Salt Marshes Cut Extreme Storm Waves Only on Quiet Coasts

A transplanted Dutch salt marsh cut two-metre storm waves by at most 6 percent after 21 hours in a giant flume, while the mud under the plants dropped 7 millimetres. The plants still took some of the punch. They did it on the quiet side of a storm, not at the height dikes are built to survive.

Pim Willemsen, a coastal researcher at Wageningen University & Research and Deltares, and colleagues published those wave cuts under extreme storm waves on 21 October 2026. Water boards have been selling salt marshes as living armour for the next round of sea-dike work. The tank says the armour is a low-energy-coast tool.

Sixty-Six Marsh Blocks Went Into a 300-Metre Tank

Dutch sea dikes are built for storms that, on paper, come once every thousands of years. Those seas almost never arrive while a field crew is standing on the marsh with instruments, so the team moved the marsh to the storm.

The dikes are designed to withstand a storm statistically occurring once every thousands of years. We cannot wait for that to happen in the field, so we had to bring the salt marsh to the storm.

Pim Willemsen, coastal researcher, Wageningen University & Research and Deltares

In September and October 2023 they cut 66 blocks from the high marsh at Peazemerlannen, a nature reserve on the Frisian Wadden Sea coast managed with It Fryske Gea. Each slab weighed 3,000 to 6,000 kilograms and came up 0.4 or 0.7 metres deep, grass, roots and clay together. Sea couch, Elymus athericus, dominated the cover, with orache and sea aster in the mix.

The blocks went into Deltares’ 300-metre-long wave flume in Delft, 5 metres wide and 9.5 metres deep, and were laid as two 71-metre profiles, 306 square metres in all, in front of a 1:3.6 model dike. A brushwood dam, the same kit used on Wadden Sea edges, held the seaward cliff so the test would measure the platform, not a collapsing front. Installation began in December 2023. Wave runs started in February 2024, with the plants in their winter state.

THE TRANSPLANT CALENDAR

1. September-October 2023: Crews lift 66 marsh blocks at Peazemerlannen and haul them to Delft.
2. December 2023: The slabs go into the flume on a sand bed with drains, in front of the model dike.
3. February 2024: Winter-state tests begin, with water 1.5 to 4 metres above the marsh and incoming significant wave heights of 0.75, 1.25 and 2 metres.
4. Spring 2024: About 29 hours of waves in total, 21 hours with the plants still standing and 8 hours after the stems were mowed to the mud.
5. 7 February 2025: University of Twente staff post the storm-peak reading at the dike toe, 1.90 metres on bare marsh and 1.79 metres with plants still in place.
6. 21 October 2026: The full analysis appears, with a global map of how long marshes sit under damaging waves.

The Dutch Flood Protection Programme paid for the tank time, as part of work that must upgrade more than 2,000 kilometres of dikes and 400 sluices and pumping stations by 2050 on a budget of 13.7 billion euros. NWO’s Living Dikes programme from 2021 to 2026, led by Bas Borsje at the University of Twente, had already put 1.9 million euros behind the wider living-dike bet.

The Clay Bed Held Through 21 Hours of Waves

Wave attenuation in a marsh comes from two things, the raised bed and the drag of stems in the water. If the bed erodes, both go. After 21 hours with the plants still in place, 32 survey pins showed a mean drop of 7 millimetres, or 0.33 millimetres an hour, smaller than the marsh’s own hummocks and hollows.

THE TANK NUMBERS

• Bed drop: 7 millimetres over 21 hours with plants, on the same order as older tests that used smaller waves.
• First-run plants: Dry biomass fell 33 percent, from 831 to 558 grams per square metre, and stems went from 56 to 45 centimetres.
• After 21 hours: 20 to 25 percent of the starting above-ground biomass was still there, and stem length was down 47 percent to an average 0.29 metres.
• Roots: A root mat sat in the top 10 centimetres and did not wash out with the leaves.

