Abstract: This paper presents a physical model study of wave induced forces on a composite vertical breakwater, where the crown wave wall is retreated with respect to the front face of the caisson. Four different configurations (one flushed wall and three retreated wall configurations) have been tested under regular wave conditions, aiming at providing a first experimental insight on the increase/reduction of the wave loads acting on the structure. The analysis of the experimental results allows to describe the basic phenomena involved and to identify the physical/geometrical drivers, which are expected to play a role on the force increase/reduction factor. Thus, detailed processing of both forces/moments (synchronous analysis) and pressures (asynchronous analysis) on the whole structure, the wall and the caisson trunk, together with the analysis of reflection coefficients as a function of the wall position, are presented and discussed in the paper. The experimental evidences suggest that, at least for the four configurations tested, the global forces acting on the caisson vary significantly depending on the wall position, resulting in a reduction between 5% and 31% for high energy sea states. A similar behavior is found considering the global moments. Furthermore, the synchronous analysis of the forces highlighted that the physical/geometrical drivers, identified in the present study, can have both a concordant and antithetical action among them, then resulting in increasing or decreasing, respectively, forces acting on the structure, if compared with the flushed wall configuration.

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