Characterizing the Impact of Wave-Current Interaction on Nearshore Sediment Transport

Predicting the fate of sand in the nearshore requires numerical models which accurately calculate shallow water sediment transport under a variety of wave, current, and combined wave-current conditions. These models typically use bulk wave and current parameters to calculate bed shear stress, and from there to simulate where and how much sediment is mobilized. Here, we use laboratory observations coupled with a numerical model to improve our understanding of how bed shear stress responds to combined wave-current conditions in a shallow sandy environment. In the experiment, called MODEX (MOrphological Diffusivity EXperiment), we forced a variety of wave and current flows in one direction over a submerged, shallow, sandy (grainsize: 200 μm) mound (Figure 1). Of the nine flow scenarios, four were combined wave-current flows and five were either waves alone or currents alone. The suite of instruments deployed in the flume measured combined wave-current velocities around the mound, vertical profiles of combined velocities near the mound crest, and mapped the eventual mound diffusion. These data are used to validate a 3D wave-resolving numerical model SWASH, which is then applied to simulate a range of wave-current conditions supplementing those measured in MODEX. For the range of conditions, the relative contributions of waves and mean currents to the boundary shear stress varies while the total boundary shear stress is kept constant. These flow conditions, encompassing waves-alone to wave-current to currents-alone, are used to compare the spatial variation and magnitude of bed shear stress as flow passes over the mound. The link between shear stress along the mound to observed mound diffusion patterns and rates for the range of wave-current conditions will be discussed.