Orifice control of bioretention

Washington State University – Puyallup will be evaluating a water-flow-rate-controlling outlet (called an orifice), which is part of a bioretention underdrain system. The project will be studying bioretention facilities with smaller orifices controlling the water-flow rate and evaluating both water quality and water quantity performance improvements.


Study questions

  • What is the water-quality-treatment performance of bioretention with the smaller outlet-control as compared to the traditional media-controlled configuration?
  • How does outlet-control vs. media-control effect the residence time and residence time distribution of water in the bioretention mesocosm?
  • Are any differences in plant health and vigor notable at a mesocosm scale between outlet-controlled and media-controlled designs?
  • Does a small orifice pose notable operations and maintenance issues compared to standard bioretention BMPs without underdrain outlet controls?
  • How are the hydraulic conditions (i.e., stage-discharge relationships) of the compared systems different? Any impacts to modeling?
  • How do hydraulic conditions vary over time and between replicate mesocosms?

Study findings

This study is ongoing until July 2023.

Project tasks and deliverables

Scope of Work, Amendment 1, Amendment 2

Task 1: Project planning and QAPP development

Task 2: Installation and startup

Task 3: Monitoring study implementation

  • Progress Report #1 summarizing vegetation establishment, water quality event #1, special testing event #1, and O&M observations
  • Progress Report #2 summarizing water quality event #2 and O&M observations
  • Progress Report #3 summarizing water quality event #3, special testing event #2, and O&M observations
  • Progress Report #4 summarizing water quality event #4, and O&M observations
  • Progress Report #5 summarizing water quality event #5, special testing event #3, and O&M observations
  • Progress Report #6 summarizing water quality event #6, and O&M observations

Task 4: Modeling study 

Task 5: Reporting and communication of findings