1.4.4 Drainage Reports

The Drainage Report shall be formatted to fit 8.5″ x 11″ paper and printed maps shall be folded to 8.5″ x 11″ size unless another format is approved prior to submittal. All Drainage Reports shall be prepared by and bear the stamp and dated signature of a professional engineer licensed in the State of Washington and shall contain the following information, at a minimum:

• Cover sheet – Include the project name, proponent's name, address and telephone number, project engineer, and date of submittal.

• Table of Contents – Show the page numbers for each section of the report, including appendices.

• Project overview – Include the following information:

  • Relevant project background information.

  • Size and location of the project site, including address and tax parcel number(s) of the property.

  • Vicinity map.

  • Project description, including proposed land uses, proposed site improvements, proposed construction of hard surfaces, proposed landscaping, proposed permanent stormwater management facilities, etc.

  • Discussion of how LID techniques were utilized to minimize impervious surfaces, loss of vegetation, and stormwater runoff. Refer to the Site Assessment and Planning Packet, to be included as an appendix, and describe how findings during the site analysis and planning stage were incorporated into preliminary and final designs.

  • List and description of the applicable design standards, documents, and requirements that were used as the basis for drainage design, including but not limited to this manual, the Ecology Manual, and the LID Technical Guidance Manual for Puget Sound (Hinman and Wulkan 2012).

• Existing site conditions – Include the following information:

  • Description of site topography, land cover, and land use.

  • Basin map:

    • Show boundaries of project, any offsite contributing drainage basins, onsite drainage basins, approximate locations of all major drainage structures within the basins, and depict the course of stormwater originating from the subject property and extending all the way to Puget Sound or to the closest receiving body of water (lakes, creeks, etc.).

    • Reference the source of the topographic base map (e.g., survey), the scale of the map, and include a north arrow.

    • Show site topography, land cover, and all drainage features on site. Depict and note on the basin map the acreage of each type of land cover (pervious, impervious, buildings, driveways, etc.).

  • Tabulation of land cover types and acreages.

  • Abutting property land cover and land use.

  • Offsite drainage tributary to the project site.

  • Existing natural and manmade drainage facilities within and immediately adjacent to the project site, points of discharge for existing drainage from the project site, and receiving water body.

  • Sensitive areas, including creeks, lakes, ponds, wetlands, ravines, gullies, steep slopes, springs, groundwater sensitive areas, and other environmentally sensitive areas on or adjacent to the project site. For groundwater sensitive areas, reference applicable reports and include well locations.

  • Existing trees and vegetation.

  • Existing drainage or erosion problems on site.

  • Existing drainage or erosion problems upstream or downstream of the project site that may impact the proposed site development and drainage designs.

  • General soil and groundwater conditions.

  • Reference to the Site Assessment and Planning Packet, to be included in the appendix. Document whether existing site conditions were found to vary from those documented in the Site Assessment and Planning Packet during the course of design, and how those variations affected minimization of impervious surfaces, loss of vegetation, and runoff generation, as well as BMP selection, if applicable.

  • References to relevant reports such as basin plans, flood studies, groundwater studies, wetland designation, critical area designation, environmental impact statements, lake restoration plans, water quality reports, etc. Where such reports impose additional conditions on the project, those conditions shall be included in the Drainage Report.

• Proposed site conditions – Include the following information:

  • Description of proposed changes to site topography as a result of grading, land cover, and land use.

  • Basin map:

    • Show boundaries of project, any offsite contributing drainage basins, onsite drainage basins, approximate locations of all major drainage structures within the basins, and depict the course of stormwater originating from the subject property and extending all the way to Puget Sound or to the closest receiving body of water (lakes, creeks, etc.).

    • Reference the source of the topographic base map (e.g., survey), the scale of the map, and include a north arrow.

    • Show proposed topography, land cover, and all proposed conveyance, on-site stormwater management, flow control, and water quality treatment facilities. Depict and note on the basin map the proposed acreage of each type of land cover (pervious, impervious, buildings, driveways, etc.).

  • For projects implementing a TDA approach (see Step 8 – Delineate Threshold Discharge Areas (if applicable)):

    • TDA Delineation map

    • For large projects that trigger MR #1-#9 (see KCC 12.08.275) and implement TDAs, provide a summary table that includes the following for each TDA (Refer to Volume III-3.2 of the Ecology Manual for additional guidance):

      • New pollution-generating hard surfaces (PGHS),

      • Replaced PGHS (where the replaced hard surfaces have been determined to be subject to requirements per Vol I–4.1 Project Applicability),

      • Effective impervious surfaces, and

      • Converted vegetated areas (to determine whether Runoff Treatment and/or Flow Control BMPs are necessary to comply with Vol I–4.2.6 Minimum Requirement #6: Runoff Treatment and Vol I–4.2.7 Minimum Requirement #7: Flow Control).

  • Tabulation of proposed land cover types and acreages.

  • Potential stormwater quantity and quality impacts resulting from the proposed project.

  • Minimum requirements that pertain to the project.

  • Drainage-related requirements beyond the minimum requirements that pertain to the project.

  • Proposed permanent stormwater management plan to address the minimum requirements and other drainage-related requirements, including conveyance, on-site stormwater management, flow control, and water quality treatment facilities.

• Infiltration feasibility assessment and infiltration BMP design – Provide a summary of the relevant Geotechnical Analysis/Soils Reports (Vol II–1.4.5 Geotechnical Analysis/Soils Reports) prepared for the project. The summary shall include discussion of the methods, assumptions, results, and recommendations regarding infiltration feasibility and design of infiltration BMPs, and how those recommendations were used to support design. Include the full Geotechnical Analysis/Soils Reports in an appendix or multiple appendices, as appropriate.

• Downstream analysis – Include a Level 1 downstream drainage analysis prepared in accordance with the requirements in Volume II, Chapter 4. This Level 1 analysis, as well as the location of the project in a drainage basin, will be reviewed by the County to determine whether a Level 2 and/or Level 3 downstream analysis will be required. Any further analysis of downstream conditions required beyond the Level 1 analysis shall become a part of the Drainage Report and shall be submitted as part of the Drainage Report.

• Hydrologic/Hydraulic analysis – Discuss the modeling methods and software programs used to size conveyance facilities, including outfalls and energy dissipation, and on-site stormwater management, flow control, and water quality treatment BMPs. Include the version of the software programs being used. Include screen shots of the facility design from the modeling software. Include complete model output reports in the appendices, annotated as appropriate to highlight assumptions, rationale for any non-standard model inputs used, interpretation of key results, etc. to aid review of the modeling.

• Operation and Maintenance (O&M) – List the standards and guidelines used to develop long-term O&M requirements for the permanent stormwater BMPs included in the designs. Provide a complete O&M manual in an appendix to the Drainage Report, detailing specific maintenance activities, frequencies, responsible parties, equipment needs, conditions triggering O&M, etc. See Volume II, Chapter 7 for additional O&M requirements.

• Appendices – Include a copy of the completed Site Assessment and Planning Packet (Vol II–1.4.2 Site Improvement Plans), relevant sheets from the site improvement plans (e.g., grading, paving, tree protection, drainage plans and profiles, etc.), Geotechnical Analysis/Soils Reports (Vol II–1.4.5 Geotechnical Analysis/Soils Reports), model outputs and reports, site photographs as appropriate, O&M manual, and any additional relevant reports that support or corroborate the findings, conclusions, or assumptions contained in the Drainage Report.