Characterisation of wave attenuation by eelgrass meadows via laboratory experiments

Coastal areas are among the most endangered zones worldwide due to ever increasing physical and environmental pressures exacerbated by the effects of climate change. Eelgrass ecosystems represent a promising nature-based solution because they promote biodiversity and carbon capture. Recent works have concluded that eelgrass can also contribute to reduce physical pressures on coastal areas by damping incoming waves. However, the large number of governing physical parameters and the limited amount of data available make it hard to quantify the wave attenuation due to eelgrass in natural scenarios.In the present work extensive laboratory experiments were conducted to characterise the wave attenuation properties of eelgrass in a range of natural scenarios. Experiments were conducted in a 50m long wave tank wherein a 4m long and 0.1m high meadow of eelgrass replicas was located. Eelgrass replicas modelled a range of eelgrass species (e.g. Cymodocea nodosa, Zostera marina) to a scale between 1:1 and 1:8 depending on the conditions considered. Replicas were designed using both Cauchy and Froude similarities and considering the morphology and flexural rigidity of eelgrass. A total of 330 experiments were performed varying the most important governing parameters, namely: water depth (from 0.15m to 0.6m), plant densities (up to 1338 plant/m2), wave height (up to 0.16m depending on the water depth) and length (between 1m and 4m). Thus, the wave Cauchy number in the tests ranged from unity up to 7000. During experiments the water surface level along the tank and the meadow was measured by means of 8 resistance gauges that recorded at 128 Hz with a relative error up to 2%. From water surface level data, the mean wave height at each location was calculated and the wave attenuation coefficient of eelgrass was estimated based on the variation of mean wave height along the patch.The resulting wave attenuation coefficients agree well with the model proposed by Lei & Nepf (2019) for submergence ratios larger than 0.2, even though for low attenuations the relative uncertainty is high. The wave attenuation caused by the eelgrass meadow is significantly larger than that due to the friction at the bed and lateral walls for submergence ratios over 0.2 and meadows denser than 251 plants/m2. These values may represent important thresholds for eelgrass contribution to wave attenuation in coastal areas.