Wave energy dissipation due to posidonia oceanica in the Mediterranean Sea

Abstract

Posidonia oceanica is a flexible subaquatic seagrass species that forms extensive meadows across the Mediterranean Sea. These are considered one of the most valuable ecosystems on Earth. This MSc. thesis examines the role of P. oceanica in protecting the coast from storm impacts. Prior studies are based on flume experiments that focused, almost exclusively, on examining the effect of P. oceanica on wave hydrodynamics in shallow waters and under mild conditions. Here, field data recorded during high wave activity periods in intermediate water over P. oceanica meadows was investigated. An assessment of the effect of P. oceanica on wave dissipation was conducted based on spectral analysis. The data analysis confirmed the effectiveness of P. oceanica meadows in reducing wave energy. Additionally, the spectral evaluation provided further evidence of the spatial distribution of the vegetation-induced wave energy dissipation, and showed that the dissipation rates were greater under the more energetic conditions. The skill of the process-based model XBeach at predicting dissipation due to flexible vegetation meadows was evaluated. Existing drag formulas from the literature and used by XBeach either underestimated or overestimated wave dissipation within the model. After calibration, quantitative comparisons between measurements and model results showed that the storm impact model XBeach was successful at reproducing the effect of P. oceanica meadows under high energetic events. Additionally, the implementation of a time-variable drag coefficient dependent on the Keulegan-Carpenter number was evaluated. This module did not significantly improve the accuracy of the results for the simulated scenarios. However, the drag effect of natural aquatic vegetation meadows is still poorly understood and field data is scarce. Therefore, it is recommended that future investigations continue experimenting with the variable drag coefficient module of XBeach; by testing different setups the use of the process-based model XBeach may give a better insight into situations where the drag coefficient might be important to consider. In light of climate change and other anthropogenic influences, it is important to consider the potential implications of the functional extinction of P. oceanica. To estimate what this might entail in terms of wave energy impact on the coast, a non-vegetated scenario was simulated with XBeach. For the setup simulated, the results indicate that the wave energy would start increasing from further offshore and, consequently, the wave energy reaching the coast is predicted to be up to 40% times higher. The latter emphasizes the role of P. oceanica as an implicit coastal defence of the Mediterranean coasts.