Salt-marsh soil withstood extreme waves in a large Dutch experiment, but damage to vegetation reduced its ability to weaken them, Nature Portfolio reported on October 22. The findings, published in Nature on October 21, help explain where these coastal ecosystems can provide the greatest protection against storms.
Pim Willemsen and colleagues installed 306 square metres of marsh and plants in the Delta Flume in Delft. According to Deltares, intact sections came from the Peazemerlannen reserve on the Frisian Wadden Sea coast. The experiment tested whether natural marsh surfaces and their vegetation could maintain their protective role under severe wave conditions.
After 21 hours of wave action, with waves reaching two metres, average surface erosion measured only seven millimetres. Deltares says the elevated marsh bed itself slows waves, while plants provide an additional effect. For waves of 75 centimetres, vegetation added 6–22% attenuation; for two-metre waves, its contribution reached at most 6%, illustrating how protection varies with wave size.
Nature Portfolio reports that, in the experiment it describes, above-ground plant biomass fell to 20–25% of its initial level after 21 hours, and stem length decreased by 47%. Researchers also combined the experimental findings with global data on storm surges, coastal elevation and marsh distribution. Their analysis suggests vegetation can reduce waves by more than 10%, and up to 50%, on lower-energy marsh coasts with waves up to one metre. Those figures are estimates across coastal settings, rather than a uniform result for every shoreline.
The researchers therefore see conservation and restoration as especially useful where wave energy is lower. On more exposed coasts, marshes could complement seawalls or dikes by reducing the waves reaching them, according to Deltares. The findings offer evidence for choosing where to combine natural and engineered protection, while showing why surviving soil alone does not mean plants retain their full protective capacity.
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