Dutch researchers found that a full-scale salt marsh lost little sediment under simulated extreme-storm waves, suggesting marshes could help protect coasts even though damaged plants reduce the largest waves less effectively.
In a wave tank, the team assembled about 306 square meters of living marsh brought from the Frisian Wadden Sea coast. They exposed the marsh to waves with significant heights of up to two meters. The test lasted about 29 hours.
After 21 hours of wave action, the marsh surface had fallen by an average of just seven millimeters, showing very little erosion in the experiment. The vegetation suffered more damage. After prolonged wave exposure, the amount of plant material above ground fell to roughly one-fifth to one-quarter of its starting level.
Plants reduced wave strength by an additional 6% to 22% in tests with 75-centimeter waves. With two-meter waves, their extra effect was at most 6%. The raised marsh platform also slows waves.
The researchers’ global analysis suggested that vegetation can reduce waves by up to 50% on coasts with lower wave energy. On higher-energy shores, the analysis suggested vegetation can reduce waves by less than 10%. The findings support conserving marshes as one part of coastal defenses.
Marshes should not replace engineered barriers, especially where the largest waves are expected.