Longshore Current Profiles and Instabilities on Plane Beaches with Mild Slopes

The accurate determination of cross-shore longshore current profiles in the surf zone is essentially important in understanding of coastal physical processes and modelling of longshore sediment transport. In this study, a comprehensive laboratory study was undertaken to directly measure longshore current profiles over plane beaches with two mild slopes, 1:40 and 1:100, in a wave basin 55 m long, 34 m wide, and 0.7 m deep. Different wave conditions with an incidence angle of 30 degrees were generated by piston-type wave makers consisting of 70 individual paddles, and two arrays of 29 Acoustic Doppler Velocimeters (ADVs) were used to measure longshore currents in the surf zone. Based on the experimental data collected in this study, three types of cross-shore longshore current profiles were found on the two plane beaches under different testing wave conditions, namely quasi-Rayleigh, quasi-Gaussian, and M-shape profiles. The quasi-Rayleigh profile was found on the beach slope of 1:40, and the other two types were found on the beach slope of 1:100. Analytical formulae were proposed to describe these profiles and agreed well with the laboratory data. The fluctuations of longshore currents observed in this study were attributed to their shear instabilities based on linear instability analysis. The results of the linear instability analysis and the spectra of measured velocities also showed that the three types of velocity profiles led to different instability characteristics.