Is storm sewer infrastructure along Lake Erie’s coast ready for the future? And what can coastal communities do to prevent an expected deluge of stormwater runoff?
That’s what a team of researchers at Youngstown State University set out to find out in a recent study funded by Ohio Sea Grant. Using feedback from local stakeholders, researchers assessed how well green infrastructure would reduce runoff. The project yielded results comparing which “green” practices are most effective.
“In the future, land use is going to change. Precipitation patterns are expected to increase,” said Suresh Sharma, an associate professor in Youngstown State’s Civil and Environmental Engineering Program, who led the study. “In that context, existing drainage infrastructure, which is primarily designed based on historical precipitation records, will not be resilient enough.”
Permeable pavement, a green infrastructure tool pictured here, can reduce runoff by storing rainwater where it falls. Stormwater soaks in at the surface and is stored in underlying layers of soil and gravel.
As urbanization increases, land becomes less able to hold onto large volumes of water from heavy rainstorms. In the future, such precipitation is only expected to increase in frequency and intensity as global temperatures rise. These factors contribute to stormwater runoff, one of the biggest sources of pollution in the U.S.
A potential way to mitigate this problem is green infrastructure, or low-impact systems that use natural processes to capture, absorb, and reduce runoff. Examples of common installations include rain gardens, bioswales, permeable pavement, green roofs, rain barrels, and others. While research on such green infrastructure is useful, Sharma said, it’s an additional challenge to incentivize local communities to actually implement the practices.
“We can conduct the research, publish the articles, and present in scientific meetings, but how can we get that information on the ground?” he said. “So that’s why we thought that it was important to engage not only the community, but also the engineers, planners, decision-makers, and mayors who are working at the policy level.”
To achieve this, Sharma’s team focused on three small cities in northeast Ohio: Willoughby, Eastlake, and Mentor, working with Keely Davidson-Bennett of Chagrin River Watershed Partners to design the study. Researchers organized stakeholder meetings, distributed questionnaires, and held intensive discussions to gauge concerns and preferences about green infrastructure. Through these steps, researchers learned that stakeholders most preferred rain gardens and permeable pavement for low-impact development.
“It was definitely helpful,” Sharma said. “Stakeholders brought up a lot of practical issues. Most importantly, they brought up the issue of long-term maintenance: how to maintain systems in the long run to ensure the efficacy of the green infrastructure.”
“If we combined all these scenarios together, like rain gardens plus permeable pavement, plus other kinds of green infrastructure like bioswales, then there would be a tremendous opportunity to mitigate runoff.”
Dr. Suresh Sharma
Using those preferences and insights, researchers developed a model to assess the effectiveness of green infrastructure in reducing runoff. First, one student researcher developed an intensity duration frequency (IDF) curve, a graphical tool that describes the likelihood of a range of extreme rainfall events.
Current drainage infrastructure along Lake Erie was designed based on an IDF curve that uses historical records of precipitation, Sharma said, meaning that in the future, those systems won’t be resilient enough. In response, his team created a curve “for the future” using projected rainfall data accounting for various emissions scenarios.
Using that IDF curve and additional data, researchers developed a stormwater management model that can calculate projected runoff and incorporate different types of green infrastructure. To do this, the team used topographical, soil, and land use data of the area surrounding the three communities as inputs and divided the watershed into smaller sections. Next, they tested the model using known precipitation data.
Youngstown State University graduate student Rajati Dahal contributed to the project’s model, which accounts for the watershed surrounding three communities in Northeast Ohio.
Based on stakeholder feedback, the model included rain gardens in residential areas — one per house — and permeable pavement in commercial parking areas. The team used the model to look at a range of different scenarios, such as whether 20% to 100% of houses added rain gardens, increasing in 20% intervals.
Notably, the model accounted for storms of various “return periods,” meaning the probability of having such a rainfall event once in every certain number of years. Specifically, the study accounted for storms of 2-, 5-, 10-, 25-, 50-, and 100-year return periods over durations of six hours and 24 hours. This process was repeated to evaluate how well the model can predict the near future (through 2059) and far future (2060-2099).
“A 100-year return period means that the storm is very unlikely to occur,” Sharma explained. “There is a probability of having such rainfall once every 100 years. We also looked at how rainfall would be distributed over 24 hours, so that’s just daily rainfall.”
From the study, Sharma’s team found that permeable pavement demonstrated notable reduction in total runoff, with peak runoff reduction ranging from 4% to 19% for a 2-year storm event and 3% to 15% for a 100-year storm event across various parts of the watershed.
Meanwhile, despite implementation across residential areas, rain gardens exhibited limited effectiveness, as their impact on peak runoff reduction was negligible for storms with a return period greater than 10 years.
“We found that the efficacy to reduce or mitigate the runoff by using permeable pavement was significantly better than the rain gardens,” Sharma said. “Rain gardens alone aren’t efficient enough to mitigate or minimize the runoff if any rainfall above a 25-year return period occurs. That was impossible.”
Rain gardens are small, shallow, sunken areas with plants that collect stormwater runoff and filter it through a mixture of soil, sand, or gravel.
“But rain gardens were effective in the majority of the time for 2-year, 5-year, and 10-year rainfall. So generally speaking, for small rainfall that we encounter every now and then, rain gardens are still efficient to minimize the runoff,” he continued.
The study also assessed the effectiveness of green infrastructure across different greenhouse gas emissions scenarios in the near and far future. Researchers found that permeable pavement worked well for certain near-future scenarios but was unable to mitigate runoff in several far-future scenarios. However, it’s worth noting that the study only considered permeable pavement in commercial areas, based on stakeholder recommendations, Sharma said.
“If we combined all these scenarios together, like rain gardens plus permeable pavement, plus other kinds of green infrastructure like bioswales, then there would be a tremendous opportunity to mitigate runoff,” he said.
After the study was completed, Sharma’s team and Chagrin River Watershed Partners worked to disseminate information to stakeholders and community members in the cities of Willoughby, Eastlake, and Mentor.
“So it started with the community, and it ended with the community to make a complete loop,” Sharma said.
For more about this research, contact Dr. Sharma at ssharma06@ysu.edu or watch his Freshwater Science webinar.
Ohio Sea Grant is supported by The Ohio State University College of Food, Agricultural, and Environmental Sciences (CFAES) School of Environment and Natural Resources, Ohio State University Extension, and NOAA Sea Grant, a network of 34 Sea Grant programs nation-wide dedicated to the protection and sustainable use of marine and Great Lakes resources. Stone Laboratory is Ohio State’s island campus on Lake Erie and is the research, education, and outreach facility of Ohio Sea Grant and part of CFAES School of Environment and Natural Resources.