A laboratory study of longshore currents over barred and non-barred beaches

Abstract

A detailed description is given of the results of laboratory experiments on wave-driven longshore currents on both barred and non-barred beaches. The objective is to examine the cross-shore distribution of the longshore current velocities for purely wave-driven currents, with emphasis on the position of maximum current velocity with respect to areas where wave energy is dissipated. Unidirectional obliquely incident waves, both regular and random, were used. The measurements were performed in a large wave basin with a pump recirculation system to create spatially homogeneous longshore currents. The experiments yielded information on wave transformation, set-up of the mean water level and the cross-shore distribution of wave-driven longshore current velocity. A number of cases are presented and compared with each other. The measurements show that in the case of purely wave-driven longshore currents, the maximum current velocities occur close to areas where wave breaking is most intense. The effect of mixing, bottom friction and wave rollers on the longshore current velocity profile are examined in more detail with help of numerical modelling. Existing model equations, based on the assumption of alongshore uniformity, are used. The results for the mean longshore current profile on a barred beach are in close agreement with the measurements.