BED SHEAR-STRESS COEFFICIENTS FOR LONGSHORE CURRENTS OVER A BARRED PROFILE

Quantitative estimates are obtained of various terms in the momentum e quation governing the mean alongshore flow in the surf zone on a barre d beach. A movable sled was instrumented with pressure, current, and w ind sensors to measure wave and wind forcing during the SUPERDUCK expe riment. Mean currents and bottom shear stress at various locations acr oss 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 forc ing, under certain conditions. Time dependence of the mean longshore c urrent is due to variable wave forcing inside the surf zone as the tid e modulates the depth of waves breaking on the bar. Bed shear stress c oefficients, determined as a residual of the local alongshore momentum balance, tended to decrease in magnitude shoreward. Mean bed shear st ress 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 immediat ely in front of the bar, and in the trough.