That platform result matches what hurricanes Katrina and Rita did in Louisiana, where 527 square kilometres of mainly brackish marsh tore out while salt marsh stayed in place. Willemsen said the strength of the bed under these waves was the first surprise, with almost no erosion in the runs.

The plants were another matter. Damage piled up with time more than with each step up in wave height, except in the last 2-metre run, and it was worst at the seaward edge. Stubble still stood. It was not the meadow that went into the tank.

Two-Metre Waves Fall by 6 Percent at Most

The team measured incoming and landward wave height with gauges, then subtracted the runs after the stems had been mowed, so the plant share could be split from the bed. On the smallest incoming waves, 0.75 metres, plants still cut 6 to 22 percent of wave height. On the 2-metre class, the plant share sat between 0 and 6 percent.

WAVE CUT BY THE PLANTS

The 1.90 and 1.79 metre pair is the height at the dike toe in the same facility, not the incoming 2-metre sea. It is the number a dike designer actually feels. A statistical model using exposure time and incoming wave height fitted the plant share tightly (pseudo R² 0.97). Plant damage in this kind of grass is already under way around 0.5 metres, and it keeps going as the sea stands up and the clock runs.

Borsje had argued in 2022 that a few hundred metres of marsh would yield enough wave cut that the dike behind would no longer need to be raised, even as the climate shifted. The tank does not wipe that idea out for a sheltered Wadden coast. It puts a hard cap on it for the seas that overtop a sea wall.

Half of Marsh Coasts Sit Below the 1-Metre Line

The group then pushed the tank model onto global storm-tide hydrographs, coast-normal elevation tracks and salt-marsh maps. They treated this winter Elymus stand as a conservative stem, on the short and weak side of marsh grasses that carry a stalk.

About half the marshes in the set sit under damaging waves for 200 to 400 minutes in a single storm, a short window for the grass to come apart. The longest exposure was 1,000 minutes for both 100-year and 1,000-year storms, and that ceiling produced at most 50 percent stem-height loss in the model. Most of the world’s peak water levels over these marshes also stayed inside the 1.5 to 4 metre depths the flume actually ran.

The split is geographic. On low-energy marsh coasts, incoming waves up to 1 metre, the plant share generally exceeds 10 percent and reaches 50 percent, and that band covers 46 percent of the 100-year transects and 34 percent of the 1,000-year transects. On high-energy marsh coasts, waves above 1 metre, the plant share does not exceed 10 percent.

That is the buried result inside a paper being read as a win for nature-based flood works. The grass earns its keep on the quiet half of marsh coasts. On the storm half it is a residual, and the raised mud is doing the job.

The same coasts are already losing marsh. Between 2000 and 2019 the planet netted a loss of 719 square kilometres of salt marsh, 0.28 percent a year, with Russia and the United States together 64 percent of the loss, driven by hurricanes and coastal erosion. The new global runs flag long exposure, and extra plant damage, in those same Atlantic and Gulf hotspots. If the marsh retreats, the wave cut retreats with it, and the dike sees more sea.

Why Marshes Fail to Form Where Dikes Need Them?

Three years before the flume paper, Beatriz Marin-Diaz and colleagues, including Tjeerd Bouma, who also signed the 2026 study, walked the Dutch Wadden dikes with wave poles. Marshes, where they already sat, cut run-up. They sat on higher tidal flats, above about 0.5 metres NAP. The dikes that took the worst run-up were the ones with no marsh in front, because the foreshore was too low for grass to take.

The areas where marshes are not developing are in turn the ones that would need more protection. Our conclusion is therefore that in some locations, we will still depend on hard engineering solutions because these nature-based defences are not effective enough.

Beatriz Marin-Diaz, University of Groningen, 2023 field study

The 2026 tank does not dissolve that bind. It measures how much of a wave the grass can still steal after it has been beaten, and it finds that the theft shrinks as the sea gets higher. Building a marsh on a coast that will not grow one, then asking it to tame 2-metre waves, is the expensive version of the same catch. A 2023 write-up of that Wadden work put the point in the open: marshes fail to form where dikes need them most, unless people force the foreshore up.

