Vortex ripples induced by oscillatory flow display straight crests at equilibrium, but they develop transient three‐dimensional (3‐D) bedform geometries when transitioning from one equilibrium state to another due to changes in the oscillatory flow. Large‐eddy simulations were performed of oscillatory flow over 3‐D bedform defects associated with an increase of the orbital motion amplitude, similar to what might happen during the waxing phase of a storm. The objective is to determine the intercorrelation between flow processes and evolution of 3‐D bedform defects toward an equilibrium two‐dimensional (2‐D) state of vortex ripples. Results are presented for oscillatory flows over fixed ripples with sinusoidal crests in the spanwise direction, which introduce slope asymmetries on opposite sides of the crests, during different transient states. It was found that steep‐sloped regions of the bedforms are associated with thicker recirculation areas, whereas in more gently sloped regions thinner recirculation vortices exist. Furthermore, streamwise vortical structures and significant secondary flow develop in the vicinity of sinusoidal crests, relative to straight crests. These flow structures might be responsible for the gradual reformation of the rippled bed, as they induce higher bed shear stresses, that is, increasing bed sediment transport, and they diffuse turbulent kinetic energy to higher levels above crests, that is, increasing suspended sediment transport, relative to equilibrium 2‐D ripples. Finally, the period‐averaged currents around the sinusoidal crests exhibit higher velocities on the steep‐sloped side than on the mild‐sloped one, also likely contributing to sediment transport and bed reformation toward a 2‐D configuration.

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Erosion and flooding are the main threats for most coastal regions. Nature-based solutions, such as coastal vegetation (CV) fields, have received significant attention in recent years because they effectively dissipate wave energy while maintaining biodiversity. In this study, a porous medium model was implemented in an in-house software to simulate the three-dimensional, two-phase (water/air) flow induced by wave propagation past CV fields. First, the model was validated against experimental measurements of wave propagation past a CV field on a horizontal bed. Then, the model was used to assess the influence on wave behaviour of the equivalent porosity, neq, and the cross-shore length, LCV, of a CV field with a constant plant height on a beach with a constant bed slope. Results were obtained for the non-vegetated case (neq = 1) and five vegetated cases with different neq and LCV values. It was found that decreasing neq moves wave breaking offshore at larger depths (40% increase for the case with neq = 0.82), while it does not affect much the breaking height (4% was the largest difference). For very high neq (0.98 in the present study) and/or small LCV (83% of the wavelength just offshore of the CV field in the present study), wave breaking may occur at depths smaller than in the non-vegetated situation due to increased wave shoaling over the corresponding CV fields. The undertow distribution is strongly modified and an increased wave setup (highest for neq = 0.82) is observed for all vegetated cases in comparison to the non-vegetated one.

An advanced numerical model is presented for the simulation of wave-induced free-surface flow, utilizing an efficient hybrid parallel implementation. The model is based on the solution of the Navier–Stokes equations using large-eddy simulation of large-scale coastal free-surface flows. The three-dimensional immersed boundary method was used for the enforcement of the no-slip boundary condition on the bed surface. The water-air interface was tracked using the level-set method. The numerical model was effectively validated against laboratory measurements involving wave propagation over a flatbed with an elliptical shoal, whose presence induces combined wave refraction and diffraction phenomena. The parallel implementation of the model enabled the efficient simulation of depth-resolved, wave-induced, three-dimensional, free-surface flow; the model parallel efficiency and strong scaling are quantitatively demonstrated.