Quantitative estimates are obtained of various terms in the momentum equation governing the mean alongshore flow in the surf zone on a barred beach. A movable sled was instrumented with pressure, current, and wind sensors to measure wave and wind forcing during the SUPERDUCK experiment. Mean currents and bottom shear stress at various locations across the surf zone were determined. Particular attention is devoted to precise estimation of the cross-shore gradient of the radiation stress. Wind forcing is found to be a first-order term, along with wave forcing, under certain conditions. Time dependence of the mean longshore current is due to variable wave forcing inside the surf zone as the tide modulates the depth of waves breaking on the bar. Bed shear stress coefficients, determined as a residual of the local alongshore momentum balance, tended to decrease in magnitude shoreward. Mean bed shear stress coefficients and estimated error are 0.004 ± 0.0013, 0.003 ± 0.0006, and 0.001 ± 0.0003 for offshore the bar, on top and immediately in front of the bar, and in the trough.
Whitford, D. J., & Thornton, E. B. (1996). Bed shear stress coefficients for longshore currents over a barred profile. Coastal Engineering, 27(3–4), 243–262. https://doi.org/10.1016/0378-3839(96)00005-1