Coastal homes being swept away by the sea due to erosion and intense storm surges are increasingly common sights as climate change leads to rising sea levels and stronger storms. In 2022 alone, coastal storms resulted in $165 billion worth of damages in the U.S.
A new study from MIT indicates that enhancing and protecting salt marshes in front of seawalls can significantly strengthen coastal defenses at a cost-effective rate.
The findings are published in the journal Communications Earth and Environment, featuring research by MIT graduate student Ernie I. H. Lee and professor of civil and environmental engineering Heidi Nepf. Nepf highlights that restoring coastal marshes “is not just an optional endeavor, but it is actually economically viable.” The investigation revealed that salt marshes can diminish wave impacts, allowing for lower seawall constructions that reduce building costs while still ensuring substantial protection during storms.
Nepf notes, “An exciting aspect of this study is that a large marsh is not necessary for good results. Even a relatively small marsh, merely tens of meters wide, can provide benefits.” This encourages her, as she believes the findings could promote the preservation of smaller marshes that might have seemed too costly to maintain. “Our results show that maintaining even a small marsh can be financially justified,” she adds.
While previous research has highlighted the advantages of expansive marshes in mitigating storm damage, Lee points out that such studies often concentrate on large marsh areas measuring hundreds of meters. “We aim to demonstrate that these benefits extend to urban areas, where marshes are limited, particularly since existing infrastructure like seawalls are typically in place,” he explains.
The study utilized computer simulations to analyze wave behaviors across various shore profiles, focusing on the characteristics of different salt marsh plants—such as their height, rigidity, and density—rather than depending solely on a traditional empirical drag coefficient. “Our model, which reflects the interaction between plants and waves, enabled us to examine how plant species and their characteristics change with seasons, without needing to collect field data for every situation,” Nepf elaborates.
The researchers conducted a benefit-cost analysis based on a straightforward metric: the potential reduction in seawall height if paired with a specific length of marsh to protect the shoreline. Other evaluation methods, like the economic impact on real estate from potential flooding, can vary widely based on how assets are valued in such situations. “We opted for a more tangible value calculation, focusing on the equivalent seawall height required to deliver the same level of protective benefit,” Lee states.
They experimented with various plant models, taking into account differences in height and stiffness throughout different seasons. The results showed a twofold variation in the effectiveness of different species in wave attenuation, although all presented some beneficial effects.
To validate their simulations with a practical case, Nepf and Lee studied local salt marshes in Salem, Massachusetts, where restoration projects are currently in progress. This concrete example provides a template for future projects, according to Nepf. In their analysis of Salem, they found that a thriving salt marsh could reduce the required seawall height by 1.7 meters (roughly 5.5 feet), ensuring pedestrian safety against wave overtopping.
Nonetheless, gathering the real-world data necessary for marsh modeling, including maps of salt marsh species, plant height measurements, and density of plant shoots, is often “very labor-intensive,” Nepf remarks. Lee is developing an approach that employs drone imagery and machine learning to simplify this mapping process. “This innovation will help researchers or planners assess a marsh area’s worth in terms of its flood-reduction capabilities,” says Nepf.
Nepf notes that the White House Office of Information and Regulatory Affairs recently issued guidance on evaluating ecosystem services when planning federal initiatives. “However, many scenarios still lack specific methods for quantifying value, and our study addresses this gap,” she comments.
Additionally, the Federal Emergency Management Agency has a benefit-cost analysis (BCA) toolkit in place. Lee adds, “They provide guidelines for quantifying various environmental services, and a unique aspect of our paper is the quantification of cost and protective value associated with marshes. Policymakers can utilize this information to better understand the environmental service value of marshes.”
The modeling software that environmental engineers can apply to specific sites is available for free on GitHub. “It’s a one-dimensional model that is user-friendly for standard consulting firms,” Nepf explains.
“This paper offers a practical tool for translating marshes’ wave attenuation abilities into economic terms, which could aid decision-makers in using marshes for natural coastal defense,” asserts Xioaxia Zhang, a professor at Shenzen University in China, who was not part of this study. “The findings suggest that salt marshes are not only ecologically beneficial but also economically advantageous.”
Lee acknowledges the support he received for this research from the Schoettler Scholarship Fund, managed by the MIT Department of Civil and Environmental Engineering.
