Development of novel instrumented Lagrangian drifters to probe the internal structure of breaking surface waves

Understanding wave breaking phenomena, especially the post-breaking turbulent dynamics, is a challenging problem that still lacks satisfactory understanding. The present study is concerned with the development of novel instrumentation adapted to obtain Lagrangian field measurements of essential variables that are intimately related to the physics of wave breaking. The subsequent turbulence generated upon the breaking point evolves into a complex hydrodynamic flow abounded by intense short-lived 3D vortex structures and the presence of air cavities and bubbles. In this article we describe the design, development and testing of miniature Lagrangian drifters equipped with 3-axis accelerometers as well as a High Definiton (HD) video camera. Two types of drifters were developed, one specifically designed to measure acceleration spectra and a larger version capable of measuring acceleration and bubble size distribution. Both versions are equipped with onboard flash memory units to record local quantities of the aforementioned variables. We have conducted lab experiments and also initial field observations in plunging waves with heights of 2 meters breaking over a coral reef. We have observed acceleration signatures which are believed to be the result of drifter entrainment into coherent vortex structures. We expect that after further development of these drifters, and the resulting field observations will likely yield substantial information about the Lagrangian frequency spectra of acceleration, time-dependent bubble size spectra and ultimately energy dissipation, giving us a new perspective on the dynamics of breaking waves. Furthermore, the device also shows promise for advancing our knowledge of the 3D distribution of vortex structures in the surf zone through the reconstruction of particles trajectories via dead reckoning. The authors are not aware of any previous study that reports this type of field measurements.