Coastal Engineering Journal

The Hilbert-Huang Transform (HHT) method for nonlinear and non-stationary time series analysis is applied to wave field data from the nearshore area. The frequency-time distribution of the energy, designated as a Hilbert spectrum is utilized for the examination of the sea waves and their group structure. The key feature of the HHT method is Empirical Mode Decomposition (EMD), which provides a unique basis for expansion of the data, derived from and based on the data. The necessary condition for the existence of wave grouping is determined based on the results of Empirical Mode Decomposition of the data. An attempt is made to investigate the transformation of the sea waves by examination of the decomposition components along the beach profile. The cross-shore variations of the group characteristics are studied. The Hilbert-Huang Transform method provides new insights on the wave and group cross-shore transformation.

The purpose of this study is to describe the characteristics of occurrence of coastal freak waves and to investigate their statistical and spectral structures. According to Ochi's definition of freak wave, 175 coastal freak waves were obtained from a long-term full scale wave data bank, in which there are more than 4500 wave records. The probability distribution of occurrence of coastal freak waves was investigated. The goodness-of-fit testing shows that the Rayleigh distribution is more appropriate for describing the occurrence probability of coastal freak waves when the ratio of its wave height to the corresponding significant wave height is more than 2.4. However, the occurrence probability of coastal freak waves remains below the Rayleigh distribution. The relationship between coastal freak waves and the related sea states shows that the occurrence possibility of coastal freak waves may significantly increase in the sea of grouping waves or in the wave field of bimodal spectra. Wavelet Transform was applied to the in situ wave records to investigate the energy distribution on the time-frequency domain. It justified the previous conclusions and demonstrated that the wave groups and superposition of the swell and wind wave energy, which occupies 70% of the coastal freak samples, play major roles of inducing the coastal freak waves.

Recently, Hanzawa et al. developed a reliability design method for the calculation of the expected damage level of armor blocks of a horizontally composite breakwater. In their method, the wave transformation from deepwater to the design site was calculated by assuming unidirectional random waves being normally incident to a straight coast with parallel depth contours. In real situations, however, directional random waves with variable principal wave directions will be incident to the shore of irregular bathymetry. In the present study, the reliability design method of Hanzawa et al. was extended to take into account the variability in wave direction in the computation of wave transformation. The directional variability includes directional spreading of waves, obliquity of the design principal wave direction from the shore-normal direction, and its variation about the design value. Even though the wave incident angle to the breakwater could be calculated, normal incidence was assumed in the calculation of the damage level of armor blocks. It was found that the inclusion of directional variability in the computation of wave transformation had great influence on the computed expected damage level of armor blocks. The previous design, which disregarded wave directionality, could either overestimate or underestimate the expected damage level by a factor of two, depending on water depth and seabed slope.

An analytical model was developed to explore the effect of swirl action on the mean evolution of a near flow field of turbulent jet. The equations of motion for turbulent flow were integrated to obtain the theoretical model with the aid of boundary layer approximations and the concept of self-preservation of velocity profiles. Vortex breakdown at the core of the jet, a phenomenon associated with swirling jets, was taken into account through the axial velocity distribution. Axial velocity profiles were described in terms of Gaussian curves and azimuthal velocity profiles described in terms of sinusoidal curves. Expressions were obtained which describe the decay of axial and azimuthal maximum velocities and the growth of jet half width. The proposed model proves that the initial swirling degree significantly influences the time-averaged jet characteristics. Collected data from various sources, covering weak to moderate degrees of swirling, were used to verify the theoretical model. The simulated results for the decay of axial and swirling velocities and the jet spread agree closely with the existing experimental findings. Compared to a non-swirling jet, the growth of the jet width is considerably large, and velocity field is suppressed in a swirling jet, and also the core length becomes shorter.

WAM (acronym for WAve Model) is a third-generation wave model used to compute the spectra of random short-crested wind-generated waves on Eulerian grids and is one of the most extensively tested and widely used wave models in the world. Waves can be propagated on a spherical or a Cartesian grid. For large ocean regions, waves must be propagated on a sphere to account for the curvature of the Earth. For coastal and nearshore regions, smaller nests with higher mesh resolution are used. As the size of the nests becomes smaller, the differences between a spherical grid and a Cartesian grid for the same nest become smaller and one would assume that the differences in the computed results would also be small. However, one should not assume that this behavior is automatically recovered in numerical wave models owing to the different nature of spherical and Cartesian grids — the wave transport equation when written in spherical coordinates is not divergence-free whereas it is divergence-free when written in Cartesian coordinates! Therefore, the transition of spherical methods to divergence-free needs to be numerically verified. The purpose of this numerical study was to examine results obtained with the spherical and Cartesian wave propagation options in the WAM code for a nearshore zone where depth effects are important. This study found that Cartesian wave propagation in the WAM code produced more accurate nearshore wave predictions than spherical wave propagation!

The 2D/3D hybrid tsunami numerical model was developed in which the conventional horizontally 2D model was adopted for the calculation in the wide region located far from the coastal structures while the 3D numerical model was used in the limited region adjacent to the structures. Applicability of the domain connection technique was examined by comparing the numerical results obtained by the present hybrid model with those obtained by applying the 3D model for the whole domain. Further, the results of the laboratory experiments were compared with the numerical results obtained by the hybrid model as well as the 2D model. It is shown that the present hybrid model reduces a calculation load significantly compared to the 3D model and is capable of reproducing the characteristics of three-dimensional and complicated flows around the structure, which cannot be reproduced by the 2D model alone.