Fresh and brackish marshes can grow taller, less flexible stems, common reed among them, and a more mixed stand, which the authors say would raise the wave cut even after damage. Organic-rich American marsh soils can also be more open, and shallower-rooted, than the 4.7 percent organic Wadden clay in the tank, so the mud-holds finding is a conservative read on European mineral marshes and a weaker one for some Gulf and East Coast peats.

Friesland Banks Centimetres on a 15-Kilometre Green Dike

The authors still argue for keeping the marshes that exist, and for putting restoration on low-energy coasts first. Even a few decimetres off the incoming wave, they note, offsets a projected 0.49 metres of sea-level rise by 2100 under RCP4.5 by decades, and stopping 1 kilometre of dike from being raised 0.2 metres already saves 1.4 million US dollars in 2005 purchasing-power terms through lower overtopping.

Wetterskip Fryslân, the water board that commissioned the flume runs, is not waiting for a global map. It is already spending the centimetres on the Koehool-Lauwersmeer sea dike, a corridor of about 50 kilometres along the Wadden shore that failed its last safety check.

WHAT FRIESLAND IS CHANGING ON THE DIKE

• Revetment: Parts of the stone face no longer need replacement, or can be built to a lower grade, because the marsh in front stays put.
• Green stretch: About 15 kilometres of hard stone sea dike is slated to become a grass and clay dike in the coming years.
• Clay thickness: Frank Jorna, a board member at Wetterskip Fryslân, said the outer clay can be 10 centimetres thinner than first planned, which saves several million euros over that length.
• Asphalt clock: Borsje has noted that asphalt on a sea dike lasts about fifty years, and that some of it is already past its strength.

Paul Buring of Wetterskip Fryslân tied those savings of millions of euros directly to the erosion resistance and wave cut measured in the tank.

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.

Paul Buring, Wetterskip Fryslân

That Frisian job sits on a Wadden coast, the quiet side of the 1-metre line, where the plants still show up in the budget. It is not a licence to plant stubble in front of a North Sea wall and call the 2-metre problem solved.

Frequently Asked Questions

Why Did the Team Test Winter Plants Instead of Summer Growth?

About 70 percent of the world’s salt marshes sit in the temperate zone and go through a winter. The February 2024 stems were checked against plants still in the field at Peazemerlannen and did not differ at the start. Winter sea couch is a low, brittle case for stem height, so the wave cuts in the paper are a floor, not a midsummer best case, though even tougher seasonal growth still breaks once an extreme storm has been through.

Would Other Marsh Grasses Have Cut More of the Two-Metre Waves?

The authors treat Elymus athericus as a conservative stand because its breaking force is weak next to several other salt-marsh species. Cordgrass is more flexible and tends to take less snap damage, so more stem stays in the water. Toward fresher water, common reed grows longer, stiffer stalks, and species mix also rises as salt stress falls, both of which would lift the plant share even after a beating.

Could the Marsh Edge Have Collapsed in a Real Storm?

A brushwood dam, copied from Wadden Sea practice, held the 0.6-metre seaward cliff in the flume, and the bare flat in front was concrete, so incoming waves did not chew a new scarp. The 7 millimetre surface drop is a platform result. In the field, cliff retreat and lateral loss remain a separate failure mode, and the authors say the wave-cut claim holds only if that front does not walk back.

Does Shredded Grass Mean the Carbon in the Mud Is Gone Too?

Root mass in the test was 78 percent in the top 5 centimetres and stayed in the clay. Mean organic matter in that Wadden soil was 4.7 percent, in the lower band for European marshes and well below the 0 to 55 percent range reported for many North American stands. Services that need a stable platform, carbon storage among them, were not stripped by the 7 millimetre skim; organic-rich or brackish peats with shallower roots could still fail in ways this mineral marsh did not.

Friesland is already taking 10 centimetres of clay off a green dike because the mud in front did not walk away. That is the useful reading of a 29-hour beating: keep the marshes that sit on quiet water, and do not ask shredded grass to replace stone where the sea comes in above 1 metre.