The coastline is eroding in Louisiana. Marshland is vanishing in New York Harbor. And in the Gulf of Mexico dead zone, fish are disappearing from a 7,000-square-mile area.
Whatever’s to blame for our threatened rivers, wetlands, and coastal zones, Heidi Nepf’s work provides rare insights into how plant life can help repair damage caused by manipulating Mother Nature.
“For more than a century, we’ve been tearing vegetation out of channels and coastal zones to facilitate navigation and development,” said Nepf, professor of civil and environmental engineering and MacVicar Faculty Fellow. “When people noticed a decline in habitat quality and an increase in uncontrolled flooding, they started to think, ‘Maybe that wasn’t such a great idea.’ Now we recognize the ecologic and economic benefits of aquatic vegetation.”
Aquatic vegetation’s roles in cycling nutrients, creating habitats, controlling erosion, and protecting coasts are estimated to contribute economic benefits worth over 10 trillion dollars per year, according to research by the University of North Carolina.
The health of large areas of ocean coastal waters and shoreline is dependent on native vegetation that ranges from tiny algae to clusters of mangroves. Nepf’s work in environmental fluid mechanics quantifies for the first time how water flow around these organisms is inextricably tied to their form and function. The work, among other things, guides restoration work by helping ecologists determine the critical mass needed to tilt the scale on a restoration effort from futile to productive.
Her work has relevance “anyplace with a coast. Marsh grasses, kelp, sea grasses — I’m interested in all of them,” she said.
To illuminate water flow around plants, Nepf and her colleagues use intriguing tools. Nepf and students in MIT’s Environmental Fluid Mechanics Laboratory created artificial sea grass. The spindly tendrils undulate in the waves just like the real thing — except they are made of the same plastic used for sandwich baggies. Nepf and her students use the model vegetation to explore changes in flow and turbulence that cause waves to flush and move sediment through the canopy. The work has implications for sea grass restoration projects, such as those in the Gulf of Mexico and in Rhode Island.
Perched on the banks of the Mississippi River in downtown Minneapolis, the Outdoor StreamLab (OSL) is a river within a river. A manmade channel that borrows part of the Mississippi’s flow, it allows researchers working on habitat restoration, stream ecology, and bank stabilization to make measurements with laboratory precision.
In summer 2008, Nepf and her students used OSL to investigate how vegetation alters flow in meandering river bends, and whether or where vegetation enhances sediment retention enough to change the river’s shape over time. “The point of our experiment was to see how vegetation on the banks of a river changes water movement and turbulence, and to connect that to sediment erosion and deposition patterns,” she said.
It turns out that nature did not intend bodies of water to have ruler-straight edges. Rivers’ and streams’ natural tendency to meander is not just the stuff of poetry; it’s key to their health.
Anybody who has ever made moats for sandcastles knows that if you dig a small channel, water will rush through, making it bigger. If vegetation is removed from a riverbed, or if muskrats track a trough along the bank, water will start to flow faster through ever-widening channels, making it harder for plants to regrow. Nepf explores engineering solutions that might offset the effects of flow so “the system does not keep resetting, but will keep performing the way you want for its whole life,” she said.
For example, Nepf is seeking to determine the critical mass a meadow of sea grass needs to repopulate itself like a shag carpet. It’s not yet fully understood at what thresholds of coverage the meadow will thrive or wash away.
Growing up on Long Island, N.Y., Nepf and her family loved the outdoors, often hiking and cross-country skiing in upstate New York. In addition to river-strewn pine forests, Nepf had easy access to two very different coastlines — the powerful Atlantic to the south and mild Long Island Sound to the north.
“I always loved nature,” she said, but it was an undergraduate class at Bucknell University that led her to the physics of fluids. “We got to put some dye into water and see how the water moves,” she said. The tendrils of blue dye swirled into intricate patterns and delicate webs. “I thought, ‘This is beautiful!’ To this day, I love to throw a little dye in and see what it does.”