The present paper provides an introduction to Bichromatic Synthetic Schlieren (BiCSS) for surface measurements, a novel extension of the Free Surface Synthetic Schlieren (FS-SS) by Moisy et al. (2009). The new technique is based on the fact that light diffraction through a medium varies with wavelength. Therefore, one may apply light at two different wavelengths to measure the change in density gradient in a medium. This paper explores the use of the difference between blue visual and near-infrared (NIR) light, but the choice of wavelengths will typically depend on the application. Calibration was performed using stationary targets of plexiglass and the results show that the new BiCSS technique improves accuracy for large surface gradients, compared to the traditional FS-SS technique. In order to test the applicability of the technique in the laboratory, two sets of experiments were performed. Firstly, an experiment using phase-locked regular waves was conducted for comparing BiCSS with FS-SS, analyze the properties and give an estimation of the error. Secondly, to investigate the applicability for more complex surface patterns, a study on a vertical surface-piercing cylinder exposed to a focused wave was conducted, obtaining the complex surface characteristics. The new technique clearly reveals nonlinear wave diffraction, in addition to cross waves and parasitic capillary waves.

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The aim of this study is to examine and measure the characteristics of air cavities generated by breaking solitary waves by utilizing a novel tomographic X-ray system. Small scale experiments of solitary waves that propagate on a 5.1 • (1:10) beach are conducted. Waves with amplitude normalized with the water depth, a/H = 0.47 on a flat bottom are investigated by two perpendicular X-ray systems. Images are captured at locations from the surf zone to the swash zone and at maximum runup. A large air tube is observed right after the plunger impacts the dry beach. Void velocity and the shape of the large air tube, are measured and reported. The large air tube evolves from a symmetrical shape with two large air pockets located close to the walls of the wave tank, to an asymmetrical shape. Contrast enhanced X-ray images reveal that the swash tongue surface is unstable and that secondary mixing of air and water occurs. X-ray images from the maximum runup reveal that the air is still entrapped by the thin swash tongue at times close to maximum runup.