Moving water exerts drag forces on vegetation. The susceptibility of vegetation to bending and breakage determines its flow resistance, and chances of survival, under hydrodynamic loading. To evaluate the role of individual vegetation parameters in this water-vegetation interaction, we conducted drag force measurements under a wide range of wave loadings in a large wave flume. Artificial vegetation elements were used to manipulate stiffness, frontal area in still water and material volume as a proxy for biomass. The aim was to compare: (i) identical volume but different still frontal area, (ii) identical stiffness but different still frontal area, and (iii) identical still frontal area but different volume. Comparison of mimic arrangements showed that stiffness and the dynamic frontal area (i.e., frontal area resulting from bending which depends on stiffness and hydrodynamic forcing) determine drag forces. Only at low orbital-flow velocities did the still frontal area dominate the force-velocity relationship and it is hypothesised that no mimic bending took place under these conditions. Mimic arrangements with identical stiffness but different overall material volume and still frontal area showed that forces do not increase linearly with increasing material volume and it is proposed that short distances between mimics cause their interaction and result in additional drag forces. A model, based on effective leaf length and characteristic plant width developed for unidirectional flow, performed well for the force time series under both regular and irregular waves. However, its uncertainty increased with increasing interaction of neighbouring mimics.

The conservation of coastal ecosystems can provide considerable coastal protection benefits, but this role has not been sufficiently accounted for in coastal planning and engineering. Substantial evidence now exists showing how, and under what conditions, ecosystems can play a valuable function in wave and storm surge attenuation, erosion reduction, and in the longer term maintenance of the coastal profile. Both through their capacity for self repair and recovery, and through the often considerable cobenefits they provide, ecosystems can offer notable advantages over traditional engineering approaches in some settings. They can also be combined in "hybrid" engineering designs. We make 10 recommendations to encourage the utilization of existing knowledge and to improve the incorporation of ecosystems into policy, planning and funding for coastal hazard risk reduction.

Coastal communities around the world face an increasing risk from flooding as a result of rising sea level, increasing storminess and land subsidence¹². Salt marshes can act as natural buffer zones, providing protection from waves during storms³⁷. However, the effectiveness of marshes in protecting the coastline during extreme events when water levels are at a maximum and waves are highest is poorly understood^8,9. Here we experimentally assess wave dissipation under storm surge conditions in a 300-metre-long wave flume tank that contains a transplanted section of natural salt marsh. We find that the presence of marsh vegetation causes considerable wave attenuation, even when water levels and waves are highest. From a comparison with experiments without vegetation, we estimate that up to 60% of observed wave reduction is attributed to vegetation. We also find that although waves progressively flatten and break vegetation stems and thereby reduce dissipation, the marsh substrate remained stable and resistant to surface erosion under all conditions. The effectiveness of storm wave dissipation and the resilience of tidal marshes even at extreme conditions suggest that salt marsh ecosystems can be a valuable component of coastal protection schemes.