Dune initiation in a bimodal wind regime

Early-stage bedforms develop into mature dunes through complex interactions between wind, sand transport and surface topography. Depending on varying environmental and wind conditions, the mechanisms driving dune formation and, ultimately, the shape of nascent dunes may differ markedly. In cases where sand availability is plentiful, the emergence and growth of dunes can be studied with a linear stability analysis of coupled transport and hydrodynamic equations. Until now, this analysis has only been applied using field evidence in uni-directional winds. However, in many areas of the world and on other planets, wind regimes are more often bimodal or multimodal. Here, we investigate field evidence of protodune formation under a bimodal wind regime by applying linear stability analysis to a developing protodune field. Employing recent development of the linear stability theory and experimental research, combined with in-situ wind, sediment transport, and topographic measurements during a month-long field campaign at Great Sand Dunes National Park, Colorado, USA, we predict the spatial characteristics (orientation and wavelength) and temporal evolution (growth rate and migration velocity) of a protodune field. We find that the theoretical predictions compare well with measured dune field attributes as characterized by high-resolution Digital Elevation Models measured using repeat terrestrial laser scanning. Our findings suggest that linear stability analysis is a quantitative predictor of protodune development on sandy surfaces with a bimodal wind regime. This result is significant as it offers critical validation of the linear stability analysis for explaining the initiation and development of dunes towards maturity in a complex natural environment.