Dutch engineers dug up 306 square metres of wild salt marsh, trucked it 250 km and hit it with the biggest waves the world’s largest wave machine could make. After hours of 2-metre storm waves, the marsh had lost an average of just 7 millimetres of soil. The result was published in Nature on 21 October 2026, and it could change how countries build their sea defences.
Key takeaways
- A full-scale salt marsh from the Dutch Wadden Sea coast was tested in Deltares’ Delta Flume, a 300-metre wave channel in Delft.
- Waves of 0.75 to 2 metres simulated storms expected only once in hundreds to thousands of years.
- The soil barely moved: about 7 mm lost after 21 hours of storm waves.
- The plants took a beating, but they matter most in calmer, low-energy places, where they can cut wave power by up to 50%.
- Water managers already count on marshes in one Frisian dike project, saving millions of euros.
What did scientists do to the salt marsh?
A salt marsh is a strip of muddy, grassy land on the coast that the tide floods and then drains. Think of it as a doormat in front of the dike. Every wave has to cross it before reaching the wall behind.
In September and October 2023, the team cut intact blocks of marsh out of the Peazemerlannen nature reserve in Friesland. Each block weighed 3,000 to 6,000 kg and held 0.4 to 0.7 metres of soil, ScienceAlert reports. Closed trucks carried them about 250 km so the wind would not damage the plants, and the crew tracked each block’s weight to make sure it did not dry out.
The blocks, mostly sea couch grass (Elymus athericus) with oraches and sea aster, were laid in the Delta Flume and tested in early 2024. According to the University of Twente, which led the work with Deltares, Wageningen University, NIOZ, TU Delft, Utrecht University and Leibniz University Hannover, the marsh took about 29 hours of waves in total.
How much can a salt marsh stop storm waves?
This is where the study gets more nuanced than the headline. A marsh slows waves in two ways. Its raised ground makes the water shallower, so waves lose height. Its plants add drag, like a hand trailing through bathwater.
The ground did most of the heavy lifting in the biggest storms. In the flume, the plants added 6% to 22% extra wave damping when waves were 75 cm high. At 2 metres, their share fell to 6% at most.
The team then scaled the result up to marsh coasts worldwide. Where waves crossing the marsh stay below about 1 metre, vegetation usually removes more than 10% of wave power, and up to 50%. Above 1 metre, it rarely tops 10%.
Did the marsh survive the storm?
The soil did, impressively. After 21 hours of storm waves, the surface had dropped by an average of 7 mm, less than the thickness of a pencil. “The marsh is even stronger than we anticipated,” lead author Pim Willemsen, now at Wageningen University and Deltares, told ScienceAlert.
The plants had a rougher time. The first wave run stripped about a third of the above-ground biomass, and stems ended up 47% shorter. But roots held, and unlike concrete, grass grows back after a storm.
How does this compare with earlier wave-machine tests?
This is not the first time a marsh has gone into a flume. In 2014, a Cambridge-led team tested German Wadden Sea marsh in Hannover’s Large Wave Flume (GWK). They found that 40 metres of marsh cut the height of large waves by 18%, with about 60% of that reduction down to plants, according to the project team.
| Test | 2014, Hannover GWK | 2024 test, Delta Flume (Nature, 2026) |
|---|---|---|
| Marsh origin | German Wadden Sea | Peazemerlannen, Dutch Wadden Sea |
| Marsh size | About 40 m long | 306 m² |
| Biggest waves | Storm-surge conditions | 2 m significant wave height |
| Headline result | 18% lower wave height | Just 7 mm soil loss in 21 hours |
| Role of plants | About 60% of wave reduction | Up to 22% extra damping at 0.75 m waves, 6% at most at 2 m |
The two results fit together. Plants matter a lot in moderate seas. In a true monster storm, it is the tough, raised soil that keeps the marsh working.
Why does this matter for dikes and coastlines?
About a quarter of the Netherlands lies below sea level, so this is not academic. Until now, Dutch dike safety checks barely counted marshes because nobody knew whether they would survive an extreme storm. Now there is evidence that they do.
That is already paying off. On the Koehool–Lauwersmeer dike reinforcement in Friesland, the marsh’s resistance meant that “part of the dike revetment does not need to be replaced, or it can be constructed to a lower strength,” said Paul Buring of the water authority Wetterskip Fryslân. He puts the savings at millions of euros.
My take: the most useful part of this study is its honesty about limits. A marsh will not replace a seawall on an exposed, high-energy coast. Put one in front of a dike, though, and you get a buffer that repairs itself, stores carbon and costs far less than poured concrete. It is the same logic behind the cheap, nature-first fixes now being priced for the Arctic.
What happens next
- Marsh and dike as one system. “It means we can start designing the marsh and the dike as one system,” said Bas Borsje, associate professor of nature-based flood protection at the University of Twente.
- Protect what is left. The world lost 719 km² of salt marsh between 2000 and 2019. NIOZ researcher Victoria Mason called conserving and restoring marshes “a priority.”
- Better forecasts. A companion paper uses plant traits such as stem flexibility to predict which marshes survive extreme storms anywhere on Earth.
- Bigger storms ahead. Stronger storm surges are expected in the coming decades as the climate warms, a trend that shows up in records like New Zealand’s 1.36°C warming report.
FAQ
Can a salt marsh replace a seawall?
No. The researchers say marshes work best alongside dikes and seawalls. They reduce the wave load on the hard defence, which can make it cheaper to build or maintain.
Where was the salt marsh experiment done?
In the Delta Flume at Deltares in Delft, the Netherlands, a wave channel about 300 metres long. The marsh came from the Peazemerlannen reserve on the Frisian Wadden Sea coast.
Do marsh plants matter if the soil does most of the work?
Yes. Where waves stay below about 1 metre, plants can cut wave power by more than 10% and up to 50%. Their roots also help bind the soil that held firm in the test.
Live near a coast? Check whether your local flood plan counts marshes and wetlands as defences, and whether there is a restoration project you can back.
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