Wave impacts on vertical breakwaters

Abstract

The need for coastal structures, such as breakwaters, at great water depths is rapidly increasing as a result ofthe increasing draught of large vessels and off-shore land reclamations which can, for instance, be used for the benefit of the expansion of harbours and related industrial activities. In water depths greater than approximately 10 m vertical breakwaters may be the best alternative compared to ordinary rubble mound breakwaters, in terms of performance, total costs, environmental aspects, construction time and maintenance. However, these breakwaters with plane vertical front walls can be exposed to enormous hydraulic loads, such as wave impacts. Wave impacts are dynamical hydraulic loads with, for instance, a very short duration (in the order of magnitude of ms) and a very high peak force which can exceed the quasi-static wave load on a vertical breakwater more than 10 times. As it is described in chapter 1 "Introduction", this report contains the reflection of a research which has been performed on the effect of wave impact loads on the stability of vertical breakwaters. This Master's thesis is divided into three parts: A - Wave impact loads on vertical breakwaters In chapter 2 "Hydraulic loads on vertical breakwaters" different types of quasi-static and wave impact loads are described. Three types of wave impact loads can be distinguished depending on the amount of trapped air between the breaking wave and the plane vertical front wall of a breakwater. In chapter 3 "Wave impact pressures", chapter 4 "Wave impactforces and momentum" and chapter 5 "Special attention to wave impacts with a trapped air pocket" different formulae are presented which can be used to calculate the characteristics of wave impact loads on vertical breakwaters. In chapter 6 "Vertical breakwater design formula and wave impacts" short attention is being paid to the calculation of wave (impact) loads according to the most widely used prediction method for wave (impact) pressures on vertical breakwaters. B - Derivation of models which describe the dynamical behaviour of a vertical breakwater In chapter 7 "Derivation of an analytical mass-spring model of a vertical breakwater", in chapter 8 "Derivation ofa mass-spring-dashpot TILLY model ofthe vertical breakwater" and in chapter 9 "Analysis ofthe structure andfoundation parameters ofthe vertical breakwater" models which can be used to describe the dynamical behaviour and stability of a vertical breakwater which is exposed to wave impacts are treated. A lot of attention is being paid to the influence of the magnitude of the different dynamical properties (mass, stiffuess and damping) of such a model. C - Analysis of different types of wave impact loads on a vertical breakwater and conclusions The "Analysis ofdifferent types ofwave impact loads on a vertical breakwater" is reflected in chapter 10. "Conclusions and recommendations" can be found in chapter 11. It can be concluded that the stability of a vertical breakwater against wave impacts entirely depends on the type ofwave impact load which is to be expected (i.e. rise time, total duration, magnitude of the peak force, the amount of trapped air) and the dynamical properties of the vertical breakwater and its foundation soil (mass, stiffuess and damping). The maximum peak force of a wave impact does not necessarily induce the maximum dynamical response of a vertical breakwater. Wave impacts with relatively low peak forces and long total durations (relative to the eigenperiod(s) of a vertical breakwater, double peaked wave impact forces and wave impacts followed by low frequency force oscillations due to large trapped air pockets seem to be more dangerous for the stability of a vertical breakwater. The amount of momentum is one of the governing properties of a wave impact load concerning the response and stability of a vertical breakwater. The suggestion commonly found in the literature that wave impacts are totally not significant and should not be used for the design of vertical breakwaters could not be confirmed. A dynamical analysis of the behaviour of a vertical breakwater by means of a mass-(elastoplastic)- spring-dashpot computer model should become a necessary part ofthe design process of vertical breakwaters which are exposed to breaking wave loads. The characteristics ofthe wave impact loads to be expected and the foundation characteristics should be obtained form large scale hydraulic model tests and site investigations.