Salt marshes are proving to be powerful allies in building a climate-resilient coastline

Salt marsh sediment and vegetation can remain largely intact even during extreme storm surges. This was demonstrated in a large-scale experiment in which researchers transported sections of salt marsh to an experimental facility capable of producing the world's largest artificial waves. The results show how salt marshes can contribute to natural coastal protection, both in the Netherlands and beyond. The results were published in the journals Nature and Communications Sustainability.

Researchers from the University of Twente, Deltares, Wageningen University & Research, NIOZ, Delft University of Technology and other institutions transported sections of intact salt marsh from the Peazemerlannen nature reserve on the Frisian Wadden Sea coast to Delft and installed them in Deltares' Delta Flume.

There, a full-scale salt marsh covering more than 300 square meters (3,230 square feet) was created. The researchers then subjected it to increasingly higher waves, with significant wave heights reaching up to 2 meters (6.6 feet). In total, the salt marsh was exposed to waves for approximately 29 hours. The experiment answered a key question for future coastal protection: Do salt marshes continue to function as natural breakwaters even during extreme storms, and can the plants that make up the marsh survive such extreme conditions?

The seabed and plants remain surprisingly stable

The stability of the salt marsh seabed was particularly striking. After 21 hours of wave action, the surface had dropped by an average of only 7 millimeters (0.28 inches). The seabed thus remained virtually intact, even with waves up to 2 meters (6.6 feet) high.

This is important for coastal protection. The elevated salt marsh seabed itself helps to slow down the waves. The plants provide additional wave attenuation on top of that, but are gradually damaged during a storm. With waves of 75 centimeters (29.5 inches), the vegetation in the experiment provided 6% to 22% additional attenuation; with waves of 2 meters (6.6 feet), that was a maximum of 6%.

In tests of different species representative of typical marsh settings around the world, surprisingly little plant material was lost—on average only 30%, and at least one-third of the plants remained even after the most extreme conditions. This means that the marsh has the potential to keep protecting our coastlines, even if more than one large storm occurs over winter.

"Firstly, it surprised us how strong the salt marsh was under these extreme waves, with almost no erosion appearing during the experiment," emphasizes Pim Willemsen, first author of the Nature paper. Willemsen did his Ph.D. at the University of Twente and then worked there as a postdoc. He is now a researcher at Wageningen University & Research and Deltares.

Victoria Mason (researcher at NIOZ) agrees: "The marsh plants were able to withstand the waves and broke surprisingly little, enabling them to consistently contribute to wave attenuation."

Significance on a global scale

To investigate what this means outside the Netherlands, the researchers combined the results from the Delta Flume with global data on salt marshes and extreme storm surges. Especially on coasts where waves over the salt marsh do not exceed about 1 meter (3.3 feet), the vegetation appears to make a significant difference: there, the additional wave attenuation typically exceeds 10% and can reach up to 50%. With higher waves, the contribution of the vegetation decreases.

Using the physical traits, like flexibility, of marsh plants like those tested in the experiment, the researchers aimed to 'forecast' how much plant material is lost in large storms, as well as forecasting the storm itself. This helps reduce uncertainty about what will happen during storms and clarify how much marshes can help to protect coastlines under extreme conditions.

The results thus show where salt marshes can contribute most to coastal protection, as a form of nature-based solutions, an approach based on employing nature. They may not replace traditional seawalls or dikes, but they can be combined with them. Because a salt marsh slows down the waves before they reach the sea defense, the load on the sea defense can be reduced. This offers opportunities to combine nature conservation with coastal safety and can also save on coastal protection costs.

"The proven erosion resistance and wave-attenuation properties of the salt marshes, which have historically been part of our coastline, have led to savings in the Koehool–Lauwersmeer dike reinforcement project. Part of the dike revetment does not need to be replaced, or it can be constructed to a lower strength, resulting in savings of millions of euros," said Paul Buring (Wetterskip Fryslân, water authority Northwest Netherlands).

"Until now, salt marshes barely counted when assessing the safety of a dike. Nobody could say for certain whether a marsh would survive an extreme storm. This full-scale experiment provides that evidence. It means we can start designing the marsh and the dike as one system," says Bas Borsje, associate professor of nature-based flood protection at the University of Twente.

Protecting what already exists

This makes the conservation and restoration of salt marshes especially important. In addition to providing coastal protection, they offer habitats for plants and animals and store carbon. Salt marshes can also grow along with rising sea levels through sediment deposition, though this is not unlimited. If there is insufficient sediment supply or erosion at the edges, salt marshes may actually retreat or disappear. Globally, 719 square kilometers (278 square miles) of salt marshes were lost between 2000 and 2019.