Measurements of surface thermal structure, kinematics, and turbulence of a large-scale solitary breaking wave using infrared imaging techniques

• Techniques to study surface turbulent flows of large-scale breaking waves are shown.
• The foamy solitary breaking wave is measured using infrared imaging velocimetry.
• Novel evolution of thermal streaks is found on the water surface of breaking wave.
• The evolution of kinematics and turbulence in the surf and swash zones is quantified.
• An increase of turbulence anisotropy from the surf to the swash zones is found.

The surface temperature fields of large-scale solitary breaking waves are measured using infrared imaging techniques in a laboratory surf and swash zone. The surface velocity fields obtained by cross-correlating the images are decomposed into wave and turbulent motions using two filtering methods in the spatial and temporal domains. The techniques presented here provide new quantitative descriptions for the evolution of the surface thermal structures, kinematics, and turbulence that are induced by unsteady and highly foamy turbulent coastal flows. Novel organized streaks of thermal structures, which exhibit a finger-like shape, are found on the water surface of the crest roller behind the head of the rebounding jet. These thermal streaks evolve with time and become isotropic when returning to the surrounding bulk water temperature. The Froude-scaled maximum flow speed, accelerations, and vorticity are O(1), and the scaled turbulent kinetic energy (TKE) is O(− 1); these results are similar to previous findings from numerical results and periodic surf-zone breakers. Significant and concentrated structures of these quantities occur in the moving wave crest during the uprush phase; however, these structures only develop during the late stages of the backwash phase. The TKE increases shoreward from the surf to the swash zones. The ratio of the averaged variance of the turbulent velocity in the wave breaking zone does not agree with the canonical prediction for plane-wake turbulence; however, the ratio is similar to that of boundary-layer turbulence and decreases in the bore region and the swash zone, indicating an increase in the turbulence anisotropy shoreward from the surf to the shallower swash flow.