The particle image velocimetry (PIV) technique was employed to measure the two-dimensional instantaneous velocity distribution under non-breaking and breaking water waves. For the breaking wave study, monochromatic waves with a wave height of 14.5 cm and a wavelength of 121 cm were generated in the intermediate water depth, h, of 20 cm. The wave train breaks at a distance of about 2h from the wave generator. Measurements were taken in the domain from about 1h upstream of the breaking point to about 4h downstream. By repeating the experiments and performing the ensemble average, the mean velocity, mean vorticity, turbulence intensity and other flow properties such as the Reynolds stress and the mean strain rate were calculated. Using the experimental data, we examined the transport equation for turbulence kinetic energy. The turbulence dissipation rate and its time scale were also estimated. In addition, the PIV technique was used to measure the instantaneous vertical vortices generated by breaking waves. The pseudo turbulence, which is inevitable with the use of a large field of view, was found in the PIV turbulence measurements. We have shown that the pseudo turbulence is mainly contributed by the bias error, which is the discrepancy between the true position of the particle image and the position calculated from the pixel array data with inadequate pixel resolution. To remedy the situation, one must first find the relation between the bias error and the mean particle displacement and then remove the pseudo turbulence in the measurements. To demonstrate the procedure, the evolution of a breaking wave was investigated.

In the second part of the thesis, the interactions of water waves and a submerged impermeable bottom-mounted rectangular obstacle were studied. Rectangular obstacles with the height to water depth ratio, D/h, of 0.25 and 0.5 were used. Both solitary waves and cnoidal waves were tested. The instantaneous velocity and vorticity were obtained by using the PIV technique. The generation and evolution of the vortices due to flow separation at the corners of the obstacles were demonstrated. The mean velocity, mean vorticity, and turbulence intensity were also computed by applying the phase-average method. A modified form of Reynolds number was found to be the dominant parameter to govern the vortex strength. The correlation between the vortex strength and the turbulence intensity was also investigated.