Turbulence measurements in the surf zone

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 meas urements along a cross-shore transect and nearby wind measurements, the dat aset provides direct estimates of the terms in simplified equations for alo ngshore momentum and turbulence energetics and permits examination of semie mpirical 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 cor related with (squared correlation coefficient r(2) = 0.63), but smaller tha n (regression coefficient b = 0.51 +/- 0.03 at 95% confidence), wind stress minus cross-shore gradient of wave-induced radiation stress, indicating th at estimates of one or more of these terms are inaccurate or that an additi onal effect was important in the alongshore momentum balance. Shear product ion of turbulent kinetic energy is well correlated (r(2) = 0.81) and consis tent in magnitude (b = 1.1 +/- 0.1) with dissipation, and both are two orde rs of magnitude smaller than the depth-averaged rate at which the shoaling wave field lost energy to breaking, indicating that breaking-induced turbul ence did not penetrate to the measurement depth. Log-profile estimates of s tress are well correlated with (r(2) = 0.75), but larger than (b = 2.3 +/- 0.1), covariance estimates of stress, indicating a departure from the Prand tl- von Karman velocity profile. The bottom drag coefficient was (1.9 +/- 0 .2) x 10(-3) during unbroken waves and approximately half as large during b reaking waves.