Observed kinematics of waves in the surf zone

Abstract

The observed kinematics of waves in the surf zone is examined with extensive measurements from the DUCK94 experiment. Field data used in the study were obtained from vertical stacks of bi-directional current meters and a pressure sensor mounted on a rigid frame at 3 locations along a cross-shore transect in depths of 2,4, and 8m. Observed pressure and velocity spectra are compared to transfer functions based on linear finite depth theory and a simple nonlinear model that accounts for harmonic generation in shallow water. At high frequencies, the observed vertical aftenuation of horizontal velocity spectra in 8 and 4m depth is much weaker than predicted by linear theory and generally in good agreement with the nonlinear model predictions. In 2m depth, differences between the linear and nonlinear transfer function are small and both predictions are in reasonable agreement with the observed weak vertical decay. At infragravity frequencies in shallow water depths, observed velocity spectra often show significant vertical decay that is not predicted by either model. Velocity and pressure spectra measured in 4m depth are in good agreement with the nonlinear transfer function. Pressure spectra levels at high frequencies are shown to be significantly reduced by the nonlinear Bernoulli term in the second order pressure field. Analysis of the slopes of the high-frequency tails of the observed velocity spectra shows considerable scatter with a general tendency for spectra to flatten as waves rotate through the surf zone.