Dutch Marsh Stands Its Ground Against the Sea

A salt marsh lifted from the Dutch coast and hit with artificial storm waves up to two metres high lost almost none of its ground, according to a full-scale experiment published in Nature on October 21, 2026 ^[2].

The result matters because coastal planners have long liked the idea of marshes as storm protection but had little proof that the ground would survive the worst weather. Until now, researchers say, evidence that marsh sediment stays stable under waves higher than one metre was lacking ^[2].

What the researchers did

The team cut intact marsh from the Peazemerlannen nature reserve on the Dutch Wadden Sea coast and rebuilt it in the Delta Flume in Delft, a very large wave channel run by the research institute Deltares ^[1][4]. The transplanted marsh covered 306 square metres ^[1][2][4], somewhat larger than a tennis court.

They then ran waves over it, raising them step by step to two metres in significant wave height, a standard measure that roughly tracks the height of the bigger waves in a sea state. In total the marsh took about 29 hours of wave attack, and for 21 of those hours its above-ground vegetation was in place ^[1][2][4].

Sensors at 32 fixed points tracked the surface. Over the 21 hours with vegetation, the marsh lowered by an average of seven millimetres ^[1][2][4]. The researchers describe the erosion of the substrate as negligible ^[2].

The vegetation was mostly Elymus athericus, a coarse grass of the upper marsh, with Atriplex prostrata, a salt-tolerant orache, at lower densities ^[2].

Ground does most of the work in big waves

The finding cuts two ways, and the second half is as useful as the first. A marsh slows waves partly because its raised, sloping ground forces water to shoal and lose energy, and partly because stems drag on the flow ^[2].

The ground held up well. The plants, however, matter less as the waves grow. At waves of 0.75 metres, vegetation added between six and 22 per cent extra wave attenuation. At two metres, it added no more than six per cent ^[1][2][4].

The paper's global assessment puts the same point in coastline terms. Where incoming waves are no higher than about one metre, vegetation can cut wave heights by more than 10 per cent and by as much as 50 per cent. On higher-energy coasts its contribution is limited ^[1][2].

So a marsh is not a magic green wall. The platform, built up over years from deposited sediment, is the part that stays put when the sea turns violent.

Do the plants survive?

A companion study in Communications Sustainability, published October 8, looked at what happens to the plants themselves ^[1][3]. Researchers tested winter-state vegetation from three marsh species, Spartina anglica, Scirpus maritimus and Phragmites australis, along with plant mimics. Conditions went up to two-metre waves, water levels of four metres and peak wave periods of eight seconds ^[1][3].

Average biomass loss was 30 per cent. At least one-third of the plants were still standing after the most extreme runs ^[1][3]. Flexible plants fared better than stiff ones, and the authors used plant traits to build a way of predicting biomass loss across marsh species ^[3].

Together the two studies suggest that after a severe storm a marsh is bruised but still there, with its ground intact and a good part of its cover.

From the flume to the dike

The researchers do not propose marshes in place of dikes or seawalls. They describe a combined system in which a marsh in front of an engineered defence reduces the wave load that reaches it ^[2][4].

Bas Borsje of the University of Twente said marshes had barely counted in dike safety assessments because nobody could be sure they would survive extreme storms. In his account, the full-scale experiment now makes it possible to design marsh and dike as a single system ^[1][4].

The water authority that commissioned the work is already counting on it. Paul Buring of Wetterskip Fryslân said the erosion resistance and wave-damping of the marsh have saved millions of euros in the Koehool-Lauwersmeer dike reinforcement project, because part of the dike revetment, the protective armour on the slope, need not be replaced or can be built to a lower strength ^[1][4]. That figure is the authority's own and was not measured in the experiment.

Wetterskip Fryslân commissioned the experiment, and the High Water Protection Program funded it. Researchers came from the University of Twente, Deltares, Wageningen University & Research, the Royal Netherlands Institute for Sea Research (NIOZ), Delft University of Technology and other institutions ^[4]. Because the University of Twente is close to the research team, its account is the team's own.

Why the ground matters

Marshes are also shrinking. The University of Twente says 719 square kilometres of salt marsh were lost worldwide between 2000 and 2019 ^[4]. A marsh can build height as sediment settles on it, but where sediment supply falls short or the edges erode, it can retreat or vanish ^[1][4].

That sets a plain condition on the engineering case. A platform that shrugs off two-metre waves is worth counting on only if it is still there when the storm arrives. Marshes also shelter wildlife and store carbon, which is a reason to keep them whether or not they help a dike ^[4].

What remains unknown

This was one Dutch marsh, one set of species and one controlled run of waves. Marshes differ in soil, tidal range, width, slope and sediment supply, and the flume cannot show how a real marsh copes with a whole winter of repeated storms.

How engineers will turn the results into design rules for marsh-and-dike systems is the next step, and the papers do not yet answer it. The Koehool-Lauwersmeer savings are also still to be shown in figures.

For a country that has spent centuries keeping the sea out, the lesson is a humble one. Some of the best sea defence may be ground the sea itself built, and the job is to leave it standing.