In August, Minnesota Department of Transportation (MnDOT) Research Services & Library released a report “Assessing and Improving Pollution Prevention by Swales.” Researchers found substantial spatial variability in swale infiltration rate. This report discusses how roadside swales can be used to their full pollution prevention potential and cost-effectiveness, covering maintenance schedules and efforts. They also developed a ditch check filtration system that can provide significant treatment of dissolved heavy metals and dissolved phosphorus. The report provides design guidelines and recommendations for these filters.

In May, the MnDOT Research Services & Library also released the report — “Reducing Construction Pollution by Skimming Stormwater Ponds.” Ponds can trap sediment from construction, and skimming devices can prevent or reduce the amount of sediment-laden water that escapes the pond. While sediment settles to the bottom, the skimmers move clean water from the top of the pond into ditches or waterways via gravity. The study identifies five floating-head skimmers that can improve the effectiveness of stormwater ponds. Though the report makes no recommendations as to the skimmers’ effectiveness, but prior research suggests that properly designed ponds with skimmers can reduce total suspended solids (TSS) by 90%. MnDOT researchers also created designs for temporary stormwater ponds that computer modeling indicated would remove 80% of TSS. Additionally, the report provides maintenance recommendations for ponds with skimmers.

Check It Out

While the MnDOT Research Services & Library helps to optimize the effectiveness of roadside stormwater controls, they recognize that not all controls perform the same treatment functions or have the same pollutant removal efficiency. Their construction and maintenance costs vary as well. In 2013, the Minnesota Local Road Research Board (LRRB) released a video covering seven stormwater management practices, including swales and ponds. The video is based on LRRB’s stormwater control decision tree. The report, released in 2011, serves as a tool to assist practitioners in selecting the right stormwater controls for specific projects.