Project overview
Bioretention basins, including rain gardens, are designed to capture and infiltrate stormwater runoff into the basin’s filter media and into the surrounding soil. Over time, sediment from the stormwater can build up in the basin and clog the filter media. Therefore, bioretention basin designs must include pretreatment devices, which are intended to capture sediment and gross solids, (floating debris) before they enter the basin, thereby preventing bioretention basins from flooding and extending their functional lifespans. However, the effectiveness of these pretreatment devices is not well documented, making it difficult for municipal planners, stormwater infrastructure managers, and stormwater engineers to make informed decisions when investing in stormwater management structures.
The purpose of this project was to field-test the performance of several pretreatment practices for bioretention commonly used in Minnesota. Five pretreatment practices for bioretention were assessed for capturing sediment and gross solids:
- Grass-lined inlet (i.e., grassed buffer strip)
- Rock-lined inlet (i.e., riprap).
- Rain Guardian Bunker proprietary device
- Rain Guardian Turret proprietary device
- In-line shallow sump grit chamber, with bypass conditions. This allowed us to determine the performance of an in-line shallow sump grit chamber under bypass conditions.
Research questions
- How well do these five pretreatment structures capture sediment?
- How well do these five pretreatment structures capture gross solids (floating debris, such as leaves and trash)?
- If the system includes an in-line sump grit chamber, do overflow conditions re-suspend sediments and gross solids such that they escape the chamber?
- How do these structures compare in terms of ease of inspection and maintenance?
Research findings
- During low-intensity testing, all five pretreatment practices captured more than 88% of the total sediment mass and more than 65% of the fine sediment fraction. During the high intensity tests, all practices captured more than 70% of the total sediment mass and greater than 30% of the fine sediment fraction.
- Four of the five pretreatment practices (the Rain Guardian Bunker, Rain Guardian Turret, rock-lined inlet, and in-line shallow sump grit chamber) captured 75% of the gross solids during low intensity tests and more than 55% of the gross solids during high intensity tests. The grass-lined inlet captured the fewest gross solids; 20% during low intensity and 30% during high intensity.
- When simulating overflow conditions in a system containing an in-line shallow sump grit chamber, overflow conditions minimally reduced the chamber’s sediment retention rate, while significantly reducing the gross solids retention rate; overall sediment captured in the chamber decreased from 95% to 80%, but gross solids capture decreased from more than 80% to below 40%.
Of the pretreatment practices tested in this study, the Bunker and Turret are among the easiest to maintain, and the shallow sump is moderately easy to maintain. The grass-lined inlet and rock-lined inlet are among the most difficult and costly to maintain.
To maintain the Rain Guardian Bunker, Rain Guardian Turret, and shallow sump grit chamber, the top grate would need to be removed to allow the collected sediment and gross solids to be shoveled or hydro-vaced from the collection chamber. The Bunker and Turret are both easily visible from the street, simplifying visual inspections of accumulated sediment depth. The shallow sump is hidden underground, which makes assessing sediment accumulation depth more challenging. The Bunker, Turret, and shallow sump appear to have ample storage volume for collection and retaining sediment and gross solids.
Key innovations/contributions
This project provided a quantitative measurement of the effectiveness of five pretreatment designs and developed a test method that can be replicated at other locations and for other pretreatment practices.
What does this mean for Minnesota?
Measuring and documenting the effectiveness of these five pretreatment structures provides designers with quantitative evidence they can use to improve their designs. This information can also be used to inform decisions about the maintenance of pretreatment and bioretention systems, which will help stormwater managers to strike the right balance of effectiveness, initial construction costs, and long-term maintenance costs. This can assist local governments to keep within tight budgets while improving the effectiveness of their pollution prevention efforts, resulting in cleaner water, healthier habitats, and improved recreational opportunities for Minnesota residents.
Reports and Presentations
- Research brief 2022 .pdf
Final report 2019