Dunes have garnered quite a bit of attention this year. Sure, some of that is due to the science fiction series of the same name– but we think the real-life dunes are much cooler. And don’t worry, there are no giant sandworms here.
Dunes are mounds or ridges that form when sand is moved by wind (also known as Aeolian transport) and deposited in an area with an obstacle, such as vegetation or rocks. The consistent deposition of sand in one spot forms a dune, commonly found along beaches and deserts.
For what may seem like a pretty basic natural feature, natural infrastructure practitioners talk about dunes a lot. We sat down with Dr. Amanda Tritinger, Professional Engineer, Deputy Program Manager of the Engineering With Nature® program, and self-proclaimed dune enthusiast to learn more about why these structures are so important for coastal resilience.
“During storm events, the dune is your foremost barrier against flooding, inundation and wave damage,” Tritinger explained. “Whatever is behind it, whether that’s an estuary or infrastructure, the dune is the first line of defense.” These shoreline sentries are one of the most natural kinds of natural infrastructure, providing an organic impediment to coastal flooding.
Dunes do occur naturally, but because of their protective value, are often helped along by a beach’s stakeholders or community.
“I mean, probably every beach you’ve ever been to has been put there by either the Army Corps or a consulting company,” Tritinger joked. Beach renourishment projects involve, no surprise, figuring out the right kind of sand to use, but it’s not as simple as it sounds. “There are certain parameters you have to meet for placing sand on a beach,” she explained. “It has to be a certain color, for example. If the sand is the wrong color, sea turtles aren’t going to nest there.” The particles also have to be the right size so that they aren’t blown away. Sand grains range greatly in size, and there are several types of geotechnology devoted to figuring out the right median diameter (or D50 value) for the sand at a given location.
Next up, you have to get that sand on the beach. “Once you’ve found that sediment source, we’ve done a lot of engineering to figure out what to do for that location. Based on wind factors and hydrodynamic factors, we’ll decide what the shape of the beach should be.” Managed beaches are usually renourished every few years.
But dunes aren’t just sand: there’s an entire ecosystem happening underneath the sand that also has to be replicated. “We’ll have tiny little plugs [of vegetation] that we put in the dunes so that we have some kind of stabilization, but we’re still studying how it works and how to maximize it,” Tritinger explained. Researchers from across the Network are working together to address this knowledge gap.
“Back in 2015 or 2016, we had a project where we looked at how above- and below-ground biomass increased the stability of the dunes,” Tritinger said. “They basically added more or less of this fiber material inside and above the dune, then ran different wave scenarios to calculate estimates, such as the amount of belowground biomass needed for effective stabilization.”
However, the team on this project knew they weren’t getting the full story. The fibers used could provide similar tensile stabilizing properties to the vegetation commonly found in dunes, but couldn’t replicate the chemical properties of biomass breakdown or the nutrition to the soil. Tritinger observed fungus growing like a root system throughout the dunes. “Like underground systems connecting,” she described.
Today, similar projects with EWN® and the University of Florida are working to better understand what additional chemistry and biology is happening under the dunes to stabilize them. “In one of their case studies, they pulled data underneath a natural dune system, and the fungus looked like concrete. There’s additional value in the stabilization of that fungus, not to mention ecosystem benefits.”
These restoration projects are looking at dunes as full ecosystems. “For example, how are these natural biomass systems providing things like shelter for ghost crabs? Then, in turn, how are the ghost crabs taking shelter there bringing more nutrients when they kill something, then supporting the vegetation?” Tritinger asked. “This collaboration is looking at more than just the impact of having biomass in the dune system, but the co-benefits to the system.”
Tritinger grew up in Florida and experienced multiple hurricane seasons firsthand. But after one particularly heavy season during her freshman year of high school, she wasn’t able to attend school for several months.
“I saw the impact of hurricanes pretty closely that year,” she said, “and then fast forward to me getting my degree, and going to the University of North Florida to study Civil Engineering with a focus on coastal modeling, we had Hurricane Matthew.” A Category 5 major hurricane that occurred in fall 2016, Matthew was the deadliest storm to hit the Gulf of Mexico in over a decade.
“It was an opportunity for one of my thesis projects to basically do a forensic study,” Tritinger recalled. “We walked the beaches, we did surveys, we talked to people, and it was pretty clear really quickly that the communities that had invested in their dune systems were the ones that were protected during the storm.” Other communities nearby had much weaker dune systems, seeing significant damage and road flooding that prevented residents from getting to work and school.
“And I knew what that impact felt like, to have your life kind of stop while a hurricane came in,” Tritinger said. “Then in 2017, Irma came. It was really fascinating; we did the same kind of forensic study in the same communities.” Ahead of Irma (an even stronger Cat. 5 hurricane than Matthew), many residents had attempted to build up their flood resilience, but there were different approaches with differing levels of success. This included hastily constructed engineering solutions that actually created more sediment erosion, ultimately causing more damage to neighboring houses when Irma made landfall.
“But some of the places that had done dune renourishment after Matthew,” she explained, “they were great– they were protected. This was the difference during Irma.” Even the city of Jacksonville had implemented dune restoration projects following Hurricane Matthew, and as a result saw significantly less inundation during Irma.
“I was in the eye of the storm,” Tritinger said. “I was able to see just how much the dunes were doing, and I’ve been kind of obsessed ever since. Dunes are beautiful, and they could be a big solution to flooding during storm events.”
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