CFD simulations of wind flow across scarped foredunes: Implications for sediment pathways and beach-dune recovery after storms

Beach-dune systems are dynamic geomorphic environments subject to erosion during storms and reconstruction during swell-dominated inter-storm periods. Wave-cut scarps are frequently carved into the beach-dune profile, occasionally eroding the stoss slope of the foredune. Although the rapid phases of erosion have been researched and modelled extensively, relatively little is known about beach-dune recovery, especially scarp healing by aeolian processes. Computational fluid dynamic (CFD) modelling was deployed in this study to yield insights into different flow patterns across beach-dune profiles with three model foredunes (unscarped, 1 m scarp, 2 m scarp) for varying wind approach angles (alpha = 0-45 degrees). The results show that the presence of a scarp substantially modifies the flow dynamics on the backbeach in front of the foredune. Vertical profiles of turbulence kinetic energy indicate that the influence of this flow-modified zone extends out to about -7h from the scarp base (where h is the scarp height). The reduction in onshore wind speed that is characteristic of this flow-modified zone favours sand deposition on the backbeach and sand ramp development at the base of the scarp. Shore-normal wind approach angles yield discontinuous roller-like vortices with reversing (offshore) flow components at the sand surface, accompanied by the evolution of echo dunes. Oblique wind approach angles yield corkscrew (helicoidal) vortices that lead to the formation of continuous sand ramps alongshore. A conceptual model of scarp healing based on ideas previously discussed in the literature and enhanced by the CFD simulation results highlights the importance of sand ramps for reconnecting the transport system on the beach to the foredune stoss slope, thereby facilitating foredune growth and maintenance.