Attenuating coastal surface gravity waves in a changing climate

In a framework of a changing climate, finding alternative solutions for coastal protection is crucial. The combined pressure of human activities and natural forces has led to both the degradation of coastal environmental quality and the triggering of erosion dynamics, resulting in the retreat of sandy shores. Wind waves are a key factor in the coastal dynamics, significantly affecting sea levels, particularly during extreme events. Ocean winds are changing as the Earth is warming, and hence the waves. In this scenario, this thesis has two major purposes. The first objective, inspired by the concept of metamaterial wave control, involves testing a floating device in a wave flume, whose aim is the attenuation of surface gravity waves. Metamaterials are engineered structures designed to interact with waves and manipulate their propagation properties. The device is built as an array of submerged and inverted cylindrical pendula, with a two-dimensional periodic configuration. The idea is to investigate the conditions which lead to the formation of band gaps, i.e. ranges of frequencies where the energy of the incoming waves is inhibited. Two experimental campaing have been carried out, exploring various configurations of the device and calculating the transmitted, reflected and dissipated energy of the waves. Experimental results demonstrated the feasibility of the concept and that wave attenuation can be significant, even using a limited number of cylinders. The analysis of the results have allowed us to assess that two leading mechanisms, dissipation and reflection, contribute to wave attenuation. The first campaign used monochromatic waves excited over a wide range of frequencies. When the incoming wave frequencies are sufficiently close to the natural frequency of the pendula, we observe considerable wave attenuation. Moreover, the device is also capable of reflecting the energy of selected frequencies of the incoming waves. These frequencies, predicted by a generalized Bragg scattering mechanism, depend on the spacing between the pendula. We also conducted tests in a long-crested irregular wave environment, using a selected configuration to achieve the best attenuation feature of the incoming JONSWAP spectrum. In these second campaing, we found out that the periodic geometry of the device could be the main contributor to the non-linear response. In conclusion, these results show the possibility to tune the position of the attenuation bands in the wave spectrum by modifying system parameters, making it possible to implement an efficient wave absorber for coastal protection. vi The second purpose of this thesis is to investigates the influence of highresolution CMIP6 10-meter surface wind fields on wave climate dynamics. In collaboration with the University of Melbourne, we nested a regional unstructured grid spectral wave climate model for the Southeast of Australia within a global state-of-the-art WAVEWATCH III spectral wave climate model to conduct our modelling experiments. The primary objective is to compare four distinct dynamical downscaling approaches of a similar General Circulation Model product, spanning 30 years (1985-2014) of historical simulations: CMIP, AMIP, HighResMIP, and a CORDEX downscaled ocean surface wind speed products. The Australian Climate Service recognised wind waves as a crucial element to support future coastal climate mitigation and adaptation strategies. Our findings emphasize the critical importance of climate model wind-forcing downscaling for ensemble statistics of future regional extreme wave climate projections, which go beyond the sole impact of spatial resolution. These insights are valuable for estimating both past and future projected coastal flooding and erosion patterns and hold relevance for coastal risk assessment studies.