Flexible, wave resistant saltmarsh vegetation survives extreme storms

Loss of coastal vegetated wetland area is a major threat in times of climate change, particularly where their contribution to coastal protection is increasingly valued. Vegetation survival under extreme conditions is largely unknown, thus vegetation is perceived to be more vulnerable to high impact storms than easily testable ‘hard’ materials, such as asphalt or concrete used in dikes and seawalls. Survival of vegetation under extreme conditions is highly challenging to test, given the unpredictable occurrence of extreme storms in the field and the logistical challenges of simulating large waves in experimental facilities. Here, we made use of the largest wave flume in the world, the Deltaflume (Deltares, the Netherlands), to test the persistence of three species of saltmarsh vegetation (Spartina anglica, Scirpus maritimus and Phragmites australis) under extreme storm conditions: a storm with a return period of >1000 years with up to 3 m high, 8 second period, waves. Persistence of vegetation was unexpectedly high. While vegetation loss increased at higher near-bed velocities, breakage was consistently lower at any given velocity for both plants and mimics with higher flexibility and/or length. The survival of longer stems likely occurred via swaying, which allowed them to effectively avoid wave forcing. Although swaying of stems will reduce their wave attenuating effect compared to (theoretically existing non-breaking) stiff stems, their persistence leads to an overall more storm-resistant marsh canopy remaining after the storm, able to withstand and attenuate repeated storm events.