restrictions 1. Delineate and label the following critical areas, and any applicable buffers, that are on or adjacent to the project site: aquatic areas, wetlands, flood hazard areas, erosion hazard areas, landslide hazards, steep slope hazard areas, and aquifer protection areas per RMC 4-3-050. 2. If construction creates disturbed areas within any of the above listed critical areas or associated buffers, specify the type, locations, and details of any measures or other provisions necessary to comply with the critical area restrictions in Appendix D and protect surface waters and steep slopes. 2.3.1.4 STORMWATER POLLUTION PREVENTION AND SPILL (SWPPS) PLAN This section details the specifications and contents for SWPPS plans, which together with ESC plans, comprise the construction stormwater pollution prevention (CSWPP) plan that must be submitted as part of the engineering plans required for drainage review . Additional guidance for developing the SWPPS plan can be found in the SWPPS Standards in Appendix D, Construction Stormwater Pollution Prevention Standards, in the King County Stormwater Pollution Prevention Manual and in the Stormwater Management Manual for Western Washington (SWMMWW) published by the Washington State Department of Ecology (Ecology). 2.3.1 ENGINEERING PLAN SPECIFICATIONS 2022 City of Renton Surface Water Design Manual 6/22/2022 2-31 The SWPPS plan must be kept on site during all phases of construction and shall address the construction-related pollution-generating activities outlined in Subsection A below. The plan must include a description of the methods the general contractor will use to ensure sub-contractors are aware of the SWPPS plan. A form or record must be provided that states all sub-contractors have read and agree to the SWPPS plan. A SWPPS plan consists of the following three elements, which are further described in Subsections B, C, and D below: 1. A site plan showing the location and description of BMPs required to prevent pollution and control spills from construction activities and from chemicals and other materials used and stored on the construction site. See Subsection B below for more specifics on the SWPPS site plan. 2. A pollution prevention report listing the potential sources of pollution and identifying the operational, source control, and treatment BMPs necessary to prevent/mitigate pollution from these sources. See Subsection C below for more specifics on the SWPPS pollution prevention report. 3. A spill prevention and cleanup report describing the procedures and BMPs for spill prevention and including provisions for cleanup of spills should they occur. See Subsection D below for more specifics on the SWPPS spill prevention and cleanup report. A. ACTIVITY-SPECIFIC INFORMATION REQUIRED At a minimum, the SWPPS plan shall address, if applicable, the following pollution-generating activities typically associated with construction and include the information specified below for each activity. If other pollution-generating activities associated with construction of the proposed project are identified, the SWPPS plan must address those activities in a similar manner. Storage and Handling of Liquids 1. Identify liquids that will be handled or stored onsite, including but not limited to petroleum products, fuel, solvents, detergents, paint, pesticides, concrete admixtures, and form oils. 2. Specify types and sizes of containers of liquids that will be stored/handled onsite. Show locations on the SWPPS site plan. 3. Describe secondary containment methods adequately sized to provide containment for all liquids stored onsite. Show the locations of containment areas on the SWPPS site plan. Storage and Stockpiling of Construction Materials and Wastes 1. Identify construction materials and wastes that may be generated or stockpiled onsite. Show the locations where these materials and wastes will be generated and stockpiled on the SWPPS site plan. 2. Specify type of cover measures to be used to keep rainwater from contacting construction materials and wastes that can contribute pollutants to storm, surface, and ground water. 3. If wastes are kept in containers, describe how rainwater will be kept out of the containers. Fueling 1. Specify method of onsite fueling for construction equipment (i.e., stationary tanks, truck mounted tanks, wet hosing, etc.). If stationary tanks will be used, show their location on the SWPPS site plan. 2. Describe type and size of tanks. 3. Describe containment methods for fuel spills and make reference to the SWPPS site plan for location information. 4. If fueling occurs during evening hours, describe lighting and signage plan. Make reference to the SWPPS site plan for location information. SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS 6/22/2022 2022 City of Renton Surface Water Design Manual 2-32 Maintenance, Repairs, and Storage of Vehicles and Equipment 1. Identify maintenance and repair areas and show their locations on the SWPPS site plan. Use of drip pans or plastic beneath vehicles is required. A note to this effect must be shown on the SWPPS site plan. 2. Describe method for collection, storage, and disposal of vehicle fluids. 3. If an area is designated for vehicle maintenance, signs must be posted that state no vehicle washing may occur in the area. A note to this effect must be shown on the SWPPS site plan. Concrete Saw Cutting, Slurry, and Washwater Disposal 1. Ensure that washout of concrete trucks is performed off-site or in designated concrete washout areas only. Identify truck washout areas to ensure such areas are not within a critical aquifer recharge area. If they are, the washout area must be lined with an impervious membrane. Show location information on the SWPPS site plan. Locate washout area at least 50 feet from sensitive areas such as storm drains, open ditches, or water bodies, including wetlands. 2. Specify size of sumps needed to collect and contain slurry and washwater. Show location information on the SWPPS site plan. 3. Identify areas for rinsing hand tools including but not limited to screeds, shovels, rakes, floats and trowels. Show the locations of these areas on the SWPPS site plan. 4. Describe methods for collecting, treating, and disposal of waste water from exposed aggregate processes, concrete grinding and saw cutting, and new concrete washing and curing water. Handling of pH Elevated Water New concrete vaults/structures may cause collected water to have an elevated pH. This water cannot be discharged to storm or surface water until neutralized. 1. Provide details on treating/neutralizing water when pH is not within neutral parameters. Written approval from Ecology is required before using chemical treatment other than CO 2 or dry ice to adjust pH. 2. Provide details on disposal of water with elevated pH or of the treated water. Application of Chemicals including Pesticides and Fertilizers 1. Provide a list of chemicals that may be used on the project site and the application rates. 2. Describe where and how chemicals will be applied. Show location information on the SWPPS site plan. 3. Describe where and how chemicals will be stored. Show location information on the SWPPS site plan. B. SWPPS SITE PLAN The site plan element of the SWPPS plan shall include all of the information required for the base map (see Table 2.3.1.A), as well as existing and proposed roads, driveways, parking areas, buildings, drainage facilities, utility corridors not associated with roadways, relevant critical areas4 and associated buffers, and proposed final topography. A smaller scale may be used to provide more comprehensive details on specific locations of each activity and specific prevention measure. In addition to this information, the following items, at a minimum, shall be provided as applicable: 1. Identify locations where liquids will be stored and delineate secondary containment areas that will be provided. 4 Relevant critical areas, for the purposes of drainage review, include aquatic areas, wetlands, flood hazard areas, erosion hazard areas, landslide hazards, steep slope hazard areas, and aquifer protection areas as described in RMC 4-3-050. 2.3.1 ENGINEERING PLAN SPECIFICATIONS 2022 City of Renton Surface Water Design Manual 6/22/2022 2-33 2. Identify locations where construction materials and wastes will be generated and stockpiled. 3. Identify location of fueling for vehicles and equipment if stationary tanks will be used. 4. Delineate containment areas for fuel spills. 5. Show location of lighting and signage for fueling during evening hours. 6. Delineate maintenance and repair areas and clearly note that drip pans or plastic shall be used beneath vehicles. Also, clearly note that signs must be posted that state no vehicle washing may occur in the area. 7. Delineate truck washout areas and identify the location of slurry/washwater sumps and rinsing areas for tools. 8. Delineate where chemicals will be applied and identify where they will be stored. 9. Identify where spill response materials will be stored. C. POLLUTION PREVENTION REPORT This report provides the specifics on pollution prevention and must include the following information in addition to the activity-specific information specified in Subsection A above: 1. List the possible sources of pollution per Subsection A above and identify the BMPs to be used for each source to prevent pollution. Include any supporting information (site conditions, calculations, etc.) for the selection and sizing of pollution prevention BMPs. 2. Identify the personnel responsible for pollution prevention and clearly list the responsibilities of each person identified. Contact information for these personnel must be clearly identified in the report and on the SWPPS site plan. 3. Describe the procedures to be used for monitoring pollution prevention BMPs and for responding to a BMP that needs attention, including keeping records/reports of all inspections of pollution prevention BMPs (see Reference Section 8-E for examples of worksheets that may be used). D. SPILL PREVENTION AND CLEANUP REPORT This report provides the specifics on spill prevention and cleanup and must include the following information in addition to any activity-specific information in Subsection A above related to spill prevention: 1. List the possible sources of a spill and identify the BMPs to be used for each source to prevent a spill. 2. Identify personnel responsible for spill prevention and cleanup and clearly list the responsibilities of each person identified. Contact information for these personnel must be clearly identified in the report and on the SWPPS site plan. (On typical projects, the primary contact for SWPPS issues will be the CSWPP supervisor, who may be managing other spill responders to ensure compliance; complex projects may warrant specialist personnel for specific site applications.) 3. Describe the procedures to be used for monitoring spill prevention BMPs and for responding to a spill incident, including keeping records/reports of all inspections and spills (see Reference Section 8-E for examples of worksheets that may be used). 4. Identify where spill response materials will be stored. Make reference to the SWPPS site plan for location information. 5. Identify disposal methods for contaminated water and soil after a spill. 2.3.1.5 LANDSCAPE MANAGEMENT PLANS (IF APPLICABLE) The City of Renton does not allow landscape management plans to be used as an alternative to the requirement to formally treat (with a facility) the runoff from pollution generating pervious surfaces SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS 6/22/2022 2022 City of Renton Surface Water Design Manual 2-34 subject to Core Requirement #8 (see Section 1.2.8). A landscape management plan is an approved plan for defining the layout and long-term maintenance of landscaping features to minimize the use of pesticides and fertilizers, and reduce the discharge of suspended solids and other pollutants. 2.3.2 PROJECTS IN TARGETED DRAINAGE REVIEW 2022 City of Renton Surface Water Design Manual 6/22/2022 2-35 2.3.2 PROJECTS IN TARGETED DRAINAGE REVIEW (TDR) This section outlines the specifications and contents of limited scope engineering plans allowed for projects in Targeted Drainage Review. Table 2.3.2.A specifies the minimum required elements of the targeted technical information report based on the type of permit or project, and on the three categories of project characteristics subject to Targeted Drainage Review per Section 1.1.2.2. TABLE 2.3.2.A MINIMUM ENGINEERING PLAN ELEMENTS(1) FOR PROJECTS IN TARGETED DRAINAGE REVIEW Type of Permit or Project Drainage Review Type Project Category 1(2) Projects in TDR that contain or are adjacent to a flood, erosion, or steep slope hazard area; or are within a Landslide Hazard Drainage Area or Aquifer Protection Area Project Category 2(2) Projects in TDR that propose to construct or modify a 12″ or larger pipe/ditch, or receive runoff from a 12″ or larger pipe/ditch Project Category 3(2) Redevelopment projects in TDR that propose $100,000 or more of improvements to an existing high-use site SINGLE- FAMILY RESIDENTIAL (SFR) BUILDING PERMITS SHORT PLATS Targeted Drainage Review ONLY  TIR Sections 1, 2, and 6 (minimum)  Simplified ESC Plan(3) and SWPPS Plan  Site Improvement Plan(5)  TIR Sections 1, 2, 3, 5, 6, 7, and 8 (minimum)  Simplified ESC Plan(3) and SWPPS Plan  ESC Plan(4) for conveyance work  Site Improvement Plan(5) N/A Targeted Drainage Review COMBINE D WITH Simplified Drainage Review  TIR Sections 1, 2, and 6 (minimum)  Simplified ESC Plan(3) and SWPPS Plan  Site Improvement Plan(5)  TIR Sections 1, 2, 3, 5, 6, 7, and 8 (minimum)  Simplified ESC Plan(3) and SWPPS Plan  ESC Plan(4) for conveyance work  Site Improvement Plan(5) N/A OTHER PROJECTS OR PERMITS Targeted Drainage Review ONLY  TIR Sections 1, 2, 6, and 8 (minimum)  ESC Plan(4) and SWPPS Plan for any site disturbance work  Site Improvement Plan(5)  TIR Sections 1, 2, 3, 5, 6, 7, and 8 (minimum)  ESC Plan(4) and SWPPS Plan for any site disturbance work  Site Improvement Plan(5)  TIR Sections 1, 2, 4, 8, and 10 (minimum)  ESC Plan(4) and SWPPS Plan for any site disturbance work  Site Improvement Plan(5) Notes: (1) The above plan elements are considered the recommended minimum for most development cases in Targeted Drainage Review. CED may add to these elements if deemed necessary for proper drainage review. Predesign meetings with CED are recommended to identify all required elements. (2) For more detailed descriptions of project categories, see Section 1.1.2.2. If the proposed project has the characteristics of more than one category, the plan elements under each applicable category shall apply. (3) Simplified ESC plans are an element of the Simplified drainage plan as explained in the Simplified Drainage Requirements booklet (Appendix C). (4) ESC plans shall meet the applicable specifications detailed in Section 2.3.1.3 (5) Site improvement plans shall meet the applicable specifications detailed in Section 2.3.1.2. CED may allow modified site improvement plans as described in Section 2.3.1.2. SECTION 2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS 6/22/2022 2022 City of Renton Surface Water Design Manual 2-36 (T his page intentionally left blan k. ) CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 2-37 2.4 PLANS REQUIRED AFTER DRAINAGE REVIEW This section includes the specifications and contents required of those plans submitted at the end of the permit review process or after a permit has been issued. 2.4.1 PLAN CHANGES AFTER PERMIT ISSUANCE If changes or revisions to the originally approved engineering plans require additional review, the revised plans shall be submitted to CED for approval prior to construction. The plan change submittals shall include all of the following: 1. The appropriate Plan Change form(s) 2. One (1) revised TIR or addendum 3. One (1) set of the engineering plans 4. One (1) Complete Electronic copy of all submittal items 5. Other information needed for review. 2.4.2 FINAL CORRECTED PLAN SUBMITTAL During the course of construction, changes to the approved engineering plans are often required to address unforeseen field conditions or design improvements. Once construction is completed, it is the applicant’s responsibility to submit to CED a final corrected plan (or record drawings). These corrected drawings must be professionally drafted revisions applied to the original approved plan, excluding the CSWPP plan, and must include all changes made during the course of construction. The final as-built plans must be stamped, signed, and dated by a civil engineer or land surveyor. A CAD drawing file (.dwg) of the final as-built plan set must be submitted. The CAD file must contain all the pages of the plan set for road and drainage infrastructure, but need not contain other sheets. A single hard copy of the draft final corrected plans may be required by the City to perform onsite field inspections to verify the final corrected plans. A final corrected TIR, updated to include all changes made to the originally approved TIR during the course of construction, must be submitted with the final corrected plan. In addition to any design changes and supporting calculations and documentation, the final corrected TIR shall include a final updated Stormwater Facility Summary Sheet (see Reference Section 8-D) and signed/recorded copies of all required easements and declarations of covenant. The electronic copy of the final corrected TIR shall be in .pdf format. Additional information regarding the final corrected plan submittal can be found on the City’s website. Disposition of Approved Engineering Plans for Subdivisions CED will retain the .pdf copy of the full record drawing set, utilizing it to make copies for public inspection, distribution, base reference, and permanent public record as required. SECTION 2.4 PLANS REQUIRED AFTER DRAINAGE REVIEW 6/22/2022 2022 City of Renton Surface Water Design Manual 2-38 2.4.3 FINAL PLAT, SHORT PLAT, AND BINDING SITE PLAN SUBMITTALS Any subdivision to be finalized, thereby completing the subdivision process and legally forming new lots, requires a final submittal for approval and recording. Binding site plans and short plats also require a final submittal for approval and recording. The final plat or map page shall contain the elements summarized and specified in detail on the City’s website. Submittals shall be accompanied by appropriate fees as prescribed by Renton Municipal Code. Final submittals will be allowed only after the approval of preliminary plans (for subdivisions only) and any required engineering plans. All final map sheets and pages shall be prepared by a land surveyor licensed in the State of Washington and shall conform to all state and local statutes. The final submittal for recording only applies to subdivisions (plats), binding site plans, and short plats. This plan is required by state and local statutes. In addition to the requirements described on the City’s website and in the City of Renton Municipal Code, submittals for final recording of subdivisions, short plats, and binding site plans must include the following information: 1. Indicate dimensions of all easements, tracts, building setbacks, tops of slopes, wetland boundaries, and floodplains. 2. Include pertinent restrictions as they apply to easements, tracts, and building setback lines. 3. State the maximum amount of added impervious surface and proposed clearing per lot as determined through engineering review. The maximum amount of impervious surface may be expressed in terms of percentage of lot coverage or square feet. 4. Include a recorded declaration of covenant and grant of easement for each lot on which on-site BMPs are installed or stipulated per Core Requirement #9, Section 1.2.9.4.1, and each lot for which on-site BMPs are installed in a separate dedicated tract per Section 1.2.9.4.1. 2022 City of Renton Surface Water Design Manual 6/22/2022 CHAPTER 3 HYDROLOGIC ANALYSIS & DESIGN CITY OF RENTON SURFACE WATER DESIGN MANUAL Section Page 3.1 Hydrologic Design Standards and Principles 3-3 3.1.1 Hydrologic Impacts and Mitigation 3-3 3.1.2 Flow Control Standards 3-5 3.1.3 Hydrologic Analysis Using Continuous Models 3-5 3.2 Runoff Computation and Analysis Methods 3-9 3.2.1 Rational Method 3-11 3.2.2 Continuous Models and the Runoff Files Method 3-19 3.2.3 The Approved Model 3-30 3.2.4 The HSPF Model 3-30 3.3 Hydrologic Design Procedures and Considerations 3-33 3.3.1 General Hydrologic Design Process 3-33 3.3.2 Flow Control Design Using the Runoff Files Method 3-34 3.3.3 Conveyance System Design with the Runoff Files Method 3-37 3.3.4 Safety Factors in Hydrologic Design 3-38 3.3.5 Design Options for Addressing Downstream Drainage Problems 3-38 3.3.6 Point of Compliance Analysis 3-38 3.3.7 Onsite Closed Depressions and Ponding Areas 3-41 CITY OF RENTON SURFACE WATER DESIGN MANUAL 6/22/2022 2022 City of Renton Surface Water Design Manual (T his page intentionally left blan k. ) CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 3-1 CHAPTER 3 HYDROLOGIC ANALYSIS & DESIGN This chapter presents the concepts and rationale for the surface water controls and designs required by this manual and the acceptable methods for estimating the quantity and characteristics of surface water runoff. These methods are used to analyze existing and to design proposed drainage systems and facilities. Hydrologic concepts, tools and methodologies, and an overview of the assumptions and data requirements of the methods, are described for the following tasks:  Calculating runoff time series and flow statistics  Designing detention and infiltration facilities Approved hydrologic modeling software are listed in Reference Section 6-D. Tools and methodologies specific to the software can be obtained from the software documentation and trainings provided by the software providers. At this writing, the approved models for stormwater runoff and water quality design include WWHM2012 and WWHM4, available from the Washington State Department of Ecology (Ecology), MGS Flood, available from MGS Engineering Consultants, Inc., and the Hydrologic Simulation Program (Fortran) (HSPF). Currently, MGS Flood is not approved for modeling bioretention. It will be allowed for modeling bioretention only at such time that it is formally approved by Ecology for that use. Hydrologic tools and methodologies, and the assumptions and data requirements of the methods, are presented for the following tasks:  Sizing conveyance facilities  Analyzing conveyance capacities. Chapter Organization The information presented in this chapter is organized into three main sections:  Section 3.1, “Hydrologic Design Standards and Principles”  Section 3.2, “Runoff Computation and Analysis Methods”  Section 3.3, “Hydrologic Design Procedures and Considerations” These sections begin on odd pages so the user can insert tabs if desired for quicker reference. Other Supporting Information For specific guidance on the mechanics of using the approved modeling software for hydrologic analysis and design, refer to the associated approved model website and program documentation. See Reference Section 6-D for limited modeling guidance and requirements as applicable for specific tasks in this manual. CHAPTER 3 HYDROLOGIC ANALYSIS & DESIGN 6/22/2022 2022 City of Renton Surface Water Design Manual 3-2 (T his page intentionally left blan k. ) CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 3-3 3.1 HYDROLOGIC DESIGN STANDARDS AND PRINCIPLES This section presents the rationale for and approach to hydrologic analysis and design. Topics covered include the following:  “Hydrologic Impacts and Mitigation,” Section 3.1.1  “Flow Control Standards,” Section 3.1.2  “Hydrologic Analysis Using Continuous Models,” Section 3.1.3 3.1.1 HYDROLOGIC IMPACTS AND MITIGATION Hydrologic Effects of Urbanization The hydrologic effects of development can cause a multitude of problems, including minor nuisance flooding, degradation of public resources, diminished fish production, and significant flooding endangering life and property. Increased stormwater flows expand floodplains, bringing flooding to locations where it did not occur before and worsening flood problems in areas already flood-prone. Increased stormwater flows also hasten channel erosion, alter channel structure, and degrade fish habitat. Human alteration of the landscape, including clearing, grading, paving, building construction, and landscaping, changes the physical and biological features that affect hydrologic processes. Soil compaction and paving reduce the infiltration and storage capacity of soils. This leads to a runoff process called Horton overland flow whereby the rainfall rate exceeds the infiltration rate, and the excess precipitation flows downhill over the soil surface. This type of flow rapidly transmits rainfall to the stream or conveyance system, causing much higher peak flow rates than would occur in the unaltered landscape. Horton overland flow is almost nonexistent in densely vegetated areas, such as forest or shrub land, where the vast majority of rainfall infiltrates into the soil. Some of this infiltrated water is used by plants, and depending on soil conditions, some of it percolates until it reaches the groundwater table. Sometimes the percolating soil water will encounter a low-permeability soil or rock layer. In this case, it flows laterally as interflow over the low-permeability layer until it reaches a stream channel. Generally, forested lands deliver water to streams by subsurface pathways, which are much slower than the runoff pathways from cleared and landscaped lands. Therefore, urbanization of forest and pasture land leads to increased stormwater flow volumes and higher peak flow rates. Land development increases not only peak flow rates but also changes annual and seasonal runoff volumes. In forested basins in King County, about 55% of the rain that falls each year eventually appears as streamflow. This percentage is called the yield of a basin. The remaining 45% of the rain evaporates and returns to the atmosphere. As trees are cleared and the soil is graded to make way for lawns and pastures, and as part of the land is covered with asphalt or concrete, the basin yield increases. More of the rain becomes streamflow, and less evaporates. In lowland King County, the yield of a basin covered with landscaped lawns would be about 65%, while the yield of an impervious basin would be about 85 to 90%. For these reasons, development without mitigation increases peak stormwater rates, stormwater volumes, and annual basin yields. Furthermore, the reduction of groundwater recharge decreases summer base flows. In summary, the following are the hydrologic impacts of unmitigated development:  Increased peak flows  Increased durations of high flows  Increased stormwater runoff volumes  Decreased groundwater recharge and base flows  Seasonal flow volume shifts  Altered wetland hydroperiods. SECTION 3.1 HYDROLOGIC DESIGN STANDARDS AND PRINCIPLES 6/22/2022 2022 City of Renton Surface Water Design Manual 3-4 The resulting economic and ecological consequences of these hydrologic changes include the following:  Increased flooding  Increased stream erosion  Degraded aquatic habitat  Changes to wetland species composition. Mitigation of Hydrologic Effects of Urbanization Engineered facilities can mitigate many of the hydrologic changes associated with development. Detention facilities can maintain the rates and/or durations of high flows at predevelopment levels. Infiltration facilities can control flow volumes and increase groundwater recharge as well as control flow rates and durations. Conveyance problems can be avoided through analysis and appropriate sizing and design of conveyance facilities. Engineered mitigation of the hydrologic impacts of development include the following:  Managing peak flow rates with detention facilities  Managing high flow durations with detention facilities  Reducing flow volumes and maintaining or enhancing groundwater recharge with infiltration facilities  Avoiding flooding problems with appropriately sized and designed conveyance systems  Bypassing erosion problems with tightlines. Engineered facilities cannot mitigate all of the hydrologic impacts of development. Detention facilities do not mitigate seasonal volume shifts, wetland water level fluctuations, groundwater recharge reductions, or base flow changes. Such impacts can be further reduced through the use of low impact development (LID) techniques, beginning with careful site planning. For instance, clustering of units to reduce impervious cover while maintaining site density is an effective way to limit hydrologic change. Preserving native vegetation and minimizing soil disturbance or compaction in pervious areas also reduces hydrologic change. Such non-engineered mitigation measures are encouraged by the City and are discussed in Core Requirement #9 and Appendix C of this manual and are referred to as on-site BMPs. Other on-site BMPs, such as permeable pavements, bioretention, vegetated roofs, and rainwater harvesting can be effective in reducing increases in surface water volumes. The incorporation of these concepts in the design of the project is required, as detailed in Core Requirement #9 and Appendix C. Many of these approaches will result in a reduction in flow control facility size, so the on-site BMP requirements in Core Requirement #9 and Appendix C should be carefully considered and applied to maximize the benefits of this approach. Detention Facility Concepts The basic concept of a detention facility is simple: water is collected from developed areas and released at a slower rate than it enters the collection system. The excess of inflow over outflow is temporarily stored in a pond or a vault and is typically released over a few hours or a few days. The volume of storage needed is determined by (1) how much stormwater enters the facility (determined by the size and density of the contributing area), (2) how rapidly water is allowed to leave the facility, and (3) the level of hydrologic control the facility is designed to achieve. To prevent increases in the frequency of flooding due to new development, detention facilities are often designed to maintain peak flow rates at their predevelopment levels for recurrence intervals of concern (e.g., 2- and 10-year). Such mitigation can prevent increases in the frequency of downstream flooding. Facilities that control only peak flow rates, however, usually allow the duration of high flows to increase, which may cause increased erosion of the downstream system. For example, the magnitude of a 2-year flow may not increase, but the amount of time that flow rate occurs may double. Therefore, stream systems, including those with salmonid habitat, which require protection from erosion warrant detention systems that control the durations of geomorphically significant flows (flows capable of moving sediment). Such detention systems employ lower release rates and are therefore larger in volume. 3.1.3 HYDROLOGIC ANALYSIS USING CONTINUOUS MODELS 2022 City of Renton Surface Water Design Manual 6/22/2022 3-5 3.1.2 FLOW CONTROL STANDARDS Refer to Chapter 1, Section 1.2.3, for flow control standards.1,2 3.1.3 HYDROLOGIC ANALYSIS USING CONTINUOUS MODELS The Need for Continuous Hydrologic Modeling This manual prescribes the use of a continuous hydrologic model for most hydrologic analyses rather than an event model. Event models such as the Santa Barbara Urban Hydrograph (SBUH), King County Runoff Series (KCRTS) and the Soil Conservation Service (SCS)3 method were used in previous versions of the King County Surface Water Design Manual. A continuous model was selected for the current version of the City of Renton SWDM because hydrologic problems in western Washington are associated with the high volumes of flow from sequential winter storms rather than high peak flows from short duration, high intensity rainfall events. The continuous hydrologic analysis tools prescribed in this manual are generically described as the “approved model”; a list of the approved models is found in Reference Section 6-D (as updated). At this writing, the approved continuous hydrologic models4 include the Western Washington Hydrologic Model (WWHM) and MGS Flood, both of which are variants of the Hydrologic Simulation Program- FORTRAN (HSPF) model. HSPF is also an approved model, but is more complex than other approved models and is typically used for basin planning and master drainage plan analyses. Continuous models are well suited to accounting for the climatological conditions in the lowland Puget Sound area. Continuous models include algorithms that maintain a continuous water balance for a catchment to account for soil moisture and hydraulic conditions antecedent to each storm event (Linsley, Kohler, Paulhus, 1982), whereas event models assume initial conditions and only address single hypothetical storm events. As a result, continuous hydrologic models are more appropriate for evaluating runoff during the extended wet winters typical of the Puget Sound area. The drawbacks of event models are summarized as follows:  Event methods inherently overestimate peak flows from undeveloped land cover conditions. The overestimation is due, in part, to the assumption that runoff from forest and pasture land covers flows across the ground surface. In actuality, the runoff from forests and pastures, on till soils, is dominated by shallow subsurface flows (interflow) which have hydrologic response times much longer than those used in event methods. This leads to an over estimation of predeveloped peak flows, which results in detention facility release rates being overestimated and storage requirements being underestimated.  A single event cannot represent the sequential storm characteristics of Puget Sound winters.  Event models assume detention facilities are empty at the start of a design event, whereas actual detention facilities may be partially full as a result of preceding storms.  Testing of event-designed detention facilities with calibrated, long-term continuous hydrologic simulations demonstrates that these facilities do not achieve desired performance goals.  Event methods do not allow analysis of flow durations or water level fluctuations. The benefits of continuous hydrologic modeling are summarized as follows: 1 Footnote 1 does not apply. 2 Footnote 2 does not apply. 3 The Soil Conservation Service (SCS) is now known as the National Resources Conservation Service (NRCS). The method described in Urban Hydrology for Small Watersheds, Technical Release 55 (TR-55), June 1986, published by the NRCS, is commonly referred to as the “SCS method.” 4 Note that MGS Flood is not currently approved for modeling bioretention. It will be allowed for modeling bioretention only at such time that it is formally approved by Ecology for that use. SECTION 3.1 HYDROLOGIC DESIGN STANDARDS AND PRINCIPLES 6/22/2022 2022 City of Renton Surface Water Design Manual 3-6  A continuous model accounts for the long duration and high precipitation volume of winter wet periods characterized by sequential, low-intensity rainfall events. Continuous simulation uses continuous long-term records of observed rainfall rather than short periods of data representing hypothetical storm events. As a result, continuous simulation explicitly accounts for the long duration rainfall events typically experienced in the Pacific Northwest as well as the effects of rainfall antecedent to major storm events.  HSPF has been shown to more accurately simulate runoff from basins with a wide range of sizes and land covers using the regional parameters developed by the United States Geologic Survey (USGS).  Continuous simulation allows direct examination of flow duration data for assessing the impacts of development on stream erosion and morphology. An event model, whether using a 1-day or a 7-day storm, cannot provide such information.  A continuous model allows water level analysis for wetlands, lakes, and closed depressions whose water level regime is often dependent on seasonal runoff rather than on 1-day or 7-day event runoff.  Continuous models produce flow control facilities that more accurately and effectively achieve desired performance goals. The importance of continuous modeling in the Puget Sound area is illustrated in Figure 3.1.3.A, which shows a small basin’s runoff response to a series of winter storms and the outflow from a detention pond designed to control the peak annual flows from this basin. Note that the largest outflow from the detention pond corresponds not to the peak inflow on 11/6/86, but rather to the high volume of flow from the sequential storms beginning on 11/19/86. This demonstrates a key difference between continuous and event based models. With an event model, designers are accustomed to working with a single design storm event (e.g., 10-year), which by definition has the same return period once routed through a reservoir (10-year inflow will always generate 10-year outflow). With a continuous model, flow recurrence estimates are based on annual peak flow rates, with each time series being analyzed independently. Events that generate annual peak inflows to a reservoir may not generate annual peak discharges from the reservoir. In other words, the runoff event containing the 10-year inflow peak, when routed, may not create the 10-year outflow peak. This is due to natural variability of storm peaks and volumes (e.g., high intensity/short duration thunderstorms as compared to moderate intensity/long duration winter storms) contained within a continuous record. Requirements of Continuous Hydrologic Modeling For the entire period of simulation, a continuous hydrologic model requires a continuous record of precipitation and evaporation at discrete time steps small enough to capture the temporal variability of hydrologic response, and it provides a continuous record of simulated flows at the same time step. The quicker a basin responds hydrologically (e.g., due to small size, land cover, or lack of detention), the smaller the time step should be. Time steps of 15 minutes are sufficient for most basins in the Puget Sound area. The continuous hydrologic model must include mathematical representations of hydrologic processes to determine the fate and movement of rainfall. For example, a good continuous hydrologic model must include representations of infiltration processes to determine how much water infiltrates the soil and how much runs off the surface. It must represent shallow and deep soil storage as well as the release of subsurface water to streams via interflow and groundwater flow, and it must also account for the loss of soil water to the atmosphere via evapotranspiration between rainfall events. The benefit of all this computation is a complete hydrologic assessment including information on peak flow rates, flow durations, storm volumes, seasonal volumes, annual volumes, and water levels of receiving bodies. 3.1.3 HYDROLOGIC ANALYSIS USING CONTINUOUS MODELS 2022 City of Renton Surface Water Design Manual 6/22/2022 3-7 FIGURE 3.1.3.A EFFECTS OF SEQUENTIAL STORMS ON DETENTION PERFORMANCE Small Basin Runoff Response: surface and interflows from 10-acre till site 0 0.5 1 1.5 2 2.5 11/2/86 11/9/86 11/16/86 11/23/86 11/30/86 DateFlow, CFSForest Condition Flows Detention Pond Outflows Pond Inflows from Residential Development SECTION 3.1 HYDROLOGIC DESIGN STANDARDS AND PRINCIPLES 6/22/2022 2022 City of Renton Surface Water Design Manual 3-8 (T his page intentionally left blan k) CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 3-9 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS This section presents the following four runoff computation methods accepted for hydrologic analysis and design:  The Rational Method described below and detailed in Section 3.2.1  The TR-55 or SBUH methods described below.  The Runoff Files Method described below and detailed in Section 3.2.2  The Hydrologic Simulation Program-FORTRAN (HSPF) model described below and detailed in Section 3.2.4.  ACCEPTABLE USES OF RUNOFF COMPUTATION METHODS Acceptable uses of the four runoff computation methods are summarized below and in Table 3.2:  Rational Method: This method is most appropriate for sizing new conveyance systems that drain smaller, quickly responding tributary areas (i.e., less than 10 acres) where very short, intense storms tend to generate the highest peak flows. The Rational Method may also be used for conveyance sizing in any size basin if the attenuation effects of existing storage features within the basin are ignored.  TR-55/SBUH Methods: The Natural Resources Conservation Service (NRCS, formerly the Soil Conservation Service (SCS)) TR-55 method or the SBUH method of the 1990 King County Surface Water Design Manual may be used for conveyance sizing where tributary areas are greater than or equal to 10 acres and if storage features are ignored. The peak flows from these single-event models are considered conservative for larger tributary areas if the flows are not routed through existing storage features. The TR-55 method is also used for water quality volume calculation in this manual. For more background information, refer to NRCS Publication 210-VI-TR-55, Second Edition (June 1986) or the 1990 SWDM.  The Runoff Files Method: This continuous modeling method using the approved model is the most versatile for quickly performing many of the computations summarized in Table 3.2. For conveyance sizing and analysis, the peak flows from the approved model are most accurate when the shortest possible time step is used. Unlike the Rational Method, the approved model may be used for tributary areas less than 10 acres where there is a significant storage feature(s). The City requires 15-minute time steps for sizing of all flow control facilities, water quality facilities and conveyance to provide consistent management of surface water and protect against cumulative increases in peak flows on a basin-wide basis (see Sections 3.3.1 and 3.3.2). Methods for analysis and design of detention storage and water levels5 require the use of the approved model. See the user’s documentation for background and guidance.  HSPF Model: For projects in Large Project Drainage Review (see Section 1.1.2.5), the City may require HSPF modeling for formulating a Master Drainage Plan (see Master Drainage Planning for Large Site Developments – Process and Requirement Guidelines available from King County). The City also generally encourages use of HSPF for tributary areas larger than 200 acres. The HSPF model can be used wherever the approved model is allowed for sizing and analysis of conveyance systems, flow control facilities, and water quality facilities using a 15-minute time step. For such projects draining to a wetland or potentially impacting groundwater resources or stream base flows, the City may require the collection of actual rainfall and runoff data to be used in developing and calibrating the HSPF model. 5 One of the simplest and most commonly used level pool routing methods is described in the Handbook of Applied Hydrology (Chow, Ven Te, 1964) and elsewhere, and summarized in Reference Section 6-C, It is based on the continuity equation and can be completed with a spreadsheet. Although not approved for design with this manual, it provides a background for modeled routing techniques. SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS 6/22/2022 2022 City of Renton Surface Water Design Manual 3-10 TABLE 3.2 ACCEPTABLE USES OF RUNOFF COMPUTATION METHODS TYPE OF COMPUTATION APPLIED TO RATIONAL METHOD TR 55/SBUH THE APPROVED MODEL HSPF PEAK FLOW CONVEYANCE SIZING INC. TESC(1) (DESIGN FLOWS) (See Chapter 4 for hydraulic analysis procedures) Tributary Areas  10 ac (measured to individual conveyance elements) REQUIRED for undetained areas,(2) and OKAY for detained areas if no storage routing(3) is performed OKAY if majority of tributary area is detained(4) OKAY if majority of tributary area is detained(4) Tributary Areas  10 ac OKAY if no storage routing(3) is performed OKAY if no storage routing(3) is performed OKAY (storage routing is allowed) OKAY (storage routing is allowed) LEVEL-POOL ROUTING FLOW CONTROL (NEW/EXIST.) & WQ FACILITY SIZING AND ANALYSIS Projects in Full Drainage Review OKAY OKAY Projects in Large Project Drainage Review MAY BE ALLOWED(5) MAY BE REQUIRED(5) DOWNSTREAM ANALYSIS Projects in Full or Targeted Drainage Review OKAY if no storage routing(3) is performed OKAY for tributary areas  10 ac. if no storage routing(3) is performed OKAY OKAY Projects in Large Project Drainage Review MAY BE ALLOWED(5) if used as described in the box above MAY BE ALLOWED(5) if as described in the box above MAY BE ALLOWED(5) if used as described in the box above PEAK FLOWS FOR APPLYING EXEMPTIONS & THRESHOLDS All Projects OKAY OKAY Notes: (1) Water quality design flow rates are determined as described in Section 6.2.1. (2) Undetained areas are those upstream of detention facilities or other storage features. (3) Storage routing uses the Level Pool Routing technique (described in Reference Section 6-C) or other similar method to account for the attenuation of peak flows passing through a detention facility or other storage feature. (4) The majority of the tributary area is considered detained if the runoff from more than 50% of the tributary area is detained by a detention facility or other storage facility. (5) For projects in Large Project Drainage Review, the selection of methodology for detention sizing and/or downstream analysis becomes a site-specific or basin-specific decision that is usually made by CED during the scoping process for master drainage plans. Guidelines for selecting the approved model, HSPF, or calibrated HSPF are found in the King County publication Master Drainage Planning for Large or Complex Site Developments, available from King County. 3.2.1 RATIONAL METHOD 2022 City of Renton Surface Water Design Manual 6/22/2022 3-11 3.2.1 RATIONAL METHOD The Rational Method is a simple, conservative method for analyzing and sizing conveyance elements serving small drainage subbasins, subject to the following specific limitations:  Only for use in predicting peak flow rates for sizing conveyance elements  Drainage subbasin area A cannot exceed 10 acres for a single peak flow calculation  The time of concentration Tc must be computed using the method described below and cannot exceed 100 minutes. It is also set equal to 6.3 minutes when computed to be less than 6.3 minutes. Note: Unlike other methods of computing times of concentration, the 6.3 minutes is not an initial collection time to be added to the total computed time of concentration.  RATIONAL METHOD EQUATION The following is the traditional Rational Method equation: QR = CIRA (3-1) where QR = peak flow (cfs) for a storm of return frequency R C = estimated runoff coefficient (ratio of rainfall that becomes runoff) IR = peak rainfall intensity (inches/hour) for a storm of return frequency R A = drainage subbasin area (acres) “C” Values The allowable runoff coefficients to be used in this method are shown in Table 3.2.1.A by type of land cover. These values were selected following a review of the values previously accepted by King County for use in the Rational Method and as described in several engineering handbooks. The values for single family residential areas were computed as composite values (as illustrated in the following equation) based on the estimated percentage of coverage by roads, roofs, yards, and unimproved areas for each density. For drainage basins containing several land cover types, the following formula may be used to compute a composite runoff coefficient, Cc: Cc = (C1A1 + C2A2 +… + CnAn)/At (3-2) where At = total area (acres) A1,2,…n = areas of land cover types (acres) C1,2,…n = runoff coefficients for each area land cover type “IR” Peak Rainfall Intensity The peak rainfall intensity IR for the specified design storm of return frequency R is determined using a unit peak rainfall intensity factor iR in the following equation: IR = (PR)(iR) (3-3) where PR = the total precipitation at the project site for the 24-hour duration storm event for the given return frequency. Total precipitation is found on the Isopluvial Maps in Figure 3.2.1.A through Figure 3.2.1.D. iR = the unit peak rainfall intensity factor SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS 6/22/2022 2022 City of Renton Surface Water Design Manual 3-12 The unit peak rainfall intensity factor iR is determined by the following equation: iR = (aR)(Tc)( )b R (3-4) where Tc = time of concentration (minutes), calculated using the method described below and subject to equation limitations (6.3  Tc  100) aR, bR = coefficients from Table 3.2.1.B used to adjust the equation for the design storm return frequency R This “iR” equation was developed by DNRP from equations originally created by Ron Mayo, P.E. It is based on the original Renton/Seattle Intensity/Duration/Frequency (I.D.F.) curves. Rather than requiring a family of curves for various locations, this equation adjusts proportionally the Renton/Seattle I.D.F. curve data by using the 24-hour duration total precipitation isopluvial maps. This adjustment is based on the assumption that the localized geo-climatic conditions that control the total volume of precipitation at a specific location also control the peak intensities proportionally. Note: Due to the mathematical limits of the equation coefficients, values of T c less than 6.3 minutes or greater than 100 minutes cannot be used. Therefore, real values of Tc less than 6.3 minutes must be assumed to be equal to 6.3 minutes, and values greater than 100 minutes must be assumed to be equal to 100 minutes. “Tc” Time of Concentration The time of concentration is defined as the time it takes runoff to travel overland (from the onset of precipitation) from the most hydraulically distant location in the drainage basin to the point of discharge. Note: When Cc (see Equation 3-2) of a drainage basin exceeds 0.60, it may be important to compute Tc and peak rate of flow from the impervious area separately. The computed peak rate of flow for the impervious surface alone may exceed that for the entire drainage basin using the value at Tc for the total drainage basin. The higher of the two peak flow rates shall then be used to size the conveyance element. Tc is computed by summation of the travel times Tt of overland flow across separate flowpath segments defined by the six categories of land cover listed in Table 3.2.1.C, which were derived from a chart published by the Soil Conservation Service in 1975. The equation for time of concentration is: Tc = T1 + T2 +…+ Tn (3-5) where T1,2,…n = travel time for consecutive flowpath segments with different land cover categories or flowpath slope Travel time for each segment t is computed using the following equation: Tt = (3-6) where Tt = travel time (minutes) Note: Tt through an open water body (such as a pond) shall be assumed to be zero with this method L = the distance of flow across a given segment (feet) V = average velocity (fps) across the land cover = kR s o where kR = time of concentration velocity factor; see Table 3.2.1.C so = slope of flowpath (feet/feet) L V60 3.2.1 RATIONAL METHOD 2022 City of Renton Surface Water Design Manual 6/22/2022 3-13 TABLE 3.2.1.A RUNOFF COEFFICIENTS – “C” VALUES FOR THE RATIONAL METHOD General Land Covers Single Family Residential Areas* Land Cover C Land Cover Density C Dense forest Light forest Pasture Lawns Playgrounds Gravel areas Pavement and roofs Open water (pond, lakes, wetlands) 0.10 0.15 0.20 0.25 0.30 0.80 0.90 1.00 0.20 DU/GA (1 unit per 5 ac.) 0.40 DU/GA (1 unit per 2.5 ac.) 0.80 DU/GA (1 unit per 1.25 ac.) 1.00 DU/GA 1.50 DU/GA 2.00 DU/GA 2.50 DU/GA 3.00 DU/GA 3.50 DU/GA 4.00 DU/GA 4.50 DU/GA 5.00 DU/GA 5.50 DU/GA 6.00 DU/GA 0.17 0.20 0.27 0.30 0.33 0.36 0.39 0.42 0.45 0.48 0.51 0.54 0.57 0.60 * Based on average 2,500 square feet per lot of impervious coverage. For combinations of land covers listed above, an area-weighted “Cc x At” sum should be computed based on the equation Cc x At = (C1 x A1) + (C2 x A2) + …+(Cn x An), where A8 = (A1 + A2 + …+An), the total drainage basin area. TABLE 3.2.1.B COEFFICIENTS FOR THE RATIONAL METHOD “IR” EQUATION Design Storm Return Frequency aR bR 2 years 5 years 10 years 25 years 50 years 100 years 1.58 2.33 2.44 2.66 2.75 2.61 0.58 0.63 0.64 0.65 0.65 0.63 TABLE 3.2.1.C KR VALUES FOR TT USING THE RATIONAL METHOD Land Cover Category kR Forest with heavy ground litter and meadow 2.5 Fallow or minimum tillage cultivation 4.7 Short grass pasture and lawns 7.0 Nearly bare ground 10.1 Grassed waterway 15.0 Paved area (sheet flow) and shallow gutter flow 20.0 SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS 6/22/2022 2022 City of Renton Surface Water Design Manual 3-14 FIGURE 3.2.1.A 2-YEAR 24-HOUR ISOPLUVIALS 3.2.1 RATIONAL METHOD 2022 City of Renton Surface Water Design Manual 6/22/2022 3-15 FIGURE 3.2.1.B 10-YEAR 24-HOUR ISOPLUVIALS SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS 6/22/2022 2022 City of Renton Surface Water Design Manual 3-16 FIGURE 3.2.1.C 25-YEAR 24-HOUR ISOPLUVIALS 3.2.1 RATIONAL METHOD 2022 City of Renton Surface Water Design Manual 6/22/2022 3-17 FIGURE 3.2.1.D 100-YEAR 24-HOUR ISOPLUVIALS SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS 6/22/2022 2022 City of Renton Surface Water Design Manual 3-18  RATIONAL METHOD EXAMPLE Compute the peak flow Q25 to size a new roadway cross culvert for a 9.8-acre drainage basin east of Kent, P25 = 3.42 inches. Given: AREAS A1 = 4.3 acres of single family residential area at 3.8 DU/GA A2 = 2.3 acres of light forest A3 = 3.2 acres of pasture At = 9.8 total acres DESCRIPTION OF FLOWPATH SEGMENTS FOR Tc L1 = 300 feet s1 = 0.08 forest land cover kR = 2.5 L2 = 200 feet s2 = 0.03 meadow kR = 2.5 L3 = 1000 feet s3 = 0.015 grassed waterway (ditch) kR = 15.0 Compute: COMPOSITE RUNOFF COEFFICIENT Cc A1: C1 = From Table 3.2.1.A, C for 4.00 DU/GA = 0.48, C for 3.50 DU/GA = 0.45. Therefore, C1 for 3.80 DU/GA = 0.47 by visual interpolation. A2: C2 = 0.15 A3: C3 = 0.20 Cc = [(C1 x A1) + (C2 x A2) + (C3 x A3)]/At = [(0.47 x 4.3) + (0.15 x 2.3) + (0.20 x 3.2)]/9.8 = 0.31 PEAK RAINFALL INTENSITY IR First, compute Tc: T1 = = 7 minutes T2 = = 8 minutes T3 = = 9 minutes Tc = T1 + T2 + T3 = 7 + 8 + 9 = 24 minutes Second, compute iR for R = 25: i25 = (aR)(Tc)(-bR) = (2.66)(24)- (0.65) = 0.34 Third, compute IR for R = 25: I25 = (P25)(i25) = (3.42)(0.34) = 1.16 PEAK RUNOFF RATE Q25 = C I25 A = Cc I25 A = (0.31)(1.16)(9.8) = 3.5 cfs L V L k sR 1 1 1 16060 300 60 25 0 08 ( ) ( . . ) L V L k sR 2 2 2 26060 200 60 25 003 ( ) ( . . ) L V L k sR 3 3 3 36060 1000 60 15 0015 ( ) ( . ) 3.2.2 CONTINUOUS MODELS AND THE RUNOFF FILES METHOD 2022 City of Renton Surface Water Design Manual 6/22/2022 3-19 3.2.2 CONTINUOUS MODELS AND THE RUNOFF FILES METHOD The approved continuous model/runoff files implementations of HSPF were developed as tools that have the accuracy and versatility of HSPF but are much simpler to use and provide a framework for efficient design of onsite stormwater detention facilities. This section describes the Runoff Files Method. The term runoff files refers to a database of continuous flows presimulated by HSPF. The KCRTS software package has formerly been a tool for using this flow database. Current approved continuous models are listed in Reference Section 6-D (as updated); as of this writing, they include the Western Washington Hydrology Model (WWHM) and MGSFlood6. Projects are required to use the same model throughout unless otherwise approved through the adjustment process described in Section 1.4. The Runoff Files method was developed as a hydrologic modeling tool for western King County to produce results (design flows, detention pond sizing, etc.) comparable to those obtained with the U.S. Environmental Protection Agency’s HSPF model but with significantly less effort. This is achieved by providing the user with a set of time series files of unit area land surface runoff (“runoff files”) presimulated with HSPF for a range of land cover conditions and soil types within King County. The design flows are estimated and detention facilities are designed by directly accessing and manipulating the runoff file data by means of the continuous modeling software. Typical basic capabilities of the continuous modeling software include:  Estimating time series of flows for a specified land use and location within King County  Analyzing flow frequency and duration  Analyzing water surface frequency and duration  Plotting analysis results  Sizing detention facilities.  DEVELOPMENT OF THE RUNOFF FILES To compile the runoff files, the land surface hydrologic response (represented by a time series of unit area land surface runoff) was generated by HSPF with regional parameters for a variety of land use classifications and for a long-term (over 50-year) rainfall station representing the western lowlands of King County (Sea-Tac Airport). A 158-year extended precipitation timeseries (Puget East) was also developed by MGS Consulting. The City allows the use of either the 50-year Sea-Tac Airport gage data or the 158-year simulated timeseries for sizing. The methods for developing the runoff files are specific to the individual approved models. Consult the program documentation and the software provider’s website information for the particular model for background on the development of the runoff files for that model. Runoff time series were generated with data from these and other stations for the following eight soil/land cover types:  Impervious  Till forest  Till pasture  Till grass  Outwash forest  Outwash pasture  Outwash grass  Wetland. HSPF and the approved models simulate surface runoff, interflow, and groundwater flow. Groundwater flow, induced by surface runoff or occurring naturally, is usually lost from the system through the 6 Note that MGS Flood is not currently approved for modeling bioretention. It will be allowed for modeling bioretention only at such time that it is formally approved by Ecology for that use. SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS 6/22/2022 2022 City of Renton Surface Water Design Manual 3-20 analysis, but may require consideration in the analysis if it expresses to the surface. Consult the user’s guide for application of the interflow and groundwater components of runoff in the approved continuous model. 3.2.2.1 GENERATING TIME SERIES Most hydrologic analyses will require time series of flows for different land use conditions. For example, to size a flow control detention facility to meet the Peak Rate Flow Control Standard, the 2-, 10-, and 100-year peaks from the facility discharge time series must be compared with 2-, 10-, and 100-year peaks from the predevelopment time series. To generate a flow time series with the approved continuous model, depending on the model used, the program applies the following: 1. As determined by selecting the project’s location on a map,  The rainfall region within which the project lies (i.e., Sea-Tac) and multiplier (a regional scale factor applied to the runoff files) to account for variations in rainfall volumes between the project site and the rainfall station, or  A calibrated area-specific rainfall map developed from the Sea-Tac rainfall data, or  A long-term (158-year) simulated precipitation timeseries (i.e., Puget East), or  Site specific calibrated rainfall data. See the approved model’s documentation for background on the development of the runoff files for the model. 2. The time step to be used in the analysis. As of this manual update, 15-minute time steps are required for all applications including detention sizing and volume analysis . 3. The complete historical runoff record used in the analysis: 4. The amount of land (acreage) of each soil/cover group for the subbasin under study, as calculated per model methodology and the methods described in this chapter. 5. If applicable, the percentage of impervious area that is effectively connected to the drainage system, typically accounted for by adjusting actual impervious area for the model inputs. See the user’s documentation for the approved model for methodology and guidance for generating a new time series. See Reference Section 6-D for specific guidance to be used with this manual.  SELECTION OF PRECIPITATION RECORD AND REGIONAL SCALE FACTOR As noted in the previous section, runoff files were developed using rainfall data from Sea-Tac Airport. The regional scale factor is a geographically variable multiplier applied to the flow time series to account for the variations in rainfall amounts, and hence runoff. Whereas previous models (e.g., KCRTS) required determination by mapped values as data input, the scaling effects are determined in the currently approved continuous models (e.g., WWHM and MGS Flood) by selecting the project location within the model. See the approved model user’s documentation for background and guidance. Alternatively, the user can select the 158-year simulated precipitation timeseries (Puget East) for sizing. This precipitation timeseries can be found by selected “Use WS-DOT data” in WWHM or under “Extended Timeseries” in MGS Flood. A scaling factor does not need to be applied to the Puget East precipitation timeseries.  CATEGORIZATION OF SOIL TYPES AND LAND COVER The Runoff Files method typically supports several land use classifications, including till forest, till pasture, till grass, outwash forest, outwash pasture, outwash grass, wetland, and impervious. These classifications incorporate both the effects of soil type and land cover. In the SCS method, four different hydrologic soil groups are defined (A, B, C, and D) based on soil type as mapped by the SCS. The SCS also defines hydrologic response for about a dozen different land use or cover types. The SCS method 3.2.2 CONTINUOUS MODELS AND THE RUNOFF FILES METHOD 2022 City of Renton Surface Water Design Manual 6/22/2022 3-21 therefore allows the user a considerably greater degree of flexibility in defining land cover and soil types than do continuous models. However, the flexibility and apparent detail available with the SCS method cannot be supported on the basis of the data used to develop that method. The Runoff Files method minimizes the number of land use classifications, thereby simplifying both the analysis and review of development proposals. Soil Groups for the Continuous Model The following soil characterization is generally true for continuous models; however, consult the model documentation for specific applicability. Till Soils Till soils are underlain at shallow depths by relatively impermeable glacial till. The principal SCS soil group within the City classified as a till soil is the Alderwood series (SCS hydrological soil group C). The hydrologic response of till soils in an undeveloped, forested state is characterized by relatively slight surface runoff, substantial interflow occurring along the interface between the till soil and the underlying glacial till, and slight groundwater seepage into the glacial till. Bedrock soils, primarily Beausite and Ovall soils in King County, are underlain by either sandstone or andesite bedrock, and a large group of alluvial soils. Alluvial soils are found in valley bottoms. These are generally fine-grained and often have a high seasonal water table. There has been relatively little experience in calibrating the HSPF model to runoff from these soils, so in the absence of better information, these soils have been grouped as till soils. Most alluvial soils are classified by the SCS in hydrologic soil groups C and D. Outwash Soils Outwash soils are formed from highly permeable sands and gravels. The principal SCS soil group classified as an outwash soil is the Everett series. Where outwash soils are underlain at shallow depths (less than 5 feet) by glacial till or where outwash soils are saturated, they may need to be treated as till soils for the purpose of application in the model. Refer to the model documentation for specifics. Wetland Soils Wetland soils have a high water content, are poorly drained, and are seasonally saturated. For the purposes of applying continuous modeling in King County, wetland soils can be assumed to coincide with wetlands as defined in the critical areas code (RMC 4-3-050 ). The approximate correspondence between SCS soil types and the appropriate soil group for typical continuous modeling is given in Table 3.2.2.A (refer to the model documentation for specific soil group application for the model). If the soils underlying a proposed project have not been mapped, or if existing soils maps are in error or not of sufficient resolution, then a soils analysis and report shall be prepared and stamped by a civil engineer with expertise in soils to verify underlying soil conditions. SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS 6/22/2022 2022 City of Renton Surface Water Design Manual 3-22 TABLE 3.2.2.A EQUIVALENCE BETWEEN SCS SOIL TYPES AND TYPICAL CONTINUOUS MODELING SOIL TYPES SCS Soil Type SCS Hydrologic Soil Group Soil Group for Continuous Model Notes Alderwood (AgB, AgC, AgD) C Till Arents, Alderwood Material (AmB, AmC) C Till Arents, Everett Material (An) B Outwash 1 Beausite (BeC, BeD, BeF) C Till 2 Bellingham (Bh) D Till 3 Briscot (Br) D Till 3 Buckley (Bu) D Till 4 Earlmont (Ea) D Till 3 Edgewick (Ed) C Till 3 Everett (EvB, EvC, EvD, EwC) A/B Outwash 1 Indianola (InC, InA, InD) A Outwash 1 Kitsap (KpB, KpC, KpD) C Till Klaus (KsC) C Outwash 1 Neilton (NeC) A Outwash 1 Newberg (Ng) B Till 3 Nooksack (Nk) C Till 3 Norma (No) D Till 3 Orcas (Or) D Wetland Oridia (Os) D Till 3 Ovall (OvC, OvD, OvF) C Till 2 Pilchuck (Pc) C Till 3 Puget (Pu) D Till 3 Puyallup (Py) B Till 3 Ragnar (RaC, RaD, RaC, RaE) B Outwash 1 Renton (Re) D Till 3 Salal (Sa) C Till 3 Sammamish (Sh) D Till 3 Seattle (Sk) D Wetland Shalcar (Sm) D Till 3 Si (Sn) C Till 3 Snohomish (So, Sr) D Till 3 Sultan (Su) C Till 3 Tukwila (Tu) D Till 3 Woodinville (Wo) D Till 3 Notes: 1. Where outwash soils are saturated or underlain at shallow depth (<5 feet) by glacial till, they should be treated as till soils. 