Morphodynamics of shadow dunes

Shadow dunes – simple linear dunes that are obstacle-related – occur on the non-erodible bedform, mostly occur under unimodal wind regime but also bimodal wind regime, and extend parallel to the resultant drift direction. Their morphology is dominantly controlled by the incident wind regime, the topographic obstacle dimensional parameters (e.g., height, width and aspect ratio), spatial alignment and porosity (e.g., blunt body or vegetation covered). Although previous studies have focused on the development of shadow dunes, there are still open issues to be solved. The aim of this study is to review the major studies on shadow dune morphodynamics and to demonstrate up-to-date knowledge on this topic. New results from field observation suggest that shadow dunes could elongate under unimodal wind regimes due to the collective effects of flow deflection around the sand ridge and the intermittent occurrence of salt crust on the dune surface. Shadow dunes always develop in fingering mode on non-erodible bedforms regardless of wind regimes. While obstacle morphology and spatial alignment contribute to shadow dune's initial morphology, the wind regime and sand availability determine their long-term stability. The driving force of shadow dune formation may vary depending on the obstacle scale. For example, the shrub- or nebkha-scale shadow dunes are dominantly controlled by the horizontal separation flow. However, mountain-scale shadow dunes may be caused by the wind seasonal circulation. This study provides new insights into shadow dune morphodynamics and identifies critical future research directions on obstacle-related dunes.