Heidi Nepf, left, and Judy Qingjun Yang SM ’15. Photo: Sarah Bastille
Scores of people died. Thousands of homes were lost. And it would have been even worse without the wetlands hugging the coastline. Marshlands prevented an additional $625 million of property damage, one study found. But more than half the wetlands in the United States have disappeared in the last half century, denuding many miles of coastline and rendering seaside land vulnerable to storms in the future. (The same goes for countries all over the world— wetland loss is widespread.) A big culprit, says Judy Qingjun Yang SM ’15, a graduate student in the Department of Civil and Environmental Engineering at MIT, is erosion of the soils and sands holding coastal plants in place. That means “vegetation could fall down, collapse, and be washed away,” says Yang, who works in the Environmental Fluid Mechanics Laboratory under the direction of Heidi Nepf, the Donald and Martha Harleman Professor of Civil and Environmental Engineering.
Researchers have worked out numerous equations to model how much sediment gets lost in a system that lacks coastal vegetation. But remarkably, few studies until now have examined what happens when there are plants present (though it’s clear that the formulas based on a world without them don’t work at all).
Stepping into the stream
Grasses, seaweeds, and kelps make the computations of fluid dynamics and flow rates much harder. Just think of the whorls you introduce into a river when you pull a single oar through the water. Now imagine each frond or blade throwing off a steady stream of vortices as the water pulses, flowing past and around it. The result is literally dizzying. Yet Yang took on the challenge headfirst, successfully developing a model describing how the movement of sediment is impacted by the presence of vegetation. She’s added in a parameter for plant density, and she’s wrestled with the turbulence that the plants introduce.
To dial in the parameters of her model, Yang set out to build a flume—a water tank 30 feet long. She poured a layer of sand across the bottom, carefully aligned a phalanx of hundreds of thin aluminum tubes to serve as imitation plants, and pumped water through the whole operation. Yang used what she observed in the flume to tweak and perfect her equations. She discovered that the eddies and swirls stirred up by vegetation in the water cause quite a bit of sediment to be transported away. “Previous models underpredicted how much sediment is eroded by three orders of magnitude,” Yang says. Her new model gets it right.
Many state and local governments are trying to restore their wetlands. Yang’s model will help inform these efforts, indicating where sediment should be deposited along the coast to maximize retention of the plants. And the benefits of bringing back wetlands extend beyond protecting coastlines from storm surge, erosion, and flooding, says Nepf. They improve water quality and clarity, provide nurseries for certain species of fish, and store a hefty amount of “blue” carbon (an acre of marsh stores more carbon than an acre of forest on land). For a long time, communities developed and implemented human engineering solutions like levees, dikes, and seawalls to deal with erosion and flooding. But people have come to recognize the benefits of a natural solution. “That’s where my lab comes in,” says Nepf.
Beauty in motion
Yang’s modeling work contributes to a growing set of recommendations for wetland restoration that Nepf and her team are developing empirically. For instance, Nepf explains, her lab is working to determine the specific shoot density and linear extent of marsh or seagrass necessary so that the “flow and wave conditions converge on a sweet spot where the plants have altered the habitat enough to sustain themselves.” Answering such questions allows developers and local governments to make better predictions about how the landscape will evolve with time.
The research has obvious practical applications for protecting coastal communities. But Nepf also does the work because she’s always had a thing for fluid motion. “Some people like to cook—I like to look at water moving,” she says with a chuckle. “It’s very beautiful. It’s so visually intuitive.” Scattered around her office is photographic evidence of her enthusiasm. In a couple of images, fluorescent green dye streams and swirls in blue water. Another photo framed on the wall shows imitation seagrass— a flurry of white ribbons tethered to brown corks—billowing in a controlled current. These snapshots reveal the stunning complexity and splendor of fluid flow.
Yang has seen that complexity up close. Last fall, she spent a few days on Plum Island on the North Shore of Massachusetts, gazing at the marsh grasses dancing in the seawater. She has invested so much time in the lab simplifying the system. But when she watched all that natural movement firsthand, she thought, “Wow. There are so many parameters, so many variables I have to consider.” Yang dipped her instruments into the water and was surprised to find how large a role the waves were playing within the marsh in generating fluid motion near the bed. “In the future, I will consider these waves to make the model more reliable,” she says. The more reliable it becomes, the better off people will be the next time the skies darken and a violent storm makes landfall, encountering a small but ready squadron of green standing guard.
Spotlight on Professorships
Building Collaborations
Marshland in the Gulf of Mexico, for instance, has eroded over the last century— depriving Gulf states of a protective buffer from storm surge—due to man-made rerouting of the Mississippi River, which once provided a constant supply of sediment and freshwater to the marshes. Nepf has a project with Christopher Esposito of the Water Institute of the Gulf to understand the role vegetation plays in trapping sediment to rebuild marshes. This will inform the design of water diversions that will restore some of the Mississippi’s flow back to the marshes.
This collaboration wouldn’t have been possible without her professorship, she explains. It gave her the discretionary funding to attend a set of preliminary meetings with the Water Institute, which led to the grant that funds the project. Nepf finds it especially meaningful that, while a professor at MIT, Donald Harleman SM ’47, ScD ’50 also worked on modeling natural systems to improve human lives. “It’s an honor to have his chair,” she says.
Spotlight on Fellowships
Enabling Exploration
Yang’s choice of PhD project has been a crucial part of her formation as a young researcher. When training to become a scientist, Yang says, “one of the most important things to learn is how to find the question that you’re interested in.” Graduate student aid she received from MIT—including the Edward H. Linde (1962) Presidential Fellowship and the Martin Fellowship—afforded her the time and freedom to explore a variety of topics until she uncovered the one that became her focus. Yang says the fellowships made it possible to attend seminars and to find communities of like-minded recipients, and this led to her cofounding MIT-TREES, aimed at communicating topics in environmental science and sustainability to a general audience.
Once Yang graduates, she’ll have a handful of strong publications to show for her efforts at MIT. They’ll be evidence of her ability to model the real world. But Yang’s real power will be in her ability to find her next big idea.