According to a new study, salt marshes in front of the dykes can dampen the energy of waves even under changing climatic conditions and thus make a valuable contribution to coastal protection.
In the study recently published in the scientific journal "Scientific Reports", researchers from Lower Saxony and Schleswig-Holstein come to the conclusion that higher water temperatures and a higher CO2 content in the water as a result of climate change have no negative effects on the wave-dampening function of two important salt marsh plant species have.
Buffer zone between sea and mainland
Salt marshes on the North Sea coast serve as a buffer zone between the sea and the mainland, in which the impact of the waves is weakened during high tide. Because the plants that grow there form a rough structure, explained Maike Paul from the Ludwig Franzius Institute for Hydraulic Engineering and Estuary and Coastal Engineering at the University of Hanover. "By standing in the way of the water, the plants ensure that wave energy is mitigated." In addition, a difference in height from the offshore tidal flats to the salt marshes means that waves break and less wave energy arrives at the dike.
"With our study, we were able to show that the coastal protection function of the salt marshes will not deteriorate, even if climate change happens as we all fear," said Paul. "Even then we can still rely on the protection of the salt marshes." Salt marshes are characterized by being regularly flooded by salty sea water. According to the national park administration, there are more than 13,000 hectares of salt marshes in front of the Schleswig-Holstein dikes and on the Halligen. In Lower Saxony there are about 8400 hectares in front of the coastal dikes and on the southern sides of the islands. Altogether, this corresponds to an area of almost 30,000 football pitches.
Three degrees higher water temperature
In the study, the researchers from Hanover, together with scientists from the Wadden Sea station of the Bremerhaven Alfred Wegener Institute in List on Sylt, investigated to what extent the biomechanical properties of two salt marsh plant species - the salt marsh grass (Spartina anglica) and the couch grass (Elymus athericus) - - change under future climatic conditions. For the experiment, they based the assumptions of a scenario by the Intergovernmental Panel on Climate Change (IPCC) and exposed the plants in a laboratory to a water temperature three degrees higher and a CO2 content of 800 ppm (parts of CO2 per million parts) for 13 weeks.
"We then examined how stiff these plants are - in other words, how much force it takes to snap them off," Paul said. Stiffness is an important parameter for estimating how well the plants can dampen waves. The result: the higher temperature and the higher CO2 content did not affect the elasticity of the stalks. In the case of the salt marsh grass, an increase in the stalk diameter and the bending stiffness was even observed. The stiffer and more rigid a plant is, the higher its wave-dampening effect, Paul said. "But if the plant buckles during the first wave, it can only produce less wave damping for the following waves."
From the researchers' point of view, the findings can be of great importance for coastal protection in future adaptation strategies to climate change. Because the data is valuable for making predictions about the protective effect of salt marshes, Paul said. Existing salt marshes should therefore be preserved and included in coastal protection plans according to the "building with nature" approach - i.e. taking natural ecosystem services into account. "We should also consider whether we could not enlarge these areas in order to use even more of their protective effect," said the scientist.
However, the question remains to what extent salt marshes can grow with sea level rise, similar to tidal flats. Studies have already shown that salt marshes are basically capable of doing this, for example by storm surges washing up sediments on the salt marshes, said Paul. "This is how the salt marshes slowly grow upwards. However, it assumes that there is sediment that can be washed onto the salt marshes." The sediment availability varies locally. The extent to which salt marshes could also grow along the German coasts is currently being investigated.
National Park Administration on salt marshes in the Wadden Sea