Turbulence Measurements in the Surf Zone

Abstract

Velocity measurements within 1 m of the bottom in approximately 4.5-m water depth on a sand beach provide estimates of turbulent Reynolds shear stress, using a dual-sensor technique that removes contamination by surface waves, and inertial-range estimates of dissipation. When combined with wave measurements along a cross-shore transect and nearby wind measurements, the dataset provides direct estimates of the terms in simplified equations for alongshore momentum and turbulence energetics and permits examination of semiempirical relationships between bottom stress and near-bottom velocity. The records are dominated by three events when the measurement site was in the outer part of the surf zone. Near-bottom turbulent shear stress is well correlated with (squared correlation coefficient r2 = 0.63), but smaller than (regression coefficient b = 0.51 ± 0.03 at 95% confidence), wind stress minus cross-shore gradient of wave-induced radiation stress, indicating that estimates of one or more of these terms are inaccurate or that an additional effect was important in the alongshore momentum balance. Shear production of turbulent kinetic energy is well correlated (r2 = 0.81) and consistent in magnitude (b = 1.1 ± 0.1) with dissipation, and both are 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 measurement depth. Log-profile estimates of stress are well correlated with (r2 = 0.75), but larger than (b = 2.3 ± 0.1), covariance estimates of stress, indicating a departure from the Prandtl?von Kármán velocity profile. The bottom drag coefficient was (1.9 ± 0.2) ? 10?3 during unbroken waves and approximately half as large during breaking waves.