2. These are bedrock soils, but calibration of HSPF by King County shows bedrock soils to have similar hydrologic response to till soils. 3. These are alluvial soils, some of which are underlain by glacial till or have a seasonally high water table. In the absence of detailed study, these soils should be treated as till soils. 4. Buckley soils are formed on the low-permeability Osceola mudflow. Hydrologic response is assumed to be similar to that of till soils. 3.2.2 CONTINUOUS MODELS AND THE RUNOFF FILES METHOD 2022 City of Renton Surface Water Design Manual 6/22/2022 3-23 Land Cover Types in Continuous Modeling Continuous models support land cover types including forest, pasture, grass, and impervious. These cover types shall be applied in accordance with Core Requirement #3 and as specified in Table 3.2.2.B. Predevelopment land cover types are determined by whether the project is in a Peak Rate Flow Control Standard Area or Flow Control Duration Standard Area and whether the area in question is a target surface, as defined in Section 1.2.3.1. Target surfaces within Peak Rate Flow Control Standard Areas and Flow Control Duration Standard Matching Existing Condition Areas and non-target surfaces are modeled as existing site conditions; for target surfaces in Flow Control Duration Standard Matching Forested Condition Areas, the predeveloped condition is assumed to be forested (historical) site conditions. TABLE 3.2.2.B CONTINUOUS MODEL COVER GROUPS AND AREAS OF APPLICATION Continuous Model Application Cover Group Predevelopment Post-Development Forest All forest/shrub cover, irrespective of age. All permanent (e.g., protected by covenant or critical area designation) onsite forest/shrub cover, irrespective of age, planted at densities sufficient to ensure 80%+ canopy cover within 5 years. Pasture All grassland, pasture land, lawns, and cultivated or cleared areas, except for lawns in redevelopment areas with predevelopment densities in excess of 4 DU/GA. Unprotected forest in rural residential development shall be considered half pasture, half grass. Grass Lawns in redevelopment areas with predevelopment densities in excess of 4 DU/GA. All post-development grassland and landscaping and all onsite forested land not protected by covenant or designated as a protection area (wellhead, wetland, or buffer) in RMC 4-3-050. For purposes of runoff modeling, underdrained pervious areas may be modeled explicitly to account for attenuation and infiltration, or may be modeled as 50% impervious/50% grass where either: (a) there is no added liner, (b) where the added liner is a treatment liner, or (c) where the added liner is one that does not restrict infiltration rates below the in situ soil infiltration rate. Other lined underdrained systems must be modeled explicitly or as 100% impervious. Wetland All delineated wetland areas. All delineated wetland areas. Impervious(1) All impervious surfaces, including compacted dirt roads, parking areas, etc., and open water bodies (ponds and lakes). For purposes of runoff modeling, gravel lots, roads and parking areas shall be modeled as 50% impervious/50% pasture. All impervious surfaces, including compacted gravel and dirt roads, parking areas, etc., and open water bodies, including onsite detention and water quality ponds.(2) (1) Impervious acreage used in computations should be the effective impervious area (EIA). This is the effective area as determined through layouts of the proposal and on-site BMP credit reductions from Table 1.2.9.A in Chapter 1. Non- effective impervious areas are considered the same as the surrounding pervious land cover. (2) To avoid iterations in the facility sizing process, the “assumed size” of the facility need only be within 80% of the final facility size when modeling its contribution of runoff from direct rainfall. SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS 6/22/2022 2022 City of Renton Surface Water Design Manual 3-24 The following factors are considered in specifying the above land cover types to be used in hydrologic analysis with continuous modeling:  Cover types are applied to anticipate ultimate land use conditions. For example, probable clearing of woodland after development is nominally complete suggests that the post-development land use be specified as grassland (either pasture or grass) unless the forest cover is protected by covenant.  In areas of redevelopment, there are often significant changes between the predevelopment and post- development efficiencies of the drainage system. For example, in conversion of low density residential areas to higher density land use, impervious areas prior to redevelopment may not be efficiently connected to a drainage system (e.g., downspouts draining to splash blocks, ditched instead of piped roadway systems). These problems are addressed by defining an “effective impervious fraction” for existing impervious areas and by generally requiring predevelopment grasslands to be modeled as pasture land.  All onsite, predevelopment forest/shrub cover and all offsite forest/shrub cover is defined as “forest,” irrespective of age. Post-development onsite land use is defined as forested only if forested areas are in a critical area buffer or are otherwise protected and will have a minimum 80% canopy cover within 5 years. In urban areas, unprotected onsite forest cover should be treated as grass in the post- development analysis. In rural areas, unprotected forest cover should be assumed 50% grass, 50% pasture.  The HSPF grass parameters were developed by the USGS study of regional hydrology and have generally been interpreted as providing the hydrologic response for “urban” grasslands (lawns, etc.), which have relatively low infiltration rates and are drained effectively. The HSPF “pasture” parameters were developed to provide a hydrologic response intermediate to the USGS forest and grass parameters, as might be typified by ungrazed or lightly grazed pasture with good grass cover. Because it is impossible to adequately control grassland management after development, all post- development grassland should be modeled as “grass” (with the exception of unprotected forest, and pasture areas on large lots, in rural development as noted above). All predevelopment grassland should be modeled as “pasture” except for redevelopment of areas with predevelopment land use densities of 4 DU/GA or greater (which are modeled as grass).  CALCULATION OF IMPERVIOUS AREA Total Impervious Coverage Table 3.2.2.C lists percent impervious coverage for use in continuous runoff modeling analysis of existing residential areas. The tabulated figures are useful in offsite analysis that includes large developed residential areas, making a detailed survey of impervious coverage impractical. Impervious coverage for proposed residential, commercial, and industrial development must be estimated for each specific proposal. Impervious coverage of streets, sidewalks, hard surface trails, etc., shall be taken from layouts of the proposal. House/driveway or building coverage shall be as follows:  For residential development, the assumed impervious coverage shall be 4,000 square feet per lot or the maximum impervious coverage permitted by RMC 4-2-110A, whichever is less.  For commercial, multi-family, and industrial development, impervious coverage shall be estimated from layouts of the proposal. 3.2.2 CONTINUOUS MODELS AND THE RUNOFF FILES METHOD 2022 City of Renton Surface Water Design Manual 6/22/2022 3-25 TABLE 3.2.2.C ESTIMATED PERCENT IMPERVIOUS COVERAGE FOR OFFSITE EXISTING RESIDENTIAL AREAS Zoning Designation Percent Impervious Surface Area Resource Conservation (RC) Lots 5 acres or more: 20% Lots 10,000 sq ft: 55%. For each additional 10,000 sq ft increase in lot size, the impervious coverage shall be decreased by 1.75% to a minimum of 20% for a 5-acre lot Lots 10,000 sq ft or less: 55% Residential-1 (R-1) 30% Residential-4 (R-4) 55% Residential-6 (R-6) 65% Residential-8 (R-8) 75% Residential-10 (R-10) Detached units: 75% Attached units: 65% Residential-14 (R-14) 85% Effective Impervious Area The net hydrologic response of an impervious area depends on whether that area is effectively connected (usually by pipes or a channel) to a storm drainage system. The impervious area that the user inputs to the continuous model is the “Effective Impervious Area” (EIA). Non-effective impervious area (i.e., total impervious area less EIA) is assumed to have the same hydrologic response as the immediately surrounding pervious area. For example, for existing residential areas with rooftops draining to splash pads on lawns or landscaping, the non-effective portion of the roof areas would be treated as pasture for predevelopment conditions (if DU/GA < 4.0) and grass for post-development conditions. Note: Credits for infiltration/dispersion of downspouts on individual lots in proposed single family residential subdivisions are applied separately on a site-specific basis. Core Requirement #9 outlines where the use of on-site BMPs may be used to reduce the effective impervious area of the project.. The effective impervious area can be determined from detailed site surveys. 3.2.2.2 TIME SERIES STATISTICAL ANALYSIS When using a continuous runoff model to size flow control, water quality, and conveyance facilities, design flows and durations must be determined through statistical analysis of time series data generated by the software. Flow frequency analysis is used for determining design peak flows while flow duration analysis is used for determining durations of flow exceedance.  FLOW FREQUENCY ANALYSIS Flow frequency is a commonly used but often misunderstood concept. The frequency of a given flow is the average return interval for flows equal to or greater than the given flow. The flow frequency is actually the inverse of the probability that the flow will be equaled or exceeded in any given year (the exceedance probability). For example, if the exceedance probability is 0.01, or 1 in 100, that flow is referred to as the 100-year flow. Assuming no underlying changes in local climate, one would expect to see about 10 peak annual flows equal to or greater than the 100-year flow in a 1,000-year period. Similarly, the 2-year flow is the flow with a probability of 0.5, or 1 in 2, of being equaled or exceeded in any given year. In a 100-year period, one would expect to observe 50 peak annual flows greater than or equal to the 2-year flow. The number of peak annual flows actually equal to the 2-year flow may be zero, since peak annual flows come from a continuous spectrum. SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS 6/22/2022 2022 City of Renton Surface Water Design Manual 3-26 There are many methods for estimating exceedance probabilities and therefore flow frequencies. The USGS Bulletin 17B methods are commonly used, as are graphical methods using the Gringorten, Cunane, or Weibull plotting schemes (Maidment, 1993). Graphical methods for flow frequency estimation involve assigning exceedance probabilities, and therefore return intervals, to each annual peak in a series of annual peak observations, and then plotting the peak flows against their assigned return periods. This plot is known as a flow-frequency curve, and it is a very useful tool for analyzing flood probabilities. Examples of flow-frequency curves for a small basin under various conditions are shown in Figure 3.2.2.A. Flow-frequency curves are used in continuous flow simulations to determine the effect of land use change and assess the effectiveness of detention facilities. Using continuous methodology to design detention facilities to control peak flows, the analyst must match (i.e., not exceed) the post-development (detained) and predevelopment flow-frequency curves at the frequencies of interest, as shown in Figure 3.2.2.A, rather than match specific design events as when using an event model. 3.2.2 CONTINUOUS MODELS AND THE RUNOFF FILES METHOD 2022 City of Renton Surface Water Design Manual 6/22/2022 3-27 FIGURE 3.2.2.A EXAMPLE FLOW FREQUENCY ANALYSIS Post-developed Pre-developed Undetained Developed Detained Developed Detained Post-developed Pre-developed The 2- and 10-year annual peak flows are matched; however, the 100-year peak flow is only partially attenuated in this example, so the detention volume would need to be increased to fully meet the Peak Rate Flow Control Standard SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS 6/22/2022 2022 City of Renton Surface Water Design Manual 3-28 Flow frequency information is derived from the time series flow file by plotting the peak annual events in the runoff file and calculating runoff frequencies using a Log Pearson distribution or other statistical analysis. Typical return periods calculated in continuous models are the 100-year, 50-year, 25-year, 10-year, 5-year, 3-year, 2-year, and lesser storms for low-flow regime, LID and water quality applications.  FLOW DURATION ANALYSIS Flow duration analysis is important because it identifies the changes in durations of all high flows rather than simply the change in frequency of the peak annual flows. Channel scour and bank erosion rates rise proportionally with increases in flow durations. Flow duration analysis can only be conducted with continuous flow models or from gage records. A flow duration curve is a plot of flow rate against the percentage of time that the flow rate is exceeded. In a continuous flow model, the percent exceedance of a given flow is determined by counting the number of time steps during which that flow is equaled or exceeded and dividing that number by the total number of time steps in the simulation period. Flow duration curves are usually plotted with a linear flow scale versus a log scale of percent exceedance. The log scale for exceedance percentage is used because geomorphically significant flows (flows capable of moving sediment) and flows that exceed the 2-year flow typically occur less than one percent of the total time.  DURATIONS AND PEAKS FOR FLOW CONTROL STANDARDS The Flow Control Duration Standard matching existing site conditions and Flow Control Duration Standard matching forested site conditions per Section 1.2.3.1 requires matching predevelopment and post-development flow duration curves for all flows from 50% of the 2-year flow up to the full 50-year flow. To simplify design, brief excursions7 of post development durations above the target predevelopment durations are allowed for matching flows greater than 50% of the predevelopment 2-year peak flow. These excursions shall not increase the duration of discharge by more than 10% at any flow level and must be strictly below the target duration curve at the low end of the range of control from 50% of the 2-year peak flow to the 2-year peak flow. This allows efficient design using only two orifices for most applications, although two-orifice designs may not allow sizing with automatic pond sizing routines; see the software documentation for guidance. An example of a flow duration analysis is shown in Figure 3.2.2.B. The Flood Problem Flow Control Standard matches predevelopment and post-development flow durations over the same range of predevelopment flows as the Flow Control Duration Standard and requires matching the 100-year post-development peak flow. This standard provides additional storage volume over the Flow Control Duration Standard facility, which substantially mitigates the impacts of increased volumes of surface runoff on downstream, volume-sensitive flooding problems. The Peak Rate Flow Control Standard does not require flow duration analysis because it addresses peak flows only (the 2-year, 10-year, and 100-year peaks). The Low Impact Development (LID) performance standard requires that stormwater discharges shall match (i.e., not exceed) developed discharge durations to pre-developed durations for the range of pre- developed discharge rates from 8% of the 2-year peak flow to 50% of the 2-year peak flow. No excursions above the pre-developed durations are allowed. 7 Brief excursions may not result in more than 50% of the target duration curve being exceeded. 3.2.2 CONTINUOUS MODELS AND THE RUNOFF FILES METHOD 2022 City of Renton Surface Water Design Manual 6/22/2022 3-29 FIGURE 3.2.2.B EXAMPLE FLOW DURATION ANALYSIS Post-developed Pre-developed Pre-developed FC Duration Existing Target Predeveloped Return Frequencies 50% 2-year 2-year 10-year 50-year 10% allowable horizontal tolerance along portion of target curve above 2-year predevelopment peak flow Strictly below target curve at low end of range of control (50% of 2-year peak flow to 2-year peak flow). SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS 6/22/2022 2022 City of Renton Surface Water Design Manual 3-30 When evaluating impacts to closed depressions, ponding areas and wetlands, or when evaluating for tightlined system requirements in critical areas per Core Requirement #1, frequencies of water levels or determination of average annual runoff volumes must be determined through statistical analysis of time series data generated using a continuous runoff model.  ASSESSING WATER LEVEL STATISTICS Stage frequency analysis consists of estimating and plotting recurrence estimates for water levels within a storage feature in the same manner as flow frequency analysis is conducted for discharges. Stage frequency analysis is required for assessing runoff impacts to offsite closed depressions and ponding areas as required under Core Requirements #2 and #3, and as discussed Section 3.3.6, “Point of Compliance Analysis,” or as required for analyses of wetland impacts pursuant to Core Requirement #9.  ASSESSING ANNUAL AVERAGE RUNOFF VOLUMES To compute the annual average runoff volume, the volume of runoff (surface + interflow) of a time series must be computed using the approved model. The analysis is performed using the entire period of record. The total volume is divided by the number of full water years being analyzed to determine the annual average runoff volume. 3.2.3 THE APPROVED MODEL The continuous hydrologic analysis tools prescribed in this manual are generically described as the “approved model”; a list of the approved models is found in Reference Section 6-D. At this writing, the approved continuous hydrologic models8 include the Western Washington Hydrologic Model (WWHM) and MGS Flood, both of which are variants of the Hydrologic Simulation Program- FORTRAN (HSPF) model. HSPF is also an approved model, but is more complex than other approved models and is typically used for basin planning and master drainage plan analyses. General instruction and guidance for use of the approved model is found in the user’s documentation for the model. Guidance specific to the City for the continuous runoff models approved for use with this manual is contained in Reference Section 6-D. A brief overview of HSPF follows below. 3.2.4 THE HSPF MODEL HSPF is the parent model from which the other approved model methods are built. It is a very versatile continuous hydrologic/hydraulic model that allows for a complete range of hydrologic analysis. This model has been extensively used in King, Snohomish, and Thurston counties and found to be an accurate tool for representing hydrologic conditions in this area. The USGS has developed regional parameters to describe the common soil/cover combinations found in this area. In many cases, these regional parameters can be used to represent rainfall/runoff relationships in lieu of site-specific calibration parameters. Unfortunately, the HSPF model is very difficult to use. Design engineers using HSPF should study this model in detail and obtain training before using it on a project. For these reasons, the HSPF model is recommended only for large and complex projects where the capabilities of the approved model are too limited. The strengths of HSPF relative to the approved model are as follows: 1. HSPF can be calibrated to local conditions. 2. HSPF can model, link, and route many separate subbasins. 8 Note that MGS Flood is not currently approved for modeling bioretention. It will be allowed for modeling bioretention only at such time that it is formally approved by Ecology for that use. 3.2.4 THE HSPF MODEL 2022 City of Renton Surface Water Design Manual 6/22/2022 3-31 3. HSPF includes the groundwater component of streamflow. 4. HSPF can address groundwater connections and perform low-flow analysis. 5. HSPF can handle more complex hydrologic routing (e.g., evaporation, seasonal infiltration, etc.). The HSPF model is generally recommended for large sites where these additional features are required for comprehensive hydrologic and/or hydraulic analysis. Anyone planning a project that is large enough to require Large Project Drainage Review and submittal of a Master Drainage Plan (MDP) per Section 1.1.2.5 should meet with CED review staff regarding appropriate hydrologic analysis prior to initiating such analysis. If a project subject to Large Project Drainage Review drains to a wetland, a salmonid stream with low-flow sensitivities, or a ground water protection area, it is likely that the City will require a calibrated HSPF model. If such a project drains to erosion-sensitive streams or has features with complex hydraulics, the City may recommend or require an HSPF model using the USGS regional parameters. Smaller or less sensitive subbasins within a MDP area can be analyzed with the approved model. Additional data is required to develop an HSPF model. At a minimum, development of an HSPF model requires collection of onsite rainfall data for a period from seven to twelve months. This data is used to determine which regional long-term rainfall record is most appropriate for modeling the site and for determining transposition factors for the long-term records. If calibration is required, the onsite rainfall data is used. Calibration also requires the installation of flow gages and the collection of flow data against which simulated flows can be compared. HSPF analysis is based on simulations with long-term rainfall records (greater than 30 years). Long-term precipitation records in HSPF format can be obtained from King County for the Sea-Tac rain gage and the Puget East 158-year simulated precipitation timeseries. Land surface representation with HSPF follows the same procedures and classification as used with the approved model. Conceptually, the outputs required from an HSPF analysis are consistent with those required from an approved model analysis, including frequency and durational analysis. Flow and/or water level frequencies shall be estimated using the full set of annual peaks from the long-term simulations using the USGS Bulletin 17B methods as well as the Gringorten or Cunane graphical methods. Durational analyses can be produced from the HSPF model and the results presented graphically. If a wetland is modeled, water level analyses may be required. Monthly, seasonal, and annual water balance and flow information, if appropriate, can be calculated with the HSPF model. SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS 6/22/2022 2022 City of Renton Surface Water Design Manual 3-32 (T his page intentionally left blan k. ) CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 3-33 3.3 HYDROLOGIC DESIGN PROCEDURES AND CONSIDERATIONS This section presents the design procedures and considerations for sizing flow control facilities to meet the required hydrologic performance specified in Core Requirement #3, Section 1.2.3. It includes the following procedures and special considerations for proper hydrologic design:  “General Hydrologic Design Process,” Section 3.3.1  “Flow Control Design Using the Runoff Files Method,” Section 3.3.2  “Conveyance System Design with the Runoff Files Method,” Section 3.3.3  “Safety Factors in Hydrologic Design,” Section 3.3.4  “Design Options for Addressing Downstream Drainage Problems,” Section 3.3.5  “Point of Compliance Analysis,” Section 3.3.6  “Onsite Close Depressions and Ponding Areas,” Section 3.3.7 3.3.1 GENERAL HYDROLOGIC DESIGN PROCESS This section presents the general process involved in conducting a hydrologic analysis using the runoff computation and analysis tools described in Section 3.2 to design flow control facilities for a project. The process is described as follows: 1. Review the core and special requirements in Chapter 1 to determine all requirements that will apply to the proposed project. a) Determine the applicable flow control standard (outflow performance criteria and land cover assumptions). b) If downstream drainage problems are identified through offsite analysis per Core Requirement #2, determine if they will necessitate additional onsite flow control or other measures as described in Section 3.3.5. 2. Determine and demonstrate in the Technical Information Report (see Section 2.3) the predeveloped conditions per Core Requirement #3, Flow Control (see Section 1.2.3). 3. Identify and delineate the drainage basin for each natural discharge location from the project site. a) Identify existing drainage features such as streams, conveyance systems, detention facilities, ponding areas, depressions, wetlands, etc. b) Identify existing land uses. c) Identify soil types using SCS soil survey or onsite evaluation. d) Convert SCS soil types to soil classifications for the approved model. 4. Select and delineate appropriate subbasins, including subbasins tributary to major drainage features and important conveyance points, and subbasins for separate computation of onsite flows and offsite flows. 5. Determine hydrologic parameters for each subbasin under predeveloped conditions. a) Categorize soil types and land cover. b) Determine total impervious areas and effective impervious areas within each subbasin. c) Determine areas for each soil/cover type in each subbasin. SECTION 3.3 HYDROLOGIC DESIGN PROCEDURES AND CONSIDERATIONS 6/22/2022 2022 City of Renton Surface Water Design Manual 3-34 6. Determine the runoff time series for predeveloped conditions at each natural discharge location. a) Compute the predeveloped condition runoff time series for each subbasin using 15-minute time steps. b) For subbasins that drain to a drainage feature with significant detention storage (e.g., existing detention facilities, ponding areas, closed depressions), route the runoff time series through the feature per the storage routing methods in the approved model. This will yield an attenuated flow series, which becomes the effective runoff time series for that subbasin. c) Sum the appropriate subbasin runoff time series to obtain the total runoff time series for each natural discharge location. d) Determine the 100-year peak flow for each natural discharge location. 7. Repeat Steps 4 through 6 for the proposed post-development condition. 8. Compare the 100-year peak flows for the appropriate predeveloped and post-development conditions at each natural discharge location. a) Check the “Discharge Requirements” criteria in Core Requirement #1 to determine the acceptable manner of discharge from the project site (using existing conditions). b) Check the flow control exemptions in Core Requirement #3 to determine if a flow control facility is required (using existing site or historical site conditions, as specified in Core Requirement #3). c) Check the requirement for bypass of runoff from non-target surfaces in Core Requirement #3 to determine if runoff from non-target surfaces must be conveyed around onsite flow control facilities (using existing conditions). 9. If flow control facilities are required, determine their location and make any necessary adjustments to the developed condition subbasins. 10. Design and size each flow control facility using the methods described in Section 3.2 and the Runoff Files Method design procedure in Section 3.3.2. a) Analyze the appropriate predeveloped condition runoff time series to determine target release rates for the proposed facility. Note: If the target release rates are zero, an infiltration facility will be required. b) Compute the post-development runoff time series for the proposed facility. c) Use the post-development runoff time series and an iterative process to size the facility to meet the required level of performance set forth in Core Requirement #3. See the approved model user’s documentation for procedures in sizing flow control facilities using continuous flow time series. 11. Design required onsite conveyance systems using the appropriate runoff computation method (either the Rational method or the Runoff Files method with 15-minute time steps) as specified in Section 3.2. 3.3.2 FLOW CONTROL DESIGN USING THE RUNOFF FILES METHOD Flow control facility design using the approved modeling software involves four basic steps: 1. Determining the statistical characteristics (peaks or durations) of predevelopment flows (using 15-minute time steps) which set the targets for the facility release rates, 2. Developing preliminary facility volume and orifice configuration, 3. Routing post-development flow time series through the preliminary facility to check performance, and 4. Iteratively revising the facility and checking performance until the target flow conditions are achieved. 3.3.2 FLOW CONTROL DESIGN USING THE RUNOFF FILES METHOD 2022 City of Renton Surface Water Design Manual 6/22/2022 3-35 Instead of using individual design rainfall events as in an event model, the design of the facility is based on simulation of the facility’s performance using the full historical (over 50-years) time series record of simulated post-development flows, and also on comparison of the outflow record to characteristics of the predevelopment flow record. Final design is achieved when the outflow time series meets the target flow specifications. Detention facility design with a continuous model is based on aggregate flow statistics, not upon individual storms. When designing detention facilities with a continuous model, the return period of the peak flow leaving the facility for a particular event may not have the same return period as the peak flow entering the facility during the same event. Unlike event models, continuous models have natural variability in the ratio of storm peak and volume. This lack of correspondence in the return periods of peak inflows and outflows in continuous models means that facility design using a continuous runoff model is more complicated than with an event method and in general has to be done on an iterative trial-and-error basis to obtain an optimal (i.e., least volume) design. The effect of detention facilities in controlling peak flows is dependent on both the volume and peak of the inflowing hydrograph. Generally, it is high volume storms rather than high intensity storms that cause detention facilities to fill and overtop. The hydrographs produced by a continuous runoff model show considerable variability in the relationships between peak flows and storm volumes. For example, one event produced by high rainfall intensities in a relatively short duration storm may produce high peak flows with a relatively small hydrograph volume. By contrast, a second rainfall event may have relatively low intensities but long duration, producing a runoff hydrograph with large volumes and relatively small peak. Due to this natural variability, the peak annual outflows from a detention facility may not correspond in time to the annual peaks of the inflow record. Similarly, the predevelopment peak annual flows may not occur during the same storm as the peak annual flows for the post-development flow series. This is because the types of storms that produce high flows from undeveloped land covers are different from those that produce high flows from impervious surfaces. Forests generate high streamflows in response to long-duration, high-volume rainfall events that soak the soil profile, whereas impervious surfaces produce the highest flow rates in response to high precipitation intensity. This is another reason why detention facility design with a continuous runoff model is based on aggregate flow statistics, not upon individual storm hydrographs. The following is a typical procedure for hydrologic design of detention/infiltration facilities using a continuous runoff model. Specific guidance for conducting hydrologic analysis and design with the approved model is provided in the approved model user’s documentation. 1. Create time series of flows from the predevelopment area using graphic elements that detail the predevelopment land cover, the post-development area tributary to the facility, any onsite post- development bypass area, and any offsite flow-through areas. 2. Add any offsite flow-through time series to the predevelopment flow time series using similar graphic elements to produce a time series of total predevelopment outflows from the project site. Similarly, add the same offsite flow-through time series to the time series of post-development flows tributary to the facility to produce a time series of total post-development inflows to the facility. 3. Generate peak annual flow estimates, flow duration curves and flow frequency curves for pre- and post-development time series. 4. Enter the Facility element for the scenario and specify initial facility specifications for the type of facility proposed. Use of two orifices is usually sufficient for most designs. If designing an infiltration facility, the bottom orifice may be elevated or zero orifices may be specified. 5. Route the complete facility inflow time series through the facility. The outflow time series is automatically saved. Use the analysis tools to evaluate facility performance. When sizing the facility to account for credits from on-site BMPs per Core Requirement #9 and Appendix C, note that it is necessary to turn infiltration off for on-line on-site BMPs draining to the facility, to avoid SECTION 3.3 HYDROLOGIC DESIGN PROCEDURES AND CONSIDERATIONS 6/22/2022 2022 City of Renton Surface Water Design Manual 3-36 counting the flow reduction effect twice. For facilities designed using this manual, explicit modeling of infiltrative BMPs for downstream flow control facility sizing is not allowed. 6. Adjust orifice configuration and facility size, iterate until desired performance is achieved. Use of the automatic facility sizing routine in the approved model is helpful. 7. Verify the facility performance by routing the complete time series of inflows and checking the post-development peak flows and/or durations at the project site boundary against the target flows and/or durations (see the criteria for “Evaluating Flow Control Performance” provided below). When explicitly modeling BMPs for compliance with the LID Performance standard, two separate routings are necessary to evaluate the flow control credit based facility performance and the explicitly modeled BMPs for the LID Performance standard. Evaluating Flow Control Performance Evaluating the performance of facility designs intended to provide flow frequency control is comparatively straightforward: the post-development facility annual peak flows should be strictly less than or equal to predevelopment annual peak flows at each of the specified return periods. Note: Peak flow matching is required per Core Requirement #3. The automatic sizing routines in the approved continuous runoff models are based on duration matching and do not evaluate for peak flow compliance. The user must complete this evaluation as an additional step to verify compliance. Evaluating the design performance of detention facilities providing flow duration control, however, generally requires several iterations. In fact, considerable time could be spent attempting to match predevelopment and post-development duration curves. Some flexibility in assessing the adequacy of fit is clearly needed to expedite both design and review. Therefore, flow duration designs will be accepted as meeting performance standards when the following conditions are met: 1. The post-development flow duration curve lies strictly on or below the predevelopment curve at the lower limit of the range of flow control (between 50% of the 2-year and the 2-year).9 2. At any flow value within the upper range of flow control (from the 2-year to the 50-year), the post- development duration of the flow is no more than 1.1 times the predevelopment flow duration. 3. The target duration curve may not be exceeded along more than 50% of the range of control. 4. Where a facility or BMP is used to meet the LID Performance Standard, the post-development flow duration curve lies strictly on or below the predevelopment curve for the range of pre-developed discharge rates for the LID Performance standard (from 8% of the 2-year peak flow to 50% of the 2-year peak flow).10 9 For small projects, the lower limit of the range of control for flow control duration standard matching existing site conditions is considered met with a minimum diameter (0.25 inches) lower orifice in a low head facility (maximum effective storage depth of 3 feet) where full duration control cannot be achieved at the lower limit. Predeveloped flow durations, within allowed tolerances, must be met for all flows above the best achievable lower limit. The LID Performance standard must also be met; performance results could be influenced by the minimum diameter. 10 See Core Requirement #9 and Appendix C for application of pre-sized on-site BMPs for mitigating the LID Performance standard in lieu of explicit modeling 3.3.3 CONVEYANCE SYSTEMS DESIGN WITH THE RUNOFF FILES METHOD 2022 City of Renton Surface Water Design Manual 6/22/2022 3-37 3.3.3 CONVEYANCE SYSTEM DESIGN WITH THE RUNOFF FILES METHOD This section provides guidance for use of the Runoff Files method in determining peak flows for the design and analysis of conveyance elements, overflow structures, and other peak flow sensitive drainage features. Rainfall events that create the highest rates of runoff from developed areas are typically shorter in duration and are characterized by brief periods of high intensity rainfall. To simulate the runoff from higher intensity, shorter duration rainfall events, a 15-minute time series is used. The following is the typical procedure for hydrologic design and analysis of conveyance facilities using the Runoff Files method: 1. Select and delineate appropriate subbasins. a) Select separate subbasins for major drainage features and important conveyance points. b) Identify existing land covers offsite and post-development land covers onsite. c) Identify soil types by using the SCS soil survey or by directly evaluating the site. d) Convert SCS soil types to the approved model soil classifications. 2. Determine hydrologic parameters for each subbasin. a) Within the approved model, locate the project to determine appropriate rainfall region and/or regional scale factor. b) Categorize soil types and land cover per Table 3.2.2.A and Table 3.2.2.B. c) Determine total impervious areas and effective impervious areas within each subbasin. d) Determine areas for each soil/cover type in each subbasin. 3. Determine peak flows for the conveyance element being analyzed. a) Following the approved model guidance, assemble the post-development scenario including an element for each subbasin and using 15-minute time steps. b) Set the point of compliance at the confluence of the post-developed subbasins being routed to the conveyance element. Run the scenario for the developed subbasins and conduct a flow frequency analysis on the results of the scenario run. From this analysis the 10-year, 25-year, and 100-year peak flows can be determined. These design flows can then be used to size or assess the capacity of pipe systems, culverts, channels, spillways, and overflow structures. SECTION 3.3 HYDROLOGIC DESIGN PROCEDURES AND CONSIDERATIONS 6/22/2022 2022 City of Renton Surface Water Design Manual 3-38 3.3.4 SAFETY FACTORS IN HYDROLOGIC DESIGN It is often appropriate to apply safety factors to detention volumes or conveyance design flows. This manual does not require safety factors for detention or conveyance design, but it does recommend the use of safety factors when the designer believes the results of the approved model are not sufficiently conservative given local conditions. The approved model methodology does not include inherent safety factors as it is meant to account for “average” conditions. On a particular site, the approved model may overestimate or underestimate flow rates and detention volumes. Within any soil/cover group, there is a range of hydrologic response dependent on local soil and geologic conditions for which the approved model methodology does not account. The USGS regional parameters for HSPF that were used to create the runoff files produce “average” runoff time series that overestimate peak flows in some basins and underestimate them in others. Similarly, the detention volumes designed with the approved model for a given conversion type are in the middle of the range of volumes that would be created if exact local hydrologic conditions were known for every project of that type. Therefore, some of the detention facilities designed with the approved model are oversized and some are undersized, depending on variable site conditions. Because of the uncertainty in local hydrologic response, the City recommends, but does not require, that a volume safety factor of 10% be applied to all detention facilities. If downstream resources are especially sensitive, or if the designer believes that the approved model significantly overestimates predevelopment flows or underestimates post-development flows, a volume safety factor of up to 20% may be appropriate. If a volume safety factor is applied to a detention facility, the volume should be increased by the given percentage at each one-foot stage increment. Safety factors for conveyance systems should be evaluated with respect to the potential damages and costs of failures due to backwatering, overtopping, etc. Applications of safety factors fall strictly within a professional engineer’s judgment and accountability for design. Section 4 of the Technical Information Report should state what safety factor was applied to the design of the flow control facility. 3.3.5 DESIGN OPTIONS FOR ADDRESSING DOWNSTREAM DRAINAGE PROBLEMS See Chapter 1, Table 1.2.3.A for options for addressing downstream drainage problems. 3.3.6 POINT OF COMPLIANCE ANALYSIS The point of compliance is the location where flow control performance standards are evaluated. In most cases, the point of compliance is the outlet of a proposed detention facility where, for example, 2- and 10-year discharges must match predevelopment 2- and 10-year peak flow rates. The point of compliance for hydrologic control moves downstream of the detention facility outlet or the property boundary under the following circumstances: 1. The proposed project discharges to an offsite closed depression with a severe flooding problem per Section 1.2.2, and the project adds impervious surface greater than or equal to 10% of the 100-year water surface area of the closed depression (see Table 1.2.3.A). In these cases, the closed depression becomes the point of compliance, and the engineer must ensure that project site runoff does not aggravate the flooding problem (or create a new flooding problem). 2. The proposed project includes an onsite runoff bypass, a small developed area that bypasses the flow control facility (see Section 1.2.3.2). In such cases, runoff from the remainder of the project site is overdetained so that the sum of the detained and undetained flows meets the required flow control performance standard. The point of compliance for such projects is where the onsite bypass flows join the detained flows. 3.3.6 POINT OF COMPLIANCE ANALYSIS 2022 City of Renton Surface Water Design Manual 6/22/2022 3-39 3. The proposed project bypasses offsite flows around an onsite closed depression, ponding area, or wetland (see Section 3.3.7). As with onsite bypasses, the point of compliance in this case is where detained flows converge with the bypassed flows. The approved model allows multiple points of compliance for evaluating runoff performance within a scenario. The automatic facility sizing routine in the approved model requires a point of compliance to size an individual facility; a separate point of compliance is required for downstream evaluation. See the approved model user’s documentation for modeling application of points of compliance to meet the requirements of this manual. Note: When controlling flow durations at a downstream point of compliance to demonstrate no adverse impact, the 10% tolerance specified for the Flow Control Duration Standard may not be used. Predevelopment condition flow durations should be matched to the extent feasible for all flows above the level of concern. The resultant facility should also be checked to verify that the minimum onsite performance standard has also been met.  OFFSITE CLOSED DEPRESSIONS If a project drains to an offsite closed depression with existing or potential flooding problems, then the water surface levels of the closed depression must not be allowed to increase for return frequencies at which flooding occurs, up to and including the 100-year frequency. This section describes the point of compliance analysis necessary to size detention facilities discharging to such a closed depression. If the closed depression is classified as a wetland, other requirements apply per Section 1.2.2, Core Requirement #2. The closed depression is first modeled (using the site’s predevelopment condition) to determine the return frequency at which flooding currently occurs and the water levels associated with return frequencies in excess of this frequency. These flooding levels and their probabilities dictate the detention performance for the proposed development. The proposed detention facility is then iteratively sized such that discharge from the site’s post-development condition does not increase water surface levels for the frequencies at which flooding occurs—that is, after development, water level frequency curves must match for all frequencies equal to or greater than the frequency at which flooding occurs (up to the 100-year water level). The infiltration rate must be determined in order to accurately model the closed depression. In the case of a closed depression with an existing flooding problem, the infiltration rate is most realistically depicted by calibrating the model to known flooding events. This should be done using the full historical runoff files and setting the closed depression outflow (infiltration) such that recorded or anecdotal levels of flooding occur during the same storm events in the historical record. Where a flooding problem might be created by discharge of post-development flows to a closed depression, and in the absence of information on dates and water surface levels in the closed depression during past runoff events, infiltration rates must be determined through testing as follows:  For a closed depression without standing water, two or more test pits should be dug in the bottom of the closed depression to a depth of 10 feet or to the water table, whichever is reached first. The test pits shall be dug under the supervision of a geotechnical engineer, and a test pit log shall be kept. Evidence of high water table shall be noted.  If the test pit reveals deep homogeneous permeable material with no evidence of a high water table, then infiltration tests shall be performed in the bottom of the closed depression at locations of similar elevation and on opposite sides of the bottom area (as feasible). Surface infiltration rates shall be determined using the methods for assessing measured infiltration rates included in Section 5.2. The measured rates should be used directly, without applying correction factors.  If the closed depression has standing water or is a defined as a wetland according to RMC 4-3-050, or if test pits show evidence of a high water table or underlying impermeable material, then procedures for determining infiltration rates will be established on a case-by-case basis in coordination with CED. SECTION 3.3 HYDROLOGIC DESIGN PROCEDURES AND CONSIDERATIONS 6/22/2022 2022 City of Renton Surface Water Design Manual 3-40  In the event that a closed depression with a documented severe flooding problem is located on private property and all reasonable attempts to gain access to the closed depression have been denied, the Flood Problem Flow Control Standard shall be applied with a 20% factor of safety on the storage volume.  ONSITE RUNOFF BYPASS It is sometimes impractical to collect and detain runoff from an entire project area, so provisions are made to allow undetained discharge from onsite bypass areas (see Section 1.2.3.2) while overdetaining the remainder of the runoff to compensate for unmitigated flows. A schematic of an onsite runoff bypass is shown in Figure 3.3.6.A. For projects employing onsite runoff bypass, flow control performance standards are evaluated at the point of compliance, the point where detained and undetained flows from the project site are combined. Point of Compliance Analysis for Onsite Bypass Areas 1. In the approved model, create a predeveloped condition element for the entire project area including the predevelopment detained area and the predevelopment bypass area. Route the scenario and apply the analysis tools to determine flow targets (either flow frequencies or durations, depending on the applicable design standard) from the predeveloped condition runoff time series. 2. Create and route separate developed condition elements for the detained area and the bypass area, producing a separate time series for each area. 3. Ensure that the flow characteristics of the developed runoff element for the bypass area do not exceed the targets determined in Step 1 or the 0.4 cfs threshold in Core Requirement #3. If the bypass area flows exceed the targets or threshold, then the bypass is not feasible. 4. Estimate allowable release rates from the detention facility for each return period of interest with the following equation: Allowable release = (Total Project Area Flow)predeveloped cond. – (Bypass Area Flow)developed cond. Note: WWHM 2012 and later supports the direct sizing of onsite detention facilities based on the results at a downstream point-of-compliance. See the WWHM user’s documentation for further details. 1. Develop a preliminary design of the flow control facility based on the estimated release rate(s). 2. Route post-development flows from the detained area through the detention facility to create a detention facility outflow time series. Provide a downstream point of compliance and route the bypass area and the facility outflow to the downstream POC. 3. The approved model determines the total project post-development outflow by adding the detention facility outflow runoff time series to the post-development runoff time series from the bypass area at the downstream point of compliance. Check characteristics of the total project post-development outflow against the targets determined in Step 1. 4. If compliance is not achieved (e.g., 2- and 10-year post-development flows exceed 2- and 10-year predevelopment flows), revise the facility design (or revise the project design to reduce the bypass area) and repeat Steps 6 through 8. For WWHM 2012 and later, Steps 6 through 8 have been automated for facility sizing by using the point of compliance option in the facility element of the model. See the WWHM user’s documentation for guidance. 3.3.7 ONSITE CLOSED DEPRESSIONS AND PONDING AREAS 2022 City of Renton Surface Water Design Manual 6/22/2022 3-41 FIGURE 3.3.6.A SCHEMATIC OF AN ONSITE RUNOFF BYPASS 3.3.7 ONSITE CLOSED DEPRESSIONS AND PONDING AREAS Onsite closed depressions, ponding areas, and wetlands require special consideration when determining detention performance targets; if altered, they can shift the point of compliance downstream. However, the critical areas code (RMC 4-3-050) regulates wetlands (note that most closed depressions and ponding areas are wetlands by definition) and generally does not permit alteration through either filling or gross hydrologic changes such as bypassing offsite flows. Note: Post-development discharges to offsite closed depressions, ponding areas, or wetlands (with the exception of those in Flood Problem Flow Control Areas per the Flow Control Applications Map or those discussed in Section 3.3.6) are normally not required to meet special performance standards unless there is a severe flooding problem as defined in Section 1.2.2.  GENERAL REQUIREMENTS The following general requirements apply to onsite closed depressions, ponding areas, and wetlands (referred to below as “features”): 1. Flow attenuation provided by onsite wetlands and ponding areas, and storage provided by onsite closed depressions must be accounted for when computing both existing onsite and offsite flows.  Existing onsite flows must be routed through onsite wetlands and ponding areas to provide accurate target release rates for the developed site. Note: Closed depressions will have no outflow for some portions of the site for some events, although overflow may occur during extreme events.  Existing offsite flows will increase at the project boundary if the feature is filled or if the offsite flows are bypassed around the feature. To compensate, post-development onsite flows must be stream NG PE bufferdetention tract open space BYPASS AREA DETAINED AREA SECTION 3.3 HYDROLOGIC DESIGN PROCEDURES AND CONSIDERATIONS 6/22/2022 2022 City of Renton Surface Water Design Manual 3-42 overdetained, and the point of compliance will shift downstream to where the detained flows converge with the bypassed offsite flows. 2. If the onsite feature is used for detention, the 100-year floodplain must be delineated considering developed onsite and existing offsite flows to the feature. Note: Additional storage volume may be necessary within the feature, and the point of compliance is the discharge point from the feature. 3. If the detention facility for the proposed project discharges to an onsite wetland, ponding area, or closed depression that is not altered11 by the proposed project, AND Flow Control Duration or Flood Problem Flow Control is provided, the point of compliance is the discharge point of the detention facility, not the outlet of the onsite feature. If Peak Rate Flow Control is being provided, the point of compliance is the outlet of the onsite feature.  FLOODPLAIN DELINEATION FOR LAKES, WETLANDS, CLOSED DEPRESSIONS, AND PONDING AREAS A minor floodplain analysis is required for onsite or adjacent lakes, wetlands, and closed depressions that do not have an approved floodplain or flood hazard study (see Section 4.4.2; note the exceptions). Minor floodplain studies establish an assumed base flood elevation below which development is not allowed. The following are guidelines for minor floodplain analysis of volume sensitive water bodies: 1. Create time series representing tributary flows to the feature from the entire tributary area. Where the feature is contained entirely onsite and where no offsite flows exist, use the tributary area for the proposed developed condition. 2. Where the feature is only partially onsite, or where there are offsite flows to the feature, assume the entire tributary area is fully built out under current zoning , accounting for required open space and protected critical areas in the basin as well as impervious surfaces and grass. 3. For potential future development, assume detention standards per Section 1.2.3.1. For simplicity the proposed detention may be simulated with a single assumed detention pond just upstream of the feature. This pond should be sized to the appropriate detention standard and predevelopment condition assumption as noted in Section 1.2.3.1 and will require generating a predevelopment time series for the basin. Large water bodies may provide significant floodwater storage and may also be included in the analysis. Most existing detention in the basin, with exception of that providing duration control, will have little effect on the analysis and should be discounted. 4. Sum all subbasin time series to create a single composite time series for the drainage feature. 5. Develop routing curves for the feature. As appropriate, consider infiltration as an outflow for closed depressions. 6. Route the time series through the storage feature, generate water surface frequency curves, and note the 100-year water surface elevation. 11 Not altered means existing on- and offsite flows to the feature will remain unchanged and the feature will not be excavated or filled. 2022 City of Renton Surface Water Design Manual 6/22/2022 CHAPTER 4 CONVEYANCE SYSTEM ANALYSIS & DESIGN CITY OF RENTON SURFACE WATER DESIGN MANUAL Section Page 4.1 Route Design and Easement Requirements 4-3 4.1.1 Route Design 4-3 4.1.2 Easement and Setback Requirements 4-3 4.2 Pipes, Outfalls, and Pumps 4-7 4.2.1 Pipe Systems 4-7 4.2.2 Outfall Systems 4-30 4.2.3 Pump Systems 4-36 4.3 Culverts and Bridges 4-37 4.3.1 Culverts 4-37 4.3.2 Culverts Providing for Fish Passage/Migration 4-50 4.3.3 Bridges 4-52 4.4 Open Channels, Floodplains, and Floodways 4-55 4.4.1 Open Channels 4-55 4.4.2 Floodplain/Floodway Analysis 4-71 CITY OF RENTON SURFACE WATER DESIGN MANUAL 6/22/2022 2022 City of Renton Surface Water Design Manual (T his page intentionally left blan k. ) CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 4-1 CHAPTER 4 CONVEYANCE SYSTEM ANALYSIS & DESIGN This chapter presents the City of Renton’s approved methods for the hydraulic analysis and design of conveyance systems. A conveyance system includes all portions of the surface water system, either natural or man-made, that transports surface and storm water runoff. This chapter contains the detailed design criteria, methods of analysis, and schematic representations for all components of the conveyance system. In some cases, reference is made to other adopted or accepted design standards and criteria such as the City of Renton Standard Details and the requirements of the City of Renton Transportation department and Surface Water Utility as applicable. The figures included in this chapter are provided as schematic representations and should not be used for design. Refer to the City of Renton Standard Details for specific design information. The figures provided in this chapter illustrate one example of how the conveyance system design criteria may be applied. Although the figures are meant to illustrate many of the most important design criteria, they may not show all criteria that apply. In general, the figures are not used to specify requirements unless they are indicated elsewhere in this manual. If this manual refers to a standard detail not included in the City of Renton Standard Details, the applicant shall use the figure provided in this manual. Chapter Organization The information presented in this chapter is organized into four main sections:  Section 4.1, “Route Design and Easement Requirements“  Section 4.2, “Pipes, Outfalls, and Pumps”  Section 4.3, “Culverts and Bridges”  Section 4.4, “Open Channels, Floodplains, and Floodways” These sections begin on odd pages so the user can insert tabs if desired for quicker reference. Required vs. Recommended Design Criteria Both required and recommended design criteria are presented in this chapter. Criteria stated using “shall” or “must” are mandatory, to be followed unless there is a good reason to deviate as allowed by the adjustment process (see Section 1.4). These criteria are required design criteria and generally affect facility performance or critical maintenance factors. Sometimes options are stated as part of the required design criteria using the language “should” or “may.” These criteria are recommended design criteria, but are closely related to the required criteria, so they are placed in the same section. SECTION 4.1 ROUTE DESIGN AND EASEMENT REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-2 (T his page intentionally left blan k) CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 4-3 4.1 ROUTE DESIGN AND EASEMENT REQUIREMENTS This section presents the general requirements for aligning conveyance systems and providing easements and setbacks to allow for proper maintenance and inspection of all conveyance system elements. 4.1.1 ROUTE DESIGN The most efficient route selected for new conveyance systems will result from careful consideration of the topography of the area to be traversed, the legal property boundaries, and access for inspection and maintenance. Additionally, topography and native soil characteristics beneficial to Low Impact Development (LID) applications may influence the route. The general requirements for route design are as follows: 1. Proposed new conveyance systems should be aligned to emulate the natural conveyance system to the extent feasible. Inflow to the system and discharge from the system should occur at the natural drainage points as determined by topography and existing drainage patterns. 2. New conveyance system alignments in residential subdivisions should be located adjacent and parallel to property lines so that required drainage easements can be situated along property lines. Drainage easements should be located entirely on one property and not split between adjacent properties. Exception: Streams and natural drainage channels shall not be relocated to meet this requirement. 3. Aesthetic considerations, traffic routes and on-site BMP strategies may dictate the placement and alignment of open channels. Appropriate vehicular and pedestrian traffic crossings must be provided in the design. 4. For any reach or partial reach of new conveyance (ditch, channel or closed pipe system) proposed by a project, a geotechnical analysis and report is required if the conveyance is located within 200 feet of a steep slope hazard area or landslide hazard area, OR if the conveyance is located within a setback distance from top of slope equal to the total vertical height of the slope area that is steeper than 15%. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built-out conditions. A low-permeability liner per Section 6.2.4 for the trench or channel may be required if warranted by soil stability conditions. 4.1.2 EASEMENT AND SETBACK REQUIREMENTS Proposed projects must comply with the following easement and setback requirements unless otherwise approved by the City: 1. Any onsite conveyance system element (including on-site BMPs used as conveyance) constructed as part of a subdivision project shall be located in a dedicated drainage easement, tract, or right-of-way that preserves the system's route and conveyance capacity as follows:  Onsite conveyance systems within the right-of-way will be inspected and maintained by the City.  Onsite conveyance systems within the drainage easements or tracts will be maintained by the property owners through the Homeowners Association created by the subdivision (with each property owner having equal responsibility for maintenance). These conveyance elements include those entering and exiting the tract from/to the public right-of-way. For conveyance pipes entering the tract from right-of-way, responsibility begins at the last structure prior to entering the tract. For conveyance pipes exiting the tract to right-of-way, responsibility ends at the next downstream structure. The easement shall grant the City rights for inspection. SECTION 4.1 ROUTE DESIGN AND EASEMENT REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-4 Exception: Roof downspout, minor yard, and footing drains do not require easements, tracts, or right- of-way. If easements are provided for these minor drains (or for other utilities such as power, gas or telephone), they need not comply with the requirements of this section. Note: except for those facilities that have been formally accepted for maintenance by the City, maintenance and repair of drainage facilities and BMPs on private property is the responsibility of the property owner. Except for the inflow pipe and discharge pipe of a City-accepted flow control or water quality facility, the City does not normally accept maintenance of conveyance systems constructed through private property. 2. Any onsite conveyance system element (including on-site BMPs used as conveyance) constructed under a commercial building or commercial development permit shall be covered by the drainage facility declaration of covenant and grant of easement in Reference Section 8-J (or equivalent) that provides the City right of access for inspection, maintenance, and repair. Note: except for those facilities that have been formally accepted for maintenance by the City, maintenance and repair of drainage facilities on private property is the responsibility of the property owner. 3. Retained or replaced 12-inch or greater pipe diameter (or equivalent) conveyance system elements that convey offsite flows on a project site on private property shall be covered by the drainage facility declaration of covenant and grant of easement in Reference Section 8-J (or equivalent) that provides the City right of access for inspection, maintenance, and repair. For projects with conveyance system elements as described above that cannot meet or be relocated to meet the easement and building setback line (BSBL) requirements in Table 4.1 due to the presence of existing structures, applicants are required only to record a notice on title that identifies the subject conveyance elements and states that maintenance and repair of those elements is the responsibility of the property owner. For conveyance system elements as described above that are on a site but not within the project site, applicants are required only to record a notice on title that identifies the subject conveyance elements and states that maintenance and repair of those elements is the responsibility of the property owner. Note: except for those facilities that have been formally accepted for maintenance by the City, maintenance and repair of drainage facilities on private property is the responsibility of the property owner. 