The wavedrifter: a low-cost IMU-based Lagrangian drifter to observe steepening and overturning of surface gravity waves and the transition to turbulence

Waves and wave breaking are important to many deep and shallow water processes. We describe the wavedrifter, an in situ water-following inertial measurement unit (IMU)-based drifter that measures wave steepening and overturning kinematics, and the subsequent transition to turbulence. The wavedrifter has 5 cm diameter, 77 g mass, and 0.84 saltwater specific gravity. GPS provides time synchronization. MATLAB's Attitude, Heading, Reference System (AHRS) library provides wavedrifter orientation. Laboratory experiments quantify the wavedrifter vertical oscillation mode, water following properties, and ability to reproduce wave spectra for small, f = 1:5 Hz waves. The wavedrifter observed wave overturning and the transition to turbulence at the Surf Ranch wave basin. Synchronized video observations provide context. The upper-back of the overturn had large (approximate to 4g) accelerations and 14g acceleration magnitude occurs with the impact of the overturning jet. Trajectories reveal the Lagrangian structure of the overturn and subsurface vortex. Although it has limitations, the wavedrifter is a powerful in situ tool to probe wave processes.