Response of vegetated dune–beach systems to storm conditions

• Vegetation can help reduce beach and coastal dune erosion as well as shoreline retreat.
• Dune vegetation increases and maintains the resilience of coastal zones.
• Vegetation is an alternative ecosystem based solution.

Human encroachment on the coasts is extensive and expected to increase over the coming decades. This proximity to the sea is coupled with potentially more frequent strong tropical cyclones and eustatic sea level rise, and thus human life, property and infrastructure are threatened. In this scenario, it is vital to find the means to maintain or increase the resilience and resistance of coastal zones. As an alternative to engineering solutions, ecosystem-based coastal defence strategies have been recommended as better and more sustainable solutions, although it has not been clearly demonstrated that they really work. In 24 wave flume experiments, we studied the effects of four densities of vegetation cover (none, low, medium, high) on the movement of sediment along two beach–dune profiles (with and without a berm), under three storm conditions (mild, moderate and intense). Erosion regimes of collision and overwash were observed in the dune profiles with a berm, whereas swash and overwash regimes were observed when no berm was present. In the profiles with a collision regime, vegetation decreased the amount of erosion. For both profiles, in the tests with the largest wave conditions, vegetation prevented overwash and thereby erosion of the landward side of the dune. Erosion of the dune face was reduced by vegetation, particularly with a strong storm, when Iribarren numbers were greater. In summary, vegetation reduced net erosion on the dune face, regardless of the wave conditions, the morphology of the beach–dune profile, or the mode of erosion.