Model wave impulse loads on caisson breakwaters : aeration, scale and structural response
The prediction of wave impact loads on prototype caisson breakwaters from the results of physical model tests is considered, with particular attention given to the effects of air, breaker shape, structural response and scale. A review of related literature is presented from which it is concluded that the different aspects of the problem may be related through the force impulse. Large scale soliton impacts are used to show the importance of entrapped air in determining the form of the load time history. Small scale waves with artificially high levels of entrained air and highly controlled drop impacts are used to show and quantify an inverse relationship between entrained air and impact load maxima. Specially developed aeration probes and analysis techniques are used to show the influence of entrapped air on pressure maxima and quantify entrained air levels in small scale fresh water breaking waves. A definition of the force impulse is proposed and used to investigate its variation with breaker shape. The impulse magnitude is shown to be relatively invariant for regular wave impacts compared to a large scatter in impulse form. A numerical model of caisson dynamics is used to predict structural motion and to calculate a series of dynamic amplification factors. The prediction of structural response to obtain effective static loads through the use of these factors is investigated and achieved through the adoption of an 'equivalent impulse' concept. The scatter in impulse form is found to cause large variations in effective static loads between nominally identical impact events. The equivalent impulse concept is used to solve this problem. A comparison is made between the form and magnitude of the force impulses of the small and large scale waves. The results indicate that the impulse magnitude may be relatively free of scale effects. An example is given in which the results of a small scale test are interpreted, scaled and processed to account for the effects of entrained air and structural response in order to predict large scale effective static loads. These are shown to compare well with predictions made using measured large scale force time histories and the numerical caisson model.