This article was originally published in Catalyst, April 2020.
The Minnesota Department of Transportation (MnDOT) is exploring innovative ways to filter pollutants from stormwater runoff and meet permit requirements for construction projects. In a recent study, U of M researchers documented the performance of an iron-enhanced ditch check filter to remove phosphates from stormwater. They found that the filter was effective, though its performance decreased over time.
Researchers determined that
natural soil amended with locally sourced materials performed well in bioslopes
and bioswales. This practice will allow MnDOT to avoid hauling in costly
commercial materials for stormwater management installations.
performance of an iron-enhanced ditch check filter to remove phosphorus from
stormwater over three years. The filter was effective, but its performance
decreased over time, and it will require relatively frequent maintenance. Several
design changes may be considered.
Soil carried away in stormwater runoff from road construction sites can pollute lakes and rivers.
Stormwater settling ponds provide a place for this sediment to settle before the water is discharged into local bodies of water. However, since stormwater ponds have limited space, a mechanism is needed to remove clean water from the pond to prevent the overflow of sediment-laden water.
MnDOT-funded researchers have designed temporary stormwater ponds with floating head skimmers that can remove clean water from the surface of the settling pond, using gravity to discharge water into a ditch or receiving body.
This is a new approach for MnDOT and Minnesota cities and counties, so research was needed to provide practical guidance for how to use these devices on construction sites.
“This was a small-scope implementation project for professionals to use as they design temporary stormwater ponds that meet state parameters,” said Dwayne Stenlund, MnDOT Erosion Control Engineering Specialist.
A new MnDOT study identifies five methods for “skimming” stormwater ponds that can improve a pond’s effectiveness by 10 percent. MnDOT researchers also created designs for temporary stormwater ponds on construction sites with the capacity to remove approximately 80 percent of suspended solids.
These designs will help contractors meet federal requirements for stormwater pond dewatering. Researchers also determined how often a pond’s deadpool must be cleaned, based on watershed size and pool dimensions.
“When sediment settles, it’s hard to determine when to clean out a pond. Based on the density of the sediments in the Minnesota River and the loading rates we computed, we were able to calculate how often we need to clean out a pond so sediment doesn’t reach the height of the skimmer,” said Joel Toso, principal of Wenck Associates and a consultant for the project.
Earlier this week, the Minnesota Local Road Research Board released this new video showcasing best practices for local stormwater management. Although it’s primarily a training video for engineers and other public works professionals, non-transportation geeks might also enjoy learning about some of the interesting, innovative techniques being employed in cities and counties across the state.
Those who’d prefer not to watch the whole 14-minute video can skip ahead by clicking on these highlights:
Overall, the video gives you an appreciation for the incredible amount of planning and work that goes into managing stormwater runoff — a task that’s critical to protecting the state’s waterways from pollution (but which many people no doubt take for granted). For those who want to learn more, the best management practices showcased here are examined in greater detail in a recent LRRB report, “Decision Tree for Stormwater BMPs,” which is available for free on the LRRB and MnDOT Research Sevices websites: