Understanding wind-blown sand: Six vexations

Abstract

The study of aeolian geomorphological systems over the last half century has advanced along several important fronts, supported by developments in technologies and analytic methods and the evolution of modeling approaches. Remote sensing has led to a wealth of knowledge concerning the distribution and dynamics of dunes on Earth and elsewhere, and wind tunnels have elucidated the roles of aeolian processes in reshaping surfaces. Improvements in laboratory and field instrumentation allow measurement of aeolian sand transport systems at levels of detail unimaginable when the Binghamton Geomorphology Symposium began. We have seen, for example, the advent of large, sophisticated field campaigns and advanced modeling approaches using cellular automata and computational fluid dynamics methods. Select avenues of development, especially concerned with depositional environments, are briefly summarized. Advances in many fields have not been matched with proportional advances in our ability to predict rates of sand transport in natural environments, one of the most fundamental challenges. At least eight, commonly cited, sand transport-rate models do not agree with each other and do not replicate field observations even under near-ideal conditions. This review focuses on select reasons why that might be so, and categorizes those reasons into six, principal theoretical and empirical vexations that have made sand transport an intractable problem: predicting rates of transport; characterizing grain size; characterizing fluid and sand densities; estimating shear velocity; predicting the threshold of motion, including the effects of sand moisture content; and understanding granular electrification. The examination of potential sources of error in the rate of transport and threshold of motion models leads to a point-by-point consideration of our current assumptions and protocols (or lack thereof). The objective is to help us identify the hard targets and the soft targets for future research.