Observations of Breaking Wave Energy Dissipation on a Rocky Coast

Over half of the World's coastline is rocky, characterized by non-planar bathymetry and high roughness as a function of quasi-randomly distributed rocks of different scales. Yet, nearly no observations of hydrodynamic processes on rocky coastlines have previously been obtained. As part of the Office of Naval Research-funded Rocky Shorelines Experiments and Simulations (ROXSI) project, we have made observations of breaking wave energy dissipation along the rocky coastline near Monterey, California. A small uncrewed aerial system (sUAS) equipped with a lidar sensor and 6k resolution video camera collected time-synchronized elevation and video observations of hundreds of individual wave breaking events across a range of breaking wave heights. These observations are being used to estimate breaking wave energy dissipation rates using two theories based on different physical mechanisms. In the first, the breaking wave energy dissipation rate is estimated through measurements of the roller geometry. In the second, turbulent velocities on the roller surface are obtained by applying particle image velocimetry (PIV) algorithms to surface bubbles, and these are linked to the energy dissipation rate through the Kolmogorov wavenumber spectrum. In combination with observations of the wave friction factor from an array of in situ instruments as well spatial maps of wave breaking and qualitative observations of circulation from a multi-camera Argus station, these results are providing a comprehensive picture of the processes by which waves dissipate their energy and force nearshore currents on rocky coastlines. This work has important implications for the Navy's operational ability in rocky coastal environments.