Observations from wave basin experiments and wave-resolving numerical simulations demonstrate the effect of wave-current interaction on shear stress around a sandy mound. Observations from the wave basin show that the mound deformation rate and morphological patterns depend on the mixture of waves and currents in the incident flow conditions. A SWASH nonhydrostatic numerical model was used to expand the parameter space of wave-current conditions observed in the flume and characterize the response of the near-bed shear stress to the mound. The model was validated with observations from wave-alone, current-alone, and wave-current flume tests and then ran for a suite of numerical flow conditions which isolate the impact of the ratio of wave-current energy on the bed shear stress. Results show how the current-to-wave ratio impacts the spatial heterogeneity of shear stress across the mound, with the region of shear stress intensification around the mound and the location of the peak shear stress becoming asymmetric with more mixed wave-current flows. These results show the nonlinear response of shear stress patterns to combined wave-current flows and how these patterns may impact eventual sediment transport and mound evolution.

Morphodynamics of sandy beaches are governed by sediment transport processes. Conceptualising the process of subaqueous and subaerial sediment transport and quantifying the magnitude of sediment transport have proven to be a difficult challenge. The challenge is reflected by the many concepts available that describe sediment transport and its relationship with hydrodynamic processes and sediment compositions. This chapter gives an overview of the most important concepts that are currently available and widely used to describe sediment transport processes.

Concepts include timescale-dependent sediment transport as a function of different hydrodynamic processes such as waves, currents and turbulent forces. Also, the role of sediment properties and the most important modes of sediment transport, bedload and suspended load transport are discussed. In a further conceptualisation, cross-shore processes are distinguished from longshore processes given the nature of varying hydrodynamic gradients along the sandy beach. The rare measurements of sediment transport provide some evidence that help quantify the concepts.

Finally, reflection on current challenges that need to be addressed to achieve a more comprehensive and detailed model for sediment transport processes is made.