Large-scale wave breaking over a barred beach: SPH numerical simulation and comparison with experiments

Abstract

Wave breaking plays a crucial role in several areas of interest in coastal engineering, such as flooding, wave loading on structures and coastal morphodynamics. In the present study, Smoothed Particle Hydrodynamics (SPH) simulations of monochromatic waves breaking over a rigid barred beach profile are presented. The numerical results comprise wave heights, phase average velocities, time-averaged velocities, vorticity dynamics, and radiation stress, and are validated versus detailed water surface and velocity measurements carried out in a large-scale laboratory wave flume. The experimental data include velocity profiles below the wave trough measured at 22 cross-shore locations in the bar region using acoustic and optical techniques and water surface elevation measured along the flume using resistive gauges, acoustic gauges and pressure sensors. This study is novel in that it analyses the hydrodynamics of wave breaking at a scale close to natural conditions, thus significantly reducing the scale effects of most previous studies, which were conducted at a much smaller scale. In general, water surface elevation is well reproduced by SPH, but discrepancies with the experiments are observed in the highly aerated breaking region, depending on the measurement technique. The SPH simulation shows that wave breaking generates a recirculating cell, immediately above the trough of the bar. Within this cell, near the bed, the flow is offshore directed, while in the upper part of the water column it is onshore oriented. This flow is probably one of the mechanisms that determine the growth of the bar when the bed is made of mobile material. The time-averaged velocity profiles are reproduced with reasonable accuracy by the numerical model, except at the edges of the bar trough, where discrepancies with respect to the measurements are observed. The numerical results provide detailed information, particularly interesting in areas lacking experimental data. One of the main surprising features revealed by the SPH simulations is the generation of a vortex pair that occurs when the cavities formed by the plunge jet collapse. These vortices can occasionally deform the free surface. Based on the numerical results, an analysis of the terms contributing to radiation stress shows that the product between the horizontal and the vertical velocity components does not make a significant contribution. Through comparisons with the SPH results, it is observed that the linear wave theory provides correct estimates of the radiation stress in the shoaling region sufficiently far from the bar crest, while in the surf zone it reproduces incorrect results. Information about the appropriate SPH model setup to correctly capture the physical processes involved in the breaking phenomenon are also presented.