4. Any offsite conveyance system element (including on-site BMPs used as conveyance) constructed through private property as part of a proposed project that conveys runoff from public roads within the project site shall be located in a drainage easement dedicated to the City. If an offsite conveyance system through private property is proposed by a project to convey runoff diverted from the natural discharge location, the City may require a drainage release covenant per Reference Section 8-K as a condition of approval of the adjustment required in Section 1.2.1. 5. A river protection easement shall be required for all properties adjoining or including major rivers1 that may be dedicated to the City or County as applicable. The County shall review and approve river protection easements dedicated to the County. 6. Table 4.1 lists the required widths and BSBLs for drainage easements. For all pipes or any channels or constructed swales greater than 30 feet wide, facilities must be placed in the center of the easement. For channels or constructed swales less than or equal to 30 feet wide, the easement extends to only one side of the facility. Note: The requirement for drainage easements with accompanying widths and BSBLs per Table 4.1 also applies to existing and replaced conveyance elements as described in #3 above. 1 Major rivers are defined in the King County Flood Hazard Management Plan. 4.1.2 EASEMENT AND SETBACK REQUIREMENTS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-5 7. Any portion of a conveyance system drainage easement (shown in Table 4.1) shall not be located within an adjacent property or right-of-way. Building setback lines may cross into adjacent property. 8. The distance between the easement line and building or other structure footings shall be no less than the BSBL distance shown in Table 4.1. Exception: The BSBL distance indicated in Table 4.1 may be measured from the edge of a pipe in the easement plus 2 feet if all of the following conditions are met: a) As-builts showing the location of the pipe are submitted b) A geotechnical/structure analysis demonstrates stability of the proposed structure c) Access for maintenance/replacement remains unobstructed. TABLE 4.1 EASEMENT WIDTHS AND BUILDING SETBACK LINES For Pipes:(1) Inside Diameter (ID) Easement Width BSBL (From Easement) ID  36″ depth to invert < 8′: 10 feet(2) depth to invert > 8′: 15 feet 5 feet 36″ < ID  60″ depth to invert < 8′: 10 feet(2) depth to invert > 8′: 15 feet 7.5 feet ID > 60″ ID plus 10 feet 10 feet For Channels and Swales: Top Width of Channel (W) Easement Width BSBL (From Easement) W  10 feet W plus 10 feet on one side W if no access required(3) 5 feet 10 feet < W  30 feet W plus 15 feet on one side 5 feet W > 30 feet W plus 15 feet on both sides 5 feet For Major Rivers Easement Width BSBL (From Easement) See the King County Flood Hazard Management Plan for a list of the major rivers Varies per site conditions Minimum 30 feet from stable top of bank(4) 5 feet Notes: (1) Pipes installed deeper than 10 feet require one of the following actions:  Increase the BSBL such that the distance from the BSBL to the centerline of the pipe is at least 1.5 times the depth to pipe invert, or  Place a restriction on adjacent lots that the footings be placed at a specific elevation, deep enough that the closest horizontal distance from the footing to the pipe centerline is 1.5 times the difference in elevation of the footing and pipe invert, or  Place a restriction on adjacent lots that the footings be designed by a geotechnical engineer or licensed engineering geologist, such that excavation of the pipe may be performed without necessitating shoring of adjacent structures. (2) Fifteen-foot easement width is required for maintenance access to all manholes, inlets, and culverts. (3) Access is not required for small channels if the channel gradient is greater than 5% (assumes steep channels will be self-cleaning). (4) Stable top of bank shall be as determined by King County. SECTION 4.1 ROUTE DESIGN AND EASEMENT REQUIREMENTS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-6 (T his page intentionally left blan k. ) CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 4-7 4.2 PIPES, OUTFALLS, AND PUMPS This section presents the methods, criteria, and schematic representations for analysis and design of pipe systems, outfalls, and pump-dependent conveyance systems. The information presented is organized as follows: Section 4.2.1, “Pipe Systems” “Design Criteria,” Section 4.2.1.1 “Methods of Analysis,” Section 4.2.1.2 Section 4.2.2, “Outfall Systems” “Design Criteria,” Section 4.2.2.1 Section 4.2.3, “Pump Systems” “Design Criteria,” Section 4.2.3.1 “Methods of Analysis,” Section 4.2.3.2. 4.2.1 PIPE SYSTEMS Pipe systems are networks of storm drain pipes, catch basins, manholes, inlets, and outfalls designed and constructed to convey surface water. The hydraulic analysis of flow in storm drain pipes typically is limited to gravity flow; however, in analyzing existing systems it may be necessary to address pressurized conditions. A properly designed pipe system will maximize hydraulic efficiency by utilizing proper material, slope, and pipe size. 4.2.1.1 DESIGN CRITERIA General In addition to the design criteria described below, pipe systems shall be design to meet the hydraulic criteria as described in Section 1.2.4.1. These criteria supersede the methodology descriptions contained in Chapter 4. All pipe material, joints, protective treatment, construction workmanship, and inspection requirements shall be in accordance with the City of Renton Standard Details and the requirements of the City of Renton Transportation Department of Surface Water Utility as applicable. Note: The pipe materials and specifications included in this section are for conveyance systems installed according to engineering plans required for City permits/approvals. Other pipe materials and specifications may be used by private property owners for drainage systems they construct and maintain when such systems are not required by or granted to City. Acceptable Pipe Sizes and Length Between Structures Acceptable pipe sizes and maximum lengths between structures shall be per Table 4.2.1.A1. SECTION 4.2 PIPES, OUTFALLS, AND PUMPS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-8 TABLE 4.2.1.A1 PIPE SIZES AND LENGTHS BETWEEN STRUCTURES Upstream Structure to Downstream Structure Pipe Diameter (in) Where Allowed Maximum Length (ft) Inlet to CB/MH 8 Private 40 12 Private 60 CB/MH to CB/MH 8 Private/Public(1) 100/66(1) 12 or greater Private/Public 300(2) Notes: (1) Minimum diameter for public pipes shall be 12 inches. However, 8-inch diameter public pipes may be permitted on cross street laterals less than 66 feet to avoid utility conflicts or to meet shallow grade. (2) Maximum spacing on surface drainage course between catch basins or manholes shall be 150 on grades less than 1% and 200 feet on grades from 1% to 3%. Otherwise, maximum spacing shall be 300 feet on grades over 3%, or as required by grate flow capacities. Maximum spacing may need to be reduced depending on street width and inlet capacity analysis in Section 4.2.1.2. Allowable Pipe Materials and Minimum Cover Requirements 1. The designer shall have the option of constructing storm sewers, drains and culverts of the pipe types listed below within the cover limits specified. In addition, concrete pipe shall be rubber gasketed and metal pipe shall be gasketed and securely banded. Leak testing shall be conducted if required by the City Engineer. 2. The pipe materials included in Table 4.2.1.A2 are allowed for use in meeting the requirements of this manual. Refer to the current edition of WSDOT/APWA Standard Specifications 7-02, 7-03 and 7-04 for detailed specifications for acceptable pipe materials. TABLE 4.2.1.A2 ALLOWABLE PIPE MATERIALS AND MINIMUM COVER Pipe Type Minimum Cover (ft) Public Private Allowed in Zone 1 of the APA Corrugated Steel Pipe 2.0 Yes Yes Yes Spiral Rib Steel Pipe 2.0 Yes Yes Yes Plain Concrete Pipe (PCP) 2.0 No Yes No Reinforced Concrete Pipe (RCP) 1.0 Yes Yes No Ductile Iron 1.0 Yes Yes Yes Line Corrugated Polyethylene Pipe (LCPE) 2.0 Yes Yes Yes Corrugated Polyethylene Pipe (CPE) – Triple Wall 2.0 Yes Yes Yes Polypropylene Pipe (PP) – Dual Wall 2.0 Yes Yes Yes Polyvinyl Chloride Pipe (PVC) 3.0 Yes Yes Yes Solid Wall High Density Polyethylene Pipe (HDPE) 2.0 Yes Yes Yes Allowable Pipe Joints 1. Concrete pipe shall be rubber gasketed. 2. Corrugated steel pipe shall be rubber gasketed and securely banded. 3. Spiral rib steel pipe shall be “hat-banded” with neoprene gaskets. 4. Ductile iron pipe joints shall be flanged, bell and spigot, or restrained mechanical joints. 5. PP and CPE pipe joints (lined and single wall, fully corrugated) shall conform to the current WSDOT/APWA Standard Specifications. 4.2.1 PIPE SYSTEMS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-9 6. PVC pipe, CPE pipe and PP pipe shall be installed following procedures outlined in ASTM D2321.Solid wall HDPE pipe shall be jointed by butt fusion methods or flanged according to the City of Renton Standard Details. Pipe Alignment 1. Pipes must be laid true to line and grade with no curves, bends, or deflections in any direction. 2. Exception: Vertical deflections in solid wall HDPE and ductile iron pipe with flanged restrained mechanical joint bends (not greater than 30) on steep slopes, provided the pipe drains. 3. A break in grade or alignment, or changes in pipe material shall occur only at catch basins or manholes. Allowable Pipe Slopes and Velocities Table 4.2.1.A3 presents allowable pipe slopes and velocities by pipe material. TABLE 4.2.1.A3 ALLOWABLE PIPE SLOPES AND VELOCITIES Pipe Material Minimum Slope Allowed(3) Pipe Slope above which Pipe Anchors Required and Minimum Anchor Spacing Maximum Slope Allowed Minimum Velocity at Full Flow(3) Maximum Velocity at Full Flow Steel or PVC(1) 0.5% 20% (1 anchor per 100 LF of pipe) 30%(4) 3 fps 30 fps Concrete, CPE, or PP(1) 0.5% 10% (1 anchor per 50 LF of pipe) 20%(4) 3 fps 30 fps Ductile Iron(2) 0.5% 20% (1 anchor per pipe section) None 3 fps None Solid wall HDPE(2) 0.5% 20% (1 anchor per 100 LF of pipe, cross-slope installations only) None 3 fps None Notes: (1) These materials are not allowed in landslide hazard areas. (2) Butt-fused or flanged pipe joints are required; above ground installation is recommended on slopes greater than 40%. (3) Minimum slope and full flow velocity is required unless it cannot be achieved due to outlet control, site topography, burial depth or other situations or conditions. (4) A maximum slope of 200% is allowed for these pipe materials with no joints (one section), with structures at each end, and with proper grouting. Changes in Pipe Size 1. Increase or decreases in pipe size are allowed only at structures. Exceptions may be allowed as follows: Connections to pipe systems may be made without placing a catch basin or manhole on the mainline by meeting all of the following conditions: a) The mainline pipe is 48 inches or greater and at least two times the size of the connecting pipe. b) Make connections in accordance with the manufacture’s recommendations. Standard shop fabricated tees, wyes and saddles shall be used, except for concrete connections constructed in accordance with the City of Renton Standard Details. c) There shall be a catch basin or manhole on the connecting pipe within 2 to 10 feet of the external wall of the main line. SECTION 4.2 PIPES, OUTFALLS, AND PUMPS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-10 d) Offset angle of connecting pipe to mainline, horizontally and vertically shall be less than 45 degrees. e) Two-point survey control shall be used to set catch basin locations. 2. When connecting pipes at structures, match any of the following (in descending order of preference): crowns, 80% diameters,2 or inverts of pipes. Side lateral connections3, 12 inches and smaller, are exempt from this requirement. 3. Drop manholes may be used for energy dissipation when pipe velocities exceed 10 feet per second. External drop manholes are preferred where maintenance access to the upstream pipe is preserved by use of a tee section. Internal drop structures may be approved only if adequate scour protection is provided for the manhole walls. Drop structures must be individually engineered to account for design variations, such as flow rates, velocities, scour potential and tipping forces. 4. Downsizing pipes larger than 12 inches may be allowed provided pipe capacity is adequate for design flows. Note: The above criteria do not apply to detention tanks. Structures Table 4.2.1.B lists typical drainage structures with corresponding maximum allowable pipe sizes. 1. Catch basin (or manhole) diameter shall be determined by pipe orientation at the junction structure. A plan view of the junction structure, drawn to scale, will be required when more than four pipes enter the structure on the same plane, or if angles of approach and clearance between pipes is of concern. The plan view (and sections if necessary) must ensure a minimum distance (of solid concrete wall) between pipe openings of 8 inches for 48-inch and 54-inch catch basins, and 12 inches for 72-inch and 96-inch catch basins. 2. Evaluation of the structural integrity for H-20 loading, or as required by the City of Renton Standard Details, may be required for multiple junction catch basins and other structures. 3. All solid wall HDPE pipe systems (including buried solid wall HDPE pipe) must be secured at the upstream end. The downstream end shall be placed in a 4-foot section of the next larger pipe size. This sliding sleeve connection allows for the high thermal expansion/contraction coefficient of this pipe material. 4. The maximum slope of the ground surface for a radius of 5 feet around a catch basin grate or solid lid should be 5:1 (H:V) to facilitate maintenance access. Where not physically feasible, a maximum slope of 3:1 (H:V) shall be provided around at least 50% of the catch basin circumference. 5. Catch basins (see City of Renton Standard Details) rather than inlets shall be used to collect storm water from road surfaces, unless approved by the City Engineer. 6. Type 2 (see City of Renton Standard Details) catch basins shall be used where the depth to the invert of the pipe exceeds 5 feet. 7. Manholes (see City of Renton Standard Details) may be used in lieu of catch basins if they do not collect surface water. Manholes must be used if inverts are greater than 18 feet. 8. Roof and yard drains, or other concentrated flow from adjacent property shall not discharge over the surface of roadways, sidewalks, walkways, or shoulders. 2 Match point is at 80% of the pipe diameter, measured from the invert of the respective pipes. 3 Side laterals include any 8-inch or smaller pipe connected to the main conveyance system at a catch basin, or manhole, as allowed under this manual and/or the City of Renton Standard Details. In addition, 12-inch and smaller pipes that serve a single inlet point (e.g., roadway simple inlets, footing drains, and lot stubouts including manifold systems serving multiple residential lots) are also included. Excluded from this definition are inlet pipes that contribute 30% or more of the total flow into a catch basin, or that collect or convey flows from a continuous source. 4.2.1 PIPE SYSTEMS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-11 9. Catch basins or manholes are required when joining differing types of pipes. 10. The location of at least two points of all catch basins shall be surveyed to ensure that the catch basin, frame and grate will properly align with finished curb, horizontally and vertically. 11. Metal castings for drainage structures shall not be dipped, painted, welded, plugged or repaired. 12. Porosity in metal castings for drainage structures shall be considered a workmanship defect subject to rejection by the inspector. 13. Grates and covers shall be seated properly to prevent rocking, including the replacement of existing covers with solid metal covers. 14. Unless otherwise specified, vaned grates (see City of Renton Standard Details), shall be used with standard frame in the traveled way, gutter, or shoulder. Vaned grates shall not be located within crosswalks. 15. At sag vertical curves, on the end of downgrade cul-de-sacs, or before intersections with a grade four percent or greater, an analysis shall be done to assure that typical catch basin grates will collect the surface runoff. To collect excessive volumes of runoff or protect against plugged grates and overflow situations, the City Road Engineer will require the use of through inlet frames on vertical curbs, (see City of Renton Standard Details). Where the through-curb inlets cannot be used, place a catch basin at the low point and two extra inlets located not greater than 0.1 foot above the low point grate within a spacing of 25 feet. 16. New catch basins and manholes that do not collect runoff shall use solid locking covers (per City of Renton Standard Details). Existing catch basins, which no longer collect runoff, shall have their frame and grates replaced with solid covers. 17. All storm drain covers and grates need to be locking regardless of their location. 18. Slot drains may be used when approved by the City Engineer. At a minimum, slot drains shall have catch basins at either end unless used as a driveway culvert. The maximum distance between catch basins along a slot drain shall be 50 feet. TABLE 4.2.1.B ALLOWABLE STRUCTURES AND PIPE SIZES Catch Basin Type(1) Maximum Pipe Diameter Steel, Solid Wall HDPE, PVC, and Ductile Iron(2) Concrete, CPE, PP Inlet(4) 12″ 12″ Type 1(3) 18″ 12″ Type 1L(3) 24″ 18″ Type 2 – 48-inch dia. 30″ 24″ Type 2 – 54-inch dia. 36″ 30″ Type 2 – 72-inch dia. 54″ 42″ Type 2 – 96-inch dia. 72″ 60″ Notes: (1) Catch basins (including manhole steps, ladder, and handholds) shall conform to the City of Renton Standard Details. (2) Generally these pipe materials will be one size larger than concrete, CPE or PP due to smaller wall thickness. However, for angled connections or those with several pipes on the same plane, this will not apply. (3) A maximum of 5 vertical feet is allowed between finished grade and invert elevation. (4) Inlets are normally allowed only for use in privately maintained drainage systems and must discharge to a catch basin immediately downstream. SECTION 4.2 PIPES, OUTFALLS, AND PUMPS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-12 Pipe Design between Structures 1. Minimum velocity at full flow should be 3.0 feet per second (fps). If site constraints result in velocities less than 3 fps at full flow, impacts from sedimentation in the pipe system shall be addressed with larger pipes, closer spacing of structures, sediment basins, or other similar measures. 2. Minimum slope for pipes shall be 0.5%, unless otherwise approved by CED in locations with site constraints provided that the minimum velocity requirement of 3 fps is met. 3. Maximum lengths between structures shall meet the requirements in Table 4.2.1.A1 above. Solid wall HDPE tightlines down steep slopes are self-cleaning and do not require structures for maintenance. Pipe Cover 1. Pipe cover, measured from the finished grade elevation to the top of the outside surface of the pipe, shall be 2 feet minimum unless otherwise specified or allowed below or as allowed above in Table 4.2.1.A2. Under drainage easements, driveways, parking stalls, or other areas subject to light vehicular loading, pipe cover may be reduced to 1 foot minimum if the design considers expected vehicular loading and the cover is consistent with pipe manufacturer’s recommendations. Pipe cover in areas not subject to vehicular loads, such as landscape planters and yards, may be reduced to 1 foot minimum. 2. All flexible storm sewer pipe and culvert material shall be covered by a minimum of 2 feet of cover unless the applicant submits detailed plans accompanied by manufacturer’s recommendations specifying allowable cover less than 2 feet in depth. All non-flexible storm sewer pipe and culvert material shall be covered by a minimum of 1 foot of cover. Pipe cover over concrete pipe shall comply with Table 4.2.1.C. For other pipe types, the manufacturer’s specifications or other documentation shall be provided for proposed cover in excess of 30 feet. Caution: Additional precautions to protect against crushing during construction may be needed under roadways if the road bed is included to meet minimum cover requirements. Damaged pipe shall be replaced. 3. For proposed pipe arches, the manufacturer’s specifications or other documentation shall be provided for proposed cover in excess of 8 feet. 4. Pipe cover over PVC SDR 35 shall be 3 feet minimum and 30 feet maximum. TABLE 4.2.1.C MAXIMUM COVER (FEET) FOR CONCRETE PIPE Pipe Diameter (inches) Plain Class II Class III Class IV Class V 12 18 10 14 21 26 18 18 11 14 22 28 24 16 11 15 22 28 30 11 15 23 29 36 11 15 23 29 48 12 15 23 29 60 12 16 24 30 72 12 16 24 30 84 12 16 24 30 96 12 16 24 30 108 12 16 24 30 Note: See Figure 4.2.1.A for a schematic representation. Only Class IV and V are allowed in public right-of-way. Pipe Clearances A minimum 7-foot horizontal separation and 1-foot vertical separation (measured wall to wall) is required between storm pipe and other utilities with the exception of water lines where a minimum 10-foot 4.2.1 PIPE SYSTEMS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-13 horizontal separation and 1.5-foot vertical separation (measured wall to wall) is required, unless otherwise approved by CED in locations with site constraints. Pipe Bedding, Backfill and Compaction Pipe bedding and backfill shall be in accordance with the City of Renton Standard Details. Pipe compaction shall follow the current WSDOT Standard Specifications. Where pipes pass through flood containment structures, these standards shall be supplemented and modified as necessary in accordance with standards set forth in Corps of Engineers Manual for Design and Construction of Levees (EM 1110-2-1913). Pipe System Connections Connections to a pipe system shall be made only at catch basins or manholes. No wyes or tees are allowed except on roof/footing/yard drain systems on pipes 8 inches in diameter or less, with clean-outs upstream of each wye or tee. Additional exceptions may be made provided the following conditions are met: 1. The mainline pipe is 48 inches or greater and at least two times the size of the connecting pipe. 2. Make connections in accordance with the manufacturer’s recommendations. Standard shop fabricated tees, wyes and saddles shall be used, except for concrete connections constructed in accordance with the City of Renton Standard Details. 3. There shall be a catch basin or manhole on the connecting pipe within 2 to 10 feet of the external wall of the main line. 4. Offset angle of connecting pipe to mainline, horizontally and vertically shall be less than 45 degrees. Storm drainage pipe systems shall not penetrate building foundations, except for sump pump discharge lines used to drain crawl spaces, provided the sump pump system includes a backflow prevention or a check valve. An area drain within a crawl space is allowed to connect into the storm drainage system provided the following conditions are met: 1. The connection is to be tight lined a minimum of ten feet outside of the foundation, and in no case connected upstream of the footing drain system for the house. 2. The area drain must be routed through a sump pump system prior to connection to the public storm drainage system, to help prevent backup during surcharge conditions and of potential sewer gas into the crawl space area. Pipe Anchors Table 4.2.1.A3 presents the requirements, by pipe material, for anchoring pipe systems. Figure 4.2.1.B and Figure 4.2.1.C show schematic representations of pipe anchors. Spill Control Where spill control is required as specified in Section 1.2.4.3.G, allowable options are as follows: a) A tee section (see Figure 5.1.4.A) in or subsequent to the last catch basin or manhole that collects runoff from non-roof-top pollution-generating impervious surface prior to discharge from the site or into an onsite natural drainage feature.4 The tee section typically provided in a wetvault or detention facility may be used to meet the intent of this requirement. Unless otherwise specified, the riser top of the tee section shall be at or above the headwater elevation for the 10-year design 4 Natural onsite drainage feature means a natural swale, channel, stream, closed depression, wetland, or lake. SECTION 4.2 PIPES, OUTFALLS, AND PUMPS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-14 flow and a minimum of 6 inches below the ceiling of the catch basin or manhole. The bottom end of the tee section shall be as illustrated in Figure 5.1.4.A. b) A wall section or other device as approved by the City that provides spill control equivalent to that of the tee section specified in a) above. c) A baffle or coalescing plate oil/water separator at or subsequent to the last catch basin or manhole that collects runoff from non-roof-top pollution-generating impervious surface prior to discharge from the site or into an onsite natural drainage feature. d) An active spill control plan. To use this option, the spill control plan and summary of an existing or proposed training schedule must be submitted as part of the drainage review submittal. At a minimum, such plans must include the following:  Instructions for isolating the site to prevent spills from moving downstream (shutoff valves, blocking catch basins, etc.)  Onsite location of spill clean-up materials  Phone numbers to call for emergency response  Phone numbers of company officials to notify  Special safety precautions, if applicable. Debris Barriers Debris barriers (trash racks) are required on all pipes 18 to 36 inches in diameter entering a closed pipe system. Debris barriers for pipes smaller than 18 inches and larger than 36 inches in diameter may be required depending on conditions and safety concerns. Debris barriers shall have a bar spacing of 6 inches. See Figure 4.2.1.D for a schematic representation of debris barriers on pipe ends outside of roadways. See Figure 4.2.1.E and Section 4.3 for a schematic representation of debris barriers on pipe ends (culverts) projecting from driveway or roadway side slopes. Outfalls Outfalls shall be designed as detailed in Section 4.2.2. Other Details In addition to the schematic representations provided in Figure 4.2.1.A through Figure 4.2.1.E , standard construction details are available in the City of Renton Standard Details and APWA/WSDOT Standard Plans for Road, Bridge and Municipal Construction . Commonly used details include field tapping of concrete pipe, catch basins and catch basin details, manholes and manhole details, curb inlets, frames, grates, and covers. 4.2.1 PIPE SYSTEMS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-15 FIGURE 4.2.1.A SCHEMATIC REPRESENTATION OF PIPE BEDDING AND BACKFILL DESIGNS 6"6"6"6"6"6"85% RISE6"FOUNDATION LEVEL FOUNDATION LEVEL FOUNDATION LEVEL GRAVEL BACKFILL FOR PIPE ZONE BEDDING (SEE NOTE 2) PIPE ZONE BACKFILL (SEE NOTE 1) NOTES: 1. SEE CURRENT WSDOT STANDARD SPECIFICATIONS SECTION 7-08.3(3) FOR PIPE ZONE BACKFILL. 2. SEE CURRENT WSDOT STANDARD SPECIFICATIONSSECTION 9-03.12(3) FOR GRAVEL BACKFILL FOR PIPE ZONE BEDDING. 3. SEE CURRENT WSDOT STANDARD SPECIFICATIONSSECTION 2-09.4 FOR MEASUREMENT OF TRENCH WIDTH. 4. SEE KCSWDM 4.2.1.1 FOR CLEARANCE BETWEEN PIPES AND OTHER UTILITIES.METAL PIPE THERMOPLASTIC PIPE CONCRETE AND DUCTILE IRON PIPE PIPE ARCHES TRENCH WIDTH (SEE NOTE 3)PIPE ZONETRENCH WIDTH (SEE NOTE 3) TRENCH WIDTH (SEE NOTE 3)PIPE ZONEGRAVEL BACKFILL FOR PIPE ZONE BEDDING (SEE NOTE 2) PIPE ZONE BACKFILL (SEE NOTE 1) GRAVEL BACKFILL FOR PIPE ZONE BEDDING (SEE NOTE 2) GRAVEL BACKFILL FOR PIPE ZONE BEDDING (SEE NOTE 2) PIPE ZONE BACKFILL (SEE NOTE 1)PIPE ZONEPIPE ZONETRENCH WIDTH (SEE NOTE 3) NOTE: ALL DETAILS NOT TO SCALE6"15% RISE 15% O.D. 85% O.D.O.D.50%O.D.50%O.D.FOUNDATION LEVEL SECTION 4.2 PIPES, OUTFALLS, AND PUMPS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-16 FIGURE 4.2.1.B SCHEMATIC REPRESENTATION OF A PIPE ANCHOR DETAIL 6" MIN. NOTE: FOR SOLID WALL HDPE, PIPE MUST BE FREE TO SLIDE INSIDE A 4' LONG SECTION OF PIPE ONE SIZE DIAMETER LARGER. SECTION B-B NTS SECTION A-A NTS CONCRETE BLOCK ANCHOR NTS STRAP-FOOTING ANCHOR NTS 1" MIN. DIAMETER STEEL ROD (STRAP) CLAMPED SECURELY TO PIPE. CONCRETE FOOTING KEYED INTO UNDISTURBED SOIL AS SHOWN PIPE BEDDING CONCRETE FOOTING KEYED INTO UNDISTURBED SOIL AS SHOWN CONCRETE BLOCK 2 x PIPE DIA. MIN.1' MIN.1' MIN. 6" MIN. 6" MIN. 2' MIN. 6" MIN. 3' MIN. 6" MIN. 6" MIN. (TYPICAL) 4.2.1 PIPE SYSTEMS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-17 FIGURE 4.2.1.C SCHEMATIC REPRESENTATION OF CORRUGATED METAL PIPE COUPLING AND/OR GENERAL PIPE ANCHOR ASSEMBLY NOTES: 1. THE SMOOTH COUPLING BAND SHALL BE USED IN COMBINATION WITH CONCRETE PIPE. 2. CONCRETE PIPE WITHOUT BELL AND SPIGOT SHALL NOT BE INSTALLED ON GRADES IN EXCESS OF 20%. 3. THE FIRST ANCHOR SHALL BE INSTALLED ON THE FIRST SECTION OF THE LOWER END OF THE PIPE AND REMAINING ANCHORS EVENLY SPACED THROUGHOUT THE INSTALLATION. 4. IF THE PIPE BEING INSTALLED HAS A MANHOLE OR CATCH BASIN ON THE LOWER END OF THE PIPE, THE FIRST PIPE ANCHOR MAY BE ELIMINATED. 5. WHEN CMP IS USED, THE ANCHORS MAY BE ATTACHED TO THE COUPLING BANDS USED TO JOIN THE PIPE AS LONG AS THE SPECIFIED SPACING IS NOT EXCEEDED. 6. ALL PIPE ANCHORS SHALL BE SECURELY INSTALLED BEFORE BACKFILLING AROUND THE PIPE. COUPLING BAND 1 1 2" X 6' PIPE STAKES EACH SIDE OF CULVERT FLATTEN TO POINT MATERIAL TO BE ASTM A36 1 4" PLATE GALVANIZED AFTER FABRICATION PER ASTM A123 SLOTS TO BE 119 32 X 3 4"78"SMOOTH COUPLING BAND FOR SMOOTH PIPE NTS PLATE DETAIL NTS ANCHOR ASSEMBLY CORRUGATED METAL PIPE NTS 3" MATERIAL TO BE ASTM A36 GALVANIZED AFTER FABRICATION PER ASTM A153 PLATE (SEE DETAIL) WELD COLLAR (2" PIPE) WELD PIPE STAKES6 58"ALL HOLES 3 4" DIAM.4 18"1 " R 2"3/4"12" 4 12"4 12" 17 8"17 8" 1" 12" 7" COUPLING BAND 12" OR 24" COUPLING BAND SECTION 4.2 PIPES, OUTFALLS, AND PUMPS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-18 FIGURE 4.2.1.D SCHEMATIC REPRESENTATION OF A DEBRIS BARRIER (OFF-ROAD RIGHT-OF- WAY) 3/4" DIAMETER SMOOTH BARS CMP OR LINED CPE PIPE PIPE COUPLING SPOT WELD BARS TO AT LEAST 2 CORREGATIONS OF METAL PIPE (TYPICAL) BOLT TO LINED CPE PIPE NOTES: 1. THIS DEBRIS BARRIER IS FOR USE OUTSIDE ROADWAYS ON PIPES 18" DIA. TO 36" DIA.. SEE FIGURE 4.2.1.E FOR DEBRIS BARRIERS ON PIPES PROJECTING FROM DRIVEWAY OR ROADWAY SIDE SLOPES. 2. ALL STEEL PARTS MUST BE GALVANIZED AND ASPHALT COATED (TREATMENT 1 OR BETTER). 3. LINED CPE PIPE REQUIRES BOLTS TO SECURE DEBRIS BARRIER TO PIPE. PLAN NTS ISOMETRIC NTS END VIEW NTS SIDE VIEW NTS 45°3"L/2L 6" 12" MIN. 6" MAX. (TYP.) 4.2.1 PIPE SYSTEMS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-19 FIGURE 4.2.1.E SCHEMATIC REPRESENTATION OF A DEBRIS BARRIER (IN ROAD RIGHT-OF-WAY) MAY BE REMOVED 3/4" DIAMETER BAR FRAME PIPE COUPLING 2" X 5" ANCHOR STRIPS WELDED TO 3/4" DIA. BAR-FRAME 4 PLACES SPACED UNIFORMLY. FASTEN W/ 1/2" GALV. OR NON-CORROSIVE BOLTS & NUTS. 3 1 3/4" DIA. SMOOTH BARS WITH ENDS WELDED TO BAR-FRAME NOTES: 1. CMP OR LINED CPE PIPE END-SECTION SHOWN; FOR CONCRETE PIPE BEVELED END SECTION, SEE KCRDCS DRAWING NO. 7-001. 2. ALL STEEL PARTS MUST BE GALVANIZED AND ASPHALT COATED (TREATMENT 1 OR BETTER). BEVELED PIPE END SECTION 6" O.C. MAX. BAR SPACING 3"-5" FOR 18" DIA. 5"-8" FOR 24" DIA. 7"-9" FOR 30" DIA. & GREATER 1' MIN. SECTION 4.2 PIPES, OUTFALLS, AND PUMPS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-20 4.2.1.2 METHODS OF ANALYSIS This section presents the methods of analysis for designing new or evaluating existing pipe systems for compliance with the conveyance capacity requirements set forth in Section 1.2.4, “Core Requirement #4: Conveyance System.”  DESIGN FLOWS Design flows for sizing or assessing the capacity of pipe systems shall be determined using the hydrologic analysis methods described in Chapter 3.  INLET GRATE CAPACITY The methods described in Chapter 5, Sections 4 and 5, of the Washington State Department of Transportation (WSDOT) Hydraulics Manual may be used in determining the capacity of inlet grates when capacity is of concern, with the following exceptions: 1. Use design flows as required in Section 1.2.4 of this manual. 2. Assume grate areas on slopes are 80% free of debris; “vaned” grates, 95% free. 3. Assume grate areas in sags or low spots are 50% free of debris; “vaned” grates, 75% free.  CONVEYANCE CAPACITY Two methods of hydraulic analysis using Manning's equation are used sequentially for the design and analysis of pipe systems. First, the Uniform Flow Analysis method is used for the preliminary design of new pipe systems. Second, the Backwater Analysis method is used to analyze both proposed and existing pipe systems to verify adequate capacity. See Core Requirement #4, Section 1.2.4, for sizing requirements of pipe systems. Note: Use of the Uniform Flow Analysis method to determine preliminary pipe sizes is only suggested as a first step in the design process and is not required. Results of the Backwater Analysis method determine final pipe sizes in all cases. Uniform Flow Analysis Method In addition to the design criteria described below, new pipe systems shall be design to meet the hydraulic criteria as described in Section 1.2.4.1. This method is used for preliminary sizing of new pipe systems to convey the design flow (i.e., the 10-year or 25-year peak flow rate as specified in Core Requirement #4, Section 1.2.4). Assumptions:  Flow is uniform in each pipe (i.e., depth and velocity remain constant throughout the pipe for a given flow).  Friction head loss in the pipe barrel alone controls capacity. Other head losses (e.g., entrance, exit, junction, etc.) and any backwater effects or inlet control conditions are not specifically addressed. Each pipe within the system is sized and sloped such that its barrel capacity at normal full flow (computed by Manning's equation) is equal to or greater than the design flow. The nomograph in Figure 4.2.1.F may be used for an approximate solution of Manning's equation. For more precise results, or for partial pipe full conditions, solve Manning's equation directly: 4.2.1 PIPE SYSTEMS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-21 V = R2/3 S1/2 (4-1) or use the continuity equation, Q = AV, such that: Q = A R2/3 S1/2 (4-2) where Q = discharge (cfs) V = velocity (fps) A = area (sf) n = Manning's roughness coefficient; see Table 4.2.1.D below R = hydraulic radius = area/wetted perimeter (ft) S = slope of the energy grade line (ft/ft) For pipes flowing partially full, the actual velocity may be estimated from the hydraulic properties shown in Figure 4.2.1.G by calculating Qfull and Vfull and using the ratio Qdesign/Qfull to find V and d (depth of flow). Table 4.2.1.D provides the recommended Manning's “n” values for preliminary design using the Uniform Flow Analysis method for pipe systems. Note: The “n” values for this method are 15% higher in order to account for entrance, exit, junction, and bend head losses. TABLE 4.2.1.D MANNING’S “n” VALUES FOR PIPES Type of Pipe Material Analysis Method Uniform Flow (preliminary design) Backwater Flow (capacity verification) A. Concrete pipe, lined CPE pipe and lined PP pipe B. Annular Corrugated Steel Pipe or Pipe Arch: 1. 2-2/3″ x 1/2″ corrugation (riveted): a. plain or fully coated b. paved invert (40% of circumference paved): 1) flow at full depth 2) flow at 80% full depth 3) flow at 60% full depth c. treatment 5 2. 3″ x 1″ corrugation 3. 6″ x 2″ corrugation (field bolted) C. Helical 2-2/3″ x 1/2″ corrugation and unlined CPE pipe D. Spiral rib metal pipe and PVC pipe E. Ductile iron pipe cement lined F. Solid wall HDPE pipe (butt fused only) 0.014 0.028 0.021 0.018 0.015 0.015 0.031 0.035 0.028 0.013 0.014 0.009 0.012 0.024 0.018 0.016 0.013 0.013 0.027 0.030 0.024 0.011 0.012 0.009 n 49.1 n 49.1 SECTION 4.2 PIPES, OUTFALLS, AND PUMPS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-22 Backwater Analysis Method This method is used to analyze the capacity of both new and existing pipe systems to convey the required design flow (i.e., either the 10-year or 25-year peak flow, whichever is specified in Core Requirement #4, Section 1.2.4). In either case, pipe system structures must be demonstrated to contain the headwater surface (hydraulic grade line) for the specified peak flow rate. Structures may overtop for the 100-year peak flow as allowed by Core Requirement #4. When this occurs, the additional flow over the ground surface is analyzed using the methods for open channels described in Section 4.4.1.2 and added to the flow capacity of the pipe system. This method is used to compute a simple backwater profile (hydraulic grade line) through a proposed or existing pipe system for the purposes of verifying adequate capacity. It incorporates a re-arranged form of Manning's equation expressed in terms of friction slope (slope of the energy grade line in ft/ft). The friction slope is used to determine the head loss in each pipe segment due to barrel friction, which can then be combined with other head losses to obtain water surface elevations at all structures along the pipe system. The backwater analysis begins at the downstream end of the pipe system and is computed back through each pipe segment and structure upstream. The friction, entrance, and exit head losses computed for each pipe segment are added to that segment's tailwater elevation (the water surface elevation at the pipe's outlet) to obtain its outlet control headwater elevation. This elevation is then compared with the inlet control headwater elevation, computed assuming the pipe's inlet alone is controlling capacity using the methods for inlet control presented in Section 4.3.1.2. The condition that creates the highest headwater elevation determines the pipe's capacity. The approach velocity head is then subtracted from the controlling headwater elevation, and the junction and bend head losses are added to compute the total headwater elevation, which is then used as the tailwater elevation for the upstream pipe segment. The Backwater Calculation Sheet in Figure 4.2.1.H may be used to compile the head losses and headwater elevations for each pipe segment. The numbered columns on this sheet are described in Figure 4.2.1.I. An example calculation is performed in Figure 4.2.1.J. Note: This method should not be used to compute stage/discharge curves for level pool routing purposes . Instead, a more sophisticated backwater analysis using the computer software provided with this manual is recommended as described below. Computer Applications The King County Backwater (KCBW) computer program includes a subroutine BWPIPE, which may be used to quickly compute a family of backwater profiles for a given range of flows through a proposed or existing pipe system. A schematic description of the nomenclature used in this program is provided in Figure 4.3.1.G. Program documentation providing instructions on the use of this and the other KCBW subroutines is available from King County Department of Natural Resources and Parks (DNRP). 4.2.1 PIPE SYSTEMS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-23 FIGURE 4.2.1.F NOMOGRAPH FOR SIZING CIRCULAR DRAINS FLOWING FULL SECTION 4.2 PIPES, OUTFALLS, AND PUMPS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-24 FIGURE 4.2.1.G CIRCULAR CHANNEL RATIOS 0 0.2 0.4 0.6 0.8 1 1.2 1.4 00.10.20.30.40.50.60.70.80.91 RATIO OF FLOW DEPTH TO DIAMETER (d/D)PROPORTIONAL AREA, DISCHARGE, VELOCITY, HYDRAULIC RADIUSPROPORTIONAL AREA PROPORTIONAL DISCHARGE PROPORTIONAL HYDRAULIC RADIUS 4.2.1 PIPE SYSTEMS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-25 FIGURE 4.2.1.H BACKWATER CALCULATION SHEET SECTION 4.2 PIPES, OUTFALLS, AND PUMPS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-26 FIGURE 4.2.1.I BACKWATER CALCULATION SHEET NOTES Column (1) - Design flow to be conveyed by pipe segment. Column (2) - Length of pipe segment. Column (3) - Pipe Size; indicate pipe diameter or span x rise. Column (4) - Manning's “n” value. Column (5) - Outlet Elevation of pipe segment. Column (6) - Inlet Elevation of pipe segment. Column (7) - Barrel Area; this is the full cross-sectional area of the pipe. Column (8) - Barrel Velocity; this is the full velocity in the pipe as determined by: V = Q/A or Col.(8) = Col.(1) / Col.(7) Column (9) - Barrel Velocity Head = V2/2g or (Col.(8))2/2g where g = 32.2 ft/sec2 (acceleration due to gravity) Column (10) - Tailwater (TW) Elevation; this is the water surface elevation at the outlet of the pipe segment. If the pipe's outlet is not submerged by the TW and the TW depth is less than (D+dc)/2, set TW equal to (D+dc)/2 to keep the analysis simple and still obtain reasonable results (D = pipe barrel height and dc = critical depth, both in feet. See Figure 4.3.1.F for determination of dc). Column (11) - Friction Loss = Sf x L [or Sf x Col.(2)] where Sf is the friction slope or head loss per linear foot of pipe as determined by Manning's equation expressed in the form: Sf = (nV)2/2.22 R1.33 Column (12) - Hydraulic Grade Line (HGL) Elevation just inside the entrance of the pipe barrel; this is determined by adding the friction loss to the TW elevation: Col.(12) = Col.(11) + Col.(10) If this elevation falls below the pipe's inlet crown, it no longer represents the true HGL when computed in this manner. The true HGL will fall somewhere between the pipe's crown and either normal flow depth or critical flow depth, whichever is greater. To keep the analysis simple and still obtain reasonable results (i.e., erring on the conservative side), set the HGL elevation equal to the crown elevation. Column (13) - Entrance Head Loss = Ke x V2/2g [or Ke x Col.(9)] where Ke = Entrance Loss Coefficient (from Table 4.3.1.B). This is the head lost due to flow contractions at the pipe entrance. Column (14) - Exit Head Loss = 1.0 x V2/2g or 1.0 x Col.(9) This is the velocity head lost or transferred downstream. Column (15) - Outlet Control Elevation = Col.(12) + Col.(13) + Col.(14) This is the maximum headwater elevation assuming the pipe's barrel and inlet/outlet characteristics are controlling capacity. It does not include structure losses or approach velocity considerations. Column (16) - Inlet Control Elevation (see Section 4.3.1.2, for computation of inlet control on culverts); this is the maximum headwater elevation assuming the pipe's inlet is controlling capacity. It does not include structure losses or approach velocity considerations. Column (17) - Approach Velocity Head; this is the amount of head/energy being supplied by the discharge from an upstream pipe or channel section, which serves to reduce the headwater elevation. If the discharge is from a pipe, the approach velocity head is equal to the barrel velocity head computed for the upstream pipe. If the upstream pipe outlet is significantly higher in elevation (as in a drop manhole) or lower in elevation such that its discharge energy would be dissipated, an approach velocity head of zero should be assumed. Column (18) - Bend Head Loss = Kb x V2/2g [or Kb x Col.(17)] where Kb = Bend Loss Coefficient (from Figure 4.2.1.K). This is the loss of head/energy required to change direction of flow in an access structure. Column (19) - Junction Head Loss. This is the loss in head/energy that results from the turbulence created when two or more streams are merged into one within the access structure. Figure 4.2.1.L may be used to determine this loss, or it may be computed using the following equations derived from Figure 4.2.1.L: Junction Head Loss = Kj x V2/2g [or Kj x Col.(17)] where Kj is the Junction Loss Coefficient determined by: Kj = (Q3/Q1)/(1.18 + 0.63(Q3/Q1)) Column (20) - Headwater (HW) Elevation; this is determined by combining the energy heads in Columns 17, 18, and 19 with the highest control elevation in either Column 15 or 16, as follows: Col.(20) = Col.(15 or 16) - Col.(17) + Col.(18) + Col.(19) 4.2.1 PIPE SYSTEMS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-27 FIGURE 4.2.1.J BACKWATER PIPE CALCULATION EXAMPLE SECTION 4.2 PIPES, OUTFALLS, AND PUMPS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-28 FIGURE 4.2.1.K BEND HEAD LOSSES IN STRUCTURES 4.2.1 PIPE SYSTEMS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-29 FIGURE 4.2.1.L JUNCTION HEAD LOSS IN STRUCTURES SECTION 4.2 PIPES, OUTFALLS, AND PUMPS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-30 4.2.2 OUTFALL SYSTEMS Properly designed outfalls are critical to ensuring no adverse impacts occur as the result of concentrated discharges from pipe systems and culverts, both onsite and downstream. Outfall systems include rock splash pads, flow dispersal trenches, gabion or other energy dissipaters, and tightline systems. A tightline system is typically a continuous length of pipe used to convey flows down a steep or sensitive slope with appropriate energy dissipation at the discharge end. In general, it is recommended that conveyance systems be designed to reduce velocity above outfalls to the extent feasible. 4.2.2.1 DESIGN CRITERIA General At a minimum, all outfalls shall be provided with a rock splash pad (see Figure 4.2.2.A ) except as specified below and in Table 4.2.2.A: 1. The flow dispersal trench shown in Figure 4.2.2.B shall only be used as an outfall as described in Core Requirement #1, Section 1.2.1. 2. For outfalls with a velocity at design flow greater than 10 fps, a gabion dissipater or engineered energy dissipater shall be required. Note the gabion outfall detail shown in Figure 4.2.2.D is illustrative only; a design engineered to specific site conditions is required. Gabions shall conform to WDSOT/APWA specifications. 3. Engineered energy dissipaters, including stilling basins, drop pools, hydraulic jump basins, baffled aprons, and bucket aprons, are required for outfalls with velocity at design flow greater than 20 fps. These should be designed using published or commonly known techniques found in such references as Hydraulic Design of Energy Dissipaters for Culverts and Channels , published by the Federal Highway Administration of the United States Department of Transportation; Open Channel Flow, by V.T. Chow; Hydraulic Design of Stilling Basins and Energy Dissipaters, EM 25, Bureau of Reclamation (1978); and other publications, such as those prepared by the Soil Conservation Service (now Natural Resource Conservation Service). Alternate mechanisms, such as bubble-up structures (which will eventually drain) and structures fitted with reinforced concrete posts, may require an approved adjustment and must be designed using sound hydraulic principles and considering constructability and ease of maintenance. 4. Tightline systems shall be used when required by the discharge requirements of Core Requirement #1 or the outfall requirements of Core Requirement #4. Tightline systems may also be used to prevent aggravation or creation of a downstream erosion problem. 5. Flood closure devices shall be provided on new outfalls passing through existing levees or other features that contain floodwaters. Such structures shall be designed to the Corps of Engineers Manual for Design and Construction of Levees (EM 1110-2-1913). 6. Backup (secondary gate) closure devices shall be required for new outfalls through flood containment levees unless this requirement is specifically waived by the City. 7. New outfalls through levees along the Green River between River Mile 6 and State Route 18 shall comply with the terms of the adopted Lower Green River Pump Operation Procedures Plan. Tightline Systems 1. Outfall tightlines may be installed in trenches with standard bedding on slopes up to 40%. In order to minimize disturbance to slopes greater than 40%, it is recommended that tightlines be placed at grade with proper pipe anchorage and support. 4.2.2 OUTFALL SYSTEMS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-31 2. Solid wall HDPE tightlines must be designed to address the material limitations, particularly thermal expansion and contraction and pressure design, as specified by the manufacturer. The coefficient of thermal expansion and contraction for solid wall HDPE is on the order of 0.001 inch per foot per Fahrenheit degree. Sliding sleeve connections shall be used to address this thermal expansion and contraction. These sleeve connections consist of a section of the appropriate length of the next larger size diameter of pipe into which the outfall pipe is fitted. These sleeve connections must be located as close to the discharge end of the outfall system as is practical. 3. Solid wall HDPE tightlines shall be designed and sized using the applicable design criteria and methods of analysis specified for pipe systems in Section 4.2.1. 4. Due to the ability of solid wall HDPE tightlines to transmit flows of very high energy, special consideration for energy dissipation must be made. A schematic representation of a “gabion mattress energy dissipater” has been provided as Figure 4.2.2.D . Flows of very high energy will require a specifically engineered energy dissipater structure, as described above in General Criterion #3. Caution, the in-stream sample gabion mattress energy dissipater may not be acceptable within the ordinary high water mark of fish-bearing waters or where gabions will be subject to abrasion from upstream channel sediments. A four-sided gabion basket located outside the ordinary high water mark should be considered for these applications. TABLE 4.2.2.A ROCK PROTECTION AT OUTFALLS Discharge Velocity at Design Flow (fps) REQUIRED PROTECTION Greater than Less than or equal to Minimum Dimensions(1) Type Thickness Width Length Height 0 5 Rock lining(2) 1 foot Diameter + 6 feet 8 feet or 4 x diameter, whichever is greater Crown + 1 foot 5 10 Riprap(3) 2 feet Diameter + 6 feet or 3 x diameter, whichever is greater 12 feet or 4 x diameter, whichever is greater Crown + 1 foot 10 20 Gabion outfall As required As required As required Crown + 1 foot 20 N/A Engineered energy dissipater required (1) These sizes assume that erosion is dominated by outfall energy. In many cases sizing will be governed by conditions in the receiving waters. (2) Rock lining shall be quarry spalls with gradation as follows: Passing 8-inch square sieve: 100% Passing 3-inch square sieve: 40 to 60% maximum Passing 3/4-inch square sieve: 0 to 10% maximum (3) Riprap shall be reasonably well graded with gradation as follows: Maximum stone size: 24 inches (nominal diameter) Median stone size: 16 inches Minimum stone size: 4 inches Note: Riprap sizing governed by side slopes on outlet channel is assumed to be approximately 3:1. SECTION 4.2 PIPES, OUTFALLS, AND PUMPS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-32 FIGURE 4.2.2.A SCHEMATIC REPRESENTATION OF PIPE/CULVERT DISCHARGE PROTECTION PLACE ROCK 1' ABOVE CROWN BOTH SIDES OF CHANNEL FOR "A" < 8' ONE SIDE OF CHANNEL FOR "A" > 8' DISCHARGE PIPE FILTER FABRIC LINER UNDER ROCK SECTION A-A NTS PLAN NTSTOE OF BANKTOE OF BANKTOP OF BANKTOP OF BANKFLOWCL REQUIRED DIMENSIONS 8' FOR ROCK LINING 12' FOR RIP RAP SEE TABLE 4.2.2.A. "A"+"B" CHANNEL "B" (4' MIN.) 2' MIN. 30° MIN. 1' MIN. 1' OR 2' ROCK THICKNESS SEE TABLE 4.4.1.A A A "A" 4.2.2 OUTFALL SYSTEMS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-33 FIGURE 4.2.2.B SCHEMATIC REPRESENTATION OF A FLOW DISPERSAL TRENCH 1. THIS TRENCH SHALL BE CONSTRUCTED TOPREVENT POINT DISCHARGE AND/OR EROSION. 2. TRENCHES MAY BE PLACED NO CLOSER THAN 50 FEET TO ONE ANOTHER (100 FEET ALONG FLOWLINE).3. TRENCH AND GRADE BOARD MUST BE LEVEL. ALIGN TO FOLLOW CONTOURS OF SITE. 4. VERTICALLY SLOTTED BOLT HOLES, 2" SLOT LENGTH, ALLOWED FOR GRADE BOARD LEVELADJUSTMENT. PROVIDE BACKING WASHERS AND FASTEN SECURELY. 5. SUPPORT POST SPACING AS REQUIRED BY SOIL CONDITIONS TO ENSURE GRADE BOARDREMAINS LEVEL. 6. 15% MAX UNLESS OTHERWISE EVALUATED AND APPROVED, SEE SECTION C.2.1.1 NOTES: MIN 6" PERFORATED PIPE LAID FLAT CLEAN (<5% FINES) 34" - 1 12" WASHED ROCK FILTER FABRIC TRENCH LINER GALVANIZED BOLTS, SEE NOTE 4 4" X 4" SUPPORT POST 2 0% MAX , S E E NO T E 6 15% M A X A NOTCHED GRADE BOARD 2" X 2" NOTCHES 18" O.C.* END CAP OR PLUG CLEAN OUT WYE FROM PIPE MIN 6" PERFORATED PIPE LAID FLAT/LEVEL TYPE I CB W/SOLID COVER (LOCKING) INFLUENT PIPE (MAX DESIGN FLOW <0.5 CFS PER TRENCH) CLEAN OUT WYE FROM PIPE FLOW TO SECOND DISPERSAL TRENCH IF NECESSARY FLOW TO OTHER BRANCHING CB'S AS NECESSARY TYPE I CB W/SOLID COVER 18" O.C.* 2" A 50' MAX. PLAN NTS SECTION A-A NTS 1'- 0 MIN. 1'- 0 MIN. PIPE O.D. 2" GRADE BOARD NOTCHES*1'- 0 MIN. PIPE O.D. 1'-0 MIN.12" MIN.36" MAX.6" MIN. 2"x12" GRADE BOARD *FOR WATER QUALITY FACILITIES, SEE SECTION 6.2.6.1, OPTION A FOR NOTCH DIMENSIONS AND SPACING SECTION 4.2 PIPES, OUTFALLS, AND PUMPS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-34 FIGURE 4.2.2.C SCHEMATIC REPRESENTATION OF AN ALTERNATIVE FLOW DISPERSAL TRENCH 1. THIS TRENCH SHALL BE CONSTRUCTED TO PREVENT POINT DISCHARGE AND /OR EROSION. 2. TRENCHES MAY BE PLACED NO CLOSER THAN 50 FEET TO ONE ANOTHER (100 FEET ALONG FLOWLINE). 3. TRENCH AND GRADE BOARD MUST BE LEVEL. ALIGN TO FOLLOW CONTOURS OF SITE. 4. VERTICALLY SLOTTED BOLT HOLES, 2" SLOT LENGTH, ALLOWED FOR GRADE BOARD LEVEL ADJUSTMENT. PROVIDE BACKING WASHERS AND FASTEN SECURELY. 5. PROVIDE SUPPORT POST SPACING AS REQUIRED BY SOIL CONDITIONS TO ENSURE GRADE BOARD REMAINS LEVEL. 6. 15% MAX UNLESS OTHERWISE EVALUATED AND APPROVED, SEE SECTION C.2.1.1 NOTES: 15% M A X 20% M A X SEE N O TE 6 GALVANIZED BOLTS, SEE NOTE 4 2" X 12" GRADE BOARD 2" X 2" NOTCHES 18" O.C.CLEAN (< 5% FINES) 3 4" - 112" WASHED ROCK FILTER FABRIC SECTION A-A NTS 18" O.C.* 4" X 4" SUPPORT POST 2" 1'-6" MIN.12" MIN.36" MAX.2" GRADE BOARD NOTCHES* *FOR WATER QUALITY FACILITIES, SEE SECTION 6.2.6.1, OPTION A FOR NOTCH DIMENSIONS AND SPACING NOTCH DETAIL NTS 4.2.2 OUTFALL SYSTEMS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-35 FIGURE 4.2.2.D SCHEMATIC REPRESENTATION OF A GABION MATTRESS ENERGY DISSIPATOR A SLEEVE OF NEXT LARGER SIZE DIAMETER PIPE FOR THERMAL EXPANSION AND CONTRACTION EXISTING GROUND LINE PLACE FILTER FABRIC BETWEEN GABIONS AT SOIL BEDDING GABION MATTRESSPIPE ANCHOR RIP RAP NOTE: IF PIPE DISCHARGES PERPENDICULAR TO STREAM, OR GABIONS ARE LOCATED WITHIN THE ORDINARY HIGH WATER MARK (OHWM) OR WILL BE SUBJECT TO ABRASION FROM UPSTREAM SEDIMENTS, A FOUR-SIDED GABION BASKET LOCATED OUTSIDE THE OHWM SHOULD BE CONSIDERED. GABIONS (TYPICAL) (SIZE AS REQUIRED) SOLID WALL HDPE PIPE GABION MATTRESS SECTION C-C NTS SECTION B-BSECTION A-A 2 x D A PLAN VIEW NTS B BA 45° PREFERRED C 1' B 3 x D D B C A SECTION 4.2 PIPES, OUTFALLS, AND PUMPS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-36 4.2.3 PUMP SYSTEMS As allowed in Core Requirement #4, Section 1.2.4.3, pump systems may be used for conveyance of flows internal to a site if located on private property and privately maintained. Pump systems discharging to the Green River between River Mile 6 and State Route 18 (within the Green River Flood Control Zone District) shall comply with the standards of the adopted Green River Pump Operation Procedures Plan. 4.2.3.1 DESIGN CRITERIA Proposed pump systems must meet the following minimum requirements: 1. The pump system must be privately owned and maintained. 2. The pump system shall be used to convey water from one location or elevation to another within the site. 3. The pump system must have a dual pump (alternating) equipped with an external alarm system. 4. The pump system shall not be used to circumvent any other City drainage requirements, and construction and operation of the pump system shall not violate any other City requirements. 5. The gravity-flow components of the drainage system to and from the pump system must be designed so that pump failure does not result in flooding of a building or emergency access, or overflow to a location other than the natural discharge point for the site. 6. The pump system shall have either installed emergency backup power or the ability for portable backup power generator in the event of a loss of primary power. If portable backup emergency power is provided, the appliance must include a description of how the backup power will be brought to the site during an emergency within an emergency response plan (discussed below). 7. The applicant must provide an emergency response plan that details how backup power will be activated during an emergency and include method for delivering to the site and energizing portable backup power. The emergency response plan must also describe response for pump failures including repair and replacement of damaged pumps/motors and generators. 4.2.3.2 METHODS OF ANALYSIS Pump systems must be sized in accordance with the conveyance capacity requirements for pipe systems set forth in Section 1.2.4, “Core Requirement #4: Conveyance System.” CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 4-37 4.3 CULVERTS AND BRIDGES This section presents the methods, criteria, and details for hydraulic analysis and design of culverts and bridges. The information presented is organized as follows: Section 4.3.1, “Culverts” “Design Criteria,” Section 4.3.1.1 “Methods of Analysis,” Section 4.3.1.2 Section 4.3.2, “Culverts Providing for Fish Passage/Migration” “Design Criteria,” Section 4.3.2.1 “Methods of Analysis,” Section 4.3.2.2 Section 4.3.3, “Bridges” “Design Criteria,” Section 4.3.3.1 “Methods of Analysis,” Section 4.3.3.2. 4.3.1 CULVERTS Culverts are relatively short segments of pipe of circular, elliptical, rectangular, or arch cross section. They are usually placed under road embankments or driveways to convey surface water flow safely under the embankment. They may be used to convey flow from constructed or natural channels including streams. The Critical Areas Code (RMC 4-3-050) contains definitions of streams (termed “aquatic areas”) and requirements for crossing of streams. In addition to those requirements and the design criteria described below, other agencies such as the Washington State Department of Fish and Wildlife (WDFW) may have additional requirements affecting the design of proposed culverts. 4.3.1.1 DESIGN CRITERIA General In addition to the design criteria described below, culverts shall be designed to meet the hydraulic criteria as described in Section 1.2.4.1. 1. All circular pipe culverts shall conform to any applicable design criteria specified for pipe systems in Section 4.2.1. 2. All other types of culverts shall conform to manufacturer's specifications. See the City of Renton Standard Details for types of culverts allowed in City right-of-way. Headwater 1. For culverts 18-inch diameter or less, the maximum allowable headwater elevation (measured from the inlet invert) shall not exceed 2 times the pipe diameter or arch-culvert-rise at design flow (i.e., the 10-year or 25-year peak flow rate as specified in Core Requirement #4, Section 1.2.4). 2. For culverts larger than 18-inch diameter, the maximum allowable design flow headwater elevation (measured from the inlet invert) shall not exceed 1.5 times the pipe diameter or arch-culvert-rise at design flow. 3. The maximum headwater elevation at design flow shall be below any road or parking lot subgrade. SECTION 4.3 CULVERTS AND BRIDGES 6/22/2022 2022 City of Renton Surface Water Design Manual 4-38 Inlets and Outlets 1. All inlets and outlets in or near roadway embankments must be flush with and conforming to the slope of the embankment. 2. For culverts 18-inch diameter and larger, the embankment around the culvert inlet shall be protected from erosion by rock lining or riprap as specified in Table 4.2.2.A, except the length shall extend at least 5 feet upstream of the culvert, and the height shall be at or above the design headwater elevation. 3. Inlet structures, such as concrete headwalls, may provide a more economical design by allowing the use of smaller entrance coefficients and, hence, smaller diameter culverts. When properly designed, they will also protect the embankment from erosion and eliminate the need for rock lining. 4. In order to maintain the stability of roadway embankments, concrete headwalls, wingwalls, or tapered inlets and outlets may be required if right-of-way or easement constraints prohibit the culvert from extending to the toe of the embankment slopes. All inlet structures or headwalls installed in or near roadway embankments must be flush with and conforming to the slope of the embankment. 5. Debris barriers (trash racks) are required on the inlets of all culverts that are over 60 feet in length and are 18 to 36 inches in diameter. Debris barriers for pipes smaller than 18 inches and larger than 36 inches in diameter may be required depending on conditions and safety concerns. Debris barriers shall have a bar spacing of 6 inches. This requirement also applies to the inlets of pipe systems. See Figure 4.2.1.D and Figure 4.2.1.E for schematic representations of debris barriers. 6. For culverts 18-inch diameter and larger, the receiving channel of the outlet shall be protected from erosion by rock lining specified in Table 4.2.2.A, except the height shall be one foot above maximum tailwater elevation or one foot above the crown, whichever is higher (See Figure 4.2.2.A ). 4.3.1.2 METHODS OF ANALYSIS This section presents the methods of analysis for designing new or evaluating existing culverts for compliance with the conveyance capacity requirements set forth in Section 1.2.4, “Core Requirement #4: Conveyance System.”  DESIGN FLOWS Design flows for sizing or assessing the capacity of culverts shall be determined using the hydrologic analysis methods described in Chapter 3.  CONVEYANCE CAPACITY The theoretical analysis of culvert capacity can be extremely complex because of the wide range of possible flow conditions that can occur due to various combinations of inlet and outlet submergence and flow regime within the culvert barrel. An exact analysis usually involves detailed backwater calculations, energy and momentum balance, and application of the results of hydraulic model studies. However, simple procedures have been developed where the various flow conditions are classified and analyzed on the basis of a control section. A control section is a location where there is a unique relationship between the flow rate and the upstream water surface elevation. Many different flow conditions exist over time, but at any given time the flow is either governed by the culvert's inlet geometry (inlet control) or by a combination of inlet geometry, barrel characteristics, and tailwater elevation (outlet control). Figure 4.3.1.A illustrates typical conditions of inlet and outlet control. The procedures presented in this section provide for the analysis of both inlet and outlet control conditions to determine which governs. 