Near bed cross-shore velocity profiles, bed shear stress and friction on the foreshore of a microtidal beach

New, high-resolution acoustic Doppler current profilers are used to quantify the vertical structure of cross-shore fluid velocities in the very inner surf and swash zones on two west-central Florida beaches. Field data are collected at an unprecedented 0.001 m vertical resolution over the lower 0.02 m of the water column and at up to 100 Hz. Swash events (defined based on the timing of sensor submergence and emergence) and inner surf zone flow cycles (defined based on the timing of zero upcrossings) are normalized around flow reversal and ensemble-averaged to investigate the vertical profiles of cross-shore flow. The ensemble-averaged cross-shore velocity profile is generally well-represented by the logarithmic model (r2 > 0.9) within 0.02 m of the bed; the maximum elevation over which the model was applied. The logarithmic model fails near flow reversal when velocities are weak. Friction velocities are used to estimate the corresponding bed shear stress with maximum values of 6.45 and 4.25 N m− 2 for swash events and inner surf zone wave cycles respectively. Friction coefficient estimates adopting the quadratic drag law and the velocity at 0.02 m above the bed are 0.024 ± 0.008 and 0.022 ± 0.007 (mean ± standard deviation) for onshore-directed motion and 0.034 ± 0.005 and 0.027 ± 0.006 for offshore-directed motion for swash zone events and inner surf zone flow cycles respectively.

► Vertical structure of cross-shore fluid velocities on the foreshore are quantified.
► Velocities generally are well-represented by law of the wall.
► Friction coefficients are similar for onshore- and offshore-directed motion.