Title
Drainage Design and Erosion Control Manual Update Project Briefing
Recommended Action
Discussion only. No action requested.
Report Issue:
Receive a briefing on the Drainage Design and Erosion Control Manual update project.
Staff Contact:
James Patton, PE, Senior Engineer, Public Works Water Resources, 360.753.8749
Fran Love, Project Engineer, Public Works Water Resources, 360.753.8397
Presenter:
James Patton, PE
Background and Analysis:
The City of Olympia (City) Drainage Design and Erosion Control Manual (DDECM) sets standards and provides guidance on the measures necessary to control the quantity and quality of stormwater produced by new development and redevelopment within the City of Olympia.
To ensure that the City continues to effectively manage stormwater from new development, redevelopment and construction activities, the City’s 2024-2029 Western Washington Phase II Municipal Stormwater Permit (Permit) requires a review of our DDECM for equivalency with the 2024 Stormwater Management Manual for Western Washington (Ecology Manual). The review and any necessary revisions must be completed by June 30, 2027 to maintain compliance with the Permit.
The City’s DDECM was last updated in November 2022 and is equivalent to the 2019 Ecology Manual, as was required by the City’s previous Permit.
A workgroup was formed to review and update the DDECM. The workgroup consists of City staff responsible for public and private development review, permitting, inspection, maintenance and Permit compliance.
To ensure that the 2027 DDECM is fully consistent with the 2024 Ecology manual, the workgroup used the 2024 Ecology manual as its “base” and then added Olympia-specific requirements. By taking this approach, the usability enhancements in the 2024 Ecology Manual, which include consolidating repetitive information, revising text for clarity, and reordering sections for a better flow of concepts, are incorporated.
Revisions to the 2024 Ecology Manual resulted in several significant changes to stormwater requirements applicable to the City. The following areas of change made to the 2024 Ecology manual are meaningful for new development and redevelopment in Olympia:
• Lower re-development project thresholds
• Reductions to project exemptions
• Less stringent wetland hydroperiod protection
• Source control Best Management Practices (BMPs) - PCB edits
• New bioretention design options
• Updated Underground Injection Control (UIC) program guidelines
In addition to incorporating changes from the 2024 Ecology Manual, the DDECM workgroup is considering some new and revised City-specific requirements, including the following:
• Revising and streamlining the Stormwater Maintenance Agreement template
• Adding a new drainage report type specifically for Middle Housing*
• Adding new BMPs specifically for Middle Housing*
• Adding an alternative method and advisory language for calculating soil infiltration rates, in light of some recent failures
* These City-specific sections allow an increase in impermeable surface coverage for Middle Housing developments that use these BMPs.
A complete list of changes will be made available after the internal review is completed and will be provided to the UAC at the February 2027 meeting.
Climate Analysis:
The DDECM is an important part of climate infrastructure resilience. The DDECM sets the requirements for new development and re-development to mitigate impacts on water quality and flooding.
The DDECM update seeks to strike a balance between stringent environmental protection and promotion of beneficial development. The DDECM must require developers to adequately mitigate their impacts on flooding and water quality. However, overly stringent requirements in the DDECM could discourage density and promote sprawl. The 2027 DDECM is being developed with the goal of balancing these considerations while maintaining applicable stormwater management and environmental protection requirements.
Equity Analysis:
The DDECM supports equitable outcomes by requiring developers to mitigate stormwater impacts caused by development in accordance with consistent City-wide standards. In this way, water quality and flooding protection measures are applied on an equitable and consistent basis.
Neighborhood/Community Interests (if known):
No specific neighborhood or community concerns have been identified at this time. A public and stakeholder review period will be included as of the DDECM update process.
Attachments:
None