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Posted March 28, 2019
Undulating coastal sand dunes make for beautiful landscapes, but they are also a critical coastal defense system. Sand dunes provide natural protection for communities from the devastating forces of waves, storm surge, flooding, and sand inundation. However, dunes are far from indestructible themselves. They can be breached, eroded, or even destroyed by the forces of large waves and coastal storms.
Without the dunes, these coastal communities can be exposed to future hazards. Can human intervention restore dunes after storms? Should coastal communities just wait while nature rebuilds the dune for them, or should they take action? How is a dune built, anyway?
Until recently, most work on this question suggested that dunes are mainly constructed by wind-born, or aeolian, transport of sand. But CEOAS’ Peter Ruggiero, an expert in the study of the evolution of coastal landscapes, knew that wind is not the only force that contributes to building dunes. In a groundbreaking set of interdisciplinary studies, Ruggiero, his collaborators, and their students have been studying the interplay of aeolian transport, ocean processes, and the effects of beach grasses on retaining sand to develop a more complete picture of dune growth.
Ruggiero came to the project from the ocean side of the dune. For decades, he has studied how water shapes beaches, which usually means he’s studying erosion – water as a destructive, rather than a constructive, force. But he and his former Ph.D. student, Nick Cohn, thought that waves could, under some conditions, contribute to dune-building as well. To add to the recipe, Ruggiero’s research group partnered with colleagues that have expertise in other dune ingredients. Laura Moore, at the University of North Carolina, was developing a model of aeolian transport first designed for use in desert ecosystems to explore the role of wind in coastal dune construction. Sally Hacker, a faculty member in Oregon State’s Department of Integrative Biology, is an expert in the ecology of dune grasses; she and Ruggiero had frequently collaborated to study the differing dune-building abilities of Oregon invasive and native dune grass species.
This trio, along with other collaborators and their students, began to collect field data and construct a new mathematical model of beach and dune development by connecting three existing models that describe the effects of wind, water, and sand retention by dune grasses. The overarching model, dubbed Windsurf, takes into account factors like wave height and direction, distance from the ocean’s edge to the vegetation line, wind velocity, and even evolving distribution of sediment sizes on the beach (important because wind can carry small grains farther than large grains).
“This is one of the first quantitative, process-based tools that allows dunes to develop from a flat beach, taking into account the effects of wind, water, and plants,” Ruggiero explained. “It’s an ecomorphodynamic approach.”
How good is Windsurf at predicting dune growth? The team calibrated and verified the model by “hindcasting” dune growth in an area they already knew well: Oysterville, WA, on the Long Beach Peninsula. Using data collected there in 2016 and 2017, the team chose input variables for the model, and were successfully able to recreate the evolution of the entire beach profile, including the dunes. While model results indicated that wind was the predominant force creating dunes in Oysterville, waves and other marine processes were significant as well, especially during fall and winter.
“As part of his Ph.D. thesis, Nick was able to tease apart the aeolian and marine contributions to dune growth for the first time,” Ruggiero said. “The water, as you can imagine, isn’t getting up to the crest of the dune – wind processes are doing the work up there. But at the base of the dune, the water is contributing sand, not just eroding the beach.”
Ruggiero and his team are now using Windsurf in another modeling framework called CReST to examine the impacts of climate change and management scenarios on dune construction. They will be comparing and contrasting model projections for managed and unmanaged beaches by testing model scenarios for another place they have studied extensively: North Carolina’s Outer Banks, a 200-mile-long (320 km) string of barrier islands and spits. Beaches on the area of the Outer Banks called Bogue Banks are managed to maintain the shoreline: beach nourishment is carried out regularly by movement of sand collected offshore onto the eroding beach. In contrast, at the relatively pristine beaches of Cape Lookout National Seashore, nature is allowed to take its course, whether nature strips the beach of sand or piles it up. Parameterizing the model for these sites will provide information about how fast dunes will rebuild themselves after a storm, or what management interventions might be most successful and cost-effective.
Managers and coastal residents will be very interested in the results, and Ruggiero and his team have already been sharing their research with stakeholders in North Carolina. “We can ask questions like, can we let nature do part of the work here? Can we time beach nourishment better? Can we figure out an optimal way to plant dune grasses so they will build a dune that will protect human infrastructure and simultaneously provide ecosystem value?” Ruggiero said.
This kind of scenario development can be used in Oregon, too, which differs from North Carolina in a number of important ways. For example, many Oregon coast towns are small and rural, without the resources to carry out extensive beach nourishment programs like the one in North Carolina. Ruggiero is leading an Oregon Sea Grant-funded project that will use these modeling tools and other information to help managers and residents envision a resilient future for Oregon coast communities as they are exposed to hazards ranging from sea level rise to earthquakes.
Ruggiero thinks that a major strength of all of this work is its interdisciplinary nature. “The students involved have a truly transdisciplinary experience on these projects,” he said. “The team includes ecologists, geomorphologists, economists, modelers, and others, so the students are all learning these disciplines together and applying them to real-world problems.”
As climate change continues to shape coastal landscapes in Oregon, North Carolina, and elsewhere, this work will become increasingly crucial to increase coastal resilience and help coastal communities make good choices about their future.