Self-Calibration of the Breaker Index from Fixed-Camera Wave Heights

Ketron Mitchell-Wynne, Michael Reinwald , Francisco Sansana Silva, Daniel Thompson, Zane D'Amico, Mercedes Stringer, Benjamin Freeston

Depth inversions from breaking waves usually assume a breaker index γ, the ratio of breaking height to water depth. Where breaking is depth-limited, the height at a fixed location rises and falls with the tide at the rate γ. Breaking waves tracked in video from fixed coastal cameras therefore can give the index from this tide response and the depth from the mean breaking height, with no survey, beach slope or wave model. This method is tested on eleven cameras around Oceanside, California, against two bathymetric surveys a season apart, using 14 days of video before each survey. The RMSE is 0.36 m on three cameras in Fall and 0.47 m on ten in January. The spatial validation shows a better agreement and reaches shallower depths than the satellite derived bathymetry using Sentinel-2. At Tamarack Beach, a beach break, the self-calibrated breaker index is 0.50 in Fall and 0.59 in January. With these indices the recovered mean depth agrees with both surveys to within 0.1 m, and cell by cell the RMSE is 0.32 and 0.42 m.