Numerical Investigations On The Influences Of Two-Dimensional Bathymetry On The Occurrence Probability Of Freak Waves

Freak waves are extremely large ocean waves with disasters and higher than surrounding waves.Such waves have large height and destructive force,which can bring serious damages to marine operations,ships and ocean platform structures.Due to complexity of freak waves,low occurrence probability and difficulty of observing,although scholars have done extensive works on them,them are still not clear about the occurrence probability,mechanism and influence factors of freak wave.Therefore,a model of occurrence probability of freak waves will be conducted from the influence of bathymetry on wave propagation in this thesis and the mechanism will be discussed.A numerical model for wave propagation over variable bathymetry was developed based on High Order Spectral(HOS)method,considering the wave-maker boundary and variable bathymetry.The typical cases,such as wave Bragg reflection,wave propagation over submerged bar and tsunami caused by submarine bathymetry,were conducted with the numerical model.The results showed that the numerical model was valid for the wave generation,propagation and simulating wave over variable bathymetry.Tsunami caused by the submarine bathymetry was investigated based on the model and the influence of velocity and width of bathymetry on the maximum amplitude of wave propagation was analyzed.The results showed that the width of submarine bathymetry was certain,the maximum amplitude of stable wave surface decreased with the increase of characteristic time after a period of time.When the characteristic time of submarine bathymetry movement was certain,the maximum amplitude of stable wave surface increased with the increase of width of submarine bathymetry.But the bathymetry width increased to a certain value,the maximum amplitude of stable wave surface had remained constant.Finally,the influence of two-dimensional Bragg bathymetry on occurrence probability of freak waves was simulated by High-order spectral(HOS)model.Firstly,the variety of reflection coefficient of regular waves in front of Bragg topography was checked and compared with the reflection coefficient of irregular waves.Then the variety of square wave height and frequency spectrum were analyzed under bathymetry conditions and the effect of steepness,wave number and length of Bragg bathymetry on square wave height and frequency spectrum were studied.Lastly,the occurrence probability of freak waves in irregular wave was discussed under the effect of different steepness,wave number and length of Bragg bathymetry.The results showed that the bathymetry parameters had a great influence on the wave reflection,the bathymetry steepness was greater,the relationship of wave number k and bathymetry number kb was closer to 2k/kb=1,the bathymetry length was greater,the reflection of bathymetry on waves was more obvious.The square wave height increased in front of the bathymetry and the one decreased behind the bathymetry.Because the energy distribution of irregular waves was wide,the Bragg bathymetry only reflected the reflection of specific waves obviously.Therefore,the influence of bathymetry on irregular wave trains was less obvious than that of regular waves.Although wave reflection increased wave height in the front domain of bathymetry,the occurrence probability of freak waves(H/Hs>2)did not increase significantly from the cumulative probability distribution of wave height.