Experimental investigation on non-breaking wave forces and overtopping at the recurved parapets of vertical breakwaters

Abstract

This paper aims at investigating the intensity of the load applied by non-breaking waves on the recurved parapet wall of vertical breakwaters. For this purpose, 2-D experiments were carried out on a simple vertical breakwater physical model with four different geometries of the parapet (with an “exit angle” of recurve of 0°, 45°, 60° and 90°), under both regular and irregular waves.The tests showed the presence of loads of a quasi-static nature and loads with a typical church roof impulsive behaviour, depending on the (non breaking) wave characteristics and on the shape of the parapet. This impulsive force resulted significantly larger than the quasi-static load of the pure vertical wall case. The maximum measured total force under regular waves was compared with the one obtained by a numerical model based on IHFOAM.For irregular waves with Hs equal to 80% of the freeboard, the maximum measured horizontal load on a 90° recurved wall is equivalent to the force applied by a constant pressure of order 5 ρgHs. It was found, by analysing waves near the leading edge of a wave-group, that history effects are important, i.e., in certain circumstances, a high wave preceded by lower waves generated a larger force than if it had been preceded by waves of the same height. For regular tests, the largest impulsive forces occurred basically at the beginning of the tests, during the initial transient ramp-up period.The irregular wave overtopping was also measured, showing good agreement with the existing predictive formulae and confirming the significant reduction of mean discharge with increasing exit angle and overhanging extension of the parapet.