4.3.1 CULVERTS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-39 Inlet Control Analysis Nomographs such as those provided in Figure 4.3.1.B and Figure 4.3.1.C may be used to determine the inlet control headwater depth at design flow for various types of culverts and inlet configurations. These nomographs were originally developed by the Bureau of Public Roads—now the Federal Highway Administration (FHWA)—based on their studies of culvert hydraulics. These and other nomographs can be found in the FHWA publication Hydraulic Design of Highway Culverts, HDS No. #5 (Report No. FHWA-IP-85-15) (September 1985), or the WSDOT Hydraulic Manual. Also available in the FHWA publication, are the design equations used to develop the inlet control nomographs. These equations are presented below. For unsubmerged inlet conditions (defined by Q/AD0.5 ≤ 3.5); Form 1*: HW/D = Hc/D + K(Q/AD0.5)M - 0.5S** (4-3) Form 2*: HW/D = K(Q/AD0.5)M (4-4) For submerged inlet conditions (defined by Q/AD0.5 ≥ 4.0); HW/D = c(Q/AD0.5)2 + Y - 0.5S** (4-5) where HW = headwater depth above inlet invert (ft) D = interior height of culvert barrel (ft) Hc = specific head (ft) at critical depth (dc + Vc2/2g) Q = flow (cfs) A = full cross-sectional area of culvert barrel (sf) S = culvert barrel slope (ft/ft) K,M,c,Y = constants from Table 4.3.1.A. The specified head Hc is determined by the following equation: Hc = dc + Vc 2/2g (4-6) where dc = critical depth (ft); see Figure 4.3.1.F Vc = flow velocity at critical depth (fps) g = acceleration due to gravity (32.2 ft/sec2). * The appropriate equation form for various inlet types is specified in Table 4.3.1.A below. ** For mitered inlets, use +0.7S instead of -0.5S. Note: Between the unsubmerged and submerged conditions, there is a transition zone (3.5 < Q/AD0.5 < 4.0) for which there is only limited hydraulic study information. The transition zone is defined empirically by drawing a curve between and tangent to the curves defined by the unsubmerged and submerged equations. In most cases, the transition zone is short and the curve is easily constructed. SECTION 4.3 CULVERTS AND BRIDGES 6/22/2022 2022 City of Renton Surface Water Design Manual 4-40 TABLE 4.3.1.A CONSTANTS FOR INLET CONTROL EQUATIONS* Unsubmerged Submerged Shape and Material Inlet Edge Description Equation Form K M c Y Circular Concrete Square edge with headwall 1 0.0098 2.0 0.0398 0.67 Groove end with headwall 0.0078 2.0 0.0292 0.74 Groove end projecting 0.0045 2.0 0.0317 0.69 Circular Corrugated Steel Pipe Headwall 1 0.0078 2.0 0.0379 0.69 Mitered to slope 0.0210 1.33 0.0463 0.75 Projecting 0.0340 1.50 0.0553 0.54 Rectangular Box 30 to 75 wingwall flares 1 0.026 1.0 0.0385 0.81 90 and 15 wingwall flares 0.061 0.75 0.0400 0.80 0 wingwall flares 0.061 0.75 0.0423 0.82 CM Boxes 90 headwall 1 0.0083 2.0 0.0379 0.69 Thick wall projecting 0.0145 1.75 0.0419 0.64 Thin wall projecting 0.0340 1.5 0.0496 0.57 Arch Corrugated Steel Pipe 90 headwall 1 0.0083 2.0 0.0496 0.57 Mitered to slope 0.0300 1.0 0.0463 0.75 Projecting 0.0340 1.5 0.0496 0.53 Bottomless Arch 90 headwall 1 0.0083 2.0 0.0379 0.69 Corrugated Steel Pipe Mitered to slope 0.0300 2.0 0.0463 0.75 Thin wall projecting 0.0340 1.5 0.0496 0.57 Circular with Smooth tapered inlet throat 2 0.534 0.333 0.0196 0.89 Tapered Inlet Rough tapered inlet throat 0.519 0.64 0.0289 0.90 * Source: FHWA HDS No. 5 Outlet Control Analysis Nomographs such as those provided in Figure 4.3.1.D and Figure 4.3.1.E may be used to determine the outlet control headwater depth at design flow for various types of culverts and inlets. Outlet control nomographs other than those provided can be found in FHWA HDS No.5 or the WSDOT Hydraulic Manual. The outlet control headwater depth may also be determined using the simple Backwater Analysis method presented in Section 4.2.1.2 for analyzing pipe system capacity. This procedure is summarized as follows for culverts: HW = H + TW - LS (4-7) where H = Hf + He + Hex Hf = friction loss (ft) = (V2n2L)/(2.22R1.33) Note: If (Hf+TW-LS) < D, adjust Hf such that (Hf+TW-LS) = D. This will keep the analysis simple and still yield reasonable results (erring on the conservative side). He = entrance head loss (ft) = Ke(V2/2g) Hex = exit head loss (ft) = V2/2g 4.3.1 CULVERTS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-41 TW = tailwater depth above invert of culvert outlet (ft) Note: If TW < (D+dc)/2, set TW = (D+dc)/2. This will keep the analysis simple and still yield reasonable results. L = length of culvert (ft) S = slope of culvert barrel (ft/ft) D = interior height of culvert barrel (ft) V = barrel velocity (fps) n = Manning's roughness coefficient from Table 4.2.1.D R = hydraulic radius (ft) Ke = entrance loss coefficient (from Table 4.3.1.B) g = acceleration due to gravity (32.2 ft/sec2) dc = critical depth (ft); see Figure 4.3.1.F Note: The above procedure should not be used to develop stage/discharge curves for level pool routing purposes because its results are not precise for flow conditions where the hydraulic grade line falls significantly below the culvert crown (i.e., less than full flow conditions). SECTION 4.3 CULVERTS AND BRIDGES 6/22/2022 2022 City of Renton Surface Water Design Manual 4-42 TABLE 4.3.1.B ENTRANCE LOSS COEFFICIENTS Type of Structure and Design Entrance Coefficient, Ke Pipe, Concrete, PVC, Spiral Rib, DI, and Lined CPE Projecting from fill, socket (bell) end 0.2 Projecting from fill, square cut end 0.5 Headwall, or headwall and wingwalls Socket end of pipe (groove-end) 0.2 Square-edge 0.5 Rounded (radius = 1/12D) 0.2 Mitered to conform to fill slope 0.7 End section conforming to fill slope* 0.5 Beveled edges, 33.7 or 45 bevels 0.2 Side- or slope-tapered inlet 0.2 Pipe, or Pipe-Arch, Corrugated Metal and Other Non-Concrete or D.I. Projecting from fill (no headwall) 0.9 Headwall, or headwall and wingwalls (square-edge) 0.5 Mitered to conform to fill slope (paved or unpaved slope) 0.7 End section conforming to fill slope* 0.5 Beveled edges, 33.7 or 45 bevels 0.2 Side- or slope-tapered inlet 0.2 Box, Reinforced Concrete Headwall parallel to embankment (no wingwalls) Square-edged on 3 edges 0.5 Rounded on 3 edges to radius of 1/12 barrel dimension or beveled edges on 3 sides 0.2 Wingwalls at 30 to 75 to barrel Square-edged at crown 0.4 Crown edge rounded to radius of 1/12 barrel dimension or beveled top edge 0.2 Wingwall at 10 to 25 to barrel Square-edged at crown 0.5 Wingwalls parallel (extension of sides) Square-edged at crown 0.7 Side- or slope-tapered inlet 0.2 * Note: “End section conforming to fill slope” are the sections commonly available from manufacturers. From limited hydraulic tests they are equivalent in operation to a headwall in both inlet and outlet control. Some end sections incorporating a closed taper in their design have a superior hydraulic performance. Computer Applications The “King County Backwater” (KCBW) computer program available with this manual contains two subroutines (BWPIPE and BWCULV) that may be used to analyze culvert capacity and develop stage/discharge curves for level pool routing purposes. A schematic description of the nomenclature used in these subroutines is provided in Figure 4.3.1.G. The KCBW program documentation available from King County Department of Natural Resources and Parks (DNRP) includes more detailed descriptions of program features. 4.3.1 CULVERTS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-43 FIGURE 4.3.1.A INLET/OUTLET CONTROL CONDITIONS SECTION 4.3 CULVERTS AND BRIDGES 6/22/2022 2022 City of Renton Surface Water Design Manual 4-44 FIGURE 4.3.1.B HEADWATER DEPTH FOR SMOOTH INTERIOR PIPE CULVERTS WITH INLET CONTROL 4.3.1 CULVERTS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-45 FIGURE 4.3.1.C HEADWATER DEPTH FOR CORRUGATED PIPE CULVERTS WITH INLET CONTROL SECTION 4.3 CULVERTS AND BRIDGES 6/22/2022 2022 City of Renton Surface Water Design Manual 4-46 FIGURE 4.3.1.D HEAD FOR CULVERTS (PIPE W/“n”= 0.012) FLOWING FULL WITH OUTLET CONTROL 4.3.1 CULVERTS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-47 FIGURE 4.3.1.E HEAD FOR CULVERTS (PIPE W/“n”= 0.024) FLOWING FULL WITH OUTLET CONTROL SECTION 4.3 CULVERTS AND BRIDGES 6/22/2022 2022 City of Renton Surface Water Design Manual 4-48 FIGURE 4.3.1.F CRITICAL DEPTH OF FLOW FOR CIRCULAR CULVERTS 4.3.1 CULVERTS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-49 FIGURE 4.3.1.G COMPUTER SUBROUTINES BWPIPE AND BWCULV: VARIABLE DEFINITIONS FLOW DATA COEFFICIENTS / INLET DATA DC - Critical Depth (ft) KE - Entrance Coefficient under Outlet Control DN - Normal Depth (ft) KB - Bend Loss Coefficient TW - Tailwater Depth (ft) KJ - Junction Loss Coefficient DO - Outlet Depth (ft) K - Inlet Control Equation parameter (See Table 4.3.1.A) DE - Entrance Depth (ft) M - Inlet Control Equation parameter (See Table 4.3.1.A) HWO - Headwater (ft) assuming Outlet Control C - Inlet Control Equation parameter (See Table 4.3.1.A) HWI - Headwater (ft) assuming Inlet Control Y - Inlet Control Equation parameter (See Table 4.3.1.A) DXN - Distance (expressed as a fraction of the pipe length) from the outlet to where the flow profile intersects with normal depth. DXN will equal one under full-flow conditions and will equal zero when a hydraulic jump occurs at the outlet or when normal depth equals zero (normal depth will equal zero when the pipe grade is flat or reversed). Q-Ratio - Ratio of tributary flow to main upstream flow (Q3/Q1) VBH - Barrel Velocity Head (ft) based on the average velocity determined by V=Q/Afull VUH - Upstream Velocity Head (ft) based on an inputted velocity. EHU - Upstream Energy Head (ft) available after bend losses and junction losses have been subtracted from VUH. VCH - Critical Depth Velocity Head (ft) VNH - Normal Depth Velocity Head (ft) VEH - Entrance Depth Velocity Head (ft) VOH - Outlet Depth Velocity Head (ft) SECTION 4.3 CULVERTS AND BRIDGES 6/22/2022 2022 City of Renton Surface Water Design Manual 4-50 4.3.2 CULVERTS PROVIDING FOR FISH PASSAGE/MIGRATION In fish-bearing waters, water-crossing structures must usually provide for fish passage as required for Washington State Department of Fish and Wildlife (WDFW) Hydraulic Project Approval or as a condition of permitting under the critical areas code (RMC 4-3-050). Culverts designed for fish passage must also meet the requirements of Section 1.2.4, “Core Requirement #4: Conveyance System.” Fish passage can generally be ensured by providing structures that do not confine the streambed—that is, a structure wide enough so that the stream can maintain its natural channel within the culvert. Bridges, bottomless arch culverts, arch culverts, and rectangular box culverts (“utility vaults”) can often be used to accommodate stream channels. Where it is unfeasible to construct these types of structures, round pipe culverts may be used if high flow velocities are minimized and low flow depths are maximized. The Hydraulic Code Rules (Title 220 WAC) detail requirements for WDFW Hydraulic Project Approval. See the WDFW manual “Design of Road Culverts for Fish Passage” for detailed design methodologies. Materials Galvanized metals leach zinc into the environment, especially in standing water situations. High zinc concentrations, sometimes in the range that can be toxic to aquatic life, have been observed in the region. Therefore, use of galvanized materials in stormwater facilities is not allowed, and their use in conveyance systems is discouraged. Where other metals, such as stainless steel, or plastics are available, they should be used. 4.3.2.1 DESIGN CRITERIA Table 4.3.2.A lists allowable velocities, flow depths, and hydraulic drops for culverts in fish-bearing streams. Velocities are for the high flow design discharge; water depths are for the low flow design discharge. The hydraulic drop (a vertical drop in the water surface profile at any point within culvert influence) is for all flows between the high and low flow design discharges. TABLE 4.3.2.A FISH PASSAGE DESIGN CRITERIA Adult Trout Adult Pink, Chum Salmon Adult Chinook, Coho, Sockeye, Steelhead 1. Max Velocity (fps) Culvert Length: 10–60 ft 4.0 5.0 6.0 60–100 ft 4.0 4.0 5.0 100–200 ft 3.0 3.0 4.0 2. Min Flow Depth (ft) 0.8 0.8 1.0 3. Max Hydraulic Drop (ft) 0.8 0.8 1.0 Source: WDFW manual “Design of Road Culverts for Fish Passage” (2003), Chapter 5, p. 21, Table 5-1. 4.3.2 CULVERTS PROVIDING FOR FISH PASSAGE/MIGRATION 2022 City of Renton Surface Water Design Manual 6/22/2022 4-51 4.3.2.2 METHODS OF ANALYSIS High Flow Design Discharge For gaged streams, the high flow design discharge shall be estimated by the 10% exceedance flow for October through April inclusive, proportioned by tributary area to the culvert using the technique described in Section 4.4.2.4 under “Flood Flows from Stream Gage Data.” For ungaged streams, the high flow design discharge shall be estimated by one of the following:  The 10% exceedance flow for October through April inclusive for the nearest hydrologically similar gaged stream, proportioned by tributary area  The 5% exceedance flow determined through duration analysis with the approved model  The 10% exceedance flow for October through April inclusive determined with the HSPF model or the approved model using the full historical record. Low Flow Design Discharge For gaged streams, the low flow design discharge shall be estimated by the 95% exceedance flow for October through April inclusive, proportioned by tributary area. For ungaged streams, the low flow design discharge shall be estimated by one of the following:  The 95% exceedance flow for October through April inclusive for the nearest hydrologically similar gaged stream, proportioned by tributary area  The 95% exceedance flow for October through April inclusive, determined by the HSPF model or the approved model using the full historical record  The following equation, using input data from the approved model (Note: Equation 4-9 is not used): For the Sea-Tac rainfall region: Ql = fr (0.46Atf + 0.56Atp + 0.46Atg + 0.72Aof + 0.96Aop + 1.10Aog) / 1000 (4-7) where Ql = low flow design discharge (cfs) fr = regional rainfall scale factor from the WWHM2012 Site Information map screen Atf = area of till forest (acres) Atp = area of till pasture (acres) Atg = area of till grass (acres) Aof = area of outwash forest (acres) Aop = area of outwash pasture (acres) Aog = area of outwash grass (acres) Note: Minimum depths may also be met by providing an “installed no-flow depth,” per Title 220 WAC, where the static water surface level meets minimum flow depth criteria. SECTION 4.3 CULVERTS AND BRIDGES 6/22/2022 2022 City of Renton Surface Water Design Manual 4-52 4.3.3 BRIDGES Bridges over waterways are considered conveyance structures and are generally constructed to allow the continuation of a thoroughfare (such as a road). They generally consist of foundation abutments and/or piers that support a deck spanning the waterway. In addition to the design criteria for conveyance described below, bridge designs must meet the City of Renton Transportation requirements, AASHTO Standard Specifications for Highway Bridges or AASHTO LRFD Bridge Design Specifications and the most current WSDOT/APWA Standard Specifications as well as the requirements of other agencies such as the Washington State Department of Fish and Wildlife (WDFW). 4.3.3.1 DESIGN CRITERIA Bridges shall be designed to convey flows and pass sediments and debris for runoff events up to and including the 100-year event in a manner that does not increase the potential for flooding or erosion to properties and structures near or adjacent to the bridge, or cause bridge failure. Inadequate conveyance capacity may cause flooding to increase by restricting flow through the hydraulic openings, by placing approach fill or abutments in floodplains, by causing changes in channel gradient and alignment or by trapping debris. A common mode of bridge failure involving debris is the resultant scour and undermining of piers or abutments where debris accumulates. Openings between the structural elements of the bridge and the bottom of the channel or floodplain ground surface must be large enough to allow for passage of water, sediment, and debris. The horizontal openings are defined by the bridge span, the horizontal distances between piers or abutments. Bridge clearance is the vertical distance between the 100-year water surface and the low chord of the bridge. For stream crossing locations where the 100-year peak flow exceeds 100 cfs, the height of a bridge clearance above rivers and streams shall be a minimum three feet above the 100-year water surface elevation unless otherwise required by the City based on evaluation of the design criteria in this section. For stream crossing locations where the 100-year peak flow is 100 cfs or less, there is no specific clearance requirement. Hydraulic Capacity Bridge and approach roads must pass the 100-year flow without creating hydraulic restrictions that cause or increase flooding. Design of bridge and approach roads shall demonstrate compliance with the compensatory storage provisions of RMC 4-3-050. Of necessity, bridge and approach roads are sometimes constructed within 100-year floodplains. In some cases, approach roads will be inundated and the bridge will not be accessible during extreme events. In other cases, both the bridge and approach roads will be inundated by the 100-year flood. In these cases, the bridge shall be designed to withstand the expected condition while inundated. The design shall employ means to facilitate flow over the bridge and to minimize the potential for erosion of the roadway fill in the approach roads. Bed Aggradation Where bed aggradation is probable, the analysis of hydraulic capacity shall assume the bed raised by an amount expected during a suitable design life (40 years minimum) of the bridge. Aggradation estimates shall be based on a sediment transport analysis that, where possible, is calibrated to direct cross-section comparisons over time. This analysis shall extend upstream and downstream a sufficient distance to adequately characterize bed aggradation that may affect the hydraulic capacity at the bridge location. Bed aggradation is frequently associated with channel migration. The location and design of bridges and approach roads shall consider channel migration hazards. 4.3.3 BRIDGES 2022 City of Renton Surface Water Design Manual 6/22/2022 4-53 Debris Passage Since debris can pass through an opening either partly or totally submerged, the total vertical clearance from the bottom of the structure to the streambed needs to be considered. Required clearance for debris shall include an assessment of the maximum material size available, the ability of the stream to transport it, and the proximity of debris sources. The following factors also must be considered: history of debris problems in the river reaches upstream and downstream of the proposed bridge location, history of debris accumulations on an existing bridge structure or nearby structures upstream and downstream from the proposed bridge location, mapped channel migration hazard and channel migration history of the reach of stream, and skew of the bridge alignment such that piers in floodplain may be in the path of the debris. For a detailed qualitative analysis of debris accumulation on bridges, see the U.S. Department of Transportation, Federal Highway Administration Publication FHWA-RD-97-028, Potential Drift Accumulation at Bridges, by Timothy H. Diehl (1997). Safety Margin When designing bridges to convey flows and pass sediments and debris, a safety margin shall be considered by the design engineer to account for uncertainties in flow rates, debris hazards, water surface elevations, aggradation, and channel migration over time. The safety margin should be increased when the surrounding community is especially susceptible to flood damages that could be exacerbated by a debris jam at the bridge. Section 5 of the Technical Information Report submitted with the project’s engineering plans shall include a discussion of the need for a safety margin and the rationale for its selection. Bridges and Levees Where bridge structures and approach roads intersect flood containment levees, the bridge structure and approach roads shall be designed and constructed to preserve existing levels of flood containment provided by the existing levee. Where the existing levee currently provides containment of the 100-year flood, the bridge structure and approach roads shall be designed and constructed to meet FEMA levee and structural performance standards, including sufficient freeboard on the levee in the bridge vicinity, as provided for in 44 CFR (also see Section 1.3.3, Special Requirement #3, Flood Protection Facilities). Bridge Piers and Abutments Bridge pier and abutment locations are governed by provisions of the City’s critical areas code, RMC 4-3-050. 4.3.3.2 METHODS OF ANALYSIS The following methods are acceptable for hydraulic analysis of bridges and approach roads: 1. The Direct Step backwater method described in Section 4.4.1.2 shall be used to analyze the hydraulic impacts of bridge piers, abutments, and approach roads to the water surface profile. 2. The Army Corps of Engineers Hydraulic Engineering Center publishes technical papers on methods used to address the hydraulic effects of bridge piers, abutments, and approach roads. The book Open Channel Hydraulics by V.T. Chow also contains techniques for analyzing hydraulic effects. SECTION 4.3 CULVERTS AND BRIDGES 6/22/2022 2022 City of Renton Surface Water Design Manual 4-54 (T his page intentionally left blan k. ) CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 4-55 4.4 OPEN CHANNELS, FLOODPLAINS, AND FLOODWAYS This section presents the methods, criteria, and details for hydraulic analysis and design of open channels, and the determination and analysis of floodplains and floodways. The information presented is organized as follows: Section 4.4.1, “Open Channels” “Design Criteria,” Section 4.4.1.1 “Methods of Analysis,” Section 4.4.1.2 Section 4.4.2, “Floodplain/Floodway Analysis” “No Floodplain Study Required,” Section 4.4.2.1 “Approximate Floodplain Study,” Section 4.4.2.2 “Minor Floodplain Study,” Section 4.4.2.3 “Major Floodplain/Floodway Study,” Section 4.4.2.4. 4.4.1 OPEN CHANNELS Open channels may be classified as either natural or constructed. Natural channels are generally referred to as rivers, streams, creeks, or swales, while constructed channels are most often called ditches, or simply channels. The Critical Areas, Shorelines, and Clearing and Grading Codes as well as Chapter 1 of this manual should be reviewed for requirements related to streams. Natural Channels Natural channels are defined as those that have occurred naturally due to the flow of surface waters, or those that, although originally constructed by human activity, have taken on the appearance of a natural channel including a stable route and biological community. They may vary hydraulically along each channel reach and should be left in their natural condition, wherever feasible or required, in order to maintain natural hydrologic functions and wildlife habitat benefits from established vegetation. Constructed Channels Constructed channels are those constructed or maintained by human activity and include bank stabilization of natural channels. Constructed channels shall be either vegetation-lined, rock-lined, or lined with appropriately bioengineered vegetation5.  Vegetation-lined channels are the most desirable of the constructed channels when properly designed and constructed. The vegetation stabilizes the slopes of the channel, controls erosion of the channel surface, and removes pollutants. The channel storage, low velocities, water quality benefits, and greenbelt multiple-use benefits create significant advantages over other constructed channels. The presence of vegetation in channels creates turbulence that results in loss of energy and increased flow retardation; therefore, the design engineer must consider sediment deposition and scour, as well as flow capacity, when designing the channel.  Rock-lined channels are necessary where a vegetative lining will not provide adequate protection from erosive velocities. They may be constructed with riprap, gabions, or slope mattress linings. The rock lining increases the turbulence, resulting in a loss of energy and increased flow retardation. Rock lining also permits a higher design velocity and therefore a steeper design slope than in grass-lined 5 Bioengineered vegetation lining as referenced here applies to channel stabilization methods. See Appendix C, Simplified Drainage Requirements for bioswale design criteria. Note, for bioswales and other infiltrative BMPs that may be placed in-line with conveyance, any infiltration option in the modeling shall be turned off when evaluating conveyance capacity. SECTION 4.4 OPEN CHANNELS, FLOODPLAINS, AND FLOODWAYS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-56 channels. Rock linings are also used for erosion control at culvert and storm drain outlets, sharp channel bends, channel confluences, and locally steepened channel sections.  Bioengineered vegetation lining is a desirable alternative to the conventional methods of rock armoring. Soil bioengineering is a highly specialized science that uses living plants and plant parts to stabilize eroded or damaged land. Properly bioengineered systems are capable of providing a measure of immediate soil protection and mechanical reinforcement. As the plants grow they produce a vegetative protective cover and a root reinforcing matrix in the soil mantle. This root reinforcement serves several purposes: a) The developed anchor roots provide both shear and tensile strength to the soil, thereby providing protection from the frictional shear and tensile velocity components to the soil mantle during the time when flows are receding and pore pressure is high in the saturated bank. b) The root mat provides a living filter in the soil mantle that allows for the natural release of water after the high flows have receded. c) The combined root system exhibits active friction transfer along the length of the living roots. This consolidates soil particles in the bank and serves to protect the soil structure from collapsing and the stabilization measures from failing. The vegetative cover of bioengineered systems provides immediate protection during high flows by laying flat against the bank and covering the soil like a blanket. It also reduces pore pressure in saturated banks through transpiration by acting as a natural “pump” to “pull” the water out of the banks after flows have receded. The King County publication Guidelines for Bank Stabilization Projects primarily focuses on projects on larger rivers and streams, but the concepts it contains may be used in conjunction with other natural resource information for stabilization projects on smaller systems. The WDFW Integrated Streambank Protection Guidelines is another useful reference. 4.4.1.1 DESIGN CRITERIA General 1. Open channels shall be designed to provide required conveyance capacity and bank stability while allowing for aesthetics, habitat preservation, and enhancement. Open channels shall be consistent with the WDFW Integrated Streambank Protection Guidelines. 2. An access easement for maintenance is required along all constructed channels located on private property. Required easement widths and building setback lines vary with channel top width as shown in Table 4.1. 3. Channel cross-section geometry shall be trapezoidal, triangular, parabolic, or segmental as shown in Figure 4.4.1.C through Figure 4.4.1.E. Side slopes shall be no steeper than 3:1 for vegetation-lined channels and 2:1 for rock-lined channels. Note: Roadside ditches shall comply with the City of Renton Standard Details. 4. To reduce the likelihood that pollutants will be discharged to groundwater when untreated runoff is conveyed in ditches or channels constructed in soils with high infiltration rates, a low permeability liner or a treatment liner shall be provided for any reach of new ditch or channel proposed by a project in which the untreated runoff from 5,000 square feet or more of pollution-generating impervious surface comes into direct contact with an outwash soil, except where it can be demonstrated that the soil meets the soil suitability criteria listed in Section 5.2.1. The low permeability liner or treatment liner shall be consistent with the specifications for such liners in Section 6.2.4. 5. Vegetation-lined channels shall have bottom slope gradients of 6% or less and a maximum velocity at design flow of 5 fps (see Table 4.4.1.A). 4.4.1 OPEN CHANNELS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-57 6. Rock-lined channels or bank stabilization of natural channels shall be used when design flow velocities exceed 5 feet per second. Rock stabilization shall be in accordance with Table 4.4.1.A or stabilized with bioengineering methods as described above in “Constructed Channels.” TABLE 4.4.1.A CHANNEL PROTECTION Velocity at Design Flow (fps) Required Protection Greater than Less than or Equal to Type of Protection Thickness Minimum Height Above Design Water Surface 0 5 Grass lining or Bioengineered lining N/A N/A 5 8 Rock lining(1) or Bioengineered lining 1 foot 1 foot 8 12 Riprap(2) 2 feet 2 feet 12 20 Slope mattress gabion, etc. Varies 2 feet (1) Rock Lining shall be reasonably well graded as follows: Maximum stone size: 12 inches Median stone size: 8 inches Minimum stone size: 2 inches (2) Riprap shall be reasonably well graded as follows: Maximum stone size: 24 inches Median stone size: 16 inches Minimum stone size: 4 inches Note: Riprap sizing is governed by side slopes on channel, assumed to be approximately 3:1. SECTION 4.4 OPEN CHANNELS, FLOODPLAINS, AND FLOODWAYS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-58 Riprap Design6 When riprap is set, stones are placed on the channel sides and bottom to protect the underlying material from being eroded. Proper riprap design requires the determination of the median size of stone, the thickness of the riprap layer, the gradation of stone sizes, and the selection of angular stones that will interlock when placed. Research by the U.S. Army Corps of Engineers has provided criteria for selecting the median stone weight, W50 (Figure 4.4.1.A). If the riprap is to be used in a highly turbulent zone (such as at a culvert outfall, downstream of a stilling basin, at sharp changes in channel geometry, etc.), the median stone W50 should be increased from 200% to 600% depending on the severity of the locally high turbulence. The thickness of the riprap layer should generally be twice the median stone diameter (D50) or at least that of the maximum stone. The riprap should have a reasonably well graded assortment of stone sizes within the following gradation: 1.25 ≤ Dmax/D50 ≤ 1.50 D15/D50 = 0.50 Dmin/D50 = 0.25 Detailed design methodology may be found in the Corps publication EM 1110-02-1601, Engineering and Design – Hydraulic Design of Flood Control Channels. For a more detailed analysis and design procedure for riprap requiring water surface profiles and estimates of tractive force, refer to the paper by Maynord et al. in Journal of Hydraulic Engineering (A.S.C.E.), July 1989. Riprap Filter Design Riprap should be underlain by a sand and gravel filter (or filter fabric) to keep the fine materials in the underlying channel bed from being washed through the voids in the riprap. Likewise, the filter material must be selected so that it is not washed through the voids in the riprap. Adequate filters can usually be provided by a reasonably well graded sand and gravel material where: D15 < 5d85 The variable d85 refers to the sieve opening through which 85% of the material being protected will pass, and D15 has the same interpretation for the filter material. A filter material with a D50 of 0.5 mm will protect any finer material including clay. Where very large riprap is used, it is sometimes necessary to use two filter layers between the material being protected and the riprap. Example: What embedded riprap design should be used to protect a streambank at a level culvert outfall where the outfall velocities in the vicinity of the downstream toe are expected to be about 8 fps? From Figure 4.4.1.A, W50 = 6.5 lbs, but since the downstream area below the outfall will be subjected to severe turbulence, increase W50 by 400% so that: W50 = 26 lbs, D50 = 8.0 inches The gradation of the riprap is shown in Figure 4.4.1.B, and the minimum thickness would be 1 foot (from Table 4.4.1.A); however, 16 inches to 24 inches of riprap thickness would provide some additional insurance that the riprap will function properly in this highly turbulent area. Figure 4.4.1.B shows that the gradation curve for ASTM C33, size number 57 coarse aggregate (used in concrete mixes), would meet the filter criteria. Applying the filter criteria to the coarse aggregate demonstrates that any underlying material whose gradation was coarser than that of a concrete sand would be protected. 6 From a paper prepared by M. Schaefer, Dam Safety Section, Washington State Department of Ecology. 4.4.1 OPEN CHANNELS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-59 FIGURE 4.4.1.A MEAN CHANNEL VELOCITY VS. MEDIUM STONE WEIGHT (W50) AND EQUIVALENT STONE DIAMETER SECTION 4.4 OPEN CHANNELS, FLOODPLAINS, AND FLOODWAYS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-60 FIGURE 4.4.1.B RIPRAP/FILTER EXAMPLE GRADATION CURVE 0.1 1 10 0 10 20 30 40 50 60 70 80 90 100 % Finer by WeightGrain Size (inches)20 RIP-RAP Coarse Aggregate size number 57 ASTM 14 C-33 4.4.1 OPEN CHANNELS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-61 4.4.1.2 METHODS OF ANALYSIS This section presents the methods of analysis for designing new or evaluating existing open channels for compliance with the conveyance capacity requirements set forth in Section 1.2.4, “Core Requirement #4: Conveyance System.”  DESIGN FLOWS Design flows for sizing and assessing the capacity of open channels shall be determined using the hydrologic analysis methods described in Chapter 3.  CONVEYANCE CAPACITY There are three acceptable methods of analysis for sizing and analyzing the capacity of open channels: 1. Manning's equation for preliminary sizing 2. Direct Step backwater method 3. Standard Step backwater method. Manning's Equation for Preliminary Sizing Manning's equation is used for preliminary sizing of open channel reaches of uniform cross section and slope (i.e., prismatic channels) and uniform roughness. This method assumes the flow depth (or normal depth) and flow velocity remain constant throughout the channel reach for a given flow. The charts in Figure 4.4.1.C and Figure 4.4.1.D may be used to obtain graphic solutions of Manning's equation for common ditch sections. For conditions outside the range of these charts or for more precise results, Manning’s equation can be solved directly from its classic forms shown in Equations (4-1) and (4- 2). Table 4.4.1.B provides a reference for selecting the appropriate “n” values for open channels. A number of engineering reference books, such as Open-Channel Hydraulics by V.T. Chow, may also be used as guides to select “n” values. Figure 4.4.1.E contains the geometric elements of common channel sections useful in determining area A, wetted perimeter WP, and hydraulic radius (R= A/WP). If flow restrictions occur that raise the water level above normal depth within a given channel reach, a backwater condition (or subcritical flow) is said to exist. This condition can result from flow restrictions created by a downstream culvert, bridge, dam, pond, lake, etc., and even a downstream channel reach having a higher flow depth. If backwater conditions are found to exist for the design flow, a backwater profile must be computed to verify that the channel's capacity is still adequate as designed. The Direct Step or Standard Step backwater methods presented in this section may be used for this purpose. SECTION 4.4 OPEN CHANNELS, FLOODPLAINS, AND FLOODWAYS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-62 TABLE 4.4.1.B VALUES OF ROUGHNESS COEFFICIENT “n” FOR OPEN CHANNELS Type of Channel and Description Manning's “n”* (normal) Type of Channel and Description Manning's “n”* (normal) A. Constructed Channels a. Earth, straight and uniform 1. Clean, recently completed 2. Gravel, uniform section, clean 3. With short grass, few weeds b. Earth, winding and sluggish 1. No vegetation 2. Grass, some weeds 3. Dense weeds or aquatic plants in deep channels 4. Earth bottom and rubble sides 5. Stony bottom and weedy banks 6. Cobble bottom and clean sides c. Rock lined 1. Smooth and uniform 2. Jagged and irregular d. Channels not maintained, weeds and brush uncut 1. Dense weeds, high as flow depth 2. Clean bottom, brush on sides 3. Same as #2, highest stage of flow 4. Dense brush, high stage B. Natural Streams B-1 Minor streams (top width at flood stage < 100 ft.) a. Streams on plain 1. Clean, straight, full stage no rifts or deep pools 2. Same as #1, but more stones and weeds 3. Clean, winding, some pools and shoals 4. Same as #3, but some weeds 5. Same as #4, but more stones 0.018 0.025 0.027 0.025 0.030 0.035 0.030 0.035 0.040 0.035 0.040 0.080 0.050 0.070 0.100 0.030 0.035 0.040 0.040 0.050 6. Sluggish reaches, weedy deep pools 7. Very weedy reaches, deep pools, or floodways with heavy stand of timber and underbrush b. Mountain streams, no vegetation in channel, banks usually steep, trees and brush along banks submerged at high stages 1. Bottom: gravel, cobbles, and few boulders 2. Bottom: cobbles with large boulders B-2 Floodplains a. Pasture, no brush 1. Short grass 2. High grass b. Cultivated areas 1. No crop 2. Mature row crops 3. Mature field crops c. Brush 1. Scattered brush, heavy weeds 2. Light brush and trees 3. Medium to dense brush 4. Heavy, dense brush d. Trees 1. Dense willows, straight 2. Cleared land with tree stumps, no sprouts 3. Same as #2, but with heavy growth of sprouts 4. Heavy stand of timber, a few down trees, little undergrowth, flood stage below branches 5. Same as #4, but with flood stage reaching branches 0.070 0.100 0.040 0.050 0.030 0.035 0.030 0.035 0.040 0.050 0.060 0.070 0.100 0.150 0.040 0.060 0.100 0.120 * Note: These “n” values are “normal” values for use in analysis of channels. For conservative design of channel capacity, the maximum values listed in other references should be considered. For channel bank stability, the minimum values should be considered. 4.4.1 OPEN CHANNELS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-63 Direct Step Backwater Method The Direct Step backwater method may be used to compute backwater profiles on prismatic channel reaches (i.e., reaches having uniform cross section and slope) where a backwater condition or restriction to normal flow is known to exist. The method may be applied to a series of prismatic channel reaches in secession beginning at the downstream end of the channel and computing the profile upstream. Calculating the coordinates of the water surface profile using this method is an iterative process achieved by choosing a range of flow depths, beginning at the downstream end, and proceeding incrementally up to the point of interest or to the point of normal flow depth. This is best accomplished by the use of a table (see Figure 4.4.1.G) or computer programs (as discussed in “Computer Applications” in Section 4.4.1.2). To illustrate analysis of a single reach, consider the following diagram: Equating the total head at cross sections 1 and 2, the following equation may be written: Sox + y1 + = y2 + + Sf x (4-8) where, x = distance between cross sections (ft) y1, y2 = depth of flow (ft) at cross sections 1 and 2 V1, V2 = velocity (fps) at cross sections 1 and 2  = energy coefficient at cross sections 1 and 2 So = bottom slope (ft/ft) Sf = friction slope = (n2V2)/(2.21R1.33) g = acceleration due to gravity, (32.2 ft/sec2) If the specific energy E at any one cross-section is defined as follows: E = y + (4-9) and assuming  = 1 = 2 where is the energy coefficient that corrects for the non-uniform distribution of velocity over the channel cross section, Equations 4-10 and 4-11 can be combined and rearranged to solve for x as follows: g V 2 2 1 1g V 2 2 2 2 g V 2 2  SECTION 4.4 OPEN CHANNELS, FLOODPLAINS, AND FLOODWAYS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-64 Dx = (E2 - E1)/(So - Sf) = E/( So - Sf) (4-10) Typical values of the energy coefficient  are as follows: Channels, regular section 1.15 Natural streams 1.3 Shallow vegetated flood fringes (includes channel) 1.75 For a given flow, channel slope, Manning's “n,” and energy coefficient , together with a beginning water surface elevation y2, the values of x may be calculated for arbitrarily chosen values of y1. The coordinates defining the water surface profile are obtained from the cumulative sum of x and corresponding values of y. The normal flow depth, yn, should first be calculated from Manning's equation to establish the upper limit of the backwater effect. Standard Step Backwater Method The Standard Step Backwater Method is a variation of the Direct Step Backwater Method and may be used to compute backwater profiles on both prismatic and non-prismatic channels. In this method, stations are established along the channel where cross section data is known or has been determined through field survey. The computation is carried out in steps from station to station rather than throughout a given channel reach as is done in the Direct Step method. As a result, the analysis involves significantly more trial-and-error calculation in order to determine the flow depth at each station. Computer Applications Because of the iterative calculations involved, use of a computer to perform the analysis is recommended. The King County Backwater (KCBW) computer program included in the software package available with this manual includes a subroutine, BWCHAN, based on the Standard Step backwater method, which may be used for all channel capacity analysis. It can also be combined with the BWPIPE and BWCULV subroutines to analyze an entire drainage conveyance system. A schematic description of the nomenclature used in the BWCHAN subroutine is provided in Figure 4.4.1.H. See the KCBW program documentation for further information. There are a number of commercial software programs for use on personal computers that use variations of the Standard Step backwater method for determining water surface profiles. The most common and widely accepted program is called HEC-RAS, published and supported by the United States Army Corps of Engineers Hydraulic Engineering Center. It is one of the models accepted by FEMA for use in performing flood hazard studies for preparing flood insurance maps. 4.4.1 OPEN CHANNELS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-65 FIGURE 4.4.1.C DITCHES — COMMON SECTIONS PROPERTIES OF DITCHES DIMENSIONS HYDRAULICS NO. Side Slopes B H W A WP R R(2/3) D-1 – – 6.5″ 5′-0″ 1.84 5.16 0.356 0.502 D-1C – – 6″ 25′-0″ 6.25 25.50 0.245 0.392 D-2A 1.5:1 2′-0″ 1′-0″ 5′-0″ 3.50 5.61 0.624 0.731 B 2:1 2′-0″ 1′-0″ 6′-0″ 4.00 6.47 0.618 0.726 C 3:1 2′-0″ 1′-0″ 8′-0″ 5.00 8.32 0.601 0.712 D-3A 1.5:1 3′-0″ 1′-6″ 7′-6″ 7.88 8.41 0.937 0.957 B 2:1 3′-0″ 1′-6″ 9′-0″ 9.00 9.71 0.927 0.951 C 3:1 3′-0″ 1′-6″ 12′-0″ 11.25 12.49 0.901 0.933 D-4A 1.5:1 3′-0″ 2′-0″ 9′-0″ 12.00 10.21 1.175 1.114 B 2:1 3′-0″ 2′-0″ 11′-0″ 14.00 11.94 1.172 1.112 C 3:1 3′-0″ 2′-0″ 15′-0″ 18.00 15.65 1.150 1.098 D-5A 1.5:1 4′-0″ 3′-0″ 13′-0″ 25.50 13.82 1.846 1.505 B 2:1 4′-0″ 3′-0″ 16′-0″ 30.00 16.42 1.827 1.495 C 3:1 4′-0″ 3′-0″ 22′-0″ 39.00 21.97 1.775 1.466 D-6A 2:1 – 1′-0″ 4′-0″ 2.00 4.47 0.447 0.585 B 3:1 – 1′-0″ 6′-0″ 3.00 6.32 0.474 0.608 D-7A 2:1 – 2′-0″ 8′-0″ 8.00 8.94 0.894 0.928 B 3:1 – 2′-0″ 12′-0″ 12.00 12.65 0.949 0.965 D-8A 2:1 – 3′-0″ 12′-0″ 18.00 13.42 1.342 1.216 B 3:1 – 3′-0″ 18′-0″ 27.00 18.97 1.423 1.265 D-9 7:1 – 1′-0″ 14′-0″ 7.00 14.14 0.495 0.626 D-10 7:1 – 2′-0″ 28′-0″ 28.00 28.28 0.990 0.993 D-11 7:1 – 3′-0″ 42′-0″ 63.00 42.43 1.485 1.302 SECTION 4.4 OPEN CHANNELS, FLOODPLAINS, AND FLOODWAYS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-66 FIGURE 4.4.1.D DRAINAGE DITCHES — COMMON SECTIONS 4.4.1 OPEN CHANNELS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-67 FIGURE 4.4.1.E GEOMETRIC ELEMENTS OF COMMON SECTIONS SECTION 4.4 OPEN CHANNELS, FLOODPLAINS, AND FLOODWAYS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-68 FIGURE 4.4.1.F OPEN CHANNEL FLOW PROFILE COMPUTATION Q = ____________ n = ____________ So = ____________  = ____________ Yn = ____________ y A R R4/3 V V2/2g E E Sf _ Sf _ So - Sf x x (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) 4.4.1 OPEN CHANNELS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-69 FIGURE 4.4.1.G DIRECT STEP BACKWATER METHOD – EXAMPLE y A R R4/3 V V2/2g E E Sf _ Sf _ So - Sf x x (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) 6.0 72.0 2.68 3.72 0.42 0.0031 6.0031 - 0.00002 - - - - 5.5 60.5 2.46 3.31 0.50 0.0040 5.5040 0.4990 0.00003 0.000025 0.00698 71.50 71.5 5.0 50.0 2.24 2.92 0.60 0.0064 5.0064 0.4976 0.00005 0.000040 0.00696 71.49 142.99 4.5 40.5 2.01 2.54 0.74 0.0098 4.5098 0.4966 0.00009 0.000070 0.00693 71.64 214.63 4.0 32.0 1.79 2.17 0.94 0.0157 4.0157 0.4941 0.00016 0.000127 0.00687 71.89 286.52 3.5 24.5 1.57 1.82 1.22 0.0268 3.5268 0.4889 0.00033 0.000246 0.00675 72.38 358.90 3.0 18.0 1.34 1.48 1.67 0.0496 3.0496 0.4772 0.00076 0.000547 0.00645 73.95 432.85 2.5 12.5 1.12 1.16 2.40 0.1029 2.6029 0.4467 0.00201 0.001387 0.00561 79.58 512.43 2.0 8.0 0.89 0.86 3.75 0.2511 2.2511 0.3518 0.00663 0.004320 0.00268 131.27 643.70 The step computations are carried out as shown in the above table. The values in each column of the table are explained as follows: Col. 1. Depth of flow (ft) assigned from 6 to 2 feet Col. 2. Water area (ft2) corresponding to depth y in Col. 1 Col. 3 Hydraulic radius (ft) corresponding to y in Col. 1 Col. 4. Four-thirds power of the hydraulic radius Col. 5. Mean velocity (fps) obtained by dividing Q (30 cfs) by the water area in Col. 2 Col. 6. Velocity head (ft) Col. 7. Specific energy (ft) obtained by adding the velocity head in Col. 6 to depth of flow in Col. 1 Col. 8. Change of specific energy (ft) equal to the difference between the E value in Col. 7 and that of the previous step. Col. 9. Friction slope Sf, computed from V as given in Col. 5 and R4/3 in Col. 4 Col. 10. Average friction slope between the steps, equal to the arithmetic mean of the friction slope just computed in Col. 9 and that of the previous step Col. 11. Difference between the bottom slope, So, and the average friction slope, Sf Col. 12. Length of the reach (ft) between the consecutive steps; Computed by x = E/(So - Sf) or by dividing the value in Col. 8 by the value in Col. 11 Col. 13. Distance from the beginning point to the section under consideration. This is equal to the cumulative sum of the values in Col. 12 computed for previous steps. SECTION 4.4 OPEN CHANNELS, FLOODPLAINS, AND FLOODWAYS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-70 gVECY2*1 2 1 FIGURE 4.4.1.H BWCHAN COMPUTER SUBROUTINE – VARIABLE DEFINITIONS BWCHAN – VARIABLE DEFINITIONS YC-IN Critical Depth (ft) at current section based on incoming flow rate. YC-OT Critical Depth (ft) at current section based on outgoing flow rate. YN-IN Normal Depth (ft) at current section based on incoming flow rate/channel grade. YN-OT Normal Depth (ft) at current section based on outgoing flow rate/channel grade. Y1 Final Water Depth (ft) at current cross section N-Y1 Composite n-factor of current section for final depth, Y1. A-Y1 Cross-sectional Area of current section for final depth, Y1. WP-Y1 Wetted Perimeter (ft) of current section for final depth, Y1. V-Y1 Average Velocity (fps) of current section for final depth, Y1. E1 Total Energy Head (ft) at current section E2 Total Energy Head (ft) at pervious or downstream section. SF1 Friction Slope of current section. SF2 Friction Slope of previous or downstream section. DXY Distance (expressed as a fraction of the current reach length) from the previous or downstream section to where the flow profile would intersect the final water depth, Y1, assuming Y1 were to remain constant EC Energy Coefficient ““ Q-TW The flow rate used to determine Tailwater Height from an inputted HW/TW Data File. TW-HT Tailwater Height. Q-Y1 Flow rate (cfs) in channel at current section, for depth, Y1 VU-Y1 Upstream Velocity (fps) at current section for depth, Y1 (“Adjust” option). V1-HD Channel Velocity Head (ft) at current section. VU-HD Upstream Velocity Head (ft) at current section. 4.4.2 FLOODPLAIN/FLOODWAY ANALYSIS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-71 4.4.2 FLOODPLAIN/FLOODWAY ANALYSIS This section describes the floodplain/floodway studies required by Special Requirement #2, Flood Hazard Area Delineation, in Section 1.3.2. Floodplain/floodway studies, as required by this manual, establish base flood elevations and delineate floodplains and/or floodways when the City determines that a proposed project contains or is adjacent to a flood hazard area for a river, stream, lake, wetland, closed depression, marine shoreline, or other water feature. Furthermore, when development is proposed within the floodplain, the floodplain/floodway study is used to show compliance with the critical areas code (RMC 4-3-050) flood hazard area regulations. There are four conditions affecting the requirements for floodplain/floodway studies. Each condition is considered a threshold for determining the type of studies required and the documentation needed to meet the study requirements. Each study threshold and related study requirements are shown in the table below, and described further in this section. Note that any projects or related flood studies that are expected to result in a change to Base Flood Elevations published in FEMA Flood Insurance Studies and Rate Maps, must also comply with 44 CFR Part 65. TABLE 4.4.2.A FLOODPLAIN/FLOODWAY STUDY THRESHOLDS AND REQUIREMENTS Threshold Study Requirements The project site is on land that is outside of an already delineated floodplain and above the floodplain's base flood elevation based on best available floodplain data. No floodplain study required  Show delineation of floodplain on the site improvement plan and indicate base flood elevation  Record a notice on title See Section 4.4.2.1 for more details The project site is on land that is at least 10 feet above the ordinary high water mark or 2 feet above the downstream overflow elevation of a water feature for which a floodplain has not been determined. Approximate Floodplain Study per Section 4.4.2.2  Submit an engineering plan with approximate base flood elevation  Record a notice on title See Section 4.4.2.2 for more details The project site does not meet the above thresholds and is either on land that is outside of an already delineated Zone A floodplain (i.e., without base flood elevations determined), or is adjacent to a water feature for which a floodplain has not been determined. Minor Floodplain Study per Section 4.4.2.3  Backwater model  Submit an engineering plan with determined base flood elevation1  Record a notice on title See Section 4.4.2.3 for more details The project site is on land that is partially or fully within an already delineated floodplain of a river or stream, or is determined by a Minor Floodplain Study to be partially or fully within the floodplain of a river or stream. Major Floodplain/Floodway Study per Section 4.4.2.4  Show mapped floodplain/floodway on the site improvement plan and indicate base flood elevation  Record a notice on title  See further requirements in Section 4.4.2.4 For any project site or study that is intended to result in a change to FEMA Flood Insurance Study or Rate Maps, including changing published based flood elevations, the applicant must comply with documentation and approval requirements of FEMA regulations 44 CFR Part 65. 1 For marine shorelines, refer to the FEMA Guidelines and Specifications for Flood Hazard Mapping Partners. SECTION 4.4 OPEN CHANNELS, FLOODPLAINS, AND FLOODWAYS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-72 4.4.2.1 NO FLOODPLAIN STUDY REQUIRED IF the proposed project site is on land that is outside of an already delineated floodplain and is above the already determined base flood elevation for that floodplain, based on best available floodplain data, THEN no floodplain study is required. In this situation, if the already determined floodplain covers any portion of the site, the boundary of that floodplain and its base flood elevation must be shown on the project's site improvement plan. In addition, a notice on title must be recorded for the site, alerting future property owners of the presence of a flood hazard area on the site and its base flood elevation. The notice on title requirement may be waived if the floodplain is not on any portion of the site. 4.4.2.2 APPROXIMATE FLOODPLAIN STUDY If the proposed project site is on land that is at least 10 feet above the ordinary high water mark or 2 feet above the downstream overflow elevation of a water feature for which the floodplain has not been delineated, then an Approximate Floodplain Study may be used to determine an approximate floodplain and base flood elevation. The intent of the Approximate Floodplain Study is to reduce required analysis in those situations where the project site is adjacent to a flood hazard area, but by virtue of significant topographical relief, is clearly in no danger of flooding. The minimum 10 feet of separation from ordinary high water reduces the level of required analysis for those projects adjacent to streams confined to deep channels or ravines, or near lakes or wetlands. The minimum 2 feet clearance above the downstream overflow elevation is intended to avoid flood hazard areas created by a downstream impoundment of water behind a road fill or in a lake, wetland, or closed depression. Use of the Approximate Floodplain Study requires submittal of an engineering plan7 showing the proposed project site is at least 10 feet above the ordinary high water elevation of the water feature in question, or at least 2 feet above the downstream overflow elevation of the water feature, whichever is less, subject to the following conditions: 1. The design engineer preparing the engineering plan shall determine an approximate base flood elevation and include a narrative describing his/her level of confidence in the approximate base flood elevation. The narrative must include, but is not limited to, an assessment of potential backwater effects (such as might result from nearby river flooding, for example); observations and/or anecdotal information on water surface elevations during previous flood events; and an assessment of potential for significantly higher future flows at basin build out. Note: Many of these issues will have been addressed in a Level 1 downstream analysis, if required. Acceptance of the approximate base flood elevation shall be at the sole discretion of the City. If the approximate base flood elevation is not acceptable, a Minor Floodplain Study or Major Floodplain/Floodway Study may be required. 2. That portion of the site that is at or below the assumed base flood elevation must be delineated and designated as a floodplain on the engineering plan, and a notice on title must be recorded for the site, notifying future property owners of the approximate floodplain and base flood elevation. 4.4.2.3 MINOR FLOODPLAIN STUDY IF the proposed project site does not meet the conditions for “no floodplain study required” per Section 4.4.2.1 or for use of the Approximate Floodplain Study per Section 4.4.2.2, AND the project site is either on land that is outside of an already delineated Zone A floodplain (i.e., without base flood 7 Engineering plan means a site improvement plan, including supporting documentation, stamped by a licensed civil engineer. In some instances, CED review staff may determine that the proposed project is sufficiently above the clearances specified in this exception and may not require an engineering plan. Typically, this is done for projects in Simplified Drainage Review that clearly exceed minimum clearances and otherwise would not require engineering design. 4.4.2 FLOODPLAIN/FLOODWAY ANALYSIS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-73 elevations determined) or is adjacent to a water feature for which a floodplain has not been determined, THEN a Minor Floodplain Study may be used to determine the floodplain. However, if the Minor Floodplain Study determines that all or a portion of the project site is at or below the base flood elevation of a river or stream and thus within the floodplain, then the applicant must either redesign the project site to be out of the floodplain or complete a Major Floodplain/Floodway Study per Section 4.4.2.4. Use of the Minor Floodplain Study requires submittal of an engineering plan and supporting calculations. That portion of the site that is at or below the determined base flood elevation must be delineated and designated as a floodplain on the engineering plan, and a notice on title must be recorded for the site, notifying future property owners of the floodplain and base flood elevation. Methods of Analysis For streams without a floodplain or flood hazard study, or for drainage ditches or culvert headwaters, the base flood elevation and extent of the floodplain shall be determined using the Direct Step backwater method, Standard Step backwater method, or the King County Backwater computer program, as described in Section 4.4.1.2. For lakes, wetlands, and closed depressions without an approved floodplain or flood hazard study, the base flood elevation and the extent of the floodplain shall be determined using the “point of compliance technique” described in Section 3.3.6. 4.4.2.4 MAJOR FLOODPLAIN/FLOODWAY STUDY The floodplain analysis shall be based on the 100-year storm event using existing land use hydrology except as noted in the paragraph titled “Flood Flows from Adopted Basin Plan Information.” IF the proposed project site is on land that is partially or fully within an already delineated floodplain of a river or stream, or determined by a Minor Floodplain Study to be partially or fully within the floodplain of a river or stream, THEN a Major Floodplain/Floodway Study is required to determine the floodplain, floodway, and base flood elevation in accordance with the methods and procedures presented in this section. This information will be used by the City to evaluate the project's compliance with regulations for development or improvements within the floodplain. Major Floodplain/Floodway Studies must conform to FEMA regulations described in Part 65 of 44 Code of Federal Regulations (CFR). In addition, the following information must be provided and procedures performed.  INFORMATION REQUIRED The applicant shall submit the following information for review of a floodplain/floodway analysis in addition to that required for the drainage plan of a proposed project. This analysis shall extend upstream and downstream a sufficient distance to adequately include all backwater conditions that may affect flooding at the site and all reaches that may be affected by alterations to the site. Floodplain/Floodway Map A Major Floodplain/Floodway Study requires submittal of five copies of a separate floodplain/floodway map stamped by a licensed civil engineer and a professional land surveyor registered in the State of Washington (for the base survey). The map must accurately locate any proposed development with respect to the floodplain and floodway, the channel of the stream, and existing development in the floodplain; it must also supply all pertinent information such as the nature of any proposed project, legal description of the property on which the project would be located, fill quantity, limits and elevation, the building floor elevations, flood-proofing measures, and any use of compensatory storage. SECTION 4.4 OPEN CHANNELS, FLOODPLAINS, AND FLOODWAYS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-74 The map must show elevation contours at a minimum of 2-foot vertical intervals and shall comply with survey and map guidelines published in the FEMA publication Guidelines and Specifications for Flood Hazard Mapping Partners. The map must show the following:  Existing elevations and ground contours;  Locations, elevations and dimensions of existing structures, and fills;  Size, location, elevation, and spatial arrangement of all proposed structures, fills and excavations, including proposed compensatory storage areas, with final grades on the site;  Location and elevations of roadways, water supply lines, and sanitary sewer facilities, both existing and proposed. Study Report A Major Floodplain/Floodway Study also requires submittal of two copies of a study report, stamped by a licensed civil engineer, which must include calculations or any computer analysis input and output information as well as the following additional information: 1. Valley cross sections showing the channel of the river or stream, the floodplain adjoining each side of the channel, the computed FEMA floodway, the cross-sectional area to be occupied by any proposed development, and all historic high water information. 2. Profiles showing the bottom of the channel, the top of both left and right banks, and existing and proposed base flood water surfaces. 3. Plans and specifications for flood-proofing any structures and fills, construction areas, materials storage areas, water supply, and sanitary facilities within the floodplain. 4. Complete printout of input and output (including any error messages) for HEC-RAS. Liberal use of comments will assist in understanding model logic and prevent review delays. 5. One ready-to-run digital copy of the HEC-RAS input file used in the study. Data shall be submitted in an electronic format. 6. The applicant shall prepare a written summary describing the model development calibration, hydraulic analysis, and floodway delineation. The summary shall also include an explanation of modeling assumptions and any key uncertainties.  DETERMINING FLOOD FLOWS The three techniques used to determine the flows used in the analysis depend on whether gage data is available or whether a basin plan has been adopted. The first technique is for basins in adopted basin plan areas. The second technique is used if a gage station exists on the stream. The third technique is used on ungaged catchments or those with an insufficient length of record. In all cases, the design engineer shall be responsible for assuring that the hydrologic methods used are technically reasonable and conservative, conform to the Guidelines and Specifications for Flood Hazard Mapping Partners, and are acceptable by FEMA. Flood Flows from Adopted Basin Plan Information For those areas where the City or King County has adopted a basin plan since 1986, flood flows may be determined using information from the adopted basin plan. The hydrologic model used in the basin plan shall be updated to include the latest changes in zoning, or any additional information regarding the basin that has been acquired since the adoption of the basin plan. Flood Flows from Stream Gage Data Flood flows from stream gage data may be determined using HEC-FFA, which uses the Log-Pearson Type III distribution method as described in Guidelines for Determining Flood Flow Frequency, Bulletin 17B of the Hydrology Committee, prepared by the Interagency Advisory Committee on Water Data (1982). Refer 4.4.2 FLOODPLAIN/FLOODWAY ANALYSIS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-75 to the FEMA Guidelines and Specifications for Flood Hazard Mapping Partners to verify the most current requirements. Use of HEC-FFA is subject to the following requirements: 1. This technique may be used only if data from a gage station in the basin is available for a period of at least ten years that is representative of the current basin conditions. 2. If the difference in the drainage area on the stream at the study location and the drainage area to a gage station on the stream at a different location in the same basin is less than or equal to 50 percent, the flow at the study location shall be determined by transferring the calculated flow at the gage to the study location using a drainage area ratio raised to the 0.86 power, as in the following equation: QSS = QG (ASS/AG)0.86 (4-11) where QSS = estimated flow for the given return frequency on the stream at the study location QG = flow for the given return frequency on the stream at the gage location ASS = drainage area tributary to the stream at the study location AG = drainage area tributary to the stream at the gage location 3. If the difference in the drainage area at the study location and the drainage area at a gage station in the basin is more than 50 percent and a basin plan has not been prepared, a continuous model shall be used as described below to determine flood flows at the study location. 4. In all cases where dams or reservoirs, floodplain development, or land use upstream may have altered the storage capacity or runoff characteristics of the basin so as to affect the validity of this technique, a continuous model shall be used to determine flood flows at the study location. Flood Flows from a Calibrated Continuous Model Flood flows may be determined by utilizing a continuous flow simulation model such as HSPF. Where flood elevations or stream gage data are available, the model shall be calibrated; otherwise, regional parameters8 may be used.  DETERMINING FLOOD ELEVATIONS, PROFILES, AND FLOODWAYS Reconnaissance The applicant's design engineer is responsible for the collection of all existing data with regard to flooding in the study area. This shall include a literature search of all published reports in the study area and adjacent communities, and an information search to obtain all unpublished information on flooding in the immediate and adjacent areas from federal, state, and local units of government. This search shall include specific information on past flooding in the area, drainage structures such as bridges and culverts that affect flooding in the area, available topographic maps, available flood insurance rate maps, photographs of past flood events, and general flooding problems within the study area. A field reconnaissance shall be made by the applicant's design engineer to determine hydraulic conditions of the study area, including type and number of structures, locations of cross sections, and other parameters, including the roughness values necessary for the hydraulic analysis. Base Data Cross sections used in the hydraulic analysis shall be representative of current channel and floodplain conditions obtained by surveying. When cross-sections data is obtained from other studies, the data shall be confirmed to represent current channel and floodplain conditions, or new channel cross-section data shall be obtained by field survey. Topographic information obtained from aerial photographs may be used in combination with surveyed cross sections in the hydraulic analysis. The elevation datum of all 8 Dinacola, 1990. U.S.G.S., Characterization and Simulation of Rainfall-Runoff Relations for Headwater Basins in Western King and Snohomish Counties, Washington. SECTION 4.4 OPEN CHANNELS, FLOODPLAINS, AND FLOODWAYS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-76 information used in the hydraulic analysis shall be specified. All information shall be referenced directly to NAVD 1988 (and include local correlation to NGVD 1929) unless otherwise approved by the City. See Table 4.4.2.B for correlations of other datum to NAVD 1988. Methodology Flood profiles and floodway studies shall be calculated using the U.S. Army Corps of Engineers’ HEC- RAS computer model (or subsequent revisions). Floodway Determination 1. Flood hazard areas are identified by the Federal Insurance Administration in a scientific and engineering report entitled the Flood Insurance Study for the City of Renton, dated September 29, 1989, and any subsequent revision, with accompanying flood insurance maps which are hereby adopted by reference and declared to be a part of this Section. The flood insurance study is on file at the Public Works Department. Previous Floodplain Studies If differences exist between a study previously approved by the City and the applicant’s design engineer’s calculated hydraulic floodways or flood profiles, the design engineer shall provide justification and obtain City approval for these differences. Zero-Rise Calculation For a zero-rise analysis, the flow profile for the existing and proposed site conditions shall be computed and reported to the nearest 0.01 foot. A zero-rise analysis requires only comparisons of the computed water surface elevations and energy grade lines for the existing and proposed conditions. Such comparisons are independent of natural dynamics and are not limited by the accuracy of the model’s absolute water surface predictions. Adequacy of Hydraulic Model At a minimum, the City considers the following factors when determining the adequacy of the hydraulic model and flow profiles for use in floodway analysis: 1. Cross section spacing 2. Differences in energy grade Note: Significant differences in the energy grade from cross section to cross section are an indication that cross sections should be more closely spaced or that other inaccuracies exist in the hydraulic model. 