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

Abstract

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 0.001m vertical resolution over the lower 0.02m of the water column and at up to 100Hz. 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 0.02m 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 4.25Nm−2 for swash events and inner surf zone wave cycles respectively. Friction coefficient estimates adopting the quadratic drag law and the velocity at 0.02m 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.