An experimental study on a tethered floating metamaterial breakwater to attenuate surface gravity waves in a shallow water environment

In a framework of changing climate, it is important to find alternative solutions for coastal protection. Indeed, the pressure of anthropogenic origin, combined with natural forcings, has contributed both to a worsening of the environmental quality of coastal areas, and to the triggering of erosion dynamics, with retreat of the sandy coast.

Tethered floating breakwaters, built as a regular lattice of reversed pendula, can provide an alternative solution for beach management. With respect to rubble mound breakwaters, this type of structures has small impact on water circulation, and, in the presence of rising sea levels, it is much more adaptable than an artificial reef. Depending on their efficiency, the cost-effectiveness of beach protection using tethered floats is favorable compared to the current hard-engineering strategies.

For this purpose, inspired by the concept of metamaterial wave control, a floating device has been tested in a wave flume with a two-dimensional periodic configuration. Metamaterials are engineered structures designed to interact with waves and manipulate their propagation properties. The device is built as an array of tethered submerged cylinders, with their axes parallel to the wave crests. The idea is to investigate the conditions which lead to the formation of bandgaps, which is the key factor for mitigating the incoming energy of the surface gravity 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. If the cylinders are fully immersed, dissipation induces a broad bandgap around a resonance frequency that depends on the characteristics of the single pendulum. Instead, wave scattering (reflection) is relevant around frequencies that can be predicted using the generalized Bragg condition and the second harmonic generation, where the most significant bandgaps in attenuation can be observed. The second harmonic generation effect is a typical non-linear phenomenon, linked to the geometrical configuration of the device. In our tests, this behavior can be observed even for very small amplitude incident waves.

In conclusion, these results show the possibility to tune the position of the attenuation bands in the wave spectrum by modifying system parameters, to make possible the implementation of an efficient wave absorber for coastal protection.