1. Methods for analyzing the hydraulics of structures such as bridges and culverts 2. Lack of flow continuity 3. Use of a gradually-varied flow model Note: In certain circumstances (such as weir flow over a levee or dike, flow through the spillway of a dam, or special applications of bridge flow), rapidly-varied flow techniques shall be used in combination with a gradually-varied flow model. 1. Manning's “n” values 2. Calibration of the hydraulic model with past flood events 3. Special applications. In some cases, HEC-RAS alone may not be sufficient for preparing the floodplain/floodway analysis. This may occur where sediment transport, two-dimensional flow, or other unique hydraulic circumstances affect the accuracy of the HEC-RAS hydraulic model. In these cases, the applicant shall obtain City approval of other methods proposed for estimating the water surface profiles. 4.4.2 FLOODPLAIN/FLOODWAY ANALYSIS 2022 City of Renton Surface Water Design Manual 6/22/2022 4-77 TABLE 4.4.2.B DATUM CORRELATIONS (for general reference use only, values are approximate) Correlation From To (Snoq. Valley) NAVD 1988* KCAS U.S. Engineers City of Seattle NGVD, USGS & USC & GS 1947 Seattle Area Tide Tables & Navigation Charts 1954 & Later NAVD 1988* (Upper Snoqualmie Valley) – -3.58 3.44 -9.54 -3.49 2.98 KCAS 3.58 – 7.02 -5.96 0.09 6.56 U.S. Engineers -3.22 -7.02 – -12.98 -6.93 -0.46 City of Seattle 9.54 5.96 12.98 – 6.05 12.52 NGVD, USGS & USC& GS 1947 (adjusted to the 1929 datum) 3.49 -0.09 6.93 -6.05 – 6.47 Seattle Area Tide Tables & Navigation Charts 1954 & Later (based on epoch 1924-1942) -2.98 -6.56 0.46 -12.52 -6.47 – Design Tidal Tailwater Elevation 12.08 8.50 15.52 2.54 8.59 15.06 Mean Higher High Water (MHHW) 8.34 4.76 11.78 -1.20 4.85 11.32 Mean High Water (MHW) 7.49 3.91 10.93 -2.05 4.00 10.47 Mean Low Water (MLW) -0.16 -3.74 3.28 -9.70 -3.65 2.82 Mean Lower Low Water (MLLW) -2.98 -6.56 0.46 -12.52 -6.47 0.00 * Varies, contact the City of Renton for datum correlation for this and other areas. KCAS datum = Sea Level Datum 1929 (a.k.a. NGVD 1929) KCAS = King County Aerial Survey NAVD = North American Vertical Datum NGVD = National Geodetic Vertical Datum USGS = United States Geologic Survey USC & GS = US Coast and Geodetic Survey SECTION 4.4 OPEN CHANNELS, FLOODPLAINS, AND FLOODWAYS 6/22/2022 2022 City of Renton Surface Water Design Manual 4-78 (T his page intentionally left blan k. ) 2022 City of Renton Surface Water Design Manual 6/22/2022 CHAPTER 5 FLOW CONTROL DESIGN CITY OF RENTON SURFACE WATER DESIGN MANUAL Section Page 5.1 Detention Facilities 5-3 5.1.1 Detention Ponds 5-3 5.1.2 Detention Tanks 5-17 5.1.3 Detention Vaults 5-21 5.1.4 Control Structures 5-25 5.1.5 Parking Lot Detention 5-35 5.1.6 Roof Detention 5-35 5.1.7 Simple Detention Pond for Cleared Areas 5-35 5.1.8 Alternative Detention Systems 5-42 5.2 Infiltration Facilities 5-45 5.2.1 General Requirements for Infiltration Facilities 5-45 5.2.2 Infiltration Ponds 5-56 5.2.3 Infiltration Tanks 5-59 5.2.4 Infiltration Vaults 5-62 5.2.5 Infiltration Trenches 5-64 5.2.6 Alternative Infiltration Systems 5-65 5.2.7 Small Infiltration Basins 5-66 CITY OF RENTON SURFACE WATER DESIGN MANUAL 6/22/2022 2022 City of Renton Surface Water Design Manual (T his page intentionally left blan k. ) CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 5-1 CHAPTER 5 FLOW CONTROL DESIGN This chapter presents the City approved methods, criteria, and details for hydraulic analysis and design of flow control facilities pursuant to Core Requirement #3, “Flow Control” (see Section 1.2.3). Flow control facilities, as described in this manual, are detention or infiltration facilities engineered to meet a specified discharge performance. Four terms are commonly used to describe flow control facilities in the City: detention facilities, retention facilities, infiltration facilities, and R/D (Retention/Detention) facilities. A detention facility, by definition, temporarily stores surface water runoff and discharges it at a reduced rate. A retention facility stores water longer and effectively has no surface outflow (outflow occurs by evaporation or soaking into the ground). Infiltration facilities are retention facilities that rely entirely on the soaking of collected surface water into the ground. The term R/D facility has been used in previous versions of this manual to generally refer to all flow control facilities. On-site BMPs, also known as low impact development (LID) BMPs, are methods and designs for dispersing, infiltrating, or otherwise reducing or preventing development-related increases in runoff at or near the sources of those increases. On-site BMPs include, but are not limited to, preservation and use of native vegetated surfaces to fully disperse runoff; use of other pervious surfaces to disperse runoff; roof downspout infiltration; permeable pavement; bioretention; and reduction of development footprint. On- site BMPs are required pursuant to Core Requirement #9, “On-Site BMPs” (see Section 1.2.9). Design criteria for on-site BMPs are included in Appendix C of this manual. The figures included in this chapter are provided as schematic representations and should not be used for design. Refer to the City of Renton Standard Details for specific design information. The figures provided in this chapter illustrate one example of how the flow control facility design criteria may be applied. Although the figures are meant to illustrate many of the most important design criteria, they may not show all criteria that apply. In general, the figures are not used to specify requirements unless they are indicated elsewhere in the manual. If this manual refers to a standard detail not included in the City of Renton Standard Details, the applicant shall use the figure provided in this manual. Chapter Organization The information in this chapter is organized into the following four main sections:  Section 5.1, “Detention Facilities”  Section 5.2, “Infiltration Facilities” These sections begin on odd pages so the user can insert tabs if desired for quicker reference. Required vs. Recommended Design Criteria Both required and recommended design criteria are presented in this chapter. Criteria stated using “shall” or “must” are mandatory, to be followed unless there is a good reason to deviate as allowed by the adjustment process (see Section 1.4). These criteria are required design criteria and generally affect CHAPTER 5 FLOW CONTROL DESIGN 6/22/2022 2022 City of Renton Surface Water Design Manual 5-2 facility performance or critical maintenance factors. Sometimes options are stated as part of the required design criteria using the language “should” or “may.” These criteria are recommended design criteria, but are closely related to the required criteria, so they are placed in the same section. Use of Materials Galvanized metals leach zinc into the environment, especially in standing water situations. High zinc concentrations, sometimes in the range that can be toxic to aquatic life, have been observed in the region. Therefore, use of galvanized materials in flow control facilities and on-site BMPs should be avoided. Where other metals, such as aluminum or stainless steel, or plastics are available, they shall be used. Allowable materials are specified in the Design Criteria for the facility. CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 5-3 5.1 DETENTION FACILITIES This section presents the methods, criteria, and details for design and analysis of detention facilities. These facilities provide for the temporary storage of increased surface water runoff resulting from development pursuant to the performance standards set forth in Core Requirement #3, “Flow Control” (see Section 1.2.3). There are three primary types of detention facilities described in this section: detention ponds, tanks, and vaults. The information presented in this section is organized as follows: Section 5.1.1, “Detention Ponds” “Design Criteria,” Section 5.1.1.1 “Methods of Analysis,” Section 5.1.1.2 Section 5.1.2, “Detention Tanks” “Design Criteria,” Section 5.1.2.1 “Methods of Analysis,” Section 5.1.2.2 Section 5.1.3, “Detention Vaults” “Design Criteria,” Section 5.1.3.1 “Methods of Analysis,” Section 5.1.3.2 Section 5.1.4, “Control Structures” “Design Criteria,” Section 5.1.4.1 “Methods of Analysis,” Section 5.1.4.2 Section 5.1.5, “Parking Lot Detention” Section 5.1.6, “Roof Detention” Section 5.1.7, “Simple Detention Pond for Cleared Areas” “Design Criteria,” Section 5.1.7.1 “Methods of Analysis,” Section 5.1.7.2 Section 5.1.8, “Alternative Detention Systems” “Design Criteria,” Section 5.1.8.1 “Methods of Analysis,” Section 5.1.8.2. 5.1.1 DETENTION PONDS Open ponds are the most desirable detention facilities for controlling runoff from developed areas. The design criteria in Section 5.1.1.1 are for detention ponds. However, many of the criteria also apply to infiltration ponds (Section 5.2.2), and water quality wetponds and combined detention/wetponds (Section 6.4). Dam Safety Compliance Detention ponds and other open impoundment facilities must comply with requirements for dam safety (WAC 173-175). Under current regulations (as of February 2012), if the impoundment has a storage capacity (including both water and sediment storage volumes) greater than 10 acre-feet above natural ground level and a dam height of more than 6 feet, then dam safety design and review are required by the Washington State Department of Ecology (Ecology). If the storage capacity is less than 10 acre-feet above SECTION 5.1 DETENTION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-4 natural ground level, then the facility is exempt from Ecology review. If the dam height is less than 6 feet but capacity is greater than 10 acre-feet, then Ecology reviews on a case-by-case-basis to determine the hazard potential downstream in the event of a failure. 5.1.1.1 DESIGN CRITERIA Schematic representations of detention ponds are shown in Figure 5.1.1.A through Figure 5.1.1.D. Control structure details are described in Section 5.1.4. General 1. Ponds must be designed as flow-through systems (however, parking lot storage may be utilized through a back-up system; see Section 5.1.5). Developed flows must enter through a conveyance system separate from the control structure and outflow conveyance system. Maximizing distance between the inlet and outlet is encouraged to promote sedimentation. 2. Pond bottoms shall be level and be located a minimum of 0.5 feet below the inlet and outlet to provide sediment storage. 3. Outflow control structures shall be designed as specified in Section 5.1.4. 4. Detention ponds preceding required water quality treatment facilities must meet the liner requirements described in Section 6.2.4 (Facility Liners) to ensure groundwater protection. 5. A geotechnical analysis and report is required if located within 200 feet of a steep slope hazard area or landslide hazard OR if the facility is located within a setback distance from top of slope equal to the total vertical height of the slope area that is steeper than 15%. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built-out conditions. 6. The perimeter of all new flow control and/or water quality treatment facilities shall be landscaped in accordance with RMC 4-4-070 and Section 5.1. Side Slopes 1. Side slopes (interior and exterior) shall be no steeper than 3H:1V. 2. Pond walls may be vertical retaining walls, provided: (a) they are constructed of reinforced concrete per Section 5.1.3; (b) a fence is provided along the top of the wall; (c) at least 25% of the pond perimeter will be a vegetated soil slope not steeper than 3H:1V; and (d) the design is stamped by a licensed structural civil engineer. Embankments 1. Pond berm embankments higher than 6 feet shall require design by a geotechnical engineer. 2. For berm embankments 6 feet or less, the minimum top width shall be 6 feet, or as recommended by a geotechnical engineer. 3. Pond berm embankments must be constructed on native consolidated soil (or adequately compacted and stable fill soils analyzed by a geotechnical engineer) free of loose surface soil materials, roots, and other organic debris. 4. Pond berm embankments greater than 4 feet in height must be constructed by excavating a key equal to 50% of the berm embankment cross-sectional height and width. This requirement may be waived if specifically recommended by a geotechnical engineer. 5. The berm embankment shall be constructed of soil placed in 6-inch lifts compacted to at least 95% of maximum dry density, within 2 percentage points of the optimum moisture content, modified proctor method ASTM D1557. Density tests shall be performed for each lift to confirm compliance with this specification. The soil used for construction shall have the following soil characteristics: a minimum of 20% silt and clay, a maximum of 60% sand, a maximum of 60% silt and clay, with 5.1.1 DETENTION PONDS 2022 City of Renton Surface Water Design Manual 6/22/2022 5-5 nominal gravel and cobble content. Note: In general, excavated glacial till is well suited for berm embankment material. 6. Anti-seepage collars must be placed on outflow pipes in berm embankments impounding water greater than 8 feet in depth at the design water surface. Overflow 1. In all ponds, tanks, and vaults, a primary overflow (usually a riser pipe within the control structure; see Section 5.1.4.2) must be provided to bypass the 100-year, 15-minute developed peak flow over or around the restrictor system. This assumes the facility will be full due to plugged orifices or high inflows; the primary overflow is intended to protect against breaching of a pond embankment (or overflows of the upstream conveyance system, in the case of a detention tank or vault). The design must provide controlled discharge directly into the downstream conveyance system or another acceptable discharge point. 2. A secondary inlet to the control structure must be provided in ponds as additional protection against overtopping should the inlet pipe to the control structure become plugged. A grated opening (“jailhouse window”) in the control structure manhole functions as a weir (see schematic representation in Figure 5.1.1.B) when used as a secondary inlet. Note: The maximum circumferential length of this opening shall not exceed one-half the control structure circumference. A “birdcage” overflow structure as shown in the schematic representation in Figure 5.1.1.C may also be used as a secondary inlet. Emergency Overflow Spillway 1. In addition to the above overflow requirements, ponds must have an emergency overflow spillway sized to pass the 100-year, 15-minute developed peak flow in the event of total control structure failure (e.g., blockage of the control structure outlet pipe) or extreme inflows. Emergency overflow spillways are intended to control the location of pond overtopping and direct overflows back into the downstream conveyance system or other acceptable discharge point. 2. Emergency overflow spillways must be provided for ponds with constructed berms over 2 feet in height, or for ponds located on grades in excess of 5%. As an option for ponds with berms less than 2 feet in height and located at grades less than 5%, emergency overflow may be provided by an emergency overflow structure, such as a Type II manhole fitted with a birdcage as shown in the schematic representation in Figure 5.1.1.C. The emergency overflow structure must be designed to pass the 100-year developed peak flow, with a minimum 6 inches of freeboard, directly to the downstream conveyance system or another acceptable discharge point. Where an emergency overflow spillway would discharge to a slope steeper than 15%, consideration should be given to providing an emergency overflow structure in addition to the spillway. 3. The emergency overflow spillway shall be armored in conformance with Table 4.2.2.A. The spillway shall be armored full width, beginning at a point midway across the berm embankment and extending downstream to where emergency overflows re-enter the conveyance system (see Figure 5.1.1.B). 4. Design of emergency overflow spillways requires the analysis of a broad-crested trapezoidal weir as described in Section 5.1.1.2. Either one of the weir sections shown in the schematic representations in Figure 5.1.1.B may be used. Access Requirements 1. Maintenance access road(s) shall be provided to the control structure and other drainage structures associated with the pond (e.g., inlet, emergency overflow or bypass structures). Manhole and catch basin lids must be in or at the edge of the access road and at least three feet from a property line. Rims shall be set at the access road grade. 2. An access ramp is required for removal of sediment with a trackhoe and truck. The ramp must extend to the pond bottom if the pond bottom is greater than 1,500 square feet (measured without the ramp) SECTION 5.1 DETENTION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-6 and it may end at an elevation 4 feet above the pond bottom, if the pond bottom is less than 1,500 square feet (measured without the ramp), provided the pond side slopes are 3H:1V or flatter. 3. Intent: On large, deep ponds, truck access to the pond bottom via an access ramp is necessary so loading can be done in the pond bottom. On small deep ponds, the truck can remain on the ramp for loading. On small shallow ponds, a ramp to the bottom may not be required if the trackhoe can load a truck parked at the pond edge or on the internal berm of a wetpond or combined pond (trackhoes can negotiate interior pond side slopes). 4. The internal berm of a wetpond or combined detention and wetpond may be used for access if it is no more than 4 feet above the first wetpool cell, if the first wetpool cell is less than 1500 square feet (bottom area measured without the ramp), and if it is designed to support a loaded truck, considering the berm is normally submerged and saturated. 5. Access ramps shall meet the requirements for design and construction of access roads specified below. 6. All control structures shall have round, solid locking lids with 5/8-inch diameter Allen head cap screws (see the City of Renton Standard Details). 7. Access shall be limited by a double-posted gate if a fence is required, or by bollards. Bollards shall be designed in accordance with the City of Renton Standard Details. Design of Access Roads Access roads shall meet the following design criteria: 1. Maximum grade shall be 15% for asphalt paving and 12% for gravel or modular grid paving. 2. Outside turning radius shall be 40 feet, minimum. 3. Fence gates shall be located only on straight sections of road. 4. Access roads shall be 15 feet in width on curves and 12 feet on straight sections. 5. A paved apron shall be provided where access roads connect to paved public roadways. The apron shall be consistent with driveway details in the City of Renton Standard Details. Construction of Access Roads Access roads shall be constructed with an asphalt, concrete or gravel surface, or modular grid pavement. Access roads must conform to the City of Renton Standard Details for residential or rural minor access streets. Modular grid pavement shall meet manufacturer’s specifications. Where access roads pass over emergency overflow spillways, a HMA wearing course is required (see Figure 5.1.1.B). Fencing 1. All ponds and other similar facilities, as determined by the City, shall be fenced. On stormwater facilities to be maintained by the City, a fence shall be placed at the top of the berm with the maintenance access road in the inside of the fence; or 5 feet minimum from the top of berm if there is no maintenance access road allowing proper maintenance access of the facility. 2. Fences shall be 6 feet in height. For example designs, see WSDOT Standard Plan L-2, Type 1 or Type 3 chain link fence. 3. Access road gates shall be 16 feet in width consisting of two swinging sections 8 feet in width. Additional vehicular access gates may be required as needed to facilitate maintenance access. 4. Pedestrian access gates (if needed) shall be 4 feet in width. 5. Fence material shall be black or green bonded vinyl chain link. The following apply: 5.1.1 DETENTION PONDS 2022 City of Renton Surface Water Design Manual 6/22/2022 5-7 a) Vinyl coating shall be compatible with the surrounding environment (e.g., green in open, grassy areas and black in wooded areas). All posts, cross bars, and gates shall be coated the same color as the vinyl clad fence fabric. b) Fence posts and rails shall conform to WSDOT Standard Plan L-2 for Types 1, 3, or 4 chain link fence. 6. Metal baluster fences are allowed where the fence will be owned and maintained by a private property owner or homeowners association. Fence maintenance requirements shall be a condition of subdivision approval, and a statement detailing maintenance responsibility of the fence will be placed in the final plat. Uniform Building Code standards shall apply. 7. Wood fences are allowed in subdivisions where the fence will be owned and maintained by a private property owner or homeowners association. Fence maintenance requirements shall be a condition of subdivision approval, and a statement detailing maintenance responsibility of the fence will be placed in the final plat. 8. Wood fences shall have pressure treated1 posts (ground contact rated) either set in 24-inch deep concrete footings or attached to footings by steel brackets. Rails and fence boards shall be cedar. Signage Detention ponds, infiltration ponds, wetponds, and combined ponds to be maintained by the City shall have a sign placed for maximum visibility from adjacent streets, sidewalks, and paths. The sign shall meet the design and installation requirements illustrated in the City of Renton Standard Details. The fence gate must be posted with a 12 inch by 18 inch “No Trespassing” sign, unless otherwise approved by the City. Right-of-Way 1. Open detention ponds shall not be located in dedicated public road right-of-way. 2. Detention ponds to be maintained by the City, along with the perimeter landscaping shall be in a stormwater tract granted and conveyed with all maintenance obligations (excluding maintenance of the drainage facilities contained therein) to the property owners. Each property lot owner within the subdivision shall have an equal and undivided interest in the maintenance of the stormwater tract and landscaping features. Any tract not abutting public right-of-way will require a 15-foot-wide extension of the tract to an acceptable access location. An underlying easement under and upon said tract shall be dedicated to the City for the purpose of operating, maintaining, improving and repairing the drainage facilities contain therein. 3. Detention ponds to be maintained by a private property owner or homeowners association shall create stormwater facilities within a private tract or easement or construct the detention pond onsite. Setbacks 1. A setback of 5 feet from the toe of the exterior slope, retaining walls and rockeries to the tract or property line is required for City-maintained ponds and recommended for privately maintained ponds. 2. The tract or property line on a detention pond cut slope shall be setback 5 feet from the emergency overflow water surface. 3. The detention pond water surface at the pond outlet invert elevation shall be setback 100 feet from proposed or existing septic system drainfields. This setback may be reduced with written approval of the Public Health – Seattle & King County. 4. The detention pond design water surface shall be a minimum of 200 feet from any steep slope hazard area or landslide hazard. Upon analysis and approval of a licensed geotechnical engineer or engineering geologist, this setback may be reduced to 50 feet. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built- out conditions. 1 Fence posts represent a rare exception to the rule of no treated lumber. Ground contact requires pressure treatment. SECTION 5.1 DETENTION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-8 5. The detention pond design water surface shall be set back a minimum distance from top of slope equal to the total vertical height of a slope area that is steeper than 15%. Upon analysis and approval of a licensed geotechnical engineer or engineering geologist, this setback may be reduced to 50 feet. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built- out conditions. Seeps and Springs Intermittent seeps along cut slopes are typically fed by a shallow groundwater source (interflow) flowing along a relatively impermeable soil stratum. These flows are storm driven and should discontinue after a few weeks of dry weather. The approved continuous runoff model accounts for this shallow groundwater component and no special provisions are needed when directing these flows through the flow control facility. However, more continuous seeps and springs, which extend through longer dry periods, are likely from a deeper groundwater source. When continuous flows are intercepted and directed through flow control facilities, adjustments to the approved facility design may be required to account for the additional base flow (unless already considered in design). If uncertain at the time of construction, the situation may be monitored while the facility is under maintenance and defect financial guarantee. Adjustments to the facility may be required prior to the release of the financial guarantee. Planting Requirements Exposed earth on the pond bottom and interior side slopes shall be planted or seeded with an appropriate seed mixture. All remaining areas of the tract must either be planted with grass, or be landscaped in accordance with the standards below and mulched with a 4-inch cover of hog fuel or shredded wood mulch.2 Landscaping Landscaping is not optional; it is required on all stormwater/landscaping tracts. Landscaping is required in those areas of the tract that will not impact the functionality or maintenance of the drainage facilities. For stormwater ponds to be maintained by the City, landscaping inside the fence shall be planted with grass, low-growing shrubs, or groundcovers that are no- to low-maintenance and do not impede other facility maintenance activities (as required in Section 5.1). Landscaping maintained by the City and comprised of species other than grass is subject to City approval. Landscaping inside the fence is allowed for storm water facilities to be privately maintained provided that the landscaping complies with the requirements of RMC 4‐4‐070F8, Storm Drainage Facilities. The following requirements shall apply: 1. No trees or shrubs may be planted within 10 feet of inlet or outlet pipes or manmade drainage structures such as catch basins, spillways or flow spreaders. Species with roots that seek water, such as willow or poplar, should be avoided within 30 feet of pipes or manmade structures. 2. Planting is restricted on berms that impound water either permanently or temporarily during storms. If the pond is City-maintained, then landscaping with trees and large shrubs that may compromise berm integrity are prohibited in the inside slope of the pond and trees are prohibited on any drainage-related berms. a) Trees or tall shrubs may not be planted on portions of water-impounding berms taller than four feet high. Only grasses and low-growing shrubs or groundcovers may be planted on berms taller than four feet. Intent: Grasses and low-growing groundcovers allow unobstructed visibility of berm slopes for detecting potential dam safety problems such as animal burrows, slumping, or fractures in the berm. 2 Shredded wood mulch is made from shredded tree trimmings, usually from trees cleared onsite. It must be free of garbage and weeds and may not contain excessive resin, tannin, or other material detrimental to plant growth. 5.1.1 DETENTION PONDS 2022 City of Renton Surface Water Design Manual 6/22/2022 5-9 b) Trees planted on portions of water-impounding berms less than 4 feet high must be small, not higher than 20 feet mature height, and have a fibrous root system. Table 5.1.1.A gives some examples of trees with these characteristics. Intent: These trees reduce the likelihood of blow-down trees, or the possibility of channeling or piping of water through the root system, which may contribute to dam failure on berms that retain water. 3. All landscape material, including grass, must be planted in good topsoil. Native underlying soils may be made suitable for planting if amended with 2 inches of mature and stable compost tilled into the top six inches of soil. Compost used should meet specifications in Reference Section 11-C. 4. Soil in which trees or shrubs are planted may require additional enrichment or additional compost top-dressing. Consult a landscape professional or arborist for site-specific recommendations. 5. For a naturalistic effect as well as ease of maintenance, trees or shrubs must be planted in clumps to form “landscape islands” rather than evenly spaced. 6. The landscaped islands must be planted above the 100-year water surface and must be a minimum of six feet apart, and if set back from fences or other barriers, the setback distance must also be a minimum of six feet. Where tree foliage extends low to the ground, the six feet of setback should be counted from the outer drip line of the trees (estimated at maturity). Intent: This landscape design must allow a 6-foot wide mower to pass around and between clumps. 7. Evergreen trees and trees that produce relatively little leaf-fall such as Oregon ash, Cascara, or Western crabapple are preferred. Large-leaf deciduous trees may not be planted where branches could extend over interior pond slopes. 8. All trees shall be set back so branches do not extend over the 100-year water surface of the pond to prevent leaf-drop into the water. 9. Drought tolerant species are recommended. 10. Landscape areas within the tracts of City-maintained ponds in residential subdivision developments shall be designated “to be maintained by the homeowner’s association.” 11. For ponds to be maintained by the City, landscaping with trees or large shrubs is not allowed inside the fence. TABLE 5.1.1.A SMALL TREES AND SHRUBS WITH FIBROUS ROOTS Small Trees/High Shrubs Low Shrubs *Red twig dogwood (Cornus stolonifera) *Snowberry (Symphoricarpus albus) *Serviceberry (Amelanchier alnifolia) *Salmonberry (Rubus spectabilis) Strawberry tree (Arbutus unedo) Rosa rugosa (avoid spreading varieties) Highbush cranberry (Vaccinium opulus) Rock rose (Cistus spp.) Blueberry (Vaccinium spp.) Ceanothus spp. (choose hardier varieties) *Filbert (Corylus cornuta, others) New Zealand flax (Phormium penax) Fruit trees on dwarf rootstock Rhododendron (native and ornamental varieties) Ornamental grasses (e.g., Miscanthis, Pennisetum) *Native species. Guidelines for Naturalistic Planting Two generic kinds of naturalistic planting are outlined below, but other options are also possible. A booklet discussing stormwater ponds and landscaping possibilities is available at the King County Water SECTION 5.1 DETENTION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-10 and Land Resources Division that can be consulted for additional ideas. Native vegetation is preferred in naturalistic plantings. Note: These landscaping criteria must be followed unless a landscape professional judges that long-term quality of the open space would be improved by deviating from the criteria, AND that if the facility is maintained by the City, maintenance would not be made more difficult by the deviations. Open Woodland In addition to the general landscaping criteria above, the following requirements must be met: 1. Landscaped islands (when mature) should cover a minimum of 30% or more of the tract, exclusive of the pond area. 2. Tree clumps should be underplanted with shade-tolerant shrubs and groundcover plants. The goal is to provide a dense understory that need not be weeded or mowed. 3. Landscaped islands should be placed at several elevations rather than “ring” the pond, and the size of clumps should vary from small to large to create variety. 4. Not all islands need have trees. Shrub or groundcover clumps are acceptable, but lack of shade should be considered in selecting vegetation. Note: Landscaped islands are best combined with the use of hog fuel or shredded wood mulch for erosion control (only for slopes above the flow control water surface). It is often difficult to sustain a low-maintenance understory if the area was previously hydroseeded. Northwest Savannah or Meadow In addition to the general landscape criteria above, the following requirements must be met: 1. Landscape islands (when mature) should cover 10% or more of the tract, exclusive of the pond area. 2. Planting groundcovers and understory shrubs is encouraged to eliminate the need for mowing under the trees when they are young. 3. Landscape islands should be placed at several elevations rather than “ring” the pond. 4. The remaining tract area should be planted with an appropriate grass seed mix, which may include northwest meadow or wildflower species. Native or dwarf grass mixes are preferred. Table 5.1.1.B below gives one acceptable dwarf grass mix. Grass or meadow seed should be applied at a rate of 80 to 100 seeds per square foot. Actual pounds of seed mix per acre will depend on specific species composition. Note: Amended soil or good topsoil is required for all plantings. Creation of areas of emergent vegetation in shallow areas of the pond is recommended. Native wetland plants, such as sedges (Carex sp.), bulrush (Scirpus sp.), water plantain (Alisma sp.), and burreed (Sparganium sp.) are recommended. If the pond does not hold standing water, a clump of wet- tolerant, non-invasive shrubs, such as salmonberry or snowberry, is recommended below the detention design water surface. Note: This landscape style is best combined with the use of grass for site stabilization and erosion control. Table 5.1.1.B lists a mix for stormwater tracts and other intermittently wet areas that should be applied at a rate of 31 pounds of pure live seed per acre. 5.1.1 DETENTION PONDS 2022 City of Renton Surface Water Design Manual 6/22/2022 5-11 TABLE 5.1.1.B STORMWATER TRACT SEED MIX. Common Name Species Percent Species Composition American sloughgrass Beckmannia syzigachne 15% Tufted hairgrass Deschampsia cespitosa 20% Blue wildrye Elymus glaucus 18% Native red fescue Festuca rubra var. rubra 20% Meadow barley Hordeum brachyantherum 12% Northwestern mannagrass Glyceria occidentalis 15% Table 5.1.1.C lists a recommended mix for landscaping seed and should be applied at 19 pounds of pure live seed per acre. TABLE 5.1.1.C LANDSCAPING SEED MIX. Common Name Species Percent Species Composition Sideoats grama Bouteloua curtipendula 20% California oatgrass Danthonia californica 20% Native red fescue Festuca rubra var. rubra 30% Prairie Junegrass Koeleria macrantha 30% Table 5.1.1.D lists a turf seed mix that should be applied at a rate of 10 pounds of pure live seed per acre. This mix is for use in dry situations where there is no need for watering. This mix requires very little maintenance. TABLE 5.1.1.D LOW-GROWING TURF SEED MIX. Common Name Species Percent Species Composition Hard fescue Festuca brevipila 25% Sheep fescue Festuca ovina 30% Native red fescue Festuca rubra var. rubra 25% Prairie Junegrass Koeleria macrantha 20% SECTION 5.1 DETENTION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-12 FIGURE 5.1.1.A TYPICAL SCHEMATIC REPRESENTATION OF A DETENTION POND FLOW12'/15' ACCESS MAINTENANCE ROADFLOWFLOWFLOWPOND INLET PIPE 15% MAX. SLOPE ACCESS RAMP INTO POND SEE SECTION 5.1.1.1 FOR SPECIFICATIONS LEVEL BOTTOM SEE FIGURE 5.1.1.B FOR SECTION CUT DIAGRAMS NOTE: THIS DETAIL IS A SCHEMATIC REPRESENTATION ONLY. ACTUAL CONFIGURATION WILL VARY DEPENDING ON SPECIFIC SITE CONSTRAINTS AND APPLICABLE DESIGN CRITERIA. A A BB C C ALTERNATE EMERGENCY OUTFLOW STRUCTURE FOR PONDS NOT REQUIRED TO PROVIDE A SPILLWAY (FIGURE 5.1.1.C) CONTROL STRUCTURE POND DESIGN WATER SURFACE 6" SEDIMENT STORAGE COMPACTED EMBANKMENT 5' MIN. SETBACK BETWEEN TOE OF SLOPE AND TRACT BOUNDARY EMERGENCY OVERFLOW SPILLWAY ROCK LINING PER TABLE 4.2.2.A TRACT LINES AS REQUIRED 5.1.1 DETENTION PONDS 2022 City of Renton Surface Water Design Manual 6/22/2022 5-13 FIGURE 5.1.1.B TYPICAL SCHEMATIC REPRESENTATION OF DETENTION POND SECTIONS OVERFLOW WS DESIGN WS ROCK LININGPER TABLE 4.4.1.A COMPACTEDEMBANKMENT EMERGENCY OVERFLOW WS (AS REQUIRED) DESIGN WS OVERFLOW WS EMERGENCY OVERFLOW WS (SEE FIGURE 5.3.1.E) EMERGENCY OVERFLOW SPILLWAY NTS SECTION B-B SECTION B-B HAS 2 OPTIONS SECTION a-a NTS SECTION C-C NTS SECTION A-A NTS FRAME/GRATE FOR SECONDARY INLET. PROVIDE VERTICAL BARS IN FRAME @4" O.C. (OTHER FLOW SYSTEMS ACCEPTABLE IF APPROVED BY DPER).BARS SHALL BE STAINLESS STEEL OR ALUMINIZED STEEL.SEE ALSO THE SEPARATE OVERFLOW STRUCTURE SHOWN IN FIGURE 5.3.1.C CIRCUMFERENCE LENGTH OF OPENING SIZED FOR 100 YR FLOW OVERFLOW WS 6" SEDIMENT STORAGE OVERFLOW WS POND DESIGN WS EMERGENCY OVERFLOW WS CONTROL STRUCTURE 6' MIN. 12'/15' MIN. FORACCESS ROAD EXISTINGGROUND PROFILE BERM EMBANKMENTDEBRIS BARRIER SEE FIGURE 4.2.1.D TOP WIDTH OF BERM L a a 2 MIN. 1 KEY, IF REQUIRED POND DESIGN WS 1 10 1 10 2" ASPHALT (FOR SPILLWAY ON ACCESS ROAD) 113 36" MIN. FREEBOARD 1' ROCK LINING EVALUATEDOWNSTREAM CONVEYANCE PERCORE REQS #2 AND #4 POND OUTLET SIZED TO CONVEY THE100-YR DEVELOPED PEAK FLOW SECTION 5.1 DETENTION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-14 FIGURE 5.1.1.C SCHEMATIC REPRESENTATION OF AN OVERFLOW STRUCTURE UPPER STEEL BAND 34" X 4" WIDE LOWER STEEL BAND 34" X 4" WIDE FORMED TO FIT IN GROOVE OF C.B. RISER 34" DIA. SMOOTH ROUND BARS WELDED EQUALLY SPACED. BARS SHALL BE WELDED TO UPPER & LOWER BANDS (24 BARS EVENLY SPACED SEE NOTE 1) STANDARD STEPS OR LADDER SEE NOTE 2 HOOK CLAMP ANCHORED TO C.B. RISER TYPE 2 CB C.B. RISER SMOOTH VERTICAL BARS 1. DIMENSIONS ARE FOR ILLUSTRATION ON 54" DIAMETER CB. FOR DIFFERENT DIAMETER CB'S ADJUST TO MAINTAIN 45°ANGLE ON "VERTICAL" BARS AND 7" O.C. MAXIMUM SPACING OF BARS AROUND LOWER STEEL BAND. 2. METAL PARTS MUST BE CORROSION RESISTANT; BARS MUST BE STAINLESS STEEL OR ALUMINIZED STEEL. 3. THIS DEBRIS BARRIER IS ALSO RECOMMENDED FOR USE ON THE INLET TO ROADWAY CROSS-CULVERTS WITH HIGH POTENTIAL FOR DEBRIS COLLECTION (EXCEPT ON TYPE 2 STREAMS). 4. THIS DEBRIS BARRIER IS FOR USE OUTSIDE OF ROAD RIGHT-OF-WAY ONLY. FOR DEBRIS CAGES WITHIN ROAD RIGHT-OF-WAY, SEE KCRDCS DRAWING NO. 7-028. PROVIDE MAINTENANCE ACCESS BY WELDING 4 CROSSBARS TO 4 VERTICAL BARS AS SHOWN. HINGE UPPER ENDS WITH FLANGES/ BOLTS AND PROVIDE LOCKING MECHANISM (PADLOCK) ON LOWER END. LOCATE STEPS DIRECTLY BELOW. 34" DIAMETER SMOOTH BARS EQUALLY SPACED (4" O.C. MAX.) 4 HOOK CLAMPS EVENLY PLACED SEE DETAIL BELOW PLAN VIEW NTS SECTION A-A NTS DETAIL HOOK CLAMP NTS NOTES: A A 45 ° 15° (TYP.)SEE N O TE 1 24" SEE NOTE 1 5.1.1 DETENTION PONDS 2022 City of Renton Surface Water Design Manual 6/22/2022 5-15 FIGURE 5.1.1.D PERMANENT SURFACE WATER CONTROL POND SIGN SECTION 5.1 DETENTION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-16 5.1.1.2 METHODS OF ANALYSIS Detention Volume and Outflow The volume and outflow design for detention ponds shall be in accordance with the performance requirements in Chapter 1 and the hydrologic analysis and design methods in Chapter 3. Restrictor orifice structure design shall comply with Section 5.1.4. Note: The design water surface elevation is the highest elevation that occurs in order to meet the required outflow performance for the pond. Detention Ponds in Infiltrative Soils Detention ponds may occasionally be sited on till soils that otherwise meet the basic criteria of “sufficient permeable soil” for a properly functioning infiltration system (see Section 5.2.1). These detention ponds have a surface discharge and may also utilize infiltration as a second pond outflow. Detention ponds sized with infiltration as a second outflow must meet all the requirements of Section 5.2 for infiltration ponds, including a soils report, performance testing, groundwater protection, presettling, and construction techniques. Detention ponds are not allowed in Zone 1 of the Aquifer Protection Area. Emergency Overflow Spillway Capacity The emergency overflow spillway weir section shall be designed to pass the 100-year runoff event for developed conditions assuming a broad-crested weir. The broad-crested weir equation for the spillway section in Figure 5.1.1.E, for example, would be: Q100 = C (2g)1/2 [2/3 LH3/2 + 8/15 (Tan ) H5/2] (5-1) where Q100 = peak flow for the 100-year runoff event (cfs) C = discharge coefficient (0.6) g = gravity (32.2 ft/sec2) L = length of weir (ft) H = height of water over weir (ft)  = angle of side slopes Assuming C = 0.6 and Tan  = 3 (for 3H:1V slopes), the equation becomes: Q100 = 3.21 (LH3/2 + 2.4 H5/2) (5-2) To find width L for the weir section, the equation is rearranged to use the computed Q100 and trial values of H (0.2 feet minimum): L = [Q100 / (3.21 H3/2)] - 2.4 H or 6 feet minimum (5-3) FIGURE 5.1.1.E SCHEMATIC REPRESENTATION OF A WEIR SECTION FOR EMERGENCY OVERFLOW SPILLWAY EMERGENCY OVERFLOW WS OVERFLOW WS ROCK LINING PER TABLE 4.4.1.A 0 1 1 33 L .7' MIN. .5' MIN. .2' MIN. H 5.1.2 DETENTION TANKS 2022 City of Renton Surface Water Design Manual 6/22/2022 5-17 5.1.2 DETENTION TANKS Detention tanks are underground storage facilities typically constructed with large diameter corrugated steel pipe. Schematic representations of detention tanks are shown in Figure 5.1.2.A and Figure 5.1.2.B. Schematic representations of control structures are shown in Section 5.1.4. 5.1.2.1 DESIGN CRITERIA General 1. Tanks shall be designed as flow-through systems with manholes in line (see Figure 5.1.2.A) to promote sediment removal and facilitate maintenance. Exception: Tanks may be designed as back-up systems if preceded by water quality facilities since little sediment should reach the inlet/control structure and low head losses can be expected because of the proximity of the inlet/control structure to the tank. 2. The detention tank bottom shall be located a minimum of 0.5 feet below the inlet and outlet to provide dead storage for sediment. 3. The minimum pipe diameter allowed for a detention tank is 36 inches. 4. Tanks larger than 36 inches may be connected to each adjoining structure with a short section (2-foot maximum length) of 36-inch minimum diameter pipe. 5. Outflow control structures shall be as detailed in Section 5.1.4. Note: Control and access manholes shall have additional ladder rungs to allow ready access to all tank access pipes when the catch basin sump is filled with water (see Figure 5.1.4.A, plan view). Materials Pipe material, joints, and protective treatment for tanks shall be in accordance with Sections 7.04 and 9.05 of the WSDOT/APWA Standard Specification as modified by the City of Renton Standard Details and AASHTO designations. Such materials include the following:  Lined corrugated polyethylene pipe (LCPE)  Aluminized Type 2 corrugated steel pipe and pipe arch (meets AASHTO designations M274 and M36)  Reinforced concrete pipe  Narrow concrete vaults (see Section 5.1.3).  Corrugated steel pipe and pipe arch, Aluminized or Galvanized3 with treatments 1, 2 or 5  Spiral rib steel pipe, Aluminized or Galvanized3 with treatments 1, 2 or 5  Structural plate pipe and pipe arch, Aluminized or Galvanized3 with treatments 1, 2 or 5 Structural Stability Tanks shall meet structural requirements for overburden support, buoyancy, and traffic loading if appropriate. H-20 live loads must be accommodated for tanks lying under parking areas, roadways, and access roads. Metal tank end plates must be designed for structural stability at maximum hydrostatic loading conditions. Flat end plates generally require thicker gage material than the pipe and/or require reinforcing ribs. Tanks shall be placed on stable, well consolidated native material with a suitable bedding. Backfill shall be placed and compacted in accordance with the pipe specifications in Chapter 4. Tanks made of LCPE 3 Galvanized metals leach zinc into the environment, especially in standing water situations. High zinc concentrations, sometimes in the range that can be toxic to aquatic life, have been observed in the region. Therefore, use of galvanized materials should be avoided. Where other metals, such as aluminum or stainless steel, or plastics are available, they shall be used. If these materials are not available, asphalt coated galvanized materials may then be used. SECTION 5.1 DETENTION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-18 require inspection for deformation prior to installation as well as continuous inspection of backfilling to one foot above the top of the tank. Tanks shall not be allowed in fill slopes, unless analyzed in a geotechnical report for stability and constructability. Buoyancy In moderately pervious soils where seasonal groundwater may induce flotation, buoyancy tendencies must be balanced either by ballasting with backfill or concrete backfill, providing concrete anchors, increasing the total weight, or providing subsurface drains to permanently lower the groundwater table. Calculations must be submitted that demonstrate stability. Access Requirements 1. The maximum depth from finished grade to tank invert shall be 20 feet. 2. Access risers required within 50 feet from any location in the detention tank and within 5 feet of each terminal end. Any location within the detention tank shall have a direct line of sight from an access point, unobstructed by any restrictions such as a wall of baffle. 3. All tank access openings shall have round, solid locking lids with 5/8-inch diameter Allen head cap screws (see City of Renton Standard Details). 4. Thirty-six-inch minimum diameter CMP riser-type manholes (Figure 5.1.2.B) of the same gage as the tank material may be used for access along the length of the tank and at the upstream terminus of the tank if a backup system. The top slab is separated (1-inch minimum gap) from the top of the riser to allow for deflections from vehicle loadings without damaging the riser tank. 5. All tank access openings must be readily accessible by maintenance vehicles. Access Roads Access roads are required to all detention tank control structures and risers. The access roads shall be designed and constructed as specified for detention ponds in Section 5.1.1. Right-of-Way Detention tanks to be maintained by the City shall be located in a stormwater tract granted and converted with all maintenance obligations (excluding maintenance of drainage facilities contained therein) to the homeowners association. If perimeter landscaping is required within the stormwater tract, then said tract shall be owned by the lot owners within the subdivision. Each lot owner shall have equal and undivided interest on the plat. Any tract not abutting public right-of-way will require a 15-foot wide extension of the tract to an acceptable access location. An underlying easement under and upon said tract shall be dedicated to the City for the purpose of operating, maintaining, improving and repairing the drainage facilities contain therein. The stormwater tract must be owned by the homeowners association. Each lot owner within the subdivision shall have an equal and undivided interest in the maintenance of the stormwater tract. Detention tanks to be maintained by a private property owner or homeowners association shall create stormwater facilities within a private tract or easement or construct the detention tank onsite. Setbacks Setbacks (easement/tract width) and building setback lines (BSBLs) for tanks shall be the same as for pipes (see Section 4.1). 5.1.2.2 METHODS OF ANALYSIS Detention Volume and Outflow The volume and outflow design for detention tanks shall be in accordance with the performance requirements in Chapter 1 and the hydrologic analysis and design methods in Chapter 3. Restrictor and orifice design shall be according to Section 5.1.4. 5.1.2 DETENTION TANKS 2022 City of Renton Surface Water Design Manual 6/22/2022 5-19 FIGURE 5.1.2.A SCHEMATIC REPRESENTATION OF A TYPICAL DETENTION TANK 2" MIN. DIAMETER AIR VENT PIPE WELDED TO TANK (REQUIRED IF NO ACCESS RISER ON TANK) "FLOW-THROUGH" SYSTEM SHOWN SOLID. DESIGNS FOR "FLOW BACKUP" SYSTEM AND PARALLEL TANKS SHOWN DASHED "FLOW-THROUGH" SYSTEM SHOWN SOLID. CONTROL STRUCTURE (FROP-T SHOWN) MIN. 54" DIA. TYPE 2 CB SEE SECTION 5.1.4 SECTION A-A NTS PLAN VIEW NTS 36" MIN. DIAMETER (TYP.) 0.5' SEDIMENT STORAGE ACCESS RISERS ACCESS RISERS (MAX SPACING SHOWN BELOW) ACCESS RISERS SEE FIGURE 5.1.2.B CONTROL STRUCTURE INLET PIPE (BACKUP SYSTEMS, WHERE ALLOWED) TYPE 2 CB REQUIRED FOR FLOW THROUGH SYSTEM ONLY INLET PIPE (FLOW THROUGH) MIN. DIAMETER SAME AS INLET PIPE OPTIONAL PARALLEL TANK NOTES: ALL METAL PARTS CORROSION RESISTANT. STEEL PARTS STAINLESS STEEL OR ALUMINIZED STEEL, EXCEPT TANK MAY BE GALVANIZED AND ASPHALT COATED (TREATMENT 1 OR BETTER). 100' MAX.50' MAX. 36" 36" FLOW A A 2' MAX. 2' MAX.2' MIN. 2' MIN. 2 MIN. DETENTION TANK SIZE AS REQUIRED SECTION 5.1 DETENTION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-20 FIGURE 5.1.2.B SCHEMATIC REPRESENTATION OF A DETENTION TANK ACCESS DETAIL 1. USE ADJUSTING BLOCKS AS REQUIRED TO BRING FRAME TO GUIDE. 2. ALL MATERIALS TO BE ALUMINUM OR GALVANIZED AND ASPHALT COATED (TREAMENT 1 OR BETTER), OR STAINLESS STEEL OR ALUMINIZED STEEL. 3. MUST BE LOCATED FOR ACCESS BY MAINTENANCE VEHICLES. 4. MAY SUBSTITUTE WSDOT SPECIAL TYPE IV MANHOLE (RCP ONLY). PLAN NTS SECTION NTS 36" 24" MAX. NOTES: WELD OR BOLT STANDARD M.H. STEPS M.H. STEPS 12" O.C. COMPACTED PIPE BEDDING STANDARD LOCKING M.H. FRAME & LID SEE KCRDCS DWG. NO. 7-022 DETENTION TANK RISER, 36" DIAM. MIN., SAME MATERIAL & GAGE AS TANK WELDED OR FUSED TO TANK MAINTAIN 1" GAP BETWEEN BOTTOM OF SLAB & TOP OF RISER - PROVIDE PLIABLE GASKET TO EXCLUDE DIRT FRAME WITH LOCKING LID (MARKED "DRAIN") MOUNTED OVER 24" DIAM. ECCENTRIC OPENING. ALIGN VERTICALLY WITH ACCESS LADDER TO PROVIDE 2-FOOT ACCESS CLEARANCE STANDARD TYPE 2-60" DIAM. CB CONCRETE TOP SLAB 36" CMP RISER 5.1.3 DETENTION VAULTS 2022 City of Renton Surface Water Design Manual 6/22/2022 5-21 5.1.3 DETENTION VAULTS Detention vaults are box-shaped underground storage facilities typically constructed with reinforced concrete. A schematic representation of a detention vault is shown in Figure 5.1.3.A. Schematic representations of a control structures are shown in Section 5.1.4. 5.1.3.1 DESIGN CRITERIA General 1. Detention vaults shall be designed as flow-through systems with bottoms level (longitudinally) or sloped toward the inlet to facilitate sediment removal. Distance between the inlet and outlet shall be maximized (as feasible). 2. The detention vault bottom shall slope at least 5% from each side towards the center, forming a broad “v” to facilitate sediment removal. Note: More than one “v” may be used to minimize vault depth. Exception: The vault bottom may be flat if removable panels are provided over the entire vault. Removable panels shall be at grade, have stainless steel lifting eyes, and weigh no more than 5 tons per panel. 3. The invert elevation of the outlet shall be elevated above the bottom of the vault to provide an average 6 inches of sediment storage over the entire bottom. The outlet must also be elevated a minimum of 2 feet above the orifice to retain oil within the vault. 4. The outflow system and restrictor device shall be designed according to the applicable requirements specified for control structures in Section 5.1.4. Materials Minimum 3,000 psi structural reinforced concrete must be used for all detention vaults. All construction joints must be provided with water stops. Structural Stability All vaults shall meet structural requirements for overburden support, buoyancy, and H-20 traffic loading. Cast-in-place wall sections shall be designed as retaining walls. Structural designs for vaults must be stamped by a licensed structural engineer unless otherwise approved by the City. Vaults shall be placed on stable, well-consolidated native material with suitable bedding. Vaults shall not be allowed in fill slopes, unless analyzed in a geotechnical report for stability and constructability. Access Requirements 1. Access consisting of a frame, grate and locking cover shall be provided over the inlet pipe and outlet structure and located in a manner to allow visual inspection. Access openings over control structures shall meet a minimum 2 ft. offset to any portion of the FROP-T as shown in Figure 5.1.4.A. Access openings shall be positioned a maximum of 50 feet from any location within the vault; additional access points may be required on large vaults. If more than one “v” is provided in the vault floor, access to each “v” must be provided. 2. For vaults with greater than 1250 square feet of floor area, a 5′ by 10′ removable, locking panel shall be provided. Alternatively, a separate access vault may be provided as shown in Figure 5.1.3.A. 3. For vaults under roadways, the removable panel must be located outside the travel lanes. Alternatively, multiple standard locking manhole covers (see City of Renton Standard Details) may be provided. Spacing of manhole covers shall be 12 feet, measured on center, to facilitate removal of sediment. Ladders and hand-holds need only be provided at the outlet pipe and inlet pipe, and as needed to meet OSHA confined space requirements. Vaults providing manhole access at 12-foot spacing need not provide corner ventilation pipes as specified in Item 9 below. SECTION 5.1 DETENTION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-22 4. All access openings, except those covered by removable panels, shall have round, solid locking covers (see City of Renton Standard Details), or 3-foot square, locking diamond plate covers. For raised openings where the depth from the iron cover to the top of the vault exceeds 24 inches, an access structure equivalent to a Type 2 catch basin or Type 1 manhole shall be used (see City of Renton Standard Details). The opening in the vault lid need not exceed 24 inches in diameter. 5. Vaults with widths 10 feet or less must have removable lids. 6. The maximum depth from finished grade to the vault invert shall be 20 feet. 7. Internal structural walls of large vaults shall be provided with openings sufficient for maintenance access between cells. The openings shall be sized and situated to allow access to the maintenance “v” in the vault floor. 8. The minimum internal height shall be 7 feet from the highest point of the vault floor (not sump), and the minimum width shall be 4 feet. Exceptions:  Concrete vaults may be a minimum 3 feet in height and width if used as tanks with access manholes at each end, and if the width is no larger than the height.  The minimum internal height requirement may be waived for any areas covered by removable panels. 9. Ventilation pipes (minimum 12-inch diameter or equivalent) shall be provided in all four corners of vaults to allow for artificial ventilation prior to entry of maintenance personnel into the vault. These openings shall be capped or otherwise covered, but designed so that maintenance personnel can remove (and replace) for ventilation purposes as described. Access Roads Access roads are required to the access panel (if applicable), the control structure, and at least one access point per cell, and they shall be designed and constructed as specified for detention ponds in Section 5.1.1. Right-of-Way Detention vaults to be maintained by the City shall be in a stormwater tract granted and converted with all maintenance obligations (excluding maintenance of drainage facilities contained therein) to the homeowners association. Each lot owner shall have equal and undivided interest on the plat granted and converted with all maintenance obligations (excluding maintenance of drainage facilities contained therein) to the homeowners association. Any tract not abutting public right-of-way will require a 15-foot- wide extension of the tract to an acceptable access location. An underlying easement under and upon said tract shall be dedicated to the City for the purpose of operating, maintaining, improving and repairing the drainage facilities contain therein. The stormwater tract must be owned by the homeowners association. Each lot owner within the subdivision shall have an equal and undivided interest in the maintenance of the stormwater tract. Detention vaults to be maintained by a private property owner or homeowners association shall create stormwater facilities within a private tract or easement or construct the detention vault onsite. Setbacks Setbacks to tract/easement lines for vaults shall be 5 feet; adjacent building setback lines shall be 10 feet. For privately owned and maintained vaults, building foundations may serve as one or more of the vault walls. 5.1.3 DETENTION VAULTS 2022 City of Renton Surface Water Design Manual 6/22/2022 5-23 5.1.3.2 METHODS OF ANALYSIS Detention Volume and Outflow The volume and outflow design for detention vaults shall be in accordance with the performance requirements in Chapter 1 and the hydrologic analysis and routing/design methods in Chapter 3. Restrictor and orifice design shall be according to Section 5.1.4. SECTION 5.1 DETENTION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-24 FIGURE 5.1.3.A SCHEMATIC REPRESENTATION OF A TYPICAL DETENTION VAULT FLOOR GRATE WITH 2' X 2' HINGED ACCESS DOOR. (1" X 14" METAL BARS), STAINLESS STEEL OR ALUMINIZED STEEL CAPACITY OF OUTLET PIPE NOT LESS THAN DEVELOPED 100 - YR DESIGN FLOW FLOW RESTRICTOR DESIGN W.S. HANDHOLDS, STEPS OR LADDER SEE KCRDCS DWG 7-011 12" 12" WALL FLANGE (TYP.) FRAMES, GRATES AND ROUND SOLID COVERS MARKED "DRAIN" WITH LOCKING BOLTS. SEE KCRDCS DWGS. 7-022, 7-023 FOR SPECIFICATION 5' X 10' OPENING FOR VAULTS 1250 SF OR GREATER FLOOR AREA 5% 5% NOTE: ALL VAULT AREAS MUST BE WITHIN 50' OF AN ACCESS POINT OUTLET PIPE 1. ALL METAL PARTS MUST BE CORROSION RESISTANT. STEEL PARTS MUST BE STAINLESS STEEL OR ALUMINIZED STEEL. 2. PROVIDE WATER STOP AT ALL CAST-IN-PLACE CONSTRUCTION JOINTS. PRECAST VAULTS SHALL HAVE APPROVED RUBBER GASKET SYSTEM. 3. VAULTS <10' WIDE MUST USE REMOVABLE LIDS. 4. PREFABRICATED VAULT SECTIONS MAY REQUIRE STRUCTURAL MODIFICATIONS TO SUPPORT 5' X10' OPENING OVER MAIN VAULT. ALTERNATIVELY, ACCESS CAN BE PROVIDED VIA A SIDE VESTIBULE AS SHOWN. 5. IF SUMP IS SET BACK FROM WALL, TEE WILL REQUIRE ADDITIONAL 3-POINT BRACING SECURED TO VAULT WALL. FLOW FLOW PROVIDE ACCESS ADJACENT TO TEE. ASSURE TEE IS VIEWABLE FROM SURFACE. 10' 4' min. PLAN VIEW NTS SECTION A-A NTS FLOW A A FLOW NOTES: 6" SEDIMENT STORAGE 6" MIN. 5' V-SHAPED BOTTOM OPTIONAL 5' X 10' ACCESS VAULT MAY BE USED IN LIEU OF TOP ACCESS 2' MIN. 2' MIN. 5.1.4 CONTROL STRUCTURES 2022 City of Renton Surface Water Design Manual 6/22/2022 5-25 5.1.4 CONTROL STRUCTURES Control structures are catch basins or manholes with a restrictor device for controlling outflow from a facility to meet the desired performance. The restrictor device is typically a tee section with an orifice plate welded to the bottom (called a “FROP-T”). To meet performance requirements, one or more elbow sections with orifice plates may need to be mounted on the side of the tee section. The restrictor device may also be a weir section sized to meet performance requirements. Schematic representations of control structures are shown in Figure 5.1.4.A through Figure 5.1.4.C. 5.1.4.1 DESIGN CRITERIA Multiple Orifice Restrictor In most cases, control structures need only two orifices: one at the bottom and one near the top of the riser, although additional orifices may best utilize detention storage volume. Several orifices may be located at the same elevation if necessary to meet performance requirements. 1. Minimum orifice diameter is 0.25 inches. Note: In some instances, a 0.25-inch bottom orifice may be too large to meet target release rates, even with minimal head. In these cases, the live storage depth need not be reduced to less than 3 feet to meet performance. 2. Orifices shall be constructed on a tee section as shown in Figure 5.1.4.A or on a baffle as shown in Figure 5.1.4.B. 3. In some cases, performance requirements may require the top orifice/elbow to be located too high on the riser to be physically constructed (e.g., a 13-inch diameter orifice positioned 0.5 feet from the top of the riser). In these cases, a notch weir in the riser pipe may be used to meet performance requirements (see Figure 5.1.4.E). 4. Consideration shall be given to the backwater effect of water surface elevations in the downstream conveyance system. High tailwater elevations may affect performance of the restrictor system and reduce live storage volumes. Riser and Weir Restrictor 1. Properly designed weirs may be used as flow restrictors (see Figure 5.1.4.C and Figure 5.1.4.E through Figure 5.1.4.F). However, they must be designed to provide for primary overflow of the developed 100-year peak flow discharging from the detention facility. 2. The combined orifice and riser (or weir) overflow may be used to meet performance requirements; however, the design must still provide for primary overflow of the developed 100-year peak flow assuming all orifices are plugged. Figure 5.1.4.H may be used to calculate the head in feet above a riser of given diameter and flow. Access Requirements 1. An access road to the control structure is required for inspection and maintenance, and shall be designed and constructed as specified for detention ponds in Section 5.1.1. 2. Manhole and catch basin lids for control structures shall be locking, and rim elevations shall match proposed finish grade. 3. The restrictor tee shall be located immediately adjacent to the 2-foot clear zone at a maintenance access ladder. Intent: To provide tee visibility from the surface at the access opening, especially where a solid vault lid or solid manhole lid design may block view; to provide maintenance access along the full height of the tee. SECTION 5.1 DETENTION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-26 FIGURE 5.1.4.A SCHEMATIC REPRESENTATION OF A FLOW RESTRICTOR (TEE) ISOMETRIC NTS SECTION A-A NTS PLAN VIEW NTS ELBOW RESTRICTOR DETAIL NTS 1. USE A MIMIMUM OF A 54" DIAMETER TYPE 2 CATCH BASIN. 2. OUTLET CAPACITY: 100-YEAR DEVELOPED PEAK FLOW. 3. METAL PARTS: CORROSION RESISTANT. STAINLESS STEEL OR ALUMINIZED STEEL. 4. FRAME AND LADDER OR STEPS OFFSET SO: A. CLEANOUT GATE IS VISIBLE FROM TOP. B. CLIMB-DOWN SPACE IS CLEAR OF RISER AND CLEANOUT GATE. C. FRAME IS CLEAR OF CURB. 5. IF METAL OUTLET PIPE CONNECTS TO CEMENT CONCRETE PIPE: OUTLET PIPE TO HAVE SMOOTH O.D. EQUAL TO CONCRETE PIPE I.D. LESS 1/4". 6. PROVIDE AT LEAST ONE 3" X .090 GAGE SUPPORT BRACKET ANCHORED TO CONCRETE WALL. (MAXIMUM 3'-0" VERTICAL SPACING) 7. LOCATE ELBOW RESTRICTOR(S) AS NECESSARY TO PROVIDE MINIMUM CLEARANCE AS SHOWN. 8. LOCATE ADDITIONAL LADDER RUNGS IN STRUCTURES USED AS ACCESS TO TANKS AND VAULT TO ALLOW ACCESS WHEN CATCH BASIN IS FILLED WITH WATER. 9. TEE SHALL BE CONSTRUCTED OF ALUMINUM CMP OR ALUMINIZED STEEL CMP MEETING WSDOT/APWA STANDARDS. NOTES: ADDITIONAL LADDER RUN (IN SETS) TO ALLOW ACCESS TO TANKS OR VAULTS WHEN CATCH IS FILLED WITH WATER. 2' MIN. CLEARANCE TO ANY PORTION OF FROP-T INCLUDING ELBOWS RESTRICTOR PLATE WITH ORIFICE DIAMETER AS SPECIFIED (NOT NEEDED IF FOR SPILL CONTROL ONLY) INVERT AND ELEVATION PER PLANS PIPE SUPPORTS SEE NOTE 6 ELBOW RESTRICTOR SEE DETAIL ELEVATION PER PLANS PLATE WELDED TO ELBOW WITH ORIFICE AS SPECIFIED REMOVABLE WATERTIGHT COUPLING OR FLANGE ANGLE AS NECESSARY SEE NOTE 7 ROD FOR CLEANOUT/DRAIN (ROD BENT AS REQUIRED FOR VERTICAL ALIGNMENT WITH COVER) SEE KCRDCS DWG. 7-026 HANDHOLDS, STEPS OR LADDER SEE KCRDCS DWG. 7-006 VERTICAL BAR GRATE FOR SECONDARY INLET INLET PIPE FRAME & SOLID COVER MARKED "DRAIN" WITH LOCKING BOLTS SEE NOTE 3 & KCRDCS DWGS 7-022, 7-023 12" 12" 12" A 6" MAX. DESIGN WATER SURFACE OUTLET PIPE SEE NOTES 1 & 5 1' MIN. UNDER PAVEMENT 6" MIN. 2' MIN. 1.5 x D MIN. D ELBOW RESTRICTOR SEE DETAIL 2 ' MI N .2' MIN.2' MIN.2" MIN. 6" MIN. 16" MAX. ACCESS ADJACENT TO TEE 5.1.4 CONTROL STRUCTURES 2022 City of Renton Surface Water Design Manual 6/22/2022 5-27 FIGURE 5.1.4.B SCHEMATIC REPRESENTATION OF A FLOW RESTRICTOR (BAFFLE) 1. OUTLET CAPACITY: 100 YEAR DEVELOPED PEAK FLOW. 2. METAL PARTS: CORROSION RESISTANT STEEL PARTS STAINLESS STEEL OR ALUMINIZED STEEL. 3. CATCH BASIN: TYPE 2 MINIMUM 72" DIAMETER TO BE CONSTRUCTED IN ACCORDANCE WITH KCRDCS DWG. 7-005 AND AASHTO M-199 UNLESS OTHERWISE SPECIFIED. 