Field Observations of the Evolution of Plunging‐Wave Shapes

• Published in: Geophysical research letters Vol. 48; no. 16
• Main Authors: O'Dea, Annika, Brodie, Katherine, Elgar, Steve
• Format: Journal Article
• Language: English
• Published: Wiley 28-08-2021
• Subjects: breaking waves; lidar; plunging waves; remote sensing; surf‐zone processes
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2021GL093664

There are few high‐resolution field observations of the water surface during breaking owing to the difficulty of collecting spatially dense measurements in the surf zone, and thus the factors influencing breaking‐wave shape in field conditions remain poorly understood. Here, the shape and evolution of plunging breakers is analyzed quantitatively using three‐dimensional scans of the water surface collected at high spatial and temporal resolution with a multi‐beam terrestrial lidar scanner. The observed internal void shapes in plunging breakers agree well with previously developed theoretical shapes at the onset of breaking, and become more elongated and less steep as breaking progresses. The normalized void area increases as the local bottom slope steepens and as the breaking depth decreases. The void shape becomes more circular as the local bottom slope and the ratio of breaking water depth to wavelength increase, as well as in conditions with opposing winds.
Plain Language Summary
Breaking waves inject momentum into the surf zone that drives nearshore currents, raises water levels, and generates turbulence. However, there are few high‐resolution field observations of the water surface during breaking because of the difficulty of collecting spatially dense measurements of the water‐surface elevation at the location of wave breaking, leaving open questions related to the factors controlling the shape and properties of breaking waves. In this study, three‐dimensional scans of the water surface in the surf zone collected using a multi‐beam terrestrial lidar scanner are used to analyze quantitatively the shape and evolution of plunging breakers. The shape of the internal barrel (or void) created as the plunging lip intersects with the front face of the wave agrees well with theoretical void shapes at the start of the breaking process, and then becomes more elongated and less steep through the breaking process. The normalized area of the void is larger when waves break on steeper bottom slopes and in shallower water. The void is more circular when waves break on steeper bottom slopes and with larger ratios of breaking water depth to wavelength, and also when the wind is blowing in a direction opposing the direction of wave propagation.
Key Points
Three‐dimensional lidar scans of the water surface allow for quantitative analyses of the shape and evolution of plunging breakers
The observed internal void shapes in plunging breakers agree well with theoretical void shapes at the onset of breaking
The internal void shape and normalized area are influenced by bathymetry, wave parameters, and cross‐crest wind stress