Coastal freak waves are dangerous to people, ships and buildings in the coastal zone. Their mechanism thus is a very important issue. In this study, the data of around eight million waves collected by Harbor and Marine Technology Center in Taichung are analyzed. The statistical analysis shows that the occurrence probability of freak wave is lower than Rayleigh¡¦s distribution. The corresponding sea-state parameters show freak waves tend to occur more frequently when the nonlinearity are higher, the wave groupiness are higher, or the FFT spectrum from wave records are more narrow-banded. HHT analysis shows wave superposition and wave nonlinearity are both possible mechanisms. Freak waves propagating from a wave source are also simulated based on the linear dispersion relation. In half of the simulations, the maximum wave height decreases to only 70% of the original value.

This thesis studies Lamb waves for long-range pipeline inspection. The property of Lamb waves propagation and the sensitivity of defect detection will be evaluated in this thesis. There are two groups of waves propagation in plate, first group of waves are called symmetrical Lamb waves, and the second group are called anti-symmetrical Lamb waves. Furthermore, there are three different mode types exist in hollow cylinder, such as the longitudinal, torsional, and flexural modes. By solving the characteristic equation of the Lamb wave problem, the dispersion character of the Lamb waves can be found. The phase and group velocity for different Lamb modes obtained from the dispersion curve are also studied in this thesis. In the experimental setup, the longitudinal wave is incident at certain incident angles and velocities to generate Lamb waves propagated on the stainless steel plate and carbon steel hollow cylinder. When the longitudinal waves are in the constructive interference in the medium, only one Lamb wave will propagate. As a result, compare with the measured data and theoretical predictions of the group velocities, it is observed that angle beam longitudinal waves offer an accurate and workable method for Lamb waves generation. To evaluate the sensitivity and resolution of the Lamb waves testing, additional experiments for detecting defects are carried out in this thesis. It is found that a single and pure Lamb wave is very useful for detection defects. Moreover, selected the non-dispersive Lamb modes for detection can keep the propagating wave shape without changing; also, only minimum energy is decayed as wave propagated.

A comprehensive experimental study was conducted to produce benchmark wave kinematics data for five different regular waves and the maxima of four different irregular wave trains. Two of the irregular waves generated are in the category of rogue waves. A series of experiments were conducted in a 2-D wave tank at Texas A&M University to measure wave velocities and accelerations using LDV and PIV systems. The wave crests of regular and rogue waves are the focus of this study. With the measured wave velocity field, the wave accelerations were computed using a centered finite difference scheme. Both local and convective components of the total accelerations are obtained from experimental data. Also, the nonlinear wave forces on a truncated slender cylinder are computed by applying the obtained wave kinematics to the Morison equation. The force results based on measured wave kinematics are compared with those based on the kinematics of linear extrapolation, Wheeler stretching, and modified stretching. The Wheeler stretching method generally underestimates the actual wave kinematics. The linear extrapolation method is very sensitive to the cutoff frequency of the wave spectrum. The modified stretching method tends to predict the maximum value of wave kinematics above the still water level (SWL) well except for the convective acceleration. The magnitude of convective acceleration in the regular waves was negligibly small, whereas the magnitudes of horizontal and vertical convective accelerations in the rogue wave were increased rapidly above the SWL.

In this thesis, passive and active 3-branch Ti:LiNbO$ sb3$ waveguide devices were investigated. A multimode field-matching technique was employed to predict the power division in a passive 3-branch waveguide power divider. The limitation and range of validity of this method were pointed out. An alternative method was also employed to predict the power division when coupling between two neighbouring branches is not negligible, as is the case when the branching angle becomes small. The multimode field-matching method was also employed to predict the power division in an active 3-branch waveguide switch, where switching is achieved through the electrooptic effect of LiNbO$ sb3$. The edge effect of the electrodes on the beam steering was also considered. A linear mode-confinement modulator was then proposed and investigated as a derivative of the 3-branch waveguide switch. / Fabrication and experimental characterization of the three devices were also carried out. Key functional parameters were measured and compared with theoretical predictions. In general experimental results have borne out theoretical prediction. / Some calibration of the fabrication techniques employed, were also performed. An outline of the various experimental techniques used is also given. / Finally, some possible improvements are suggested for further work.

Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 140-146). Available also in a digital version from Dissertation Abstracts.

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From this review, a FORTRAN code was developed that generates time and position dependent distributed loads based on linear wave theory for shallow water conditions. The routine was integrated into to the Abaqus finite element analysis framework, and used to evaluate the structural response of a representative bridge section impacted by tidal surge.

Rogue waves are unexpected, individual ocean surface waves that are disproportionately large compared to the background sea state. They present considerable risk to mariners and offshore structures when encountered in large seas. Rogue waves have gone from seafarer’s folktales to an actively researched and debated phenomenon. In this work an easily derived spectral parameter, as an indicator of rogue wave risk, is presented, and further evidence for the generation mechanism responsible for these abnormal waves is provided. With the additional goal of providing a practical rogue wave forecast, the ability of a standard wave model to predict the rogue wave probability is assessed. Current forecasts, like those at the European Centre for Medium-Range Weather Forecasts (ECMWF), rely on the Benjamin Feir Index (BFI) as a rogue wave predictor, which reflects the nonlinear process of modulation instability as the generation mechanism for rogue waves. However, this analysis finds BFI has little predictive power in the real ocean. From the analysis of long term sea surface elevation records in nearshore areas and hourly bulk statistics from open ocean and coastal buoys in the Northeast Pacific, crest-trough correlation shows the highest correlation with rogue wave probability. These results provide evidence in support of a probabilistic prediction of rogue waves based on random linear superposition and should replace forecasts based on modulation instability. Crest-trough correlation was then forecast by a regional WAVEWATCH III ® wave model with moderate accuracy compared with the high performance of forecasting significant wave height. Results from a case study of a large fall storm October 21-22, 2021, are presented to show that the regional wave model produces accurate forecasts of significant wave height at high seas and presents a potential rogue wave probability forecast. / Graduate

