It is well known that second order effects may in many cases be important for the nonlinear hydrodynamic problems arising in ocean engineering. Despite considerable efforts having been made in the past in calculating second order unsteady forces, similar studies are rare for the actual second order velocity potential itself, which is important for the understanding of wave kinematics. A mathematical model has been developed for the calculation of the second order sum frequency diffraction potential for fixed bodies in waves. It is believed that a first step towards the solution of the second order problem is the accurate evaluation of the first order quantities. By the use of Green's second identity, the first order problem can be cast into the form of a Fredholm integral equation and then solved by the Boundary Element Method. Some new developments based on this technique have been undertaken in this work, and as a result, there is a major improvement in the accuracy of the first order analysis. For the second order problem, the solution procedures are similar to those used for the first order problem except that special techniques have been developed to calculate efficiently the additional free surface integral which decays slowly to infinity in a highly oscillatory manner. In addition, an effective method has also been implemented to calculate the second derivative term in the free surface integral. From the numerical results presented, a number of interesting findings are illustrated. A closed form expression for a vertical circular cylinder has also been developed which not only furnishes a valuable check on the general numerical model but also provides some physical explanation for the second order phenomena. Moreover, it has been used to investigate some theoretical problems which (in the past) have caused confusion and error in the second order analysis. They are mainly associated with the troublesome nonhomogeneity presented in the free surface boundary condition.

Several studies have previously demonstrated that sediment transported as bed load under a standing wave will be moved from between node and antinode towards the node where it accumulates. The end result is the generation of areas of scour in the bed between node and antinode and areas of sediment accretion around the node. However, these studies have failed to provide an adequate description of the mechanisms which led to this so-called "N-type" response. Consequently it has been the purpose of this study to examine the phenomenon of N-type beach response in some detail. As a first step an experimental programme was conducted in a random wave flume using a model beach and vertical, impermeable wall N-type conditions were produced, observations are made on the mechanisms of sediment transport and flow-field measurements are also reported. The experimental results demonstrate that N-type beach response is the direct result of an asymmetry in the main flow-field caused by the superposition of incident and reflected non-linear waves. A second experimental programme examines the N-type equilibrium profile shape using measurements of profiles generated under a range of wave conditions. An equation defined for a given profile amplitude and profile limits is found to predict the underlying characteristic N-type profile shape very well. Finally, a model of N-type beach response is developed. This model follows a relatively simple "grab-and-dump" concept with coefficients based on the understanding gained of the N-type response processes. It is shown that N-type profiles can be predicted reasonably well for the range of wave conditions used in the experiments here. Outside of this range predictions are less good, suggesting a re-examination of the model coefficients is required.

A series of detailed laboratory investigations were conducted to examine low solubility gas transfer across wind-forced wavy air-water interfaces. The study focuses on the increase in gas flux associated with the microphysical interfacial wind momentum exchange and the complex wave coupled hydrodynamics. Key elements of the laboratory investigations included the measurement of hydrodynamic behaviour within the aqueous viscous sub-layer using a particle image velocimetry (PIV) system and the development of a Laser Induced Fluorescent (LIF) system capable of measuring reliable dissolved oxygen concentration profiles to within 28??m of the air-water interface. Major achievements and findings included: 1. The first phase resolved gas flux measurements along wind forced microscale waves, indicating the highest mean gas fluxes are located in the wave troughs. This finding demonstrated the relative importance of wave orbital straining in gas flux enhancement; a wave coupled hydrodynamic process whose significance has previously been neglected. 2. The relative contributions to gas flux from wind shear, wave orbital straining, increased surface area of the waves, parasitic capillary ripples and microscale breaking are quantified with respect to friction velocity, wave steepness and an efficiency of microscale wave breaking. The parasitic capillary ripples are shown to have a negligible role in gas enhancement. A hybrid model is developed to estimate the gas flux based on both wind and wave characteristics. 3. Gas enhancement due to microscale wave breaking and the significance of the highly localised subduction at the toe of the spilling region on the leeward face of the wave crests was investigated using data from the LIF experiments. The highly localised subduction was shown to substantially reduce the thickness of the diffusion sub-layer, resulting in an increase in gas flux when waves transitioned from the incipient breaking to the microscale breaking wave form. 4. Consideration of previously unidentified optical distortions in LIF imagery due to non-linear effects is presented that is critical for robust LIF data processing and experimental design. A formal mathematical description of optical distortions has been developed and presented.

This thesis describes experiments that were carried out using focused wave groups in the UK Coastal Research Facility (UKCRF). Considerable effort was put into calibrating the UKCRF to determine the relationship between the input signals sent to the paddles and the waves generated in the facility. Focused wave groups of various sizes and phases, based on NewWave theory were generated, and measurements were made of the resulting surface elevation data, water particle kinematics, wave runup and overtopping volumes. NewWave theory models the profile of extreme waves in a Gaussian (random) sea. The thesis describes the first time this model has been applied in the context of coastal wave transformation. A method for the separation of the underlying harmonic structure of a focused wave group is described and results presented. This technique has been used in relatively deep water but is shown to work successfully in the coastal zone until wave overturning. A method has been devised to provide a theoretical Stokes-like expansion of the free and bound waves to model the surface elevation and water particle kinematics of the focused wave groups. Satisfactory agreement is achieved between the theoretical predictions of UKCRF measurements. Suggestions are made for an improved model. The underlying harmonic structure of the focused wave groups is presented as stacked time histories that give insight into the wave transformation process from deep to shallow water. Particular attention is paid to the low frequency wave generated as the wave group interacts with the beach. This is compared to the low frequency wave that is generated by a solitary wave in the UKCRF. Runup and overtopping measurements are in reasonable agreement with predictions based on certain empirical formulae, but not others. These comparisons are useful in identifying those formulae able to predict runup and overtopping of extreme waves in the coastal zone.