4. ORIFICES: SIZED AND LOCATED AS REQUIRED WITH LOWEST ORIFICE A MINIMUM OF 2' FROM BASE. ELBOW RESTRICTOR DETAIL NTS ISOMETRIC NTS PLAN VIEW NTS SECTION B-B NTS SECTION A-A NTS NOTES: ORIFICE PLATE 10 GAGE MINIMUM GALVANIZED STEEL WITH ORIFICE DIAMETER 1" MINIMUM LESS THAN DIAMETER OF CONCRETE HOLE FRAME AND ROUND SOLID COVER MARKED "DRAIN" WITH LOCKING BOLTS. SEE KCRDCS DWGS. 7-022, 7-023. FRAME ELEVATION PER PLANS DESIGN W.S.MAX W.S. OVERFLOW CONDITIONS ELBOW RESTRICTORS SEE DETAIL BELOW HANDHOLDS, STEPS OR LADDER SEE KCRDCS DWG. 7-011 ATTACH SHEAR GATE CONTROL ROD TO SUPPORT BRACKET ON INSIDE OF ACCESS OPENING SHEAR GATE WITH CONTROL ROD FOR DRAIN. SEE KCRDCS DWG. 7-026 REMOVABLE WATER-TIGHT COUPLING GROUTED PLATE WELDED TO ELBOW WITH ORIFICE AS SPECIFIED A A B B FLOW 6" MIN. 6" MIN. 6" MAX. 2' MIN. 2' MIN. 1' MIN. 1' MIN. UNDER PAVEMENT 16" MAX. 6" SECTION 5.1 DETENTION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-28 FIGURE 5.1.4.C SCHEMATIC REPRESENTATION OF A FLOW RESTRICTOR (WEIR) LOCATE ADDITIONAL LADDER RUNGS (IN SETS) TO ALLOW ACCESS TO TANKS OR VAULTS WHEN SUMP IS FILLED WITH WATER SHEAR GATE LOCATE HORIZONTAL FOR CLEARANCE WITH LADDER. ATTACH ROD TO SUPPORT BRACKET ON INSIDE OF ACCESS OPENING HANDHOLDS, STEPS OR LADDER (2 PLACES) SEE KCRDCS DWG. 7-006 FRAME ELEVATION PER PLANS OUTLET PIPE SHEAR GATE WITH CONTROL ROD FOR DRAIN. SEE KCRDCS DWG. 7-026 I.E. WEIR, INLET PIPE AND DRAIN = CROWN OUTLET PIPE DESIGN W.S. WEIR SHAPE AS NEEDED FOR PERFORMANCE FRAME AND ROUND SOLID COVER MARKED "DRAIN" WITH LOCKING BOLTS. SEE KCRDCS DWGS. 7-022, 7-023 1. OUTLET CAPACITY: 100-YEAR DEVELOPED PEAK FLOW. 2. METAL PARTS: CORROSION RESISTANT STEEL PARTS, STAINLESS STEEL OR ALUMINIZED STEEL. 3. CATCH BASIN: TYPE 2 MIN. 72" DIAMETER TO BE CONSTRUCTED IN ACCORDANCE WITH KCRDCS DWG 7-005 AND AASHTO M-199 UNLESS OTHERWISE SPECIFIED. 4. BAFFLE WALL: TO BE DESIGNED WITH CONCRETE REINFORCING AS REQUIRED. 5. SPILL CONTROL REQUIREMENTS: SEE SECTION 4.2.1 PIPE SYSTEMS - DESIGN CRITERIA, SPILL CONTROL NOTES: SECTION B-B NTS SECTION A-A NTS ISOMETRIC NTS PLAN VIEW NTS FLOW B B A A 6" MIN. 1' MIN. UNDER PAVEMENT W 2' MIN. 2' MIN. 5.1.4 CONTROL STRUCTURES 2022 City of Renton Surface Water Design Manual 6/22/2022 5-29 5.1.4.2 METHODS OF ANALYSIS This section presents the methods and equations for design of control structure restrictor devices. Included are details for the design of orifices, rectangular sharp-crested weirs, v-notch weirs, sutro weirs, and overflow risers. Orifices Flow through orifice plates in the standard tee section or turn-down elbow may be approximated by the general equation: Q = CA (5-4) where Q = flow (cfs) C = coefficient of discharge (0.62 for plate orifice) A = area of orifice (sf) h = hydraulic head (ft) g = gravity (32.2 ft/sec2) Figure 5.1.4.D illustrates a simplified application of the orifice equation, assuming a water surface at the top of the riser and that the 2-year water surface represents the head in the outlet pipe. FIGURE 5.1.4.D SIMPLE ORIFICE The diameter of the orifice is calculated from the flow. The orifice equation is often useful when expressed as the orifice diameter in inches: d = (5-5) where d = orifice diameter (inches) Q = flow (cfs) h = hydraulic head (ft) gh2 h Q88.36 h = DISTANCE FROM HYDRAULIC GRADE LINE AT THE 2-YEAR FLOW OF THE OUTFLOW PIPE TO THE OVERFLOW ELEVATION. = C Q = CA 2gh 2g h h(A + A ) t2gh b b b + CA t b t Q h b ht ORIFICE (t) ORIFICE (b) SECTION 5.1 DETENTION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-30 Rectangular, Sharp-Crested Weir The rectangular, sharp-crested weir design shown in Figure 5.1.4.E may be analyzed using standard weir equations for the fully contracted condition. FIGURE 5.1.4.E RECTANGULAR, SHARP-CRESTED WEIR Q = C (L - 0.2H)H3/2 (5-6) where Q = flow (cfs) C = 3.27 + 0.40 H/P (ft) H,P are as shown above L = length (ft) of the portion of the riser circumference as necessary not to exceed 50% of the circumference D = inside riser diameter (ft) Note that this equation accounts for side contractions by subtracting 0.1H from L for each side of the notch weir. V-Notch, Sharp-Crested Weir V-notch weirs, as shown in Figure 5.1.4.F, may be analyzed using standard equations for the fully contracted condition. SECTION NTS PLAN VIEW NTS P H LD RISER 5.1.4 CONTROL STRUCTURES 2022 City of Renton Surface Water Design Manual 6/22/2022 5-31 FIGURE 5.1.4.F V-NOTCH, SHARP-CRESTED WEIR Ratio of H/Y Where values of C may be taken from the following chart: Q = C Tan (0/2)H , in cfsd 52 SECTION A-A NTS d dC A A Y 0.2 0.4 0.6 0.8 2.9 2.8 2.7 2.6 2.5 2.4 60° 20° 45° 90° H 0 0 SECTION 5.1 DETENTION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-32 Proportional or Sutro Weir Sutro weirs are designed so that the discharge is proportional to the total head. This design may be useful in some cases to meet performance requirements. The sutro weir consists of a rectangular section joined to a curved portion that provides proportionality for all heads above the line A-B (see Figure 5.1.4.G). The weir may be symmetrical or non-symmetrical. FIGURE 5.1.4.G SUTRO WEIR For this type of weir, the curved portion is defined by the following equation (calculated in radians): = 1 - Tan-1 (5-7) where a, b, x and Z are as shown in Figure 5.1.4.G. The head-discharge relationship is: Q = Cd b Q = Cd b (5-8) Values of Cd for both symmetrical and non-symmetrical sutro weirs are summarized in Table 5.1.4.A. Note: When b > 1.50 or a > 0.30, use Cd = 0.6. b x  2 a Z ag2   31 ah SEE EQUATION BELOW TOTAL HEADDISCHARGE SYMMETRICAL NON-SYMMETRICAL SEE EQUATION BELOW A B x h1 b a Z x b a 5.1.4 CONTROL STRUCTURES 2022 City of Renton Surface Water Design Manual 6/22/2022 5-33 TABLE 5.1.4.A VALUES OF CD FOR SUTRO WEIRS Cd Values, Symmetrical b (ft) a (ft) 0.50 0.75 1.0 1.25 1.50 0.02 0.608 0.613 0.617 0.6185 0.619 0.05 0.606 0.611 0.615 0.617 0.6175 0.10 0.603 0.608 0.612 0.6135 0.614 0.15 0.601 0.6055 0.610 0.6115 0.612 0.20 0.599 0.604 0.608 0.6095 0.610 0.25 0.598 0.6025 0.6065 0.608 0.6085 0.30 0.597 0.602 0.606 0.6075 0.608 Cd Values, Non-Symmetrical b (ft) a (ft) 0.50 0.75 1.0 1.25 1.50 0.02 0.614 0.619 0.623 0.6245 0.625 0.05 0.612 0.617 0.621 0.623 0.6235 0.10 0.609 0.614 0.618 0.6195 0.620 0.15 0.607 0.6115 0.616 0.6175 0.618 0.20 0.605 0.610 0.614 0.6155 0.616 0.25 0.604 0.6085 0.6125 0.614 0.6145 0.30 0.603 0.608 0.612 0.6135 0.614 Riser Overflow The nomograph in Figure 5.1.4.H may be used to determine the head (in feet) above a riser of given diameter and for a given flow (usually the 100-year peak flow for developed conditions). SECTION 5.1 DETENTION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-34 FIGURE 5.1.4.H RISER INFLOW CURVES 1 10 100 0.1 1 10HEAD IN FEET (measured from crest of riser) Qweir=9.739 DH3/2 Qorifice=3.782 D2H1/2 Q in cfs, D and H in feet Slope change occurs at weir-orifice transition Q (cubic feet per second)18 21 24 27 30 42 487254 10 12 15 33 36 RISER DIAMETER (inches) 5.1.7 SIMPLE DETENTION POND FOR CLEARED AREAS 2022 City of Renton Surface Water Design Manual 6/22/2022 5-35 5.1.5 PARKING LOT DETENTION Private parking lots may be used to provide additional detention volume for runoff events greater than the 2-year runoff event provided all of the following conditions are met: 1. The depth of water detained does not exceed 1 foot at any location in the parking lot for runoff events up to and including the 100-year event. 2. The gradient of the parking lot area subject to ponding is 1 percent or greater. 3. The emergency overflow path is identified and noted on the engineering plan, and the path complies with Core Requirements #1 and #2 (see Sections 1.2.1 and 1.2.2). 4. Fire lanes used for emergency equipment are free of ponding water for all runoff events up to and including the 100-year event. Note: Flows may be backed up into parking lots by the control structure (i.e., the parking lot need not function as a flow-through detention pond). 5.1.6 ROOF DETENTION Detention ponding on roofs of structures may be used to meet flow control requirements provided all of the following conditions are met: 1. The roof support structure is analyzed by a structural engineer to address the weight of ponded water. 2. The roof area subject to ponding is sufficiently waterproofed to achieve a minimum service life of 30 years. 3. The minimum pitch of the roof area subject to ponding is 1/4-inch per foot. 4. An overflow system is included in the design to safely convey the 100-year peak flow from the roof. 5. A mechanism is included in the design to allow the ponding area to be drained for maintenance purposes or in the event the restrictor device is plugged. 5.1.7 SIMPLE DETENTION POND FOR CLEARED AREAS This simplified alternative to the standard detention pond (Section 5.1.1) may be used to satisfy the flow control facility requirement only for a conversion of forest to pasture or grass, provided that all of the following conditions are met: 1. The total area draining to any one pond must be no larger than 3 acres and must consist primarily of vegetated land (e.g., forest, meadow, pasture, grass, garden, crops, etc.) free of impervious surface. If more than 3 acres of cleared area (i.e., area converted from forest to pasture/grass) is proposed to be served, multiple simple detention ponds must be used. 2. The area served by the pond must not be located within a Flood Problem Flow Control Area as determined in Section 1.2.3.1. 3. The pond must not drain to a severe erosion problem or a severe flooding problem as defined in Section 1.2.2, Core Requirement #2. 4. The pond is not located with Zone 1 of the Aquifer Protection Area. 5. The pond must be constructed in accordance with the design criteria and methods of analysis specified in this section. 5.1.7.1 DESIGN CRITERIA Schematic representations of a simple detention pond are shown in Figure 5.1.7.A and Figure 5.1.7.B. SECTION 5.1 DETENTION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-36 General 1. A geotechnical analysis and report is required if located within 200 feet of a steep slope hazard area or landslide hazard OR if the facility is located within a setback distance from top of slope equal to the total vertical height of the slope area that is steeper than 15%. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built- out conditions. 2. The detention pond design water surface shall be a minimum of 200 feet from any steep slope hazard area or landslide hazard. Upon analysis and approval of a licensed geotechnical engineer or engineering geologist, this setback may be reduced to 50 feet. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built- out conditions. 3. The detention pond design water surface shall be set back a minimum distance from top of slope equal to the total vertical height of a slope area that is steeper than 15%. Upon analysis and approval of a licensed geotechnical engineer or engineering geologist, this setback may be reduced to 50 feet. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built- out conditions. 4. The dispersal trench at the outlet from the storage pond may not be placed closer than 50 feet from the top of slopes, 20% or greater. 5. The pond, berm, and dispersal trench must be fenced to prevent livestock disturbance. 6. Runoff discharge toward landslide hazard or steep slope hazard areas must be evaluated by a geotechnical engineer or a qualified geologist. The discharge point may not be placed on or above slopes greater than 20% or above erosion hazard areas without evaluation by a geotechnical engineer or qualified geologist and City approval. Berming and Excavation 1. To the extent feasible, the pond shall be excavated into the ground with minimal berming on the downslope (outlet) end of the pond. An excavated pond is easier to construct and maintain and is less likely to cause problems during severe storm events. 2. Where berms are used, the top of berm shall be a minimum of 3 feet wide. The soil shall be well compacted and planted with an erosion-control seed mix as soon as possible. 3. Whether created by excavation or berming, all pond side-slopes shall be gently sloped, no steeper than 3 feet horizontal per 1 foot of vertical drop. 4. Prior to constructing the berm, the underlying ground shall be scrapped clean of organic material. 5. At a minimum, a hand-level shall be used to ensure the berm and outlet structure are constructed at the correct relative elevations. 6. The bottom 6 inches of the pond shall retain standing water in the pond between storms to create a permanent pool. The volume of the permanent pool is not counted towards the required detention volume, which is above the permanent pool. 7. The water depth of required detention volume above the permanent pool should average about 18 inches and must be no deeper than 24 inches. Simple Outlet Control Structure 1. Materials Required: a) PVC pipe, 4 inch diameter or greater as needed. b) PVC pipe cap. c) Small plastic or concrete catch basin with grate, minimum 12-inch width. 5.1.7 SIMPLE DETENTION POND FOR CLEARED AREAS 2022 City of Renton Surface Water Design Manual 6/22/2022 5-37 2. Construction Method: a) Drill or cut a hole just below the rim of the catch basin, sized to connect the PVC pipe. b) Install the catch basin into the bottom of the pond. The catch basin should be located within a few feet of the berm at the downslope end of the pond. The top of catch basin must be a minimum of 6 inches above the bottom of the pond to create the permanent pool. Align the hole in the downslope direction of discharge. c) Dig a trench for the pipe from the catch basin to the location of the flow spreader. d) Connect the PVC pipe to the catch basin. PVC pipe should extend about 4 inches into the basin. e) Drill the appropriate size hole into the PVC cap. Clean hole to remove burrs, without increasing the size of the opening. f) Connect the drilled cap to the end of the PVC pipe extending into the catch basin. g) Extend the PVC pipe to the location of the flow spreader. The pipe shall be laid with a slight slope towards the flow spreader. A slope of ¼ inch per foot of pipe is recommended and should not exceed 2 inches per foot. h) Backfill the trench over the PVC pipe and compact well. Avoid placing large and/or sharp rocks in the trench to minimize potential for damaging the pipe during compaction. SECTION 5.1 DETENTION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-38 FIGURE 5.1.7.A SCHEMATIC REPRESENTATION OF A SIMPLE DETENTION POND – PLAN VIEW SLOPEPLAN VIEW NTS A A B B C C 3' MIN BERM TOP WIDTH 3H:1V MAX. SIDE SLOPE (TYP.) SPILLWAY SMALL CATCH BASIN FLOW SPREADER 5.1.7 SIMPLE DETENTION POND FOR CLEARED AREAS 2022 City of Renton Surface Water Design Manual 6/22/2022 5-39 FIGURE 5.1.7.B SCHEMATIC REPRESENTATION OF A SIMPLE DETENTION POND – SECTION VIEWS 4" 6" ALL SLOPES 3:1 OR FLATTER ALL SLOPES 3:1 OR FLATTER 10" HIGH BERM SECTION A-A NTS SECTION B-B NTS SECTION C-C NTS 3 1 3'6" 18" 6"6" 4" 6' 3 1 6" 10" BERM COMPACTED EARTHEN MATERIAL 8' BOARD 2" X 10" BOTTOM EMBEDDED 6" INTO THE GROUND FLOW SPREADER6'1'1' PVC OUTLET PIPE COMPACTED EARTHEN MATERIAL SPILLWAY 6" LOWER THAN BERM 4"4"4"4" CATCH BASIN GRATE GRASS TYPICAL OF ALL DISTURBED AREAS SMALL CATCH BASINNOTCHED 2" x 10" BY 8' LONG SPREADER BOARD 4" PVC PIPE CAP WITH DRILLED ORIFICE COMPACTED EARTHEN BERM 10" HIGH BERM NATURAL GROUND LINE END OF BOARD EMBEDDED 1' INTO BERM SECTION 5.1 DETENTION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-40 5.1.7.2 METHODS OF ANALYSIS The detention volume and orifice sizing for the simple detention pond shall be determined as described in this section. This determination is based on where the pond is located within the City and how much cleared area (i.e., area of forest converted to pasture or grass) is served by the pond. Detention Volume The map in Figure 5.1.7.C provides the minimum pond volume required based on 10,000 square feet of cleared area. To determine the total pond volume required, locate the project site on the map and multiply the number from the map by the amount of cleared area that will be served by the pond (if the cleared area is measured in units of square feet, remember to divide the actual area by 10,000 before multiplying with map value). If the project site is located between the lines shown on the map, select the larger of the two pond unit volumes associated with the lines. Do not interpolate the volume if located midway between two lines. To determine if the constructed pond has adequate storage, the pond area must be determined by field measurements. If all side slopes are at 3H:1V or flatter, the pond’s bottom area may be used to determine the pond volume, Vt, above the permanent pool using the following equation. The resulting volume, Vt, must be equal to or greater than the required volume determined from Figure 5.1.7.C. Vt = 1.5 Ab + 3.4 P (5-9) where Vt = total pond volume available (cu ft) Ab = bottom area of pond (sq ft) P = bottom perimeter of pond (ft) A more accurate volume determination can be made with field measurements and area calculations taken at two elevations. The first elevation at which the pond area is measured is at the top of the permanent pool. The second area measurement is taken at the overflow spillway elevation. Vt = d (5-10) where Vt = total pond volume available (cu ft) Aw = area of pond (sq ft) measured at the lowest elevation of the overflow spillway (Ab) Ab = area of pond (sq ft) measured at the top of the permanent pool d = depth of reservoir (ft) = 1.5 feet Orifice Sizing Table 5.1.7.A provides the orifice diameter to be drilled into the PVC cap. If the orifice diameter matches the PVC pipe diameter, no cap is required. Otherwise, the PVC pipe diameter must be greater than the required orifice diameter. Select the orifice diameter based on the cleared area tributary to the pond, interpolating between the values when designing for intermediate tributary acreage. 2 )(bwAA CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 5-41 FIGURE 5.1.7.C SIMPLE DETENTION POND – MINIMUM VOLUME SECTION 5.2 INFILTRATION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-42 TABLE 5.1.7.A SIMPLE DETENTION POND – UNIT VOLUME AND ORIFICE SIZE Rainfall Region Seatac West Seatac Landsburg West Landsburg East King County Unit Volume per Acre Cleared* 6300 cft 5400 cft 6300 cft 6500 cft 4700 cft** Acres Cleared Orifice Diameter (decimal inches and equivalent fractional inches, 1/16″ increments) 10,000 sq ft (0.23 ac) 0.4375″ (7/16″) 0.375″ (3/8″) 0.4375″ (7/16″) 0.5625″ (9/16″) 0.8125″ (13/16″) 0.25 ac 0.4375″ (7/16″) 0.375″ (3/8″) 0.4375″ (7/16″) 0.5625″ (9/16″) 0.8125″ (13/16″) 1 ac 0.875″ (7/8″) 0.75″ (3/4″) 0.875″ (7/8″) 1.1875″ 1-3/16″) 1.6875″ (1-11/16″) 2 ac 1.25″ (1-1/4″) 1.0625″ (1-1/16″) 1.25″ (1-1/4″) 1.6875″ (1-11/16″) 2.4375″ (2-7/16″) 3 ac 1.5625″ (1-9/16″) 1.3125″ (1-5/16″) 1.5625″ (1-9/16″) 2.0625″ (2-1/16″) 3.0″ (3″) * Unit Volume per acre is based on modeling cleared areas as pasture, assuming soil amendment requirements are met, and 1.5 feet of storage depth in pond with 3:1 side slopes ** Volume variability in regions of increasing rainfall reflects limited single-orifice riser efficiency at shallow storage depths, particularly in western regions where runoff peaks and volumes are smaller. HOW TO USE THIS TABLE:  Locate the project on Figure 5.1.7.C.  Design unit volume per acre cleared is selected from the larger of the two values (i.e., not interpolated) associated with the Rainfall Region isopluvials bracketing the project location.  Determine design volume by multiplying unit volume by cleared acres tributary to facility.  Select and interpolate the orifice diameter based on acreage cleared for the selected region unit volume. NOTE: Projects proposing to clear an acre or less may qualify for a flow rate increase exception and waiver of the flow control facility requirement per SWDM 1.2.3, Core Requirement #3. An engineering analysis specific to the project site or other approval from CED review staff is required to qualify for the exception. 5.1.8 ALTERNATIVE DETENTION SYSTEMS Manufactures have developed other systems that have properties in common with vaults and tanks, but that do not conform to the standards for those facility types. These systems may be approved by CED using suitable design standards adapted from the established standards for similar systems. 5.1.8.1 DESIGN CRITERIA General 1. Alternative detention systems shall be designed as flow-through systems to promote sediment removal and facilitate maintenance. 2. Outflow control structures shall be as detailed in Section 5.1.4. 5.2.1 GENERAL REQUIREMENTS FOR INFILTRATION FACILITIES 2022 City of Renton Surface Water Design Manual 6/22/2022 5-43 Access Requirements 1. The maximum depth from finished grade to invert shall be 20 feet. 2. Access openings required within 50 feet from any location in the facility and within 5 feet of each terminal end. Any location within the facility shall have a direct line of sight from an access point, unobstructed by any restrictions such as a wall of baffle. The facility must be able to be flushed without restriction from an access point. 3. All access openings, except those covered by removable panels, shall have round, solid locking covers (see City of Renton Standard Details), or 3-foot square, locking diamond plate covers. For raised openings where the depth from the iron cover to the top of the alternative detention system exceeds 24 inches, an access structure equivalent to a Type 2 catch basin or Type 1 manhole shall be used (see City of Renton Standard Details). The opening in the lid need not exceed 24 inches in diameter. 4. All access openings must be readily accessible by maintenance vehicles. Structural Stability, Access Roads, Right-of-Way, and Setbacks Alternative detention systems shall comply with the structural stability, access road, right-of-way, and setback criteria consistent with either detention tanks (Section 5.1.2) or detention vaults (Section 5.1.3), whichever is most similar to the alternative detention system. 5.1.8.2 METHODS OF ANALYSIS The volume and outflow design for alternative detention systems shall be in accordance with the performance requirements in Chapter 1 and the hydrologic analysis and routing/design methods in Chapter 3. Restrictor and orifice design shall be according to Section 5.1.4. SECTION 5.2 INFILTRATION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-44 (T his page intentionally left blan k. ) 5.2.1 GENERAL REQUIREMENTS FOR INFILTRATION FACILITIES 2022 City of Renton Surface Water Design Manual 6/22/2022 5-45 5.2 INFILTRATION FACILITIES This section presents the methods, criteria, and details for design and analysis of infiltration facilities. These facilities are used where soils are suitable for soaking the increased runoff from development into the ground. Such facilities usually have a detention volume component to allow for temporary storage of runoff while it is being infiltrated. This detention volume is typically dependent on the infiltration capacity of the soils and the required facility performance. There are five types of infiltration facilities allowed for use in complying with Core Requirement #3, “Flow Control”: infiltration ponds, infiltration tanks, infiltration vaults, infiltration trenches, and small infiltration basins. In general, ponds are preferred because of the ease of maintenance and the water quality treatment that surface soil and vegetation provide. Tanks and trenches are useful where site constraints prevent use of a pond, and small infiltration basins are simple to design but have limited uses. Infiltration facilities are not allowed in Zone 1 of the Aquifer Protection Area. The information presented in this section is organized as follows: Section 5.2.1, “General Requirements for Infiltration Facilities” Section 5.2.2, “Infiltration Ponds” “Design Criteria,” Section 5.2.2.1 “Methods of Analysis,” Section 5.2.2.2 Section 5.2.3, “Infiltration Tanks” “Design Criteria,” Section 5.2.3.1 “Methods of Analysis,” Section 5.2.3.2 Section 5.2.4, “Infiltration Vaults” “Design Criteria,” Section 5.2.4.1 “Methods of Analysis,” Section 5.2.4.2 Section 5.2.5, “Infiltration Trenches” “Design Criteria,” Section 5.2.5.1 “Methods of Analysis,” Section 5.2.5.2 Section 5.2.6, “Alternative Infiltration Systems” “Design Criteria,” Section 5.2.6.1 “Methods of Analysis,” Section 5.2.6.2 Section 5.2.7, “Small Infiltration Basins” “Design Criteria,” Section 5.2.7.1. 5.2.1 GENERAL REQUIREMENTS FOR INFILTRATION FACILITIES This section presents the design requirements generally applicable to all infiltration facilities. Included are the general requirements for determining acceptable soil conditions, determining infiltration rates, and providing overflow protection, spill control, presettling, groundwater protection, protection from upstream erosion, and construction. For site selection and design decisions, a geotechnical and hydrogeologic evaluation and report should be prepared by a licensed engineer with geotechnical and hydrogeologic experience, or a licensed geologist, SECTION 5.2 INFILTRATION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-46 hydrogeologist, or engineering geologist. The design engineer may utilize a team of certified or registered professionals in soil science, hydrogeology, geology, and other related fields.  SOILS The applicant must demonstrate through infiltration testing, soil logs, and the written opinion of a geotechnical professional that sufficient permeable soil exists at the proposed facility location to allow construction of a properly functioning infiltration facility. At a minimum, test pits or borings shall extend 5 feet below the bottom of the infiltration facility, and at least one test hole should reach the water table. If the water table is very deep, the test hole need not extend more than one-fourth the maximum width of the pond below the bottom of a pond, or more than 5 feet below the bottom of a tank. Measurements shall be made during the period when the water level is expected to be at a maximum (usually in late winter or early spring). Projects performing a groundwater mounding analysis may be required to provide more extensive subsurface exploration as described in the “Groundwater Mounding Analysis” section below. For projects that perform a groundwater mounding analysis that demonstrates the design is adequate and that overtopping does not occur, the basic requirement is a minimum of 3 feet of permeable soil below the bottom of the facility (bottom of pond or excavation for tank) and at least 3 feet between the bottom of the facility and the maximum wet-season water table. For projects that do not perform a groundwater mounding analysis as allowed and described in the “Design Infiltration Rate” section below, the basic requirement is a minimum of 5 feet of permeable soil below the bottom of the facility (bottom of pond or excavation for tank) and at least 5 feet between the bottom of the facility and the maximum wet-season water table. Any requirements associated with impacts to an erosion hazard area, steep slope hazard area, or landslide hazard should also be addressed in the soil study. The geotechnical professional shall provide a report stating whether the location is suitable for the proposed infiltration facility, and shall recommend a design infiltration rate (see “Design Infiltration Rate” below).  MEASURED INFILTRATION RATES Infiltration rate tests are used to help estimate the maximum sub-surface vertical infiltration rate of the soil below a proposed infiltration facility (e.g., pond or tank); an infiltrative BMP serving either more than one lot, 10,000 square feet or more of impervious surface, 3/4 acre or more of pervious surface or 5,000 square feet or more of pollution generating impervious surface; any BMP explicitly modeled to accomplish LID Performance Standard criteria (see Section 1.2.9); or a closed depression. The tests are intended to simulate the physical process that will occur when the facility is in operation; therefore, a saturation period is required to approximate the soil moisture conditions that may exist prior to the onset of a major winter runoff event. Testing Procedure 1. Excavations shall be made to the bottom elevation of the proposed infiltration facility. The measured infiltration rate of the underlying soil shall be determined using one of the following: a small or large scale Pilot Infiltration Test (PIT) as described in the 2014 Stormwater Management Manual for Western Washington and Reference Section 6-A of this manual. The PIT tests have been shown to more closely match actual full-scale facility performance than other test methods. A single ring percolation test using a ring at least 3 feet in diameter (see Reference Section 6-A), may be used to determine BMP infiltration rates used to demonstrate compliance with the LID Performance Standard. 2. The test hole or apparatus shall be filled with water and maintained at depths above the test elevation for the saturation periods specified for the appropriate test. 3. Following the saturation period, the rate shall be determined in accordance with the specified test procedures, with a head of 6 inches of water. 5.2.1 GENERAL REQUIREMENTS FOR INFILTRATION FACILITIES 2022 City of Renton Surface Water Design Manual 6/22/2022 5-47 4. The design engineer shall perform sufficient tests at multiple locations in a proposed facility footprint to determine a representative infiltration rate. At least one test per 5,000 square feet (or fraction thereof) of proposed facility footprint shall be performed, with a minimum of one test for each proposed infiltration facility location; and at least 2 tests per acre shall be performed for a closed depression. Proposed bioretention swales require a minimum of 1 test per 200 linear feet of swale; with a minimum of one test performed per site. Proposed bioretention facilities require a minimum of 1 test per 5,000 square feet of facility footprint; with a minimum of one test performed per site. 5. At a minimum, a soils log shall be obtained for each required infiltration test location. Additional tests shall be obtained as necessary to capture significant soil variations in the facility footprint. Soils shall be logged for a minimum of 5 feet below the bottom of each proposed infiltration facility. The logs shall describe the SCS series of the soil, indicate the textural class of the soil horizons throughout the depth of the log, note any evidence of high groundwater level (such as mottling), and estimate the maximum groundwater elevation, if within the limits of the log.  DESIGN INFILTRATION RATE- INFILTRATION FACILITIES AND CLOSED DEPRESSIONS In the past, many infiltration facilities have been built that have not performed as the designer intended. This has resulted in flooding and substantial public expenditures to correct problems. Monitoring of actual facility performance has shown that the full-scale infiltration rate is far lower than the rate determined by small-scale testing. Actual measured facility rates of 10% of the small-scale test rate have been seen. It is clear that great conservatism in the selection of design rates is needed, particularly where conditions are less than ideal. The design infiltration rate determination shall include a groundwater mounding evaluation using an analytical groundwater model to investigate the effects of the local hydrologic conditions on facility performance. Groundwater modeling will not be required for facilities serving less than 1 acre of tributary area and where there is at least 5 feet of separation between the bottom of the proposed facility and the maximum seasonal groundwater table or low permeability stratum, unless requested by CED review staff, or as part of an analysis in the event of facility failure at performance testing. A ground water mounding analysis is advisable for facilities with drainage areas smaller than 1 acre if the depth to a low permeability layer (e.g., less than 0.1 inches per hour) is less than 10 feet. If the ground water in the area is known to be greater than 50 feet below the proposed facility, detailed investigation of the ground water regime for flow control design is not necessary. The preliminary design infiltration rate is determined by applying correction factors to the measured infiltration rate. The correction factors account for uncertainties in testing, depth to the water table or impervious strata, infiltration receptor geometry, and long-term reductions in permeability due to biological activity and accumulation of fines. Equation 5-11 has been developed to account for these factors. This equation estimates the maximum design infiltration rate (Idesign); additional reduction in rate beyond that produced by the equation may be appropriate. Note that the design infiltration rate Idesign must not exceed 20 inches/hour. Idesign = Imeasured x Ftesting x Fgeometry x Fplugging (5-11) Correction factor Ftesting accounts for uncertainties in the testing methods. For the small and large scale Pilot Infiltration Test (PIT), Ftesting = 0.50. For the Single Ring Percolation Test (See Reference Section 6-A) (used only for determining BMP infiltration rates for demonstrating compliance with the LID Performance Standard), Ftesting = 0.30. When expanding an existing infiltration facility, the historical full-scale infiltration performance of the existing facility may be considered in lieu of the testing procedures above. However, determination of Ftesting for the expanded facility shall include consideration of the existing facility and site characteristics, existing infiltration performance relative to the original design, facility maintenance and site maintenance history, and any other factors influencing the performance of the existing facility. A value for Ftesting between 0.5 and 1.0, as determined by CED review staff, reflecting the existing facility history shall be applied to the historical full-scale measured infiltration rate. SECTION 5.2 INFILTRATION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-48 Fgeometry accounts for the influence of facility geometry and depth to the water table or impervious strata on the actual infiltration rate. A shallow water table or impervious layer will reduce the effective infiltration rate of a large pond, but this will not be reflected in a small scale test. Clearly, a large pond built over a thin pervious stratum with a shallow water table will not function as well as the same pond built over a thick pervious stratum with a deep water table. Fgeometry must be between 0.25 and 1.0 as determined by the following equation: Fgeometry = 4 D/W + 0.05 (5-12) where D = depth from the bottom of the proposed facility to the maximum wet-season water table or nearest impervious layer, whichever is less W = width of the facility Note: When conducting a mounding analysis, apply Fgeometry in the mounding analysis only if facility geometry is not captured in the groundwater model inputs. Fplugging accounts for reductions in infiltration rates over the long term due to plugging of soils. This factor is:  0.7 for loams and sandy loams  0.8 for fine sands and loamy sands  0.9 for medium sands  1.0 for coarse sands or cobbles, or any soil type in an infiltration facility preceded by a water quality facility.  DESIGN INFILTRATION RATE – BIORETENTION AND PERMEABLE PAVEMENT For bioretention facilities used to meet the LID Performance Standard, a corrected design infiltration rate shall be used for the standard bioretention soil mix (BSM) cited in Reference Section 11-C. The corrected rate assumes a correction factor of either 2 or 4 is applied to the standard BSM uncorrected rate of 12 inches per hour. A corrected design rate of 3 inches per hour is used where the drainage area to the bioretention device exceeds any of the following:  10,000 sq. ft. of impervious surface  5,000 sq. ft. of pollution-generating impervious surface  3/4 acre of pervious surface A corrected BSM design rate of 6 inches per hour is used if the contributing drainage area does not exceed any of the above-listed areas, OR for bioretention where the contributing area exceeds any of the thresholds above AND the design includes a presettling facility for solids removal. The design rate of the in situ soils underlying the bioretention soil mix shall be the measured infiltration rate multiplied by a correction factor ranging from 0.33 to 1 as recommended by a geotechnical professional. The selected correction factor should be based on the number of tests in relation to the size of the bioretention facility and site variability. For permeable pavement used to meet the LID Performance Standard, the design rate of the in situ soils underlying the permeable pavement shall be the measured infiltration rate multiplied by a correction factor ranging from 0.33 to 1 (no correction) as recommended by a geotechnical professional. The selected correction factor should be based on the number of tests in relation to the size of the permeable pavement and site variability. A further correction factor of 0.9 to 1 (no correction) is determined based on the quality of the aggregate base material. A correction factor of 1 for the quality of pavement aggregate base material is allowable if the aggregate base is clean washed material with 1% or less fines passing the 200 sieve. 5.2.1 GENERAL REQUIREMENTS FOR INFILTRATION FACILITIES 2022 City of Renton Surface Water Design Manual 6/22/2022 5-49  GROUNDWATER MOUNDING ANALYSIS Groundwater mounding analysis is generally required for infiltration facilities that serve 1 acre or more of tributary area and have less than 15 feet of separation to a restrictive layer or groundwater table, as described in the “Design Infiltration Rate” section above. Groundwater modeling (mounding analysis) of the proposed infiltration facility shall be done using the design infiltration rate (i.e., reduction factors applied to the measured rate) modified to exclude the correction factor for geometry (Fgeometry) and the estimated maximum groundwater elevation determined for the proposed facility location. It is assumed the groundwater mounding model inputs will capture the facility geometry for the analysis, however if this is not true for the chosen model, the correction factor for geometry shall be included in the infiltration rate. Note the use of the design infiltration rate (rather than the measured rate) results in a conservative analysis of the pond design, but may not be representative of the lateral extent of the actual groundwater mounding effect. The design professional is advised to evaluate the true extent of the mound and its effects on adjacent structures, properties, etc. MODRET or an equivalent model must be used unless CED approves an alternative analytic technique. More complex analyses (e.g., MODFLOW) may warrant preliminary discussion with CED to ensure the modeling strategies are acceptable. Developed condition hydrographs of the project site shall be exported from the approved model for the groundwater mounding analysis. Hydrographs for the mounding analysis input shall include, at a minimum, the complete water year (October 1 through September 30) records containing a) the 100-year peak rate event and b) the cumulative highest 30-day volume event identified through analysis of the developed condition runoff (the two events are usually in different water years). The peak rate water year is readily determined from the flow frequency analysis in the approved model. The cumulative highest 30-day volume analysis can be completed in a spreadsheet using the developed condition hydrograph for the full historical record exported from the approved model. Due to model limitations on the size of the input files, a 1-hour timestep shall be used to generate the hydrographs to be exported, unless otherwise required by CED. The exported hydrograph file will require minor modification in preparation for import into the groundwater model; see the specific model’s documentation for guidance (MODRET file preparation for hydrograph input is described in the appendix for the software user’s guide). See Reference Section 6-D for modeling guidelines specific for use with this manual. Note that an iterative process may be required beginning with an estimated design rate, facility sizing with the approved runoff model, then groundwater model testing. The mounding analysis report shall be included in the Special Reports section of the technical information report (TIR, see Section 2.3.1.1). All mounding analysis submittals shall have at least the following information in one package:  Test pit and boring logs, including actual elevations used on the design plans (not just relative elevations) documenting subsurface explorations to a depth below the base of the infiltration facility of at least 5 times the maximum design depth of ponded water proposed for the infiltration facility, but not less than 10 feet below the base of the facility. At sites with shallow ground water (less than 15 feet from the estimated base of facility), if a ground water mounding analysis is necessary, determine the thickness of the saturated zone. Note that documentation of the thickness and location of the saturated zone can generally be beneficial to mounding analysis results.  Logs must include at a minimum, depth of pit or hole, soil descriptions, depth to ground water table and/or bedrock/impermeable layers, presence of stratification. (Note: Logs must substantiate whether stratification does or does not exist. The licensed professional may consider additional methods of analysis to substantiate the presence of stratification that will significantly impact the design of the infiltration facility).  Continuous sampling (representative samples from each soil type and/or unit within the infiltration receptor) to a depth below the base of the infiltration facility of 2.5 times the maximum design ponded SECTION 5.2 INFILTRATION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-50 water depth, but not less than 10 feet. For large infiltration facilities serving drainage areas of 10 acres or more, perform soil grain size analyses on layers up to 50 feet deep (or no more than 10 feet below the water table).  Map showing location of test pits, borings and infiltration facility  Wet season (October 1 to April 30) maximum water table elevation. Monitoring through at least one wet season is required, unless substantially equivalent site historical data regarding ground water levels is available.  If mottling or iron oxide staining is present, and that elevation does not reflect the wet season maximum water table elevation, include a detailed justification.  Description and documentation supporting all modeling input parameters  LS stamped letter documenting constructed volume, elevations, infiltration area (constructed facilities only)  PE stamped letter documenting TIR volume, elevations and infiltration area (design reviews only)  PE stamped letter (may be the same letter as the previous bullet) documenting rainfall data and infiltration rate determination used in the analysis. Rainfall data shall be at a minimum, the complete water year (October 1 through September 30) records containing a) the 100-year peak rate event and b) the cumulative highest volume event identified through analysis of the developed condition runoff, both using 1-hour timesteps minimum. Infiltration rate description shall include the initial measured rate and details of the reduction factors applied per Section 5.2.1, Design Infiltration Rate.  Actual inflow data (electronic files prepared for model input) used in the mounding analysis modeling runs.  Separate model runs for the peak rate and highest 30-day cumulative volume periods (two runs unless the events occur in the same water year).  Justifications for safety factors applied to the infiltration rate applied in the modeling.  Geotechnical professional summary and conclusions  Small scale infiltration test data (inches/hour) with calibration factor for test type, then converted to Vertical Hydraulic Conductivity (feet/day)  Geotechnical professional documentation of why a particular Horizontal Hydraulic Conductivity to Vertical Hydraulic Conductivity (HHC:VHC) ratio is applicable. Without detailed justification, the City will accept for MODRET input an HHC:VHC ratio of 1.5:1 for homogeneous soils and 3:1 for layered soils. Note, however, the vertical conductivity input KVU is for the unsaturated condition (typical of small-scale or PIT test results), while the horizontal conductivity input KHS is for the saturated condition. Alternatively, if small-scale or PIT is the only test information available, the saturated horizontal hydraulic conductivity could be estimated by applying two adjustment factors as follows:4 KVS (vertical, saturated) = 1.5 KVU (vertical, unsaturated) (5-13) KHS (horizontal, saturated) = 1.5 KVS (vertical, saturated) (5-14)  PERFORMANCE TESTING Performance testing and verification for a facility shall be conducted before final construction approval by the City, or prior to construction of other project improvements or recording of a subdivision as required by RMC 4-4-060. For projects where a mounding analysis is not required at the design phase (i.e., facilities serving less than 1 acre of tributary area and where there is at least 5 feet of separation between the bottom of the proposed facility and the maximum seasonal groundwater table or low permeability stratum), the completed facility 4 Source: State of Florida Dept. of Transportation, Stormwater Management Facility Drainage Handbook, Jan 2004, p. 70 5.2.1 GENERAL REQUIREMENTS FOR INFILTRATION FACILITIES 2022 City of Renton Surface Water Design Manual 6/22/2022 5-51 must be tested and monitored to demonstrate that the facility performs as designed. If the facility performance is not satisfactory, the facility will need to be modified or expanded as needed in order to make it function as designed. Where a groundwater mounding analysis was used in the design, performance testing and verification in the bottom of the facility to demonstrate that the soils in the constructed facility are representative of the design assumptions is required. The evaluation shall include measured infiltration rate testing and evaluation of in-situ soil characteristics and groundwater table location as described in this section. The measured infiltration rate test procedure should follow the same methodology as during the design phase to be comparable. If the facility performance evaluation is not satisfactory, the facility will need to be modified or expanded as needed in order to make it function as designed.  100-YEAR OVERFLOW CONVEYANCE An overflow route shall be identified for stormwater flows that overtop the facility when infiltration capacity is exceeded or the facility becomes plugged and fails. The overflow route must be able to safely convey the 100-year developed peak flow to the downstream conveyance system or other acceptable discharge point in accordance with conveyance requirements in Section 1.2.4. Where the entire project site is located within a closed depression (such as some gravel pits), the requirement to identify and analyze a 100-year overflow pathway may be waived by CED if (1) an additional correction factor of 0.5 is used in calculating the design infiltration rate, (2) the facility is sized to fully infiltrate the 100-year runoff event, and (3) the facility is not bermed on any side. Intent: to address situations where the infiltration facility may be a highly permeable onsite closed depression, such as a gravel pit, where all stormwater is currently, and will remain, fully infiltrated.  SPILL CONTROL DEVICE All infiltration facilities must have a spill control device upstream of the facility to capture oil or other floatable contaminants before they enter the infiltration facility. The spill control device shall be a tee section per Figure 5.1.4.A or an equivalent device approved by CED. If a tee section is used, the top of the riser shall be set above the 100-year overflow elevation to prevent oils from entering the infiltration facility.  PRESETTLING Presettling must be provided before stormwater enters the infiltration facility. This requirement may be met by either of the following:  A water quality facility from the Basic WQ menu (this alternative is recommended; see Section 6.1.1 for facility options).  A presettling pond or vault with a treatment volume equal to 0.25 times the basic water quality design volume (see Section 6.4.1.1 for information on computing this volume). If water in the WQ facility or presettling facility will be in direct contact with the soil, the facility must be lined according to the liner requirements in Section 6.2.4. If the presettling facility is a vault, design of the vault shall be the same as required for presettling cells in sand filter vaults (see Section 6.5.3.2). The settling pond or vault shall be designed to pool water 4 to 6 feet deep with an overflow capacity sufficient to pass the developed 100-year peak flow. Settling facilities must have a length-to-width ratio of at least 3:1. The inlet(s) and outlet should be situated to maximize the length of travel through the settling pond or vault. Berms or baffles may be used to lengthen the travel distance if site constraints limit the inlet/outlet placement. Inlets should be designed to minimize velocity and turbulence. SECTION 5.2 INFILTRATION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-52  PROTECTION FROM UPSTREAM EROSION Erosion must be controlled during construction of areas upstream of infiltration facilities since sediment- laden runoff can permanently impair the functioning of the system. Erosion control measures must be designed, installed and maintained with great care. Various strategies may be employed to protect infiltration facilities during construction, as described below. Projects may be phased to limit clearing and minimize the time that soils are exposed. An alternative to this approach is to serve the undeveloped area with a large sediment trap on an undeveloped tract with the trap left in place until all clearing and construction is complete and all permanent landscaping is in place. See Erosion and Sediment Control Standards (Appendix D) for design details. At the completion of all construction, the sediment trap must be cleaned out (taking care that no sediment enters the drainage system) and filled in, and the flow routed to the permanent drainage system. Another alternative for subdivisions is to stage excavation of the pond as follows: 1. Bottom elevation of the pond prior to paving of plat roadways: 3 feet above the final pond bottom elevation. At this stage of rough grading, the facility may be used to meet sediment retention requirements. 2. Bottom elevation of the pond during and after paving and prior to construction of 80% of the houses: 18 inches above the final pond bottom elevation with upstream sediment retention, as needed. At this stage, the pond will serve as an interim flow control facility pending final stabilization of the site. Note that RMC 4-4-060 requires that flow control facilities be operational prior to the construction of any improvements.  FACILITY CONSTRUCTION GUIDELINES Excavation of infiltration facilities should be done with a backhoe working at “arm’s length” to minimize disturbance and compaction of the completed infiltration surface. If the bottom of the facility will be less than three feet below final grade, the facility area should be cordoned off so that construction traffic does not traverse the area. The exposed soil should be inspected by a soils engineer after excavation to confirm that soil conditions are suitable. Two simple staff gages for measuring sediment depth should be installed at opposite ends of the bottom of ponds. The gages may consist of 1-inch pipe driven at least one foot into the soil in the bottom of the pond, with 12 inches of the pipe protruding above grade.  OFFSITE GROUNDWATER LEVEL IMPACTS Potential impacts to groundwater levels off the project site should be considered. In general, replacing vegetation with impervious cover will increase the total annual volume of runoff generated on a site. Infiltrating this runoff will tend to increase ground water recharge, which may affect groundwater levels offsite. The impacts of infiltration could include increased water to landslide hazards, increased groundwater resources available, increased water levels in closed depressions, and higher groundwater levels. Higher groundwater levels offsite could result in increased flooding of basements, or impaired functioning of infiltration systems resulting in surface water flooding. Evidence of offsite groundwater flooding problems should be examined during the offsite analysis required under Core Requirement #2 (see Section 1.2.2). In general, groundwater level impacts will be very difficult to reduce, and there are no specific requirements that apply in many cases. The design engineer is encouraged to consider whether there are any feasible approaches to reduce groundwater flooding impacts, such as moving facilities or changing facility geometry, retaining forest cover, minimizing impervious coverage, or fixing downstream problems. 5.2.1 GENERAL REQUIREMENTS FOR INFILTRATION FACILITIES 2022 City of Renton Surface Water Design Manual 6/22/2022 5-53  GROUNDWATER PROTECTION The protection of groundwater quality is recognized as an issue of greater concern that in the past, and groundwater protection standards are changing rapidly, see Section 1.3.6 Core Requirement #6: Aquifer Protection Area. Increased safeguards are often required. The applicant should refer to the Wellhead Protection Area Zones layer of COR Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>) to determine if the project lies within a groundwater protection area. In combination with the general requirements for infiltration facilities, compliance with Core Requirement #8 (Water Quality Facilities) and Special Requirement #5 (Oil Control), where applicable, of this manual is required to address protection of groundwater quality where infiltrating runoff from pollution generating surfaces. Water quality facility requirements, exemptions, and exceptions in Core Requirement #8 are influenced by whether a project is infiltrating within a groundwater protection area, whether the project is infiltrating into soils with properties required for groundwater protection, which water quality treatment menu is applicable, and the infiltration site’s measured distance to either a sensitive lake; a fresh water that has an existing or designated aquatic life use; or a surface water body impaired for phosphorus or metals. Soil Properties Required for Groundwater Protection Soil properties required for groundwater protection both outside of and within groundwater protection areas are listed below. Groundwater protection areas include the Cedar Valley Sole Source Aquifer Review Area, Wellhead Protection Areas, and the Aquifer Protection Area. Note: The soil properties given are primarily for groundwater protection and do not necessarily satisfy other protection needs. For example, projects infiltrating runoff within a quarter-mile of a Sensitive Lake may still be required to provide water quality treatment to meet the resource protection needs of the Sensitive Lake. See Core Requirement #8 (Section 1.2.8) for additional WQ requirements. Soil Properties Required for Groundwater Protection Outside of Groundwater Protection Areas For infiltration facilities located outside of groundwater protection areas, acceptable groundwater protection is provided by the soil if the first two feet or more of the soil beneath the infiltration facility has a cation exchange capacity5 greater than 5 and an organic content6 of 1.0% or greater, AND meets one of the following criteria: 1. The soil has a measured infiltration rate less than or equal to 9 inches per hour7 or is logged as one of the classes from the USDA Textural Triangle (Figure 5.2.1.A,), excluding sand and loamy sand (Note: soil texture classes other than sand and loamy sand may be assumed to have an infiltration rate of less than or equal to 9 inches per hour without doing field testing to measure rates.8), OR 2. The soil is composed of less than 25% gravel by weight with at least 75% of the soil passing the #4 sieve. The portion passing the #4 sieve must meet one of the following gradations:  At least 50% must pass the #40 sieve and at least 2% must pass the #100 sieve, or  At least 25% must pass the #40 sieve and at least 5% must pass the #200 sieve. 5 Cation exchange capacity shall be tested using EPA Laboratory Method 9081. Note that per EPA method 9081 guidance, distinctly acidic soils require “the method of cation-exchange capacity by summation (Chapman, 1965, p. 900; see Paragraph 10.1).” 6 Organic content shall be measured on a dry weight basis using method ASTM D2974 for the fraction passing the #40 sieve. 7 See discussion of the measured infiltration rate in Section 5.2.1. 8 Criteria (a) is based on the relationship between infiltration rates and soil texture. However, there are many other factors, such as high water table, presence of impervious strata or boulders close to the surface, etc., which also affect infiltration rate. When any such condition is suspected because soils are coarser than expected from the measured infiltration rate, a sieve analysis should be done to establish soil characteristics. The judgment of a geotechnical professional shall determine whether a sieve analysis is warranted. The sieve analysis must meet Criteria (b) above to be considered protective. SECTION 5.2 INFILTRATION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-54 Note: These soil properties must be met by the undisturbed native soils onsite (i.e. in situ). Soil may not be imported in order to meet groundwater protection criteria. Soil Properties Required within Groundwater Protection Areas For projects located within groundwater protection areas (excluding Zone 1 of the Aquifer Protection Area), acceptable groundwater protection is provided by the soil if the first two feet or more of the soil beneath the infiltration facility has a cation exchange capacity greater than 5 and an organic content of 1% or greater, AND meets one of the following criteria: 1. The soil has a measured infiltration rate less than or equal to 2.4 inches per hour or is logged as one of the classes from the USDA Textural Triangle (Figure 5.2.1.A), excluding sand, loamy sand, and sandy loam (Note: soil triangle texture classes other than sand, loamy sand, and sandy loam may be assumed to have an infiltration rate of less than or equal to 2.4 inches per hour without doing field testing to measure rates), OR 2. The soil has a measured infiltration rate less than or equal to 9 inches per hour, and it must be composed of less than 25% gravel by weight with at least 75% of the soil passing the #4 sieve. The portion passing the #4 sieve must meet one of the following gradations:  At least 50% must pass the #40 sieve and at least 2% must pass the #100 sieve, or  At least 25% must pass the #40 sieve and at least 5% must pass the #200 sieve. Note: The above soil properties must be met by the undisturbed native soils onsite (i.e. in situ). Soil may not be imported in order to meet groundwater protection criteria. FIGURE 5.2.1.A USDA TEXTURAL TRIANGLE 5.2.1 GENERAL REQUIREMENTS FOR INFILTRATION FACILITIES 2022 City of Renton Surface Water Design Manual 6/22/2022 5-55 Infiltration near Water Supply Wells The design engineer should consider the following when designing infiltration facilities near water supply wells: 1. In no case should infiltration facilities be placed closer than 100 feet from drinking water wells and 200 feet from springs used for drinking water supplies. Where water supply wells exist nearby, it is the responsibility of the applicant’s engineer to locate such wells, meet any applicable protection standards, and assess possible impacts of the proposed infiltration facility on groundwater quality. If negative impacts on an individual or community water supply are possible, additional runoff treatment must be included in the facility design, or relocation of the facility should be considered. 2. All infiltration facilities located within the one-year capture zone of any well should be preceded by a water quality treatment facility. Infiltration near Steep Slope Hazard Areas and Landslide Hazards The following restrictions apply to the design of infiltration systems located near a slope steeper than 15%. 1. Where infiltration facilities are proposed within 200 feet of a steep slope hazard area or a landslide hazard, OR closer to the top of slope than the distance equal to the total vertical height of a slope area that is steeper than 15%, a detailed geotechnical evaluation is required. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built- out conditions. 2. Individual lot infiltration and dispersion systems rather than a centralized infiltration facility should be used to the extent feasible, except for lots immediately adjacent to a landslide hazard. The runoff from such lots should be discharged into a tightline system, if available, or other measures should be implemented as recommended by a geotechnical engineer, engineering geologist, or CED.  UNDERGROUND INJECTION CONTROL WELL REGISTRATION The Underground Injection Control Program (UIC) administered by WA Ecology protects groundwater quality by regulating discharges to UIC wells. WA Ecology adopted revisions to Chapter 173-218 WAC, the UIC program rules, on January 3, 2006. The newly adopted revisions went into effect on February 3, 2006. These rules require the registration of new injection wells that manage stormwater. Information regarding these new regulations may be found at Ecology’s UIC Program website, <https://ecology.wa.gov/Regulations-Permits/Guidance-technicalassistance/Underground-injection- control-program>. UIC wells are manmade structures used to discharge fluids into the subsurface. Examples are drywells, infiltration trenches with perforated pipe, and any structure deeper than the widest surface dimension (see Chapter I-4 UIC Program in the 2019 Stormwater Management Manual for Western Washington (SWMMWW). For single family projects, drywells that are located immediately adjacent to buildings and infiltrate roof runoff directly from the gutters and downspouts do not need Ecology registration. Open ponds are not considered injection wells. UIC Program rule requirements apply to all UIC wells. If an existing UIC well receives stormwater and was in use before 2/3/2006, the well owner must complete a well assessment with Ecology to determine if the UIC well is a high threat to groundwater. See Chapter 173-218-090 (2) WAC UIC Program, <http://app.leg.wa.gov/WAC/default.aspx?cite=173-218-090> or visit <https://ecology.wa.gov/Regulations-Permits/Guidance-technical-assistance/Underground- injectioncontrol-program> for more information. An assessment example is available at https://apps.ecology.wa.gov/publications/documents/1210012.pdf. If UIC registration is required by Ecology for the proposed design, a copy of the registration, or the Ecology-issued System ID provided at registration, shall be provided by the applicant prior to plan approval or permit issuance by the City (see Section 2.3.1.1 Technical Information Report (TIR), TIR Section 7 Other Permits and Section 5.4.1). Note that existing UIC wells that are unable to obtain Ecology rule authorization and UIC Site ID number without modification may require design review and permit approval per City requirements for such SECTION 5.2 INFILTRATION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-56 modifications. Permitting for the modified facility shall follow the UIC registration requirements guidance for new facilities. 5.2.2 INFILTRATION PONDS Infiltration ponds may be constructed by excavating or constructing berms. A schematic representation of a typical infiltration pond is shown in Figure 5.2.2.A. Infiltration ponds are not allowed in Zone 1 of the Aquifer Protection Area. 5.2.2.1 DESIGN CRITERIA General The following criteria for ponds are in addition to the general requirements for infiltration facilities specified in Section 5.2.1: 1. The proposed pond bottom must be at least 3 feet above the seasonal high groundwater level and have at least 3 feet of permeable soil beneath the bottom. 2. Infiltration ponds are not allowed on slopes greater than 25% (4H:1V). A geotechnical analysis and report is required if located within 200 feet of a steep slope hazard area or landslide hazard OR if the facility is located within a setback distance from top of slope equal to the total vertical height of a slope area that is steeper than 15%. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built- out conditions. 3. The infiltration surface must be in native soil (excavated at least one foot in depth). 4. Maintenance access shall be provided to both the presettling pond or vault (if provided) and the infiltration pond. 5. An overflow structure such as that shown in Figure 5.1.1.C shall be provided. In addition, infiltration ponds shall have an emergency spillway as required for detention ponds in Section 5.1.1.1. 6. The criteria for general design, side slopes, embankments, planting, maintenance access, access roads, fencing, signage, and right-of-way shall be the same as for detention ponds (see Section 5.1.1), except as required for the infiltration design. Setbacks 1. The toe of the exterior slope of an infiltration pond berm embankment shall be set back 5 feet from the tract, easement, or property line. 2. The tract, easement, or property line on an infiltration pond cut slope shall be set back 5 feet from the emergency overflow water surface. 3. The infiltration pond design water surface shall be set back 100 feet from proposed or existing septic system drainfields. This setback may be reduced to 30 feet with approval from the Public Health – Seattle & King County. 4. The infiltration pond design water surface shall be a minimum of 200 feet from any steep slope hazard area or landslide hazard. Upon analysis and approval of a licensed geotechnical engineer or engineering geologist, this setback may be reduced to 50 feet. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built- out conditions. 5. The infiltration pond design water surface shall be set back a minimum distance from top of slope equal to the total vertical height of a slope area that is steeper than 15%. Upon analysis and approval of a licensed geotechnical engineer or engineering geologist, this setback may be reduced to 50 feet. