Controls on Yardang Morphology: Insights from Field Measurements, Lidar Topographic Analyses, and Numerical Modeling

Abstract:

Yardangs are streamlined bedforms sculpted by the wind and wind-blown sand. They can form as relatively resistant exposed rocks erode more slowly than surrounding exposed rocks, thus causing the more resistant rocks to stand higher in the landscape and deflect the wind and wind-blown sand into adjacent troughs in a positive feedback. How this feedback gives rise to streamlined forms that locally have a consistent size is not well understood theoretically. In this study we combine field measurements in the yardangs of Ocotillo Wells SVRA with analyses of airborne and terrestrial lidar datasets and numerical modeling to quantify and understand the controls on yardang morphology. The classic model for yardang morphology is that they evolve to an ideal 4:1 length-to-width aspect ratio that minimizes aerodynamic drag. We show using computational fluid dynamics (CFD) modeling that this model is incorrect: the 4:1 aspect ratio is the value corresponding to minimum drag for free bodies, i.e. obstacles around which air flows on all sides. Yardangs, in contrast, are embedded in Earth’s surface. For such rough streamlined half-bodies, the aspect ratio corresponding to minimum drag is larger than 20:1. As an alternative to the minimum-drag model, we propose that the aspect ratio of yardangs not significantly influenced by structural controls is controlled by the angle of dispersion of the aerodynamic jet created as deflected wind and wind-blown sand exits the troughs between incipient yardang noses. Aerodynamic jets have a universal dispersion angle of 11.8 degrees, thus predicting a yardang aspect ratio of ~5:1. We developed a landscape evolution model that combines the physics of boundary layer flow with aeolian saltation and bedrock erosion to form yardangs with a range of sizes and aspect ratios similar to those observed in nature. Yardangs with aspect ratios both larger and smaller than 5:1 occur in the model since the strike and dip of the resistant rock unit also exerts a first-order control. We report on the analysis of a 1 m/pixel airborne-lidar-derived DEM of all of the yardangs in Ocotillo Wells, a map of erosion/deposition obtained from terrestrial-laser-scanning-derived DEMs obtained 1 year apart, 4D data for wind velocities and aeolian sediment fluxes around the yardangs, and the orientations and strengths of rock strata.