A CFD STUDY ON THE STRUCTURAL RESPONSE OF A SLOPING TOP CAISSON SUBJECT TO WAVE OVERTOPPING

Vertical breakwaters are frequently employed for the protection of harbors in many areas around the world. Their success is due to the fact that they represent, especially in relatively deep water, a better alternative in terms of performances, construction speed, and maintenance costs compared to traditional rubble mound breakwaters. However, they have suffered in the past from severe damage caused by storms, which have led, in some cases, to catastrophic failures. In such seas, where the large wave heights generate tremendous wave forces acting on the breakwaters, a monolithic structure expressly designed to face rough seas is the so-called sloping top caisson. A sloping top caisson has a superstructure that is sloped to reduce the wave forces, i.e., the downward forces on the slope cancel the uplift pressure, thereby reducing the wave pressure on the upright wall. Although this approach may seem rational, Walkden el al., (2001) warned that, while decreasing the landward thrust, a large amount of overtopping may cause strong seaward directed impulsive loadings. During an extremely small set of physical experiments (3 tests only) conducted on a small scale model of a Hanstholm type breakwater, the authors measured rear pressure peaks so intense to lead to a seaward sliding force 40 percent larger than the landward one. In this study, the role of air contribution was significantly highlighted. After that pioneering study, however, the nature of the overtopping generated impacts has surprisingly not deepened further. Despite sloping top breakwater has long been used in the engineering practice (the first was constructed in Naples, Italy, in 1906), detailed reports on their structural response are very few. In 1994, Takahashi et al. proposed a prediction method to calculate wave force under the crest phase, which modifies the well-known Goda formula for vertical walls. The method is based on few regular wave experiments and, at the author knowledge, has been not further verified. The present thesis discusses main results of a numerical investigation conducted with the aim to have a deeper insight on the structural response of the sloping top caissons subjected to wave overtopping. A numerical suite, namely Flow 3D (developed by flow science Inc.), have been employed. Using 11 random sea states driven by extremely narrow banded spectra, the general characteristics of the hydrodynamic loadings are studied. Following chapters form the basis of this thesis: Chapter 1 provides a general overview of the vertical and composite breakwaters with their historical background. Chapter 2 gives a literature review of vertical breakwater failures and their respective origin. Then the previous theoretically and experimentally studies regarding wave force including pulsating and impulsive load are covered. A summary of pressure impulse theory, developed by Cooker and peregrine, is also presented in Chapter 2. Chapter 3 discusses sloping top breakwaters and current design method proposed by Takahashi et.al (1994). The problem of impulsive load particularly in seaward direction is also treated. Chapter 4 presents numerical setup of CFD based simulation of wave-structure interaction. In this phase, much attention has been drawn to take the effect of the air into account. Primary results including evaluation of hydraulic performance and comparison with previous experiment methods are also reported Chapter 5 gives main results of a CFD study on the structural response of a sloping top breakwater subject to wave overtopping. The analysis showed that the transmitted wave field act to increase both the landward and the seaward forces and that the conventional design methods may be not adequate to guarantee an appropriate degree of safety to the structure. Three impulsive wave loading mechanisms on the back of the structure due to wave overtopping were identified. The study also confirmed the previous finding by Walkden et al. (2001), which noticed the existence of strong impulsive loadings on the inner face of the wall, due to violent overtopping events. However, a large underestimation was observed to estimate pressure impulse using the proposed method.