Exploring wind flow dynamics in foredune notches using Computational Fluid Dynamics (CFD)

Coastal dunes offer a wide range of valuable ecosystem services such as protection from erosion, flooding, sea-level rise, and provision of specialised habitat for endangered, endemic, or migratory species. Foredune blowouts and landward migrating parabolic dunes play an important role in many coastal dune settings creating ecological heterogeneity associated with inland sand transport, nutrient supply, and geomorphic disturbance processes. However, as coastal dunes globally are being increasingly stabilised by vegetation and declining in their ecological resilience and functionality, anthropogenic interventions, such as the removal of invasive species and excavation of foredune notches, have emerged to simulate and restore critical aeolian processes required to maintain dune morphodynamics and onshore sediment transport between the beach and inland dunes. This study employed computational fluid dynamics (CFD) modelling to investigate key controls on the wind flow dynamics and sand transport potential within idealised foredune notches of varying widths, slopes, and planform shape (rectangular vs. trapezoidal) for perpendicular and oblique incident wind directions. Compared with empirical findings from similarly engineered notches, our results show that notch width significantly influences shear velocity in the excavated notch 'slot', with narrower notches (25 m wide) enhancing wind flow acceleration and inland sediment transport potential. Spatial patterns of shear velocity throughout notches were also sensitive to incident wind direction, with maximum shear velocities, and consequent inland sand transport potential, occurring when winds were parallel to the orientation of the notch. On the lobes of the notches, shear velocity and sand transport potential were greatest during oblique winds. Our results suggest that a relatively narrow notch (e.g. 25 m as opposed to 50 m or 100 m), aligned with the prevailing wind direction, creates the most favourable conditions for transporting sediment from the beach to the dune behind. These findings underscore the importance of notch design in coastal dune restoration, offering critical insights for optimising interventions aimed at sustaining aeolian sediment transport from the beach to the hinterdune.