EP010-02
The emergence of proto-bedforms in a bimodal wind regime
Tuesday, 8 December 2020: 04:04
Virtual
Pauline Delorme1, Giles Wiggs2, Matthew C Baddock3, Philippe Claudin4, Joanna M Nield1 and Andrew Valdez5, (1)University of Southampton, Southampton, SO14, United Kingdom, (2)University of Oxford, Oxford, United Kingdom, (3)Loughborough University, Geography, Loughborough, United Kingdom, (4)Laboratoire de Physique, ENS - PSL Research University, Université de Paris, CNRS, Sorbonne Université, Paris, France, Paris Cedex 05, France, (5)Great Sand Dunes National Park and Preserve, Resource Management Division, Mosca, CO, United States
Abstract:
Early-stage aeolian bedforms develop into sand dunes through complex interactions between flow, sediment transport and surface topography. Depending on site-specific environmental and wind conditions, the mechanisms of dune formation, and ultimately the shape of the nascent dunes, differ. When the sand availability is unlimited (a sand bed) the emergence of a dune can be studied in the scope of the linear stability analysis. In this analysis the wind is accelerated upwind of the crest of a developing dune and decelerated downwind. This process shifts the location of the maximum basal shear stress experienced on the bedform to upwind of the crest. In addition, the sand flux does not adjust instantaneously and requires some distance (the saturation length, L
sat) to relax towards its saturated value, thus inducing a downwind lag in the position of the maximum sand flux. The balance between the destabilizing hydrodynamic process and the stabilising transport process results in bedform growth and nucleation.
Here, we present the first field evidence for the operation of the linear stability process resulting in the emergence of proto-bedforms in a bimodal wind regime. Employing recent theoretical and experimental research, combined with in-situ wind, sediment transport, and topographic measurements during a month-long field campaign, we use a development of the linear stability theory to predict the spatial characteristics (orientation and wavelength) and temporal evolution (growth rate and migration velocity) of a proto-dune field.
We find that the output of the linear stability analysis compares well to high-resolution Digital Elevation Models measured using terrestrial laser scanning. Our findings suggest that the bed instability mechanism is a quantitative predictor of proto-dune development on sandy surfaces with a bimodal wind regime and supports the application of the analysis in determining dune growth characteristics in complex wind environments elsewhere.
