La risposta idraulica di barriere sommerse in campo non lineare

Abstract

Submerged breakwaters are a very common structural solution employed for shore erosion control. Predicting wave climate leeward the barriers is indeed the main task to the functional design of such structures; this because it rules the distribution of wave thrusts in the protected area that, in turn, governs hydrodynamic and morphodynamic processes. In this regard it should be noticed that the interaction between waves and submerged breakwaters is a strongly non-linear process. In fact, when water waves come across the structure, multiple-frequency harmonic wave components arise, generating a transmitted wave field very different from the incident one. The super-harmonics consist of phase-locked waves to the fundamental one plus free waves that travels with their own phase speed in the shadow zone. These components interact leading all kinematic and dynamic quantities to spatially fluctuate behind the structure. The aim of this work is to investigate the main characteristics of the non-linear interaction between waves and submerged barriers for conditions of both breaking and no-breaking waves over the structure crown. For this purpose a long series of laboratory experiments have been carried out at the small scale wave flume of the Hydraulic, Geotechnical and Environmental Engineering Department (DIGA) of University of Naples “Federico II”. Models of rubble-mound submerged breakwaters with different geometries and porosity have been employed under the action of regular waves with different height and period. The free surface elevation process has been acquired at several positions leeward of structure models. Experimental results clearly show that the wave shape in the sheltered area is non-permanent leading the profile asymmetry coefficients spatially fluctuate as well as the twice frequency Fourier modes. This behavior, due to the interaction between phase-locked waves and free waves, leads the variance profile (i.e. the spectrum area) to fluctuate in the same way making the definition itself of transmitted wave height quite uncertain. As a further result it has been noticed that higher harmonics transmitted rate reaches a maximum value when waves start breaking over the structure and then, with increasing of dissipation process, to be reduced lying in a saturation curve.