MODEX: Using Wave-Resolving Models to Understand Observed Sand Mound Deformation

Characterizing the diffusion of nearshore sediment features such as mounds and bars is critical to manage natural coastal environments as well as human interventions such as foreshore nourishments. Here, laboratory and numerical results are presented to describe the link between hydrodynamic structures, specifically wave-induced vorticity and wave-current interaction, and the deformation of a submerged mound.

During MODEX (MOrphological Diffusivity EXperiment), we measured the diffusion of a mound in a wave flume under a variety of wave, current, and combined flow regimes. Hydrodynamic conditions around the mound were recorded using wave rods (wave shape and height), velocity point measurements, and near-bed velocity profilers. Mound deformation was monitored with two Ripple Profiling Scanners (RPS) which captured 3D mound shape at 5-15 minute intervals. Here, these data are used in combination with wave-resolving models (FUNWAVE-TVD and SWASH) to capture the hydrodynamic structures (wave-diffraction, vortical motion), wave-current interaction, and sediment transport observed around the mound.

We present results from these numerical models to show how wave-driven hydrodynamic structures link to patterns of mound deformation and how these structures change with wave-current interaction. In particular, vorticity generated by wave diffraction is shown to induce sediment transport perpendicular to the flow direction. When waves and currents are combined, waves become more irregular with variable wave height across the flume, and as a result these vortices change in size and location. The impact of wave-current interaction on vorticity and sediment transport in the flume will be discussed using results from numerical models and lab observations.