Stormwater Drainage Manuals
The Thurston County Drainage Design and Erosion Control Manual (DDECM) spells out regulatory standards for reducing and managing stormwater runoff on new development and redevelopment projects. It applies in unincorporated Thurston County and in cities and urban growth areas that adopt it. The 2022 Drainage Manual is the current standard.
Due to project vesting provisions contained in Thurston County's Municipal Stormwater Permit, the 2022 Drainage Manual requirements apply to projects located within the Municipal Stormwater Permit boundary, with applications submitted:
- On or after July 1, 2022.
- Before January 1, 2017, those who have not started construction by January 1, 2022.
- Before July 1, 2022, those who have not started construction by July 1, 2027.
The 2022 Drainage Manual also applies to projects located outside the Municipal Stormwater Permit boundary with applications submitted on or after July 1, 2022. For projects with applications submitted prior to July 1, 2022, the applicable County code will apply.
Read the 2022 DDECM Vesting Memorandum.
Current Manual
Volume 1: Minimum Technical Requirements and Site Planning
Volume 2: Construction Stormwater Pollution Prevention
Volume 3: Hydrologic Analysis and Stormwater Conveyance
Volume 5: Stormwater Best Management Practices
2022 Drainage Manual Update video
Supporting Documents
Documents and Forms
- Engineers Construction Inspection Report
- Facility Summary Form
- Standard Stormwater Notes
- Stormwater Conveyance System Connection Application
- Stormwater Pollution Prevention Site Plan Checklist
- Ecology's Construction Stormwater Pollution Prevention Plan (CSWPP) template (Word)
- Use of this template is not required but could help prepare a required Construction SWPPP.
- Abbreviated CSWPP Narrative for Small Residential Projects
- Maintenance Activity Log
Maintenance Checklists for Agreement to Maintain
- Aqua Shield Aqua Swirl
- Baffle Oil/Water Separator
- Basic and Compost-Amended Biofiltration Swales
- Biofiltration Swales (Wet and Continuous Inflow)
- Bioretention (Cells, Swales, and Planter Boxes)
- Catch Basins
- CDS Media Filtration System
- Cisterns
- Closed Detention Systems (Tanks and Vaults)
- Coalescing Plate Oil/Water Separator
- Control Structure/Flow Restrictor
- Conveyance Systems (Pipes and Ditches)
- Dead-End Sump Vaults
- Debris Barriers (Trash Racks)
- Detention Ponds
- Downspout Full Infiltration Systems
- Downspout, Sheet Flow, and Concentrated Dispersion Systems
- Energy Dissipators
- Fencing/Shrubbery Screen/Other Landscaping
- Filterra
- Filter Strips
- Gates
- Grounds (Landscaping)
- Infiltration Basins and Trenches
- Media Filter Drain
- Permeable Pavement
- Rain Gardens
- Sand Filters (Basin, Linear, and Vault)
- Stormceptor System
- Stormwater Treatment Wetlands
- Trees
- Vegetated Roof
- Vortechs Stormwater Treatment System
- Wet Ponds
- Wet Vaults
- D.01 Detention Ponds Checklist
- IN.01 Infiltration Basins Checklist
- LID.02 Post Construction Soil Quality and Depth Checklist
- LID.04 Downspout Dispersion Systems Checklist
- LID.06 Sheet Flow Dispersion Checklist
- LID.07 Concentrated Flow Dispersion Checklist
- LID.08 Bioretention Checklist
- LID.08A Rain Gardens Checklist
- LID.09 Permeable Pavements Checklist
- LID.11 Full Dispersion Checklist
- LID.14 Tree Retention and Planting Checklist
- Project Review Flowchart
- Project Elements Submission Completeness Checklist
- Drainage Report Required Elements
- LID Infeasibility Checklist
- Infiltration Facility Procedures Checklist
- Methods for Determining Infiltration Rates Checklist
- Construction Plans Required Elements
- Geotechnical and Infiltration Facility Checklist (Word)
- Agreement to Maintain Commercial Fillable PDF Form
- Agreement to Maintain Subdivision-Residential Fillable PDF Form
- Bond Quantities Worksheet Fillable Excel Worksheet
- Checklist - Final Acceptance Requirements (Due prior to release of Financial Guarantees)
- Drainage Marking Detail
- Low Pressure Air Test Report Form
- Operation & Maintenance Cost Estimate Worksheet
- Source Control Plan for Commercial Word | PDF
- Source Control Plan BMPs Word | PDF
- Agreement to Maintain Fillable PDF Form
- Source Control Manual, Residential Stormwater Pollution Prevention
Past Manuals
Individual Volumes of the 2016 Drainage Manual
Individual Volumes of the 2009 Drainage Manual