How roadside drainage ditches reduce pollution

Stormwater can pick up chemicals and sediments that pollute rivers and streams. Roadside drainage ditches, also known as swales, lessen this effect by absorbing water. But until recently, MnDOT didn’t know how to quantify this effect and incorporate it into pollution control mitigation measures.

In a recently completed study, researchers evaluated five Minnesota swales, measuring how well water flows through soil at up to 20 locations within each swale.

“There’s a big push in Minnesota, and probably everywhere, to do more infiltration,” Barbara Loida, MS4 Coordinator Engineer, MnDOT Metro District, said. “We know that our ditches are doing some of that, but we wanted to look at how much infiltration these ditches are providing.”

A key finding: grassed swales are significantly better at absorbing water than expected, which may reduce the need for other, more expensive stormwater management practices, such as ponds or infiltration basins.

This could save MnDOT and counties significant right-of-way and construction costs currently expended on more expensive stormwater management techniques. While swales were recognized in the Minnesota Pollution Control Agency’s new Minimal Impact Design Standards, there was a need to quantify the amount of water a swale can absorb so it could receive the appropriate MIDS credits.

Researchers also tested the ability of carbon, iron chips, steel wool and other materials to remove pollutants as ditch check filters—material put into swales to enhance removal of pollutants.

Gradations on a Modified Philip Dunne infiltrometer allow the measurement of stormwater infiltration.
Gradations on a Modified Philip Dunne infiltrometer allow the measurement of stormwater infiltration.
What’s Next?

A follow-up project, which the MPCA is participating in, will seek to clarify the impact of swale roughness on infiltration rates. The goal is a calculator for real-world infiltration rates that MnDOT and local agencies would be able to implement.

MPCA, MnDOT and the city of Roseville are also partnering on a project to install and test the effectiveness of ditch check filters in real-world locations.

Maintenance recommendations should help MnDOT and local agencies ensure that swales operate at maximum efficiency. These recommendations should continue to be revised as knowledge evolves.

Related Resources

*Editor’s note: This article was adapted from our upcoming edition of the Accelerator. Read the newsletter online, or sign up to receive by mail. 

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