Project overview
Sediment is a regulated pollutant that is carried by urban stormwater runoff. Many stormwater treatment systems therefore incorporate pretreatment structures, such as pond forebays, deep manhole sumps, or shallow catch basin sumps, to capture sediment before it reaches the main treatment system.
Although these structures are commonly used, there is limited guidance for determining which type of pretreatment structure to select, how large it should be, and whether the additional treatment benefits justify the additional maintenance costs.
To address this gap, the researchers collected maintenance data from 87 ponds, 64 deep manhole sumps, and 13 shallow catch basin sumps. They used the data to develop a model that estimates sediment accumulation and maintenance costs over a 30-year period and then applied the model to compare different pond forebay sizes.
Research questions
- How effective are pond forebays at capturing sediment and reducing sediment accumulation in the main pond?
- How does pond forebay size affect long-term maintenance costs?
- How do the maintenance costs of pond forebays compare with other pretreatment options, such as deep manhole sumps and shallow catch basin sumps?
Research findings
- Forebays can increase sediment capture. After modeling three Minnesota ponds, the project team found that adding a forebay increased modeled sediment removal from 95–98% to 100% (by mass). Larger forebays generally captured more sediment, reducing the amount that accumulated in the main pond.
- Forebay size affects long-term maintenance costs. The research team modeled pond systems with no forebay and with forebays making up 5%, 10%, and 15% of total pond surface area. Maintenance costs initially decreased as forebay size increased, then leveled off or increased. The data suggest that choosing a forebay size based on long-term maintenance costs could reduce estimated 30-year maintenance costs by 20–30%.
- Forebays may have lower sediment-removal maintenance costs than other capture structures. Data from Minnesota stormwater systems suggest maintenance rates were much higher for deep manhole sumps—about 100 times those of forebays—and for shallow catch basin sumps—about 4–7 times higher. This comparison does not consider construction or land acquisition costs or potential benefits beyond sediment removal.
What does this mean for Minnesota?
Minnesota stormwater managers and designers can use this research to compare pretreatment structure options and evaluate forebay sizes based on sediment loading, storage capacity, and long-term maintenance costs. This can help local governments make more informed infrastructure and maintenance decisions, use public resources more efficiently, and protect Minnesota’s lakes and rivers from sediment and other pollutants.
Project assets
Reports and presentations
The documents (PDFs) below were created before current policy requirements took effect, and therefore may not be accessible. To request this content in an accessible format, contact [email protected]. To view the video of this presentation please visit the 2026 MSRC Annual Meeting site.
- Research brief 2026
- Project update 2026 .pdf
Tools and guidance
- Practice brief 2026