In a twist, beach grass could make dunes more vulnerable to storms | Science

When a hurricane blows over a sandy shore, dunes take the brunt of the waves. That’s why land managers hoping to prevent damage farther onshore often embrace planting grasses and other vegetation to boost dune strength. But now a massive laboratory experiment, described in Science Advances this week, suggests those grasses may accelerate erosion of dunes during severe storms.

The finding “is quite controversial,” says Sierd de Vries, an expert in coastal engineering sciences at the Delft University of Technology who was not involved in the study. Still, “I think this will be a very influential paper,” he says, with implications for how engineers plan coastal protection projects that can cost billions of dollars.

Scientists have previously studied dunes in the lab by creating vegetated dunes in metal troughs called wave tanks. This research has shown that plants slow down the waves and prevent erosion of the sand. The benefit increases with more and bigger plants. But the wave tanks are relatively small—typically a few meters long—and the experiments often last less than an hour. Rusty Feagin, a coastal ecologist at Texas A&M University, wondered what would happen if the dunes were pummeled with bigger waves over a longer duration. He teamed up with more than a dozen colleagues to design a grander experiment. “It was like, just go big.”

The team carried out the work at Oregon State University, which runs a 104-meter-long flume—the largest experimental wave laboratory in the country. They trucked in 500 cubic meters of sand and spent weeks with a bulldozer and backhoe sculpting a beach and dune to look like shores along the northeastern United States. Then they transplanted bitter panicgrass (Panicum amarum), a common dune plant, and let it grow and establish roots inside the flume for 6 months.

Once the grass had taken hold, they cranked up the wave machine to simulate Hurricane Sandy, which struck the northeastern U.S. in 2012. The experiment simulated 19 hours of wave battering in total. But it took about a month to do that, because after every half-hour of storm waves, they would stop to scan the shape of the dune with a laser. Then they repeated the experiment with an identical dune that was just bare sand.

On the bare dune, the waves rolled up the face of the dune and then back down. This removed sand from the dune, but the slope remained about the same.

Something different happened on the grassy dune: When waves hit the grass, they dug sand out from in front of the plants. These divots grew into larger depressions as more erosive energy was concentrated in a smaller area, because the grass blocked part of each wave. After 14 hours, the waves had carved a small cliff face called a scarp. Then the erosion rapidly accelerated because the vertical face of sand, 79 centimeters high, was undercut by the waves. “The whole thing just fails,” Feagin says.

On the bare dune, a similar scarp formed after nearly 18 hours, but it was less than half the height. And, by the end of the storm, the bare dune retained more sand than the grassy dune. Overall, the vegetated dune lost 22% more sand than the bare dune because of erosion.

It’s very unusual to observe vegetation accelerating and increasing erosion, de Vries says. “That’s something I’ve never thought of, or heard of, before seeing this work of these very highly respected scientists.” But he’s uncertain about whether or how often it might happen in the real world.

“The findings are very interesting as they challenge the ideas we have about dune vegetation and erosion,” says Julie Zinnert, a coastal ecologist at Virginia Commonwealth University who was not involved. But she notes that the type of grass used in the experiment has fewer roots than other dominant species on dunes, and roots may help prevent erosion. Dunes may also be better protected when they have many types of vegetation, she says, and plants help dunes rebuild after wave erosion because they trap wind-blown sand.

Feagin doesn’t dispute the benefits of verdant dunes for storm protection. Even if the plants cause dunes to erode in severe conditions, he says, it’s still worth having them for protection from milder storms. “When that medium-small storm hits, [the dune] is going to do its job, but the giant one every 10 years is gonna take it out.” And vegetated dunes have advantages over concrete walls for storm protection, because native plants can grow and spread. That means the plants can help dunes increase in size as they trap sand, and perhaps keep up with rising sea levels.

The take-home point of paper is “more about managing expectations” of vegetated dunes, Feagin says. They are not a panacea for coastal protection, he suggests—and land managers shouldn’t expect them to deliver permanent stability.

