Preventing erosion of urban streams
Rain falling over an urban area follows pathways laid out by city infrastructure to guide it to gutters and storm sewers that help keep water off the streets and away from homes, and direct it into local streams. There, the stormwater runoff rushes away to join larger and larger bodies of water.
This system has unintended consequences for urban streams, which erode under the resulting high volume and flow of water. The issues don’t end with urban streams, however; they also flow to waterbodies downstream.
Dr. Rod Lammers, an Institute for Resilient Infrastructure Systems research and teaching associate, explained, “In urban areas, we put down a lot of pavement and concrete. When it rains, water runs off into streams instead of soaking into the ground. Stream channels tend to erode in response to that runoff, which in turn releases sediment and phosphorus into the water, where it may cause algal blooms downstream and make it more difficult to treat the water for drinking.”
Examples of algal blooms range from the seasonal blooms that Lake Erie experiences to Florida’s red tide. These events have the potential to close beaches and wreak havoc on ecosystems. As streams become deeper and wider due to erosion, the areas around them become less effective at removing excess nutrients, further exacerbating the problem.
Lammers examined a new approach to preventing this chain of events by combining two tactics for ameliorating stream erosion: installing stormwater controls like rain gardens, which lets runoff soak into the ground near its source, and restoring stream channels to a more stable form with vegetated banks.
“The goal of this project was to look at how we could integrate those two things better. Especially because if you’re doing restoration, you’re usually addressing the symptom of the problem, whereas by installing natural infrastructure, you’re addressing the cause of the problem—which is the altered runoff in urban areas,” Lammers said.
In order to examine how these two approaches worked together to regulate stormwater runoff, the research team linked two models to depict different runoff scenarios in Big Dry Creek, a watershed north of Denver, Colorado. The first was the Stormwater Management Model (SWMM), developed by the United States Environmental Protection Agency, which allowed them to model how installation of natural infrastructure, such as rain gardens, manages stormwater.
The second model is one that Dr. Brian Bledsoe, director of the Institute for Resilient Infrastructure Systems and Georgia Athletic Association Distinguished professor, and Lammers developed together: the River Erosion Model (REM), which simulated how Big Dry Creek eroded in different stormwater control and restoration scenarios.
“Modeling scenarios can be useful to see the potential impacts of different techniques,” Lammers said. “This research was somewhat unique because it combines two models together.”
Ultimately, they found that stormwater control has the biggest effect on preventing erosion.
“Our major conclusion was that stormwater control alone has a much bigger effect on preventing erosion than channel restoration. It goes back to the fact that stormwater controls address the root cause, whereas stream restoration is often a bandaid,” Lammers said.
However, combining the two methods yielded the best overall results.
“In our modeling scenarios, the restoration projects failed if flows and stresses on the channel were too high. And when we combined both stormwater controls and restoration, we saw the lowest rates of failure,” Lammers explained.
This work was funded by the EPA-supported Comprehensive Optimal and Effective Abatement of Nutrients (CLEAN) Center and the NSF-supported Urban Water Innovation Network (UWIN).