VERTICAL PROFILES OF LONGSHORE CURRENTS AND RELATED BED SHEAR-STRESS AND BOTTOM ROUGHNESS

The vertical structure of the mean wave-driven longshore current over a barred beach is examined on three strong current days during the DUC K94 experiment, and it is found that the bottom boundary layer is well described by a logarithmic profile (mean correlation coefficient for all 22 profiles, 0.98). The logarithmic profile fits better in the tro ugh, where turbulent bottom boundary layer processes predominate, than over the bar, where breaking-wave-induced turbulence generated at the surface modifies the profile. The surface layer in the presence of wa ves is well described by adjusting the logarithmic profile for the int ermittent presence of water and adding the alongshore component of the mass transport velocity (slope of the least squares linear regression between model predictions and observations, 1.005 and root-mean-squar e (rms) error of 7%). Bed shear stresses calculated from logarithmic v elocity profiles are equated to a quadratic bottom shear stress formul ation. The associated bed shear stress coefficients vary by more than an order; of magnitude across the surf zone (0.0006-0.012). Bottom rou ghness was measured throughout the nearshore using a sonic altimeter m ounted on a moving platform. The bed shear stress coefficients are pos itively correlated with bottom roughness (linear correlation coefficie nt, 0.6). A higher linear correlation coefficient (0.8) is obtained by subtracting skin friction from the total bed shear stress.