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Managing stormwater and waterborne pollution

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Stormwater Services and Information for King County, Washington

1998 Surface Water Design Manual

Summary of Changes from the 1990 Design Manual

Restructuring of Chapters and Appendices

  • The 1990 Design Manual Chapter 5 (TESC Standards) was moved to Appendix D, which is now a separate stand-alone document.
  • The 1990 Chapter 4 was divided into 3 chapters as follows:
    • Chapter 4, Conveyance System Analysis and Design
    • Chapter 5, Flow Control Design
    • Chapter 6, Water Quality Design
  • A new Appendix C was added to house the new proposed Small Site Drainage Requirements. Like Appendix D, it is a separate stand-alone document.

Details of changes for each chapter are below. Click on the Chapter Title to go to the changes for a particular chapter, or scroll down the page:

Chapter 1 - Drainage Review and Requirements

This chapter describes the basic drainage requirements that implement King County adopted surface water runoff policies and explains how these requirements are applied to proposed projects through the drainage review process. The most significant changes in Chapter 1 were:

  • Added a new drainage review threshold for applying drainage requirements to smaller projects that drain to landslide hazard areas.
  • Added four new drainage review types, each with a different set of applied requirements based on project scope and characteristics.
  • Revised the threshold for non-UPD projects requiring a Master Drainage Plan.
  • Added a new tightline requirement for larger projects discharging to landslide hazard areas.
  • Added more guidance in Core Requirement #2 on how to identify and mitigate impacts to downstream drainage problems.
  • Added three new area-specific flow control standards to replace the 10 standards now applied by 23 different regulations. These new standards are housed in Core Requirement #3 along with special modifications of the standards to address specific downstream problems or concerns.
  • Changed the runoff computation methodology used in the sizing of most drainage facilities.
  • Eliminated the half-cfs flow control facility exemption for all projects except redevelopments.
  • Added a new flow control and water quality facility exemption for rural area plats and short plats that implement clearing restrictions and flow control BMPs (e.g., flow dispersion and infiltration).
  • Added a new flow control facility exemption for certain infill projects in highly urbanized areas.
  • Added new requirements for roof downspout controls in subdivisions to increase infiltration opportunities and eliminate mandatory connection of downspouts to the local drainage system.
  • Simplified and clarified existing conveyance standards in Core Requirement #4.
  • Added four new area-specific water quality standards targeted to the protection needs of downstream receiving waters and resources. These standards are housed in the newly added Core Requirement #8.
  • Added options and increased flexibility for achieving required flow control and water quality protection.
  • Added new water quality requirements for pervious areas and for certain redevelopment projects.
  • Established new water quality requirements for developments that generate large quantities of oil due to high traffic turnover.
  • Consolidated 12 special requirements (of the 15 in code) into 5 through editing and reformatting.
  • Edited and reformatted existing language to improve clarity and user friendliness.

Chapter 2 - Drainage Plan Submittal

This chapter describes the requirements and specifications for submittal of design plans for drainage review, including report and plan formats, and scopes. It was changed as follows:

  • Added language to accommodate the new drainage review types proposed in Chapter 1, Section 1.1.
  • Modified the TIR contents to reflect other proposed changes in Chapter 1.
  • Edited and reformatted existing language to improve clarity.

Chapter 3 - Hydrologic Analysis and Design

This chapter presents the acceptable methods of hydrologic analysis used to estimate runoff and design flow control, conveyance and water quality facilities. It was substantially rewritten to accommodate the new runoff computation methodology and other changes to Core Requirement #3 of Chapter 1.

Chapter 4 - Conveyance System Analysis and Design

This chapter presents the acceptable methods, details and criteria for analysis and design of conveyance systems. It contains most of the text from Section 4.3 of the 1990 Design Manual except for the following changes:

  • Moved the Rational Method discussion (Section 4.3.3) to Chapter 3.
  • Modified easement and building setback requirements.
  • Edited and reformatted language to improve clarity and user friendliness, and to accommodate changes to Core Requirement #4 of Chapter 1.
  • Removed some infrequently used figures and tables.

Chapter 5 - Flow Control Design

This chapter presents the acceptable methods, details and criteria for analysis and design of flow control facilities. It contains much of the text from Sections 4.4 and 4.5 of the 1990 Design Manual except for the following changes:

  • Added language to accommodate the new roof downspout control requirements for subdivisions as specified in Core Requirement #3 of Chapter 1.
  • Added language to detail implementation of "flow control BMPs" used to meet exemption conditions in Core Requirements #3 and #8 of Chapter 1.
  • Added an infiltration trench design and vault design to the list of acceptable infiltration facilities.
  • Added new criteria for protection of groundwater when using infiltration facilities.
  • Modified setbacks and fencing requirements for detention and infiltration ponds.
  • Removed the internal berm/wall requirement for detention ponds and vaults.
  • Edited and reformatted existing language to improve clarity and user friendliness.

Chapter 6 - Water Quality Design

This chapter presents the acceptable methods, details and criteria for analysis and design of water quality facilities. It totally replaced and substantially expanded upon the water quality design information in Section 4.6 of the 1990 Design Manual.

Definitions Section

This section provides a formal list of the words, terms and abbreviations accompanied by their meaning as applied in the 1998 manual. New definitions were added to accommodate the changes to the Design Manual.

Appendix A - Maintenance Standards for Privately Maintained Drainage Facilities

This appendix contains the frequency, thresholds and standards for maintenance of all privately maintained drainage facilities. The scope of these standards was expanded to include the new water quality designs in Chapter 6. Also included were new guidelines for developing and implementing landscape management plans.

Appendix B - Master Drainage Plan Objective, Criteria Components and Review Process

This appendix 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. No changes were made to this appendix.

Appendix C - Small Site Drainage Requirements

This new appendix describes the simplified drainage requirements for projects that qualify for the small site drainage review.

Appendix D - Erosion and Sedimentation Control Standards

This appendix 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 which may be needed to supplement the required erosion and sedimentation control measures. It contains all the text from Chapter 5 of the 1990 Design Manual except that the language was edited and reformatted to improve clarity, user friendliness, and to make this part of the 1998 Design Manual as a stand-alone document.

Reference Section

This section includes materials which are strictly for reference only and have not been adopted by the public rule adopting this manual. The applicant is responsible to insure that the most current materials are used in preparing a permit application. The following changes were made:

  • Reorganized and updated the included materials, eliminating those no longer needed or applicable.
  • Revised Section 1 to be a list of rural zoned areas subject to areal clearing limits under KCC 16.82.150.C and a list of adopted critical drainage areas.
  • Revised Section 2 to be a summary of other adopted requirements in basin and community plans, etc.
  • Revised Section 3 to reflect the proposed public rule repeal of four critical drainage areas and edits made to one other critical drainage area to eliminate duplicative drainage requirements.
  • Replaced Reference Section 4, "Overview of King County Surface Water Policies," with a bibliography of supporting studies and research used to develop the 1998 Design Manual update.

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