Presenter: Falk Feddersen (Scripps Institution of Oceanography)

Description:
Surfers know that wind direction (onshore vs. offshore) strongly affects surfing wave quality by affecting wave shape. Scientists, however, have yet to thoroughly research this effect. Wave shape is not only important to surfers but has implications for breaking wave turbulence and (sediment) transport.  A previous study related the break point location to the on/offshore wind direction with offshore winds leading to plunging waves that break closer to shore. Additional research noted that onshore wind speed affected the shoaling wave shape (skewness and asymmetry), but did not include offshore wind. These controlled laboratory experiments used small (~10 cm) waves. Field scale ocean measurements by O'Dea et al. (GRL, 2021) show that overturning void shapes (area, eccentricity) depend on bottom slope and breaking kh. Void eccentricity is suggested to be a function of cross-shore wind. Ocean measurements though are challenging due the non-stationarity of the wind, wave field, and underlying bathymetry. Here, we present observations of wave shape for steady field-scale waves (~2 m breaking waves) on a fixed bathymetry with naturally variable wind at the WSL Wave Company facility in Lemoore CA. Waves were observed with wave staffs, drone-based visible imagery,  and multiple lidars focussed on a detailed region of wave shoaling and breaking. One lidar was mounted on a drone and another 32-beam Velodyne lidar was mounted on pole looking down at the breakpoint. During measurements, the wind varied substantially from 10 m/s out of the North and 3 m/s out of the South, resulting in a range of offshore and onshore wind conditions. Here we examine the wind effect on the shape of overturning waves.  Funded by the Mark Walk Wolfinger Surfzone Processes Research Fund.