When a hurricane blows over a sandy shore, dunes take the brunt of the waves. That’s why land managers hoping to prevent damage farther onshore often embrace planting grasses and other vegetation to boost dune strength. But now a massive laboratory experiment, described in Science Advances this week, suggests those grasses may accelerate erosion of dunes during severe storms.

The finding “is quite controversial,” says Sierd de Vries, an expert in coastal engineering sciences at the Delft University of Technology who was not involved in the study. Still, “I think this will be a very influential paper,” he says, with implications for how engineers plan coastal protection projects that can cost billions of dollars.

Scientists have previously studied dunes in the lab by creating vegetated dunes in metal troughs called wave tanks. This research has shown that plants slow down the waves and prevent erosion of the sand. The benefit increases with more and bigger plants. But the wave tanks are relatively small—typically a few meters long—and the experiments often last less than an hour. Rusty Feagin, a coastal ecologist at Texas A&M University, wondered what would happen if the dunes were pummeled with bigger waves over a longer duration. He teamed up with more than a dozen colleagues to design a grander experiment. “It was like, just go big.”

The team carried out the work at Oregon State University, which runs a 104-meter-long flume—the largest experimental wave laboratory in the country. They trucked in 500 cubic meters of sand and spent weeks with a bulldozer and backhoe sculpting a beach and dune to look like shores along the northeastern United States. Then they transplanted bitter panicgrass (Panicum amarum), a common dune plant, and let it grow and establish roots inside the flume for 6 months.

Once the grass had taken hold, they cranked up the wave machine to simulate Hurricane Sandy, which struck the northeastern U.S. in 2012. The experiment simulated 19 hours of wave battering in total. But it took about a month to do that, because after every half-hour of storm waves, they would stop to scan the shape of the dune with a laser. Then they repeated the experiment with an identical dune that was just bare sand.

On the bare dune, the waves rolled up the face of the dune and then back down. This removed sand from the dune, but the slope remained about the same.

Something different happened on the grassy dune: When waves hit the grass, they dug sand out from in front of the plants. These divots grew into larger depressions as more erosive energy was concentrated in a smaller area, because the grass blocked part of each wave. After 14 hours, the waves had carved a small cliff face called a scarp. Then the erosion rapidly accelerated because the vertical face of sand, 79 centimeters high, was undercut by the waves. “The whole thing just fails,” Feagin says.

On the bare dune, a similar scarp formed after nearly 18 hours, but it was less than half the height. And, by the end of the storm, the bare dune retained more sand than the grassy dune. Overall, the vegetated dune lost 22% more sand than the bare dune because of erosion.

It’s very unusual to observe vegetation accelerating and increasing erosion, de Vries says. “That’s something I’ve never thought of, or heard of, before seeing this work of these very highly respected scientists.” But he’s uncertain about whether or how often it might happen in the real world.

“The findings are very interesting as they challenge the ideas we have about dune vegetation and erosion,” says Julie Zinnert, a coastal ecologist at Virginia Commonwealth University who was not involved. But she notes that the type of grass used in the experiment has fewer roots than other dominant species on dunes, and roots may help prevent erosion. Dunes may also be better protected when they have many types of vegetation, she says, and plants help dunes rebuild after wave erosion because they trap wind-blown sand.

Feagin doesn’t dispute the benefits of verdant dunes for storm protection. Even if the plants cause dunes to erode in severe conditions, he says, it’s still worth having them for protection from milder storms. “When that medium-small storm hits, [the dune] is going to do its job, but the giant one every 10 years is gonna take it out.” And vegetated dunes have advantages over concrete walls for storm protection, because native plants can grow and spread. That means the plants can help dunes increase in size as they trap sand, and perhaps keep up with rising sea levels.

The take-home point of paper is “more about managing expectations” of vegetated dunes, Feagin says. They are not a panacea for coastal protection, he suggests—and land managers shouldn’t expect them to deliver permanent stability.