It’s September 2023 and we’re in a nature reserve on the Frisian coast. Several excavators are used to drive two-meter by two-meter steel molds into the ground. This is distance saving, which Pim Willemsen, a researcher at Wageningen and Deltares, calls a “giant cookie cutter.” The only problem is that each cookie weighs three to six tons.
This is the story of how Dutch researchers transported 66 of these “cookies” in about 20 trucks and took them to Delft to be placed in a wave channel. The reason? demonstrate that this Phrygian marsh was one of our best defenses against storms.
Is all this really necessary? Sometimes in science, it’s not enough to just sit back and watch. Sometimes the only way we can find out what’s going on is by testing. This is an illustrative case: we already knew that wetlands slow down waves; We had no idea about actual storm resistance. Similar to the storm generated by the type of canal used at Delft University.
However, this is not the first time. In 2013, researchers brought a similar wetland block to the Grand Canal in Hanover. There they discovered that about 40 meters of wetlands reduced wave height by 18%. But these waves were much smaller than what the Netherlands needed (90 cm compared to two meters for extreme storms for which the Dutch dikes are designed).
So they decided to investigate it themselves.
And in rare greenhouses in the country. The Delta Flume is one of the largest wave channels in the world: a “bug” 300 meters long, five wide and nine and a half and deep. What they did was combine the blocks inside (under grow lights, with irrigation and strict salinity controls) until creating two 71-meter strips with a dike behind. Then they start the machine.
Increasingly large waves, ranging from 0.75 to two meters, rush against the strip of land. They always look for the worst combination of the worst possible damage factors.
And what did they discover? The roots in the first five centimeters of soil formed a strong network that protected the soil. The vegetation thing is more complicated, but it also works against waves (although less because they are bigger).
And then? The team cross-referenced their results with global marsh maps and storm surge simulations for 100- and 1,000-year return periods (that is, events that could occur on average once every 100 or every 1,000 years). Their conclusion was that on coasts with waves of up to one meter, vegetation reduced waves by between 10 and 50% (with large waves, less than 10%).
This has a direct consequence. To strengthen the Koehool-Lauwersmeer dam, the Frisian Water Authority is already using this data to avoid replacing parts of the lining and save “millions of euros”.
Why are we interested? Because we know a lot about Spain’s wetlands and we know even more about eliminating them: according to the wetland assessment of the Junta de Andalucia, 138,300 of the 138,300 hectares of the original Guadalquivir wetlands have been lost, 83%. It is not only a major environmental problem, but (as we see) it poses a risk to everything around it. And that, on the Spanish coast, many many things.
Picture | Rijk van de Ka
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