Researchers in the Netherlands lifted sections of living salt marsh from the Wadden Sea coast, drove them to a lab in Delft and subjected them to waves that statistically arrive once in hundreds to thousands of years. The marsh soil barely moved. The plants on top of it did not fare as well. The experiment, published on October 21 in Nature, tests how salt marshes behave in storms of that size, and its answer is more interesting than the usual claim that marshes are natural sea walls.

Bringing the marsh to the wave machine

Intact blocks of salt marsh from the Peazemerlannen nature reserve on the Frisian coast were installed in Deltares' Delta Flume, a facility capable of producing the largest artificial waves in the world, according to TU Delft's account of the study. The marsh was exposed to increasing wave heights from 0.75 metres up to 2 metres, first author Pim Willemsen told ScienceAlert, with water depths over the marsh from 1.5 to 4 metres, according to the study's conference abstract. Those are conditions that occur once every hundred to thousands of years, Willemsen said.

The work was announced by Delft University of Technology and the University of Twente, and the results appeared in two journals: the Nature paper, and a companion study in Communications Sustainability.

The soil held

The most striking result concerned the ground, not the grass. Willemsen said in the university releases that it surprised the team how strong the marsh was under these waves, with almost no erosion during the experiment. The raised marsh platform itself slows the waves, the researchers explain, and it stayed stable throughout.

That matters because erosion is the failure that would turn a protective marsh into an open mudflat. The finding suggests the underlying structure of a healthy marsh survives events that people assume would destroy it.

The plants wore down

Vegetation was a different story. On average only about 30 percent of plant material was lost in the experiment, and at least one third of the plants remained even after the most extreme conditions, according to TU Delft. But the damage accumulated, and the additional wave reduction it provided shrank as the waves grew.

With 75-centimetre waves, the vegetation provided 6 to 22 percent additional attenuation. With 2-metre waves, the contribution was at most 6 percent, TU Delft reported. Stem shortening tracked the loss of effect closely at the lowest wave height, with an r² of 0.89, which gives managers a simple field indicator: shorter stems mean less protection.

Where marshes actually help

To work out what this means outside the Netherlands, the team combined the flume results with global data on salt marshes and extreme storm surges. The map that comes out is split by wave energy. On coasts where waves over the marsh stay below about 1 metre, vegetation adds more than 10 percent additional attenuation and up to 50 percent for 100-year storms, according to the researchers. Where waves exceed 1 metre, the vegetation's contribution does not exceed 10 percent, the paper's abstract states.

In other words, marshes are most useful on sheltered, low-energy shores, such as estuaries and lagoons, and least useful on open, exposed coasts. This is a more specific claim than "nature protects coasts," and it gives planners a way to decide where restoration money is likely to pay off.

Complement, not substitute

The authors are explicit that marshes work alongside engineered defences. Salt marshes can complement dikes by reducing wave loads, and conserving and restoring them can combine nature protection with coastal safety and may cut costs, the Delft team said. In the Netherlands, marsh restoration and conservation could be used in tandem with dikes.

There is also a timing point. Plants are most effective before the storm season starts, because intact vegetation provides the strongest initial wave reduction, and damage might reduce attenuation during consecutive storms later in the winter, the paper cautions. A marsh that is already degraded before winter starts may offer less than the experiment suggests.

What this does not show

Treat the numbers as a strong experiment, not a universal rule. The test used sections of one type of marsh from one coast, transplanted into a flume. The conditions were designed to maximise wave height relative to water depth, which the authors describe as conservative for vegetation damage. The global extrapolation relies on a statistical model fitted to those results, with a high fit to the flume data but still dependent on how well marshes elsewhere resemble the Wadden Sea sample.

The research also does not claim that marshes can stop a storm surge on their own. Willemsen expected the marsh to absorb some wave power, and it did. The magnitude depends on the wave climate.

Why it matters now

Sea levels are rising, and storm planning increasingly assumes that today's once-in-a-century event will recur sooner. Our earlier report on how waves, not just tides, decide where mangroves can grow pointed to the same principle from a different coast: wave energy sets the limits of coastal vegetation. The trajectory behind those risks is set out in UNEP's emissions gap report, which has current policies heading toward 2.8°C.

The practical takeaway is modest and useful. Protect the marsh platform, keep the vegetation healthy going into winter and put restoration where waves are small. On exposed coasts, the dike still has to do the work.

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