Field Observations of the Evolution of Plunging‐Wave Shapes

Annika O'Dea,Steve Elgar,Katherine Brodie

doi:10.1029/2021gl093664

Abstract

AbstractThere 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.

Highlights

Depth-induced wave breaking transfers momentum into the surfzone water column, driving cross- and alongshore currents, increasing shoreline water levels, and generating turbulence and vorticity (Peregrine, 1998)
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
Observing the rapid transformation of the shapes of waves as they shoal and begin to break requires spatially dense measurements in and near the surf zone, which are difficult to obtain in the field, in high-energy, plunging conditions

Summary

Introduction

Depth-induced wave breaking transfers momentum into the surfzone water column, driving cross- and alongshore currents, increasing shoreline water levels, and generating turbulence and vorticity (Peregrine, 1998). A depth-independent parametric form of the equations of motion (Longuet-Higgins, 1982) has been used to assess depth-limited plunging wave shapes in the laboratory (Blenkinsopp & Chaplin, 2008) and the field (Mead & Black, 2001). In these studies, a modified curve was fit to optical images of breaking waves, and the resulting curve parameters were used to characterize the plunging wave shapes, and to analyze the factors contributing to the differences between observed and theoretical shapes. The shape of the internal void created by the plunging lip is compared with theory (Longuet-Higgins, 1982), and the spatio-temporal evolution of the plunging wave and the factors influencing the void shape, angle from horizontal, and area are assessed at the onset of breaking

Observations

Plunging Wave Identification

Void Fitting

Wave-By-Wave Parameters

Results and Discussion