Abstract
Vegetation can contribute to coastal defence by damping incoming waves. However, prior studies have shown that attenuation varies greatly among plant species. Plant flexibility is a mechanical property that is commonly omitted, but varies considerably between shrubs and grasses on salt marshes. Therefore, we present an experimental study in a laboratory wave flume with artificial vegetation that differs in flexibility only. We measured wave attenuation and water particle velocities around rigid and flexible salt marsh vegetation. Waves were measured using a series of gauges and Particle Image Velocimetry (PIV) was used to measure spatio-temporal variations of water particle velocities in the x-z plane around the vegetation. Our results show that flexible vegetation attenuates waves up to 70% less than rigid vegetation due to swaying of flexible plants. Furthermore, we find that rigid vegetation modifies the velocity structure, whereas flexible vegetation does not. Specifically, a mean current in the direction of wave propagation develops around the canopy and the horizontal particle velocities are amplified directly above the canopy. These results indicate that plant flexibility is a key parameter in the wave-vegetation interaction that controls wave damping and velocity structure.
Highlights
Nature-based coastal defences in the form of vegetated foreshores are increasingly common in coastal protection schemes
A laboratory study under controlled conditions using artificial rigid and flexible vegetation has provided us with the opportunity to study the impact of plant flexibility on the drag coefficient and the velocity structure
We test mimics that differ in flexural rigidity only under conditions that have been directly derived from the field
Summary
Nature-based coastal defences in the form of vegetated foreshores are increasingly common in coastal protection schemes. Salt marshes are vegetated tidal wetlands that can be part of a nature-based coastal defence solution. The potential of their vegetation to damp waves has been shown in the field (Jadhav et al, 2013) and in large-scale experiments (Losada et al, 2016; Mo€ller et al, 2014). They capture and bind sediments (Fagherazzi et al, 2012), which contributes to coastal stability (Bouma et al, 2014) and provides adaptation to sea level rise (French, 1993)
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