Hydrologic performance of bioretention

The City of Olympia will conduct a controlled field study of infiltration rate of the oldest bioretention facilities in Western Washington to evaluate infiltration over time, influential factors on system performance, and provide design and maintenance recommendations.


Study objectives

  • Assess up to 50 bioretention facilities and the effectiveness across their lifespan. Will measure infiltration rates, soil composition, vegetation, and maintenance practices
  • Evaluate this dataset for the influential factors on system performance 
  • Conduct a point-in-time evaluation at bioretention facilities that are 10 years of age or older
  • Provide engineering guidance on the critical factors that prevent bioretention facility failure and recommendations for future designs to build confidence in bioretention systems. 

Study findings

The study will continue until June 2023. 

Project tasks and deliverables

Contract IAA & Scope of Work 

  1. Project Management
    1.1 Scope of contract 
    1.2 Quarterly progress reports
    1.3 Communication coordination with Ecology, partner jurisdictions, and consultants
  2. Study design, QAPP, and Site Selection 
    2.1 Summarize meeting notes
    2.2 Draft QAPP
    2.3 Comment response and final QAPP
    2.4 Communication, site selection, and checklist
    2.5 Site selection technical memorandum 
  3. Field Assessment, Data Collection, and Analysis
    3.1 Hydrologic design review technical memorandum
    3.2 Geotechnical assessment and facility condition technical memorandum
    3.3 Vegetation assessment and maintenance survey summary technical memorandum
  4. Summary Analysis and Report  
    4.1 Summary notes of Stormwater Workgroup/Ecology meeting to discuss initial results, adequacy, and analysis
    4.2. Electronic draft report
    4.3 Summary of meeting notes for discussion of draft report
    4.4 Final report
  5. Distribution of findings
    5.1 Presentation to Stormwater Work Group 
    5.2 Two-page SAM fact sheet summary of project results/findings 
    5.3 Six virtual presentations for counties and city permittees