Determining the largest breaking wave height which can occur in water of finite depth is a fundamental reference quantity for the design of coastal structures. Current design guidelines are based on investigations which predominantly used monochromatic waves, thereby neglecting group effects which are inherent to the free propagation of waves in deep water. The Coastal Engineering Manual (CEM) states that wave grouping and its consequences is of significant concern, with breakwater armour damage being generally attributed to higher waves associated with wave groups. However, the CEM also acknowledges that there is little guidance and few formulae for use in practical engineering. This thesis describes a laboratory-based investigation into the effect of wave groupiness on wave shoaling, breaking and surf zone processes. New optical-based techniques for data abstraction, developed within this study, have allowed examination of the interaction between deep water intra-wave group processes and shallow water shoaling processes. The applicability of existing methods for predicting breaking wave height and position is evaluated, along with the implications of groupiness on engineering design in the nearshore. The effect of wave groupiness on overtopping and hazard on emerged rock platforms is similarly assessed. Wave group testing has revealed that the spatial phasing of intra-group processes during shoaling can result in considerably different shoaling and breaking regimes. Under certain regimes, wave breaking occurred further shoreward and in a more plunging manner than under other regimes. Within the mid to inner surf zone, waves were also observed to propagate into shallower water before breaking than is predicted by existing design guidelines. This could result in under-prediction of wave height by up to 100%. Expressions are developed for the prediction of maximum wave heights and surface elevation on plane slopes. These expressions implicitly include non-linear group effects and group-induced water-level variations within the surf zone, and are found to provide conservative upper envelopes for the range of data observed within the current testing regimes. Predictive schemes are similarly developed for overtopping hazard on emerged rock platforms based on critical wave and water-level conditions. Variations in maximum overtopping flow values due to intra-wave group processes of up to +/-35% were found. These group effects were found to reduce by up to 30% the threshold wave conditions before the initiation of hazard.

Numerous studies have identified the west coast of Canada as an attractive place for the development of wave energy projects. To evaluate the viability of these projects, an accurate description of the wave resource is crucial. Most of the previous efforts to characterize the wave climate in B.C. at shallower waters, where wave energy converters (WECs) are most likely to be deployed, lack the necessary nearshore spatial resolution, and were driven by overly simplistic wave boundary conditions. In addition, none of the previous studies have included the effect of tidal currents, which have been proven to be significant in wave resource characterizations in other locations. This work increased the fidelity of the wave resource characterization and developed an understanding of the impact of tidal currents on the wave conditions in this region by generating two most accurate, long-term (14 years, 2004 to 2017), high resolution (in space and time) datasets of the wave resource for the west coast of Canada. The two datasets were generated using nearly identical SWAN wave models, which their only difference was that one of them (V5), did not incorporate the effect of currents, while the other (V6) included tidal currents as forcing. Thus, the pure influence of the tidal currents on the wave characteristics was able to be identified when comparing the two wave model results. This study developed simple, robust, and objective metrics to support the calibration process and to evaluate the performance of the models. Utilizing these metrics, the V5 and V6 models presented substantial improvements in reproducing the wave conditions of about 18% and 20%, respectively and in relation to the previous most complete and accurate wave model of the region (V4). Their better performance was largely achieved by a significant increment in their ability to reproduce the significant wave height (H_m0) and energy period (T_e). The inclusion of tidal currents in the wave model increased the accuracy of the wave resource characterization, mainly by improving the model’s ability in simulating T_e by 5.1%. The most sensitive wave parameter to the tidal currents was the peakedness of the wave spectrum (Q_p), which was consistently and significantly reduced by values even larger than 2.5. In some regions, directions characterized by the mean wave direction (D_m) and the directional spreading (D_spr) were also noticeably very sensitive to the currents, which even deflected D_m to its opposite direction and drove changes in D_spr that reached values of up to 40°. However, these significant transformations were less frequent and reduced in magnitude at exposed (to swell-waves) sites, where strong currents have affected waves in a reduced part of their trajectory. Typically, tidal currents had the effect of reducing the wave power density (P), but in a relatively small amount, however, during rare events, tidal currents were able to induce changes in this parameter ranging -140 kW/m to 75 kW/m. At these extreme events, it was observed that the peak of the wave spectra became flatter, with some of its wave height variance redistributed to near increasing and decreasing frequencies and directions, regardless to the magnitude and direction of the local tidal currents. Impacts of the tidal currents on P were largely attributed to the induced changes in H_m0 and T_e. Although D_spr and Q_p were greatly transformed by the action the tidal currents, they account very little in explaining the variations in P. These four wave parameters together, and how they are transformed under the presence of currents, can explain a large part of the changes in P, however, other transformations of the wave spectrum due to the currents, not investigated in this study, must account for a considerable part of the changes in P. / Graduate