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built- out conditions. 5.2.2 INFILTRATION PONDS 2022 City of Renton Surface Water Design Manual 6/22/2022 5-57 6. Building setback lines for adjacent internal lots shall be 20 feet. These may be reduced to the minimum allowed by zoning if the facility soils report addresses the potential impacts of the facility phreatic surface on structures so located. 7. The infiltration pond design water surface shall be set back 20 feet from external tract, easement or property lines. This may be reduced to 5 feet if the facility soils report addresses the potential impacts of the facility phreatic surface on existing or future structures located on adjacent external lots. 5.2.2.2 METHODS OF ANALYSIS The size of the pond shall be determined using the hydrologic analysis and routing methods described for detention ponds in Chapter 3. The storage volume in the pond is used to detain runoff prior to infiltration. The stage/discharge curve shall be developed from the design infiltration rate determined according to Section 5.2.1. At a given stage the discharge may be computed using the area of pervious surface through which infiltration will occur (which will vary with stage) multiplied by the recommended design infiltration rate (in appropriate units). Berms (which should be constructed of impervious soil such as till), maintenance access roads, and lined swales should not be included in the design pervious surface area. SECTION 5.2 INFILTRATION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-58 FIGURE 5.2.2.A SCHEMATIC REPRESENTATION OF A TYPICAL INFILTRATION POND NOTE: DETAIL IS A SCHEMATIC REPRESENTATION ONLY. ACTUAL CONFIGURATION WILL VARY DEPENDING ON SPECIFIC SITE CONSTRAINTS AND APPLICABLE DESIGN CRITERIA. SECTION A-A NTS PLAN VIEW NTS OVERFLOW/EMERGENCY OVERFLOW PROVIDED PER SECTION 5.1.1.1 INFLOW PIPE ACCESS ROAD SEE SECTION 5.1.1.1 FOR SPECIFICATIONS TRACT/EASEMENT LINES AS REQUIRED CONNECTING SPILLWAY INFILTRATION POND OUTFLOW/ OVERFLOW STRUCTURE SEE FIGURE 5.1.1.B FOR DETAILS SEE FIGURE 5.1.1.B SETTLING POND IF REQUIRED GEOTECHNICAL DESIGN REQUIRED FOR BERM HEIGHT>6' 6' MIN. 5' MINIMUM A A EXISTING GROUND EMERGENCY OVERFLOW SPILLWAY KEY REQUIRED FOR BERM HEIGHT>4' SLOPES 3H:1V (TYP.) 5.2.3 INFILTRATION TANKS 2022 City of Renton Surface Water Design Manual 6/22/2022 5-59 5.2.3 INFILTRATION TANKS Infiltration tanks consist of underground pipe that has been perforated to allow detained stormwater to be infiltrated. A schematic representation of a typical infiltration tank is shown in Figure 5.2.3.A . Infiltration tanks are not allowed in Zone 1 of the Aquifer Protection Area. 5.2.3.1 DESIGN CRITERIA General The following criteria for tanks are in addition to the general requirements for infiltration facilities specified in Section 5.2.1: 1. The proposed tank trench bottom shall be at least 3 feet above the seasonal high groundwater level and have at least 3 feet of permeable soil beneath the trench bottom. 2. Infiltration tanks are not allowed on slopes greater than 25% (4H:1V). A geotechnical analysis and report is required if located within 200 feet of a steep slope hazard area or landslide hazard OR if the facility is located within a setback distance from top of slope equal to the total vertical height of a slope area that is steeper than 15%. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built-out conditions. The infiltration surface elevation (bottom of trench) must be in native soil (excavated at least one foot in depth). 3. Spacing between parallel tanks shall be calculated using the distance from the lowest trench bottom to the maximum wet season ground water surface (D) and the design width of the trench for a single tank (W). The tank spacing S = W2/D, where S is the centerline spacing between trenches (or tanks) in feet. S shall not be less than W, and S need not exceed 2W. 4. Tanks shall be bedded and backfilled with washed drain rock that extends at least 1 foot below the bottom of the tank, at least 2 feet but not more than 5 feet beyond the sides, and up to the top of the tank. 5. Drain rock (3 to 11/2 inches) shall be completely covered with filter fabric prior to backfilling. 6. The perforations (holes) in the tank must be one inch in diameter and located in the bottom half of the tank starting at an elevation of 6 inches above the invert of the tank. The number and spacing of the perforations should be sufficient to allow complete utilization of the available infiltration capacity of the soils with a safety factor of 2.0 without jeopardizing the structural integrity of the tank. 7. Infiltration tanks shall have an overflow structure equipped with a solid bottom riser (with clean-out gate) and outflow system for safely discharging overflows to the downstream conveyance system or another acceptable discharge point. 8. The criteria for general design, materials, structural stability, buoyancy, maintenance access, access roads, and right-of-way shall be the same as for detention tanks (see Section 5.1.2,), except for features needed to facilitate infiltration. Setbacks 1. Tanks shall be set back 100 feet from proposed or existing septic system drainfields. This setback may be reduced to 30 feet with approval from the Public Health – Seattle & King County. 2. All tanks shall be a minimum of 200 feet from any steep slope hazard area or landslide hazard. Upon analysis and approval of a licensed geotechnical engineer or engineering geologist, this setback may be reduced to 50 feet. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built- out conditions. 3. All tanks shall be set back a minimum distance from top of slope equal to the total vertical height of a slope area that is steeper than 15%. Upon analysis and approval of a licensed geotechnical engineer or SECTION 5.2 INFILTRATION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-60 engineering geologist, this setback may be reduced to 50 feet. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built- out conditions. 4. Building setback lines for adjacent internal lots shall be 20 feet. These may be reduced to the minimum allowed by zoning if the facility soils report addresses the potential impacts of the facility phreatic surface on structures so located. 5. Infiltration tanks shall be set back 20 feet from external tract, easement, or property lines. This may be reduced to 5 feet if the facility soils report addresses the potential impacts of the facility phreatic surface on existing or future structures located on adjacent external lots. 5.2.3.2 METHODS OF ANALYSIS The size of the tank shall be determined using the hydrologic analysis and routing methods described in Chapter 3, and the stage/discharge curve developed from the recommended design infiltration rate as described in Section 5.2.1. The storage volume in the tank is used to detain runoff prior to infiltration with the perforations providing the outflow mechanism. At any given stage, the discharge may be computed using the area of pervious surface through which infiltration will occur multiplied by the recommended design infiltration rate (in appropriate units). The area of pervious surface used for determining the potential infiltration from the tank shall be computed by taking the lesser of the trench width, or two times the width of the tank, and then multiplying by the length of the tank (assuming infiltration through the bottom of the trench only). 5.2.3 INFILTRATION TANKS 2022 City of Renton Surface Water Design Manual 6/22/2022 5-61 FIGURE 5.2.3.A SCHEMATIC REPRESENTATION OF A TYPICAL INFILTRATION TANK NOTES: 1. ALL METAL PARTS CORROSION RESISTANT. STEEL PARTS STAINLESS STEEL OR ALUMINIZED STEEL, EXCEPT TANK MAY BE GALVANIZED AND ASPHALT COATED (TREATMENT 1 OR BETTER). 2. FILTER FABRIC TO BE PLACED OVER WASHED ROCK BACKFILL PRIOR TO BACKFILLING OVER FACILITY. OVERFLOW STRUCTURE MIN. 54" DIA. TYPE 2 C.B. SEE SECTION 5.2.3.1 OUTLET PIPE OUTLET/OVERFLOW STRUCTURE WASHED ROCK BEDDING AND BACKFILL TO TOP OF TANK, MIN. 2' BEYOND EDGES SETTLING VAULT OR TYPE 2 C.B. IF REQUIRED OPTIONAL PARALLEL TANK ACCESS RISERS (MAX SPACING 100-FT) ACCESS RISERS SEE FIGURE 5.1.2.B TYPE 2 C.B. OR SETTLING VAULT IF REQUIRED WASHED ROCK BEDDING AND BACKFILL TO TOP OF TANK FILTER FABRIC TOP ONLY RISER FOR INFILTRATION ONLY; FOR COMBINED DETENTION / INFILTRATION SEE FIGURE 5.1.4.A 36" MIN. DIA. (TYP.) DETENTION TANK SIZE AS REQUIRED 1" HOLES AS REQUIRED 6" MIN. DEAD STORAGE PLAN VIEW NTS SECTION A-A NTS INLET PIPE (FLOW THROUGH) 100' MAX. 50' MAX. 4' MIN. A A 2.0' MAX. 2' MIN. 2" MIN. DIAMETER AIR VENT PIPE WELDED TO TANK (REQUIRED IF NO ACCESS RISER ON TANK) SECTION 5.2 INFILTRATION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-62 5.2.4 INFILTRATION VAULTS Infiltration vaults consist of a bottomless concrete vault structure placed underground in native infiltrative soils9. Infiltration is achieved through the native soils at the bottom of the structure. Infiltration vaults are similar to detention vaults. A schematic representation of a detention vault is shown in Figure 5.1.3.A. Schematic representations of overflow riser are shown in Section 5.1.4. Infiltration vaults are not allowed in Zone 1 of the Aquifer Protection Area. 5.2.4.1 DESIGN CRITERIA General The following criteria for vaults are in addition to the general requirements for infiltration facilities specified in Section 5.2.1: 1. The proposed vault bottom shall be at least 3 feet above the seasonal high groundwater level and have at least 3 feet of permeable soil beneath the bottom. 2. Infiltration vaults are not allowed on slopes greater than 25% (4H:1V). A geotechnical analysis and report is required if located within 200 feet of a steep slope hazard area or landslide hazard OR if the facility is located within a setback distance from top of slope equal to the total vertical height of the slope area that is steeper than 15%. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built- out conditions. 3. The vault bottom must be in native soil (excavated at least one foot in depth). 4. A suitable means to dissipate energy at the inlet is required to prevent scour and may be accomplished by using the detail for the sand filter vault (see Figure 6.5.3.A). 5. Infiltration vaults shall have a solid bottom riser (with clean-out gate) and outflow system for safely discharging overflows to the downstream conveyance system or another acceptable discharge point. Structural Stability All vaults shall meet structural requirements for overburden support, buoyancy, and H-20 vehicle loading. Cast-in-place wall sections shall be designed as retaining walls. Structural designs for vaults must be stamped by a licensed structural engineer unless otherwise approved by CED. Bottomless vaults shall be provided with footings placed on stable, well-consolidated native material and sized considering overburden support, traffic loading (assume maintenance traffic, if placed outside ROW), and lateral soil pressures when the vault is dry. Infiltration vaults shall not be allowed in fill slopes unless analyzed in a geotechnical report for stability. The infiltration surface at the bottom of the vault must be in native soil. Access Requirements Same as specified for detention vaults in Section 5.1.3.1. Access Roads Same as specified for detention vaults in Section 5.1.3.1. Right-of-Way Infiltration vaults to be maintained by the City shall be in a stormwater tract granted and converted with all maintenance obligations (excluding maintenance of drainage facilities contained therein) to the homeowners association. Any tract not abutting public right-of-way will require a 15-foot wide extension of the tract to accommodate an acceptable access location. An underlying easement under and upon said tract shall be dedicated to the City for the purpose of operating, maintaining, improving and repairing the drainage facilities contain therein. The stormwater tract must be owned by the homeowners association. Each lot owner within the subdivision shall have an equal and undivided interest in the maintenance of the 9 See Section 5.2.1 and Reference Section 6 for UIC definition and UIC well registration requirements for infiltration vaults 5.2.4 INFILTRATION VAULTS 2022 City of Renton Surface Water Design Manual 6/22/2022 5-63 stormwater tract. Infiltration vaults to be maintained by a private property owner or homeowners association shall create stormwater facilities within a private tract or easement or construct the infiltration vault onsite. Setbacks 1. Infiltration vaults shall be set back 100 feet from proposed or existing septic system drainfields. This setback may be reduced to 30 feet with approval from the Public Health – Seattle & King County. 2. Infiltration vaults shall be a minimum of 200 feet from any steep slope hazard area or landslide hazard. Upon analysis and approval of a licensed geotechnical engineer or engineering geologist, this setback may be reduced to 50 feet. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built- out conditions. 3. Infiltration vaults shall be set back a minimum distance from top of slope equal to the total vertical height of a slope area that is steeper than 15%. Upon analysis and approval of a licensed geotechnical engineer or engineering geologist, this setback may be reduced to 50 feet. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built- out conditions. 4. Building setback lines for adjacent internal lots shall be 20 feet. These may be reduced to the minimum allowed by zoning if the facility soils report addresses the potential impacts of the facility phreatic surface on structures so located. 5. Infiltration vaults shall be set back 20 feet from external tract, easement, or property lines. This may be reduced to 5 feet if the facility soils report addresses the potential impacts of the facility phreatic surface on existing or future structures located on adjacent external lots. 5.2.4.2 METHODS OF ANALYSIS The size of the vault shall be determined using the hydrologic analysis and routing methods described in Chapter 3 and the stage/discharge curve developed from the recommended design infiltration rate as described in Section 5.2.1. The storage volume in the vault is used to detain runoff prior to infiltration. At any given stage, the discharge may be computed using the area of pervious surface through which infiltration will occur (the exposed soil comprising the vault bottom) multiplied by the recommended design infiltration rate (in appropriate units). SECTION 5.2 INFILTRATION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-64 5.2.5 INFILTRATION TRENCHES Infiltration trenches can be a useful alternative for developments with constraints that make siting a pond difficult. Infiltration trenches may be placed beneath parking areas, along the site periphery, or in other suitable linear areas. Infiltration trenches are not allowed in Zone 1 of the Aquifer Protection Area. 5.2.5.1 DESIGN CRITERIA General The following criteria for trenches are in addition to the general requirements for infiltration facilities specified in Section 5.2.1: 1. The proposed trench bottom must be at least 3 feet above the seasonal high groundwater level and 3 feet below finished grade. 2. There must be at least 3 feet of permeable soil beneath the trench bottom. 3. The infiltration surface elevation (bottom of trench) must be in native soil (excavated at least one foot in depth). 4. Infiltration trenches are not allowed on slopes greater than 25% (4H:1V). A geotechnical analysis and report is required if located within 200 feet of a steep slope hazard area or landslide hazard OR if the facility is located within a setback distance from top of slope equal to the total vertical height of the slope area that is steeper than 15%. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built- out conditions. Trenches shall be a minimum of 2 feet wide and no more than 5 feet wide10. 5. Trenches shall be backfilled with 11/2 – 3/4-inch washed rock, completely surrounded by filter fabric and overlain by a minimum 1 foot of compact backfill. 6. Level 6-inch minimum diameter rigid perforated distribution pipes shall extend the length of the trench. Distribution pipe inverts shall be a minimum of 2 feet below finished grade. Provisions (such as clean-out wyes) shall be made for cleaning the distribution pipe. The pipe capacity shall be calculated to verify that the distribution pipe has capacity to handle the maximum design flow. 7. Alternative trench-type systems such as pre-fabricated bottomless chambers that provide an equivalent system may be used at the discretion of CED. 8. Two feet minimum cover shall be provided in areas subject to vehicle loads. 9. Trenches shall be spaced no closer than 10 feet, measured on center. Setbacks 1. Trench systems shall be set back 100 feet from proposed or existing septic system drainfields. This setback may be reduced to 30 feet with approval from the Public Health – Seattle & King County. 2. Trench systems shall be a minimum of 200 feet from any steep slope hazard area or landslide hazard. Upon analysis and approval of a licensed geotechnical engineer or engineering geologist, this setback may be reduced to 50 feet. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built- out conditions. 3. Trench systems shall be setback a minimum distance from top of slope equal to the total vertical height of a slope area that is steeper than 15%. Upon analysis and approval of a licensed geotechnical engineer or engineering geologist, this setback may be reduced to 50 feet. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built- out conditions. 10 See Section 5.2.1 and Reference Section 6 for UIC definition and UIC well registration requirements for infiltration trenches. 5.2.6 ALTERNATIVE INFILTRATION SYSTEMS 2022 City of Renton Surface Water Design Manual 6/22/2022 5-65 4. A minimum 5-foot setback is required between any part of the trench system and any property line. 5. Structures shall be set back 20 feet from individual trenches. This may be reduced if the facility soils report addresses potential impacts of trench phreatic surface on structures so located. 5.2.5.2 METHODS OF ANALYSIS The sections and lengths of trenches shall be determined using the hydrologic analysis and routing methods for flow control design described in Chapter 3. The stage/discharge curve shall be developed from the design infiltration rate recommended by the soils engineer, as described in Section 5.2.1. Storage volume of the trench system shall be determined considering void space of the washed rock backfill and maximum design water surface level at the crown of the distribution pipe. At any given stage, the discharge may be computed using the area of pervious surface through which infiltration will occur (trench bottom area only) multiplied by the recommended design infiltration rate (in appropriate units). 5.2.6 ALTERNATIVE INFILTRATION SYSTEMS Manufactures have developed other systems that have properties in common with vaults, tanks, and trenches, but that do not conform to the standards for those facility types. These systems may be approved by CED using suitable design standards adapted from the established standards for similar systems 11. Alternative infiltration systems are not allowed in Zone 1 of the Aquifer Protection Area. 5.2.6.1 DESIGN CRITERIA General The following criteria for alternative infiltration systems are in addition to the general requirements for infiltration facilities specified in Section 5.2.1: 1. The proposed infiltration surface must be at least 3 feet above the seasonal high groundwater level. 2. There must be at least 3 feet of permeable soil beneath the infiltration surface. 3. The infiltration surface elevation must be in native soil (excavated at least one foot in depth). 4. Infiltration systems are not allowed on slopes greater than 25% (4H:1V). A geotechnical analysis and report is required if located within 200 feet of a steep slope hazard area or landslide hazard OR if the facility is located within a setback distance from top of slope equal to the total vertical height of the slope area that is steeper than 15%. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built- out conditions. 5. Systems shall be backfilled with 11/2 – 3/4-inch washed rock or similar material, completely surrounded by filter fabric and overlain by a minimum 1 foot of compact backfill. 6. Two feet minimum cover shall be provided in areas subject to vehicle loads. 7. Chambers shall be spaced no more than 10 feet apart as measured from the adjacent edges. Inflow pipes or a manifold system shall be connected to each infiltration chamber. Inspection and maintenance access to each chamber shall be provided as deemed necessary by the City. 11 See Section 5.2.1 and Reference Section 6 for UIC definition and UIC well registration requirements for alternative infiltration systems. SECTION 5.2 INFILTRATION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-66 Access Requirements 1. The maximum depth from finished grade to invert shall be 20 feet. 2. Access openings required within 50 feet from any location in the facility and within 5 feet of each terminal end. Any location within the facility shall have a direct line of sight from an access point, unobstructed by any restrictions such as a wall of baffle. The facility must be able to be flushed without restriction from an access point. 3. All access openings must be readily accessible by maintenance vehicles. Structural Stability, Access Roads, Right-of-Way, and Setbacks Alternative infiltration systems shall comply with the structural stability, access road, right-of-way, and setback criteria consistent with either infiltration tanks (Section 5.2.3), infiltration vaults (Section 5.2.4), infiltration trenches (Section 5.2.5) whichever is most similar to the alternative infiltration system. 5.2.6.2 METHODS OF ANALYSIS The sizing and layout of the system shall be determined using the hydrologic analysis and routing methods for flow control design described in Chapter 3, using the approved continuous runoff model. The stage/discharge curve shall be developed from the design infiltration rate recommended by the soils engineer, as described in Section 5.2.1. Storage volume of the system shall be determined considering void space of the washed rock backfill and the volume contained in system elements. At any given stage, the discharge may be computed using the area of pervious surface through which infiltration will occur multiplied by the recommended design infiltration rate (in appropriate units). 5.2.7 SMALL INFILTRATION BASINS Small infiltration basins consist of a bottomless, precast concrete catch basin or equivalent structure placed in an excavation filled with washed drain rock. Stormwater infiltrates through the drain rock into the surrounding soil. This facility is intended for use with contributing surface areas of less than 5,000 square feet. Presettlement is most easily provided by a catch basin or manhole with a turned-down elbow; see Figure 5.2.7.Afor a schematic representation. If water quality treatment is required by Core Requirement #8 or Special Requirement #5, runoff from pollution-generating impervious surfaces must be treated before it enters the infiltration portion of the system. Small infiltration basins are not allowed in Zone 1 of the Aquifer Protection Area. 5.2.7.1 DESIGN CRITERIA The design criteria for small infiltration basins are the same as for infiltration tanks (see Sections 5.2.1 and 5.2.3), except that only one infiltration rate test and soil log is required for each small infiltration basin. Access into the basins shall be provided for inspection and maintenance. Designs may incorporate Type II catch basins, but equivalent designs using other materials may be accepted 12. 12 See Section 5.2.1 and Reference Section 6 for UIC definition and UIC well registration requirements. Careful consideration of the catch basin or structure to be used may avoid the requirement to register. 5.2.7 SMALL INFILTRATION BASINS 2022 City of Renton Surface Water Design Manual 6/22/2022 5-67 FIGURE 5.2.7.A SCHEMATIC REPRESENTATION OF A SMALL INFILTRATION BASIN INLET SLOPE = 2% SECTION NTS FILTER FABRIC 48" PRECAST CATCH BASIN W/O BOTTOM FILL EXCAVATION BELOW AND SURROUNDING BASIN WITH 1-1/2" TO 3" WASHED DRAIN ROCK 24" DIA. CATCH BASIN LID 8" PVC ELBOW SHORT BEND 8" PVC PIPE 20"X24" STANDARD CATCH BASIN LID WITH OVERFLOW GRATE OIL COLLECTION SEDIMENT COLLECTION CATCH BASIN WITH BOTTOM 6" 3' 2' 2' 6" SECTION 5.2 INFILTRATION FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 5-68 (T his page intentionally left blan k. ) 2022 City of Renton Surface Water Design Manual 6/22/2022 CHAPTER 6 WATER QUALITY DESIGN CITY OF RENTON SURFACE WATER DESIGN MANUAL Section Page 6.1 Water Quality Menus 6-3 6.1.1 Basic Water Quality Menu 6-5 6.1.2 Enhanced Basic Water Quality Menu 6-8 6.1.3 Sensitive Lake Protection Menu 6-10 6.1.4 Sphagnum Bog Protection Menu 6-14 6.1.5 High-Use Menu 6-16 6.1.6 Pretreatment Facilities 6-18 6.2 General Requirements for WQ Facilities 6-19 6.2.1 Water Quality Design Flows and Treatment Volumes 6-19 6.2.2 Sequence of Facilities 6-22 6.2.3 Setbacks, Slopes, and Embankments 6-24 6.2.4 Facility Liners 6-28 6.2.5 Flow Splitter Designs 6-32 6.2.6 Flow Spreading Options 6-36 6.3 Vegetated Flowpath Facility Designs 6-41 6.3.1 Basic Bioswales 6-41 6.3.2 Wet Bioswales 6-57 6.3.3 Lateral Inflow Bioswales 6-59 6.3.4 Standard Filter Strips 6-60 6.3.5 Narrow Area Filter Strips 6-68 6.4 Wetpool Facility Designs 6-69 6.4.1 Wetponds — Basic and Large 6-69 6.4.2 Wetvaults 6-84 6.4.3 Stormwater Wetlands 6-90 6.4.4 Combined Detention and Wetpool Facilities 6-96 6.5 Filtration Facility Designs 6-101 6.5.1 General Requirements For Filtration Facilities 6-101 6.5.2 Sand Filters — Basic and Large 6-102 6.5.3 Sand Filter Vaults 6-118 6.5.4 Linear Sand Filters 6-123 6.6 Oil Control Facility Designs 6-127 6.6.1 Catch Basin Inserts 6-127 6.6.2 Oil/Water Separators 6-127 6.7 Proprietary Facility Designs 6-141 6.7.1 Ecology Requirements 6-141 6.7.2 City of Renton Requirements 6-141 6.8 Bioretention Facility Designs 6-145 6.8.1 Bioretention 6-145 CITY OF RENTON SURFACE WATER DESIGN MANUAL 6/22/2022 2022 City of Renton Surface Water Design Manual 6.9 WSDOT WQ Facility Designs 6-159 6.9.1 Media Filter Drain 6-159 6.9.2 Compost-Amended Filter Strips 6-169 6.9.3 Compost-Amended Biofiltration Swales 6-170 CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 6-1 CHAPTER 6 WATER QUALITY DESIGN This chapter presents the City of Renton approved methods, criteria, and details for analysis and design of water quality facilities pursuant to Core Requirement #8, discussed in Section 1.2.8, and Special Requirement #5, discussed in Section 1.3.5. Chapter Organization The information in this chapter is organized into the following nine main sections.  Section 6.1, “Water Quality Menus,” details the area-specific water quality menus referred to in Core Requirement #8 of Chapter 1, and the High-Use Menu referred to in Special Requirement #5, also in Chapter 1.  Section 6.2, “General Requirements for WQ Facilities,” presents general design requirements and details pertinent to all water quality facilities.  Section 6.3, “Vegetated Flowpath Facility Designs,” presents the details for analysis and design of bioswales and filter strips.  Section 6.4, “Wetpool Facility Designs,” presents the details for analysis and design of wetponds, wetvaults, stormwater wetlands, and combinations of these facilities with detention facilities.  Section 6.5, “Filtration Facility Designs,” presents the details for analysis and design of sand filters.  Section 6.6, “Oil Control Facility Designs,” presents the details for analysis and design of coalescing-plate oil/water separators.  Section 6.7, “Proprietary Facility Designs,” discusses general considerations for proprietary manufactured facilities, including summary notes regarding City requirements for approval for use of these systems. This section points to Reference Section 14-A and Reference Section 14-B, which includes design and maintenance considerations for proprietary facilities which have been approved by the City.  Section 6.8, “Bioretention Facility Designs,” presents the details for analysis and design of bioretention facilities.  Section 6.9, “WSDOT WQ Facility Designs,” presents the details for analysis and design of media filter drains, compost-amended vegetated filter strips, and compost-amended biofiltration swales. Required vs. Recommended Design Criteria Both required and recommended design criteria are presented in this chapter. Criteria stated using “shall” or “must” are mandatory, to be used unless there is a good reason to deviate as allowed under the adjustment process in Section 1.4. These criteria are required design criteria and generally affect facility performance or critical maintenance factors. Sometimes options are stated as part of the required design criteria using the language “should” or “may.” These criteria are recommended design criteria, but are closely related to the required criteria, so they CHAPTER 6 WATER QUALITY DESIGN 6/22/2022 2022 City of Renton Surface Water Design Manual 6-2 are placed in the same section. In some cases, recommended design features are presented under a separate heading in the “Design Criteria” sections. Design Criteria Applicable To All Facilities All facilities must be designed and constructed to allow inspection and maintenance. Use of Chapter 6 Figures The figures included in this chapter are provided as schematic representations and should not be used for design. Refer to the City of Renton Standard Details for specific design information. The figures provided in this chapter illustrate one example of how the WQ facility design criteria may be applied. Although the figures are meant to illustrate many of the most important design criteria, they may not show all criteria that apply. In general, the figures are not used to specify requirements unless they are indicated elsewhere in the manual. If this manual refers to a standard detail not included in the City of Renton Standard Details, the applicant shall use the figures provided in the manual.1 Water Quality Facility Sizing Worksheets Sizing worksheets for the major water quality facilities can be found in Reference Section 8-C of the 2021 King County Surface Water Design Manual at: <https://kingcounty.gov/services/environment/water-and- land/stormwater/documents/surface-water-design-manual.aspx>. These worksheets are based on the step by step sizing methods given for the water quality facilities in this Chapter. Most design criteria that are not required for facility sizing are omitted from the worksheets. It is the designer’s responsibility to make sure that all the required design criteria for each water quality facility are provided on submitted plans. Facility sizing credits for water quality facilities may be used as allowed and specified in Chapter 1, Section 1.2.9.3 “Requirements for Use of BMP Credits.” Please note that the worksheets are dated in the footer of each page. It is the designer’s responsibility to ensure that any Manual updates affecting the sizing procedure or design criteria after that date are incorporated into the worksheet. Updates, errata, and clarifications are posted at the City of Renton’s Surface Water Design Standards website: <www.rentonwa.gov/swdm>. If there are instances in which the worksheet differs from the design criteria in the text of this Chapter, the criteria as given in this Chapter, and as modified by subsequent updates, shall be considered the governing criteria. 1 Footnote 1 is not used. CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 6-3 6.1 WATER QUALITY MENUS This section identifies facility choices and, in some cases, non-structural options that comprise the water quality (WQ) menus referred to in Chapter 1. The menus covered in this section and summarized in Figure 6.1.A are as follows:  “Basic Water Quality Menu,” Section 6.1.1  “Enhanced Basic Water Quality Menu,” Section 6.1.2  “Sensitive Lake Protection Menu,” Section 6.1.3  “Sphagnum Bog Protection Menu,” Section 6.1.4  “High-Use Menu,” Section 6.1.5 Guide to Applying Water Quality Menus 1. Check the exemption language on Section 1.2.8 to determine if or which threshold discharge areas of the project site must provide WQ treatment per Core Requirement #8. 2. Use the Basic WQ treatment areas Section 1.2.8.1.A to determine if basic or enhanced treatment is required. 3. Consult Section 1.2.8.1 for other design requirements, allowances, and flexible compliance provisions related to implementing water quality treatment. 4. Read the implementation requirements in Chapter 1 (Section 1.2.8.2) that address pollution generating pervious surface. For some WQ menus, and in some situations, the facility requirements for these surfaces are eased. 5. Determine if your project fits the definition of a high-use site (see Special Requirement #5 in Chapter 1). If it does, or if you elect to provide enhanced oil pollution control, choose one of the options presented in the High-Use menu, Section 6.1.5. Detailed designs for oil control facilities are given in Section 6.6. 6. General water quality facility requirements (see Section 6.2) apply to all menus and may affect the placement of facilities on your site. SECTION 6.1 WATER QUALITY MENUS 6/22/2022 2022 City of Renton Surface Water Design Manual 6-4 FIGURE 6.1.A WATER QUALITY TREATMENT FACILITY SELECTION FLOW CHART 6.1.1 BASIC WATER QUALITY MENU 2022 City of Renton Surface Water Design Manual 6/22/2022 6-5 6.1.1 BASIC WATER QUALITY MENU Where applied: Basic WQ Treatment Areas are designated by the City of Renton where a general, cost- effective level of treatment is sufficient for most land uses. Some land uses, however, will need an increased level of treatment because they generate high concentrations of metals in stormwater runoff and acute concentrations of metals in streams are toxic to fish. The treatment facility requirements for Basic WQ Treatment Areas provide for this increase in treatment. For precise details on the application of this and other water quality menus, refer to Section 1.2.8, “Core Requirement #8: Water Quality.” Treatment goal: The Basic Water Quality menu facility choices are designed to remove 80 percent of total suspended solids2 (TSS) for flows or volumes up to and including the WQ design flow or volume (defined in Section 6.2.1). Flows and volumes in excess of the WQ design flow or volume may be routed around the WQ facility or may be passed through untreated. Basis: “The use of TSS as an ‘indicator’ pollutant for sediment is well established.”3 “The control of TSS leads to indirect control of other pollutants of concern that can adhere to suspended solids in stormwater runoff.”3 “80% TSS removal level is reasonably attainable using properly designed, constructed and maintained structural stormwater BMPs (for typical ranges of TSS concentration found in stormwater runoff).” 3 For higher removal rates, there are diminishing returns, and relatively less treatment is gained for incremental increases in facility size. WA Ecology’s TAPE4 guidance finds 80% removal to be achievable by and a suitable criterion for proprietary “emerging” technologies.  BASIC WQ OPTION 1  BIOSWALE A bioswale is a long, gently sloped, vegetated ditch designed to settle out pollutants from stormwater. Grass is the most common vegetation used. Design details are given in Section 6.3.1. The wet bioswale (see Section 6.3.2) is a variation of the basic bioswale for use where soils drain poorly, the longitudinal slope is slight (1.5 percent or less), water tables are high, or continuous base flow is likely to result in saturated soil conditions. Under such conditions, healthy grass growth is not possible and wetland plants are used instead. The lateral inflow bioswale (see Section 6.3.3) may be used in situations such as roadways and parking lots where water enters the swale along the side rather than at one discrete inflow point at the head of the swale summarizes when the bioswale and its variations are to be applied. 2 The influent concentration range for demonstrated pollutant removal is 100 to 200 mg/L. For influent concentrations lower than 100mg/l the effluent goal is equal to or less than 20 mg/l. For influent concentrations greater than 200 mg/l, the goal is greater than 80% TSS removal. 3 Source: Knox County Tennessee Stormwater Management Manual, Volume 2, Technical Guidance. Date unknown. Accessed 2014/02/14. 4 Ecology, WA. 2011. Technical Guidance Manual for Evaluating Emerging Stormwater Treatment Technologies: Technology Assessment Protocol – Ecology (TAPE). In Publication No. 11-10-061, 1-73. Lacey, WA: Washington State Department of Ecology. <https://fortress.wa.gov/ecy/publications/summarypages/1110061.html>; <https://fortress.wa.gov/ecy/publications/publications/1110061.pdf>. SECTION 6.1 WATER QUALITY MENUS 6/22/2022 2022 City of Renton Surface Water Design Manual 6-6 TABLE 6.1.1.A SELECTION OF BIOSWALE TYPE APPROPRIATE FOR SITE Site Circumstances Bioswale Type Flow enters at head of swale  Longitudinal slope 1.5% or less OR  Located downstream of a Flow Control Duration Standard or Flood Problem Flow Control detention facility Wet bioswale (Section 6.3.1) Flow enters at head of swale  Longitudinal slope between 1 and 2%  Soil saturation or base flows likely in wet season EITHER wet bioswale (Section 6.3.2), OR basic bioswale (Section 6.3.1), depending on site; may require underdrain or low-flow drain. Flow enters at head of swale  Longitudinal slope between 2% and 5%  Base flows may or may not be likely in wet season  Not downstream of Flow Control Duration Standard or Flood Problem Flow Control detention facility Basic bioswale (Section 6.3.1); may require low-flow drain, depending on site Along a roadway or parking lot with:  Sheet inflow into the bioswale, OR  Numerous discrete inflows with no single inflow contributing more than about 10% of total swale flow Lateral inflow bioswale (Section 6.3.3)  BASIC WQ OPTION 2  FILTER STRIP A filter strip is a gently sloped grassed area which treats stormwater runoff from adjacent paved areas before it concentrates into discrete channels; see Section 6.3.4 for design details. TSS removal is achieved by particle settling.  BASIC WQ OPTION 3  WETPOND Wetponds are stormwater ponds that maintain a pool of water for most of the year. Stormwater entering the pond is treated during the relatively long residence time within the pond. Wetpond volume described in Section 6.2.1 for the Basic treatment menu is determined directly by the approved continuous runoff model. Alternatively, the manual sizing method provided for use in this manual calculates the wetpond volume based on a method developed by the Natural Resources Conservation Service (NRCS, formerly the Soil Conservation Service [SCS]). See Section 6.4.1 for design details.  BASIC WQ OPTION 4  WETVAULT An underground vault may be used to comply with the Basic Water Quality menu. The treatment volume is the same as for the basic wetpond; see Section 6.4.2 for design details.  BASIC WQ OPTION 5  STORMWATER WETLAND A stormwater wetland uses biological processes of plant uptake and bacterial degradation as well as physical and chemical processes, e.g., stilling, and gravity settling to remove pollutants. The footprint of the stormwater wetland is sized based on the wetpond sizing, but the depth of water in the second cell is reduced to encourage plant growth; see Section 6.4.3 for design details.  BASIC WQ OPTION 6  COMBINED DETENTION AND WETPOOL FACILITIES This option allows the wetpond, wetvault, or stormwater wetland to be placed under the detention facility live storage. Where site conditions permit its use, this option occupies less space than separate siting of 6.1.1 BASIC WATER QUALITY MENU 2022 City of Renton Surface Water Design Manual 6/22/2022 6-7 detention and water quality facilities. The basic wetpond portion of the combined facility is sized using the same method as the wetpond in Option 3; see Section 6.4.4 for design details.  BASIC WQ OPTION 7  SAND FILTER A sand filter is a land depression, pond, or vault, with a bed of sand near the bottom. Stormwater is treated as it percolates downward through the sand layer. Removal efficiency for sand filters is much less sensitive to particle density distribution as compared to that of particle settling facilities (e.g., ponds, vaults, bioswales), which include an assumption that the particle density is close to that of silica sand. Sand filters may be built as open ponds, underground vaults or linear perimeter trenches; see Section 6.5.2 for basic and large sand filters, Section 6.5.3 for sand filter vaults, and Section 6.5.4 for linear sand filters. A sand layer may also be installed above an infiltration pond or vault to treat stormwater before it infiltrates. Note: Presettling is required prior to sand filtration as described in Section 6.5.1.  BASIC WQ OPTION 8  PROPRIETARY FACILITIES Most proprietary facilities for basic treatment are cartridge filters, although there are some media filter designs that do not involve cartridges. A cartridge filter system is a flow-through stormwater filtration system comprised of a manhole or vault that houses one or more media-filled or porous membrane cartridges through which stormwater is filtered. Note: a presettling cell or facility is required for both cartridge filters and for non-cartridge media filters. Approved proprietary facilities are listed in Table 6.1.1.B as well as in Reference Section 14-A and 14-B of this manual. The City reserves the right to modify the list of proprietary facilities approved for public maintenance at any time. Section 1.4 of Chapter 1 and Reference Section 8 provide relevant information on the process necessary to obtain approvals of other proprietary facilities. TABLE 6.1.1.B PROPRIETARY FACILITIES ON THE BASIC WQ MENU Proprietary Facility Name Publicly Maintained Privately Maintained BayFilter Stormwater Treatment System w/ Enhanced 545 Media Catridge X X BioPod X X Boxless BioPod X X EcoStorm Plus X Filterra X X Filterra Bioscape X Jellyfish Filter X Kraken Filter X Modular Wetlands Linear X X PerkFilter w/ ZPC Media X X StormFilter w/ PhosphoSorb Media X StormFilter w/ ZPG Media X X StormGarden Biofilter X StormTree X Up-Flo Filter w/ Filter Ribbons X WetlandMod X X Other Facilities with a General Use Level Designation (GULD) for Basic Treatment X SECTION 6.1 WATER QUALITY MENUS 6/22/2022 2022 City of Renton Surface Water Design Manual 6-8  BASIC WQ OPTION 9  BIORETENTION A bioretention facility is a shallow landscaped depression designed to temporarily store and promote infiltration of stormwater runoff; see Section 6.8. Where bioretention is intended to fully meet treatment requirements for its drainage area, it must be designed, using an approved continuous runoff model, to pass at least 91% of the influent runoff file through the imported soil mix.  BASIC WQ OPTION 10  WSDOT WQ FACILITIES WSDOT has developed several water quality facilities that may be used to meet basic water quality. These facilities include the media filter drain or MFD (formerly known as the Ecology Embankment), compost- amended vegetated filter strips (CAVFS), and compost-amended biofiltration swales (CABS); see Section 6.9. The MFD is a linear flow-through treatment facility that includes four basic components: a gravel no- vegetation zone, a grass strip, the MFD mix bed, and a conveyance system for flows leaving the MFD mix (typically a gravel-filled underdrain trench or a layer of crushed surfacing base course). MFDs are typically used in areas with limited right-of-way such as highway side slopes, medians, ditches and other linear depressions. CAVFS and CABS are variations of the basic vegetated filter strip and bioswale, respectively, which incorporate compost to provide Enhanced Basic WQ treatment. The addition of compost into native soils also improves plant health and sustainability, increases surface roughness, and improves infiltration capacity. 6.1.2 ENHANCED BASIC WATER QUALITY MENU Where applied: The Enhanced Basic Water Quality menu5 is applied where an enhanced level of treatment is required for those development sites with land uses that generate the highest concentrations of metals in stormwater runoff and drain by surface flows to a fish-bearing stream. Metals including but not limited to copper and zinc are toxic to fish and other aquatic biota.6 For precise details on the application of this and other water quality menus, refer to Section 1.2.8, “Core Requirement #8: Water Quality Facilities.” Note: The Enhanced Basic menu is a stand-alone menu. It integrates the Basic menu level of protection (TSS removal) and the additional measures needed to achieve a higher level of metals removal. When this menu is required in Basic WQ Treatment Areas per Section 1.2.8.1.A of Core Requirement #8, it is intended to replace the Basic WQ menu on development sites or portions of development sites that generate the highest concentrations of metals in stormwater runoff. When this menu is required in Sensitive Lake WQ Treatment Areas per Section 1.2.8.1.B, it is intended to be combined with the Sensitive Lake Protection Menu such that a facility design option common to both menus must be used. Treatment goal: The Enhanced Basic WQ menu is designed to achieve > 30% dissolved copper removal and > 60% dissolved zinc removal; in addition to Basic treatment (80% TSS removal) for flows up to and including the WQ design flow or volume (defined in Section 6.2.1). The goal assumes that dissolved copper concentrations for untreated runoff are between 5 and 20 micrograms per liter (u/L), and that dissolved zinc concentrations for untreated runoff are between 20 and 300 micrograms per liter (ug/L). Basis: The treatment goal is expressed in terms of dissolved copper and zinc removal. Copper and zinc are reliable indicators of a wider range of heavy metals and are typically found in stormwater runoff from industrial, commercial, and high density residential land uses at levels that are toxic to fish and other aquatic biota. Many metals are readily adsorbed onto particulates in the runoff, usually the finer fraction of the particulates. Facility combinations that remove more of the particulate load than the Basic menu, including the finer fraction, are specified by the Enhanced Basic menu. Facilities providing organic 5 The Enhanced Basic WQ menu targets different pollutants than the lake or bog protection menus. It does not necessarily provide a higher level of treatment except for the target pollutant, metal contaminants. 6 Other metals, e.g., lead, are toxic to humans and may build up in sediments. 6.1.1 BASIC WATER QUALITY MENU 2022 City of Renton Surface Water Design Manual 6/22/2022 6-9 binding sites that enhance metal adsorption are also specified. The treatment goals have been found by WA Ecology to be achievable.  ENHANCED BASIC OPTION 1  LARGE SAND FILTER This option includes use of a large sand filter, large sand filter vault, or large linear sand filter. Sizing specifications for these facilities can be found in Sections 6.5.2, 6.5.3, and 6.5.4, respectively. Note: Presettling is required prior to sand filtration as described in Section 6.5.1.  ENHANCED BASIC OPTION 2  STORMWATER WETLAND Provision of a stormwater wetland (see Section 6.4.3) or combined detention and stormwater wetland (see Section 6.4.4) satisfies the Basic (TSS) and Enhanced Basic (dissolved copper and zinc) removal goals without additional facilities.  ENHANCED BASIC OPTION 3  TWO-FACILITY TREATMENT TRAIN This option uses one of the basic water quality treatment options listed in Table 5.1.2.A followed by a basic sand filter (see Section 6.5.2), sand filter vault (see Section 6.5.3), or a linear sand filter (see Section 6.5.4). TABLE 6.1.2.A PAIRED FACILITIES FOR ENHANCED BASIC TREATMENT TRAIN, OPTION 3 First Basic WQ Facility: Second WQ Facility: Bioswale (Sections 6.3.1, 6.3.2, and 6.3.3) Basic sand filter or sand filter vault (Section 6.5.2 or 6.5.3) or proprietary facility7 Filter strip (Sections 6.3.4 and 6.3.5) Linear sand filter (Section 6.5.4) with no presettling cell needed Linear sand filter (Section 6.5.4) Filter strip (Sections 6.3.4 and 6.3.5) Basic wetpond (Section 6.4.1) Basic sand filter or sand filter vault (Section 6.5.2 or 6.5.3) or proprietary facility7 Wetvault (Section 6.4.2) Basic sand filter or sand filter vault (Section 6.5.2 or 6.5.3) or proprietary facility7 Basic combined detention and wetpool facility (Section 6.4.4) Basic sand filter or sand filter vault (Section 6.5.2 or 6.5.3) or proprietary facility7 Basic sand filter or sand filter vault (Sections 6.5.2 or 6.5.3). A presettling cell is required if the sand filter is not preceded by a detention facility. Proprietary facility7 Proprietary facility approved by the City for Basic WQ7 (Section 6.7) Basic sand filter or sand filter vault (Section 6.5.2 or 6.5.3)  ENHANCED BASIC OPTION 4  BIORETENTION Provision of a bioretention facility (see Section 6.4.3) satisfies the Basic (TSS) and Enhanced Basic (dissolved copper and zinc) removal goals without additional facilities. Stormwater runoff that infiltrates through the imported soil mix will have received Enhanced Basic treatment.  ENHANCED BASIC OPTION 5  PROPRIETARY FACILITY Section 6.7, “Proprietary Facility Designs,” discusses general considerations for proprietary manufactured facilities. Current approvals for publicly and privately maintained systems are included in Table 6.1.2.B 7 See Reference Section 14-A for City-approved proprietary facilities. SECTION 6.1 WATER QUALITY MENUS 6/22/2022 2022 City of Renton Surface Water Design Manual 6-10 and Reference Section 14-A and 14-B. The City reserves the right to modify the list of proprietary facilities approved for public maintenance at any time. TABLE 6.1.2.B PROPRIETARY FACILITIES ON THE ENHANCED BASIC WQ MENU Proprietary Facility Name Publicly Maintained Privately Maintained BioPod X X Boxless BioPod X X Filterra X X Filterra Bioscape X Modular Wetlands Linear X X StormTree X WetlandMod X X Other Facilities with a General Use Level Designation (GULD) for Enhanced Treatment X  ENHANCED BASIC OPTION 6  WSDOT WQ FACILITIES Provision of a MFD, CAVFS, or CABS (see Section 6.4.3) satisfies the Basic (TSS) and Enhanced Basic (dissolved copper and zinc) removal goals without additional facilities. 6.1.3 SENSITIVE LAKE PROTECTION MENU This section is not currently applicable to the City of Renton. Where applied: The Sensitive Lake Protection menu is applied to the watersheds of lakes that have been determined to be particularly sensitive to phosphorus and that are being managed to reduce water quality impacts. This menu applies to stormwater conveyed to the lake by surface flow as well as to stormwater infiltrated within one-quarter mile of the lake in soils with high infiltration rates (i.e., measured rate exceeding 9 inches per hour). If stormwater is infiltrated further than one-quarter mile from the lake, then the Basic WQ menu is applied unless the project is exempt from Core Requirement #8 per Section 1.2.8. For precise details on the application of this and other area-specific water quality menus, refer to Section 1.2.8, “Core Requirement #8: Water Quality.” Note: The Sensitive Lake Protection menu is a stand-alone menu. It integrates the Basic WQ menu level of protection (TSS removal) and the additional protection needed to achieve lake protection goals in the options described below. When this menu is required as specified in Core Requirement #8 (see Section 1.2.8), it is intended to replace the Basic WQ menu in the watersheds of sensitive lakes. Treatment goal: The Lake Protection menu is designed to achieve a goal of 50 percent total phosphorus (TP) removal for the WQ design flow or volume (defined in Section 6.2.1), assuming typical forms and concentrations of phosphorus in untreated stormwater runoff.8 Basis: The Lake Protection menu will result in removal of more of the TSS load, including more of the finer fraction of TSS, than the Basic menu. The additional increment of solids removal will also provide enough phosphorus removal to meet the TP goal stated above. 8 Typical TP concentrations in untreated Seattle-area runoff are considered to be between 0.10 and 0.50 mg/L. For projects that are expected to generate higher levels of TP, such as animal husbandry operations, a higher treatment goal may be appropriate. 6.1.2 ENHANCED BASIC WATER QUALITY MENU 2022 City of Renton Surface Water Design Manual 6/22/2022 6-11  LAKE PROTECTION OPTION 1  LARGE WETPOND The 50 percent TP removal goal can be satisfied by use of a large wetpond or large combined detention and wetpond sized so that the wetpond volume is 1.5 times the Basic water quality volume as determined either by the approved continuous runoff model or as calculated using the manual method described in Section 6.4.1. See Section 6.4.1.1 for the large wetpond design, and Section 6.4.4.1 for the large combined pond design. Note: A large wetvault option is not included in this menu since the biological processes thought to remove phosphorus do not take place in underground vaults.  LAKE PROTECTION OPTION 2  LARGE SAND FILTER This option includes use of a large sand filter, large sand filter vault, or large linear sand filter. Sizing specifications for these facilities can be found in Sections 6.5.2, 6.5.3, and 6.5.4, respectively. Note: Presettling is required prior to sand filtration as described in Section 6.5.1.  LAKE PROTECTION OPTION 3  TWO-FACILITY TREATMENT TRAIN This option involves use of one of the basic water quality treatment options, listed in Table 6.1.3.A, followed by either a basic sand filter (Section 6.5.2) or basic sand filter vault (Section 6.5.3). For dispersed flows, a linear sand filter may be used as the second facility. TABLE 6.1.3.A PAIRED FACILITIES FOR LAKE PROTECTION TREATMENT TRAIN, OPTION 3 First Basic WQ Facility Second WQ Facility Bioswale (Sections 6.3.1, 6.3.2, and 6.3.3) Basic sand filter or sand filter vault (Section 6.5.2 or 6.5.3) Filter strip (Sections 6.3.4 and 6.3.5) Linear sand filter (no presettling cell needed) (Section 6.5.4) Linear sand filter (Section 6.5.4) Filter strip (Sections 6.3.4 and 6.3.5) Basic wetpond (Section 6.4.1) Basic sand filter or sand filter vault (Section 6.5.2 or 6.5.3) Wetvault (Section 6.4.2) Basic sand filter or sand filter vault (Section 6.5.2 or 6.5.3) Stormwater wetland (Section 6.4.3) Basic sand filter or sand filter vault (Section 6.5.2 or 6.5.3) Basic combined detention and wetpool facility (Section 6.4.4) Basic sand filter or sand filter vault (Section 6.5.2 or 6.5.3) Proprietary facility approved by the City for Basic WQ9 9 (Section 6.7) Basic sand filter or sand filter vault (Section 6.5.2 or 6.5.3)  LAKE PROTECTION OPTION 4  BASIC MENU PLUS PHOSPHORUS CREDIT This option provides credit to developments that integrate land use and site design measures to prevent or reduce the levels of phosphorus leaving the site. Credit is also given for the voluntary use of extra levels of onsite detention, since less in-stream erosion is likely to take place with more highly controlled stormwater releases. This reduction in in-stream erosion and bank failure translates directly into control of the phosphorus load delivered to downstream lakes. The measures for which credit is given are detailed below, along with the point values assigned to each of the actions. Providing any combination of these measures equaling 10 points or more earns this credit. The 9 See Reference Section 14-A for City-approved proprietary facilities. SECTION 6.1 WATER QUALITY MENUS 6/22/2022 2022 City of Renton Surface Water Design Manual 6-12 credit excuses the applicant from the requirement to provide a second water quality facility. Thus, even though the development is located in the watershed of a sensitive lake, the water quality requirements can be fully met with the provision of a single water quality facility from the Basic Water Quality menu. Credit-Earning Actions Several land use actions and source controls are particularly effective in reducing phosphorus. These actions are not required by this manual or other regulations; they are an alternative to end-of-the-pipe treatment of stormwater. Credit options for phosphorus-reducing actions are described below. 1. Leaving at least 65 percent of the site undisturbed, including undevelopable land. Full credit, or 10 points, is awarded for leaving 65 percent of a site in undisturbed native vegetation or allowing native vegetation to re-establish. Critical areas and their buffers may be counted. All areas for phosphorus credit must be in tracts dedicated to the City or protected by covenant. A descending scale of points applies where lower percentages of the site are left undisturbed. Possible credit = 1 to 10 points. 2. Providing extra flow control. Credit for providing extra flow control applies only in cases where site runoff travels via stream or open drainage system to the sensitive lake. Voluntary use of the Flow Control Duration Standard when the Peak Rate Flow Control Standard would be required = 5 points. Voluntary use of the Flood Problem Flow Control Standard when the Peak Rate Flow Control Standard would be required = 8 points. Voluntary use of the Flood Problem Flow Control Standard when the Flow Control Duration Standard would be required = 3 points. Possible credit = 3 to 8 points. 3. Directing runoff from target pollution-generating surfaces to grassy areas with level spreading. Directing runoff from target pollution-generating areas to grassy areas that are not routinely fertilized or to areas of native vegetation results in pollutant removals similar to those obtained in swales while also providing an increased opportunity for infiltration. To use this option, flows must remain unconcentrated and be spread uniformly over the intended area. (Flow spreader details are given in Section 6.2.6.) In general, the vegetated area receiving dispersed flows should be at least 25 percent as large as the area contributing flow. The receiving area should be increased by one percent for each percent increase in slope over four percent. The area should be configured so that the length of the flow path is no longer than the width over which flows are dispersed. Example: Assume a parking lot is 100′  600′, or 60,000 sf. Flows will be dispersed through an adjacent area of native vegetation with a slope of 8 percent. The area of vegetation must be at least 17,400 sf (i.e., 25% + 4% (for the 8% slope)  60,000 sf). Assuming runoff is dispersed continuously along the wider edge of the parking lot, the flow path would need to be at least 29 feet (17,400′  600′). If the water were dispersed along the shorter edge, flow path would be 174 feet (17,400′  100′). However, this flow path would be longer than the width over which flows were dispersed (100′), and would not be a satisfactory option. The parking lot could be graded, however, so that flows would be dispersed at both of the 100 foot ends, making each flow path 87 feet, which would be acceptable. Credit is proportional to the total volume of runoff diverted; one point is earned for every 25 percent of total volume so directed. Possible credit = 1 to 4 points. 4. Providing covered vehicle washing areas connected to the sanitary sewer system. This credit applies to commercial, industrial, and multifamily sites excluding commercial car washes or other operations where this action is already required by other regulations . Frequent car-washing can contribute significant amounts of phosphorus to stormwater. Note that sewer districts may have pretreatment requirements before allowing connection to the sanitary sewer. Possible credit = 3 points. Table 6.1.3.B details the credit options and associated point totals. 6.1.3 SENSITIVE LAKE PROTECTION MENU 2022 City of Renton Surface Water Design Manual 6/22/2022 6-13 Credit may be applied to the whole site or to a natural discharge area within the site. It may be advantageous for a developer to concentrate only on a natural discharge area if the point total for that particular area could equal 10. For example, assume a particular natural discharge area is one half the total site area. If 65 percent of the land area in the natural discharge area will remain undisturbed, that natural discharge area is eligible for 10 points (see Table 6.1.3.B). The stormwater from that natural discharge area could be treated with a single water quality facility from the Basic WQ menu; the second facility could be waived. The rest of the site would still have the two-facility requirement. Alternatively, if the entire site were considered, the undisturbed area decreases to 35 percent, eligible for only 3 points. In this case, the developer would need to implement other controls worth 7 points in order to waive the second water quality facility for the entire site. If the credit option is used, it shall be applied for during initial drainage review by CED. The application shall include a written request for credit based on either the site plan or the grading plan for the project, and the threshold discharge areas shall be delineated on the plans. The request shall outline where the credit would be applied and how the point totals are to be achieved. CED would then evaluate the request and may waive the second water quality treatment requirement for the site or threshold discharge area based on point totals outlined in Table 6.1.3.B (below). Credit is not given unless requested. TABLE 6.1.3.B WATER QUALITY CREDIT FOR PHOSPHORUS CONTROL Credit Option Points Leaving site undisturbed, in native vegetation. At least 65 % = 10 60% = 9 50% = 7 40% = 5 30% = 3 20% = 1 Directing road runoff to pervious, non-pollution-generating vegetated area. 100% of volume = 4 75% of volume = 3 50% of volume = 2 25% of volume = 1 Covered car wash area connected to sanitary sewer (multifamily, commercial, or industrial sites, except for commercial car-wash businesses). 3 Extra detention with next most restrictive release rate (if discharge to stream). Peak Rate Flow Control  Flow Control Duration Standard = 5 Peak Rate Flow Control  Flood Problem Flow Control = 8 Flow Control Duration Standard  Flood Problem Flow Control = 3  LAKE PROTECTION OPTION 5  PROPRIETARY FACILITY Section 6.7, “Proprietary Facility Designs,” discusses general considerations for proprietary manufactured facilities. Current approvals for publicly and privately maintained systems are included in Table 6.1.3.C and Reference Section 14-A and 14-B. The City reserves the right to modify the list of proprietary facilities approved for public maintenance at any time. SECTION 6.1 WATER QUALITY MENUS 6/22/2022 2022 City of Renton Surface Water Design Manual 6-14 TABLE 6.1.3.C PROPRIETARY FACILITIES ON THE SENSITIVE LAKE PROTECTION MENU Proprietary Facility Name Publicly Maintained Privately Maintained BayFilter Stormwater Treatment System w/ Enhanced 545 Media Cartrdige X X BioPod X X Boxless BioPod X X Filterra X X Filterra Bioscape X Jellyfish Filter X Kraken Filter X Modular Wetlands Linear X X PerkFilter w/ ZPC Media X X StormFilter w/ PhosphoSorb Media X StormGarden Biofilter X StormTree X Up-Flo Filter w/ Filter Ribbons X Other Facilities with a General Use Level Designation (GULD) for Phosphorus Treatment X  LAKE PROTECTION OPTION 6  WSDOT WQ FACILITIES WSDOT has developed the media filter drain that may be used to meet lake protection. 6.1.4 SPHAGNUM BOG PROTECTION MENU This section is not currently applicable to the City of Renton. Where applied: The Sphagnum Bog Protection menu10 covers sphagnum bog wetlands11 greater than 0.25 acres in size.12 It applies to stormwater conveyed by surface flow to the sphagnum bog vegetation community. If stormwater is infiltrated by the project per Section 5.2, then the Basic WQ menu is applied unless the project is exempt from Core Requirement #8, “Water Quality.” For precise details on the application of this and other area-specific water quality menus, refer to Section 1.2.8. Note: The Sphagnum Bog Protection menu is a stand-alone menu. It integrates the Basic WQ menu level of protection and the additional measures needed to achieve bog protection goals in the options described below. When this menu is required as specified in Core Requirement #8 (see Section 1.2.8), it is intended to replace the Basic WQ menu in areas draining to sphagnum bogs. Treatment goal: If surface water must be discharged to a bog, the treatment goal is to reduce total phosphorus by 50 percent, reduce nitrate + nitrite by 40 percent, maintain alkalinity below 10 mg/L, calcium concentrations should be less than 2 mg/L, and maintain pH below 6.0.13 10 The Bog Protection menu targets a different set of pollutants than the Sensitive Lake or Enhanced Basic menus. Since the targeted pollutants are more difficult to remove, use of larger and/or additional water quality facilities is required. 11 A sphagnum bog wetland is defined as a wetland having a predominance of sphagnum moss creating a substrate upon which a distinctive community of acid-loving plants is established (see Section 1.2.8.C and "Definitions" for more detail). 12 The size of a sphagnum bog wetland is defined by the boundaries of the sphagnum bog plant community. 13 Calcium, alkalinity, and pH values are from : Kulzer, L., S. Luchessa, S. Cooke, R. Errington, F. Weinmann, and D. Vitt. 2001. Characteristics of the low-elevation sphagnum-dominated peatlands of western Washington: A community profile. King County, WA: King County Water and Land Resources Division. 