Depth-Dependence of Surfzone Eddies: Generation and Destruction of Vertical Shear

Wave breaking, bottom friction, and advection likely influence the vertical structure of surfzone currents and eddies. Vertical structure was measured for 21 days on a natural beach using 12 Acoustic Doppler Current Profilers. Eddy-frequency (0.0005-0.01 Hz) velocity fluctuations, which dominate nearshore lateral mixing, were nearly vertically-uniform far inside the surfzone, but displayed vertical variability near, and offshore of, the outer edge of the surfzone . At lower frequencies, `mean currents’ inside the surfzone were sheared in the across-shore direction. A simple model suggests a balance between generation and destruction of mean-squared vertical shear. Generation may occur through breaker-induced momentum injection, bottom friction, advection, and wind stress, while vertical turbulent mixing may destroy vertical structure. Modeled shear is influenced by the ratio between near-bed and mid-water-column eddy viscosity. Inside the surfzone, simple models suggest that breaker-injected turbulence reduces this ratio, weakening vertical structure. Using model-data comparisons, we test this potential explanation for observed variability in vertical shear, and examine the role played by additional factors such as advection.