The contribution of salt marshes to wave attenuation during extreme storms.

Salt marshes protect coastlines by attenuating wind waves with their elevated marsh platform and above-ground vegetation1. Confidence in the persistence of this service under a changing climate requires knowledge on how extreme storms damage the platform and vegetation; however, a comprehensive, quantified understanding of salt marsh-wave interactions under extreme events is lacking. A unique experiment was conducted by transferring intact salt marsh to one of the world's largest wave flumes, and subsequently exposing it to increasingly higher waves that reach up to 2 m significant wave height. We show that marsh substrates can persist with negligible erosion, despite cumulative wave exposure. By contrast, the vegetation was progressively damaged during continued wave exposure, gradually reducing vegetation-related wave attenuation. Results emphasize that vegetation was most effective in attenuating waves at low-energy marsh coasts (up to 1 m), where attenuation exceeded 10% (up to 50%), whereas attenuation at high-energy marsh coasts (wave height > 1 m) does not exceed 10%. Our global assessment supports the differentiation between low- and high-energetic conditions resulting from the flume experiment. Our results suggest that the marsh platform remains stable under extreme wave events and that substantial wave attenuation by marsh vegetation is limited to low-energy marsh coasts.