Boussinesq modeling of longshore currents in the SandyDuck experiment under directional random wave conditions

Abstract

To investigate surf zone hydrodynamics in a directional random wave environment, a numerical simulation of the SandyDuck experiment was performed for 2 October 1997 using the Boussinesq model FUNWAVE. This simulation utilized the field topography and the offshore spectrum constructed from observed datasets. The simulation results agreed well with the experimental data to reveal a wave height distribution of the random waves as well as the well-developed longshore current and its energetic fluctuation. The simulation also revealed that longshore variations were induced by not only the field topography but also the phase interaction of multidirectional random waves in the surf zone wavefield, and subsequently complicated the mixing process with fluctuations of longshore currents and rip currents in the nearshore current field. Based on the Boussinesq modeling, further simulations were performed using incident wave spectra that varied in the frequency and directional spreading. The simulation results confirmed that random waves with broader frequency-directional spectra tended to produce weaker mean longshore currents, which were associated with the early dissipation of relatively short wave components and the small moments of broad wave spectra. In addition, it was found that the energetic nearshore fluctuations, which involved complicated evolutions of vortices, did not vary significantly according to the frequency and directional spreading. However, the results of the monochromatic unidirectional wave revealed relatively weak and organized nearshore fluctuations (i.e., instabilities), even though this simulation also revealed a stronger longshore current with stronger wave breaking than did the random wave simulations.