The Oregon Dunes Day Use Area in Douglas County, Sept. 15, 2022. Researchers at Oregon State University found that newly planted vegetation can actually accelerate dune erosion in the face of extreme storms.
Rolando Hernandez / OPB
One of the best-known strategies to prevent dune erosion is to introduce plants whose roots will help hold sand in place and stabilize the dune. But experiments conducted at Oregon State University show that may not always be the case. Researchers found that newly planted vegetation can actually accelerate erosion in the face of extreme storms. Meagan Wengrove is an assistant professor of civil and construction engineering at OSU. She was involved in the experiments and joins us to talk about what the findings could mean for coastal management.
Note: This transcript was computer generated and edited by a volunteer.
Dave Miller: This is Think Out Loud on OPB. I’m Dave Miller. One of the best-known strategies to prevent dune erosion is to introduce plants whose roots can help hold sand in place and stabilize the dune. But experiments conducted at Oregon State University show that that may not always be the case. Researchers found that newly planted dune grasses actually accelerated erosion in the face of a simulated extreme storm. Meagan Wengrove is an assistant professor of civil and construction engineering at Oregon State University. She was involved in these experiments and she joins us now. It’s great to have you on the show.
Meagan Wengrove: Thanks for having me, Dave.
Miller: What did you set out to study with this experiment?
Wengrove: Yeah, good question. We were looking at different dune archetypes, different shapes of dunes and vegetation or different engineering features of dunes, coastal dunes, to look at how they erode during extreme wave conditions. And so we looked at a bare dune, a dune with only sand, and a vegetated dune, and then also a dune that was more highly engineered.
Miller: Can you remind us why dune ecosystems like those on the Oregon coast, but you know, you worked with researchers on the Gulf Coast, we also could see them in various parts on the Atlantic side. Why are they so important?
Wengrove: Yeah. So our coastal dunes are really important for coastal protection. That’s what we think of them from a civil engineering framework, so when you have a big storm, the dunes and the beach are often a communities or a habitat’s first line of defense against the wave attack and erosion. And so they’re there for protection. But then from an ecological side, they also provide a lot of habitat to various different types of species. So they have different economic benefits.
Miller: And what’s at stake now going forward, given rising sea levels and more intense storms?
Wengrove: So with rising sea levels, we’ll see water, high water levels approach the toe of the dune, is what we call the very bottom of the dune, more frequently when you have a storm event and it’s really the combination of these higher water levels that come with a storm and wave action that cause erosion of the dunes. And so what we would expect over time, like many decades, is that the dunes would like to migrate landward, step backwards, with increasing sea levels. But sometimes we might not see that because there’s houses or walls or whatever in the way behind the dunes.
Miller: How did you actually set up this study?
Wengrove: The Wave Lab is a National Science Foundation funded facility at Oregon State University. It’s really cool. It’s 104 meters long, 15 feet deep and 12 feet wide. And you can test all different, almost two scale or prototype scale applications in the flume. And so what we did is we built an actual dune that was half the size of a dune we would find out on the open coast, inside of the flume. And that meant we trucked in 76 dump trucks full of sand. We compacted each one foot level of sand and lifts and placed instruments inside of the dune.
And then for the vegetated dune experiment, we actually grew the plants in place on the dune. We constructed a greenhouse over the dune and the plants grew there for about six months before we started the actual experiment where we ran waves and increased the water level to look at the combined influence of surge, increased water levels on the coast and wave attack during a storm.
Miller: How big a wave did you subject the dunes to?
Wengrove: We pushed the limits of the wave lab paddle. So it has this paddle that’s basically a giant hydraulic moving wall at the far end, and it can generate waves up to just less than two meters in wave height and that was the highest wave that we ran for this storm.
Miller: And so something like six feet or so.
Wengrove: Yeah.
Miller: That doesn’t sound like a gigantic wave. But in the paper, this was listed as something that would come from a really serious storm. So what am I missing?
Wengrove: Yeah, good question. So, we had to scale the dune and the waves down to fit in the flume. So everything was scaled down, excluding the vegetation, because we don’t know how to scale plants, I guess. And so the sediment size was scaled to the wave conditions which was scaled to the size of the dune. And so this would, we modeled this off of Hurricane Sandy scale waves, which hit the New Jersey and New York coast pretty hard and a dune on the New Jersey coast, so it was scaled from a real life situation to fit into the flume, so we could test the extreme conditions that were seen out there.
Miller: OK. So a much smaller dune than might naturally exist and a smaller wave, but scaled to be actually a really immense, in this case, a famous and very destructive wave, or scaled to that equivalent. What did you find?
