Hathaway's Substitute for Rain Gardens Helps Underserved Communities

University of Tennessee, Knoxville professor Jon Hathaway exemplifies the balance of disciplines in the Department of Civil and Environmental Engineering (CEE).

Hathaway specializes in urban hydrology, or how water moves through urban spaces. He is particularly interested in how green spaces can add multiple dimensions of value to the concrete jungle.

"Natural areas contribute a lot of good to the overall city," Hathaway said. "It's partly recreation, partly aesthetics, partly habitat for animals, and there's also some really tangible benefits for things that civil engineers are interested in, like stormwater management."

Rain gardens and other specially designed "green infrastructure" store and filter water, reducing flooding and water pollution during storms. That reduces strain on the built water management system, including pipes, municipal water recycling plants, and storm drains.

Recognizing the many important functions of green infrastructure, several organizations in Knoxville have been pushing to increase tree coverage in the city. Installing trees in historically underserved areas, which often have very few or no trees, is one of the highest priorities of the 2024 Knoxville Urban Forest Master Plan.

In 2023, Hathaway and his colleagues at the Tennessee Water Resources Research Center, part of the University of Tennessee's Institute for a Secure & Sustainable Environment (ISSE), received a five-year, $2.6 million grant from the United States Forest Service (USFS) to install green infrastructure around Knoxville—especially in underserved communities.

Hathaway's team includes researchers from the College of Arts and Sciences and the School of Natural Resources and collaborators from many local organizations, including Socially Equal Energy Efficient Development (SEEED) Knox.

"This is a super interdisciplinary project," Hathaway said. "Nothing like this works unless you have a good team of people who care and get things done."

Now halfway through the grant, the team has constructed four novel gravel-tree stormwater systems (GTSSs) around the city, bringing green spaces and all their benefits to underserved areas at a fraction of the usual cost.

What Makes GTSS Special

Rain gardens and similar green infrastructure require specialized soil, putting the cost of installation beyond the reach of some underserved communities.

"That soil can be fairly expensive relative to other basic building materials," Hathaway said. "If you can replace that with something like rock, that's a major cost savings."

That idea inspired Eric Kuehler, the City of Nashville's urban forester, to develop the GTSS. Each GTSS has only a small section of topsoil, which hosts a sapling; the rest of the 15×15-foot site is filled with gravel. As the tree grows, its roots reach down and take up water between the rocks, like they would if growing near a rocky riverbed.

When Hathaway and Kuehler piloted the GTSS, their preliminary results suggested that the new system achieves the same hydrological benefits as other green infrastructure at about one-third the cost.

Stormwater Management with Community Impact

GTSS site selection starts with looking at maps and aerial photos for plots of the right size on land owned by the city, county, or state. Since the USFS grant includes K-12 education on stormwater management and urban watersheds, schoolyards are particularly desirable.

Hathaway and his collaborators visit those candidate sites and consider how much stormwater the site is likely to receive; whether the water would be able to drain out of a GTSS after a storm; if there are overhead power lines or other infrastructure that could block tree growth; and many other factors.

If all those boxes are checked and the land's controllers sign off, Hathaway's team at TNWRRC designs the GTSS for a site.

Construction for the first four GTSSs was handled by SEEED's Green Construction Program, which pays young adults in Knoxville to learn sustainable construction skills—so the GTSS can start benefitting the community before it's even finished.

The grant also funds an ongoing training series for Knoxville community members interested in learning more about tree care, green infrastructure, and urban hydrology. UT and SEEED instructors collaborate to teach the free courses, which often include field trips to in-progress GTSS sites.

Once Hathaway and his team are satisfied with GTSS performance, they will release the plans for free online, so engineers landscaping sites for office buildings, housing developments, parks, and more will have more options for incorporating inexpensive, effective green infrastructure.

"The goal is to have people in Tennessee and beyond be able to design their own effective, low-cost systems based on GTSS," said Hathaway. "Rain gardens are always going to have their place. But for places trying to do stormwater management in a more economical way, they can still make a real difference by implementing a system like this."

Provided by University of Tennessee at Knoxville