Steady-State Dune Morphodynamics Through the Barchan-Parabolic Transition

Dunes in the presence of vegetation exhibit a variety of shapes, from barchan to parabolic forms. Given the rapid fluctuations in environmental conditions across space and time, it is challenging to ascertain whether these dune shapes are merely transient or indicative of a dynamic equilibrium between sediment transport and vegetation growth. In this study, plant cover is introduced into a 3D cellular automaton dune model to numerically investigate the influence of vegetation on dune morphodynamics. Numerical simulations show that isolated parabolic dunes are unstable, increasing or decreasing in size according to the volume of vegetated sediment they remobilize downstream or deposit upstream in their horns. When specific boundary conditions are applied, dunes converge on steady states that accurately capture the barchan-parabolic transition. Most of the isolated dune shapes observed in nature are reproduced at steady state in the model by increasing the intensity of vegetation processes, from the typical migrating barchans to the fully stabilized parabolic dunes. As the impact of vegetation increases, the steepness of the steady-state dune slopes changes, and the crest curvature reversal occurs. This ensures that all the longitudinal sections of the dune migrate at the same rate by reorienting the transverse sediment fluxes. In the model parameter space, sudden jumps in steady-state properties are associated with the stabilization of upstream horns or crest curvature reversals. These results illustrate why transitional dune shapes between barchans and parabolic dunes are less common in natural environments where environmental conditions are heterogeneous and variable in both space and time.