Wengrove: So we found a few different things related to the topic of this paper. We found that the vegetated dune actually scarped. So the word scarp, meaning that steep step that you might find on the beach when you’re walking around. It looks like a wall, sometimes, of sand. So the dune scarped sooner for the vegetated dune case than for the only sand, the bare dune case. And the reasons we think that happened are because when the waves rush up on the beach, they cause two things: they cause instability of sediment offshore of what we call the run up 2% line. So where 2% of all the waves reach on the beach, they cause erosion offshore of that area and a little bit of accretion onshore of that area. And that creates this steepening, this juxtaposition between erosion and accretion creates a little bit of steepening on the beach which we saw eventually formed into a scarp. So that was consistent with the bare dune observations.
The vegetated dune, however, we saw that the water actually seeped into areas around the stems of the plants and caused some instability at those stem interfaces which caused this scarp to form much sooner than it did on the bare dune. So…
Miller: If I could interrupt, so I could understand here, because it seemed like, if I understand the old paradigm correctly, it was that the roots of these plants, they would grab onto some of the sand and they would make it sort of a little bit of a stronger wall, they would prevent erosion by holding onto some of the sand in the face of the oncoming water. But instead, what you found is that the roots or stems made almost passageways for the water to trickle down into. And then when that water would go, would follow the stems or the roots down, it would increase instability in the dune. Did I say that right?
Wengrove: Yeah, that was a great summary. So we found that, what you summarized, and what happens when a scarp forms is it’s a lot easier for the dune to erode if the water levels and the waves stay high enough to keep hitting that scarp, because it’s a really unstable face and the sand can just kind of keep slumping off of that face when the waves impact it. But I would say there is a caveat to our findings in that our plants, as I said earlier, only were growing for six months. So the plant root mat, what we would call it, is not very established at that point. So if you had a dune that had been growing in like the real environment for over two years, the root mat of the plants would be much more dense. And so we can’t say whether or not the roots would have held the sediment in place more for a dune that was more established. But what we did find is for a newly planted dune, which does happen a lot around communities that are trying to reestablish their dunes for various reasons in Oregon and on the east and the Gulf Coast that there could be some vulnerability there, if a newly planted dune is hit by a big storm.
Miller: It seems like then there are still some really important unanswered questions. But let’s say that what you just outlined is correct, that for the first two years after, following the new planting of dune grasses, the dune is actually more susceptible to erosion than before. But then after that, maybe it’s less susceptible. I mean, it seems like there’s maybe, if that all that is true, then you just, there’s no way to get around that. You just have to hope for the best for the first couple years of the planting before the roots can really establish themselves?
Wengrove: Yeah, that’s what my personal takeaway is, that you really have to hope for the best. Or maybe if you did see a big storm coming, go push more sand up on the beach, that in that kind of road before it hits your baby plants that you’re trying to establish or think about ways of maintaining your newly planted dune, within that period when it’s trying to establish itself, to enable it to be successful in the longer run.
Miller: It also makes me think that the best day to plant dune grasses is two years ago, but the second best day is today.
Wengrove: Yeah, that’s true.
Miller: What are you interested in studying next?
Wengrove: Well related to this problem, I do have a colleague on the east coast who is looking at the erosion of established dunes with more established root mats. So I’m pretty interested in what they find as their results. I think that’s a great next step.
And another current project that we have now, which is equally important to dune growth and resilience, is looking at wind blown sand feeding the dunes. So these plants are not only there to protect the community or the dune from wave attack, but they also are really good at capturing sand in windy conditions. And so we’re looking here in Oregon, actually how different types of dune grasses, different species, capture sand differently from one another and what that might mean for the resilience of a dune.
Miller: And what about the rooting properties of different kinds of grasses? I mean, there are native grasses on the Oregon coast, there are introduced ones that people 100 years ago thought maybe were a good idea, that have drastically changed dunes, say in, on the south coast. How much do we know about how quickly different grasses will actually provide erosion protection?
Wengrove: Yeah, so that is a good question. The resilience of different species is an open question, for sure. Especially due to wave attacks. And so I think that is another next step, looking at how different species and their root mats behave in these big extreme storm events. And yeah, most of the invasive grasses are actually all mostly on the northern coast of Oregon and into Washington. And we’re working with them, along with a great ecologist here at OSU, Sally Hacker.
Miller: Meagan Wengrove, thanks very much.
Wengrove: Thank you so much, Dave. Thanks for having me.
Miller: Meagan Wengrove is an assistant professor of coastal and ocean engineering at Oregon State University. She’s part of a team that found that dunes with recently planted grasses eroded faster when hit with large waves than a bare dune.
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