Characteristics of turbulence in the surf zone

Near-bottom measurements from an array of acoustic Doppler velocimeters over a sand beach produce direct estimates of turbulent shear stress, by means of a differencing procedure that rejects contributions from surface waves, and indirect estimates of dissipation, by means of an inertial-range model. Measurements of winds, waves, and currents provide a context for analysis of the turbulence measurements. The analysis focuses on an alongshore momentum equation, semiempirical relationships between turbulent shear stress and alongshore velocity, and simplified budgets for energetics. The records are dominated by three events when the instruments were in the surf zone. Near-bottom turbulent shear stress was well correlated with, but smaller than, the sum of wind and wave forcing. Vertical gradient of alongshore velocity was larger than indicated by the Prandtl–Karman law of the wall. Drag coefficient was smaller during breaking waves than during unbroken waves. Shear production of turbulent kinetic energy was approximately equal to dissipation, and both were two orders of magnitude smaller than the depth-averaged rate at which the shoaling wave field lost energy to breaking, indicating that breaking-induced turbulence did not penetrate to the seabed.