Patricio Catalán

Remote sensing of wave roller lengths in the laboratory

[1]xa0The optical intensity signals from surf zone waves in a laboratory flume are analyzed using several different phase-averaging techniques, and a methodology is developed for estimating wave roller lengths and local wave dissipation. The intensity signals (i.e., phase-averaged intensity profiles) of individual breaking waves are compared with the wave profiles measured by in situ wave gauges, and the optical signal of the wave roller is shown to ramp up from the toe of the wave roller on the front face of the wave to a maximum intensity at the wave crest. The remote sensing observations capture the growth, equilibrium, and decay phases of the roller as it propagates over a fixed bed arranged in a bar/trough morphology. Next, for the regular wave conditions considered here, the local maxima of the phase-averaged intensities are shown to better indicate the initial onset of wave breaking and the occurrence of wave breaking in the bar trough, as compared to the more commonly used time-averaged mean intensity. In addition, the phase-averaged profiles are used to measure the size of the roller, and these measurements are compared to previous observations of smaller-scale rollers in equilibrium. The observed roller lengths are shown to agree with predictions from a wave roller model and to provide a new physical link between the remotely sensed signal and roller dissipation. Finally, as an example application of these new data, a simple wave height inversion model is presented that allows an estimation of surf zone wave heights from the remotely sensed roller lengths.

MODELING OF SURFZONE BUBBLES USING A MULTIPHASE VOF MODEL

The ability to make optically-based observations in the surf-zone is strongly influenced by the presence of suspended sediment particles and of air bubbles, both of which are present due to the action of breaking waves. Wave breaking is instrumental in injecting large volumes of air into the water column. This air volume subsequently evolves into a distribution of bubble sizes that interact with the fluid turbulence and are advected by the organized flow. The bubble population in the surf-zone is intensified due to the greater intensity of breaking processes, leading to increase in turbulence intensity and associated energy dissipation. The bubble sizes are also affected by the densely sedimented flows that could alter the relationship between turbulent perturbing forces and surface tension-based restoring forces leading to the determination of critical bubble diameters.

Nonlinear Phase Speeds and Depth Inversions

Experiments were conducted in a laboratory facility under with a fixed bathymetric profile containing a single bar. A hybrid data set was collected consisting of remotely sensed wave data combined with modelgenerated wave amplitude profiles. The data is used to test the improvements in depth retrieval through the use of a nonlinear composite dispersion that is applicable in arbitrary water depths. The dispersion relation relates the wave properties with the local water depth and the results suggest that inclusion of nonlinearity improves depth estimation on shallow water.

SURF ZONE WAVE BREAKING IDENTIFICATION USING MARINE RADAR

This paper presents a novel combination of synchronous, large coverage marine radar, calibrated pulsed Doppler radar, and video remote sensing observations from a nearshore field site. The data enable the analysis of the evolution and c haracteristics of the remotely observed signals from surf zone waves. The combination of different remote sensors allows a better discrimination between breaking and non-breaking waves, such a discrimination method offers the potential of more quantitative analyses of the wave breaking roller at field scales. In the present work, we concentrate on quantifying the contribution of the wave breaking roller to the total microwave backscatter and optical intensity signals. Analysis of the power outputs of both breaking and non-breaking events demonstrates that the backscatter from the wave roller is distinguishable from relict foam and appears to be independent of environmental conditions. Finally, we test a volumetric scattering model against the wave breaking observations. This is a new application for this ty pe of scattering model. The model treats the wave roller as a multi-phase body where most of the scattering arises from a layer of water droplets representing the wave roller. The model shows an improved agreement with the data as compared to the traditional surface scattering model.

Measurements of Shallow Water Breaking Wave Rollers

Large scale laboratory experiments concerning breaking wave propagation over a fixed-bed, barred beach were performed. The primary observations of interest were video intensity time series obtained from a set of high-resolution video cameras in order to study the onset of wave breaking in shallow water and wave roller transformation in the surf zone. The approach is new in the sense that the analysis concentrates on individual breaking waves, as opposed to the more commonly used timeexposure technique, which averages the information over wave group time scales. A new parameter of interest is derived from the video observations based on the instantaneous intensity maximum, which propagates with the roller. The parameter is shown to provide high-resolution information regarding the onset of wave breaking and the spatial evolution of the wave roller. These observations provide a new and quite rigorous test for phaseresolving shallow water wave models.