6.1.3 SENSITIVE LAKE PROTECTION MENU 2022 City of Renton Surface Water Design Manual 6/22/2022 6-15 Basis: In their undeveloped condition, bogs are isolated from surface water, being supplied almost solely by rainwater. The best strategy for protection of bog water quality is to infiltrate the water quality design volume while routing high flows around the bog. Although it is not known whether alkalinity or nitrogen can be reduced sufficiently by the options outlined below, there are no other technologically-feasible alternatives at this time. An adjustment (see Section 1.4) could be pursued as additional technologies become available.  SPHAGNUM BOG PROTECTION OPTION 1  LARGE WETPOND FOLLOWED BY LARGE SAND FILTER This option uses a large wetpond (see Section 6.4.1) or a large combined detention and wetpond (see Section 6.4.2), sized so that wetpond volume is 1.5 times the Basic water quality volume as determined either by the approved continuous runoff model or as calculated using the manual method described in Section 6.4.1. A large sand filtration facility (see Section 6.5.2 or 6.5.3) must follow the pond. In order to ensure that algae and sources of alkalinity from the pond are not washed from the pond into the bog, the sand filter must be the last facility. The sand used for filtration must be silica-based sand rather than an aragonite14 sand.  SPHAGNUM BOG PROTECTION OPTION 2  STORMWATER WETLAND IN SERIES WITH A LARGE SAND FILTER This option uses a stormwater wetland (see Section 6.4.3) or combined detention and stormwater wetland (see Section 6.4.4) to remove solids and enhance the concentration of organic acids, and a large sand filter (see Section 6.5.2) to remove the finer sediment for alkalinity and nutrient reduction. The sand used for filtration must be silica-based sand rather than an aragonite sand. The order of facilities is interchangeable since there are both advantages and disadvantages to having the sand filter last in the train. Note: Presettling is required prior to sand filtration as described in Section 6.5.1 .  SPHAGNUM BOG PROTECTION OPTION 3  LARGE SAND FILTER IN SERIES WITH A PROPRIETARY FACILITY This option uses a large sand filter or large sand filter vault followed by a proprietary facility. Sizing specifications for the large sand filters can be found in Sections 6.5.2 and 6.5.3. Proprietary facilities are detailed in Reference Section 14-A and 14-B. The sand used for filtration must be silica-based sand rather than an aragonite sand. Note: Presettling is required prior to sand filtration as described in Section 6.5.1.  SPHAGNUM BOG PROTECTION OPTION 4  THREE-FACILITY TREATMENT TRAIN This option uses one of the basic water quality treatment options followed by two other facilities. Table 6.1.4.A lists the possible choices of facilities for this option. 14 Aragonite is the second most common type of sand, and is composed of calcium carbonate from biota including but not limited to coral and shellfish. (Sand. (2014, April 12). In Wikipedia, The Free Encyclopedia. Retrieved 20:38, April 15, 2014, from <http://en.wikipedia.org/w/index.php?title=Sand&oldid=603938376>) SECTION 6.1 WATER QUALITY MENUS 6/22/2022 2022 City of Renton Surface Water Design Manual 6-16 TABLE 6.1.4.A FACILITY COMBINATIONS FOR BOG PROTECTION TREATMENT TRAIN, OPTION 4 First Facility Second Facility Third Facility Bioswale (Sections 6.3.1, 6.3.2, and 6.3.3) Basic sand filter (Sections 6.5.2, 6.5.3, or 6.5.4) Proprietary facility15 Filter strip (Sections 6.3.4 and 6.3.5) Basic wetpond (Section 6.4.1) Basic combined detention and wetpool facility (Section 6.4.4) Wetvault (Section 6.4.2) Stormwater wetland (Section 6.4.3) Proprietary facility 16F 16 (Section 6.7) * Other treatment options may be pursued through an adjustment per Section 1.4. 6.1.5 HIGH-USE MENU Where applied: The High-Use menu is applied to all new development and redevelopment projects that have high-use site characteristics, as defined in Chapter 1 (see “Special Requirement # 5, Oil Control”). Oil control devices are to be placed upstream of other facilities, as close to the source of oil generation as practical. Gasoline service stations will likely exceed the high-use site threshold. Note: Where this menu is applicable, it is in addition to the area-specific WQ menus. Treatment goal: Oil control options given in the High-Use menu are designed to meet the goals of no visible sheen or less than 10 mg/L total petroleum hydrocarbons (TPH) leaving the site.  OIL CONTROL OPTION 1  CATCH BASIN INSERT This oil control option is not allowed in the City of Renton.  OIL CONTROL OPTION 2  BAFFLE OIL/WATER SEPARATOR Baffle oil/water separators (see Section 6.6.2) may be used to treat stormwater runoff from high-use developments and facilities that produce relatively high concentrations of oil and grease. Baffle separators historically have been effective in removing oil having droplet sizes of 150 microns or larger. If sized properly, they can achieve effluent concentrations as low as 10 to 15 mg/L.  OIL CONTROL OPTION 3  COALESCING PLATE OIL/WATER SEPARATOR Coalescing plate separators (see Section 6.6.2) may be used to treat stormwater runoff from high-use developments and facilities that can produce relatively high concentrations of oil and grease. Current technology and design of coalescing plate separators achieve effluent concentrations as low as 10 mg/L with removal of oil droplet sizes as small as 20 to 60 microns.  OIL CONTROL OPTION 4  LINEAR SAND FILTER The linear sand filter (see Section 6.5.4) is used in the Core Requirement #8 water quality menus (i.e., the Basic, Enhanced Basic, Sensitive Lake, and Sphagnum Bog menus), as well as for oil control in the High- Use menu (Special Requirement #5). However, if used to satisfy Core Requirement #8, the same facility shall not also be used to satisfy the oil control requirement (Special Requirement #5) unless enhanced maintenance is ensured. This is to prevent clogging of the filter by oil so that it will function for suspended solids, metals, and phosphorus removal as well. Quarterly cleaning is required at a minimum unless more frequent cleaning is specified otherwise by the designer. 15 See Reference Section 14-A for City-approved proprietary facilities. 16 See Reference Section 14-A for approved proprietary facilities. 6.1.6 PRETREATMENT FACILITIES 2022 City of Renton Surface Water Design Manual 6/22/2022 6-17  OIL CONTROL OPTION 5  WETVAULT WITH BAFFLE A wetvault may be modified to fulfill requirements for oil control provided the following are true: 1. The criteria given at the end of Section 6.4.2.2 for modification of wetvaults for use as a baffle oil/water separators shall be met, and 2. Assurance is provided that the maintenance frequency and oil removal frequency for baffle oil/water separators will be followed (see Section 6.6.2).  OIL CONTROL OPTION 6  PROPRIETARY FACILITIES Section 6.7, “Proprietary Facility Designs,” discusses general considerations for proprietary manufactured facilities. Current approvals for publicly and privately maintained systems are included in Table 6.1.5.A and Reference Section 14-A and 14-B. The City reserves the right to modify the list of proprietary facilities approved for public maintenance at any time. TABLE 6.1.5.A PROPRIETARY FACILITIES ON THE HIGH-USE MENU Proprietary Facility Name Publicly Maintained Privately Maintained Filterra X X Filterra Bioscape X Other Facilities with a General Use Level Designation (GULD) for Oil Treatment X  OIL CONTROL OPTION 7  COMPLIANCE WITH OTHER AGENCY REQUIREMENTS If the site has a National Pollutant Discharge Elimination System (NPDES) industrial stormwater permit that specifically addresses oil control for the target pollution-generating impervious surface of the site, compliance with NPDES permit conditions may be adequate to comply with the oil control requirements of Special Requirement #5. Copies of the site’s NPDES permit requirement and the best management practices specifically addressing oil control shall be submitted to determine adequacy. If the area under the covered fueling island drains to the sanitary sewer, then only the remaining high-use area actually draining to the storm drainage system (normally ingress and egress routes) need comply with the High-Use menu. Note: Ecology requires that fueling islands be paved with Portland cement concrete (or equivalent, not including asphaltic concrete) and must drain to a dead-end sump or spill control separator in compliance with the UFC or IFC, and recommends draining from the sump to a sanitary sewer. An alternative to discharge to a sanitary sewer is to collect stormwater from the fuel island containment pad and hold for proper off-site disposal. Drains to treatment facilities must have a normally closed shutoff valve. The spill control sump must be sized in compliance with Section 7901.8 of the Uniform Fire Code (UFC). Alternatively the fueling island must be designed as a spill containment pad with a sill or berm raised to a minimum of four inches (Section 7901.8 of the UFC) to prevent the runoff of spilled liquids and to prevent run-on of stormwater from the surrounding area. (See Ecology’s Stormwater Management Manual for Western Washington, Volume IV, Section 2.2, S409 BMPs for Fueling At Dedicated Stations. These BMPs are also required by the City of Renton for new construction. SECTION 6.1 WATER QUALITY MENUS 6/22/2022 2022 City of Renton Surface Water Design Manual 6-18 6.1.6 PRETREATMENT FACILITIES  PRETREATMENT FACILITIES OPTION 1  PROPRIETARY FACILITY DESIGN Current approvals for publicly and privately maintained systems are included in Table 6.1.6.A and Reference Section 14-A and 14-B. The City reserves the right to modify the list of proprietary facilities approved for public maintenance at any time. TABLE 6.1.6.A PROPRIETARY FACILITIES ON THE PRETREATMENT FACILITIES MENU Proprietary Facility Name Publicly Maintained Privately Maintained Aqua-Swirl CONCENTRATOR Stormwater Treatment System X BaySeparator Stormwater Treatment System X CDS X Downstream Defender X Stormceptor X Vortechs X Other Facilities with a General Use Level Designation (GULD) for Pretreatment X CITY OF RENTON SURFACE WATER DESIGN MANUAL 2022 City of Renton Surface Water Design Manual 6/22/2022 6-19 6.2 GENERAL REQUIREMENTS FOR WQ FACILITIES This section presents general requirements and other information applicable to the design of water quality (WQ) facilities. Topics covered include the following:  “Water Quality Design Flows,” Section 6.2.1  “Sequence of Facilities,” Section 6.2.2  “Setbacks, Slopes, and Embankments,” Section 6.2.3  “Facility Liners,” Section 6.2.4  “Flow Splitter Designs,” Section 6.2.5  “Flow Spreading Options,” Section 6.2.6 When detail in the WQ designs is lacking, refer to Chapter 5 for guidance. In cases where requirements are extremely costly, a less expensive alternative that is functionally equivalent in terms of performance, environmental effects, health and safety, and maintenance may be sought through the adjustment process (see Section 1.4). Proprietary Facility Designs Current proprietary facility approvals for publicly and privately maintained systems are included in Reference Section 14-A and 14-B. Other proprietary facilities that have received a general use level designation (GULD) through the state Department of Ecology’s Technology Assessment Protocol – Ecology (TAPE) program will be considered for approval by the City through an adjustment process for water quality treatment. A list of Ecology GULD approved proprietary facilities can be found on the Department of Ecology website at <http://www.ecy.wa.gov/programs/wq/stormwater/newtech/index.html>.17F 17 Use of Materials Galvanized metals leach zinc into the environment, especially in standing water situations. High zinc concentrations, sometimes in the range that can be toxic to aquatic life, have been observed in the region.18 Therefore, use of galvanized materials in stormwater facilities and conveyance systems is discouraged. Where other metals, such as aluminum or stainless steel, or plastics are available, they shall be used. Groundwater Protection Water quality facilities that allow runoff to have direct contact with the soil, such as wetponds, biofiltration swales, bioretention facilities, infiltration facilities and stormwater wetlands, are prohibited in Zone 1 of the Aquifer Protection Area. 6.2.1 WATER QUALITY DESIGN FLOWS AND TREATMENT VOLUMES Water Quality Design Flow The water quality design flow is defined as follows:  Downstream of detention: The full 2-year release rate from the detention facility, determined using the approved continuous runoff model.  Preceding detention, or when detention facilities are not required : The flow rate from the drainage basin at or below which 91% of the total runoff volume will be treated. Design criteria for treatment facilities are assigned to achieve the applicable performance goal at the water quality design flow rate (e.g., 80 percent TSS removal). At a minimum, 91% of the total runoff volume, as estimated 17 Footnote 18 is not used. 18 Finlayson, 1990. Unpublished data from reconnaissance of Metro Park and Ride lot stormwater characteristics. SECTION 6.2 GENERAL REQUIREMENTS FOR WQ FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 6-20 by an approved continuous runoff model with 15-minute time steps calibrated to site conditions, must pass through the treatment facility(ies) at or below the approved hydraulic loading rate for the facility(ies). Design flow rates for water quality facilities designed using this manual are calculated using a continuous simulation model. Most of the performance research on biofiltration BMPs has been conducted on facilities that used event-based designs. The volume of treatment runoff can be predicted from a 24-hour storm with a 6-month return frequency (a.k.a., 6-month, 24-hour storm). However, the 91st percentile flow event (as calculated by the continuous model) tends to be less than the estimated 6-month, 24-hour event flow rate in most cases. To maintain sizing comparable to the performance research, Ecology has developed a correlation between the 91st percentile flow event calculated using the approved models and the single-event predicted flow event. This correlation is used in sizing water quality flow-based facilities in Section 6.3 and is presented in Table 6.2.1.A below. Intermediate values of the ratio k for WWHM are calculated by linear interpolation. (Note: This table does not apply to flow-based non-water quality BMPs; see relevant flow rate requirements for flow-based non-water quality BMPs in Appendix C.) TABLE 6.2.1.A ADJUSTMENT FACTOR k FOR CALCULATING MODIFIED WATER QUALITY FLOW RATE FROM MODELED ON-LINE/OFF-LINE RATES SBUH Peak/WWHM On-Line 15-Min WQ Flow Ratio vs 6-Month Precipitation for 0% to 100% Impervious Areas SBUH Peak/WWHM Off-Line 15-Min WQ Flow Ratio vs 6-Month Precipitation for 0% to 100% Impervious Areas 6-Month, 24-Hr Precipitation (72% of the 2-yr), Inches Ratio, k 6-Month, 24-Hr Precipitation (72% of the 2-yr), Inches Ratio, k 0.80 1.01 0.80 1.95 1.00 1.30 1.00 2.44 1.50 2.02 1.50 3.68 2.00 2.74 2.00 4.92 2.50 3.45 2.50 6.16 2.90 4.03 2.90 7.15 Intermediate values of k for WWHM are calculated by linear interpolation. SBUH Peak/MGSFlood On-Line and Off-Line 15-Min WQ Flow Ratio vs 6-month Precipitation for 0% to 100% Impervious Areas For on-line facilities: k = 1.4366 (P72%, 2-yr.) – 0.1369 (Eq. 6-1) For off-line facilities: k = 2.4777 (P72%, 2-yr.) – 0.0352 (Eq. 6-2) where: P72%, 2-yr = 72% of the 2-year, 24-hour precipitation depth (in.) Note: If the 6-month, 24-hour precipitation depth (in.) is known for the project site, that value may be used instead of P72%, 2-yr. The ratio between the 91st percentile flow event and the estimated 6-month, 24-hour flow rate varies with location and percent of impervious area in the modeled drainage basin. The correlations in the table account for these variations. When designing bioswales and other flow rate based facilities, multiply the on-line or off-line water quality design flow rate determined with the approved model by the coefficient k (off-line or on-line) determined from the associated table (see Methods of Analysis for guidance on selection of on-line or off-line flow rate and application of the associated correlation). Unless amended to reflect local precipitation statistics, the 6-month, 24-hour precipitation amount 6.2.1 WATER QUALITY DESIGN FLOWS AND TREATMENT VOLUMES 2022 City of Renton Surface Water Design Manual 6/22/2022 6-21 may be assumed to be 72 percent of the 2-year, 24-hour amount determined either with the approved model or by interpolating between isopluvials for the 2-year, 24-hour precipitation. Isopluvials for 2-year, 24-hour amounts for Western Washington are reprinted in Section 3.2.1, Figure 3.2.1.A. Flow Volume to be Treated When water quality treatment is required pursuant to the core and special requirements of this manual, the water quality design storm volume, when using an approved continuous runoff model, shall be equal to the simulated daily volume that represents the upper limit of the range of daily volumes that accounts for 91% of the entire runoff volume over a multi-decade period of record. Alternatively, the water quality design volume of runoff can be predicted from a 24-hour storm with a 6-month return frequency (a.k.a., 6-month, 24-hour storm). Wetpool facilities are sized based upon use of the NRCS (formerly known as SCS) curve number equations for the 6-month, 24-hour storm.19 Treatment facilities sized by this simple runoff volume-based approach are the same size whether they precede detention, follow detention, or are integral with the detention facility (i.e., a combined detention and wetpool facility). The approved model calculates the water quality design volume directly. Alternatively, the NRCS method described in Section 6.4.1.1 may be used. Unless amended to reflect local precipitation statistics, the 6-month, 24-hour precipitation amount may be assumed to be 72 percent of the 2-year, 24-hour amount. Interpolating between isopluvials for the 2-year, 24-hour precipitation and multiplying by 72% yields the appropriate storm size. Isopluvials detailed for 2-year, 24-hour amounts for western King County (including the City of Renton) are reprinted in Section 3.2.1, Figure 3.2.1.A. For locations east of the figure limits, precipitation amounts are more variable; use the 2-year, 24-hour isopluvial map located on the National Oceanic and Atmospheric Administration (NOAA) website at <http://www.nws.noaa.gov/oh/hdsc/PF_documents/Atlas2_Volume9.pdf>. Note that facilities which are sized based on volume and which include routing of flows through a detention facility, such as the detailed sand filter method, are significantly smaller when located downstream of detention, even though the same volume of water is treated in either situation. This is because the detention facility routing sequence stores peaks within the pond and releases them at a slow rate, reducing the size of the sand filter pond subsequently needed (the volume needed to store the peaks need not be provided again in the sand filter pond). Treatable Flows As stated in Chapter 1, only runoff from target pollution-generating surfaces must be treated using the water quality facility options indicated in the applicable water quality menu. These surfaces include both pollution-generating impervious surface and pollution-generating pervious surface. “Target” means that portion from which runoff must be treated using a water quality facility as specified in Chapter 1. Pollution-generating impervious surfaces are those impervious surfaces which are subject to vehicular use, industrial activities, or storage of erodible or leachable materials, wastes, or chemicals; and which receive direct rainfall or the run-on or blow-in of rainfall. Target pollution-generating impervious surfaces typically include right-of-way improvements (roads), parking areas and driveways that are not fully dispersed as specified in Section 1.2.3.2. Metal roofs are also considered to be pollution-generating impervious surface unless they are coated with an inert, non-leachable material (see Reference Section 11-E); or roofs that are subject to venting significant amounts of dusts, mists, or fumes from manufacturing, commercial, or other indoor activities. Pollution-generating pervious surfaces are those non-impervious surfaces subject to use of pesticides and fertilizers, loss of soil, or the use or storage of erodible or leachable materials, wastes, or chemicals. Target pollution-generating pervious surfaces typically include lawns and landscaped areas that are not fully dispersed and from which there will be some concentrated surface discharge in a natural channel or man-made conveyance system from the site. 19 For more information, see Urban Hydrology for Small Watersheds, Technical Release 55 (TR-55), June 1986, published by the NRCS. See Table 6.4.1.1.xx for CN values to be used with this manual. SECTION 6.2 GENERAL REQUIREMENTS FOR WQ FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 6-22 The following points summarize which site flows must be treated and under what circumstances:  All runoff from target pollution-generating impervious surfaces is to be treated through the water quality facility or facilities required in Chapter 1 and specified in the Chapter 6 menus.  Runoff from lawns and landscaped areas often overflows toward street drainage systems where it is conveyed to treatment facilities along with the road runoff. However, sometimes runoff from commercial areas and residential backyards drains into open space or vegetated buffer areas. In these cases, buffers may be used to provide the requisite water quality treatment provided: 1. Runoff sheet flows into the buffer or a dispersal trench is provided to disperse flows broadly into the buffer, and 2. The flow path through the pollution-generating area is limited to 200 feet, and 3. The buffer contains only native vegetation and is not itself subject to application of any fertilizers or pesticides.  Drainage from impervious surfaces that are not pollution-generating (such as patios, walkways, and some roofs) or are not target pollution-generating surfaces may bypass the water quality facility. However, this allowance to bypass does not excuse target impervious surfaces from, meeting the flow control requirements per Core Requirement #3. Note that metal roofs are considered pollution- generating unless they are treated to prevent leaching (see Reference Section 11-E), as are roofs that are subject to venting significant amounts of dusts, mists, or fumes from manufacturing, commercial, or other indoor activities.  Drainage from areas in native vegetation should not be mixed with untreated runoff from streets and driveways, if possible. It is best to infiltrate or disperse this relatively clean runoff to maximize recharge to shallow groundwater, wetlands, and streams.  Where runoff from non-pollution-generating impervious areas (non-PGIS), areas in native vegetation, or any other area not targeted for water quality treatment reaches a water quality facility, flows from those areas must be included in the sizing calculations for the facility. Once runoff from non- pollution-generating areas and non-target pollution-generating areas is combined with runoff from target pollution-generating areas, it cannot be separated before treatment. 6.2.2 SEQUENCE OF FACILITIES As specified in the water quality menus, where more than one water quality facility is used, the order is often prescribed. This is because the specific pollutant removal role of the second or third facility in a treatment train often assumes that significant solids settling has already occurred. For example, phosphorus removal using a two-facility treatment train relies on the second facility (sand filter) to remove a finer fraction of solids than those removed by the first facility. There is a larger question, however, of whether water quality facilities should be placed upstream or downstream of detention facilities. In general, all water quality facilities may be installed upstream of detention facilities, although presettling basins are needed for sand filters and infiltration basins. Not all water quality facilities, however, can be located downstream of detention facilities. Those facilities that treat sheet flows, such as filter strips and narrow-area filter strips, will seldom be practical downstream of detention facilities. Other facilities present special problems that must be considered before placement downstream is advisable. Two facilities that fall into this latter category are the basic bioswale (see Section 6.3.1) and the sand filter or sand filter vault (see Sections 6.5.2 or 6.5.3). For these facilities, the prolonged low flows resulting from Flow Control Duration Standard or Flood Problem Flow Control Standard may interfere with facility operation. In the case of basic bioswales, prolonged flows, generally in excess of about two weeks, will cause the grass to die. This can be dealt with by using the wet bioswale design. In the case of sand filters, prolonged flows may result in the sand being saturated for long periods. Saturated sand can become hypoxic or anoxic (lose most or all oxygen) when dissolved oxygen in the pore water becomes depleted. Under these conditions, some previously trapped phosphorus can become soluble 6.2.2 SEQUENCE OF FACILITIES 2022 City of Renton Surface Water Design Manual 6/22/2022 6-23 and be released,20 resulting in phosphorus releases in excess of influent concentrations. To prevent long periods of sand saturation, adjustments may be necessary after the sand filter is in operation to bypass some areas of the filter, allowing them to drain completely. If saturated conditions are present after facility operation, adjustments to the design shall be required. It may also be possible to employ a different alternative that uses facilities less sensitive to prolonged flows. Table 6.2.2.A summarizes placement considerations of water quality facilities in relation to detention. Oil control facilities must be located upstream of water quality facilities and as close to the source of oil- generating activity as possible. They should also be located upstream of detention facilities, if possible. TABLE 6.2.2.A WATER QUALITY FACILITY PLACEMENT IN RELATION TO DETENTION Water Quality Facility Preceding Detention Following Detention Basic bioswale (Section 6.3.1) OK OK if downstream of detention sized to meet Peak Rate Flow Control Standard. However, prolonged flows may cause soil saturation and injure grass. If downstream of a pond sized to meet Flow Control Duration Standard or Flood Problem Flow Control Standard, the wet bioswale may be needed (see Section 6.3.2) Wet bioswale (Section 6.3.2) OK OK Lateral inflow bioswale (Section 6.3.3) OK No—must be installed before flows concentrate. Filter strip or roadway filter strip (Sections 6.3.4 and 6.3.5) OK No—must be installed before flows concentrate. Basic or large wetpond (Section 6.4.1) OK OK—less water level fluctuation in ponds downstream of detention may improve aesthetic qualities. Basic or large combined detention and wetpond (Section 6.4.4) Not applicable Not applicable Wetvault (Section 6.4.2) OK OK Basic or large sand filter or sand filter vault (Section 6.5.2 or 6.5.3) OK, but presettling and control of floatables needed OK—sand filters downstream of a pond sized to meet Flow Control Duration Standard or Flood Problem Flow Control Standard may require field adjustments if prolonged flows cause sand saturation and resultant hypoxic, anoxic or anaerobic conditions, interfering with the phosphorus removal mechanism and likely resulting in episodic phosphorus releases in excess of influent concentrations. 20 Bicudo, D. D. C., et al. (2007). "Undesirable side-effects of water hyacinth control in a shallow tropical reservoir." Freshwater Biology 52(6): 1120-1133. SECTION 6.2 GENERAL REQUIREMENTS FOR WQ FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 6-24 TABLE 6.2.2.A WATER QUALITY FACILITY PLACEMENT IN RELATION TO DETENTION Water Quality Facility Preceding Detention Following Detention Stormwater wetland/pond (Section 6.4.3) OK OK—less water level fluctuation and better plant diversity are possible if the stormwater wetland is located downstream of the detention facility. Proprietary facility (Section 6.7) OK OK Bioretention (Section 6.8) OK No 6.2.3 SETBACKS, SLOPES, AND EMBANKMENTS This section presents the general requirements for water quality facility setbacks, side slopes, fencing, and embankments. When locating water quality facilities near wetlands and streams, there is a potential that the wetland or stream water level may be lowered by draining to the facility. Care in the design and siting of the facility or conveyance elements associated with the facility is needed to ensure this impact is avoided. Sufficient setback of the facility from the water body is one method to prevent impact. When locating water quality facilities near steep slopes, there is a potential for slope erosion or destabilization as a result of seepage, infiltration or overflow.  SETBACKS FROM TRACT LINE Water quality facilities that are maintained by the City must be in tracts dedicated to the City. Different water quality facilities and different types of side slopes (bermed vs. cut) have somewhat different requirements for setback from the tract line or setbacks for structures on adjacent tracts; these various requirements are given in Table 6.2.3.A. Most setbacks from tract lines are for maintenance equipment maneuverability. Setback requirements do not apply to water quality facilities that are privately maintained, but adequate room for maintenance equipment shall be considered during site design. Restrictions on the placement of structures on adjacent internal lots, as specified for infiltration facilities in Sections 5.2.2, 5.2.3, and 5.2.4, do however apply to privately maintained facilities.  FACILITY SITING New residential subdivisions with drainage facilities that collect public runoff must place water quality treatment ponds, vaults, and other similar drainage facilities, along with the required perimeter landscaping in a separate stormwater tract per RMC 4-6-030. The stormwater tract, including the landscaped area, must be owned by the homeowners association. Other types of new development shall create stormwater facilities either within an easement or within a tract not dedicated to the City per RMC 4-6-030.  SIDE SLOPES, FENCING, AND EMBANKMENTS Side slopes for water quality facilities should not exceed a slope of 3H:1V. Moderately undulating slopes are acceptable and can provide a more natural setting for the facility. In general, gentle side slopes improve the aesthetic attributes of the facility and enhance safety. Fencing may be required for public safety and/or protecting the integrity and function of the facility. 6.2.3 SETBACKS, SLOPES, AND EMBANKMENTS 2022 City of Renton Surface Water Design Manual 6/22/2022 6-25 Intent: The requirements for slopes, fencing, and embankments are intended to accomplish the following objectives:  To prevent persons from inadvertently slipping into the pond, either by providing gentle interior side slopes (3H:1V or gentler) or by fencing or other barrier  To allow easy egress from the pond (gentle side slopes, safety benches, etc.) when access is not restricted by a fence or other barrier  To ensure interior and exterior slopes or embankments are stable and will not create a hazardous or damaging situation. Water quality facilities must meet the following requirements for side slopes, fencing, and embankments: 1. All wetponds, stormwater wetlands, and similar facilities shall be fenced per Section 5.1.1. A 6-foot tall chain link fence shall be provided around the facility with access gate(s) to allow maintenance per City of Renton Standard Details. 2. All open (uncovered) sand filters require fencing. The intent is to prevent sand filters from being used recreationally and to exclude domestic animals. 3. Where required, fencing shall be placed at the top of the berm with the maintenance access road on the inside of the fence or 5 feet minimum from top of berm if there is no maintenance access road allowing proper maintenance access of the facility. The specific fencing requirements in Chapter 5 (see Section 5.1.1) also apply to WQ facilities. Non-residential commercial or industrial facilities that are privately owned and maintained must still comply with the fencing requirements in RMC 4-6-030. 4. Side slopes (interior and exterior) shall be no steeper than 3H:1V. 5. Pond walls may be vertical retaining walls, provided: (a) they are constructed of reinforced concrete per Section 5.3.3; (b) a fence is provided along the top of the wall; (c) at least 25% of the pond perimeter will be a vegetated soil slope not steeper than 3H:1V; and (d) the design is prepared and stamped by a licensed structural civil engineer. 6. Water quality facilities with embankments that impound water must comply with Washington State dam safety regulations (WAC 173-175). The cited language below is as of February 2012 and is excerpted verbatim from the Washington Administrative Code except for substitution of Department of Ecology for department. When reading, substitute facility for dam, and overflow water surface for crest: (1) These regulations are applicable to dams which can impound a volume of ten acre-feet or more of water as measured at the dam crest elevation. The ten acre-feet threshold applies to dams which can impound water on either an intermittent or permanent basis. Only water that can be stored above natural ground level or which could be released by a failure of the dam is considered in assessing the storage volume. The ten acre-feet threshold applies to any dam which can impound water of any quality, or which contains any substance in combination with sufficient water to exist in a liquid or slurry state at the time of initial containment. (2) For a dam whose dam height is six feet or less and which meets the conditions of subsection (1) of this section, the Washington Department of Ecology (Ecology) may elect to exempt the dam from these regulations. The decision by Ecology to exempt a dam will be made on a case-by-case basis for those dams whose failure is not judged to pose a risk to life and minimal property damage would be expected. If the storage capacity is less than 10 acre-feet above natural ground level, then the facility is exempt from Ecology review. SECTION 6.2 GENERAL REQUIREMENTS FOR WQ FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 6-26 TABLE 6.2.3.A SETBACK REQUIREMENTS * WATER QUALITY FACILITY SETBACK FROM TRACT LINE At Grade or Underground If Facility Slope is Cut into Grade If Slope is an Embankment Bioswale N/A See conveyance system requirements (Section 4.1) 5 feet from toe of exterior slope Filter strip 5 feet from toe 5 feet from toe N/A Wetpond N/A 5 feet from emergency overflow water surface (WS) 5 feet from toe of exterior slope Combined detention and wetpond N/A 5 feet from emergency overflow WS 5 feet from toe of exterior slope Stormwater wetland N/A 5 feet from emergency overflow WS 5 feet from toe of exterior slope Wetvault or sand filter vault 5 feet from property line N/A N/A Sand filter ponding area N/A 5 feet from emergency overflow WS 5 feet from toe of exterior slope Linear sand filter 5 feet from property line N/A N/A Proprietary facility21 5 feet from property line N/A N/A Bioretention N/A 5 feet from emergency overflow WS 5 feet from toe of exterior slope * Greater setback distances are required whenever expressly stated or referenced in this manual or when required by other City codes or other agencies. Steep slopes, land slide areas, open water features, springs, wells, and septic tank drainfields are features that often have additional setback requirements. Geotechnical Setbacks: Except for tanks, vaults, and pipes: 1. Facilities are not allowed on slopes greater than 25% (4:1). A geotechnical analysis and report is required if located within 200 feet of a steep slope hazard area or landslide hazard OR if the facility is located within a setback distance from top of slope equal to the total vertical height of a slope area that is steeper than 15%. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built-out conditions. 2. The facility design water surface shall be a minimum of 200 feet from any steep slope hazard area or landslide hazard. Upon analysis and approval of a licensed geotechnical engineer or engineering geologist, this setback may be reduced to 50 feet. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built-out conditions. 3. The facility design water surface shall be set back a minimum distance from top of slope equal to the total vertical height of a slope area that is steeper than 15%. Upon analysis and approval of a licensed geotechnical engineer or engineering geologist, this setback may be reduced to 50 feet. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built-out conditions. Public Health Minimum Setbacks for All Facilities: Some typical setback distances imposed by the Public Health – Seattle & King County include the following:  Open water features: 100 feet.  Wells: 100 feet.  Springs used for potable water: 200 feet.  Septic tanks: 50 feet, and drainfields: 100 feet; tanks or vaults must not be located so that they could impede septic drainfield flows. WA Ecology Stormwater Facility Setback Requirements for Public Health: 21 See Reference Section 14-A for approved proprietary facilities. 6.2.3 SETBACKS, SLOPES, AND EMBANKMENTS 2022 City of Renton Surface Water Design Manual 6/22/2022 6-27 TABLE 6.2.3.A SETBACK REQUIREMENTS *  Stormwater infiltration systems shall be set back at least 100 feet from open water features and 200 feet from springs used for drinking water supply. Infiltration facilities up-gradient of drinking water supplies must comply with State Health Department requirements (Washington Wellhead Protection Program, Department of Health, 12/93).  Stormwater infiltration systems, and unlined wetponds and detention ponds shall be located at least 100 feet from drinking water wells and septic tanks and drainfields. Where one agency’s setback requirements are more or less restrictive than another’s, the more restrictive setback is required. SECTION 6.2 GENERAL REQUIREMENTS FOR WQ FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 6-28 6.2.4 FACILITY LINERS Open channel conveyance systems and facilities that allow runoff to have direct contact with the soil may require liners for any of three reasons: groundwater quality protection, steep slope or building protection, and/or stormwater treatment facility performance. Liners are intended to: 1. Reduce the likelihood that pollutants in stormwater will reach ground water by transmission through soil from earthen facilities and conveyances. 2. Prevent infiltration where underflow could cause problems with steep slopes or nearby structures. 3. Ensure permanent wet pools for proper functioning of wetponds, treatment wetlands, and pre-settling ponds. 4. Ensure wet conditions in the second cell of stormwater treatment wetland sufficient to maintain wetland plant vegetation. Types of Liners Low Permeability Liners Low permeability liners reduce infiltration to a very slow rate, generally less than 0.02 inches per hour (1.22 cm/day). Low permeability liners may be fashioned from compacted till, clay, geomembrane, or concrete as detailed in Section 6.2.4.1. Treatment Liners Treatment liners are soil layers meeting specific quality criteria. Depending on design requirements, treatment liners may include in-place native soils, amended soils, or imported soils. Treatment liners are assumed to treat infiltrating stormwater before it reaches more freely draining soils. Treatment liners have slow rates of infiltration; the initial measured rate should be less than 2.4 inches per hour (1.7 x 10 -3 cm/s), but rates are not as slow as with low permeability liners. See Section 6.2.4.2 for details. Where Liners Are Required for Groundwater Protection Outside of Groundwater Protections Areas A liner is required for facilities and conveyance systems that allow untreated runoff from PGIS to have direct contact with the soil if the soil has an initial infiltration rate22 greater than 9 inches per hour (0.15 inches per minute) and the soil suitability criteria for groundwater protection given in Chapter 5, Section 5.2.1 is not met. Inside Groundwater Protections Areas Facilities that allow runoff to have direct contact with the soil, on-site BMPs that rely on infiltration, and open channel conveyance systems that are not concrete lined are not allowed in Zone 1 of the Aquifer Protection Area. See Section 1.3.6 for details. Other areas, such as Zone 1 Modified or Zone 2 of the Aquifer Protection Area, may be required to incorporate liners for groundwater protection. A liner is required for facilities and conveyance sytems that allow untreated runoff from PGIS to have direct contact with the soil when the soil infiltration rate exceeds an initial infiltration rate of 2.4 inches per hour (0.04 inches per minute) and the soil suitability criteria for groundwater protection given in Chapter 5, Section 5.2.1, is not met. Where Liners are Required to Ensure Permanent Pools and Wet Conditions 1. Both cells of a two-cell wetpond and the single cell of a one cell wetpond must retain a permanent pool of water throughout the wet season. A wetpond is considered non-compliant if the pond level 22 Infiltration rates can either be measured in the field using methods given in Chapter 5 or inferred from the USDA soil textural triangle included in “Groundwater Protection,” Section 5.2.1. 6.2.4 FACILITY LINERS 2022 City of Renton Surface Water Design Manual 6/22/2022 6-29 drops more than 12 inches in any 7-day measurement period. A low permeability liner will be required to achieve this standard in infiltrative soils. 2. Presettling ponds must retain a permanent pool of water throughout the wet season. A presettling pond is considered non-compliant if the pond level drops more than 12 inches in any 7-day measurement period. A low permeability liner will be required to achieve this standard in infiltrative soils. 3. Both cells of a stormwater wetland shall be lined in infiltrative soils as follows: a) The first cell of a treatment wetland must retain a permanent pool of water throughout the wet season. It is considered non-compliant if the pond level drops more than 12 inches in any 7-day measurement period. A low permeability liner will be required to achieve this standard in infiltrative soils. b) The second cell must retain water for at least 10 months of the year. A low permeability liner will be required to achieve this standard in infiltrative soils. A treatment liner is an alternative where groundwater levels and/or existing soil infiltration rates are sufficient to achieve the standard. General Design Criteria 1. Table 6.2.4.A identifies the type of liner for use with various water quality treatment facilities. If a facility requires a liner, a treatment liner shall be provided, except where a low permeability liner is noted in Table 6.2.4.A. 2. Liners shall be evenly placed over the bottom and/or sides of the treatment area of the facility as indicated in Table 6.2.4.A. Areas above the treatment volume that are required to pass flows greater than the water quality treatment flow (or volume) need not be lined, except in groundwater protection areas which must be lined to the 2 year water surface in a combined facility or overflow water surface in a non-combined facility. Note: If the liner cannot be anchored at the required elevation, the lining must be extended to the top of the interior side slope and anchored. 3. For low permeability liners, the following criteria apply: a) Where the seasonal high groundwater elevation is likely to contact a low permeability liner, liner buoyancy may be a concern. A low permeability liner shall not be used in this situation unless evaluated and recommended by a geotechnical engineer. b) Where grass must be planted over a low permeability liner per the facility design, a minimum of 6 inches of good topsoil or compost-amended23 native soil (2 inches compost23 tilled into 6 inches of native soil) must be placed over the liner in the area to be planted. Twelve inches is preferred. c) If an identification sign is required for the facility (see detention pond requirements in Section 5.1.1), the face of the sign shall bear a note indicating the facility is lined to protect water quality. In addition, the back of the sign shall include information indicating which facilities are lined, the extent of lining, the liner material used, the liner thickness (if clay or till), and the type and distance of the marker above the liner (if a geomembrane). This information need only be readable by someone standing at arms-length from the sign. 4. If a treatment liner will be below the seasonal high water level, the pollutant removal performance of the liner must be evaluated by a geotechnical or groundwater specialist and found to be as protective as if the liner were above the level of the groundwater. See Sections 6.2.4.1 and 6.2.4.2 for more specific design criteria on the various options for low permeability liners and treatment liners. 23 Compost must meet the compost quality requirements in Reference Section 11-C. Compost for application of this requirement in stormwater treatment wetlands must be Specification 1 Compost detailed in Reference Section 11-C. SECTION 6.2 GENERAL REQUIREMENTS FOR WQ FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 6-30 TABLE 6.2.4.A LINING TYPES FOR STORMWATER FACILITIES Facility Type Area to be Lined Type of Liner Bioswale Bottom and sides Treatment liner Wet bioswale Bottom and sides Low permeability liner (If the swale will intercept the seasonal high groundwater table, a treatment liner is recommended.) Lateral inflow bioswale Bottom and sides Treatment liner Presettling pond or basin Bottom and sides Low permeability liner (If the cell will intercept the seasonal high groundwater table, a treatment liner is recommended.) Wetpond First cell: bottom and sides to WQ design water surface, (except in groundwater protection areas which must be lined to the overflow water surface Low permeability liner (If the cell will intercept the seasonal high groundwater table, a treatment liner is recommended.) Second cell: bottom and sides to WQ design water surface, (except in groundwater protection areas which must be lined to the overflow water surface Low permeability liner (If the cell will intercept the seasonal high groundwater table, a treatment liner is recommended.) Single cell: bottom and sides to WQ design water surface, (except in groundwater protection areas which must be lined to the overflow water surface Low permeability liner Combined detention/WQ facility First cell: bottom and sides to the 2-year live storage elevation Low permeability liner (If the cell will intercept the seasonal high groundwater table, a treatment liner is recommended.) Second cell: bottom and sides to the 2-year live storage elevation Low permeability liner (If the cell will intercept the seasonal high groundwater table, a treatment liner is recommended.) Single cell: bottom and sides to the 2-year live storage elevation Low permeability liner Wet vault Not applicable No liner needed Stormwater wetland Bottom and sides, both cells Low permeability liner (If the facility will intercept the seasonal high groundwater table, a treatment liner is recommended.) Sand filter Pond sides only Treatment liner Detention pond Bottom and sides to the 2-year live storage elevation Treatment Liner Sand filter vault Not applicable No liner needed Linear sand filter Not applicable if in vault Bottom and sides of presettling cell if not in vault No liner needed Low permeability or treatment liner Proprietary filter (in vault) Not applicable No liner needed Bioretention Bottom and sides (when required per Section 6.8) Low permeability liner 6.2.4 FACILITY LINERS 2022 City of Renton Surface Water Design Manual 6/22/2022 6-31 6.2.4.1 DESIGN CRITERIA FOR LOW PERMEABILITY LINER OPTIONS This section presents the design criteria for each of the following four low permeability liner options:  Compacted till liners  Clay liners  Geomembrane liners  Concrete liners  COMPACTED TILL LINERS 1. Liner thickness shall be 18 inches after compaction. 2. Soil shall be compacted to 95% minimum dry density, modified proctor method (ASTM D-1557). 3. Soil should be placed in 6 inch lifts. 4. Soils may be used that meet the following gradation: Sieve Size Percent Passing 6 inch 100 4 inch 90 #4 70–100 #200 30–100  CLAY LINERS 1. Minimum dry (un-swollen) thickness of 12 inches 2. Compacted to 95% minimum dry density, standard proctor method ASTM D-698 3. Clay Particles Passing, ASTM D-422, not less than 30 percent 4. Plasticity Index of Clay, ASTM D4318, not less than 15 percent 5. The slope of clay liners must be restricted to 3H:IV for all areas requiring soil cover; otherwise, the soil layer must be stabilized by another method so that soil slippage into the facility does not occur. Any alternative soil stabilization method must take maintenance access into consideration. 6. Where clay liners form the sides of ponds, the interior side slope should not be steeper than 3H:1V, irrespective of fencing. This restriction is to ensure that anyone falling into the pond may climb out safely.  GEOMEMBRANE LINERS 1. Geomembrane liners shall be UV resistant and have a minimum thickness of 30 mils. A thickness of 40 mils shall be used in areas of maintenance access or where heavy machinery must be operated over the membrane. Protect the geomembrane liner from puncture, tearing, and abrasion by installing geotextile fabric on the top and bottom of the geomembrane. 2. Geomembranes shall be bedded according to the manufacturer’s recommendations. 3. Liners shall be installed so that they can be covered with 12 inches of top dressing forming the bottom and sides of the water quality facility. Top dressing shall consist of 6 inches of crushed rock covered with 6 inches of native soil. The rock layer is to mark the location of the liner for future maintenance operations. As an alternative to crushed rock, 12 inches of native soil may be used if orange plastic “safety fencing” or another highly-visible, continuous marker is embedded 6 inches above the membrane. 4. If possible, liners should be of a contrasting color so that maintenance workers are aware of any areas where a liner may have become exposed when maintaining the facility. SECTION 6.2 GENERAL REQUIREMENTS FOR WQ FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 6-32 5. Where top dressing is required, liners shall not be used on slopes steeper than 5H:1V, to prevent the top dressing material from slipping. Textured liners may be used on slopes up to 3H:1V upon recommendation by a geotechnical engineer or engineering geologist that the top dressing will be stable for all site conditions, including maintenance.  CONCRETE LINERS 1. Portland cement concrete liners are allowed irrespective of facility size, and shotcrete may be used on slopes; however, specifications must be developed by an engineer who certifies the liner against cracking or losing water retention ability under expected conditions of operation, including facility maintenance operations. Cautionary design note: weight of maintenance equipment can be up to 80,000 pounds when fully loaded. 2. Asphaltic concrete may not be used for a liner because of asphalt’s permeability to many organic pollutants, and potential for asphalt to leach pollutants into stored or conveyed water. 3. If grass is to be grown in soil over a concrete liner, slopes must be no steeper than 5H:1V to prevent the top dressing material from slipping. 6.2.4.2 DESIGN CRITERIA FOR TREATMENT LINER OPTIONS This section presents the design criteria for the organic soil layer used as a treatment liner.  ORGANIC SOIL LAYER 1. A two-foot thick layer of soil with a minimum organic matter (OM) content of 1.0% AND a minimum cation exchange capacity (CEC) of 8 milliequivalents per 100 grams (meq/100g) can be used as a treatment layer beneath a water quality or detention facility. If the soil is amended or imported, the top 8 inches must have a minimum cation exchange capacity of 10 meq/100g and the remainder of the depth no less than 8 meq/100g. An 18-inch layer with the same CEC and OM profile will suffice for ditch conveyances, based on unsaturated flow as a result of alternating wet-dry periods. 2. To demonstrate that in-place soils meet the above criteria, one sample per 1,000 square feet of facility area, or 500 linear feet of ditch, and no fewer than three samples shall be tested. Each sample shall be a composite of equally spaced subsamples taken throughout the full extent of the treatment layer depth (usually two to six feet below the expected facility invert for facilities), except stratified composite sampling is required where the top 8 inches are required to meet a higher CEC level (composite of top eight inches and separate composite of the remainder below). 3. Organic content shall be measured on a dry weight basis using ASTM D2974. 4. Laboratory results shall be provided for cation exchange capacity (CEC). 5. Certification by a soils testing laboratory that imported soil meets the organic content and CEC criteria above shall be provided to the local approval authority. 6. Soil amendment may only be compost meeting the requirements of Reference Section 11-C. Compost for application of this requirement in stormwater treatment wetlands must be Specification 1 Compost detailed in Reference Section 11-C. 7. If a treatment liner will be below the seasonal high water level, the pollutant removal performance of the liner must be evaluated by a geotechnical or ground water specialist and found to be as protective as if the liner were above the level of the ground water. 6.2.5 FLOW SPLITTER DESIGNS Most water quality facilities may be designed as flow-through, or on-line, systems with flows above the water quality design flow or volume simply passing through the facility untreated. However, it is sometimes desirable to restrict flows to water quality treatment facilities and bypass the remaining higher flows around them (off-line facilities). This can be accomplished by splitting flows in excess of the water 6.2.5 FLOW SPLITTER DESIGNS 2022 City of Renton Surface Water Design Manual 6/22/2022 6-33 quality design flow upstream of the facility and diverting higher flows to a bypass pipe or channel. The bypass typically enters a detention facility or the downstream receiving drainage system, depending on flow control requirements. In most cases, it is a designer’s choice whether WQ facilities are designed as on-line or off-line; an exception is oil/water separators, which must be designed off-line. A crucial factor in designing flow splitters is to ensure that low flows are delivered to the treatment facility up to the WQ design flow rate. Above this rate, additional flows are diverted to the bypass system with minimal increase in head at the flow splitter structure to avoid surcharging the water quality facility under high flow conditions. Flow splitters are typically catch basins or vaults with concrete baffles. In place of baffles, the splitter mechanism may be a half tee section with a solid top and an orifice in the bottom of the tee section. A full tee option may also be used (see “Design Criteria” below). Two possible schematic representations for flow splitters are shown in Figure 6.2.5.A and Figure 6.2.5.B. Other designs that achieve the result of splitting low flows, up to the WQ design flow, into the WQ treatment facility and divert higher flows around the facility may be considered (an adjustment per Section 1.4 may be required upon evaluation by CED staff). 6.2.5.1 METHODS OF ANALYSIS Flow splitters are modeled with the approved model using the design flow rates as described in Section 6.2.1. The stage/discharge relationship of the outflow pipes should be determined using the backwater analysis techniques in Chapter 4. The orifice shall be sized per Section 5.1.4.2. Weirs should be analyzed as sharp-crested weirs. 6.2.5.2 DESIGN CRITERIA General 1. A flow splitter shall be designed to deliver the required water quality design flow rate specified in Section 6.2.1 to the WQ treatment facility. 2. The top of the weir shall be located at the water surface for the design flow. Remaining flows enter the bypass line. Flows shall be modeled using 15-minute time steps. 3. The maximum head shall be minimized for flow in excess of the water quality design flow. Specifically, flow to the WQ facility at the 100-year water surface shall not increase the design WQ flow by more than 10%. 4. Either design shown in Figure 6.2.5.A or Figure 6.2.5.B shall be used. 5. Special applications, such as roads, may require the use of a modified flow splitter. The baffle wall may be fitted with a notch and adjustable weir plate to proportion runoff volumes other than high flows. 6. For ponding facilities, backwater effects must be included in designing the height of the standpipe in the catch basin. 7. Ladder or step and handhold access (per City of Renton Standard Details) shall be provided. If the weir wall is higher than 36 inches, two ladders, one to either side of the wall, are required. Material Requirements 1. The splitter baffle shall be installed in a Type 2 catch basin or vault. 2. The baffle wall shall be made of reinforced concrete or another suitable material resistant to corrosion, and have a minimum 4-inch thickness. The minimum clearance between the top of the baffle wall and the bottom of the catch basin cover shall be 4 feet; otherwise, dual access points shall be provided. 3. All metal parts shall be corrosion resistant. Examples of preferred materials include aluminum, stainless steel, and plastic. Zinc and galvanized materials, bronze and brass, and cadmium-plated hardware shall not be used unless there is no substitute, because of aquatic toxicity. Painting or other SECTION 6.2 GENERAL REQUIREMENTS FOR WQ FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 6-34 coating of metal parts shall not be allowed because of poor longevity and lack of standardization or assurance of non-toxic coatings. FIGURE 6.2.5.A SCHEMATIC REPRESENTATION OF FLOW SPLITTER, OPTION A SECTION A-A NTS PLAN VIEW NTS INFLOW NOTE: THE WATER QUALITY DISCHARGE PIPE MAY REQUIRE AN ORIFICE PLATE TO BE INSTALLED ON THE OUTLET TO CONTROL THE HEIGHT OF THE DESIGN WATER SURFACE (WEIR HEIGHT). THE DESIGN WATER SURFACE SHOULD BE SET TO PROVIDE A MINIMUM HEADWATER/DIAMETER RATIO OF 2.0 ON THE OUTLET PIPE. TO WQ FACILITY TO BYPASS CONVEYANCE SYSTEM OR DETENTION POND A 4" MIN. THICKNESS REINFORCED CONCRETE BAFFLE WALL OR OTHER SUITABLE MATERIAL BYPASS PIPE 4' MIN. OR PROVIDE SEPARATE ACCESS TO EITHER SIDE OF BAFFLE WALL WQ DESIGN WATER SURFACE ELEVATION HANDHOLD AND STEPS OR LADDER ACCESS (PROVIDE LADDERS TO BOTH SIDES OF WALL IF WEIR >36" HIGH) ROUND SOLID LID (KCRDCS DWG 7-022 AND 7-023) INFLOW REINFORCED BAFFLE WALL GROUTED TO MANHOLE STRUCTURE (BOTH ENDS) TYPE 2 C.B.4' MIN.2' MIN. A TO WQ FACILITY 6.2.5 FLOW SPLITTER DESIGNS 2022 City of Renton Surface Water Design Manual 6/22/2022 6-35 FIGURE 6.2.5.B SCHEMATIC REPRESENTATION OF FLOW SPLITTER, OPTION B INFLOW SECTION A-A NTS PLAN VIEW NTS TO WATER QUALITY FACILITY TO WATER QUALITY FACILITY TO BYPASS CONVEYANCE SYSTEM OR DETENTION POND INFLOW *NOTE: DIAMETER OF STANDPIPE SHOULD BE LARGE ENOUGH TO MINIMIZE HEAD ABOVE WQ DESIGN WS AND TO KEEP WQ DESIGN FLOWS FROM INCREASING MORE THAN 10% DURING 100-YEAR FLOWS. A 2d*6"6"7' MIN.2' MIN.d SOLID BOTTOM (PROVIDE MAINTENANCE ACCESS) LADDER (TYP.) TOP OF PIPE AT WQ DESIGN WATER SURFACE ELEVATION ROUND SOLID LID (SEE KCRDCS DWG 7-022 AND 7-023) BAFFLE TO CONTROL FLOATABLES, OR PROVIDE SPILL CONTROL UPSTREAM "TEE" SECTION WITH CLEANOUT (OR REMOVABLE BEND-DOWN ELBOW) ORIFICE SIZED TO PASS WQ DESIGN FLOW TOP OF RISER AT DESIGN WS ELEVATION TYPE II CB NO BASE CHANNEL REQ'D DIA. OF STANDPIPE* (2 X DIA. OF OUTLET PIPE RECOMMENDED AS STARTING POINT, SEE NOTE) BAFFLE TO CONTROL FLOATABLES SECTION 6.2 GENERAL REQUIREMENTS FOR WQ FACILITIES 6/22/2022 2022 City of Renton Surface Water Design Manual 6-36 6.2.6 FLOW SPREADING OPTIONS Flow spreaders function to uniformly spread flows across the inflow portion of water quality facilities (e.g., sand filter, bioswale, or filter strip). There are five flow spreader options presented in this section:  Anchored section: Anchored plate or board (Option A)  Concrete sump box (Option B)  Notched curb spreader (Option C)  Through-curb ports (Option D)  Interrupted curbing (Option E) Options A through C may be used for spreading flows that are concentrated. Any one of these options may be used when spreading is required by the facility design criteria. Options A through C may also be used for unconcentrated (sheet) flows, and in some cases must be used, such as to correct for moderate grade changes along a filter strip. Options D and E are only for flows that are already unconcentrated when they enter a filter strip or lateral inflow bioswale. Other flow spreader options are possible with approval from CED. 6.2.6.1 DESIGN CRITERIA FOR FLOW SPREADER OPTIONS General Design Criteria 1. Flow must not escape around ends or through any breaks in a flow spreader. 2. Where flow enters the flow spreader through a pipe, it is recommended that the pipe be submerged to the extent practical to dissipate energy as much as possible. 3. For higher velocity inflows (greater than 5 cfs for the 100-yr storm), a Type 1 catch basin should be positioned in the spreader, and the inflow pipe should enter the catch basin with flows exiting through the top grate. The top of the grate should be lower than the level spreader plate, or if a notched spreader is used, lower than the bottom of the v-notches. 4. Table 4.2.2.F in Chapter 4 provides general guidance for rock protection at outfalls.  OPTION A  ANCHORED PLATE OR BOARD (FIGURE 6.2.6.A) 1. An adjustable-level anchored plate or board flow spreader shall be preceded by a sump having a minimum depth of 8 inches and minimum width of 24 inches. If not otherwise stabilized, the sump area shall be lined to reduce erosion and to provide energy dissipation. 2. The top surface of the flow spreader plate or board shall be level, projecting a minimum of 2 inches above the ground surface of the water quality facility, or v-notched with notches 6 to 10 inches on center and 1 to 6 inches deep (use shallower notches with closer spacing). Alternative designs are allowed. The anchored plate or board level shall be adjustable using slotted bolt holes in the anchored plate or board. 3. A flow spreader plate or board shall extend horizontally beyond the bottom width of the facility to prevent water from eroding the side slope. The horizontal extent should be such that the bank is protected for all flows up to the 100-year flow or the maximum flow that will enter the WQ facility. 4. Flow spreader plates or boards shall be securely fixed in place by bolts through slotted holes for adjustability in establishing and maintaining level. 5. Flow spreader plates or boards may be made of either wood, metal, fiberglass reinforced plastic, or other durable material. If wood, untreated 4 by 10-inch cedar heartwood lumber or cedar landscape timbers are acceptable. 6.2.6 FLOW SPREADING OPTIONS 2022 City of Renton Surface Water Design Manual 6/22/2022 6-37 6. Anchor posts shall be 4-inch square concrete or tubular stainless steel. Other material resistant to decay may be used if approved by CED.  OPTION B  CONCRETE SUMP BOX (FIGURE 6.2.6.B) 1. The wall of the downstream side of a rectangular concrete sump box shall extend a minimum of 2 inches above the treatment bed. This serves as a weir to spread the flows uniformly across the bed. The wall shall have an adjustable anchored plate or board as described in Option A above. The adjustable anchored plate or board shall be securely fixed to the concrete wall and meet the same material specifications as described in Option A above. 2. The downstream wall of a sump box shall have “wing walls” at both ends. Side walls and returns shall be slightly higher than the weir so that erosion of the side slope is minimized. 3. Concrete for a sump box may be either cast-in-place or precast, but the bottom of the sump shall be reinforced with wire mesh for cast-in-place sumps. 4. Sump boxes shall be placed over bases that consists of 4 inches of crushed rock, 5/8-inch minus to help ensure the sump remains level.  OPTION C  NOTCHED CURB SPREADER (FIGURE 6.2.6.C) Notched curb spreader sections shall be made of extruded concrete laid side by side and level. Typically five “teeth” per four-foot section provide good spacing. The space between adjacent “teeth” forms a v- notch.  OPTION D  THROUGH-CURB PORTS (FIGURE 6.2.6.D) Sheet flows from paved areas entering filter strips or lateral inflow bioswales may use curb ports or interrupted curbs (Option E) to allow flows to enter the strip or swale. Curb ports use fabricated openings that allow concrete curbing to be poured or extruded while still providing an opening through the curb to admit water to the WQ facility. Openings in the curb shall be at regular intervals but at least every 6 feet (minimum). The width of each curb port opening shall be a minimum of 11 inches. Approximately 15 percent or more of the curb section length should be in open ports, and no port should discharge more than about 10 percent of the flow.  OPTION E  INTERRUPTED CURB (NO FIGURE) Interrupted curbs are sections of curb placed to have gaps spaced at regular intervals along the total width (or length, depending on facility) of the treatment area. At a minimum, gaps shall be every 6 feet to allow distribution of flows into the treatment facility before they become too concentrated. The opening shall be a minimum of 11 inches. As a general rule, no opening should discharge more than 10 percent of the overall flow entering the facility.